RAMMED EARTH: FIBER-REINFORCED, CEMENT

RAMMED EARTH: FIBER-REINFORCED, CEMENT-STABILIZED
by
ERIC WALTER SIMENSON
B.S., University of Colorado Denver, 2011
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirements for the degree of
Master of Science
Civil Engineering
2013
This thesis for the Master of Science degree by
Eric Walter Simenson
has been approved for the
Civil Engineering Program
by
Dr. Frederick Rutz, Chair
Dr. Kevin Rens
Dr. Nien-Yin Chang
Date: November 12th, 2013
ii
Simenson, Eric Walter (M.S., Civil Engineering)
Rammed Earth: Fiber-Reinforced, Cement-Stabilized
Thesis directed by Assistant Professor Dr. Frederick Rutz
ABSTRACT
This thesis examines the use of cement and synthetic plastic fiber additives to
improve the strength of rammed earth walls. These additives can be an economical
solution to increasing the strength of rammed earth, which by itself is typically a lowstrength material when additives are not used. Cement can be easily incorporated into the
soil mixture and it adds strength and durability to the wall. Modern fibers, such as
polypropylene plastic, have high tensile strength, they are durable, and they can be
incorporated into rammed earth material. The fibers can add shear strength and flexural
strength to rammed earth material. Three mix designs were created: soil only, soilcement, and soil-cement with fiber. Based upon the results of this thesis, the use of
cement additive in rammed earth significantly increased the compressive, shear, and
flexural strength. The addition of fiber did not increase the ultimate strengths, but it did
provide a secondary benefit of keeping material bound together after failure and
increased residual strength after failure.
The form and content of this abstract are approved. I recommend its publication.
Approved: Dr. Frederick Rutz
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DEDICATION
I dedicate this work to the family and friends who supported my efforts.
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ACKNOWLEDGMENTS
I would like to thank my advisor Dr. Frederick Rutz. His interest in analyzing
historic structures encouraged me to do the same and follow my passion.
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TABLE OF CONTENTS
CHAPTER
I. EARTH CONSTRUCTION ............................................................................................ 1
Advantages of Earth Homes ................................................................................... 1
Disadvantages of Earth Homes ............................................................................... 2
II. HISTORICAL BACKGROUND ................................................................................... 4
III. EARTH CONSTRUCTION RESEARCH ................................................................... 9
Strength of Compacted Earth ................................................................................ 10
Soil Selection Criteria ........................................................................................... 10
Direct Tensile Testing ........................................................................................... 11
Structural Behavior of Rammed-Earth Walls ....................................................... 12
Assessing the Anisotropy of Rammed Earth ........................................................ 12
Soils Reinforced with Plastic Fiber....................................................................... 13
IV. MATERIALS ............................................................................................................. 15
Soil ........................................................................................................................ 15
Sand and Aggregate .............................................................................................. 15
Cement .................................................................................................................. 16
Reinforcement ....................................................................................................... 17
V. RAMMED EARTH PROCESS ................................................................................... 18
VI. OBJECTIVE OF STUDY........................................................................................... 24
VII. MIX DESIGN............................................................................................................ 25
Soil Selection Requirements ................................................................................. 25
Optimum Moisture Content and Maximum Dry Density ..................................... 32
Cement and Fiber Additives ................................................................................. 36
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VIII. EXPERIMENT DESCRIPTION ............................................................................. 38
Unconfined Compression Test Preparation .......................................................... 38
Direct Shear Preparation ....................................................................................... 44
Modulus of Rupture Preparation ........................................................................... 50
IX. TEST RESULTS ........................................................................................................ 53
Unconfined Compression...................................................................................... 53
Modulus of Elasticity ............................................................................................ 61
Direct Shear .......................................................................................................... 66
Modulus of Rupture .............................................................................................. 79
X. CONCLUSION ............................................................................................................ 86
Recommendations for Future Research ................................................................ 87
REFERENCES ................................................................................................................. 89
APPENDIX ....................................................................................................................... 92
New Mexico Building Code Excerpts .................................................................. 92
Fiber Specifications from Manufacturer ............................................................. 104
Unified Soil Classification System ..................................................................... 105
Gypsum Cement Specifications .......................................................................... 106
Portland Cement Specifications .......................................................................... 108
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LIST OF TABLES
TABLE
VII.1 Gradation Results from the Soil. ............................................................................ 27
IX.1 Unconfined Compressive Strength for Soil Samples. ............................................. 56
IX.2 Unconfined Compressive Strength for Soil-Cement Samples................................. 56
IX.3 Unconfined Compressive Strength for Soil-Cement-Fiber Samples. ...................... 57
IX.4 Modulus of Elasticity for Soil-Cement Samples. .................................................... 64
IX.5 Modulus of Elasticity for Soil-Cement-Fiber Samples. .......................................... 65
IX.6 Adjusted Modulus of Elasticity for Soil-Cement-Fiber Samples. ........................... 66
IX.7 Soil Direct Shear Test Results. ................................................................................ 69
IX.8 Soil-Cement Direct Shear Test Results. .................................................................. 70
IX.9 Soil-Cement-Fiber Direct Shear Test Results.......................................................... 71
IX.10 Soil Modulus of Rupture Test Results. .................................................................. 81
IX.11 Soil-Cement Modulus of Rupture Test Results. .................................................... 81
IX.12 Soil-Cement-Fiber Modulus of Rupture Test Results. .......................................... 82
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LIST OF FIGURES
FIGURE
II.1 Pueblo de Taos. ........................................................................................................... 6
II.2 Hakka Earth Houses. ................................................................................................... 7
II.3 Miller House. ............................................................................................................... 8
V.1 Rammed Earth Forms. .............................................................................................. 19
V.2 Concrete Foundation. ................................................................................................ 21
V.3 Modern adobe wall.................................................................................................... 22
V.4 Rammed Earth Wall. ................................................................................................. 23
VII.1 Sieve Stack. ............................................................................................................ 26
VII.2 Liquid Limit. ......................................................................................................... 28
VII.3 Casagrande Device. ................................................................................................ 29
VII.4 Plastic Limit. .......................................................................................................... 31
VII.5 Standard Proctor Tools. .......................................................................................... 33
VII.6 Air-drying Soil. ...................................................................................................... 34
VII.7 Compaction Curve. ................................................................................................. 35
VII.8 Polypropylene Fibers. ............................................................................................. 37
VIII.1 Color of Soil-Cement and Soil. ............................................................................. 39
VIII.2 Fibers in Soil. ........................................................................................................ 40
VIII.3 Soil-Cement in Proctor Mold. ............................................................................... 41
VIII.4 Extracted Soil-Cement Sample. ............................................................................ 41
VIII.5 Capping Cylinders................................................................................................. 43
VIII.6 Unconfined Compression Sample. ........................................................................ 44
VIII.7 Liner on Proctor Mold........................................................................................... 45
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VIII.8 Liner inside Mold. ................................................................................................. 45
VIII.9 Liner filled with Soil. ............................................................................................ 46
VIII.10 Pneumatic Jack. ................................................................................................... 47
VIII.11 Soil and Liner. ..................................................................................................... 48
VIII.12 Cut Shear Samples. ............................................................................................. 49
VIII.13 Beam Mold. ......................................................................................................... 51
VIII.14 Modulus of Rupture. ........................................................................................... 52
IX.1 MTS Machine. ......................................................................................................... 54
IX.2 Forney Testing Machine. ......................................................................................... 55
IX.3 Crushed Soil Sample................................................................................................ 59
IX.4 Crushed Soil-Cement Sample. ................................................................................. 59
IX.5 Crushed Soil-Cement-Fiber Sample. ....................................................................... 60
IX.6 Soil-Cement Stress-Strain. ....................................................................................... 62
IX.7 Soil-Cement-Fiber Stress-Strain. ............................................................................. 63
IX.8 Soil Shear Stress. ..................................................................................................... 69
IX.9 Soil-Cement Shear Stress. ....................................................................................... 70
IX.10 Soil-Cement-Fiber Shear Stress. ............................................................................ 71
IX.11 Shear Stress Failure of Soil.................................................................................... 73
IX.12 Shear Stress Failure of Soil-Cement. ..................................................................... 74
IX.13 Shear Stress Failure of Soil-Cement-Fiber. ........................................................... 74
IX.14 Soil Direct Shear Plot. ........................................................................................... 75
IX.15 Soil-Cement Direct Shear Plot............................................................................... 76
IX.16 Soil-Cement-Fiber Direct Shear Plot. .................................................................... 77
IX.17 Soil-Cement Modulus of Rupture. ......................................................................... 83
IX.18 Soil-Cement-Fiber Modulus of Rupture. ............................................................... 84
x
IX.19 Soil-Cement-Fiber Modulus of Rupture Close-Up................................................ 84
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LIST OF EQUATIONS
EQUATION
IX.1 Unconfined Compressive Strength. ......................................................................... 53
IX.2 Modulus of Elasticity. .............................................................................................. 61
IX.2 Shear Strength. ......................................................................................................... 68
IX.4 Modulus of Rupture for 4-point loading. ................................................................. 80
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LIST OF ABBREVIATIONS
in
lb
MOR
OMC
psi
SC
SCF
tsf
UCS
inch
pound
modulus of rupture
optimum moisture content
pounds per square inch
soil-cement
soil-cement with fiber
tons per square foot
unconfined compressive strength
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CHAPTER I
EARTH CONSTRUCTION
Advantages of Earth Homes
Rammed earth houses are universally available. Earth is a free building material
and no person or company can monopolize the building material. Anyone with a plot of
land and a few helping hands can construct an earthen home. These homes require only
simple building techniques, and materials can be locally sourced. Earth homes offer a
sustainable housing solution for everyone in the world.
These houses are very durable and require little maintenance over the life of the
structure. The cost of maintaining a rammed earth can be very low. Typically, the only
regular maintenance is refinishing the exterior with a fresh coat of plaster. This coating
helps protect the wall from moisture and erosion. The soil in the wall contains no organic
matter and it will not biodegrade overtime. Unlike rammed earth homes, wood homes
will slowly deteriorate as the wood rots from moisture and microorganisms. The earthen
walls do not attract rodents or insects because there is nothing in the wall for these pests
to eat. Termites can cause serious problems for wood homes, which makes earth homes
an excellent choice in termite prone regions.
The massive walls, usually at least 18 inches thick, perform well in adverse
weather conditions. The walls can handle high winds, which can be beneficial in
hurricane and tornado prone areas. The weight of walls is self-anchoring against uplift
forces and the thick walls are much more rigid than typical wood framed homes. Earth
homes offer a natural barrier from cold temperatures and extreme fluctuations in
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temperature. The thick walls act as an energy collector and store heat from the sun. This
energy is slowly radiated inside and it helps keep the indoor air temperature constant.
Rammed earth homes are very quiet inside. The mass of the walls provides
exceptional insulation from outside noise. As David Miller notes (Miller 1980), “Earth
structures are also very quiet due to mass, added insulation, and double glazing. This
lack of noisy and irksome furnaces, compressors, fans, and ducts helps contribute to
peace of mind.” Many modern earth homes typically utilize radiant heat in the floors;
thus, they do not have noisy forced-air furnaces for heating. Air-conditioning is typically
not needed, even in desert climates, because the thermal mass of the walls regulates the
indoor air temperature.
Disadvantages of Earth Homes
Earth homes are very labor intensive to build. In areas with a high labor cost,
there is a large upfront cost to construct an earthen home. The walls must be built onsite
and there is no mass production of prefabricated building materials when it comes to
building a rammed earth home. In developed countries, rammed earth homes have
difficulty competing with cheap building products, like wood. Wood homes can be
rapidly built, which reduces costs associated with labor.
Modifications are to existing rammed earth homes are challenging. Rammed
earth walls are much more permanent than typical wood framed walls. It is not easy to
cut new openings in walls for windows or doors. This can increase future construction
costs if modifications to the structure are needed.
Rammed earth is an unusual and uncommon building material. It carries a stigma
because people tend to think only the poor live in “mud huts”. The stigma is primarily a
2
result of society being unfamiliar with modern rammed earth homes. Many examples of
high-end rammed earth homes can be found in the American West. Still for many
people, building a home from uncommon materials creates some apprehension in their
mind. People might be uncertain about how the building will perform or who will be
qualified to inspect and service this unique structure. These are valid concerns and
thankfully all these concerns can be addressed through education about rammed earth.
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CHAPTER II
HISTORICAL BACKGROUND
Building a shelter is a primitive instinct in humans. In the last several thousand
years, ancient cultures across the globe built dwellings of earth and stone. More recently,
the industrial revolution brought assembly-line mechanization that produces cheap
building products of wood, steel, and concrete. As a result of the industrial revolution,
the prevalence of earth buildings has declined in the last century. A new era of earth
building is on the horizon with people looking for sustainable, economical, and durable
housing. This renewed interest brings challenges as building experts must bring this
ancient technology up to modern building codes and standards.
Examples of rammed earth can be found all around the world. Many of the
ancient examples have disappeared as they succumbed to the forces of nature. Some of
the first cities in the world were built from earth, since earth was a readily available
resource. Building materials in the Middle East were scarce and they mainly consisted of
earth and stone, as good timber was not abundantly available. The Tower of Babel in the
seventh century B.C. was constructed of sun baked bricks and parts of the Great Wall of
China were built with rammed earth (Easton 2007). In the first century A.D., the
historian Pliny the Elder documented rammed earth construction in his book, “Natural
History” (Bostock 1885).
And then, besides, have we not in Africa and in Spain walls of earth,
known as "formaceoan" walls from the fact that they are moulded, rather
than built, by enclosing earth within a frame of boards, constructed on
either side. These walls will last for centuries, are proof against rain,
4
wind, and fire, and are superior in solidity to any cement. Even at this day,
Spain still beholds watch-towers that were erected by Hannibal, and
turrets of earth placed on the very summits of her mountains. It is from the
same source, too, that we derive the substantial materials so well adapted
for forming the earth-works of our camps and embankments against the
impetuous violence of rivers. What person, too, is unacquainted with the
fact, that partitions are made of hurdles coated with clay, and that walls
are constructed of unbaked bricks?
