International Concrete Abstracts Portal

This paper deals with trial testing on concrete reinforced by different combinations of carbon-steel, polypropylene-steel or steel-steel hybrid fibres in the low volume content range to ensure high electrical resistivity of the fibre concrete. Optimum reinforcement composition is obtained for (compression, splitting tension, and flexural) strength testing conditions and for post-peak behaviour. The effect of various measures adopted to promote uniform fibre distribution are assessed by impedance measurements. During the testing program, fibre distribution is checked by microscopy and SEM.

The properties of concrete containing foundry sand as a partial replacement of fine aggregate were investigated. Three types of sand used in foundries were considered, the white fine sand without the addition of clay and coal, the foundry sand before casting (blended), and the foundry sand after casting (spent). The standard sand (Class M) was partially replaced by (O%, 25%, 50%, 75% and 100%) these types of sand. Thirteen concrete mixtures were employed to conduct this study. Concrete strength up to 90 days and length change (drying shrinkage and expansion) up to 60 days were determined. As the replacement level of standard sand with sand used in foundries increased, the strength of concrete decreased. Concrete containing white sand showed somewhat similar strength to those containing spent sand at all replacement levels. The presence of high percentage of blended sand in the concrete mixture caused a reduction in strength as compared with concrete incorporating white sand or spent sand. The increase in strength was not observed at low replacement levels (less than 50%). The length change of concrete increased -as the replacement level of standard sand with the three types of sand increased. Drying shrinkage values were higher in concrete containing spent sand and lower in concrete containing white sand. Expansion was generally lower in concrete containing white sand as compared with the other two types (blended and spent) at a low sand replacement level of 25%; and, different trend was obtained at higher levels.

The maturity approach has been used conventionally to model temperature effects on the development of concrete compressive strength. Its application to concrete technology, however, goes far beyond simply estimating compressive strength. When the maturity approach is based on cement hydration kinetics, it can be applied to any concrete property related to the extent of cement hydration. In this study, the application of the maturity approach to model the development of various concrete properties was investigated. In addition to compressive strength, other properties evaluated included degree of hydration and ultrasonic pulse velocity. Hyperbolic equations were investigated for the development of each model. Different values of apparent activation energies (Ea) were calculated according to the procedure in ASTM C 1074. It was concluded that E, is an indication of the thermal sensitivity of the concrete property investigated. The calculated value of E, depends on the specific property and the maturity model used.

Recycled aggregate concrete was used as structural concrete at site under the strict quality control system especially on water content in concrete. Two innovative methods were applied for this project: one is the rapid test method for water absorption of aggregate, and the other is the continuous real-time monitoring of water content of freshly mixed concrete at site using a radio isotope (RI) moisture meter. With regard to the rapid test method for water absorption of aggregate, hot water and pressure were applied for the test. By the rapid method, test results could be obtained within two hours. This method is very useful in water content control of concrete produced at mixing plants, especially for concrete with recycled aggregate, water absorption of which generally fluctuates more than that of normal aggregate. As for the water content control using RI moisture meter, water content of freshly mixed concrete transported to the site was being continuously monitored during pumping by a meter attached to the transporting pipe of the concrete pump. Neutron radiation intensity measured varies with the water content of concrete. Every 120 seconds, measurements were used for running mean of water content. According to some experimental studies on this method, standard deviation of the water content of concrete is about 3 kg/m3. In this paper, outline of above-mentioned methods and their application at site are described.

The Reactive Powder Concretes (RPC) are well known for their high strength, but they also present good performances regarding durability aspect due to their very dense microstructure. Studies were made to evaluate the durability of RPC to : freeze-thaw scaling, carbonation, chloride ions diffusion, abrasion coefficient and shrinkage. Their very good performance characteristics have led to a specific research programme to evaluate the long-term durability of RPC for nuclear waste containment. Several kinds of potential degradation are studied, but here only the attack by de-ionised water, a very severe leaching test, will be presented. After pre-determined periods, the material was analysed regarding its microstructure (altered depth, composition and mineralogical changes), porosity and ion transport properties. The results of SEM, X-Ray diffraction, mercury intrusion porosimetry, BET pore-size and tritium diffusion coefficient analyses are presented.