STRENGTH V ARIATIONS IN SOLID CONCRETE MASONRY BLOCKS MANUFACTURED USING MULTI-BLOCK MAKlNG MACIDNES

David M F Orr1

1. ABSTRACT

Solid concrete masonry blocks manufactured using multi-block making machines are used in masonry laid either on edge or on face. The muIti-block making machine produces a bale of blocks at each laying operation. Tests on blocks of complete bales indicate a significant systematic strength variation between the blocks in a bale. The measured strengths of the blocks is considerably higher for the on face direction than for the on edge direction. The difference in measured strengths is generally attributed to platen restraint or to aspect ratio effect, however experiments indicate that the material in the blocks has significant directional strength variations.

2. INTRODUCTION

Concrete masonry blocks are widely used in building construction in Ireland. The estimated annual output is 175 million units. A wide variety of blocks are produced but the predominantIy used unit is a nonnal density solid block measuring 440mm by 215mm by IOOmm. Some 85% of total output is manufactured using mobile multi-block making machines. The standard nonnal density solid blocks are wideIy used in buildings and engineered structures, either laid on edge to give a wall thickness of lOOmm or fiat on face to give a much more robust waIl with a thickness of215mm. The surfaces ofthe blocks are defined as end, edge and face by lrish Standard, IS20(1); the edge surface measuring 440mm by lOOmm and the face 440mm by 215mm.

The strength of the blocks when measured in the laid fiat on face direction is considerably higher than the strength measured in the on edge direction. The difference in measured strength is generaIly attributed to platen restraint or to aspect ratio effect, however factorial experiments on prisms cut trom blocks in the three orthogonal

Keywords: Concrete Masonry Blocks; Multi-Iaid; Compressive Strength.

1 Associate Professor of Civil Engineering, Department of Civil and Environrnental Engineering, University CoIlege Cork, Cork, Ireland.

1239

directions indicate that the material itself in the blocks has significant directional strength variations.

The multi-block making macrune produces a drop or bale of blocks at each laying operation; 44 in the case of standard solid blocks. Tests on blocks of complete bales indicate a significant systematic strength variation between the blocks in a bale, and trus can contribute to the variation obtained when blocks are selected at random from a number ofbales for quality control purposes.

3. MATERIALS

The blocks used in trus investigation contained coarse aggregate, maxímum particle size 14mm, obtained from glacial till gravei pits. In general, trus gravei is derived from Old Red Sandstone rock. The gravei is screened and over sized particles are crushed. Thus, both round and angular particles are present. Trus combination has been found to lower water demand; the effect of the rounded particles, and to improve bond and interlock; the effect of the angular particles. The fine aggregate used is predominantIy a natural sand, but with a small proportion of crushed fines. The grading of the sand is witrun the medium category of BS 882: 1992(2) . The quantities of materiais for mixes for 5MPa and 10MPa blocks are given in Table I. These mixes produce a very coarse textured block with surfaces that are particularly suitable for plastering. The mechanical properties ofthe coarse aggregates are given in Table 2.

Table I. Quantities of materials for mixes for concrete blocks

Material 5MPa block kg!m3

14mm 625 10mm 425 5mm 425 Sand 645

Cement 135 Water 133

Table 2. Properties of aggregates

Property Aggregate crusrung value Aggregate impact value 10% fines Relative density Water absorption

10MPa block kg!m3

Value 17% 20%

280kN 2.58 1.60%

630 400 400 600 200 160

Normal Portland cement conforming to IS1:l991(3), equivalent to Ordinary Portland cement, BSI2(4), was used.

1240

4. BATCHING AND MIXING

The batching and mixing operations are controlled by a pre-set cascade system which allows for fine adjustment to the component weights whenever necessary. The operation commences with the introduction of the coarse and fine aggregates and the cement into the mixer. These are continuously mixed while the required amount of water is added by a spray bar. The cycle period for a 1.3 m3 batch is 3 minutes. The concrete is conveyed in a 5 tonne feed bucket mounted on a forklift truck for delivery to the block malàng machine as this machine traverses the block yard slab.

5. MOBll.,E BLOCK MAK.ING PROCESS

The mobile block malàng process was introduced to intensify the production of concrete blocks. The machine used to produce the blocks used in this investigation has the advantage of placing blocks with their long axis in the vertical direction, resuIting in the largest possible number of blocks being placed in a given area. Also, the vertical placement increases the accuracy of the more important dimensions of the blocks, their width and thickness, as ali four long sides of the blocks are formed against precise mould surfaces. Slight variations in the lengths of the blocks and unevenness of the end surfaces can be readily accommodated in the perpend joints in masonry.

