The in situ compressive strength of concrete

Abstract

The following problems associated with the determination of the compressive strength of concrete in actual, full-size structures are reviewed: standard test specimens are rarely representative of in situ concrete; strength within a structure itself varies; the causes of these variations are unclear; and methods of estimating in situ strength are of doubtful accuracy. The relationship between the strengths of separately moulded test specimens and the strengths of in situ concrete is examined. In situ strengths are estimated using the ultrasonic pulse velocity (UPV) method; these estimated strengths are compared against drilled core strengths. Full-size columns are tested to determine the nature of in situ density, UPV and strength variations. The effects, on in situ variations, of heat of hydration, formwork surfaces, curing conditions, and compaction are examined. Compaction is considered to be the most critical factor because segregation of the concrete makes estimation of in situ strength by the UPV method unreliable. The strength-UPV relationship depends heavily on the aggregate concentration of concrete. Hence the "Photonegative Method" is developed as a quick, simple, and relatively cheap means of estimating in situ aggregate concentration. Finally, the "UPV-AWC Method" - in which in situ strengths are estimated from correlation curves based on the correct Aggregate Weight Concentration at each point where UPV is measured - is demonstrated and shown to give better estimations of relative in situ strengths than the UPV method that is currently used.