International Concrete Abstracts Portal

This paper presents possibilities of use of fly ashes from co-burning bituminous coal and other fuels in cement production process. Both fly ashes coming from co-burning bituminous coal and biomass and the ones from coal combustion were analysed. The physical and chemical properties of the fly ashes were examined by determination of fineness, chemical and phase composition, pozzolanic activity and structure of the glassy phase. Cement samples with different content of the fly ashes were prepared. The following properties of the samples were tested: porosity, compressive strength as well as heat of hydration. The results show that cement samples containing fly ashes from co-burning bituminous coal and biomass had demonstrated adverse features like higher porosity, lower compressive strength after specified ages, than the ones containing fly ashes from bituminous coal combustion. The investigations of microstructure of the cements were also carried out by SEM.

This paper reports high-strength concrete behavior subjected to temperatures up to 200 °C and 100 freezing and thawing cycles in regime of 8 hours in water at + 20 °C and 16 hours at - 20 °C (weekends in frost). The concrete is composed of 425 kg/m3 of portland cement of CEM I 42.5 type, 32 kg/m3 of silica fume, 5.6 L/m3 of super plasticizer Melment and has a W/C of 0.32. Compressive strength is 78.5 MPa at 28-day curing on cubes for temperature resistance tests and 63.1 MPa on prisms for freezing and thawing tests, both after 28-basic curing in 20 °C/100 % R. H. - air. Evident C-S-H dewaterization of the cement paste is observed between 100 °C and 200 °C. Initial shrinkage within 24-hour period due to rapid cooling is more detrimental on the cement paste strength than shrinkage due to C-S-H dewatering at temperature elevation from 100 °C to 200 °C. The strength, elastic modulus and volume deformation of concrete are irreversibly influenced either by temperature elevation or rapid cooling to 20 °C. Differences in strength, elastic modulus and shrinkage or expansion after 100 freezing and thawing cycles relative to those in water are negligible. The compressive strength of prisms subjected to 118-day freezing and thawing was 62.9 MPa, compared to 65.2 MPa for those kept in water.

This paper presents the results of an experimental study to develop a new accelerated Rapid Chloride Permeability Test (RCPT). This research was conducted as part of a project for the South Dakota Department of Transportation (SDDOT). The present ASTM C1202 test method "Electrical Indication of Concrete’s Ability to resist Chloride Ion Penetration" recommends that concrete specimens shall be cured for 56 days prior to testing them for chloride permeability. This curing period would have to be further extended to 90 days for high performance concretes made with fly ash and silica fume. Curing concrete specimens for 90 days would allow the internal microstructure of concrete to fully develop due to conclusion of the hydration process. Therefore it is essential to wait for 90 days to determine accurately the chloride permeability of concrete, unless there is another method, which can be used to accelerate the curing process. This study aims to use an accelerated curing process to determine the chloride permeability values within a shorter duration. Two identical batches of concrete specimens were made with various quantities of silica fume and fly ash. One batch was subjected to standard curing for 90 days and the other was subjected to accelerated curing for 7 days prior to testing them for Rapid Chloride permeability using ASTM C 1202 test method. A mathematical equation was developed to establish a relationship between permeability values of standard and accelerated cured specimens. Statistical analysis was done to validate the obtained relationship. This relationship can be used to predict the 90-day chloride permeability values within a time frame of 7 days.

The construction of concrete slabs-on-ground in Australia requires often requires good control of setting times and bleeding rates in lean concretes using air entraining admixtures to minimize the potential for plastic cracking. Whilst this has been successfully achieved on the East Coast of Australia for decades with fly ash concretes a new source of fly ash from the Collie power station located on the West Coast proved difficult to use. The University of New South Wales undertook an investigation into the cause of sensitivity and variability exhibited by key properties of concretes that contained Collie fly ash. Testing of the basic properties of three Collie fly ash samples and one sample of fly ash from Eraring power station was used to characterize the materials and the samples separated into specific size fractions with further testing undertaken. Testing was undertaken included LOI, density, colour, fineness, carbon content, sulphur content, and physical features using SEM. The investigation concluded that the causes of the admixture sensitive behavior of Collie fly ash concrete is not simply due to the presence of carbon but the high surface area and low density of these carbon particles in association with the presence of oxidisable sulphur compounds.

This paper addresses kinetics and strength build-up of pozzolanic reactions in 2-inch-cubes of biomass, coal fly ash/Ca(OH)2 (CH) and sand mortar. A comprehensive experimental design of six biomass/coal fly ashes, three temperatures, three mass ratios of fly ash/CH and six test dates of up to 1 year was set up. The results show that the compressive strength of biomass fly ash samples exceeds that of coal ash samples by factors of 2-3 and rivals that of pure cement ones. Further investigations indicate that except CH reaction extent, other factors, such as fly ash type, mixing ratio of fly ash with CH and curing temperature, all have significant effects of compressive strength build up of fly ash samples.