Question: We're supplying concrete for a tilt-up project that requires concrete with a 3500-psi (25 MPa) 28-day compressive strength. In addition to making standard test cylinders, the tilt-up contractor is also making beams that are stored in the field before they're tested. Because of nonstandard curing conditions for the beams, should we be concerned about being held accountable for the beam strengths?
Answer: No. The beams and cylinders are made for two different purposes. The tilt-up contractor's erection manual gives a strength that in-place concrete must attain before the panels can be lifted. This is not the same as the 28-day strength specified for standard-cured cylinders. Typically the required in-place compressive strength is around 2500 psi (18 MPa), which is usually reached in five to seven days.Some contractors, however, use beam tests to determine when the panels are ready for lifting. When cylinders are made, the contractor's testing laboratory also casts companion beams that are left at the jobsite so they're exposed to the same conditions as the tilt-up panels. Before lifting, laboratory personnel test the beams and calculate the modulus of rupture. Generally, panels are designed to be lifted when the modulus of rupture reaches 500 psi.You're responsible only for the strength of the cylinders cured as described in ASTM C 31, Section 9.2, Standard Curing. The beam strength is affected by jobsite conditions such as temperature and curing methods over which you have no control.