Tests for Early Acceptance of Concrete

About the report:

A literature survey and limited study of laboratory concretes were conducted to evaluate various methods for predicting concrete strengths at 28 days based on procedures for accelerated strength development or measurement of strength of normally cured specimens at early ages. A review of the literature was also made to assess the usefulness of procedures for the direct determinations of water and cement contents in quality control of hydraulic cement concrete. It is concluded that although general relationships exist as reported in the literature, none of the procedures are sufficiently precise for use in quality control or acceptance testing by the Virginia Department of Highways & Transportation. The amount of calibration testing for projects normally built by the Department would be prohibitive. Any application of reduced pay factors for strength in a statistical specification must be based on tests made at the age designated in the specification. The literature survey shows that in their present state of development existing procedures for the direct determination of water and cement in plastic concrete are not sufficiently fast or precise to be useful for quality control of hydraulic cement concrete. It is recommended that further evaluation of such procedures by the Virginia Department of Highways and Transportation be deferred until results of studies now being made for the FHWA are available. The results of 14-day and 28-day tests on the same field concretes reveal that the ratio of strengths at these two ages often varies significantly from the assumed value of 0.85. Thus, it is recommended that the practice of accepting concrete on the basis of 14-day strengths be discontinued unless the value at 14 days is equal to the required value at 28 days.

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