Structurally Efficient Hollow Concrete Blocks in Load Bearing Masonry
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Transcript of Structurally Efficient Hollow Concrete Blocks in Load Bearing Masonry
1427
STRUCTURALL Y EffICIENT HOLLOW CONCRETE BLOCKS IN
LOAD BEARING MASONRY
Dr.T.P.GANESAN Professor Dr.P.KAL YANASUNDARAM Professor
R.AMBALA VANAN Sr.Scientific Officer K.RAMAMURTHY Research Scholar
Building Technology Division Department of Civil Engineering
Indian Institute of Technology Madras-600 036, INDIA
ABSTRACT
Concrete hollow blocks for use in masonry have been produced in a large variety of shapes and sizes. Within the limits of codal requirements, it is possible to vary the overall size of blocks, face and web shell thickness. However, when these blocks are used in load bearing masonry, all the shapes with a certain void/solid ratio cannot be said to be equally efficient. In this study, it is demonstrated that the wall to block strength ratio considerably vary with different shapes of blocks and types of mortar bedding. Theoretical analysis also indicate the presence of stress concentrations in certain shapes and arrangement of blocks leading to reduction in the capacity of masonry. An efficient shape is suggested to eliminate these shortcomings when used in common running bond.
INTRODUCTION
Precast concrete hollow and cellular blocks have been widely used for masonry
work in developed countries but their use in India has been limited. Bricks
continue to be extensively used. However, it is observed that clay suitable
for making high strength bricks are not available in southE:rn parts of India
(1). Consequently, clay bricks produced and presently used in construction
are non- uniform in quality. Of late, there has been a steep rise in the cost
of bricks and clay products. These factors have encouraged the investiga
tions in the use of hollow concrete blocks, especially for load bearing walls
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of low-rise constructions like residential houses. It is also realised that in
a developing country like India, scarce resources should be put to optimum
utilisation and hence it is imperative that maximum possible capacity of
masonry is achieved with a given block unit.
SHAPES AND SIZES Of HOLLOW BLOCKS
A hollow block is defined as one having one or more large holes or cavities
which either pass through the block (open cavity) or do not effectively pass
through the block (closed cavity) and having the solid material between 50
and 75 percent of the total volume of the block caJculated from the overall
dimensions. Codes specify the block properties such as block density and
compressive strength (2). Within the limits of these requirements, it is possible
to vary the overall size of blocks and thickness of face and web shells.
A large variety of shapes and sizes have been developed in various countries.
Some examples (3 to 7) are shown in fig.!. Ali these blocks are shaped
to conforrn to the codal requirements of the respective countries.
STRENGTH Of MASCNRY UNIT/WALL PANEL
The compressive strength of the individual hollow block depends on the cross
sectional area, height, concrete mix and moulding methods used. The strength
of the wall panel depends upon severa I factors such as strength of the unit,
strength of jointing moct ar, deformation characteristics of the unit and mortar
and type of bonding of units.
In most conventional constructions with concrete bIocks running
(stretcher) bond is adopted. Mortar can be spread out on both the web and
face shells of the hollow block (full bedding) or on the face shells only (face
shell bedding). full bedding is applicable when the webs of the blocks laid
in a course bear fully or partially on the webs of blocks in the lower course.
for the three-cored blocks and many other types of blocks only face
shell bedding of mortar is used in running bond. In such arrangement, the
web shells of these blocks virtually do not carry any load except wherever
they get supported partially or fully. In the case of a typical two-cored
unit having three webs of equal thickness when used in running bond as shown
in fig.2, its mid-web rests over the end webs joined by mortar. Even in
this arrangement, however, ali the webs of ali blocks do not get uniformly
good bearing support.
Studies were undertaken at the Building Technology Laboratory, I.I.T.,
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1.1. 3 CORED WITHOUT ENC PROJECTlONS.
1.3 . 3 CORED WITH PROJECTIONS ATBOTH ENDS .
[DvO[}" ~ 'Si-m71 l'
1.5 . PEAR SHAPED CORES.
1
1.2 . 3CORED WITH PROJECTIONS AT ONE END.
~"""y .. " 1.4 . 3UNEQUAL CORES.
)OlJ'} +-- "0 t
1.6 .TAPERED CORES.
FIG .1. DIFFERENT TYPES OF BLOCKS .
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I I I 11 I I 11 I I 1 I1 I I I1 I I I
I ,
I I I I I I I I
" I I I I I
! I I . ~ I1 I I I I I I I I I
I I I I I I I I , I
I I I I ' I 1' I , I I
I 1 ' II I I I1
EL EVAT IOM
I , , ; I
, -1 I I
I I I I I II I I I I I ,I I I II I I I 1 I I II
: I I I I 1I II
[Oolo[j[ooloolô PLAN
Fig . 2. MASONRY ARRANGEMENT WITH TYPE-C BLOCKS
Madras on the performance of three different types of blocks and walls made
with them (8,9). The details of blocks are shown in Fig.3. The ratios of
wall to block strength observed for these blocks with full mortar bedding
and with face shell bedding only are given in Table-l, As seen from the
table, this ratio is about 0.44 for the case of face shell bedding and goes
upto about 0.60 to 0.65 for full bedding. Similar results have also been
observed in studies reported by others. In tests reported by Read and
Clements (lO) this ratio varies from 0.38 to 0.45 average (0.415). The unit
used is similar to Type A block (Fig.3) and possibly used in face shell bedding.
