Example: Say your sample is a cube of 5cm (0.05m) sides. So its surface area S = 5 x 5 = 25
square centimetres.
Now if the distance A on the machine is 85mm, the distance L is 1,676mm and the weight
(W) used is 50kgs when the cube starts to crush when then the compression C is given by:
C = 50 x (1,676/85) = 986kgs
So the compressive strength = ((986/(0.05 x 0.05))
= 394,400 kgs/square metre
To compare this to the strengths for blocks given in MPa (Mega Pascals, 1 Mpa = 1 Mega
Newton/square metre). Mega means 1,000,000. 1 kg is equal to approximately 10 Newtons,
so:
394,400 kgs/square metre
= 3,944,000 Newton/square metre
= 3.944 Mega Newton/square metre
= 3.944 MPa
So this sample is strong enough.
(g) A very simple test is the "ring" test. If two fully cured blocks are knocked
together a good sharp ringing noise should be heard. If the blocks ring they are
probably strong and water resistant. If they sound hollow or "dull" they are probably not
very good.
(h) Another simple test is to try and scratch with your thumbnail a fully cured
block that has been soaked overnight. If you cannot make a mark the block is probably
okay, at least for single storey buildings.
CONCLUSION
The most important things that affect the quality of SSB's are how hard it has been pressed,
the choice and amount of stabiliser used, soil preparation and block curing. All these things
are in the hands of the blockmaker to control except the compaction pressure which depends
on the machine used.
There are at least two presses made in Zimbabwe:
1 A constant volume machine - APTECH, PO Box AY 175, Amby, Harare.
2 A constant pressure machine - Ecological Designs, Box 6084, Harare.
Prepared by the Building Materials Programme, ITDG, PO Box 1744, Harare. Telephone 796420 or 796409.
14