From the Middle East, earth construction spread to parts of Africa and Asia. The
Moors brought rammed earth and mud brick technology called adobe to Spain. Earth
building technology then traveled to South America and eventually north to Mexico
during the Spanish conquests. Prior to the arrival of the Spanish conquistadors, Native
Americans, mainly the Pueblo people, used basic earth buildings techniques throughout
the American southwest around A.D. 700. Taos Pueblo community in northern New
Mexico, Figure II.1, was established in the late 13th and early 14th centuries and located
there is the oldest continuously occupied structure in North America (Easton 2007). It is
one of the best examples of prehispanic architecture in the Americas. It is made from
adobe mud bricks and some of the walls are several feet thick. The main multistory
structure consists of residential apartments that usually have two rooms, one for sleeping/
living and the other for cooking. The primary access to each apartment was by ladder
through an opening on the roof. The roof entrance was designed for defensive purposes
and the ladders could be removed if the village came under attack.
Traditional doors
5
were added later when the Spanish settled in the area and spread their architectural
influence.
Figure II.1 Pueblo de Taos.
The north side of the main structure at Taos Pueblo. Photo by Captain-tucker. Used with
permission.
For the last several centuries different methods of earth construction have been
used throughout the world. A variation of adobe called cob was common in the United
Kingdom. Cob uses softball sized lumps of wet clay stacked together to form thick walls.
Many examples of historic earthen buildings exist in Rhone River valley of France. The
French call rammed earth pisé de terre and they used wooden forms filled with
compacted layers of earth. The Chinese have used rammed earth for thousands of years
and the Hakka people of southern China currently occupy multistory buildings built from
rammed earth, Figure II.2.
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Figure II.2 Hakka Earth Houses.
A cluster of circular and square rammed earth homes in southern China. Photo by yaziswallow. Used with permission.
More recently a significant amount of research into rammed earth was performed
by CRATerre (Center for the Research and Application of Earth) in France during the
1970’s to 1990’s under the leadership of Hugo Houben, Patrice Doat, and Hubert
Guillard (Houben and Guillard 2008). CRATerre is still conducting research and
students can earn a graduate degree in Architecture. Australia is another pioneer of
modern rammed earth construction. The lack of good timber in Australia spawned a
7
building industry focused around masonry, concrete, and earth. Both Australia and New
Zealand have well established and detailed building codes for earthen structures.
Even Colorado had two modern day rammed earth pioneers, Lydia and David
Miller. The Millers built five houses around Greely, Colorado during the 1940’s and
early 1950’s (Miller 1980). The private residence of the Millers is shown in Figure III.3.
California, Arizona, and New Mexico have many examples of earth buildings and these
states have a small, but very capable community of earth building experts. New Mexico
even has a section in their current building code that specifies requirements for earth
structures. Excerpts of the New Mexico building code are in the Appendix.
Figure II.3 Miller House.
Ranch style house built with rammed earth by the Millers. Photo by Mother Earth News.
Used with permission.
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CHAPTER III
EARTH CONSTRUCTION RESEARCH
The initial work on this thesis involved the review of numerous research papers,
books, and other sources related to rammed earth. No published research was found on
the use of plastic fiber reinforcement in rammed earth. Geotechnical papers discussed the
use of fiber reinforcement, but these papers were not addressing the use of fiber
reinforcement in rammed earth construction. Building a house from vertical walls of soil
is a unique blend of structural and geotechnical engineering, and relatively few research
papers focus on this topic.
Based upon the literature review, several research papers were selected for
discussion in this thesis. These selected research papers guided the work in this thesis to
a subject that had not been investigated. Specifically, the subject of using fiber and
cement together to improve the strength of rammed had not been thoroughly investigated.
Also, there were no research papers on rammed earth that tested compressive strength,
shear strength, and flexural strength in the same study. The New Mexico and Australian
building codes both have strength requirements, so research should focus on ways to
improve the strength of rammed earth in order to meet building codes. Furthermore,
research papers did not examine how fiber reinforcement changes the material properties
of rammed earth. Fiber reinforcement could be an economical means of increasing the
strength of rammed earth.
The content of the research papers that were selected for further discussion is
summarized below. The main purpose of discussing these research papers is to help the
reader understand the current body of knowledge. The papers on rammed earth include
9
studies on compressive strength, developing soil selection criteria, tensile strength
testing, structural behavior, anisotropic properties, and fiber reinforcement.
Strength of Compacted Earth
One study (Burroughs 2006) examined the strength of rammed earth with varying
amounts of stabilizers, like Portland cement. This study is significant due to the large
number of soils tested, which numbered 104 different soils. The author performed 219
strength tests using varying amounts of lime, cement, and asphalt emulsion for
stabilization. Stabilizers were added at an amount of 0-6% per dry soil weight. The
strength test was unconfined compressive strength (UCS). Each soil was tested for liquid
limit, plastic limit, linear shrinkage, and gradation.
Burroughs was able to classify soils as favorable or unfavorable for stabilization.
To be favorable for rammed earth construction, the UCS must be over 2 MPa (~290psi).
Soils that consistently performed well had sand contents >65%, plasticity index (PI) of
<15, and a shrinkage of <6%. It was observed that these favorable soils gain relatively
little compressive strength when stabilized with lime and/or cement. Unfavorable soils
(<2 MPa) exhibited more strength gain when stabilized, but their compressive strength
still did not meet the 2 MPa criteria.
Soil Selection Criteria
Another study (Burroughs 2008) continued the research started in the 2006 study
mentioned previously. In this study by Burroughs, soil samples from other research
studies were compared to the 104 samples collected by the author. Parameters for
determining suitable soils for rammed earth include: clay/silt content, sand content,
10
gravel content, liquid limit, plastic limit, plasticity index, and linear shrinkage. Optimal
parameters are as followed: clay/silt content ranged from 21% to 35%; sand content
ranged from 30% to 70%; gravel ranged from 13% to 62%; liquid limit ≤35; plastic limit
16-19; plasticity index <15; and linear shrinkage <6%. Linear shrinkage is the best
discriminator in determining the suitability of a soil for rammed earth and it was found
that 93 of 100 samples with a LS<6% meet the 290 psi criteria for unconfined
compressive strength. In other words, soils that have low shrinkage generally have good
strength for rammed earth walls. Gravel content and plastic limit are the worst
discriminators in determining the suitability of a soil.
Direct Tensile Testing
This study by Mesbah et al. (Mesbah et al. 2006) focused on how to test a
compressed earth block containing randomly distributed natural fibers for reinforcement.
The natural fibers were sisal fibers from the Agave plant and they ranged in length from
0.8 inches to 2.0 inches long. Natural fibers have been used throughout history as a
means to reinforce earth and they are commonly found in adobe blocks. In this case,
fibers were added at a rate of 0.5% per dry weight of soil.
The authors devised a means to directly pull apart the block and test the tensile
strength. It was noted that the fibers kept cracks from propagating and thus the tensile
strength was increased. The longer fibers, 2 inches, gave the blocks better residual
strength after cracks formed. The fibers did not fail, but they did pull out of the block.
Overall, the fibers improved the tensile strength and more research is needed to determine
the optimal length and quantity of fibers.
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Structural Behavior of Rammed-Earth Walls
This study by Reddy and Kumar (Reddy and Kumar 2011) was conducted to
examine the ultimate compressive strength of full scale rammed earth walls. This is an
important study because actual full scale walls were tested. Previous studies reviewed for
this thesis tended to focus on the testing of small cylindrical samples. This study by
Reddy and Kumar draws correlations between the strengths of small samples and full
scale walls. Three different sized specimens were tested: a prism measuring 6” long x 6”
thick x 12” tall, a small wall measuring 24” x 6” x 28”, and a wall 30” x 6” x 118”. The
prism had the highest average compressive strength, followed next by the small wall, and
finally the wall had the lowest strength. Much of the variation between the strength of
the three different specimens can be attributed to the slenderness ratio of each specimen.
The walls were the most slender and this leads to buckling as the walls deflect laterally
when loaded vertically.
Some important recommendations were collected from this study. The samples
should have enough time to cure and all the samples should be roughly the same moisture
content before testing. Compaction of the soil on the wet-side of the optimum moisture
content (OMC) curve results in higher compressive strength. Testing samples that are
fully saturated is recommended since this approach is done in testing of masonry.
Testing prisms rather than cylinders will better simulate field conditions and this
approach is done in masonry testing.
Assessing the Anisotropy of Rammed Earth
This study by Bui and Morel (Bui and Morel 2009) evaluates the possible
anisotropic nature of rammed earth. Since monolithic walls are constructed with layers
12
of compacted soil, there is concern about bonding between layers and the variations
within each layer. Layers that are too thick will not be compacted evenly. The upper
portion of the layer will be more compacted than the bottom, which means the density
will be higher in the upper portion.
To evaluate the rammed earth material, samples were loaded perpendicular to the
layers. This simulated how a wall is typically loaded from the dead loads of the building.
The samples were also loaded parallel to the layers. This simulated how the horizontal
forces of wind or earthquakes might load a wall. The results for compressive strength
and failure modulus (the ratio between maximum stress and maximum deformation) were
very similar. The study concluded at low stress, before separation of layers, the rammed
earth acts as a continuous material.
The possible density variation between the top and the bottom of a lift was also
examined. A homogenization process determined an equivalent elastic modulus for a
layer of rammed earth. The homogenized sample was compared to a typical nonhomogenized rammed earth sample. Again, the compressive strengths were similar.
Ultimately this study concluded rammed earth is isotropic if the layers remain
bonded together. The results of this research have a significant impact on the study of
rammed earth. By assuming the material is isotropic, simplified models can be used to
predict the behavior of rammed earth. Anisotropic material requires more detailed and
complicated analysis.
Soils Reinforced with Plastic Fiber
This study by Jiang et al. (Jiang et al. 2010) compared the engineering properties
of soil reinforced with different lengths and percentages of polypropylene fiber. These
13
samples were not rammed earth samples, but the study did provide general information
on how fibers change the strength properties of soil. Fiber lengths were 0.4 inches, 0.6
inches, 0.8 inches, and 1.0 inch. Fiber contents as a percentage of dry soil weight were
0.1%, 0.2%, 0.3%, and 0.4%. The researchers measured the unconfined compressive
strength and the shear strength using the direct shear test. It was concluded that fibers 0.6
inches or less increased both compressive strength and shear strength. Longer fibers
decreased the compressive and shear strength. A fiber content of 0.3% was the optimal
content to increase both compressive and shear strength.
This study also examined the effect of aggregate sizes in the fiber-reinforced soil
mixture. Aggregate sizes were <1mm, 1-2mm, 2-5mm, and 5-10mm. In general,
increasing the aggregate size lowered the unconfined compressive strength. The angle of
internal friction peaked at 3.5mm aggregate. Cohesion decreased as the aggregate size
increased. Overall, the conclusion of this study is polypropylene fiber can increase the
strength and stability of soil.
14
CHAPTER IV
MATERIALS
Soil
One soil was obtained and it was the only soil used in this experiment. The soil
was a Clayey Sand (SC) as defined by the Unified Soil Classification criteria. The liquid
limit was 26 and the plasticity index was 6. The optimum moisture content (OMC) was
11.5% and the maximum dry density was 121 pounds per cubic foot. The optimum
moisture content is the percentage of moisture in the soil that allows for maximum
density after compaction. All these tests were performed by the author of this thesis.
Linear shrinkage was not tested because Atterberg tests indicate this was a low plasticity
soil and these soils have very little shrinkage; thus, the linear shrinkage was likely below
the <6% criteria defined in other research (Burroughs 2006). This soil conformed to the
ideal properties developed in other studies mentioned in this thesis. It did have a very
low clay content, which means stabilization with cement was needed to achieve the
desired strength parameters.
Sand and Aggregate
Sometimes it is necessary to add sand and aggregate to soil mixture. For this
thesis, it was not necessary to add sand and aggregate. The native soil used in this project
already had a high sand content; hence, its classification as a Clayey Sand.
15
There are instances when sand and aggregate can improve the rammed earth soil
mixture. If the native soil was pure clay, it would be beneficial to add both sand and
gravel. Pure clays have the tendency to shrink and swell, and this can lead to cracking in
the wall. Another disadvantage of pure clay soil is cement does not bind well with it. A
general rule of thumb is the clay content of the rammed earth soil should be around 20 to
30 percent.
It is worth noting that some studies (Burroughs 2008, Jiang et al. 2010) have
found large aggregate reduces the strength of rammed earth. The larger material has a
smaller surface area, which will reduce the friction surface and this can lead to lower
strength in compression and shear.
Gravel and other aggregate are not always used in rammed earth projects. One
study (Burroughs 2008) found gravel content >13% can increase the strength of rammed
earth. It should be noted that gravel, like pea gravel, is typically graded to 3/8” diameter
and smaller gravel can be about 1/8” diameter or larger. Material passing through the
No. 4 sieve (~1/8” diameter) is considered sand. Gravel used in rammed earth projects is
typically pea gravel or smaller, and the use of bigger aggregate is not common.
Cement
Type I/II Portland cement was used to stabilize the soil. For this project, cement
was used at a rate of 6% per dry soil weight. The New Mexico building code specifies a
minimum of 6% cement for stabilized rammed earth. Other studies mentioned previously
in this thesis have used ranges of 5% to 10% cement. One author found low cement
contents, ~2%, can actually lower the strength because the cement interferes with the
16
inter-particle bonding of the clay (Minke 2006). That author and other researchers
consistently find cement contents above 2% increase the strength of the soil mixture.
