Measures to Control Temperature Cracks in tbe Roof Walls of

11th INTERNA TIONAL BRlCKlBLOCK MASONRY CONFERENCE
TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997
MEASURES TO C.oNTROL TEMPERATURECRACKS INTlIE R.o.oF WAJr:.LS OF
c.oNeRETE SMALL-SIZED H.oLLOW MAS.oNRY UNIT
Haibo Zhou
1
ABSTRACT
This paper is based on the investigation of the temperature cracks in the roof walls of
residential buildings constructed of concrete small-sized hollow masonry units in
Shanghai area. We analyze the reasons why these cracks occur. Standing by the principIe
of the combination of resisting and releasing the temperature cracks, the measures and
the construction details are put forward to control the temperature cracks in the roof
masonry walls.
INTRODUCTION
In recent years, the concrete small-sized hollow masonry unit (shortened as masonry
below) has been widely used in Shanghai area as a kind ofnew load-bearing wall material
to save soil and energy resources. By means of the improvement in design, science
research, masonry products and construction,many of these masonry buildings perform
well and the cracks in these masonry buildings have been controlled gradually.
Keywords:
Concrete Small-sized HollQ9Masonry Unit;
"Resistance"; "Release"
Temperature Cracks;
Assistant Civil Engineer Of Shanghai Zhongfang Architectural Design Institute, 234
Meng Hua Street ,Shanghai 20001O,China
1
758
Section 1:Main types and analysis of temperature cracks
Main Types OfTemperature Cracks
According to the crack investigation and our return visits to those buildings constructed
of masonry units, the main types of the temperature cracks in the roof walls are shown as
below.
(1)4So diagonal crack
These cracks mainly occur at the end of interior and exterior longitudinal roof walls and
the end ofthe transverse roofwalls, as shown in Fig. l-I (a) (b) (c).
)1óD DO ODrroU:
TIU D D O OD D TID
(a)4So diagonal crack at the end ofexterior longitudinal roofwalls
~
~
,
"-
/
/I
V
(c)4So diagonal crack at the end
of transverse roof walls
(b)4So diagonal crack at the end of
interior longitudinal roof walls
Fig. l-I
(2)Horizontal crack
These cracks mainly occur between the bottom of the roof slabs and the roofbond beams,
or between the bottom of the roof bond beams and the masonry walls. As shown in Fig.
1-2(a)(b).
roo i
lab
.
~:.
"-~ "
bond
M.W.
M.W.
M.w.---masonry wall
(b)
(a)
Fig. 1-2
Analysis OfTemperature Cracks
The difference in temperature mainly leads to these cracks. In shanghai area ,especially in
the summer, The roof slabs absorb a large amount of heat in the sunlight. The highest
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temperature in the surface of the roof will amount to about 60° C, but 40° C indoors.
Because of the difference in temperature between the roof slabs and the masonry walls ,
also between indoors and outdoors, the temperature deformation of the roof slabs (~s)
will be greater than that of the masonry walls( ~w),but the deformation of the roof slabs
were restrained by the masonry walls below, shown in Fig.1-3(a). So shear forces
between the roofslabs and the masonry walls come into being, as shown in Fig.1-3(b).
roof slob
roof sl ob
6.
/
1= =z= =
--- ~~
-~~;;; --
M.W.
M.W.
(a)
(b)
Fig.1-3
When the shear stress is greater than the shear strength of the bed mortar :
(1 )If the roof slabs are precast, the horizontal crack will occur between the roof slabs and
the roofbond beams, as shown in figure 1-2(a).
(2)If the roof slabs are cast-in-place reinforced concrete, and cast monolithically with the
roof bond beams. The horizontal crack will occur between the bottom of the roof bond
beams and the masonry walls, as shown in figure 1-2(b).