Tests by Richart, Woodworth and Moorman (l1) on walls built with 8" (200
mm) thick units with three oval cores indicated an average ratio of 0.53
and the ratio ofaxial compressive strength with face shell bedding to that with full bedding was 0.8.
PROPOSED DESIGN OF UNIT TO IMPROVE STRUCTURAL EFFICIENCY
As seen from the above, it is apparent that the use of blocks of different
shapes and core sizes but keeping alI the web thicknesses uniform, results
in obtaining face shell bedding only in t he walls, and partial bedding of the
webs also in a few cases. These shapes, therefore, are inefficient structurally.
This shortcoming can be avoided if stack bond is used instead of running
bond. On the other hand, stack bond in masonry walls is undesirable owing
to the presence of continuous vertical joints. Thus there is a real need for
development of a suitable shape for the block to obtain full bedding in running
tr ~ .. , -~7 i*'2 o DJ
~L L L. I. ~ 2 \\0' 115 '''°1
115 \0 ° TVPE .A. (Height 190 mm)
~ no
[00] l L L L L Ir \0' 115 ' ao 'I 115 14ô'
TYPE.B.( Height 190mm)
l 's,!) ~ , . , ..
[Q-g] L L L L l Ir 140f 135 140f 135 f 40'
TYPE .C. (Height 190mm)
Fig.3 . PLA N OF BLOCKS USED IN EXPERIMENTAL INVESTIGATIONS. (Vide Table.1 )
+ 390 -+
f[o L~ 40
120
40
~f 130 f 70 .r-Ili;-t,#- Fig . 4. REDESIGNED BLOCKr TYPE . O
t [, r-1'0
I I
I I
I I I ~ ~
.j::. w ......
1432
bond. Wíth thís objectíve, the two-cored hollow unít was redesígned to have
a ratíonalised shape wíth a míd-web thíckness equal to twíce the end web
thíckness plus one joínt mortar thíckness (10 mm), the dímensíons of whích
are shown ín Fíg.4. Thís block when used ín runníng bond wíth a 200 mm
stagger, results ín the míd- web restíng on the two end webs joíned by mortar
and více versa ín consecutíve layers of masonry. By thís arrangement (Fíg.5)
all the webs get full bearíng support throughout the length / height of the
wall. The total core area of the redesigned hollow unít ís retained to be
almost equal to that of Type C block (Fíg.3).
ELEVAT ION
[E5Io]D!oIQ;olo!ol f PLAN
Fig . 5. MASONRY ARRANGEMENT WITH TYPE -O BLOCKS
COMPARISON OF THE PERFORMANCE OF HOLLOW BLOCKS
To compare the performance of the redesigned unit and the unit with equal
web thickness, which has been adopted ín most earlier studies (and whích
ís normally adopted ín practíce, a theoretical analysis of walls under axíal
compressíon using finite element method has been developed (12). The three
dimensional analysis of walls makes use of the two dimensional membrane
elements, ídealizíng the face and web shells of the block as membrane
elements (as plane stress elements in different planes).
Wall panels of size 1200 x 1200 mm (Fig.6), using the redesigned block,
(Type O )and Type C block were analysed. Type C blocks were chosen for
comparison since the proposed redesigned block has almost the same core
area and the same volume of material. The fíníte element analysís has been
carríed out for different loadíng conditions. The detaíls of the wall panel
analysed is shown ín Fíg.6.
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La yer.1
2
J Fig.S. WAll PANEl USED
r 4 FOR ANALYSIS.
5
o 6 o N .-
P = 1000kg.
Typical graphs showing the comparison of vertical stresses in face and
web panels at two different layers of the wal!s using both the blocks are
given in Fig.7 and 8 respectively for a concentrated vertical load. Tables
2 and 3 show the percentage deviations of vertical stresses in face and web
panels respectively for a uniformly distributed load on the wal! using the
two types of blocks.
CONCLUSIONS
1. The available experimental results of wal! to block strength indicate
the advantage of fuI! mortar bedding over face shel! bedding for obtaining
increased strength of wal!s in axial compression. The increase varies from
25 to 50%.
2. The comparative study of wal!s by theoretical analysis using two types
of blocks one with uniform web thickness and the other with the rationalised
dimenslons indicate the additional advantage of having 100% bedding of the
blocks which help to realise the fuI! potential capaclty of the blocks in wal!s.
