Hairline Cracks and Drummy Plaster After Hot-Weather Application
A plastering crew is finishing both faces of a 4.0 m long × 2.7 m high 100 mm CHB wall. The mix is 1:3 cement-sand and is applied as a single 20 mm coat in direct sun at 35 °C. The blocks were not pre-wetted, and the surface is dusty. The plasterers work from one end to the other and finish with a steel trowel. Curing consists of a light hose-down twice a day for two days. Seven days later, the owner sees hairline map cracks over much of the surface; tapping reveals hollow, drummy patches near the top of the wall. The painter wants to skim over the cracks and proceed. The contractor asks whether the plaster is acceptable and what repair is needed.
1. Identify the likely causes of the hairline cracks and hollow, drummy plaster in this scenario.
Read the worked answer
Hairline map cracks and drummy patches point to several related causes. A single 20 mm coat is too thick for one application; the mortar shrinks as it dries, and the shrinkage is restrained by the block, causing cracks. At 35 °C in direct sun, the block and mortar lose water rapidly; because the blocks were not pre-wetted and were dusty, the wall sucked water from the fresh mortar and the dust prevented a bond, producing hollow areas. The 1:3 mix is rich, which increases shrinkage. Curing twice a day for only two days is insufficient; cement-based plaster needs about seven days of moist curing. The combination-thick coat, rapid drying, poor bond, short curing-explains both the cracks and the drummy patches.
2. Describe the correct plastering procedure for this wall, including coat thickness, surface preparation, and curing.
Read the worked answer
Pre-wet the wall so it is damp but not running, and clean off dust. Apply plaster in two coats: a 10–12 mm scratch coat, scored, then cured; after it has set, apply the 8–10 mm finish coat. Use a 1:3 or 1:4 mix that is workable but not wet. Work in shade or protect the wall from direct sun and wind; do not apply at 35 °C in full sun without protection. Screed each coat to a true surface, float, and finish with a steel trowel only when the mortar has begun to stiffen. Cure by keeping the surface moist for at least seven days-wet burlap, plastic sheeting, or frequent fine misting-not just two hose-downs. These steps control suction, shrinkage, and bond.
3. How should the drummy and cracked areas be repaired and accepted?
Read the worked answer
Tap the whole surface and mark every hollow area. Cut out the drummy plaster to sound material with square edges, clean and wet the substrate, apply a bonding slurry or key coat, and fill with a matching mortar in two thin coats rather than one thick patch. Cure the patch. For hairline cracks, if they are nonstructural, open them slightly into a V, clean, and fill with a flexible filler or cement slurry; if the cracks are active or widespread, investigate the cause before skimming. The painter must not simply skim over drummy plaster, because the hollow will continue to detach. Acceptance: no drummy sound on tapping, no visible cracking after curing, and flatness within normal tolerance (for example, about 3 mm under a 3 m straightedge).