- 229 -

SHELL BEDDED SOLID CONCRETE BLOCK MASONRY

D. M. F. Orr, Assoe. Professor, Department of Civil Engineering, University ColJege Cork, Ireland.

by

ABSTRACT

M. P. Mannion, Lecturer, Regional Technical ColJege, Cork, Ireland.

The use of masomy constructed from solid bloeks measuring 440mm by 215mm by 100mm laid on face has both structural and architectural advantages. However the wide bed joint is a disadvantage for bloek laying as it makes levelJing the bloeks difficult and the area of contact between block and mortar in the bed joint is uncertain. These disadvantages can be overcome by using shelJ bedded construction, and by using a template to control the dimensions of the edge strips of mortar in the shelJ bed.

Tests on stack bonded masomy prisms indicate that the compressive strength of shelJ bedded masonry is proportional to the area of mortar in the b~d joint, provided that failure is caused by splitting of the blocks rather than by crushing of the mortar. The width of edge strip which site trials have shown to be convenient for block laying would, in general, meet this requirement.

INTRODUCTION

Solid concrete blocks, that is, blocks containing no formed voids, are used extensively in masomy construction in Ireland. Standard solid bloeks which measure 440mm by 215mm by 100mm, and with strengths of 5MPa or 10MPa are most commonly used. In small buildings, particularly dwelJing houses, the bloeks are normally laid on edge to give a wall thickness of 100mm and a coordinating size with lOmm joints of 450mm by 225mm. In large buildings and in freestanding walls, the bloeks are frequently laid on face to give a wall thickness of 215mm. When bonding blocks on face with bloeks on edge, it is necessary to increase the thickness of the bed joint to 12mm, so that the height of two bloeks laid on face is equal to the height of one bloek laid on edge. The details of an externai cavity wall with an inner leaf 215mm wide, an ou ter leaf 100mm wide and a cavity 100mm wide, inc1uding 50mm of insulation, are shown in Figure 1. In addition to being structurally robust, walls constructed with solid bloeks laid on face have architectural advantages, such as being able to accommodate deep chases to conceal services.

When laying bloeks on face on a fulJ width bed of mortar, masons tend to remove the centre portion of the bed in order to facilitate the levelJing of the bloeks. Thus contact between bloek and mortar bed may not be achieved over the fulJ width of the joint. Indeed it has been found [1] that when a fulJ width bed joint has been intended, as much as 20% of the mortar bed may be void, and the contact area may be very irregular, Figure 2. A simple tool has been developed which enables masons easily to form bed joints of two edge strips of mortar of regular width, referred to as shelJ bedding, and tests on masomy prisms have been carried out to investigate the influence of the width of the mortar strips on the compressive strength of the shell bedded masomy.

- 230 -

Figure 1 Details of cavity walI. Figure 2 Voids in full width mortar bed. [From ref. 1]

SHELL BEDDED MASONRY CONSTRUCTION

The term 'shell-bedded' was originally used in connection with hollow block construction to describe joints in which only the face shells of the blocks are bedded in mortar, Figure 3a. In solid block masonry, the term refers to bed joints consisting of two cdge strips of mortar, Figure 3b. In the case of hollow blocks, the width of the mortar strips in shell bedded joints is determined by the thickness of the face shell of the block, whereas with solid blocks on face. the width of the edge strips of mortar can be varied.

Figure 3 Shell bedding: (a) hollow block. (b) solid block.

Laboratory and site tests have shown that in solid block masonry, the dimensions of the edge strips of mortar in a shell bed can be controlled using a template. Site trials indicate that the use of a template actually increases the rate at which blocks are laid. A mortar bed prepared in the usual way for a full width joint is shown in Figure 4. Rough grooving of the mortar to facilitate levelling the blocks is evident. This grooving results in the voids shown in Figure 2.

Regular edge strips formed with the aid of a template are shown in Figure 5. and the template used is shown in Figure 6. Suitable dimensions for the template were found from site trials to be 1200mm long by 70mm wide and a thickness of 8mm for a 10mm thick joint. The mortar is placed on top of the template and spread to the edges as shown in Figure 7. In this way a mortar bed of the required thickness can be laid rapidly and also the blocks are sufficiently mobile on the shell bed to be easily positioned to line and leveI.