When the shear strength of the bed mortar is high enough, the roof slab,the roof bond
beam and the masonry wall will work as one unit, the internal forces of masonry walls can
be analyzed as following:
if a small square element is isolated from the masonry wall, the horizontal shear stresses('t)
on it, equal and opposite on the two faces for reasons of equilibrium,as shown in Fig. 1-4
T'
j
D tI
--'-
T'
p'''''''
6'i1llG1'
(a). Howeve< ,;fthe.e w...e lhe ooly """"'"
lhe
element would not be in equilibrium; it would spin. T'
fT'
Therefore ,on the two vertical faces there should exist
equilibrating vertical shear stresses (T' ) of the sarne 6 ~ 6'
Fig.1-4(a) I magnitude shown in Fig. 1-4 (a).That is, at any point in the IFig.1-4(b) I
.I
. wall, the vertical shear stresses( i )are equal in magnitude to
the horizontal shear stresses('t).These shear stresses combined in such a manner that their
effect is shown in Fig.l-4(b).That is ,the action of the two pairs of shear stresses on the
vertical and horizontal faces is the sarne as that of two pairs of normal stresses, one
tension and one compression,acting on 45° faces .When the principal tensile stress is
greater than the tensile strength ofthe masonry wall,the 45° diagonal cracks will occur in
the wall.
Section 2: Standing by the principie of the combination of resisting and releasing the
temperature cracks, the measures and construction details are put forward to control the
temperature cracks in the roof masonry wall.
The construction details of heat insulation used in shanghai area are obviously to reduce
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the difference in temperature between the roof slabs and the masonry walls.
(1) Both the elevated heat preservation layer and the insulation layer should be layed
on the roof slabs, and the ventilate ridge should be placed too. The color of elevated slabs
should be light, so as to increase heat reflection.
(2)The roof expansion joints should be used at a certain interval in the roof slab joints,
filled with flexible materials such as asphaIt mortar, caulked with water-proof ointment
on these joints, then enhance the joints with water-proof materiais.
However, these construction details of heat insulation shown above can't eliminate the
difference in temperature between the roof slabs and the masonry walls. The temperature
stresses still remain. Utilize the principIe of the combination of resisting and releasing the
temperature cracks: the bed mortar of the roof slab should be low grade or asphalt felts
should be layed on the roof bond beams, so as to allow tiny free slide on the top of the
roof bond beam, to release part of the temperature deformation, and to decrease the
temperature stresses in the masonry walls. Thus it canachieve the "release" of the roof
slab to the temperature deformation. Meanwhile ,the mortar in the masonry walls should
be high grade, and the construction operation should be standard, to ensure that mortar
joints are full . The holes of masonry units where the cracks occur shown in Fig. l-I
should be filled with low grade concrete, reinforced ifnecessary. The steel meshes should
be placed at a certain intervaI in vertical direction in the roof masonry walls. These
measures can improve the horizontal and vertical shear strength of the roof masonry
walls and achieve the "resistance" of the masonry walls against the temperature stresses.
In construction details' design, the anti-seismic( 6° or 7° earthquake resistance intensity
in shanghai area),the leak prevention of a building and construction feasibility should be
considered .
The Measures OfRoofSlabs
Standing by the principIe of the "release" of the roof slabs to the temperature
deformation,the design method and construction process should be improved.
(l)The cast-in-place reinforced concrete slabs should be designed to be simply
supported to the roofbond beams.
(2)The bed mortar should be low grade compo mortar(Ml.O) or asphalt felt should
be layed on the top of the bond beam.
(3)In order to disperse the temperature deformation of the roof and to increase
energy consumption, part of the slab joints above the masonry walls should be roof
expansion joints.
(4)In order to prevent the roof slabs from falling down in earthquake, the tie bars
should be placed between the roof slabs and the roof bond beams. Meanwhile ,in order to
allow tiny free slide of the roof slabs, the joint should be elastic.
