Mortar Mix on a Hot Day
A crew is laying a 230 mm face-brick wall on a 34 °C day with light wind. The mortar is a 1:1:6 cement–lime–sand mix gauged on a board in 0.2 m³ batches. Bricklayers report the mortar stiffening within 25 minutes and adding water to the board to keep it workable. By mid-morning two courses in a 6 m panel show hairline shrinkage cracks and a few bed joints are visibly dry and sandy when raked. The supervisor wants to finish the panel before lunch and asks whether to keep retempering or to change approach.
1. What is happening to the mortar, and what should the supervisor decide about retempering on this panel?
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The symptoms point to rapid moisture loss from a cement–lime–sand mortar in hot, windy conditions: water evaporates and is drawn into the absorbent bricks, so the mix stiffens early. Adding water to a mortar that has begun to set is retempering beyond the point where the cement has hydrated; it dilutes the binder and weakens the joint. The dry, sandy bed joints confirm the mix has lost cohesion and the bricks have sucked moisture before the mortar could cure. The supervisor should stop retempering once initial set is evident, discard stiffened mortar, and reduce batch size so each board is used within its workable life. Bricks should be wetted or the wall kept damp, and mixing should move to a shaded area. The cracked courses should be assessed and likely rebuilt rather than patched, because the bond and strength are compromised.
2. How would you restructure the morning's work to protect quality while keeping the panel progressing?
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The plan should match batch size to consumption: smaller gauges (say 0.05–0.08 m³) used quickly avoid waste and the temptation to retemper. Work in the cooler part of the day, pre-wet highly absorbent bricks, and cover the mortar board. Keep mortar off the board in direct sun and avoid laying on a hot, dry bed without damping. Progress the panel in sections so fresh mortar is not left standing, and check joints by raking a sample: if the bed is friable, the mix is failing. Record the ambient conditions and the decision to discard mortar, because the supervisor's instruction to 'keep going' conflicts with the standard requirement that mortar be used within its working life. The panel should be finished with sound joints, even if that means a later finish time.