Control de Calidad de los Pavimentos de Hormigón

Control de Calidad de los
Pavimentos de Hormigón
Mark Snyder
Quality Control of
Concrete Paving
International Seminar on Concrete Pavements:
Improving the Quality of Concrete Pavements
Viña del Mar, Chile
September 3-4, 2013
Mark B. Snyder, Ph.D., P.E.
International Society for Concrete Pavements
What is Construction Quality?
• Achieving quality equates to conformance to
requirements
• Requirements need to be well-defined,
measurable, and not arbitrary
• Quality must be built into a project. It is not a
“hit-or-miss” proposition.
3
How Do The Following People
Affect “Quality”?
o
o
o
o
o
o
o
o
Designer/Specifier
Agency Inspector
QC Technician
Loader Operator at the concrete plant
Truck Driver
Paver Operator
Concrete Finisher
Texture/Cure Machine Operator
Quality Measurement Tools
• Two principal tools used to measure
conformance with requirements:
Inspection
Testing
Inspection
• Equipment
• Environmental
Conditions
• Materials
• Product
Workmanship
Testing
• Three criteria:
Quality Characteristics
(What do we want?)
Quality Measures
(How do I measure it?)
Quality Limits
(How much is enough?)
How Much Quality Is Enough?
• Is 99.9% “good enough”?
45 minutes of unsafe drinking water every month
2 long or short landings at many American
airport each day
500 incorrect surgical operations each week
3,000 newborns accidentally falling from the
hands of nurses or doctors each year
22,000 checks deducted from the wrong bank
account each hour
QA Principles
• Types of Tests
Process Control Test
Not Random
Contractors use when needed
–Change in process or material
Independent Assurance
Not the project personnel
Random Samples
For compliance with specifications
No others count for compliance
Basis for Testing
• Random testing assumes that the results
are normally distributed
• The mean and standard deviation of test
results are used to determine if the
samples are within specified limits
• Variability is due to the operator (and
equipment), the test procedure, and the
material being tested
Quality vs. Construction Variability
• Variability is an inherent part of construction.
Many sources
• All sources of variability have a negative impact
on the property being measured.
• Need to understand the magnitude of the
different sources of variability
• Quality construction requires control over all
sources of variability.
Source: Shiraz Tayabji, Fugro Consultants, Inc.
QA Principles
N=1
• Sources of Variability
Material
Sampling
Tester
Equipment
Procedures
Sources of Variability
Material
Process
Sampling
Composite
Variability
Testing
Precision and Bias
• Established test procedures (ASTM,
AASHTO) have accounted for test
variability through precision and bias
statements
• All physical tests have built in variability
that must be accounted for in some
manner
• The following slide illustrates the problem
in determining a “right answer”
Precision and Bias
REASONABLE LEVELS OF VARIABILITY
(In terms of acceptable standard deviation)
• Subgrade Density: 1 to 3 lb/cu. ft
• Concrete Thickness: 0.25 to 0.50 in.
• Concrete Flex Strength: 40 to 60 psi
• Concrete f’c: 300 to 500 psi
16
Higher levels of variability =>> construction process
is not under control and/or testing procedures are
marginal
17
Quality Control (QC)
• QC generally refers to testing by the
contractor for the purpose of process
control and to ensure meeting or exceeding
specifications
• A comprehensive QC program is much more
involved than QA because all aspects of the
project must be proactively monitored
(materials, batching, placement, etc.)
Quality Control
• Contractor’s QC system should address:
Materials production processes
Materials transportation and handling
Field placement procedures
Calibration and maintenance of
equipment
Watching the process
Fixing the process
Quality Control Charts
• Quality control charts (QCC) are statistically
based and used primarily for process
control
• Graphical format of QCC provides a simple
and effective means to determine when a
specific process is trending out of limits
Statistical Process Control
Concrete Batching QC
• Uniformity between concrete batches is
critical in producing a smooth and longlasting pavement
• The following parameters are routinely
checked during batching:
Aggregate moisture
Water content
Water/cementitious materials ratio
• Plant calibration and continuous
monitoring are required
ASTM Batching Tolerances
Construction Operations QC
Construction operations require many varied
types of QC measures, including the
following:
• Concrete temperature at time of
placement
• Entrained air content
• Consolidation (internal vibration)
• Dowel bar placement
• Potential for many others depending on
specification requirements
Testing – Temperature
• ASTM C1064
• EASY, just place thermometer in
concrete
• Results help verify conformance to
requirements
Testing – Slump
• ASTM C143 /
AASHTO T119
• Measures consistency;
NOT QUALITY!
• Typical values:
Slipform: 0.5-1.5 in.
Fixed form: 3-4 in.
• Slump is dependent on
mixture and also on
time of testing
Testing – Density (Unit Weight)
•
•
•
•
•
ASTM C138 / AASHTO T121
Measures known volume
Typically 130 to 150 lb/ft3
Indicates batch-to-batch variability
Reduction in density may indicate:
Higher air content, higher water
content, lower cement content, change
in proportions of ingredients, or change
in aggregate specific gravity or
moisture
• One of the most valuable tests for process
control
Testing – Air Content
• ASTM C231 / AASHTO T152
• Target air depends on agg size
• Testing at plant or in front of
paver doesn’t account for air
loss of up to 2% in paver
• Quality critical to durability
• AVA and petrography are other
means to measure
Max Agg
Size
Target Air
9.5mm
7 ½%
13mm.
