Block Wall Plumb Error and Mortar Crack in Garoua
You are a CAP masonry trainee building a 3.0 m long × 2.5 m high partition wall in a classroom in Garoua. The wall has one door opening 0.9 m × 2.1 m. You are using hollow concrete blocks 40 cm long × 20 cm high × 15 cm thick, with 10 mm mortar joints, in a 1:6 cement–sand mix. The blocks were delivered two days ago and have not been wetted. The sand is dirty river sand, and some cement bags have hard lumps because they were stored on a damp floor. After three courses, a plumb line shows the wall is 8 mm out of plumb over 1 m. A fine horizontal crack has appeared along a mortar joint near the door. The site supervisor wants you to 'cover it with render and continue.'
1. Calculate the net wall area, the number of blocks required including 5% breakage, and the mortar volume using 0.03 m³ of mortar per m² of wall. For a 1:6 mix, estimate the cement and sand.
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Gross wall area = 3.0 m × 2.5 m = 7.5 m². Door area = 0.9 m × 2.1 m = 1.89 m². Net wall area = 7.5 − 1.89 = 5.61 m². One block plus its joint covers (0.40 + 0.01) m × (0.20 + 0.01) m = 0.41 × 0.21 = 0.0861 m². Blocks required = 5.61 ÷ 0.0861 = 65.2, so 66 blocks. Adding 5% breakage: 66 × 1.05 = 69.3, so 70 blocks. Mortar volume = 5.61 m² × 0.03 m³/m² = 0.1683 m³. For a 1:6 mix, allow about 1.33 times the wet volume for dry materials: 0.1683 × 1.33 = 0.2238 m³. Cement is 1 part of 7, so 0.2238 ÷ 7 = 0.0320 m³. At about 1,440 kg/m³, that is about 46 kg, practically one 50 kg bag. Sand is 6 parts of 7: 0.2238 × 6/7 = 0.192 m³. I would order one bag of cement and about 0.2 m³ of clean sand, with a little allowance for waste.
2. Diagnose the causes of the 8 mm plumb deviation and the horizontal crack. What must be done before continuing?
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The 8 mm deviation over 1 m is far too much for a fair-faced partition. Likely causes are that the first course was not level, the corners were not plumbed, the blocks were laid without checking each course, or the mortar was too stiff and the blocks were forced out of line. The horizontal crack is probably a shrinkage or bond failure caused by dry blocks sucking water from the mortar, dirty river sand reducing adhesion, and lumpy cement that has partly hydrated. The blocks should have been wetted before laying, especially in Garoua's heat. The crack must not be covered with render: render hides the defect but does not restore the bond, and the crack can widen. I would stop work, remove the cracked course and the out-of-plumb section, clean the blocks, wet them, reject the lumpy cement and dirty sand, and rebuild from a level, plumbed first course. I would check plumb and level at every course before continuing.
3. The supervisor says to render over the crack. Explain why this is unacceptable and propose a quality-control sequence for the rest of the wall.
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Rendering over the crack would only hide a live defect. The crack shows that the mortar joint has lost bond or that the wall has moved. If the cause is dry blocks, dirty sand, or bad cement, the rest of the wall may have the same weakness. Covering it would leave the client with a wall that may crack again, and the render itself would crack along the same line. The correct action is to cut out and rebuild the affected section, not to decorate over it. For the rest of the wall, I would control the first-course level and plumb, wet the blocks before laying, use clean sand and fresh cement, mix a workable 1:6 mortar, keep joints at 10 mm, check plumb and level every course with a spirit level and plumb line, and cure the wall by dampening it for several days. I would also check the door opening dimensions and keep a straight lintel bearing. The supervisor's instruction should be refused politely and reported.