Hydrated lime is another means of stabilization. Lime is well suited to soil with
high clay content. The lime does not interfere as much with inter-particle bonding of the
clay. For this study, lime was not preferred as a stabilizer because the overall clay
content of the final soil mixture was only ~20%. Also, cement seems to be the preferred
method of stabilization amongst the research studies.
Reinforcement
Polypropylene fiber was used in this study. The fibers were short (0.5 inch) in
length and mixed into the dry soil at a quantity of 0.3% per dry weight of soil. These
parameters are based upon the study of fiber-reinforced soil (Jiang et al. 2010). Other
studies (Consoli 1998, 2011) have used longer fibers and incorporated greater quantities
of fibers into the soil. Longer fibers have dispersion problems and tend to not mix well
with the dry soil as observed by the author of this thesis. Greater quantities of fibers can
lower compressive strength and shear strength.
Based upon trail batches of soil with various amounts of polypropylene fiber, it
was determined that 0.3% fiber content was acceptable for this thesis. Higher amounts
tended to ball up and disperse poorly in the soil. Lower amounts resulted pockets of soil
that had very little fiber reinforcement.
17
CHAPTER V
RAMMED EARTH PROCESS
Earth buildings are still being built all over the world. The buildings range from
simple one room homes to high-end luxury homes. Thousands of years of refining
building techniques and more recently the use of applied science have resulted in a
construction method that is a mix of art and science. Rammed earth research is ongoing at
many universities throughout the world and the body of knowledge surrounding rammed
earth is continuously expanding (Maniatidis and Walker 2003).
Rammed earth is an old building technique that has benefited from modern
science. The basic principle is compaction of a soil mixture into rigid forms to create
monolithic walls. The soil mixture is typically ~25% clay, ~60% sand, and ~15% gravel
(Burroughs, 2008). Modern practice is to stabilize the soil mixture with about 5-10%
hydrated lime or Portland cement as a percentage of the dry weight of soil. Concrete
forms or other heavy duty forms are needed to withstand the compaction force and
contain the earth wall until it hardens. Typical monolithic walls are 18 to 24 inches thick
and usually limited to two stories in height.
18
Figure V.1 Rammed Earth Forms.
These plywood and steel framed forms are held together in the middle with steel straps.
Photo by Eric Simenson.
The loose soil mixture is wetted to its optimal moisture content (OMC) and
placed in a loose layer, or “lift”, about 8 inches thick. Pneumatic “pogo-stick” tampers,
similar to those used in small back-fill compaction jobs, are used to compact the soil
(Adobe Builder 2001). Layers that are too thick will potentially have areas that are not
compacted because the tamper is not powerful enough to compact deep lifts of soil. In
practice, the final compacted layer typically is about half the thickness of the loose lift.
The forms are usually left in place for a few days until the cement partially cures.
Depending upon the wall thickness, the wall may take several months or more to fully
cure.
19
Modern rammed earth buildings can incorporate many features to improve the
performance and safety of the structure. The most common features include concrete
foundations and concrete bond beams at the bottom and top of the walls, respectively.
Other design features include the use of steel reinforcement with vertical and horizontal
rebar embedded in the wall. The use of rebar is relatively uncommon in rammed earth
construction and it is use in other types of earth construction techniques has not been
documented. The structural performance of the building can also be improved by
limiting the size and location of doors, windows, and other openings. These basic
principles are very similar to masonry design. Architectural features usually include
large roof overhangs to keep rain from eroding the walls.
To illustrate some of these modern features, Figure V.2 shows a typical rammed
earth foundation. A keyway is constructed in the top of the foundation to transfer shear
forces. Figure V.3 shows both a concrete foundation and a concrete bond beam on an
adobe house. Modern rammed earth and adobe walls have some similar features, like
bond beam and concrete foundations, because building codes require these features.
20
Figure V.2 Concrete Foundation.
Concrete foundation with a keyway serves as the foundation for a rammed earth wall.
Photo by Eric Simenson.
21
Figure V.3 Modern adobe wall.
Concrete foundation and concrete bond beam on an adobe wall. Photo by Eric Simenson.
Special admixtures can be incorporated into the soil and special coatings can be
applied to the finished wall to improve performance. Many modern rammed earth
buildings are coated with plaster and newer products like acrylic sprays are utilized as
wall coatings that provide weatherproofing. Coatings, like acrylic, act as waterproofing,
which helps prevent the wall from eroding due to moisture. Plaster and stucco coatings
22
provide a durable exterior surface that takes the wear and tear from mother nature. It is
routine maintenance to restore coatings at regular intervals.
Overall, the design of rammed earth buildings largely depends upon the soil
properties and the location of the building with regards to climate and environmental
hazards. Some earth buildings are currently hundreds of years old and they were made
without the use of cement and special coatings. Other buildings in moist climates need
additional detailing to prevent water from destroying the walls.
Figure V.4 Rammed Earth Wall.
The plywood forms were removed after one day on this cement stabilized rammed earth
wall in New Mexico. Photo by Eric Simenson.
23
CHAPTER VI
OBJECTIVE OF STUDY
The purpose of this study was to test the engineering properties of rammed earth
with polypropylene plastic fibers and Portland cement additives. Currently, there is no
published research on the use of plastic fiber reinforcement in rammed earth. The
materials included a suitable soil that meets criteria established by other researchers in
peer-reviewed journals (Bryan 1988, Burroughs 2006, 2008). Additives to the soil
included Portland cement Type I/II, polypropylene fiber, and water. The three mix
designs for the experiment were soil, soil-cement, and soil-cement-fiber. For the soilcement and soil-cement-fiber mixes, the minimum strength requirements were 300
pounds per square inch compressive strength and 50 pounds per square inch modulus of
rupture (New Mexico Building Code 2009). According to the Australian building code,
the rammed earth material was assumed to have no shear strength (HB 195 Standards
Australia 2001).
Unconfined compression, direct shear, and beam flexure tests were performed on
the three mix designs. Analysis of the test results included comparison of ultimate
strengths and also the behavior of the material during testing.
24
CHAPTER VII
MIX DESIGN
Finding a suitable soil is a crucial step in rammed earth construction because the
mix design consists of over 90% moist soil and the remainder being soil additives, like
cement. Poor soils can be used if larger quantities of cement are added to increase
strength. Cement is a costly additive and one of the advantages of rammed earth
construction is minimizing material cost; thus, minimizing cement is a goal for most
builders.
Soil Selection Requirements
Several research papers (Bryan 1988; Burroughs 2006, 2008) offer guidance on
selecting soils. The soil mixture is typically about 25% clay, 60% sand, and 15% gravel.
The gravel content needs to be fairly small with a limited amount of material being
retained on the No.4 sieve. To determine clay/sand/gravel content, dry soil was passed
through a stack of sieves with progressively smaller openings. Upon inspection, the soil
for this experiment contained some percentage of clay and this clay portion needed to be
removed from the soil before putting the soil through the sieve stack. The reason for this
procedure was the clay sticks on the finest sieve (No. 200) instead of passing through it.
To measure the clay content, the soil was washed through a No. 200 sieve. The clay
content was determined by comparing the original dry weight of the soil to the dry weight
of the soil remaining on the No. 200 sieve. The soil washed through the No. 200 sieve
was considered the clay portion of the soil. Below is Figure VII.1 showing the washed
and dried soil sample ready for gradation through the sieve stack.
25
Figure VII.1 Sieve Stack.
The soil sample is placed on the screen and it is shaken through progressively finer
screens.
For this project, approximately 3% was retained on the No.4 screen and this was
considered to be the gravel fraction of the soil. Approximately 20% passed through the
No. 200 sieve and this was the clay fraction. The remaining 77% was sand of various
grain sizes. For this experiment, the fraction passing the No. 200 sieve was considered
26
clay and not silt due to the properties exhibited during the Atterberg tests. Clay is
cohesive and it displays predictable behaviors during the Atterberg test. On the hand, silt
can be unpredictable and can be very sensitive to moisture. For example, adding one
percent moisture to silt can cause it to go from a stiff soil to almost liquid. It is very
difficult to perform an Atterberg test on silt. For these reasons, it can be justifiably
assumed that clay was in the soil and not silt.
Below are the results from the gradation test and Atterberg test. Based upon these
results, the soil could be classified with the Uniform Soil Classification System.
Table VII.1 Gradation Results from the Soil.
Sieve
Size
[mm]
Sieve Size
Cumulative
%
%
Soil Retained
Retained Passing
[g]
4.75
4
18.4
3.2
96.8
2
10
71
12.5
87.5
1.19
16
115.8
20.4
79.6
0.425
40
231.4
40.8
59.2
0.25
50
281.5
49.7
50.3
0.15
100
368.7
65.1
34.9
0.075
200
435
76.7
23.3
Pan
451.6
79.7
20.3
27
Figure VII.2 Liquid Limit.
The results of the Atterberg Liquid Limit test.
As shown in Table VII.1 above, the soil for this experiment meets the gradation
criteria explained in previous research (Bryan 1988, Burroughs 2006, 2008) for clay,
sand, and gravel. The approximate ideal quantities are 20% to 30% clay, 70% to 80%
sand, and a small percent of less than 10% as gravel. The soil was obtained from Pioneer
Sand Company at 6379 Valmont Road Boulder, Colorado 80301. Pioneer supplies a
variety of landscaping materials and they called this soil “fill dirt”. According the
supplier, it is essentially soil used for backfilling and it is not meant for growing plants.
Figure VII.2 displays the liquid limit results from the Atterberg test. The test
method used was ASTM D4318 “Standard Test Methods for Liquid Limit, Plastic Limit,
and Plasticity Index of Soils”. The test was performed using a standard device developed
by Professor Casagrande.
28
Figure VII.3 Casagrande Device.
The standard device used to test the liquid limit of soils.
A layer of moist soil was placed smoothly into the brass cup shown in Figure
VII.3. A groove was cut through the middle of the sample using a special grooving tool
as specified in ASTM D4318. The test was performed by turning a crank on the
Casagrande device, which drops the brass cup a standard distance. Each drop is called a
“blow” and the total blow count was recorded when the groove in the soil closes at which
29
time the test was completed. As seen in Figure VII.3, a portion of the groove has closed
and the test is completed. The test was performed three times using soil with different
moisture contents. The liquid limit is defined as the moisture content at which it takes 25
blows to close the groove. The purpose of testing three samples is to have one point
below 25 blow counts, one point near 25, and the last point above 25. The three points
should form a straight line and graphically the 25 blow count point on the line is used to
determine the liquid limit. ASTM does allow a liquid limit to be determined from testing
just one sample. This procedure is necessary when a limited amount of soil is available
for testing. For this thesis, 100 grams of soil was reserved for the Atterberg tests, which
allowed three liquid limit tests and the plastic limit test to be performed.
In general, pure clays tend to have a higher liquid limit than sandy soils and
testing the liquid limit of a soil can give someone a rough idea of the suitability soil for
rammed earth. Pure clays should be avoided due to their tendency to shrink and swell,
which causes cracking. On the other hand, pure sand is not wanted because it has no
cohesion between the sand grains and it cannot be formed into a wall. Suitable soil has a
small fraction of clay and the remainder is sand. The sticky, cohesive property of clay
helps bond together all the sand grains.
Three liquid limit samples were tested at varying moisture contents to establish a
linear trend line. Since the trend line closely follows the points, the sample at 25 blows is
used to determine the liquid limit of 26% for this soil.
The plastic limit was tested in accordance with ASTM D4318. The test involved
rolling thin ribbons of soil on a glass plate. The plastic limit is defined as the moisture
content at which a ribbon of soil can be rolled 1/8” thick. Higher moisture contents allow
30
the ribbons to easily be rolled. The procedure is to slowly reduce the moisture content by
actively rolling the soil on the glass plate. At the plastic limit, the soil will be at the
lowest moisture content needed to form the 1/8” thick ribbon. The plastic limit was
tested and determined to be 15.
Figure VII.4 Plastic Limit.
The 1/8” thick ribbon of soil has reached the plastic limit.
The plasticity index is determined by subtracting the plastic limit from the liquid
limit. The plasticity index was 11. According to the Unified Soil Classification System,
a soil with these gradation and Atterberg properties is classified as a Silty Sand/Clayey
Sand with the symbol SM-SC. As previously described, the soil exhibited some cohesion
during the Atterberg limit tests; thus, the soil is more likely a Clayey Sand (SC). Silt is
non-cohesive and it can be very moisture sensitive when performing these tests. For
31
example, some silt can have a liquid limit that is very close to the plastic limit. The
unusual behavior is common with silts and this behavior was not observed during testing
for this thesis. Overall, this soil passed the general requirements for rammed earth
material.
Optimum Moisture Content and Maximum Dry Density
Upon determination of a suitable soil, the next step was to calculate the optimum
moisture content and maximum dry density of the soil. These values were calculated
using a Standard Proctor test in accordance with ASTM D698 “Standard Test Method for
Laboratory Compaction Characteristics of Soil Using Standard Effort (12400 ft-lbf/ft3)”
(ASTM D698 2012). A split mold was the best choice because the compacted samples
could be easily extracted from the mold. Without a split mold, the compacted sample
was very difficult to remove from the mold. Typically, the sample would be pried out of
the mold in small pieces and a composite sample of small pieces would be used to
determine the moisture content.
The split mold, shown in Figure VII.5, was also needed to prepare rammed earth
samples for unconfined compression and direct shear testing. The unconfined
compression samples were prepared by following the standard proctor procedure and
creating a compacted cylinder of soil. Those tests are described in more detail in Chapter
VIII.
32
Figure VII.5 Standard Proctor Tools.
The split mold seen on the left and the standard 5.5 lb hammer on the right.
The soil was prepared for the standard proctor test by air-drying it for 24 hours
and the moisture content of the air-dried was measured. This was the preferred procedure
because it is relatively easy to spread the soil on a tarp in the laboratory or outside on a
sunny day. Smaller samples can be dried more rapidly by placing them in an oven set at
a low temperature. Given the volume of soil needed for this thesis, air drying was the
most practical method that satisfied the ASTM D698 standard.
