International Concrete Abstracts Portal

The use of high strength concrete has increased progressively over the past years not only for benefits of strength but also for significant improvements in service life. Nevertheless increases in strenght lead to a more brittle behavior of the material. Steel fiber reinforcement is probably the best way to improve its performance when higher toughness is required. This paper discusses the contribution of fiber reinforcement in high strength concretes. Load-deformation curves under compressive and flexural loads of concretes prepared with different types and contents of fibers are compared. The behavior of sound and microcreacked concretes exposed to high temperatures is also studied. The effect of fiber reinforcement on the compressive behavior of high performance concrete was similar to that observed on normal concrete. Fibers incorporation enhance crack control and produced significant benefits in toughness. Flexure tests performed on fiber reinforced concretes were very stable. It was possible to use different specimens sizes and loading configurations to evalute the effct of the type and content of fibers. There were some improvements when high carbon steel fibers were employeed.

Uniaxial tension tests of fiber reinforced concrete have been performed for determining the influence of several geometric parameters on the experimentally-obtained behavior. Molded cylinders were used as well as cores and panels cut from companion prisms. All specimens were notched at mid-height and the average crack opening was used for controlling the tests. The results show that the influence of the notch depth and specimen length on the stress-crack opening response is negligible, within the range studied. Comparisons with the results of cores demonstrate significant differences in the fracture behavior due to the preferential orientation of the fibers produced during compaction. It is also shown that the panels do not exhibit a representative behavior, especially when thin specimens are considered.

The use of calcined clay as a pozzolanic partial cement replacement material in concrete is currently receiving increased attention. Previous work by the authors has demonstrated the effectiveness of utilising ground waste clay bricks in mortar and concrete. This paper presents the results of an investigation of the properties of mortar in which a calcined clay was employed as a pozzolan. The clay was heated in crucibles to 800°C with a heating ramp of 100°C per hour. The furnace environment was then kept constant at 800°C for a period of two hours. After heat treatment the clay was cooled in two different ways. One batch was allowed to cool naturally to room temperature and the second was water quenched with the crucible lid on. Mortars were prepared using either the heat treated clay or ground waste clay bricks (from the same clay subjected to 1000°C calcining) as a pozzolanic partial replacement for cement at replacement levels of 10, 20 and 30%. The compressive strengths of the mortars were monitored up to 90 days and the resistance to sodium sulfate solution and synthetic seawater was monitored up to 300 days. The specimens were also monitored for weight changes. Partially replacing cement by ground brick or heat-treated brick clay gives early strengths that are lower than that of the control. At 90 days, however, the strengths are the same as or greater than that of the control. Heat-treated clay is effective in reducing expansion during exposure of the mortar to sulfate solution and synthetic seawater. The rapidly cooled clay gives better performance, in terms of strength development and resistance to harmful solutions, than the slow cooled clay.

The concrete studies for the Cana Brava Hydroelectric Plant were conducted on the basis of the following provisions: the prescriptions of the project’s Technical Specifications, with emphasis on the durability of the structures; the control of thermal-originated cracking; the use of alkali-reactive aggregates and on the Construction Consortium’s plan, which prioritized pumped concrete. These aspects led to the choice of a cementitious material ‘composed of blast furnace slag cement with addition of silica fume in all concretes used in the project. Therefore, this work will focus on the study of mixtures and the characterization of the Cana Brava concrete, both of which were held to meet the project provisions. The studies showed that the use of silica fume, through its beneficial effect in increasing concrete strength, enabled a reduction in the amount of cementitious materials, thus benefically contributing to thermal cracking control; furthermore, it proved advantageous in preventing the alkali-aggregate reaction and in reducing permeability.

The results of an experimental investigation on the behavior of high-performance concrete subjected to biaxial tension-compression stresses are presented. Short-term static tests were performed on 125 mm square by 12.5 mm thick plates. Strain controlled tests were executed in a biaxial testing machine constructed at the University of Texas. The primary studied variables were the discontinuity and the ultimate stress levels at each stress ratio. Results indicated that even small amounts of tensile stress reduced the ultimate compressive strength of the specimens substantially. The failure mode of the plate specimens fell basically into one category: tensile splitting in a plane or planes perpendicular to the direction of the principal tensile strain. The failure surface contained both fractures through coarse aggregate and mortar. These results suggest that the failure criteria for high-performance concrete, under biaxial tension-compression, is a limiting value for the tensile strain. The magnitude of the failure tensile strains is not constant, but increases with the degree of compression.