7%
19mm
6%
25mm
6%
38mm
5 ½%
Testing – Air Content (continued)
• Affected in the field by:
Cement
SCMs
Chemical admixtures
Gradation of aggregates
w/cm ratio
Temperature
Delays
Placement/consolidation
Air Content
• Why important?
• Does not address
aggregate durability
problems
• Typically specified as a
target value and range
(e.g., 5.0% 1.0%)
Nonfreeze states: 1-6%
Freeze states: 4-8%
Quality Assurance (QA)
• QA typically involves testing by the
agency or its representative to determine
compliance with specifications
• The most frequently used QA criteria for
paving jobs include
Slab thickness
Concrete strength
Entrained air content
Ride quality
Acceptance Testing
• Intent of testing is not to discriminate absolutely
between good and bad end product
Otherwise, we would be testing every cy of concrete
and every sy of the pavement
• Intent is to discriminate sufficiently to minimize
Contractor’s risk of good end product being rejected
Owner’s risk of a bad end product being accepted
• Balance is maintained by type & extent of testing
and rules used to accept test results
Source: Shiraz Tayabji, Fugro Consultants, Inc.
Qualifications
• QC/QA personnel must be adequately
trained (and, often, certified)
• Repeatability and reproducibility of results
are critical for both QC and QA functions
Key Concrete Pavement
Acceptance Items
•
•
•
•
Air content
Slab thickness
Concrete strength
Initial
smoothness
Thickness
• Probe
• Probe
• Probe
Slab Thickness
• Why important?
• Traditional method:
Cores at prescribed
locations
Governed by ASTM
C174
• NDT methods:
GPR (limitations)
Impact echo
Slab Thickness Measuring:
Magnetic Imaging Tomography
Concrete Strength
• Why important?
• Traditional measurement methods
Flexural strength of beams
Compressive strength of cylinders
or cores
Split tensile testing of cylinders or
cores
Flexural, Compressive Strength
• Strength testing of concrete is one of the
most common tests performed
• It is usually a combination of field
preparation and laboratory (or mobile
laboratory) testing
• Relatively simple, but variability in results
can often be attributed to slight variations
in procedure
Testing – Comp & Flex Strength
• ASTM C39 / AASHTO T22 for cylinders
• ASTM C78 / AASHTO T97 for beams
• Typical strength requirements (low / avg / high)
Compressive: 21 / 24.5 / 28 MPa
Flexural: 3.8 / 4.2 / 4.5 MPa
Concrete Strength Testing:
Traditional Measurement Methods
Cylinders
Beams
Cores
Concrete Strength:
Type Used for Acceptance
Number of States
20
15
3-day
7-day
14-day
28-day
Unknown
10
5
0
Compressive
Flexural
Concrete Strength:
Specifying Strength
• Strength type (flex, comp, split tensile)
• Time of measurement (3, 7, 14, 28 days)
• Strength value (average or minimum)
• Typical*:
Avg f’c = 28 MPa at 28 days
Min f’c = 24 MPa at 28 days
*ACPA Database of State Practices (2007)
Concrete Strength:
Alternative Methods
• Maturity (ASTM C1074)
Monitoring in-place concrete pavement
temperatures to estimate strength
• Pulse velocity (ASTM C597)
Measuring speed at which ultrasonic
waves travel through concrete
• Seismic pavement analyzer
Measuring resonant frequency of stress
waves imparted in concrete
Concrete Strength:
Compressive Strength (MPa)
• Strength of concrete = f(time, temp)
• Laboratory-derived correlation curve
• Monitoring of field
40
pavement temps
35
• Precision to
30
cylinders: ±5%
25
• Governed by
20
ASTM C 1074
15
5000
4000
3000
2000
10
1000
5
0
0
0 100 200 300 400 500
Temperature-Time Factor, M(t)
Compressive Strength (psi)
Maturity
Testing – Maturity
Testing – Maturity
ASTM C1074 / AASHTO T325
Testing – Maturity
Maturity Testing
•
•
•
•
Early sawing
Surface texturing
Compatible equipment
Equipment condition
Initial Smoothness
• Why important?
• Measurement methods
Straight edge
Profilograph (California)
Lightweight profilers
• Typically expressed in terms of:
Profile Index
5-mm blanking band (typ. 0.08 – 0.16 m/km)
0-mm blanking band: (typ. 0.32 – 0.47 m/km)
IRI (typ. 0.95 – 1.10 m/km)
Initial Smoothness:
Measurement Methods
California Profilograph
Lightweight Profiler
Current Smoothness Issues
• Accuracy
Wavelengths (profilograph)
Tining, joints, cracks
• Repeatability of device
Tining, joints, cracks
Coarse textured pavements
These are current topics of research…
Specification
Incentives/Disincentives
Number of States
30
25
20
Incentives
Disincentives
None
15
10
5
0
Strength
Thickness
Smoothness
Summary
• Key specification acceptance items
Strength
Thickness
Air content
Smoothness
• Conventional test methods well established
• New and innovative test methods evolving
Rapid
Nondestructive
More test points
Acknowledgments
• U.S. Federal Highway Administration Concrete
Pavement Technology Program
• CTL Group, Inc.
• U.S. National Highway Institute
• American Association of State Highway and
Transportation Officials (AASHTO)
• American Concrete Pavement Association
• Iowa State University National Concrete
Pavement Technology Center
Questions?