International Concrete Abstracts Portal

High-performance concrete (HPC) has been studied extensively at many research centres, because of its increasing use in reinforced concrete buildings. Since HPC is a brittle material, studies have been done to increase its ductility. Increases in longitudinal and /or transverse steel ratios can improve the ductility of HPC elements. The addition of fibres also increases the deformability and thus the ductility. Hence, the transverse steel ratio can be reduced by using fibres. This paper presents a study of axially loaded columns made with high-performance concrete containing steel fibres. The average compressive strength of the concrete was 80 Mpa. The volumetric ratios of fibres were: .25%; .50% and 1.00%, and the stirrup ratios were .55% and .82%. The longitudinal steel ratio was the same for all columns tests, the W/C was .37, 10% silica fume was added and it was also necessary to use about 3% superplasticizer to improve workability. A comparison was made between the results for columns in high-strength concrete with and without fibres. It was observed that only the cross-sectional core effectively contributed to the load capacity of the columns.

This paper is concerned with the structural behavior of reinforced high-strength concrete corbels. The flow of internal forces and the solution of the problem are discussed. A design approach based on strut-and tie models is proposed. The paper presents the results of experimental investigation conducted with reinforced high-strength concrete corbels and give explanation for them by means of the proposed model. At the end it is shown that, when proper main and web reinforcements are provided, the failure by yielding of the main reinforcement can be achiev3ed and that the models originally developed for normal-strength concrete corbels are suitable to those made with high-strength concrete.

This paper deals with measurement of concrete durability by means of initial absorption test of the concrete surface. The criterion used for this analysis is the amount of water absorbed after one hour. The specimens were placed in a constant water level of 1cm, and the quantity of absorbed water was measured by mass. It was observed the influence of cement type, temperature, relative humidity and surface protection in the initial absorption. Two concrete mixtures were made with ordinary portland cement blended with calcareous filler (12% and 25% by mass). These concretes were cured at two temperatures (20 degrees C and 40 degrees C) and different relative humidities (30%, 60%, and 100%). In some specimens a curing compound was used to observe its influence on surface protection from initial water absorption. The concrete strengths ranged from 28 to 62 Mpa. The experimental results showed that by changing the relative humidity from 60% to 100% during 7 days, the initial absorption can diminish by a factor of 2. The curing temperature can increase the initial absorption by 64%. The curing compound have a real influence in the concrete initial absorption.

This paper presents the results of laboratory studies conducted to determine freezing and thawing and scaling resistance of high-performance concrete. High-performance concretes were made using a combination of different cementitious materials (Blast-furnace slag and silica fume). The water-to-cementitious materials ratio was .27, and the bulk volume of coarse aggregate and fine aggregate per unit volume of concrete were fixed at .50 and .60, respectively. All mixtures used a superplasticizer and were non-air-entrained. Test cylinders were cast for testing in compression at 1 and 28 days, and test prisms were cast for determining resistance to freezing and thawing cycles in accordance with ASTM C 666, Procedure A. and for resistance to scaling from deicing chemicals according to ASTM C 672. The curing methods were water curing and steam curing. The air-void parameters of the hardened concrete were determined on the sawn sections. The test results indicate that non-air-entrained, high-performance concrete with steam curing showed low durability factors. High-performance concrete with water curing performed satisfactorily when subjected to up to 1500 cycles of freezing and thawing. Water-cured, high-performance concrete showed no appreciable scaling after 100 freezing and thawing cycles, showing high resistance to scaling.

Attempts to study the effect of vibration of fresh concrete have mainly been based on visual observation of, for example the radius of influence of the insertion vibrator, or the rate of flow of concrete down a tube when vibration is applied. The reason for this has been the difficulty of measuring the sinusoidal wave form created by mechanical vibrators. Advances in electronic equipment have made devices for measuring this wave form commercially available, and they have therefore been used in this research project to gain a better understanding of the consolidation process. The amplitude of the sinusoidal signal was calculated from the acceleration measured at distances up to 250mm from the surface of the insertion vibrator. Preliminary tests indicate that the amplitude of the vibratory wave decays exponentially with distance. The damping coefficient is greater for superplasticized high-strength concrete mixtures with low W/C than it is for normal-strength concretes. An attempt was made to relate the damping coefficients to the rheological properties, yield (g) and plastic viscosity (h) values determined from tests carried out with Tattersall's two point test apparatus. Both the yield (g) and plastic viscosity (h) values were found to increase by decreasing the W/C, despite the concrete having an equal slump of 150 mm. This shows that the slump values obtained by the use of high dosages of superplasticizers, as is the case with low W/C, are not directly comparable to those resulting from high water contents, with respect to the rheological behavior of concrete.