Q: As a contractor, I often have to dig and pour concrete footings. But after long spells of dry weather, the soil at the bottom of the excavated holes can be super dry. How does the moisture content in the soil affect how bagged concrete cures?
A: Bill Palmer Jr., an engineer and the editor-in-chief of Concrete Construction a sister publication of responds: Concrete gains strength through a reaction called hydration, which means that water is consumed as it reacts with the mix and forms the crystalline structure that makes the concrete strong. If the concrete dries out before the hydration reaction is complete, it will not reach its full strength. This holds true for curing both at the top surface and at the bottom surface of the pour.
If concrete is poured into a hole with dry soil, the soil can pull the water out of the mix and for some distance into the concrete (depending on how dry the soil is), the concrete will be weaker than intended. To prevent weakening the concrete, dampen any dry soil that will be in contact with the concrete. Dampening the soil can also help to reduce shrinkage cracks caused by water being drawn out of the mix.
Be aware that the type of soil can have a bearing on how much dampening you should do. For example, clay soil doesn’t absorb much water, so it doesn’t take much water to dampen it. Never pour concrete into a hole if there is standing water in it. Excess water increases the water-to-cement ratio in the concrete mix, which also reduces the strength of the concrete. Other important factors to consider with soil types are how much the soil will compact and settle with loads placed on top, and whether the soil will expand as it absorbs moisture, as with some clays. If you have doubts about the bearing capacity of your soils, be sure to check with an engineer.