33
Figure VII.6 Air-drying Soil.
The soil was dried outside on a sunny day.
Five samples were prepared for the standard Proctor test and the moisture content
was increased by approximately 3% for each sample. Any material larger than the No. 4
sieve was removed in accordance with the test procedure.
34
Figure VII.7 Compaction Curve.
The compaction curve was developed using five samples at varying moisture contents.
Based upon the test results, the optimum moisture content (OMC) was 11.5%
with a maximum dry density of approximately 121 pounds per cubic foot. The OMC is
the point where the soil obtains the highest density when compacted. For rammed earth,
it is very important to prepare soil at the OMC in order to achieve the highest density and
strength.
For this thesis, Portland cement was added to some of the samples being tested.
When water is added to cement, a chemical process occurs and the water is consumed
during the reaction. In effect, some portion of the water in the soil will be lost when
cement is added. For this thesis, it was decided that water will be added only to bring the
soil to its optimum moisture content. One study (Reddy and Kumar 2011) found the
optimum moisture content was slightly higher, 0.4%, for soil-cement when compared to
the original soil. It was decided that increasing the moisture content for the samples
35
containing cement would introduce a new variable and that would make it more difficult
to compare the three soil mixtures.
Cement and Fiber Additives
Three mix designs were used in this experiment. The first mix was simply soil at
the OMC of 11.5%. The second mix was soil at the OMC plus 6% Portland cement. The
6% Portland cement was calculated as a percentage of the dry weight of soil. The third
mix was soil at the OMC plus 6% Portland cement and 0.3% ½” polypropylene fiber.
The 0.3% fiber was calculated as a percentage of the dry weight of soil.
The percentage of Portland cement was specifically chosen because the NM
Building Code and the research cited in this thesis used 6% as the minimum amount of
cement required to stabilize rammed earth. Other researchers (Minke 2006) have shown
lower quantities of cement can reduce strength of the rammed earth due to chemical
interaction with the cement and clay. Higher quantities of cement will most likely
increase the strength of rammed earth, but higher quantities are not always necessary and
are not economical. Typically, a suitable soil for rammed earth will easily meet strength
requirements outlined in the NM Building Code when low quantities, such as 6%, of
Portland cement are used.
The percentage of fiber was based upon the work of other researchers (Jiang et al.
2010). Typically low percentages, such as below 1%, of fiber are used in mix designs.
High amounts of fiber can reduce workability as the fibers tend to impede the mixing of
materials. This phenomenon was observed during trail batching for this thesis. Several
trial batches were created to test the workability of the addition of fiber to the soil. Based
upon previous research and trial batches, it was concluded that 0.3% fiber reinforcement
36
would be sufficient for this thesis. Smaller quantities left pockets in the soil where there
were little or no fibers. Higher quantities tended to bunch up during mixing and this
could have created weak spots in the samples because the fibers do not bond to each
other. The 0.3% fiber generated the highest strength samples for other researchers and
the objective of this thesis was to create higher strength rammed earth by incorporating
synthetic fibers.
Figure VII.8 Polypropylene Fibers.
The ½” long polypropylene fibers used in the fiber-reinforced samples.
37
CHAPTER VIII
EXPERIMENT DESCRIPTION
Three tests were performed on the three mix designs. The first test performed
was unconfined compression on cylindrical samples. The second test performed was
direct shear on cylindrical samples. The third test performed was 3-point modulus of
rupture on rectangular beams. Both the unconfined compression and modulus of rupture
required 6 samples of each mix per test. The direct shear required 9 samples of each mix.
Unconfined Compression Test Preparation
Samples for unconfined compression were prepared in accordance with the
ASTM standard “Compressive Strength of Molded Soil-Cement Cylinder” (ASTM
D1633 2007). The samples were made by compacting soil into a standard Proctor mold
following the same procedure previously mentioned for determining the OMC and
maximum dry density. The benefit of following this procedure was the samples were
uniform size and each sample received the same amount of compactive force.
The procedure used for this experiment was to mix enough soil to make two
samples at a time. Materials were mixed in a 5-gallon bucket and stirred together with a
hand trowel. Initially, larger batches were attempted, but the problem was the soilcement would become noticeably stiffer about 20 minutes after adding water to the dry
soil-cement mixture. The stiffer soil-cement made it difficult to compact into the mold.
It was determined that only two samples could be prepared at a time before the soilcement created any issues with compaction.
38
Figure VIII.1 Color of Soil-Cement and Soil.
After adding the Portland cement, the mixture turns a gray color.
The method was to mix all the materials dry then add water. It was much less
effort to mix dry materials and materials mixed together better when dry. The fibers have
a tendency to clump together and extra mixing was needed to evenly disperse the fibers.
If clumps of fibers formed, the clumps were pulled apart by hand. Mixing was completed
once the gray color of the cement covered all the soil and the original brown color of the
soil could no longer be seen. Mixing of fibers was completed once random sampling of
the mixture showed even dispersion of fibers throughout the mixture.
39
Figure VIII.2 Fibers in Soil.
Fibers are randomly distributed in the soil and the cement has not been added.
The soil mixes were placed into the mold in three lifts following the same
procedure for the standard Proctor test. Each lift was scarified to promote bonding
between the lifts. Excess material above the mold was scraped off using a beveled edge
and the tops of the cylinders were made smooth as possible. To facilitate sample
extraction from the mold, the sides were lightly lubricated with WD-40 prior to placing
any soil mix in the mold. The lubrication was necessary because the soil-cement mixes
had a tendency to stick to the sides of the mold.
40
Figure VIII.3 Soil-Cement in Proctor Mold.
The top surface has been trimmed and sample is ready for extraction from the mold.
Figure VIII.4 Extracted Soil-Cement Sample.
Sample removed from mold and ready for curing procedure.
41
The molded samples were individually placed in sealed plastic bags and allowed
to cure for 28 days in a humid concrete curing room. This was somewhat of a redundant
procedure. The most important step in the curing procedure was seal the sample in a
plastic bag. This ensured no moisture was gained or lost from the sample. Controlling
the moisture was one variable that was held constant throughout this experiment. Placing
the samples in a humid curing room followed the ASTM standard. The reality was the
samples did not benefit from being in the humid room; however, it was convenient to
store the samples in the curing room, which was located in the laboratory where the
testing was performed. In practice, it is beneficial to moist-cure soil-cement similar to
moist-curing concrete. However, moisture content was controlled throughout the
experiment and kept at 11.5%, so moist-curing was not done on any samples.
One deviation from the ASTM procedure was made in this experiment. The
standard calls for submerging the cylinder in water for 4 hours prior to testing. It was
decided not to do this because the soil and soil-cement cylinder would absorb water at
different rates. There was also some concern that the soil cylinders would fall apart if
they were submerged. The ASTM procedure was specific to soil-cement, which can
handle being submerged for prolonged periods. It was determined that since moisture
was being held constant in the samples, it would have been contradictory to suddenly
submerged the samples immediately before testing.
Prior to testing, gypsum cement (plaster) caps were placed on both ends of the
cylinder. Several initial specimens were tested without caps and the results varied
significantly. The primary reason for this is the surface of the cylinders was inherently
uneven and compressive load was not distributed evenly, which leads to stress
42
concentrations and premature failure of the specimen. Hydrocal® gypsum cement was
used to cap all the cylinders. The end of the cap was made smooth with a piece of glass
and a small level.
Figure VIII.5 Capping Cylinders.
Glass plates were used to create smooth caps made of gypsum cement.
43
Figure VIII.6 Unconfined Compression Sample.
A capped sample of rammed earth ready for unconfined compression testing.
Direct Shear Preparation
The first steps in preparing the shear samples started with the procedure for
making an unconfined compressive sample. The soil mixture was compacted into the
standard Proctor mold following the same procedure previously mentioned. Once the
mold was filled and leveled with the beveled edge, it remained in the mold. A small soil
sample liner of stainless steel measuring 2.5 inches in diameter and 3 inches long was
placed on top of the soil in the mold. The metal liner was then forced down into the soil,
thus filling the liner with the prepared soil sample. This procedure is illustrated in
Figures VIII.7, VIII.8, and VIII.9.
44
Figure VIII.7 Liner on Proctor Mold.
Soil sample liner ready to be driven down into the Proctor mold.
Figure VIII.8 Liner inside Mold.
Soil sample liner successfully driven into the Proctor mold.
45
Figure VIII.9 Liner filled with Soil.
The liner filled with soil after the excess soil is trimmed away.
46
Figure VIII.10 Pneumatic Jack.
The jack used to extract soil sample from soil sample liner.
47
Figure VIII.11 Soil and Liner.
A stainless steel soil sample liner and the extracted sample.
The soil filled liner was placed in a sealed plastic bag and stored in a humid
concrete curing room for 7 days. After 7 days, the soil sample was removed from the
liner using a pneumatic soil extraction jack, shown in Figure VIII.10. The soil sample
measuring approximately 3 inches long was then saw-cut into smaller samples measuring
about 0.9 inches long. Typically, one 3 inch long liner would yield two shear samples.
The soil at the top and bottom of the liner was usually damaged and not intact, so those
areas were cut off. Figure VIII.12 shows examples of the cut shear samples that are
ready for testing.
48
Figure VIII.12 Cut Shear Samples.
Three shear samples have been cut from a 3 inch long soil sample liner.
The reason for using the 2.5 inch diameter stainless steel soil liner was the direct
shear machine has a 2.5 inch diameter shear box. Once the samples were saw-cut, they
neatly fit into the shear box. One difficulty was this experiment required all samples
receive the same compactive force and it would be very challenging to compact soil into
the shear box. The liner solved this problem because it was driven into a standard
Proctor mold. This method ensured all shear samples were compacted the same amount.
Another issue was the soil-cement samples needed time to cure. It would be
inefficient to compact soil-cement directly into the shear box and wait at least a week for
it to cure. There was only one shear box available and it was not practical to wait one
week to test one sample when 27 samples needed to be tested.
49
One advantage of using the liner was multiple samples were cut from one liner.
Another advantage is soil samples in the field are collected by placing these liners into
hollow stem drill rigs. Soil laboratories prepare shear samples by extracting soil out of
the liner and cutting it to the desired length. When possible, this experiment used
industry practices that could easily be replicated by other laboratories.
Shear samples were tested in accordance with ASTM standard “Direct Shear Test
of Soils Under Consolidated Drained Conditions” (ASTM D3080 2011). The shear box
on the direct shear machine forced a failure plane through the midsection of sample.
Cutting the samples to the correct thickness ensured the failure plane occurred through
middle of the sample. A force normal to the failure plane was applied by adding weights
to a moment arm on the machine. This force was varied and three samples were tested at
three different normal forces.
Modulus of Rupture Preparation
Modulus of rupture beam samples were created in custom built rectangular forms,
shown in Figure VIII.13. The testing procedure followed ASTM standard “Flexural
Strength of Soil-Cement Using Simple Beam with Third-Point Loading” (ASTM D1635
2012). The beams measured 3 inches by 3 inches by 11.25 inches. In order to achieve
uniform density, the volume of the mold was calculated and an amount of soil mixture
was weighed to fill the mold. In theory, all of the weighed soil would fit into mold if the
proper amount of compaction was applied. The soil was placed in three lifts and each lift
was scarified to promote bonding between layers. This procedure was similar to the
standard Proctor procedure used to create unconfined compression samples and direct
shear samples.
50
Figure VIII.13 Beam Mold.
Laminated melamine board was used to create a mold for the modulus of rupture test.
Before adding the soil, the sides were lightly greased with WD-40 to prevent the
soil mixture from sticking to the laminated wood. Once the mold was filled, a beveled
edge was used to scrap off excess soil and create a smooth surface on the top of the beam.
The mold filled with soil was wrapped with plastic and the sample was allowed to cure in
the mold for 7 days in a humid concrete curing room. Beam samples were removed from
the mold by unscrewing one side of the mold, which allowed the samples to easily be
removed.
51
Figure VIII.14 Modulus of Rupture.
The loading apparatus required to perform the modulus of rupture test.
The Figure VIII.14 shows a beam placed in the testing apparatus. Steel pipes with
a diameter of 1.25 inches were used instead of steel rods as specified in the ASTM
standard. The pipes were a suitable alternative because the soil mixes were expected to
fail at very low strengths and any deflection in the pipe would be insignificant. Stronger
steel rods would be needed for testing high strength material.
52
CHAPTER IX
TEST RESULTS
Unconfined Compression
All unconfined compression tests were performed in accordance with ASTM
D1633 standard “Compressive Strength of Molded Soil-Cement Cylinder” (ASTM
D1633 2007) on samples that were cured 28 days. The unconfined compressive strength
(σc) is calculated by dividing the ultimate load (Pu) by the area (A) of the cylinder.
σc = Pu/A
Equation IX.1 Unconfined Compressive Strength.
The unconfined compression tests were performed using the MTS Model
#204.63LUBT Compression Testing Machine that has a 20,000 pound capacity. In
accordance with ASTM D1633, the machine was programmed to apply a constant load
rate of 20 ± 10 psi per second. The load and vertical displacement were automatically
recorded with the machine’s computer software. The raw data was entered into Excel,
where plots of the stress versus strain were created for each mix design.
Before testing samples in the MTS machine, several trial samples were created of
each mix. The purpose of the trial samples was to understand how the samples would
perform during the compression tests and the main concern was how the plaster caps
would affect the results. It was observed that the plaster caps sometimes cracked during
the early stages of the load application from approximately zero to 100 psi compressive
stress. The cracking of the caps caused the vertical displacement to rapidly increase
53
during this initial phase of loading. After the initial settlement or cracking of the cap, the
stress-strain curve began to follow a linear path that correlated with the predicted
properties of the mix designs. The results of the trial tests are not included here because
they were simply used to calibrate and hone the procedural steps for actual testing of the
mix designs.