(5)Construction process:
After finishing construction of the roof masonry walls. First, make the roof bond
beams ,and embed the joint bars between the roof slabs and the roof bond beams in the
roof bond beams. Then, make the bed mortar of the precast slabs, and place the precast
slabs. Or lay asphalt felts on the roof bond bearns, and make the cast-in-place concrete
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slabs. And then grout the joints of the precast slabs . Finally make the heat insulation
layer on the roof slabs.
(6)Roof expansion joints
I, Location: Above ali edge bond beams ,the distance of roof expansion joints
should not exceed 6m in the two principal directions.
11, In order to ensure that the expansion joints can deform freely, when casting
concrete or constructing the roof ,the hardness such as concrete,construction garbage
should be prevented from falling into the expansion joints.
(7)The construction details of the roof slabs placed on and jointed to the bond beams
are shown in Fig.2-1 (a)(b)and Fig.2-2(a)(b):
qJ8 0ne bar each 'oinl
&<=1000
rool expansio
',oinl
roo! bond
eam
roa! M.W.
~\:)
M1.0
>
"O
D compO~Jr
r=l
rool bond
beam
morlar
lie bar
rool M.W.
(a)The roofprecast slab
to the edge bond beam
D
M1.0
compo
morlar
fie bar
>
~\:).r' ~O
r=l 51 C»«:'
I
i
i
(b) The roof precast slab
to the middle bond beam
Fig. 2-1
qJ8<=1000
roof ex ansia
jainl
rool bond
beam
roa! M.W.
D
r:l
$8 <=1000
.J;
aSha~I~\:)
(?
lelt
~
roof bond
beam
rool M.W.
tie bar
D
~\:)
~
r:l N, M
lie bar
li)
i
I
i
(b) The roof cast-in-place slab
to the middle bond beam
(a)The roof cast-in-place slab
to the edge bond beam
Fig.2-2
The Measures OfMasonry Wall
Standing by the principie ofthe "resistance" ofthe roofmasonry walls to the temperature
stresses, the measures to improve the horizontal and vertical shear strength of the roof
masonry walls are shown as follow.
(I)The mc:-!ar in the roof masonry walls should be high grade (not less than M7.5
) .The construction operation should be standard,and the mortar joints should be full.
(2)Aimed at the location ofthe temperature cracks in the roofmasonry walls:
I, at the end of the interior and exterior rooflongitudinal walls,
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11.. at the end of the roof transverse roof walls,
low grade concrete (ClS-C20) should be used to fi11 the hollow of the rnasonry unit,
reinforced if necessaI)'. Because of staggered-joint boIid ('f" the masonry walls, the
concrete fi1led in the hole acts as a bolt, thus it can improve the shear strength of the
masonry wall.
(3) The (l/l4 ) steel meshes should be placed at a certain interval (400mm) in vertical
direction in the roof masonry walls, so as to improve the shear strength of the roof
masonry walls.
2000(1600)
fi lled wilh co ncrele,
reinforced.
oof olovalion
01 w Idin sI I mosh
~I
ex terior tra nsvers
wali
o
o
...
N
floo elevalion
o
<D (2»)
SIGNAL PLAN OF CRACK RESISTANCE
QETAILS OF ROOF WALLS
The similar construction details Sho'.'ln above were used in some constructing buildings.
The temperature cracks of these buildings wi11 be carefully investigated after
construction.
The principie of the combination of resisting and releasing the temperature cracks should
be utilized proper1y. Based on the investigation of the temperature cracks, we can
improve the construction details, so ás to reduce or avoid the temperature cracks in the
roof masonry walls.Thus the concrete sma11-sized hollow masonry unit wil1 be applied
furtherly.
ACKNOWLEOOMENTS
This paper is exarnined by
Zhang Guanfu Vice President & ChiefEngineer Of Shanghai Zhongfang
Architectural Design Institute
Liu Yongfeng
Vice President, Senior Engineer Of Shanghai Zhongfang architectural
Design Institute Taking Charge OfScience & Technology .
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