International Concrete Abstracts Portal

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

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.