Brick Stock With Efflorescence and High Absorption
A load-bearing 230 mm external wall for a two-storey school building has been specified with class 7.5 bricks and 1:6 cement mortar. The first truck delivers 20,000 modular bricks (190 x 90 x 90 mm). Site tests on five random bricks give mean compressive strength 5.2 MPa, water absorption 18%, and a moderate efflorescence deposit after the standard test. The supplier's invoice says 'best quality local bricks' but no class marking. The mason proposes to soak the bricks and lay them in 1:4 cement mortar to 'make up for weakness'. The wall will be plastered externally with 12 mm cement plaster and painted. Rain is expected in three days, and the programme is already two days behind. The second truck is due tomorrow with another 20,000 bricks from the same kiln.
1. Should the consignment be accepted for the specified wall? Interpret the test results and justify your decision.
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The consignment should not be accepted for this load-bearing external wall. The specification calls for class 7.5 bricks, which requires a mean compressive strength of at least 7.5 MPa; the sample mean is 5.2 MPa, so the bricks fall short even though five bricks are only an indicative sample. Water absorption of 18% is high for a good-quality brick and indicates a relatively porous body. Moderate efflorescence is worse than the slight deposit normally tolerated in exposed brickwork, and the wall is to be external and painted.
The invoice wording 'best quality' is not a class marking and cannot substitute for test evidence. The correct action is to segregate the lot, obtain a proper random sample and test compressive strength, water absorption and efflorescence to the relevant Indian Standard methods. Until that is done, the lot should not be used in the load-bearing wall. A weaker brick might be considered for internal non-load-bearing work only after re-testing and written approval, but it is not a direct substitute here.
2. Evaluate the mason's proposal to soak the bricks and use 1:4 cement mortar. What mortar and construction precautions are appropriate?
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Using 1:4 mortar does not 'make up' for weak bricks. A richer mortar has a higher strength and modulus than the specified 1:6 mix, so it can restrain brick movement and promote cracking at the interface; it is also uneconomical. The correct approach is to use bricks of the specified class with the specified 1:6 cement mortar, or to have the wall redesigned if weaker bricks are proposed. The mortar should be compatible with the brick, not simply stronger.
Soaking needs care. Bricks should be wetted so that they do not absorb water from the mortar too quickly, but they should not be saturated or laid dripping wet. High-absorption bricks need controlled wetting and should be protected from rain. Joints should be 10 mm thick, filled fully, and the mortar should be used within its working time. The wall should be protected from rain, and curing should start after the mortar has stiffened. If rain is due in three days, the top of the wall should be covered and unfinished work protected.
3. What testing, sampling and record-keeping would you require before the second truck is allowed onto the wall?
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The second truck should be held in quarantine until it is identified by kiln, lot number, delivery date and quantity. A random sample should be drawn in the presence of the engineer, not selected by the supplier, and tested for compressive strength, water absorption and efflorescence using the relevant Indian Standard procedures. The sample should be large enough for the required tests, and the bricks should be marked so that results can be traced back to the lot.
Records should include the sampling date, location, witness names, test results, and the acceptance decision. Delivery challans, stockpile location and daily use should be recorded so that rejected bricks cannot be mixed with accepted stock. The contractor should also record the mortar mix and brick wetting practice. The second truck should be allowed only if its test results meet the specified class and durability requirements; otherwise it should be rejected and removed from site.