Q.: When we use air-entrained concrete we find that the concrete becomes crumbly when we put on our finish edging. What causes this?
A.: This problem is related to the sand gradation and it occurs when the sand is too coarse. This means that there is too small an amount of material that passes the Number 16, Number 50 and Number 100 sieves. If you can get your supplier to provide a sand which contains enough more fine material it should solve your problem. If not, you might improve the concrete by using a mix with a little bit more cement or with a small amount of fly ash in addition to the quantity of cement already in the mix.