International Concrete Abstracts Portal

Research conducted in different parts of the world has led to the development of several pmcedues for accelerated stren th tests for concrete. While these procedures most il y relate to prediction of concrete compressive strength, no such work has been reported on flexural strength of concrete, which is essential In rigid pavement design. Studies reported herein attempt to bridge this gap. Among the different promising procedures for accelerated curing, Akroyd's Modified Boiling Water Method was considered to be the most suitable for field jobs like construction of concrete pavement. The studies were conducted with three different types of coarse aggregate and two ordinary portland cements of varying compound composition. The concrete mixes were designed for three different water-cement ratios 0 The findings confirmed that the correlations between the strengths of 7 or 28 days normally cured specimens and those cured under the stipulated accelerated condition were affected by the type of coarse aggregate and characteristics of cement like fineness and compound composition, but not by the quantity of cement present In the concrete mix. I t could be generally concluded that the Modified Boiling Water Method was capable of predicting the 7 or 28 days standard compressive and f lexur al strengths with reasonably good degree of accuracy.

This study evaluates the field worthiness and accuracy of a chemical technique (Kelly/Vail) for determining water and cement contents of fresh concrete. The results are compared directly to mixture proportions of cement and water and to cement contents obtained by a nuclear cement content gauge method. The study also evaluates the accuracy of estimating 28 day compressive strengths by both accelera-ted curing technique and the Kelly/Vail technique. The study proved the Kelly/Vail system to be field worthy, rapid (less than 15 min.) and simple enough to be operated by technicians or inspectors. Kelly/ Vail water and cement contents when used in conjunction with an air content test can estimate the strength potential of fresh concrete. The Kelly/Vail strength estimates are not as accurate as the accelera-ted curing estimates of 28 day compressive strengths, but the Kelly Vail tests are sufficiently accurate to be meaningful and they are significantly more timely - 15 min. versus 24 hours than accelerated strength tests.

An accelerated-curing method to predict the 28-day strength of concrete, from 2-day self-cured test results was evaluated in the field. All concrete samples were collected at the job-site as a part of normal field control work. The method consists of casting and curing the concrete in expanded polystyrene molds which accelerates the rate of strength gain at early age and of testing the cylinders at 48 hours (24 hours). A total of 37 different mixes and 18,908 cylinders test results from four suppliers using different brands and types of cements and admixtures were studied. Particular attention was given to the influence of initial concrete temperature on thestrength prediction. Under the conditions prevailing during this study, the evaluation of the field test results indicate that with Type I cement, a) the 28-day strength can be predicted with a high degree of confidence, from the 2-day self-cured accelerated concrete strength test, when the relationship has been established with several cement factors or strength levels; ` within standard temperature placing limits of concrete 50 to 9OoF (10 to 32oC), the predicted results are consis-tent with the behaviour of concrete under those conditions; c) the addition of initial concrete temperature as a variable allows a better estimate of the 28-day strength although the improvement is not very significant from a quality control stand point. With Type II cement, prelim-inary test results obtained indicate that the strength prediction is not suitable without modification of the present method. Further studies are required for low heat cements.

A study of the method for prediction of potential strength of concrete based on the maturity concept is presented. The maturity which is expressed as the integral of the curing temperature with respect to time, is related to the compressive strength of standard cylinders cured at 33 oF (1.7 "C), 55 OF (12.8 "C), 73 OF (22.8 "C), and 90 OF (32.2 "C). The relationship between the compressive streng and maturity is obtained by regression analysis. Other published dat are also used in the analysis of the relationship. It is shown that the function relating the compressive strength to the logarithm of maturity is nonlinear and that the relationship is dependent on the strength-gain characteristics of cement and on water/cement ratio.

Description is presented of a new method and apparatus for accelerated strength testing of concrete. The method consists of subjecting the fresh concrete mixture to pressure and elevated temperature to accelerate curing. In a preferred embodiment, prediction of the 28-day strength is provided in about 5-hours. The apparatus comprises a cylindrical container with piston closures, a means to pressurize and seal the container and heating means to heat the sample within the container. Preliminary tests show that the relationship between the accelerated cured (X) and 28-day standard cured strength (Y) can be represented by the linear equation: Y = 1570 + 1.42X 500< X < 3400 psi Y = 105.9 + 1.42X 35< X < 239 kgf/cm 2 with an accuracy of + 15%. This relationship appears to be independent of the type of aggregates and admixtures used.