3. The results of the analysis (Fig.7 and 8) indicate stress concentration
and sharp variation in stresses in masonry uslng conventional blocks, whereas
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oem 20
I , 40 60 , 80 100 , , '1' rm
_ " '.o "
-O. s f-
LAYER.2
80 100 , , 120
.,., ~ lAYE;S #
;(
-1.01-
-20
, lU'
, / ,
\ /
_1.5 1- ,
-3.0
\ I
,
' I
'"
"-0 ..... __ .#.1:/
-4.0
BLOCK-C BLOCK-O
, , , 0 _
___ 0 """
I - 2D
Fig . 7. COMPARISON OF VERTICAL STRESS IN FACE PANELS
o cm 20 40 60 &0 100
12001~ 40 60 &O 100 120
I i
LAyER .2 ~
LAYER.5 - 1.0
"1 -2 .0
-1 .0 ____ -<>.. ",0---
\ I , - 3.0
\ /
-1.51- \ -J \\ ) BLOCK-C , /0.. BLOCK-D , ,
\ I \ \ I
I
- 50~ \ I \ \ /
- 2.0f- \ / \ \ ." .... 'o ~ \d
'l: ~ a> ..
Fig .8 . COMPARISON OF VERTICAL STRESS IN WEB PANELS
1435
TABLE 1 Ratios of wall to block strength
:fype of block Block strength, Q,kg/sq.cm.
Wall strength on gross are a P,kg/sq.cm.
Ratio P/Q
Remarks
Type A+ 33.0 14.6 0.442 Wall size 1200x800 mm
Type B++ 51.0 33.3 0.653 -do-
Type C++ 49.4 29.1 0.590 Average values for different wall sizes
+ face shell bedding ++ full bedding vide fig.3 for details of ':Jlocks
TABLE 2
Compar'ison of vertical stresses in face panel (percentage deviation)
Layer No. fI f2 f3 f4 f5 f6
1 -11.38 -4.20 -4.33 -4.33 -4.20 -11. 36 2 4.73 -6.57 -5.96 -5.96 -6.57 4.73 3 -13.23 -4.33 -6.00 -6.00 -4.33 -13.23 4 5.19 -6.61 -6.10 -6.10 -6.61 5.19 5 -12.67 -4.81 -6.59 -6.59 -4.81 -12.67 6 6.07 -7.46 -6.59 -6.59 -7.46 6.07
fl-f6 are elements in each layer.
TABLE 3 Comparison of vertical stresses in web panel
Layer No. Wl W2 W3 W4 W5 W6 W7
1 -4.36 -19.14 10.49 -19.57 10.49 -19.14 -4.36 2 -8.57 17.64 -31.94 19.17 -31.94 -17.64 -8.57 3 0.09 -26.48 16.86 -29.80 -16.86 -26.48 0.09 4 -6.88 16.44 -30.30 17.10 -30.30 16.44 -6.88 5 2.36 -27.52 16.88 -30. 74 16.88 -27.52 2.36 6 -3.49 15.27 -29.76 16.99 -29.76 15.27 -3.49
WI-W7 are elements in each layer. Note: Negative sign indicates that the stresses are more in the case of
wall panel with conventionally adopted blocks (with equal web thickness) than the wall using the redesigned blocks.
1436
the stress variation is smooth with the rationalised block.
It could therefore be seen that for almost the same amount of material
used in a block, a small change in the configuration of the blocks improves
its efficiency and results in increased capacity of the masonry.
REFERENCES
1. Dayaratnam, P., Brick and Reinforced Brick Structures. Oxford & IBH
Publishing Company Private Ltd., India 1987.
2. .. .. , I.S. 2185 (Part 1) - 1979. Specifications for concrete masonry units,
Part-I, Hollow and solid concrete blocks (Second Revision), Indian Standards
Institution, New Delhi, India.
3. Ehle, 1.C., Developments in manufacturing technology of concrete masonry
units. Title 45-36, lournal of American Concrete Institute, Vol.45, pp.
613-620, April 1949.
4. .. .. , Recommended Practice for laying concrete block. Indian Concrete
lournal, Vol.30, pp.18-26, 1 anuary 1956.
5. Malik, c.P., Manufacture of hollow concrete blocks in lapan. Indian
Concrete lournal, Vol.34, pp.85-86, March 1960.
6. Sawtell David, Concrete block masonry in architecture. Cement and
Concrete Association, London, 1971, p.139.
7. Uzomaka, 0.1., An lnvestigation into the production of sandcrete blocks
in the East Central State of Nigeria. Proc. Symposium on sandcrete blocks
and the construction industry. Dept. of Civil Engg. University of Nigeria,
Nsukka, pp.6-20, August 1975.
8. Kalyanasundaram, P., and Rao P.L., Concrete hollow blocks from low cost
materiaIs for economy in construction. Proc. of the lnternational Conference
on Low lncome Housing - Technology and Policy, Bangkok, Thailand.
pp.999-1013, lune 1977.
9. Rao, M.V., Development of a system of Hollow Blocks for Low-Cost
Housing. Ph.D. Thesis, Civil Engg.Department, I.!. T.,Madras,December 84.
10.Read, 1.B., and Clements, S. W., The strength of concrete block walls,
Phase 1, Technical Report, Cement & Concrete Association, London 1972.
11 . Richart, F.E., Woodworth, P.M., and Moorman, R.B.B. Tests of the stability
of concrete masonry walls, Proc. of ASTM, 1931,No.31, pp.661-680.
12 . Research work on ana lysis and test ing of hollow block masonry walls,
Building Technology Laboratory, 1.1. T., Madras - To be published.