When is it safe to remove wall forms and to backfill?

Concrete Construction, August, 2002

Stripping wall forms after 24 hours is common practice in our industry. Early formwork removal and backfilling keep construction schedules on track and provide easy access to the work--both important considerations--but safe? To avoid damage to concrete that may not have attained full strength or may be overloaded in the stripping or in subsequent construction operations, here are some guidelines *:

* The contractor is responsible for designing and constructing the formwork and making sure it is safe. For important structures, the engineer/architect should specify when forms and shores should be removed. Minimum stripping time for walls when minimum strength is not specified is 12 hours total (not necessarily consecutive), during which the ambient temperature is 50[degrees]F or greater, according to ACI.

* For basement walls, the interior braces should be left in place until after backfilling. Walls are usually braced a few days after forms have been stripped and before backfilling operations begin.

* When forms are removed, supporting forms and shores must not be removed from walls (or other structural units) until concrete is strong enough to carry its own weight and any other superimposed dead or live loads.

* Careful supervision and continuous inspection of formwork erection and removal can prevent many accidents, and these operations should be performed in a manner to ensure the safety and serviceability of the structure.

Backfilling near unsupported walls and the movement of heavy equipment nearby may cause structural failure. Backfilling should not take place until adequate support is in place for the foundation wall--from top to bottom. Bracing for residential concrete basement walls usually consists of one 2x8 or 2x12 lumber brace for every 25 linear feet of wall. Walls 30 to 60 feet long may require two such plank braces at one-third points.

A good rule of thumb is to keep all construction machinery away from foundation walls at a distance equal to the wall height. The earthmoving operator is responsible for any wall cracks created by improper backfilling methods. Cracks can occur from concentrated pressures exerted by incorrectly placed fill material. Cracks are usually seen by contractors (and owners) at outside corners and midspan on the inside face of the wall.

Some builders take risks. They have backfilled subgrade walls for years without lateral bracing and have never experienced cracks or failures. This practice is not advisable based on established industry guidelines. First, there can be significant variability in the magnitude of lateral forces placed on foundation walls, depending on the backfill material (sandy or clay soils, presence of boulders) and on whether the soil is wet or dry. Accurate estimates of live loads of heavy earthmoving equipment are often not possible. Finally, there are few jobs in which the builder can be completely confident that walls are built exactly as designed, that walls contain the recommended, precisely placed reinforcement, and that there was no human error in the construction or mix design.

* Some of the guidelines are taken from "Formwork for Concrete," ACI SP 4, by M.K. Hurd; ACI 347, "Guide to Formwork for Concrete"; ACI 332, "Guide to Residential Cast-in-Place Concrete Construction"; ACI 301, "Specifications for Structural Concrete"; ACI 318, "Building Code for Structural Concrete and Commentary."