When concrete is strong enough to remove formwork

Fresh concrete needs its formwork and shores until it can safely support the loads that will act on it. OSHA does not set one universal number of days or one compressive-strength threshold for stripping every form. The employer must determine that the concrete has gained sufficient strength for its own weight and superimposed loads using one of the methods specified in the construction standard.

The performance test

Under 29 CFR 1926.703(e)(1), forms and shores generally may not be removed until the employer determines the concrete can support its weight and superimposed loads. The focus is safe load capacity at the time of removal, not whether the surface looks hard or a customary number of hours has passed.

Two bases for determining adequate strength

1. The plans and specifications stipulate conditions for removing the forms or shores, and those conditions have been followed; or
2. The concrete is properly tested with an appropriate ASTM standard test method designed to indicate compressive strength, and the results show sufficient strength to support its weight and superimposed loads.

A calendar estimate alone is not one of the listed bases. Concrete strength development varies with mix, temperature, curing, member geometry and loading. The project plans and specifications or appropriate test evidence need to support the determination.

Removing forms is different from removing reshores

Reshores support concrete after forms or original shores are changed or removed. OSHA separately requires that reshores not be removed until the concrete they support has adequate strength to carry its own weight and all loads then in place. A slab may have enough strength for one operation and still require reshoring to carry construction loads or loads from floors above.

Plan for the loads and sequence

The determination should consider the concrete element's own weight plus loads imposed by workers, equipment, stored materials and construction sequencing. Removal can shift load to adjacent floors or remaining shores. Follow the engineered formwork and reshoring plan, preserve required drawings at the jobsite, and have a qualified person evaluate changes instead of improvising the sequence.

Common exam errors

• Using a fixed number of hours or days as the only criterion.
• Removing forms because the concrete surface appears hard.
• Ignoring superimposed construction loads.
• Treating form removal and reshore removal as the same decision.
• Assuming an unspecified cylinder test is sufficient without an appropriate ASTM method and results that demonstrate capacity.

Key takeaway

Remove forms only after the employer determines the concrete can carry its weight and added loads, based on plan conditions or appropriate strength tests. Reshores remain until the supported concrete can carry all loads in place.