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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.

Over one million cubic meters of concrete were poured during the construction of a huge project known as Deep Drainage System designed to eliminate the risk of floods in Mexico City. The main structure of this system is a tunnel 49.8 km (30.95 miles) in length, 6.50 m (21' 4") in internal diameter, and with a 0.70 m (27 l/2") average thickness of concrete lining. The most outstanding part of the product control of concrete was carried out by testing specimens cured in boiling water (procedure B, ASTM C-684). More than 1700 samples, consisting each of four specimens, were tested. Two specimens of each sample were tested at 28 l/2 hours and two at 28 days. Functional relationships were establi the 28 day compressive strength from the 28 l/2 hour test, with very satis These relationships allowed to adjust proportions to the optimum amounts of in order to fulfill the strength requ specifications. shed for predicting the data obtained in factory results. opportunely the mix ingredient materials irements of the job

Recognition of the advantages of being able to judge the compressive strength of concrete much sooner than by the traditional 28 day test has led to a 50 year search for reliable and convenient accelerated strength testing methods. The autogenous curing method was used as an integral part of the concrete quality assurance program during construction of the CN Communications Tower in Toronto. The results presented which are obtained from 547 sets of cylinders cast during the placing of 40,000 cu.yds. (30,580 m3) of slipformed concrete to a height of 1500 ft. (457 m) between July 1973 and February 1974 indicate that the 28 day or 90 day compressive strength of the concrete made with either low heat or normal Portland cements or a blend thereof can be accurately predicted from 2 day autogenous strength results. Accelerated strength determination played an important role in the control of quality and the overall structural safety of the concrete shaft of the world's tallest free standing structure.

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.

During the construction of La Angostura Hydroelectric Pro-ject, owned by Comision Federal de Electricidad (CFE) of Mexico, quality control of concrete has been achieved by means of short-termed tests, in addition to the standard 28-day tests. Two types of short-termed tests were performed: analysis of fresh concrete samples according to a non-standardized method devel-oped at the laboratories of CFE, and autogenous curing compressive strength following Procedure C of ASTM Designation C 684. This paper presents the results obtained in these tests after a three-year period of application at the job site; they have been ana-lyzed statistically and correlations among them are discussed. It was concluded that fresh concrete analysis tests were rea-sonably accurate to determine the actual composition of freshly mixed concrete, but the results did not allow the prediction of the 28-day concrete strength, because of influence of changes in cement charac-ter: istics. The autogenous curing strength test represented a good means to predict the 28-day concrete strength for all types of mixes, save for 3-in. maximum aggregate size, possibly due to wet sieving.