So a dry block of the size used in our example should weigh at least 6.19 kg.
(d) One important test is to see how the blocks stand up to getting wet and
drying out again as they would in use in a building. Take 5 fully cured blocks and put
them in water so they are completely covered. Leave them in the water for at least 12
hours overnight. Take them out and let them dry in the sun for the day. Repeat this
process every day for 7 days.
If the blocks fall apart, crack or bits flake off there is something wrong. It might be that the
mix is wrong or the soil is bad for SSB's.
(e) When the wetting and drying test is being done you can also do the test for
water absorption. Weigh the 5 blocks before they are put in water (remember to mark
each block with a number that will not wash off). Weigh the blocks again after they
have soaked overnight.
A good block will not absorb more than 12 % water by weight. So if the dry block weighs 6.2
kg and the wet block weighs 8.13 kg what is the percentage of water absorbed. The
calculation to find out the percentage water absorption is given below:
PERCENTAGE WATER = ((WET WEIGHT - DRY WEIGHT)/DRY WEIGHT) x 100
Example: ((8.13-6.2)/6.2) x 100 = 31%
This would be a bad block to use.
(f) Compressive strength testing can be done on the site but a machine like the
ones below will have to be made.
When the very large weights required
todo compression tests on full size
blocks are not available, it is common to
make small test samples, 50mm in
diameter or square, and 50mm high. If
these cannot be made on site, use a saw
or mason's axe to cut a larger sample or
block into roughly the right size and
measure its surface area.
Practical equipment for compression
testing using a lever system is shown on
the left. Cover the sample with plywood
or a board 20mm thick (more for larger
samples). Place weights on the end of
the lever arm, or suspend them from it.
Calculate the compression (C) as follows:
C = W x 0 (0 + A)
where -
W = weight;
D = distance from weight to point of resistance (variable);
A = distance from point of resistance to centre of the sample (300mm).
Compression is increased by increasing either W, or 0, or both, until the sample fails. Divide the
compression by the surface area of the sample for the result. Test five samples for an average
compressive strength.
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