- 231 -

COMPRESSIVE STRENGTH OF SHELL BEDDED MASONRY

Compressive strength tests on masonry prisms to investigate the influence of the width of mortar strips in bed joints on the compressive strength of 5MPa block masonry have been reported previously [2]. Additional tests on prisms constructed witb lOMPa blocks bave now been carried out.

Figure 4 Full width mortar bed. Figure 5 Edge strips formed by template.

Figure 6 Template. Figure 7 Use of template.

Tbe strengths of tbe materials used are summarised in Tablc 1. Tbc concrete blocks were manufactured from a mix containing Portland cement and crusbed limestone aggregate. Tbe density of the blocks was approximately 2000 kglm3. Compressive strength tests on the concrete. blocks were carried out by loading the blocks on edge and witb 12mm soft board packing between the blocks and the plattens of the testing machine. generally in accordance

- 232-

with the requirements of IS 20 [3], but with the exception that the blocks were tested in an air dry condition rather than saturated and surface dry. Three mortar mixes were used with the following proportions; (1) 1:7::cement:sand with plasticizer, (2) 1:2:9::cement:bydrated lime:sand and (3) 1:1:6::cement:bydrated lime:sand. The strengtbs of tbe mortars were further adjusted by varying the water contento Tbe compressive strengtbs of the mortars were determined in accordance with the requirements of BS 4551 [4].

Masonry prisms consisting of six blocks, stack bonded on face, were constructed witb a range of widths of mortar strips in sbell beds. The tbickness of tbe joints and tbe widtbs of tbe mortar strips were controlled by placing lOmm thick expanded polystyrene board between the blocks. Tbe board was raked out of tbe joints prior to testing tbe prisms at an age of 28 days.

Table 1 Strengtbs of materials.

Material No.oftests Mean strength (MPa) Coeff. of variation

Concrete blocks, SMPa 10 8.70 9.8 Concrete blocks, lOMPa 10 13.10 6.6 Mortarmix 1 6 1.34 3.0 Mortarmix 2 12 3.17 9.4 Mortar mix 3 12 6.38 8.5

Tbe masonry prisms were loaded in compression in a 300 tonne capacity Amsler column testing macbine and 12mm tbick softboard packing was used. Tbe combinations of block strengths and mortar strengtbs used in three series of tests are given in Table 2, and tbe results of the compressive strengtb tests are given in Table 3. For tbe larger widtbs of mortar strips, tbe mode of failure in tbe masonry prisms was splitting througb tbe blocks parallel to the direction of load and aligned with tbe inside edge of tbe mortar strips, Figure 8, wbereas for tbe narrower strips tbe mode of failure was crusbing of tbe mortar in tbe bed joints.

Table 2 Materials used in masonry prisms.

Test series

Meanblock strength (MPa)

Meanmortar strength (MPa)

Block strength/ Mortar strength

A

8.7

1.34

6.49

B c

8.7 13.1

3.17 6.38

2.74 2.05

Figure 8 Masonry prism sbowing failure by splitting of blocks.

- 233 -

INFLUENCE OF THE WIDTH OF MORT AR STRlP ON COMPRESSIVE STRENGTH OF SHELL BEDDED MASONRY

The failure stress for the masonry prisms is calculated as load divided by area of mortar in the bed joint. When this stress is plotted against width of strip the data indicate a bi-linear relationship for each series of tests, with the failure stress initially increasing with strip width before reaching a constant value. In order to combine the results for the three series of tests, the failure stresses for each series are divided by the mean values of the tests in the series for which failure occurred by splitting of blocks. The resulting values of compressive strength ratio, plotted against width of strip, are shown in Figure 9.

Prism

A5 A6 A7 A8 A9 AlO A11 A13 A14 A15 A16 B1 B2 B3 B4 B5 B6 B7 B8 B9

Table 3 Compressive strength results for masonry prisms.