A variety of moulds may be used for producing solid and hollow blocks. The standard solid block measuring 440mm by 215mm by 100mm is the most commonly used masonry unit in building construction in Ireland. The machine produces 44 blocks of this size at each cycle ofproduction, and more than two cycles per minute can be achieved.

The machine operates on a clean levei concrete slab and its movements over the surface of the slab are automatically controlled. The distance between drops is approximately 300mm and the distance between rows of drops is approximately 360mm. The slab area available for production is thus very efficiently used.

Top hopper

Tampers

--Shutter

'i'---rbl---F eed bc X

Fig. 1 Side e1evation of multi-block malàng machine

1241

The principal parts of the machine that are of interest are illustrated in the side elevation shown in Fig. 1. The cyc1e of operation is as follows : The mould box is lowered to the slab surface, the hopper shutter is opened to fill the feed box, the shutter is c10sed and the feed box is moved across the moulds, filling them with concrete. Pre-vibration is switched on and the feed box is moved back and forth across the moulds. The vibration is then switched off, the feed box is withdrawn, and the tampers are lowered. This automatically switches on the final vibration and tamping is continued by lever movement until vibration stops. With the tampers resting firmJy on the blocks within the moulds, the mould box is withdrawn upwards. The rising mould box lifts the tampers c1ear of the blocks and the machine moves forward a pre-set distance to commence the next production cyc1e.

51 Curing and Storing

The normal means of curing the blocks in an open block yard is to leave them as cast on the production slab to cure naturally for two to four days depending on weather conditions. The blocks are then strapped into bales using a mechanical banding machine, prior to transfer to a stockyard.

6. COMPRESSIVE STRENGTH TESTS

Compressive strength tests on concrete blocks were carried out by loading the blocks on edge and with 12mm soft fibreboard packing between the blocks and the platens of the testing machine, generally in accordance with the requirements of IS 20(1). As required by this Standard the blocks were conditioned by immersion in water at a temperature of 20 ± 2°C for at least 16 hours and then drained for 30 minutes before being tested. The dimensions of the blocks were also recorded.

7. V ARlATION OF COMPRESSlVE STRENGTH WITH1N BALES

Each bale of standard solid concrete blocks consists of 11 rows by 4 columns. The compressive strength of each of the 44 blocks in a bale of standard solid concrete blocks, with a nominal strength of 10MPa are given in Table 3. The mean block strength is 16.6MPa, the standard deviation is 1.95MPa and the coefficient ofvariation is 11.7%. The blocks at both ends of the bale have higher values of compressive strength than the blocks near the middle of the bale. The data were separated into three populations; I, rows 1-3; 2, rows 4-8; 3, rows 9-11 . Details ofthese populations are given in Table 4. The resuIts of a one-way analysis of variance carried out on the three populations is given in Table 5. The F test indicates that there is a highly significant difference between the strengths of concrete blocks in rows near to the ends of the bale, populations 1 and 3, and the strengths of blocks in rows near to the middle of the bale, population 2. Two­samplet t-tests indicate that populations 1 and 3 are similar and differ from population 2 at the 95% confidence leveI. A second bale of 10MPa blocks and a bale of 5MPa blocks gave similar results for within bale variation of compressive strengths.

1242

Table 3. Compressive strengths, MPa, ofblocks in a 4 column by 11 row bale

POEulation Row Column 1 Column 2 Column 3 Column4 1 19.7 17.5 20.7 17.9 2 17.7 16.9 14.7 18.8 3 18.0 16.5 16.1 15 .8 4 16.1 14.1 15 .9 14.8 5 15.3 14.2 15.3 13.9

2 6 16.1 13 .9 14.1 14.7 7 15.6 15.2 16.6 16.2 8 16.0 15.4 15.3 16.3 9 16.9 16.4 14.8 16.5

3 10 16.9 17.8 18.7 16.2 11 19.8 19.9 19.9 22.0

Table 4. Details ofbale populations

Population Number Mean strength Standard deviation

2 3

MPa MPa 12 17.5 1.69 20 15.3 0.87 12 18.0 2.08

Table 5. Analysis ofvariance ofbale populations

Source Degrees Sums of Mean Variance offreedom sguares sguare ratio

Factor 2 69.67 34.83 15.3 Error 41 93 .27 2.27 Total 43 162.94

Note: Fo.01 ,2,41 = 5.18

8. V ARIATION OF COMPRESSIVE STRENGTH WITHIN BLOCKS

Prisms measuring 50mm by 50mm by 100mm were cut from concrete blocks and tested in the air-dry condition for compressive strength, using 12mm soft fibreboard packing. Three prisms were cut for each of the three principais directions from each of three blocks, with the positions ofthe prisms changed between blocks as shown in Fig. 2. This provided test results for a factorial experiment with two variables, orientation and position, each at 3 leveis and replicated 3 times. The notation for the leveis of orientation and position is ilIustrated in Fig. 3.

