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Wilfried,
Where you plan to build the concrete sub, what is the quality of the sand and gravel ? According to concrete experts that I have talked to this is very important for the ultimate strenght of the cured concrete. For instance, here in Ventura California where I live, the geology of the area here on the coast, the rock here is not good for strong concrete. I have to make sure I get sand and gravel from the desert area where there are deposits of granite. Brian Cox |