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.

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.

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.

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