Masonry has a very long tradition of building by craftsmen, without engineering supervision of the kind applied to reinforced concrete construction. Consequently, it is frequently regarded with some suspicion as a structural material and carries very much higher safety factors than concrete. There is, of course, some justification for this, in that, if supervision is non-existent, any structural element, whether of masonry or concrete, will be of uncertain strength. If, on the other hand, the same level of supervision is applied to masonry as is customarily required for concrete, masonry will be quite as reliable as concrete. It is therefore important for engineers designing and constructing in masonry to have an appreciation of the workmanship factors which are significant in developing a specified strength. This information has been obtained by carrying out tests on walls which have had known defects built into them and comparing the results with corresponding tests on walls without defects. In practice, these defects will be present to some extent and, in unsatisfactory work, a combination of them could result in a wall being only half as strong in compression as it should be. Such a wall, however, would be obviously badly built and would be so far outside any reasonable specification as to be quite unacceptable.
It is, of course, very much better for masonry to be properly built in the first instance, and time spent by the engineer explaining the importance of the points outlined below to the brick- or block layer and his immediate supervisor will be time well spent.
DEFECTS IN BRICKWORK DUE TO WORKMANSHIP
1. Failure to fill bed joints
It is essential that the bed joints in brickwork should be completely filled. Gaps in the mortar bed can result simply from carelessness or haste or from a practice known as ‘furrowing’, which means that the bricklayer makes a gap with his trowel in the middle of the mortar bed parallel to the face of the wall. Tests show that incompletely filled bed joints can reduce the strength of brickwork by as much as 33%.
Failure to fill the vertical joints has been found to have very little effect on the compressive strength of brickwork but does reduce the flexural resistance. Also, unfilled perpendicular joints are undesirable from the point of view of weather exclusion and sound insulation as well as being indicative of careless workmanship generally.
2. Bed joints of excessive thickness
Increase in joint thickness has the effect of reducing masonry strength because it generates higher lateral tensile stresses in the bricks than would be the case with thin joints. Thus, bed joints of 16–19 mm thickness will result in a reduction of compressive strength of up to 30% as compared with 10mm thick joints.
3. Deviation from verticality or alignment
A wall which is built out of plumb, which is bowed or which is out of alignment with the wall in the storey above or below will give rise to eccentric loading and consequent reduction in strength. Thus a wall containing a defect of this type of 12–20 mm will be some 13–15% weaker than one which does not.
4. Exposure to adverse weather after laying
Newly laid brickwork should be protected from excessive heat or freezing conditions until the mortar has been cured. Excessive loss of moisture by evaporation or exposure to hot weather may prevent complete hydration of the cement and consequent failure to develop the normal strength of the mortar. The strength of a wall may be reduced by 10% as a result. Freezing can cause displacement of a wall from the vertical with corresponding reduction in strength. Proper curing can be achieved by covering the work with polythene sheets, and in cold weather it may also be necessary to heat the materials if bricklaying has to be carried out in freezing conditions.
5. Failure to adjust suction of bricks
A rather more subtle defect can arise if slender walls have to be built using highly absorptive bricks. The reason for this is illustrated in Fig-1, which suggests how a bed joint may become ‘pillow’ shaped if the bricks above it are slightly socked as they are laid. If water has been removed from the mortar by the suction of the bricks, it may have become too dry for it to recover its originally flat shape. The resulting wall will obviously lack stability as a result of the convex shape of the mortar bed and may be as much as 50% weaker than should be expected from consideration of the brick strength and mortar mix. The remedy is to wet the bricks before laying so as to reduce their suction rate below 2kg/m2/min, and a proportion of lime in the mortar mix will help to retain water in it against the suction of the bricks.
6. Incorrect proportioning and mixing of mortar
The effect of mortar strength on the strength of masonry may be judged from Fig-2 from which it may be seen with bricks having a crushing strength of 30 N/mm2 that reducing the mortar strength from 11 N/mm2to 4.5 N/mm2 may be expected to reduce the brickwork strength from 14 N/mm2 to 11N/mm2. This corresponds to a change in mortar mix from 1 : 3 cement : sand to 1 : 4.5 or about 30% too little cement in the mix. A reduction in mortar strength could also result from a relatively high water/cement ratio whilst still producing a workable mix. It is therefore important to see that the specification for mortar strength is adhered to although there is an inherent degree of tolerance sufficient to accommodate small errors in proportioning and mixing the mortar. The use of unsuitable or an excessive amount of plasticizer in place of lime will produce a porous and possibly weak mortar and has to be guarded against.