- Q. One of our customers wants to cut construction time on a job that will include several concrete retaining walls. They’ve asked us how quickly they can backfill these walls and how much sooner they can do that if they use a high early-strength mix. What should we tell them?
A. One of the great things about ACI’s “Manual of Concrete Practice” is it addresses almost everything related to concrete construction. Sometimes, it can be tricky finding the appropriate information. In this case, refer to ACI 347, “Guide to Formwork for Concrete.”
Section 18.104.22.168 recommends when formwork can be removed and loads applied. “The engineer/architect should specify the minimum strength of the concrete to be attained before removal of forms or shores,” it states.
Determining the concrete strength once it is placed can be done by testing job-cured specimens or by measuring the in-place concrete, such as by using a maturity meter. The engineer can also permit form removal after a minimum elapsed time, depending on weather and other variable circumstances.
Because concrete strength is the best criterion for form removal, using a high-early-strength mix does offer some advantages. But other factors may control when backfilling can begin.
Some retaining walls are designed as cantilevers. These typically include reinforcement and rely on the combined strength of the concrete and reinforcement to resist bending at the base of the wall. In that case, plan to wait seven days before placing the full depth of backfill.
Be careful if the wall has been designed to be supported at the top, too, rather than as a free-standing structure. In residential foundations, for example, concrete walls often are designed to act as beams that are supported on each end in resisting horizontal loads. The bottom support is there at the beginning of wall placement, but the top support is provided only by the installation of floor joists. Foundation walls that are backfilled before the joists are installed may fail simply because that top support is not there.
Suggest that your customer work with the designer to determine whether concrete strength or wall stability will be the controlling factor in determining when backfilling can begin.