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

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.

Data are presented covering the period 1974 to 1976 inclusive as collected by Manitoba Hydro and by R.M. Hardy and Associates Ltd. The former is a public utility and the latter a consulting engineering group. The Mani-toba Hydro results are from a project in Northern Manitoba utilizing CSA Type 10 (ASTM Type 1) portland cements manufactured in both Manitoba and Ontario. Results from the Hardy group were collected on a number of projects in Southern Alberta - principally sidewalk concrete control work in the City of Calgary and a gas plant near Pincher Creek, Alberta. Results from CSA Types 10 and 50 (ASTM Type I and V) portland cements are presented separately. Regression equations are developed and compared with equations prepared from earlier Western Canadian data.

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.