Accelerated testing of concrete by means of boil-ing test cylinders and testing them at the age of 28 1/2 hours to predict the 28 day strength was used by ready-mixed con-crete producer's quality control laboratory for about two years. Over 300 accelerated tests were carried out indi-cating that 28 day strength can be predicted using this method within the accuracy of +12%. Continuous updating of the accelerated to 28 day strength relationship is neces-sary. The suitability of the method for day-to-day quality control purposes is evaluated.

In recent years there has been an increasing acceptance of accelerated strength tests for routine quality control of concrete and to estimate the 28-day compressive strength. However, very little, or no data, are available as to the use of accelerated strength tests for estimating the potential splitting-tensile strength and modulus of rupture of concrete. This study reports results of an investigation to determine the possibility of using the boiling procedure as an accelerated splitting-tension test. A total of twenty-two concrete mixes were made in the laboratory using limestone and natural sand as coarse and fine aggregates respectively. A total of 176 cylinders, 6 x 12 in. (152 x 305 mm) in size, and 44 prisms, 3.5 x 4 x 16 in. (89 x 102 x 466 mm) in size, were tested. The cylinders were tested in splitting-tension after accelerated- and moist-curing, and the prisms were tested in flexure after moist-curing. The correlations between the splitting-tensile strengths of accelerated-cured specimens and those of moist-cured specimens were statistically significant. The average within-batch variation for the splitting-tensile strength of accelerated-cured specimens was 5.1 per cent; the corresponding value for the strength of the 28-day moist-cured specimens was 5.7 per cent. From the analysis of the test results, it is concluded that the accelerated splitting-tensile test appears to be an adequate means for controlling the quality of pavement concrete. Those contemplating the use of the accelerated test for predicting the later-age splitting-tensile and flexural strengths of concrete are cautioned that they should develop their own correlations to allow for the variations in aggregates and cements.

Accelerated curing and testing of concrete cylinders came into being because of the need for faster evaluation of the quality control of the concrete, as a result of accelerated construction sched-ules and increased volumes of concrete required in structures, so that it was not practical to await the standard 28-day strength results. This same speed-up of construction and increase in concrete vol-umes involved in structures, brought about faster or early evaluation needs, and the maturity concept of concrete (degree-hours) is supple-menting and displacing the accelerated tests. The continuation of this faster trend and increasing volumes has brought about immediate evaluation while the materials are still in the weighing hopper or mixer, so that if a batch is out of tolerance it can be dumped out, instead of sent out to the job. To further meet today's needs, continuous mixing plants are appear-ing on the scene. Their virtues are lower capital costs, reduced variability of the process, and thus possibility of reduced cement content, lower operation and maintenance, and more satisfied operators. And just below the horizon, as the next improvement, is a process that forcibly mixes the water and cement, so that every grain of the latter is hydrated, as against only partially hydrated in existing mixing processes, thus permitting still further reduction in cement content. This particular process is also the cheapest way to eliminate cement dust around concrete plants.

The accelerated curing methods recommended by the American Society for Testing and Materials were adopted for estimating the potential strength of con-crete used in the western South Dakota. Cylinders were made and tested from 21 different batches of concrete used in the construction of buildings, bridges, and pavements. These concretes were supplied by two local ready-mix concrete producers, one using the central-mixer technique and the other using the transit mixer operation. Ranges of variables included in this investigation were, types of cements (Type I, Type II, and Type K), types of molds (steel, plastic and cardboard), water-cement ratios (by weight) 0.41 to 0.72, aggregate-cement ratios (by weight) 2.5 to 4.1, maximum size of coarse aggregate 1 inch, and the 28-Day Compressive Strengths 3400 to 6800 psi (23500 to 46900 kPa). An equation and correlation curves are presented for the locally used materials and mixes relating the strength of Type I, Type II and Type K cement concretes obtained in the accelerated curing methods and the 28-day strength obtained with standard curing conditions. Analyzing the results from this investigation and the results from other investigators from various parts of the world, an equation applicable universally with reasonable ac-curacy is presented for estimating the potential strength of concrete using the boiling water method.

This paper presents modified boiling water methods (M.B.W. Methods) and discusses how these methods can be adapted for regular quality con-trol and quality assurance of concrete for small as well as large pro-jects. These M.B.W. Methods were found to be more suitable than other A.S.T.M. methods. Accelerated strength test data can be used in two ways: (1) for quality control, where actual accelerated strength can be used as an established target value; and, (2) for quality assurance, where, if required by the contract specification, the future strength, e.g. the 28-day strength, can be projected from the accelerated strength. In order to predict the future strength, a basic correlation curve must be established for a given set of conditions. A minimum of ten to fifteen sets of data were found to be adequate in order to es-tablish this correlation curve. The A.S.T.M. boiling water method schedule requires the strength test to be performed at 28-l/2-hour age. In order to provide the greatest flexibility in scheduling, and to eli-minate the overtime work associated with the 28-l/2-hour test, modified boiling water methods were developed in which the test is performed at either 24-hour, 28-hour, 48-hour or 72-hour age. Adaptation of these four modified boiling water methods not only eliminated overtime work required on a regular working day, but it also eliminated work on holi-days and weekends. This study showed that any one of the commergially available single use cylinder molds can be used for the M.B.W. Methods. The single use tin molds, however, were preferable. An accelerated strength testing program, along with the reliability of results obtain-ed, is discussed in detail in this paper. Some of the more important observations given in this study are: (1) overtime work can be virtu-ally eliminated by adapting the methods given; (2) influence of initial concrete temperature had minimal effect upon the accelerated strength test value; (3) a job site testing laboratory is not needed; and, (4) the danger of being exposed to steam emanating from the boiling water tank and the danger in handling boiling cylinders was ill-founded.