International Concrete Abstracts Portal

This Symposium Publication includes 30 papers selected from the Third Conference on High-Performance Concrete (HPC) and Performance and Quality of Concrete Structures, held in Brazil. Topics covered include chloride penetration models in reinforced concrete structures, high-strength concrete with crushed and natural sand, sewage sludge ash as an addition in concrete, and the mechanical properties of polymer-modified high-performance lighweight aggregate concrete. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP207

Some of the most recent developments related to the production of concrete have focused on the addition of components which can improve the mechanical, wokability and durability properties of concrete and whenever possible, to solve environmental problems in a simple and economical way. This is research work fits in this field, trying to contribute to the clearing up of the advantages and disadvantages of concrete production with the addition of fly ash (FA). High-performance concrete (HPC) is usually produced using high quality materials. These constituents drastically increase the initial cost of HPC, thus hindering its more widespread usage. This research work intends to investigate the possibility of producing low cost enhanced performance concrete or even low cost HPC, with 90 day strengths in the range of up to 60 MPa, using low quality fly ash and locally available crushed aggregates. The effect of the amount of fly ash was evaluated using 0, 20%, 40% and 60% cement replacement with different quantities of total binder of 400 kg/m3, 500 kg/m3 and 600 kg/m3, Workability, mechanical and durability properties were also studied. The results obtained indicate that it is possible to, produce HPC with up to 60 MPa by replacing up to 40% of cement by fly ash and using local available crushed granite aggregates. Furthermore, it was observed that the workability and the durability, as measured by the chloride-ion difision coefficient, increased drastically when fly ash partially replaced Portland cement. Based on the results obtained, it is possible to conclude that the use of fly ash in concrete is beneficial in terms of the workability and durability properties but was some disadvantages because early strengths are reduced.

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.

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 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.