Changes

* Use aggregate (gravel) of 5-20mm for non-porous concrete and 5-10mm for porous concrete. Aggregate can be graded using 2 locally-made sieves of 5mm and 20mm – discard any aggregate that remains on the 20mm sieve or that falls through the 5mm sieve. The reasons for having a maximum size is to do with workability, but also that the maximum size is not greater than the “cover” ofconcrete over a reinforcing bar – if a stone would span the distance from outer edge to reinforcement, this can become a zone of