Strip Failure Failure width stress mode (mm) (MPa)

15 2.27 mortar 15 1.14 mortar 20 2.73 mortar 20 2.56 mortar 30 3.60 block 30 3.22 block 50 5.11 block 70 5.11 block 70 5.03 block full bed 4.81 block full bed 4.82 block 10 3.51 mortar 10 3.40 mortar 12 3.02 mortar 12 3.11 mortar 15 3.93 mortar 15 3.55 mortar 20 3.68 mortar 20 4.25 mortar 30 4.79 block

Prism

B10 B11 B12 B13 B14 B15 B16 C5 C6 C7 C8 C9 ClO CU C12 C13 C14 C15 C16

Strip width (mm)

30 50 50 70 70 full bed full bed 15 15 20 20 30 30 50 50 70 70 full bed full bed

Failure stress (MPa)

5.13 5.71 5.01 5.73 4.89 5.25 5.39

10.95 9.06

10.20 9.91

12.08 11.89 10.87 10.65 12.22 9.72 8.22

11.59

Failure mode

block block block block block block block mortar mortar mortar mortar block block block block block block block block

The discontinuity point between the two straight lines corresponds with an observed change in mode of failure from crushing of the mortar to splitting of the blocks. This indicates that the compressive strength of shell bedded masonry is proportional to the area of mortar in the bed joints when failure occurs by splitting of the blocks. Also, the minimum width of strip required to avoid crushing of the mortar increases as the ratio of block strength to mortar strength increases.

The lower compressive strength for narrower strip widths can be attributed to the influence of aspect ratio, height of specimen to its least lateral dimension, on the strength of brittle materiais. The indicated compressive strength for a specimen with an aspect ratio of 1:1 is about half that for a specimen with an aspect ratio of greater than 1:3 [5].

To allow for the reduction in the compressive strength for shell bedded solid block

- 234-

masonry Irish Standard, IS 325 [6] requires the characteristic compressive strength for masonry with a full width bed joint to be adjusted by the ratio of the area of mortar in the bed joint to the total area of the block. While the data obtained from the masonry prism tests indicate that there is a minimum width of strip necessary for this adjustment to be valid, this minimum width is likely to be considerably less than that used for practical reasons. However, designers should be aware of this limitation, particularly when the mortar is much weaker than the blocks used in shell bedded solid block masonry.

1.2

êJ 8 ~

8 o 0.8 o ~ .t= Õ 0.6 B c ~ (j)

0.4

o 0.2

o Serias A O Sarlas B O Serias C

O O 20 40 60 80 100 120

Strip width (mm)

Figure 9 Strength ratio versus width of mortar strip for masonry prisms.

CONCLUSIONS

Shell bedding overcomes the practical difficulties which arise because of the wide bed joint when standard 440mm by 215mm by 100mm solid concrete blocks are laid on face.

The dimensions of the edge strips of mortar in a shell bedded joint are readily controlled and the levelling of blocks is facilitated, by the use of a template. Site trials have established that a 70mm wide template is suitable for a 215mm wide joint, giving edge strips approximately 70mm wide.

Tests on masonry prisms indicate that the compressive strength of shell bedded solid block masonry is proportional to the area of mortar in the bed joint, provided failure occurs by splitting of the blocks, rather than by crushing of the mortar in the joints. The width of mortar strip required to induce splitting depends on the strengths of block and mortar, and is likely to be less than the 70mm found to be suitable for block laying. Nevertheless, design standards should give guidance as to the minimum width of mortar strip permitted in shell bedded masonry, particularly when the mortar is substantially weaker than the blocks.

ACKNOWLEDGEMENTS

The experimental work was carried out in the Structural Engineering Laboratory at University College Cork with the assistance of technicians and final year civil engineering students. Materials were donated by John A. Wood Limited, and site trials were facilitated

- 235 -

by John O'Donovan and Associates, ConsuIting Engineers and John Sisk Limited. This assistance is gratefully acknowledged.

REFERENCES

1. Masonry Panel, Compressive strength of masonry eonstrueted from solid bloeles laidflat, National Standards Association oflreland, August 1988, pp 12.

2. OIT, D. M. F., Mannion M. P., Murphy J. l, Compressive strength of shell bedded eoncrete block masonry, Third International Seminar on Structural Masonry for Developing Countries, Mauritius, July 1990, pp 39-43.

3. National Standards Association of Ireland, IS 20, Concrete building blocles part 1, Normal density blocles, Dublin, 1987.

4. British Standards Institution, BS 4551, Methods for testing mortars, sereeds and plasters, London, 1980.

5. Render, S. and Phipps, M. E., The eJleet of unit aspeet ratio on the axial eompressive strength ofmasonry, Masonry International, No. 8, July 1986, pp 28-38.

6. National Standards Association of Ireland, IS 325, Code of praetice for use of masonry part 1, Structural use ofunreinforced masonry, Dublin, 1986.