International Concrete Abstracts Portal

The diffusion mechanisms of chloride ions into ordinary and high performance mortars were studied. Four different mortar mixtures were tested. Test parameters included the water/binder ratio (0.25 and 0.45) and the use of silica fume. An ASTM type III cement was used in the preparation of the 0.25 water/binder ratio mortars while the 0.45 water/binder ratio mixtures were prepared with an ASTM type I. For all mixtures, the sand volume fraction was maintained constant at 50%. The diffusion properties of the mortars were studied according to two different experimental procedures. In a first series of tests, apparent diffusion coefficients were calculated from chloride ion profiles measured after a 12-month immersion period. In a second series, a migration test (where the chloride ion penetration is accelerated by the application of an electrical potential of 10 volts) was used to investigate the transport properties of the four mortars. All test results clearly show that the reduction of the water/binder ratio and the use of silica fume contribute significantly to the reduction of the chloride ion penetration. The consequences of these results on the long-term durability of high-performance concrete structures and, more specifically, on their ability to resist to reinforcing steel corrosion are discussed. The ability of the accelerated migration test to reliably predict the penetration of chlorides in cement-based materials after only a 14-day test period is also discussed.

High performance concretes (HPC) are increasingly used in high-rise building columns. Some creep and shrinkage data of HPC has been published based on laboratory tests. Very few long term field results are available. During the construction of the new library of Concordia University (Montreal, Canada) in 1990, two pairs of circular 850 mm diameter reinforced, concrete columns made of lOO-MPa silica fume concrete and of 80.MPa non silica fume concrete were instrumented with vibrating wire extensometers. One column (called active column) of each pair was part of the structure; the other (called mockup column) was never loaded and used for shrinkage and thermal strain measurements. Strains and temperatures at different locations in concrete and in reinforcing bars as well as load in concrete were recorded for about 5 years. Data obtained on these HPC columns is presented in this paper. Creep, shrinkage, thermal expansion coefficient of the two field concretes are discussed. Stresses in concrete and in reinforcing bars are analyzed. Axial loads in active columns are calculated from experimental data and compared with the specified loads used in the structural design.

The ratio between the in-place compressive strength of high performance concretes and the strength of standard 28-day cylinders is investigated. Strength data for 771 cores from 3 1 large elements cast using 22 concrete mixes reported in five investigations by others are analysed. It is observed that the ratio of in-place strength to standard cylinder strength decreases as the maximum temperature sustained during hydration increases. If the concrete mix contains silica fume, Class C fly ash, or slag, the ratio of the in-place strength at 28 days to the standard 28-day cylinder strength of the same concrete is markedly less than that observed for concretes which do not contain supplementary cementitious materials. In all elements investigated, the average in-place strength continued to increase after 28 days. The relative strength gain of silica fume concretes after 28 days was significantly less than that of conventional concretes.

This investigation was performed to access the applicability of conventional maturity functions to high strength concretes incorporating silica fume and superplasticizer. Concrete specimens were allowed to cure under three temperatures simulating hot weather, laboratory, and cold weather conditions. Both linear and exponential strength-maturity functions predicted different values of strength for different concrete curing temperatures for the same value of maturity, as has been observed by other researchers. Of these two unctions, the exponential performed somewhat better in this regard for elatively low values of maturity. Concrete strength-gain behavior was influenced by the presence of silica ume and the high amount of super-plasticizer in the mixture. Strength-maturity equations already developed for normal strength concrete underestimated strength at low maturity ages and overestimated strength at high maturity. It is suggested that further studies should be done to evaluate such effects in those relationships.

A public-private partnership has been chosen to ignite the introduction of RPC into the United States construction market. This research and development project is being conducted under the Construction Productivity Advancement Research (CPAR) program of the US Army Corps of Engineers. The project was initiated in the fall of 1994 and it will run for three years. The program goal is to verify product integrity and gain industry acceptance and commercialization by developing and demonstrating the technical and economic viability of RPC for producing culvert/sewer pipes, pressure pipes and piles. T h e primary technology transfer has been completed, US component material source identification has been brought into action and material property verification has been initiated. Other US products development efforts have been initiated. These applications include : 0 0 spun cast concrete poles, impact resistant railroad ties and grade crossing planks.