if double layer, that's much better. proper curing of concrete is also very important. normally 30Mpa concrete will be used for foundation wall. you need make sure concrete keep moisturized for at least 7 days. study show that if not moisturized properly concrete can only reach 40% of its design strength. after 7 days only 80 percent it strength achieved. In design, we normally use its 28 days strength, that's called design strength. during curing, the cement will absorb a lot of water. proper curing will also reduce shrink crack.