Figure IX.1 MTS Machine.
MTS Machine used for unconfined compression tests.
The samples of soil only were tested using the Forney Model F-40EX-F-TPILOT
Compression Testing Machine following ASTM D1633 procedures. These samples were
the first mix design tested and the MTS machine was unavailable at the time of testing.
Unfortunately this machine does not automatically record data and only ultimate stress
54
was recorded for each sample. The ultimate strengths of the three mix designs were still
compared, but stress-strain plots for the soil only mix design were not created.
Figure IX.2 Forney Testing Machine.
Forney machine used for compression tests and modulus of rupture tests.
Below are summary tables of the unconfined compression tests for each of the
three mix designs. Six samples of each mix design were tested. The ultimate
compressive strength is listed for each sample in pounds per square inch. The average
compressive strength of the samples, the standard deviation, and the coefficient of
variation are also listed.
55
Table IX.1 Unconfined Compressive Strength for Soil Samples.
Soil
Sample
1
2
3
4
5
6
Average
Strength [psi]
Std. Deviation
[psi]
COV
Pressure
[psi]
53
74
64
68
66
67
65
7
0.11
Table IX.2 Unconfined Compressive Strength for Soil-Cement Samples.
Soil-Cement
Sample
Pressure [psi]
1
732
2
806
3
766
4
632
5
634
6
651
Average
Strength
704
[psi]
Std.
Deviation
75
[psi]
COV
0.11
56
Table IX.3 Unconfined Compressive Strength for Soil-Cement-Fiber Samples.
Soil-Cement-Fiber
Sample
Pressure [psi]
1
766
2
735
3
720
4
716
5
658
6
655
Average
708
Strength [psi]
Std.
Deviation
44
[psi]
COV
0.06
The soil-cement and soil-cement-fiber samples had nearly identical average
compressive strengths. This suggests that the fibers had little or no influence upon the
compressive strength. The fibers were found to hold cracks together after failure. The
soil-cement matrix was the sole provider of strength in compression. The strength
contribution due to stabilization with cement is clearly evident in these test results.
Adding 6% Portland cement increased the compressive strength of the original soil by
approximately 11 times.
Numerous researchers referenced in this thesis have documented the increase in
compressive strength when cement is added to soil. However, studies have not compared
the compressive strengths between soil-cement and soil-cement with fibers. This thesis
found the average strength of soil-cement and soil-cement with fibers were essentially the
same, but the standard deviation and COV was lower for the soil-cement-fiber mixture
indicating the fibers gave the material more predictable average strength. A larger
57
sample size would need to be studied before conclusively stating that fiber additives
produce samples with less strength variability.
The results of this test confirm the work that others have already done. The
primary reasons for performing this test were two-fold: 1) validate cement-stabilization is
possible with the selected soil, and 2) validate the results with other studies. Clearly,
cement-stabilization occurred because the compressive strength greatly increased when
cement was added to the soil. The results were also similar to studies referenced in this
thesis. One additional goal was achieved through performing compression tests and that
was the cement-stabilized samples all exceeded the minimum compressive strength of
600 psi specified in the New Mexico building code. This soil and mix design would be
suitable for construction in the state of New Mexico.
The failure mechanisms for the three mixes were markedly different. The soil
tended to slowly deform under light loading in a ductile, plastic type behavior. Both the
soil-cement and soil-cement-fiber displayed sudden, brittle failure at much higher
compressive stress levels than the soil only mixture.
58
Figure IX.3 Crushed Soil Sample.
Failed soil sample after completion of the unconfined compression test.
Figure IX.4 Crushed Soil-Cement Sample.
Failed soil-cement sample after completion of the unconfined compression test.
59
Figure IX.5 Crushed Soil-Cement-Fiber Sample.
Failed soil-cement-fiber sample after completion of the unconfined compression test.
The soil-cement cylinders would suddenly fail with vertical cracking along the
outer edges, which indicates a splitting type failure. Once failure occurred approximately
50% of the cylinder would crumble and fall off the platen. The soil-cement-fiber
cylinders also failed suddenly with vertical cracking; however, nearly all of the material
would remain connected through the fibers and the cylinder would remain on the platen.
The fibers were not able to increase the load capacity of the cylinder; however, they did
significantly change the behavior during failure. The plaster caps influenced the failure
type by restraining the ends of the cylinder. These restraints forced the stress path
straight-down through the cylinder. This is a known phenomenon, so the caps were
poured as thin as possible to minimize the restraining effect.
60
Modulus of Elasticity
From data collected during the compression testing, it was possible to create
stress-strain curves for the soil-cement and soil-cement-fiber mix designs. Using ASTM
standard “Static Modulus of Elasticity and Poisson’s Ratio of Concrete in Compression”
(ASTM C469 2010) the modulus of elasticity was calculated. The Poisson ratio was not
tested because it requires a special apparatus to measure lateral deformation of the
cylinder. The modulus of elasticity (E) was calculated using the chord modulus of
elasticity formula where S2 is the stress at 40% of the ultimate load, S1 is the stress at ɛ1,
ɛ2 is the strain at S2, and ɛ1 corresponds with a longitudinal strain of 0.000050.
E = (S2 - S1)/(ɛ2 - ɛ1)
Equation IX.2 Modulus of Elasticity.
The following stress-strain plots were generated from the compression testing of
the soil-cement and soil-cement-fiber mix designs.
61
Figure IX.6 Soil-Cement Stress-Strain.
The stress versus strain plots for the six soil-cement samples.
Sample #2 and Sample #6 exhibit the characteristic initial curve shape that
indicates cracking and settlement of the plaster cap. The first portion of the curve rises at
a small angle until approximately 50 psi then the curve rises steeply. The linear elastic
portion of the curve occurs after this initial portion. To avoid recording data from the
cracking of the cap, Sample #1, #3, #4, and #5 were preloaded. The preload was applied
by the machine and the load was adjusted using the machine’s computer controls. The
preload varied from less than 40 psi to 120 psi depending upon condition of the plaster
cap and how the platens on the machine contacted the plaster caps.
62
Figure IX.7 Soil-Cement-Fiber Stress-Strain.
The stress versus strain plots for the six soil-cement-fiber samples.
The preload for the soil-cement-fiber samples was much smaller. This was due to
better pouring of the caps. The plaster had a tendency to harden very quickly and within
as little as 10 minutes the material was sometimes unworkable. Practice with mixing and
pouring the plaster led to the creation of thinner, smoother, and more level plaster caps.
These higher quality caps required less preloading because they had less of a tendency to
crack and settle. Despite all this attention to detail, Sample #4 showed larger initial strain
with little loading and this indicates a potentially uneven cap. Sample #1 also showed
some initial issues with the cap and the overall curve did not take the same shape as the
63
other sample. Sample #1 reached a high strength, but the stress-strain curve indicates
some type of anomaly with this sample.
The modulus of elasticity was calculated using Equation IX.2. According to the
ASTM standard, the initial stress is taken at a strain of 0.00005. For this thesis, this
procedure was not practical because the plaster caps created some anomalies during the
first portion of the stress-strain curves. After reviewing the data and plots, it was decided
that the initial stress/strain point would be taken where the slope of curve began to rise
steeply. At this point, it was assumed that soil-cement was behaving linearly elastically.
If the initial stress/strain point was taken prior to any cracking of the plaster cap, the
result was a modulus of elasticity that was unrealistically low.
Table IX.4 Modulus of Elasticity for Soil-Cement Samples.
Soil-Cement
Sample
1
2
3
4
5
6
Average
Strength
[psi]
Modulus of
Elasticity [psi]
118,409
111,571
143,740
142,231
162,460
171,074
141,581
Std.
Deviation
[psi]
23,445
COV
0.17
64
Table IX.5 Modulus of Elasticity for Soil-Cement-Fiber Samples.
Soil-Cement-Fiber
Sample
1
2
3
4
5
6
Average
Strength
[psi]
Modulus of
Elasticity [psi]
29,965
111,394
176,875
188,992
112,752
126,053
124,339
Std.
Deviation
[psi]
56,835
COV
0.46
It is should be noted that Sample #1 of the soil-cement-fiber mix exhibited an
unusual stress-strain curve. The cylinder was able to deform much more than the other
samples and it was the strongest sample of the soil-cement-fiber mix. One possible
explanation for this anomaly was a small portion of the cylinder may have not been fully
compacted. This could happen if one of the three lifts in the cylinder was placed too
thick, which would have caused the bottom portion of the lift to be less compacted. As
the sample was tested, the testing machine compressed the weak layer until it was fully
consolidated and this caused extra deformation in the cylinder. Regardless of the
explanation, this sample was suspect and another table without Sample #1 was created.
Removal of outlying data was done in accordance with ASTM “Standard Practice for
Dealing with Outlying Observations” (ASTM E178 2008). The ASTM standard uses
statistical analysis to determine if a data point can be classified as an outlier. In this case
65
Sample #1 was determined to be an outlier and removed from the Modulus of Elasticity
calculations.
Table IX.6 Adjusted Modulus of Elasticity for Soil-Cement-Fiber Samples.
Soil-Cement-Fiber
Sample
1
2
3
4
5
6
Average
Strength
[psi]
Modulus of
Elasticity [psi]
111,394
176,875
188,992
112,752
126,053
143,213
Std.
Deviation
[psi]
36,958
COV
0.26
With Sample #1 removed, the adjusted modulus of elasticity for soil-cement-fiber
mix design in Table IX.6 was very similar to the soil-cement mix design. The average
modulus of elasticity for soil-cement-fiber was E = 143,213 psi and the soil-cement was
E = 141,581 psi. Given the results of the compression tests and the calculated modulus
of elasticity, it appeared that adding fibers to the soil-cement had little or no impact on
these material properties. Again, the biggest changes in material properties came from
the addition of cement.
Direct Shear
Direct shear tests were performed in accordance with ASTM D3080 standard
“Direct Shear Test of Soils Under Consolidated Drained Conditions” (ASTM D3080
66
2011) on samples that were cured for 7 days. The decision was made to test samples a 7
days rather than 28 days due to the large number of samples needed for testing.
Furthermore, it was very difficult to prepare shear samples and it was not feasible to wait
28 days in between batches given time constraints on the project. Many of the samples
were damaged during saw cutting or during extraction from the soil sample liner.
Typically the middle of the soil sample liner would yield one good, undamaged sample
that could be used for testing. Originally, it was thought that one soil sample liner would
yield three samples, but this proved impossible due to damage during extraction from the
liner and fracturing during saw cutting.
Trial samples were tested to gauge how the machine would handle soil-cement,
which is much stronger than typical soil tested in this machine. It was observed that high
normal force was needed to keep soil-cement samples stationary during testing. The
samples had a tendency to shift in the shear box because the shear force was large, so a
large normal force was needed to hold the sample in place. Typically for soils, a larger
normal force will cause failure at higher shear force. In order to predict the shear
strength of a material, samples were tested at different normal forces and plotted in Excel.
Typically for soils, a linear relationship occurs between shear force and normal force.
Shear stress (τ) in pounds per square inch is calculated by dividing the shear force
(Fs) in pounds by the cross-sectional area (A) of the sample measured in square inches.
Three samples were tested at the same normal stress to obtain an average shear stress for
that specific normal stress. The normal stress was applied at three different stress
increments. The purpose for varying the normal stress was to create a plot of average
67
shear stress versus normal stress. In this plot, a linear trend line was drawn and from the
trend line, the cohesion and friction angle were determined.
τ = Fs /A
Equation IX.3 Shear Strength.
All shear samples were tested in accordance with ASTM standard D3080 using a
Forney Direct Shear Machine Model #2050 SN 814. The machine has an adjustable
displacement rate, which was set at 0.02 inches per minute. The ASTM Standard has a
range of acceptable displacement rates from 0.0001 to 0.04 inches per minute. For soils,
the displacement rate is chosen based upon the time it takes the soil to consolidate. For
this thesis, the soil-cement was already consolidated and it did not consolidate even under
the large normal stress applied to the samples during shear testing. The displacement rate
calculation outlined in the ASTM standard was not applicable in this case; however, the
displacement rate within the standard’s range was selected. At this rate of displacement,
it took approximately 15 minutes to complete testing on one sample. It total, 27 samples
were tested.
Below are the summary tables and charts for direct shear testing of the three
mixes.
68
Table IX.7 Soil Direct Shear Test Results.
Soil
Normal
Stress
[psi]
Normal
Stress
[psi]
Normal
Stress
[psi]
3.5
Average
Shear Stress
[psi]
7.65
6.9
Average
Shear Stress
[psi]
13.25
13.9
Average
Shear Stress
[psi]
19
Figure IX.8 Soil Shear Stress.
Plot of shear stress and normal stress.
69
Table IX.8 Soil-Cement Direct Shear Test Results.
Normal
Stress [psi]
Normal
Stress [psi]
Normal
Stress [psi]
Soil-Cement
Average
Shear
Stress
[psi]
111.1
156
138.9
Average
Shear
Stress [psi]
194
166.7
Average
Shear
Stress [psi]
212
Figure IX.9 Soil-Cement Shear Stress.
Plot of shear stress and normal stress.
70
Table IX.9 Soil-Cement-Fiber Direct Shear Test Results.
Normal
Stress
[psi]
Normal
Stress
[psi]
Normal
Stress
[psi]
Soil-Cement-Fiber
Average
Shear
Stress
111.1
[psi]
Average
Shear
Stress
138.9
[psi]
Average
Shear
Stress
166.7
[psi]
164
190
245
Figure IX.10 Soil-Cement-Fiber Shear Stress.
Plot of shear stress and normal stress.