Test results for 5MPa blocks have previously been reported(5) . In this experiment, the orientation main effect was found to be significant at the 1 % probability levei and the position main effect at the 5% leveI.

1243

000

CJ DD

DDD Block 1

LJLJ CJ

OCO 000

Block 2

DDD 000

CJCJ Block 3

Fig. 2 Positions of prisms cut from masonry blocks

t 01

P 1

P2

P3 --.. 02

/' 03

Fig. 3 Notation for leveis of orientation and position

A similar factorial experiment has now been carried out on lOMPa blocks. The results of a two-way analysis of variance of normalised residuais with respect to block means are shown in Table 6. The F test for variance ratio shows that the interaction between the two factors is not significant. Both main effects, position and orientation, are significant at the 1 % probability leveI. This compares very c10sely with the results of the factorial experiment on 5MPa blocks(5), the only difference being that in the latter experiment the position main effect was significant at the 5% leveI. The orientation effects show that the block material is weakest in the edge direction, orientation 2, to the extent of some

1244

13% below the mean and that the material is strongest, some 8.5% above the mean, in the on face direction, orientation 3.

Table 6. Analysis ofvariance for within block compressive strength.

Source Sumof Degrees of Variance Variance squares freedom estimate ratio

Position 0.4075 2 0.2037 19.22 Orientation 0.1154 2 0.1154 10.89 Interaction 0.0478 4 0.0119 1.12 Residual 0.1905 18 0.0106 Total 0.8766 26

Note: FO.01 ,2,18 = 6.01 ; FO.01 ,4,18 = 4.58

The position effect for 10MPa blocks shows that the block material is weakest to the extent of some 15% below the mean, at the bottom of the block, as it is laid, while the material is strongest, some 15% above the mean at the top of the block. In the case of 5MPa blocks (5) this variation was very much less, 7% above and below the mean. Thus, in general, similar trends in the variations of compressive strengths within blocks occur in both 5MPa and 10MPa blocks.

9. CONCLUSIONS

Mobile multi-block making machines are capable of economically producing a variety of concrete masonry units, the most commonly used ofwhich is the standard normal density solid block, measuring 440mm by 215mm by 100mm. The results of compressive strength tests carried out on complete bales of standard solid blocks manufactured by a multi-block laying machine and on prisms cut from blocks of this type, lead to the following concIusions:

9.1 There is a significant systematic variation in compressive strength between blocks in a bale. The rows of blocks near to the ends of the bale are stronger than the rows near to the middle of the bale. Thus, for quality control purposes, extreme care is necessary when interpreting data trom compressive strength tests on blocks produced using multi­block making machines.

9.2 The compressive strengths ofblock material depends on direction; the edge direction is the weakest and the face direction is the strongest.

9.3 The compressive strength varies along the length of the block, being weakest at the bottom and strongest at the top, as laid.

10. ACKNOWLEDGEMENTS

The experimental work was carried out in the Structural Engineering Laboratory at University College Cor!e, with the assistance of technicians and final year civil

1245

engineering students. Infonnation on the use of multi-block making machines and materiais were provided by John A Wood Limited, Cork.

11. REFERENCES

1. National Standards Association of Ireland, IS20, " Concrete Building Blocks. Part 1:1987, Nonnal Density Blocks", Dublin, 1987.

2. British Standards Institution, BS882: 1992, "Specification for Aggregates iTom Natural Rock Sources for Concrete", London, UK, 1992.

3. National Standards Association ofIreland, ISl:1991 , "Portland Cement", Dublin, 1991.

4. British Standards Institution, BS 12: 1991, "Specification for Portland C ement " , London, UK, 1991

5. Orr, D. M. F., "Influence ofBlock Orientation on the Compressive Strength of Concrete Masonry Prisms", Proceedings ofthe 8th Intemational BrickIBlock Masonry Conference, Dublin, Ireland, 1988, pp. 253-260.

1246