Q.: Is there any standard or code that defines how strong the concrete must be before one can backfill a concrete wall?
A.: One of the great things about ACI’s “Manual of Concrete Practice” is that it addresses nearly 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 22.214.171.124 contains recommendations for when formwork can be removed and loads applied. Among other things, the document says, “The engineer/architect should specify the minimum strength of the concrete to be attained before removal of forms or shores.” Once it is placed, concrete strength can be determined by testing job-cured specimens or by measurements made on 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 circumstances.
Although the basic criterion for form removal is concrete strength, other factors may control when backfilling can begin.
If a retaining wall is designed as a gravity wall, it can be backfilled relatively early. The horizontal pressure on the wall from the backfill is countered by the deadweight of the concrete and of the backfill material pressing down upon its broad base, which keeps a well-designed system in equilibrium.
Sometimes 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 on waiting 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 supported at the top and bottom to resist 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. Backfilling foundation walls before installing the joists may result in failure simply because that top support is not there.
In short, if you’re in a hurry to begin backfilling, work with the engineer to understand whether concrete strength or wall stability is likely to be the controlling factor, and determine when backfilling can commence.