The soil had a friction angle of 46.4° and cohesion of 4.8 psi. This friction angle
corresponded with the typical upper limit for dense sand (Das 2008). The friction angle
was calculated by taking the inverse tangent of the slope of the line and the cohesion was
71
calculated as the y-intercept of the line. Based upon the soil classification and the
compaction applied to each sample, these results corresponded to what would be
expected for this type of soil.
The soil-cement had a friction angle of 45.2° and cohesion of 47.3 psi. The
friction angle remained close to the soil, but the cohesion increased nearly 10 times. The
increase in cohesion came from the cement. The cement created much greater bonding
between soil particles and this caused the cohesion to substantially increase.
The soil-cement-fiber and soil-cement had similar average shear strengths at
normal stresses of 111.1 psi and 138.9 psi. At a normal stress of 166.7 psi, the soilcement-fiber had a significantly higher average shear stress when compared to the soilcement, 245 psi and 212 psi shear strength respectively. This higher shear stress caused
the trend line on the soil-cement-fiber plot to rise at a much steeper angle of 55.5°. The
cohesion was calculated at the point the trend line intercepts the y-axis, and in this case,
the value was -2.8 psi. It is theoretically impossible to have a negative value for
cohesion, so it is reasonable to assume this value should be zero based upon the plot.
The anomaly for the soil-cement-fiber samples was likely the higher shear
strength at the maximum normal stress tested. This value of 245 psi seemed unusually
high and lower value that was closer to the soil-cement value of 212 psi would be
expected given the similarity in all the test results thus far. At lower normal stress, the
soil-cement and soil-cement-fiber samples produced similar shear strengths.
Furthermore, the unconfined compression tests and modulus of elasticity calculations
showed soil-cement and soil-cement-fiber samples have very similar properties. If the
average shear stress of 245 psi for the soil-cement-fiber was closer to 212 psi, the trend
72
line for the soil-cement-fiber would be very similar to the trend line for the soil-cement.
The new trend line would create a lower friction angle and higher cohesion, similar to the
soil-cement.
If shear tests were performed again, some changes could be made that might
improve the results. Letting the samples cure for 28 days would give the samples
additional time to gain strength. The extraction of the samples from the liner and also
saw-cutting the samples may have introduced micro-cracks that influenced the results.
Stronger samples might have been able to resist potential cracking caused by these
procedures. Additionally, cutting only one sample from the middle of the liner might
yield higher quality samples. The ends of the extracted samples always showed signs of
damage and these portions were cut away, but it is possible that all the damaged material
was not removed.
Figure IX.11 Shear Stress Failure of Soil.
Failed soil sample from direct shear testing.
73
Figure IX.12 Shear Stress Failure of Soil-Cement.
Failed soil-cement sample from direct shear testing.
Figure IX.13 Shear Stress Failure of Soil-Cement-Fiber.
The two halves are still connected together with fiber after failure.
74
The most significant observation between the soils tested in shear was the soilcement-fiber samples remained connected together after failure. The two halves of the
soil-cement-fiber samples had the typical failure plane through the middle of the sample
and the fibers kept the halves connected together.
Figure IX.14 Soil Direct Shear Plot.
Horizontal load and horizontal displacement plot from direct shear testing.
75
Figure IX.15 Soil-Cement Direct Shear Plot.
Horizontal load and horizontal displacement plot from direct shear testing.
76
Figure IX.16 Soil-Cement-Fiber Direct Shear Plot.
Horizontal load and horizontal displacement plot from direct shear testing.
The direct shear machine had a maximum displacement of 0.3 inches. Two of the
soil-cement tests ended before 0.3 inches because the shear box was not positioned to
allow for maximum displacement. Luckily, the samples still failed and some residual
strength was recorded, so the data from these samples was considered valid. Almost all
of the curves have an initial portion where the curve is horizontal. This is caused by the
shear box moving until it was firmly against the sample. Typically a slight gap existed
between the sample and the shear box, so the data showed some horizontal displacement
until this gap was closed. This does not alter the maximum shear stress results.
Overall, the soil-cement and soil-cement-fiber exhibited residual strength after
failure. The residual strength comes from the roughness between the two halves of the
sample sliding over each other. Three of the soil-cement samples had a rapid loss of
77
strength after failure, Sample 8tsf-01, Sample 10tsf-01, and Sample 12tsf-01. The soilcement-fiber did not have these sharp declines. It was expected that the fibers would add
more residual strength because the fibers would keep the two halves connected. It is
possible that the fibers helped prevent a sudden loss in strength.
The observation that every sample with fiber remained connected together after
shear failure indicated the fibers were well bonded in the soil-cement matrix. The fibers
have a high tensile strength of 83-96 kips per square inch as specified by the
manufacturer (see Appendix for material data sheet) and they have a large capacity to
elongate before failure. As the two halves sheared apart, the fibers were put into tension
as they tried to hold the halves together. The direct shear machine was unable to create
enough displacement to fail the fibers.
After reviewing the results, it was assumed that the residual strength of the
samples would be even more dramatic if the direct shear machine allowed for greater
displacement. Alternatively, direct tensile testing might be required to fully test the
strength and limits of the fibers in the soil-cement. In future studies, it would be
interesting to continue the test until the fibers failed. At that point of failure, it would be
possible to analyze how much strength the fibers actually contributed. Overall, the
ultimate shear stress of the samples came from the strength of the soil-cement matrix.
The fibers appeared to influence the strength only after the ultimate shear stress of the
soil/cement was reached.
78
Modulus of Rupture
The modulus of rupture (MOR) tests were performed in accordance with ASTM
D1635 standard “Flexural Strength of Soil-Cement Using Simple Beam with Third-Point
Loading” (ASTM D1635 2012). This test measures the flexural strength of beams using
a four point loading apparatus. The samples were tested using the Forney Model F40EX-F-TPILOT Compression Testing Machine. This machine had a large lower platen
that could accommodate the size of the beams. This test is specified in the New Mexico
building code and cement-stabilized rammed earth must have a minimum MOR of 50 psi.
It was one objective to determine if the cement-stabilized samples in this thesis met the
minimum strength requirement. Performing research on a material that met building
code requirements would be more relevant to the rammed earth industry than testing a
material that was unsuitable for building.
It was hypothesized that the fibers would increase the MOR by delaying the onset
of cracking and then holding the cracks together. Using natural fibers to delay the onset
of cracking was documented during direct tensile testing research performed by other
researchers (Mesbah et al. 2006). In the MOR test, the bottom of the beam was put into
tension and the top of the beam was in compression. Soil-cement was assumed to have
relatively low tensile strength, but the fibers have relatively high tensile strength. It was
assumed that the fibers would carry the tensile stresses; thus, the soil-cement-fiber beams
would have a higher MOR than the soil-cement beams.
The modulus of rupture (R) is calculated by multiplying the load (P) and span
length of the beam (L) and dividing those numbers by beam width (b) multiplied by the
beam depth squared (d). It should be noted that the span length, width, and depth are
79
measured in inches. The load is measured in pounds force. The span length is not the
overall length of the beam rather it is the distance between the lower supports on the
beam. In accordance with the ASTM standard, the span length was 9.25 inches for all the
specimens.
R = (PL)/(bd2)
Equation IX.4 Modulus of Rupture for 4-point loading.
It should be noted that Equation IX.4 was used to calculate the MOR for the
loading setup shown in Figure VIII.14, which was the loading setup specified by the
ASTM standard. The ASTM standard calls the test a three point loading test, but the
setup they specify is actually a four point loading test. The supports on the top of the
beam were spaced a distance of L/3 per the ASTM standard. Placing the top supports at
this distance modifies the 3-point MOR equation to the 4-point MOR equation.
Trial beams were not created for this test because experience had been gained
using the Forney machine during the compression tests. Also, the ASTM standard
provided an average flexural strength of 94 psi for specimens with 6% cement. This
information was helpful because it provided some estimation of the expected beam
strength. The machine was adjusted to provide a constant load rate of approximately 5
pounds per second. The ASTM standard specifies a constant load rate of 100 ± 5 psi/min
in the extreme fibers of the beam, which equates to approximately 5 pounds per second
applied by the machine.
All of the beams cracked within the middle third of the beam. Adjustments to the
MOR calculation must be made if the crack is outside of the middle third. Below is a
summary of the modulus of rupture tests for the three mixes. The maximum load was
80
recorded by the machine and Equation IX.4 was used to calculate the MOR for each
sample.
Table IX.10 Soil Modulus of Rupture Test Results.
Soil
Sample
1
2
3
4
5
6
Average
Strength
[psi]
Std.
Deviation
COV
Pressure
[psi]
6
8
4
4
6
4
5
2
31
Table IX.11 Soil-Cement Modulus of Rupture Test Results.
Soil Cement
Sample
Pressure [psi]
1
163
2
179
3
100
4
171
5
158
6
150
Average
Strength
154
[psi]
Std.
Deviation
COV
28
18
81
Table IX.12 Soil-Cement-Fiber Modulus of Rupture Test Results.
Soil Cement Fiber
Sample
Pressure [psi]
1
142
2
138
3
154
4
142
5
158
6
171
Average
Strength
151
[psi]
Std.
Deviation
COV
13
8
Both the soil-cement and soil-cement-fiber samples had very similar results with
an average modulus of rupture of 154 psi and 151 psi, respectively. The soil-cementfiber had a lower standard deviation and coefficient of variation. The soil-cement-fiber
beams remained bonded together even after failure, but the beams could not handle any
additional loading. The fibers would stretch if additional load was applied.
The soil had a modulus of rupture of only 5 psi and it could barely handle any
load. The soil almost no flexural strength as was anticipated before testing. One reason
for using reinforced concrete lintel over doors and windows is unstabilized rammed earth
homes have virtually no flexural strength and these lintels are needed to withstand
bending that occurs over the openings. The results of these tests demonstrated the benefit
of adding Portland cement to increase the flexural strength.
82
Figure IX.17 Soil-Cement Modulus of Rupture.
Failed soil-cement beam from modulus of rupture test.
83
Figure IX.18 Soil-Cement-Fiber Modulus of Rupture.
Failed soil-cement-fiber beam from modulus of rupture test.
Figure IX.19 Soil-Cement-Fiber Modulus of Rupture Close-Up.
Fibers hold the failed beam together.
84
Again the most significant difference between the soil-cement and soil-cementfiber was the failure mechanism. The soil-cement would suddenly crack and fall off the
testing apparatus. The soil-cement-fiber would suddenly crack, but the fibers would hold
the beam together and the beam would remain on the testing apparatus. The fibers held
the beam together, but the beam could not take any additional load.
The fibers did not delay the onset of cracking, which was unexpected based upon
the research mentioned in this thesis. As soon as a visible crack appeared, it would
propagate to the top of the beam and the beam would fail. If the fibers had delayed the
onset of cracking, the soil-cement-fiber beams would have handle more load, but this did
not happen.
After reviewing the results it seemed the fibers were not stiff enough to allow the
beam to take more load. If the fibers were stiffer, it is likely the crack would have not
propagated as quickly and the crack would have remained tight. Once the beam cracked,
the fibers would begin to stretch, which allowed the crack to widen and propagate.
Fiberglass or steel fibers might have increased the MOR because they are significantly
stiffer than the polypropylene fibers used in this thesis.
85
CHAPTER X
CONCLUSION
The initial hypothesis was the addition of fibers to the soil-cement would greatly
increase the shear strength and flexural strength of the material. For the soil-cement and
soil-cement-fiber samples, the results for unconfined compression, shear, and flexural
testing were similar. The soil-cement-fiber samples had statistically less variation for the
unconfined compression and modulus of rupture tests. The sample size of six is
relatively small to make statistical conclusions, but the evidence in this thesis suggests
soil-cement-fiber material has less variation in strength compared to soil-cement.
The main difference between the soil-cement and soil-cement-fiber was the
failure mechanisms. Brittle failure happened on both the soil-cement and soil-cementfiber. However, the material in the soil-cement-fiber samples remained bonded together
after failure. While the fiber held the material together, it did not significantly increase
the load capacity. The cement additive provided the additional strength and it also made
the material brittle.
One possible benefit of the fibers could be in the form of safety. During failure,
the material in a rammed earth wall with fiber reinforcement would remain connected
together rather than falling off the wall. A soil-cement wall without fiber could
potentially be hazardous if large chunks of the wall were falling down on the occupants
during an earthquake or collapse. Adding fibers seems to be a relatively easy way from a
construction standpoint to gain some additional safety.
The results were somewhat surprising in that the fiber did not increase the
strengths more than the results showed. One possible explanation is the plastic fibers are
86
much more elastic than the soil-cement. The difference in engineering properties causes
the soil-cement matrix to resist the entire load until failure. After failure the fibers are
stretched and begin to resist the applied load, but most material strength is gone once the
soil-cement matrix cracks.
This thesis explored a new topic in rammed earth research and tested the strength
of rammed earth with fiber and cement additives. No previous study had been published
on the use of synthetic fiber reinforcement in rammed earth. From the test results, the
soil-cement and soil-cement-fiber mix design had nearly identical compressive, shear,
and flexural strength. The soil-cement-fiber mixture did have less statistical variation in
the test results. The soil-cement-fiber mixture also exhibited different behavior during
failure compared to the soil-cement, and it appears that the soil-cement-fiber mixture
could improve the safety of rammed earth buildings during failure or collapse.
Recommendations for Future Research
The use of fibers in rammed earth is a topic that must be researched more
thoroughly. Currently, there is very little published research on this topic and relevant
research about fiber-reinforced soils is specific to geotechnical applications and not
rammed earth walls. The published geotechnical papers focus on a wide variety of soils
and these soils are not selected specifically for their use in rammed earth. Future research
should follow the recommended soil selection criteria mentioned in this thesis.
There are endless possibilities for conducting more tests on rammed earth.
