Effect of workmanship on strength of masonry

There is a very long tradition of masonry construction by craftsmen, without the engineering supervision applied to reinforced concrete construction. As a result, it is often regarded with some doubt as a structural material and has much higher safety factors than concrete. Of course, this has some justification, in that, if supervision is not present, any structural element, whether of masonry or concrete, will be of uncertain strength. On the other hand, if the same level of monitoring is applied to the masonry as is required for the concrete, the masonry will be as reliable as the concrete. It is therefore important for engineers who design and build masonry to appreciate the workmanship factors that are important in developing a specified strength. This information is obtained by conducting tests on walls that had known defects and comparing the results to tests performed on walls without any defects. In practice, these defects will be present to some extent and, in discontinuous work, their combination may result in the wall being only half as strong in compression as it should be. However, such a wall would clearly be badly built and so far from any reasonable specification as to be quite unacceptable.

Of course, it is much better for masonry to be built properly in the first place, and the time taken by the engineer to explain the importance of the points outlined below the brick- or block layer, and the time of his immediate supervisor, will be well spent.

1. failure to fill bed joints

It is necessary that the joints of the bed in the brickwork are completely filled. Gaps in mortar beds can be caused by carelessness or haste or by an exercise known as ‘furrowing‘, meaning 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 bedding joints can reduce the strength of brickwork by up to 33%.

Failure to fill vertical joints has been found to have little effect on the compressive strength of the brickwork, but reduces flexural resistance. In addition, incomplete vertical joints are undesirable, due to weather exclusion and sound insulation, as well as being a sign of careless workmanship.

2. excessive thickness bed joints

An increase in joint thickness has the effect of reducing the strength of the masonry as it produces higher lateral tensile stresses in the bricks, as is the case with thinner joints. Thus, Bed joints of 16-19 mm thickness will result in up to 30% reduction in compressive strength compared to 10 mm thick joints,

3. deviation from verticality or alignment

A wall that is made of plumb, which is inclined or that is out of alignment with the wall in the floor above or below, will lead to eccentric loading and result in loss of strength. Thus A wall with this type of 12-20 mm defect will be 13-15% weaker than one that does not,

4. Exposure to adverse weather after laying

Newly laid brickwork should be protected from extreme heat or cold conditions until the mortar cures. Excessive loss of moisture by evaporation or exposure to hot weather can prevent complete hydration of the cement and result in failure of the mortar to develop its normal strength. As a result the strength of a wall can be reduced by 10%. Freezing can cause vertical to wall displacement with a decrease in power. Proper curing can be achieved by covering the work with polythene sheets, and heating the material in cold weather may also be necessary if the brickwork is to be done in cold conditions.,

5. Failure to adjust the suction of bricks

Figure 1
Figure 1

A more subtle defect can arise if thin walls are to be constructed using highly absorbent bricks. The reason for this is shown in Figure 1, which explains how a bed joint can be formed’Pillow‘ The shape is given if the bricks on top of it are placed a bit. If water has been removed from the mortar by suction of bricks, it may have been too dry to recover its originally flat shape. The convex shape of the mortar bed will result in the resulting wall lacking stability and may be 50% weaker than expected considering the strength of the brick and mortar mix. the solution is Wet the bricks before laying so that their suction rate is 2kg/m. be less than2/ min, and the proportion of lime in the mortar mixture will help retain water against the suction of the bricks.

6. Incorrect proportion and mixing of mortar

The effect of mortar strength on masonry strength can be judged by line drawing number 2 Out of which it is 30 N/mm. can be seen with bricks having a crushing capacity of2 Which increases the strength of the mortar to 11 N/mm. does less than2to 4.5 N/mm2 14 N/mm . may be expected to reduce the strength of the brickwork2 to 11N/mm2, This corresponds to a change in the mortar mix. 1 : 3 cement : sand 1: 4.5 Or about 30% too little cement in the mix. The reduction in mortar strength can also result from a relatively high water/cement ratio, while still producing a workable mixture. It is therefore important to see that the specification for mortar strength is followed, although the mortar has an inherent degree of tolerance sufficient to accommodate small errors in proportion and mixing. Using an inappropriate or excessive amount of plasticizer in place of lime will produce a porous and possibly weak mortar and this has to be avoided.


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Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.