International Concrete Abstracts Portal

Silica fume (SF) and high reactivity metakaolin (HRM) are two highly reactive pozzolans that offer excellent potential for use in high-performance concrete since concrete mixtures containing them demonstrate superior performance in terms of strength, and durability. High-performance concrete applications, such as pavements and bridge decks, are also required to demonstrate superior performance against early age shrinkage cracking. This paper describes a comparative study of the effects of SF and HRM on the early age stress development and cracking in restrained mortar mixtures due to shrinkage. The restrained ring test was used to assess early age residual stress development in mortar ring specimens. In addition, free shrinkage strains and splitting tensile strength measurements were performed to assess the cracking potential. It was found that the addition of SF and HRM increased the shrinkage level in the mixtures which resulted in increases in residual tensile stress development due to restraint. In addition, their addition in the mixtures increased the cracking potential and resulted in early cracking in the ring specimens.

Ten reinforced concrete blocks (1.5 x 1.5 x 0.5 m) were cast during the autumn of 1982 and exposed in the tidal zone in the Trondheim fjord in March 1983. The blocks were made of concrete with two different strength grades and with 0, 10 and 20 % silica fume addition. The blocks have been subject to several investigations during the years, especially with respect to chloride intrusion. The last examination was executed at approximately 21 years of exposure. The results indicate that the activity factor of silica fume regarding resistance against chloride intrusion is in the same order as for compressive strength (approximately 3). The chloride diffusion coefficients have been calculated for several exposure periods. This makes a calculation of the aging effect, and long term development of the effective chloride diffusion coefficient, possible. The concrete was composed according to the common practice in Norway in the early 1980s, and the concrete proportions are therefore hardly relevant to day. The effect from addition of silica fume with respect to durability and chloride intrusion for well documented, long term field exposed concrete is however considered very valuable.

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.