Testing more samples in compression, shear, and flexure would improve the statistical
analysis of the material by providing a larger data set to analyze. Other test methods, like
triaxial testing, are strongly recommended. This test would likely produce the most
87
accurate results for compressive and shear strength. Researchers have not used this test
on rammed earth and one reason for this might be due to the difficulty and time involved
in performing this test. Triaxial testing might overcome some of the challenges
mentioned in this thesis that occurred with preparing the shear samples. Triaxial tests
could be performed on samples using the same preparation procedures for compression
test samples outlined in this thesis.
Experimenting with different mix designs is highly recommended. The use of
different fiber material is a promising area of research. Fiberglass and steel fiber could
be used instead of polypropylene fiber. These materials have been successfully used to
increase soil and concrete strengths, but their use in rammed earth has not been explored.
Studies could also examine the possibility of reducing the cement content when
increasing amounts of fiber reinforcement are used.
88
REFERENCES
Adobe Builder (2001). “Rammed Earth, Book #9.” New Mexico.
ASTM. (2007). “Standard Test Methods for Compressive Strength of Molded SoilCement Cylinders.” ASTM D1633-07, West Conshohocken, PA.
ASTM. (2007). “Standard Practice for Making and Curing Soil-Cement Compression and
Flexure Test Specimens in the Laboratory.” ASTM D1632-07, West
Conshohocken, PA.
ASTM. (2008). “Standard Practice for Dealing with Outlying Observations.” ASTM
E178-08, West Conshohocken, PA.
ASTM. (2010). “Standard Test Method for Liquid Limit, Plastic Limit, and Plasticity
Index of Soils.” ASTM D4318-10, West Conshohocken, PA.
ASTM. (2010). “Standard Test Method for Static Modulus of Elasticity and Poisson’s
Ratio of Concrete in Compression.” ASTM C469-10, West Conshohocken, PA.
ASTM. (2011). “Standard Test Method for Direct Shear Test of Soils Under
Consolidated Drained Conditions.” ASTM D3080-11, West Conshohocken, PA.
ASTM. (2012). “Standard Test Method for Flexural Strength of Soil-Cement Using
Simple Beam with Third-Point Loading.” ASTM D1635-11, West Conshohocken,
PA.
ASTM. (2012). “Standard Test Method for Laboratory Compaction Characteristics of
Soil Using Standard Effort (12400 ft-lbf/ft3 (600 kN-m/m3)).” ASTM D698-12,
West Conshohocken, PA.
Bryan, A. J. (1988). “Criteria for the suitability of soil for cement stabilization.” Building
and Environment, 23(4), 309-319.
Bryan, A. J. (1988). “Soil/cement as a walling material—I. Stress/strain properties.”
Building and Environment, 23(4), 321-330.
Bryan, A. J. (1988). “Soil/cement as a walling material—II. Some measures of
durability.” Building and Environment, 23(4), 331-336.
89
Bui, Q., and Morel, J. (2009). “Assesing the anisotropy of rammed earth.” Construction
and Building Materials, 23, 3005-3011.
Burroughs, S. (2006). “Strength of compacted earth: linking soil properties to
stabilizers.” Building Research and Information, 34(1), 55-65.
Burroughs, S. (2008). “Soil property criteria for rammed earth stabilization.” Journal of
Materials in Civil Engineering, 20(3), 264-273.
Consoli, N. C., Prietto, D. M., and Ulbrich, L. A. (1998). “Influence of fiber and cement
addition on behavior of sandy soil.” Journal of Geotechnical and
Geoenvironmental Engineering, 124(12), 1211-1214.
Consoli, N. C., Zortea, F., Souza, M. D., and Festugato, L. (2011). “Studies on the dosage
of fiber-reinforced cemented soils.” Journal of Materials in Civil Engineering, 23,
1624-1632.
Das, B. (2008). Advanced Soil Mechanics, Taylor and Francis, New York, NY.
Easton, D. (2007). The Rammed Earth House, Chelsea Green, White River Junction, VT.
Houben, H., and Guillard, H. (2008). Earth Construction—A comprehensive guide,
Intermediate Technology Publications, London.
Jaquin, P. A., Augarde, C. E., Gallipoli, D., and Toll, D. G. (2009). “The strength of
unstabilised rammed earth materials.” Géotechnique, 59(5), 487-490.
Jiang, H., Cai, Y., and Liu, J. (2010). “Engineering properties of soils reinforces by short
discrete polypropylene fiber.” Journal of Materials in Civil Engineering, 22(12),
1315-1322.
Kaniraj, S. R., and Havanagi, V. G. (2001). “Behavior of cement-stabilized fiberreinforced fly ash-soil mixtures.” Journal of Geotechnical and Geoenvironmental
Engineering, 127(7), 574-584.
King, B. (1996). Buildings of earth and straw: Structural design for rammed earth and
straw bale architecture, Ecological Design Press, Sausalito, CA.
90
Maniatidis, V., and Walker, P. (2008). “Structural capacity of rammed earth in
compression.” Journal of Materials in Civil Engineering, 20(3), 230-238.
Mesbah, A., Morel, J. C., Walker, P., and Ghavami, K. (2004). “Development of a direct
tensile test for compacted earth blocks reinforced with natural fibers.” Journal of
Materials in Civil Engineering, 16(1), 95-98.
Miller, B., and Miller, L. (1980). Manual For Building A Rammed Earth Wall, SelfPublished by Authors, Greeley, CO.
Minke, G. (2000). Earth Construction Handbook, WIT Press, Southampton, UK.
Minke, G. (2006). Building with earth: design and technology of a sustainable
architecture. Birkhäuser, Boston, MA.
NM Building Code (2009). “2009 New Mexico Earthen Building Materials Code.”
Housing and Construction Building Codes General, 14.7.4.
Reddy, B. V. V., and Kumar, P. P. (2011). “Structural behavior of story-high cementstabilized rammed-earth walls under compression.” Journal of Materials in Civil
Engineering. 23(3), 240-247.
Standards Australia. (2002). The Australian earth building handbook, Sydney, Australia.
Walker, P. J., and Dobson, S. (2001). “Pullout tests on deformed and plain rebars in
cement stabilized rammed earth.” Journal of Materials in Civil Engineering, 13,
291-297
91
APPENDIX
New Mexico Building Code Excerpts
92
TITLE 14
HOUSING AND CONSTRUCTION
CHAPTER 7 BUILDING CODES GENERAL
PART 4
CODE
2009 NEW MEXICO EARTHEN BUILDING MATERIALS
14.7.4.1
ISSUING AGENCY: Construction Industries Division of the
Regulation and Licensing Department.
[14.7.4.1 NMAC - Rp, 14.7.4.1 NMAC, 1-28-11]
14.7.4.2
SCOPE: This rule applies to all earthen building materials
contracting work performed in New Mexico on or after January 28, 2011, that is subject
to the jurisdiction of CID, unless performed pursuant to a permit for which an application
was received by CID before that date.
[14.7.4.2 NMAC - Rp, 14.7.4.2 NMAC, 1-28-11]
14.7.4.3
60-13-44.
STATUTORY AUTHORITY: NMSA 1978 Section 60-13-9 and
[14.7.4.3 NMAC - Rp, 14.7.4.3 NMAC, 1-28-11]
14.7.4.4
DURATION: Permanent.
[14.7.4.4 NMAC - Rp, 14.7.4.4 NMAC, 1-28-11]
14.7.4.5
EFFECTIVE DATE: January 28, 2011, unless a later date is
cited at the end of a section.
[14.7.4.5 NMAC - Rp, 14.7.4.1 NMAC, 1-28-11]
93
14.7.4.6
OBJECTIVE: The purpose of this rule is to establish minimum
standards for earthen building materials construction in New Mexico.
[14.7.4.6 NMAC - Rp, 14.7.4.1 NMAC, 1-28-11]
14.7.4.7
DEFINITIONS:
A.
Amended soil means improving an unqualified soil to a qualified state
with the addition of other soils or amendments.
B.
Amendments means additive elements to soil, such as lime, portland
cement, fly ash, etc. which are “dry-mixed” into the main soil body as a percentage of
total weight to achieve stabilization.
C.
Buttress means a projecting structure providing lateral support to a wall.
The buttress shall be incorporated into the foundation and wall system. (Refer to figure 1
of the earthen building figures supplement).
D.
CEB means compressed earth block.
E.
Keyway means a groove on the vertical rammed earth wall surface for
interlocking purposes. Refer to figure 3 of the earthen building figures supplement).
F.
compacted.
Lift means a course of rammed earth, placed within the forms, and then
G.
Nailer means any material rammed into the wall that serves as an
attachment device. Refer to figure 4 of the earthen building figures supplement).
H.
Optimum moisture means sufficient water (generally no more than ten
(10) percent) mixed into the soil to attain sufficient compaction.
I.
psi means pounds per square inch.
J.
Qualified soil means any soil, or mixture of soils, that attains 300 psi
compression strength and attains 50 psi. modulus of rupture.
K.
Rammed earth means qualified soil that is mechanically or manually
consolidated to full compaction.
L.
Round-cap nails means fasteners that include nails or screws in
combination with caps of at least three-fourths (3/4) inches diameter or three-fourths ( ¾)
inch square.
94
M.
Stabilization, stabilized means qualified soils that pass the wet strength
test under ASTM D1633-00 or contain a minimum of six (6) percent portland cement by
weight. Stabilization is achieved through the use of amendments.
N.
Wet strength compression test means an approved testing laboratory
process in which a fully cured rammed earth cylinder is completely submerged in water a
minimum of four hours according to ASTM D1633-00, then subjected to a compression
test.
[14.7.4.7 NMAC - Rp, 14.7.4.7 NMAC, 1-28-11]
14.7.4.8
EARTHEN BUILDING MATERIALS:
A.
General. The provisions of this rule, 14.7.4 NMAC, shall control the
design and construction of one- and two-family dwellings in which earthen building
materials form the bearing wall system.
B.
Allowable wall heights for earthen structures. All earthen structures
whether adobe, burned adobe, compressed earth block, rammed earth or terrón, shall
conform to table 1. For purposes of using table 1, height is defined as the distance from
the top of the slab or top of stem wall to the underside of the bond beam.
Table 1
ALLOWABLE WALL HEIGHTS FOR EARTHEN STRUCTURES
Maximum
Wall
Maximum
Maximum
Wall
Maximum
Sds
Thickness
Height
Sds
Thickness
Height
10
120”
10
120”
12
128
12
128
14
144
14
144
16
144
16
144
18
144
18
144
24
144
24
144
.25
.3
10
120”
10
120”
12
128
12
128
14
144
14
144
16
144
16
144
18
144
18
144
24
144
24
144
.35
.4
10
104”
10
96”
12
128
12
112
14
144
14
136
95
16
18
24
144
144
144
16
18
24
144
144
144
.45
.5
This table is based on two story maximum, one and two family residential with seismic
soil site class D1.
[14.7.4.8 NMAC - Rp, 14.7.4.8 NMAC, 1-28-11]
14.7.4.12
A.
RAMMED EARTH CONSTRUCTION:
General. The following provisions shall apply.
(1) Rammed earth shall not be used in any building more than (2) stories
in height. The height of every wall of rammed earth without lateral support is specified
in 14.7.4.8 NMAC table 1. The height of the wall is defined as the distance from the top
of the slab or top of stem wall to the underside of the bond beam.
(2) Exterior rammed earth walls shall be a minimum of eighteen (18)
inches in thickness. Exception: Exterior walls that are also designed as solar mass walls
(trombe) as defined by the passive solar heating worksheet, dated June 2004 and prepared
by the state of New Mexico energy, minerals and natural resources department, are
allowed and shall be minimum thickness of ten (10) inches, not to exceed twelve (12)
inches. They shall be fully attached to or integrated with any adjacent structural wall and
topped with a bond beam that fully attaches them to the bond beam of any adjacent
structural wall as described in 14.7.4.17 NMAC.
(3)
Interior rammed earth walls shall be a minimum of twelve (12) inches
in thickness.
(4) Unstabilized rammed earth walls must be covered to prevent
infiltration of moisture from the top of the wall at the end of each workday and prior to
wet weather conditions, whether the walls are contained within forms or not.
(5)
Fully stabilized rammed earth walls may be left unprotected from the
elements.
(6) In no case shall a rammed earth wall be reduced in thickness with back
to back channels or nailers. Channels or nailers rammed on both sides of a running wall
shall not be opposite each other to avoid an hourglass configuration in the wall section.
Channels or nailers on both sides of a running wall shall be separated from each other
vertically at a distance no less than the rammed earth wall thickness. (Refer to figure 4 of
the earthen building figures supplement).
96
(7) An architect or engineer registered in the state of New Mexico shall
design and seal structural portions of two-story residential rammed earth construction
documents.
(8) The general construction of the building shall comply with all
provisions of the 2009 New Mexico Residential Building Code (NMRBC), unless
otherwise provided for in this rule.
(9) Passive solar structures incorporating the use of solar mass walls
(trombe), direct gain arrays or sunspaces (greenhouses) as defined by the passive solar
heating worksheet, dated June 2004 and prepared by the state of New Mexico energy,
minerals and natural resources department, are allowed.
B.
Fireplaces. Adobe or masonry fireplaces and chimneys in rammed earth
structures shall comply with 14.7.3.18 NMAC. They shall be integrated into adjacent
rammed earth walls during construction or secured to them by suitable steel ladder
reinforcement or reinforcing rods.
C.
Count Rumford fireplaces. Count Rumford fireplaces are allowed as
provided in 14.7.3.18 NMAC.
D.
Stop work. The building inspector shall have the authority to issue a
“stop work” order if the provisions of this section are not complied with.
E.
