i just found a bug in the mortar (cement and sand) computation of my estimate for a 1 sqm of wall using 4" CHB while i was reviewing the formula last night. the 0.0532 cubic meter of sand and 0.3133 bag of cement from my previous posting replies was WRONG. the 12.5 4" CHB for 1 sqm of wall is still CORRECT ...

below is the correct computation for the estimate i did, after checking all my old notes from our house construction ... apologize for the wrong info before ...

4" CHB dimension ... L=400mm x H=200mm x W=100mm (approx 16" x 8" x 4")

dimension of 1 cavity in a 4" CHB ... 50mm x 90mm x 170mm

i have remove the plastering and rebar (2 pcs of 1m lenght 9mm deformed rebar for 1 sqm of wall) in the estimation of a 1 sqm of wall as it makes more sense to compute the estimate of these items separately on a per wall basis based on the house/bldg plan.

to get the amount of mortar required for a 1 sqm wall, first we need to estimate the area/volume in which the mortar will be put. these are:

- a 4" CHB normally has 3 cavities each measuring an area of approx 90mm x 50mm x 170mm.

- at the sides of a 4" CHB, you can also see a half-circle (or half-square) grove. if you set 2 CHB in a line with a 1/2" clearance between the 2 CHB, the area for the mortar can also be computed as approximately 1 cavity too. since each 1m (1000mm) length of wall will require about 2.5 CHB, this means another 3 cavities for the 2.5 CHBs in a row.

- so total cavities (vertical area) per row will be 6 (for the 2 CHB) + 1.5 (for the 1/2 CHB) + 3 (for the 2.5 CHB per row) = 10.5 cavities.

- the height of a CHB is approx 200mm, add the 1/2" (12mm) mortar height between rows, you have approx a 212mm height. in a 1m high wall, you will then need approx 5 rows (1000mm / 212mm = 4.72) of CHB.

- the area for the mortar in between rows (horizontal area) of CHB will be about 1/2" (12mm) high with a lenght of 1000mm (1m) and a width of 100mm (4")

using the same formula for computing concrete with a mix ratio of 1:6:0 (cement:sand:gravel) for mortar, you can then compute the total volume of concrete needed for the area of 1 sqm wall of 4" CHB ...

- for each cavity (vertical area), the area/volume is ... {[(90*50*170) / 100000] / 10000} = 0.0008 cubic meter * 10.5 (number of cavities in a row) = 0.0080 cubic meter of concrete

- for each row (horizontal area), the area/volume is ... {[(1000*100*12) / 100000] / 10000} = 0.0012 cubic meter * 5 (number of rows) = 0.0060 cubic meter of concrete

- total volume = 0.0080 + 0.0060 = 0.0140 cubic meter of concrete for a 1 sqm wall

having the concrete volume, we can then compute for the cement and sand using a mix ratio of 1:6:0 - gravel or aggregate is not needed for mortar ...

so, to get the amount of cement needed for a 0.0140 cubic meters of concrete the formula is ... (volume * ratio of cement) / mixture total ratio ... (0.0140 * 1) / 7 = 0.0140/7 = 0.0020 cubic meter of cement ...

so, to get the amount of sand needed for a 0.0140 cubic meters of concrete the formula is ... (volume * ratio of sand) / mixture total ratio ... (0.0140 * 6) / 7 = 0.084/7 = 0.012 cubic meter of sand ...

in summary ... 0.0140 cubic meters of concrete with a ratio of 1:6:0 will require ...

0.0020 of cement

0.0120 of sand

--------

0.0140 cubic meters of concrete (total) ...

to convert the amount of cement into bags of cement ... just multiply the cement in cubic meters by 35.31 ... 0.0020 * 35.31 = 0.0708 bags of cement ...

cheers ...

Boo!

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