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

-

1428

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 ......

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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

1434

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.