Lateral support. Lateral support shall occur at intervals not to exceed
twenty-four (24) feet. Rammed earth walls eighteen (18) inches to less than twenty-four
(24) inches thick shall be laterally supported with any one or combination of the
following: A rammed earth wall of bond beam height that intersects the running wall
with at least sixty (60) degrees of support (refer to a figure 5 of the earthen building
figures supplement); an adobe wall of bond beam height and at least ten (10) inches in
width that intersects with and attaches to the running wall with at least sixty (60) degrees
of support (refer to figure 5 of the earthen building figures supplement); a minimum
twenty 20 gauge steel frame or wood frame wall of full height that intersects with and
attaches to the running wall with ninety (90) degrees of support, that is properly crossbraced or sheathed (refer to figure 6 of the earthen building figures supplement); a
buttress configuration that intersects the running wall at ninety (90) degrees, of adobe or
rammed earth. The buttress base must project a minimum of three (3) feet (or thirty-three
(33) percent of the wall height) from the running wall and support at least seventy-five
(75) percent of the total wall height (refer to figure 7 of the earthen building figures
supplement). The thickness of a rammed earth buttress shall be at least eighteen (18)
inches. The thickness of an adobe buttress shall be a minimum fourteen (14) inches.
Rammed earth walls greater than twenty-four (24) inches in thickness are self-buttressing
and do not require lateral support provided their design adheres to 14.7.4.8 NMAC table
1 and the other applicable provisions of this rule.
97
F.
Openings. Door and window openings shall be designed such that the
opening shall not be any closer to an outside corner of the structure as follows.
(1) In rammed earth walls eighteen (18) inches to less than twenty-four
(24) inches thick, openings shall not be located within three (3) feet of any corner of the
structure. (Refer to figure 8 of the earthen building figures supplement). Exception:
Openings may be located within three (3) feet of any corner provided a buttress extending
at least three (3) feet from the structure supports the corner. A continuous footing below
and a continuous bond beam above, shall be provided across such openings.
(2) Rammed earth walls greater than twenty-four (24) inches thick are
self-buttressing, with no special consideration for placement of openings within the area
of the wall.
G.
Piers. Rammed earth piers supporting openings shall measure no less
than three (3) square feet in area and no dimension shall be less than eighteen (18) inches.
(Refer to figures 9-A and 9-B of the earthen building figures supplement).
[14.7.4.12 NMAC - Rp, 14.7.4.12 NMAC, 1-28-11]
14.7.4.13
FOUNDATIONS:
A.
General. Foundation construction shall comply with applicable
provisions of the 2009 New Mexico Residential Building Code, and the following: a
minimum of three (3) continuous #4 reinforcing rods are required in minimum 2500 psi.
concrete footings supporting rammed earth walls. Stem walls shall be the full width of
the wall supported above or wider to receive forming systems. Footings shall be a
minimum of ten (10) inches in depth.
B.
Perimeter insulation. For the purposes of placement of perimeter
insulation, rammed earth walls may overhang the bearing surface up to the thickness of
the perimeter insulation, but in no case greater than two (2) inches.
C.
Keyway. A key way shall be provided where the rammed earth wall
meets the foundation system. The keyway shall be established at the top of the stem a
minimum of two (2) inches deep by six (6) inches wide formed at the time of the pour,
and shall run continuously around the structure to include any intersecting rammed earth
wall sections. The rammed earth wall shall be fully rammed into this keyway (refer to
figure 2 of the earthen building figures supplement). Exception: Placement of vertical
reinforcing rods extending a minimum twelve (12) inches into the rammed earth wall.
The vertical rods shall be minimum #4, imbedded into the concrete and spaced fortyeighty (48) inches on center, maximum.
D.
Concrete grade beam. Rubble filled foundation trench designs with a
reinforced concrete grade beam above are allowed to support rammed earth wall
98
construction. An architect or engineer registered in the state of New Mexico shall certify
the grade beam/rubble-filled trench design portion.
[14.7.4.13 NMAC - Rp, 14.7.4.13 NMAC, 1-28-11]
14.7.4.14
RAMMED EARTH SOIL SPECIFICATIONS:
A.
General. The soil shall not contain rock more than one-and-a-half (1 1/2)
inch in diameter. The soil shall not contain clay lumps more than one-half (1/2) inch in
diameter. The soil shall be free of all organic matter. The soil shall not contain more
than two (2) percent soluble salts.
B.
Soil compressive strength. Prior to the start of construction, fully-cured
rammed earth soil samples shall be tested at an approved testing laboratory for
compressive strength. The ultimate compressive strength of all rammed earth soil,
stabilized or non-stabilized, shall be a minimum three-hundred (300) psi. The
compressive strength report shall be submitted with the permit application. This report
may be waived if the builder provides certification of compliance. The certification must
be dated within one year of the date on the application for the building permit. Samples
tested shall be representative of soil to be used on the project for which the permit
application is submitted.
C.
Stabilized rammed earth soil. The following shall apply to stabilization
of rammed earth soil: Asphalt emulsion may not be used for stabilization of rammed
earth soil. Thorough mixing of additives to the soil may be achieved by any method that
assures a complete blending to a uniform color and texture. Stabilized soil is suitable soil
that contains six (6) percent or more portland cement by weight or that passes ASTM
D1633-00. Samples tested shall be representative of soil to be used on the project for
which the permit application is submitted. The compressive strength report shall be
submitted with the permit application. Laboratory testing shall indicate rammed earth
samples attained a minimum of two-hundred (200) psi. after seven (7) days. If a different
soil is provided at any time during construction, it must meet the minimum requirements
outlined above, prior to use in the structure.
D.
Unstabilized rammed earth soil. Unstabilized rammed earth soil is that
containing less than six (6) percent portland cement by weight or that fails to pass ASTM
D1633-00. The exterior of such walls shall be protected with approved stucco systems or
other method approved by the building official. Refer to 14.7.4.19 NMAC for weatherresistive barrier requirements.
E.
Amended soil. The following guidelines shall apply when amending soils
to attain a qualified soil. Soil shall not contain rock greater than one-and-a-half (1 1/2)
inch in diameter. Soil shall not contain clay lumps greater than one-half (1/2) inch
diameter. Soil shall be free of organic matter. Soil shall not contain more than two (2)
99
percent soluble salts. Soils to be mixed shall be sufficiently dry to blend completely to
one uniform color and texture. The amended soil shall be tested prior to use as per
Subsection B of 14.7.4.14 NMAC.
F.
Forming systems. The forming system shall be adequate to contain the
material under compaction. It shall be properly plumbed and braced to withstand the soil
pressures as well as construction activity on and around it.
G.
Placement of material, compaction and curing.
(1) No amount of portland cement stabilized soil will be mixed that will
not be placed in the wall system within sixty (60) minutes of its preparation.
(2) Lifts of prepared soil shall be placed in the forms in relatively even
layers not to exceed 8 inches in depth. Each lift shall then be rammed to full compaction.
(3) Optimum moisture content as determined to meet minimum
compressive strength shall be maintained for stabilized and unstabilized walls.
(4)
Work will progress, lift-by-lift, until the work approaches bond beam
height.
(5) Forms may be stripped immediately after ramming is completed for a
section of wall, providing ramming of adjacent sections does not affect the structural
integrity of completed walls.
(6) Portland cement stabilized walls not in forms shall be lightly spraycured with water at least five (5) spaced times during daylight hours. This procedure
shall continue for at least three (3) days starting from the time that the wall is exposed to
the elements. Exception: Rammed earth walls left in forms three (3) or more days shall
not require water-spray curing.
H.
Placement of attachment materials.
(1) Nailers: Nailers incorporated into the rammed earth wall shall be
installed as follows (Refer to figure 4 of the earthen building figures supplement); the
rammed earth wall shall not be reduced in thickness with back-to-back nailers. To avoid
an hourglass configuration in the wall section, nailers on either side of a running wall
shall not be opposite each other. Nailers on either side of a running wall shall be
separated from each other vertically a distance not less than the rammed earth wall
thickness. Nailers shall be placed onto the wall such that the narrow dimension of the
nailer is exposed on the race of the wall prior to ramming. Nailers shall be cured and
sealed against moisture penetration prior to installation in forms. The nailers shall not
extend the full depth of the wall. Box wood nailers are not allowed. (Refer to figure 11
of the earthen building figures supplement). The nailer shall be no more than two (2)
inches by four (4) inches by its length.
100
(2) Channels: Channels may be incorporated into the rammed earth wall
as follows (Refer to figure 2 of the earthen building figures supplement); To avoid an
hourglass configuration in the wall section, channels on either side of a running wall shall
not be opposite each other. (Refer to figure 4 of the earthen building figures supplement).
Channels shall be no more than two (2) inches by four (4) inches by their length in
dimension. Vertical channels shall not be placed closer than twelve (12) inches to a
rammed earth wall finished edge or corner.
[14.7.4.14 NMAC - Rp, 14.7.4.14 NMAC, 1-28-11]
14.7.4.17
BOND BEAMS:
A.
General. The bond beam shall be secured to the rammed earth wall.
Refer to Subsections H and I of 14.7.4.16 NMAC above. Bond beams may be of wood
or concrete construction. Bond beams shall measure six (6) inches nominal depth and
extend the full width of the wall. Exception: The bond beam width may be reduced as
follows: Two (2) inches maximum in an eighteen (18) to less than twenty-four (24) inch
thick rammed earth wall, or three (3) inches maximum in a rammed earth wall twentyfour (24) inches or greater in thickness. Bond beams must be continuous, running the full
perimeter of the structure. Interior rammed earth or adobe walls shall be incorporated
into the bond beam. Varying height bond beams shall extend into the adjoining rammed
earth wall one-half (1/2) the thickness of the adjoining rammed earth wall. The concrete
bond beam may secure anchoring and strapping devices.
B.
Wood bond beam construction. In addition to the general requirements
of Subsection A of 14.7.4.17 NMAC, wood bond beams may be constructed as approved
by the building official. Light wood bond beam construction may be utilized as shown in
figure 10 of the earthen building figures supplement.
C.
Concrete bond beam construction. In addition to the general
requirements of Subsection A of 14.7.4.17 NMAC, concrete bond beams shall be
constructed of minimum twenty-five hundred (2500) psi. concrete and shall contain steel
reinforcement as follows: For eighteen (18) to less than twenty-four (24) inch thick
rammed earth wall construction, a minimum of two (2) continuous number four (4)
reinforcing rods shall be used. For walls equal to or greater than twenty-four (24) inches
in thickness, a minimum of two (2) continuous number five (5) reinforcing rods shall be
used. Provide two (2) inch minimum reinforcement concrete cover over all horizontal
reinforcing rods. Concrete bond beams may be used to secure anchoring and strapping
devices.
D.
Concrete bond beam cold joints. Concrete bond beam cold joints are
limited to corners of perpendicular intersections with other structural, full-height walls.
Cold joints shall be tied into the adjoining bond beam with three (3) number four (4)
reinforcing rods. The reinforcement shall extend a minimum of twenty-four (24) inches
into both portions of the concrete bond beam.
101
[14.7.4.17 NMAC - Rp, 14.7.4.17 NMAC, 1-28-11]
14.7.4.18
LINTELS OVER OPENINGS:
A.
General. All openings require a lintel or semi-circular arch over the
opening. All lintels, whether of wood or concrete shall bear a minimum of twelve (12)
inches into the length of the wall. Exception: Nichos and other shaped voids as defined
in 14.7.4.15 NMAC.
B.
Bearing limitations. Lintels shall bear a minimum of twelve (12) inches
beyond coved, splayed or rounded bearing portions of openings that are less than the full
width of the wall. (Refer to figure 15 of the earthen building figures supplement).
C.
Lintels over openings in stabilized rammed earth walls. Openings less
than twenty-four (24) inches in width shall not require a lintel or semi-circular arched
opening. Openings greater than twenty-four (24) inches in width require lintels as
defined in table 4.
Table 4 Concrete Lintels Over Openings in Rammed Earth Walls (1)
Wall Lintel span Lintel depth Reinforcement Reinforcement
width
(2)
Concrete Cover
(3)
24”
6”
3- #4 @ 4”o.c.
36”
6”
3- #4 @ 4”o.c.
48”
6”
3- #4 @ 4”o.c.
60”
6”
3- #4 @ 4”o.c.
3” minimum
72”
8”
3- #5 @ 4”o.c.
concrete
cover
84”
8”
3- #5 @ 4”o.c.
on
all
sides
18”
96”
8”
3- #5 @ 4”o.c.
20”
24”
36”
48”
60”
72”
84”
96”
6”
6”
6”
6”
8”
8”
10”
3- #4 @ 4”o.c.
3- #4 @ 4”o.c.
3- #4 @ 4”o.c.
3- #4 @ 4”o.c.
3- #5 @ 4”o.c.
3- #5 @ 4”o.c.
3- #5 @ 4”o.c.
24”
36”
48”
60”
6”
6”
6”
6”
3- #4 @ 5”o.c.
3- #4 @ 5”o.c.
3- #4 @ 5”o.c.
3- #4 @ 5”o.c.
4” minimum
concrete cover
on all sides
Uniform
Load
1000
PLF
1350
PLF
102
22”
72”
84”
96”
8”
10”
10”
3- #5 @ 5”o.c.
3- #5 @ 5”o.c.
3- #5 @ 5”o.c.
3 1/2” minimum
concrete cover
on all sides
24”
24”
36”
48”
60”
72”
84”
96”
6”
6”
6”
6”
8”
10”
12”
3- #4 @ 6”o.c.
3- #4 @ 6”o.c.
3- #4 @ 6”o.c.
3- #4 @ 6”o.c.
3- #5 @ 6”o.c.
3- #5 @ 6”o.c.
3- #5 @ 6”o.c.
3” minimum
concrete cover
on all sides
1700
PLF
2000
PLF
1. 3000 psi minimum concrete at approximately 28 days.
2. Grade 40 steel reinforcement minimum.
3. Steel reinforcement at mid-depth of lintel.
[14.7.4.18 NMAC - Rp, 14.7.4.18 NMAC, 1-28-11]
103
Fiber Specifications from Manufacturer
104
Unified Soil Classification System
105
Gypsum Cement Specifications
106
107
Portland Cement Specifications
108