Backfill timing.

Q. One of our customers wants to reduce 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.

Section 3.7.2.1 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 various circumstances.

Because concrete strength is the basic criterion for form removal, using a high-early-strength mix does offer some advantage. 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, the bottom support is there at the beginning of wall placement, but the top support is provided only by installing floor joists. Backfilling foundation walls before installing the joists may result in failure simply because that top support is not there.

Suggest that your customer work with the engineer to understand whether concrete strength or wall stability is likely to be the controlling factor and when backfilling can begin.