One of the most difficult decisions to make with concrete is deciding when it is actually ready to withstand additional pressure, whether from cars being driven on it, additional floors being built, or adding backfill. While concrete may appear dry fairly quickly, its strength may not be fully developed until quite a while later.
If you are building a home, there may be pressure to move on with the project soon after the foundation is poured to avoid delays and possible additional costs. However, it can be dangerous to rush a job and chance damaging the concrete before it is properly set. Here is some information about properly allowing concrete to set and when to backfill a concrete foundation.
It takes time
As soon as concrete is mixed, whether it is made onsite or mixed offsite then trucked in a plastic state to the construction site, the cement begins to react with the water and form molecular bonds. These molecules form crystals that develop throughout the concrete, interlocking and connecting to fill gaps in the material.
As the crystals develop, they provide the strength for which concrete is renown. The crystals continue to develop as long as water remains in the concrete. The problem is that the crystallization process may take a month or longer to fully form. This is in ideal situations, with a stable temperature around 70 degrees Fahrenheit. In colder temperatures, this process may take longer or even be arrested until the temperature warms up again. In warmer temperatures, the process can progress too quickly or the concrete may dry out prematurely, which will not allow the crystals to fully form.
Strength comes slowly
About a week after being poured, a concrete foundation will have about three-quarters of its final strength. However, it may take 28 to 30 days or longer for the walls to gain their full strength. In that time, it can be dangerous to add backfill. Soil, especially if it is wet, can be tremendously heavy and exert extreme forces on the foundation walls.
The strength of concrete is in its ability to withstand compression, and you can squeeze concrete with very large forces before it cracks. However, try to bend concrete with the same force, and it cracks very easily. The force of backfill against the wall can push the foundation in its most vulnerable spot. One of the biggest causes of cracked foundations is the combination of an improperly cured wall and an inexperienced worker pushing backfill against the foundation, rather than dropping it gently.
Tips to avoid cracked foundations
The best advice is to wait until the concrete has fully cured and gained its maximum strength before backfilling the area. It is also important to ensure that, if the foundation is not poured on a slab, the interior floor slab is poured to provide a base against which the concrete can rest. A contractor should know that the first floor level should be completed before any backfilling is considered. This locks the foundation in place along the bottom and top of the wall to minimize movement when the backfill is applied. Without the basement slab and the pressure from the first floor, the wall can crack or even slide out of place when the backfill is applied.
Finally, if time is a big issue, consider using a reinforced concrete foundation. The tensile strength of the steel rods will allow the concrete to withstand the pressure from the backfill much earlier than would otherwise be possible and my help to prevent cracks in the future.