International Concrete Abstracts Portal

Concretes having strengths ranging from 54 to 73 MPA and densities ranging from 1920 to 2080 kg/m3 were produced from two lightweight aggregate sources. Supplementary cementitious materials (including silica fume, fly ash, and ground granulated blast furnace slag) were used in the mixtures. Test specimens were subjected to a variety of freezing and thawing test procedures and conditioning methods. These included standard ASTM test procedures as well as procedures designed to simulate service in arctic offshore environments. The high-strength lightweight concretes exhibited outstanding performance, with virtually no degradation during standard freezing and thawing testing. Prolonged exposure was needed to cause significant damage under simulated arctic offshore conditions. Durability was found to be a strong function of cumulative freezing and thawing cycles and moisture content, with saturation of aggregates prior to test leading to premature failure.

Roller-compacted concrete (RCC) is a non-air-entrained concrete of no-slump consistency, which is placed by spreading in horizontal layers and consolidated by smooth-drum, vibrating rollers. Though used in other applications for many years, RCC in its present form has been used to construct concrete gravity dams by taking advantage of the high production rates and attendant cost savings associated with the use of earthwork equipment to transport, place, and "compact" the material. RCC is finding increasing use in thick paving applications, taking advantage of the high production rates of asphalt paving equipment. The earlier attempts to entrain air in lean, dry RCC mixtures were nsuccessful, and the freeze-thaw (FT) durability of RCC was considered poor. Dam structures were designed so that the RCC was protected from weathering by zones of conventional air-entrained concrete, or "sacrificial" RCC was placed beyond the design cross section. This raised the cost of these structures due to the higher unit cost of the conventional concrete or the added costs associate with the larger cross section. With the expanding use of RCC, particularly in paving applications, it is desirable to attempt to develop air entrainment to satisfy FT durability criteria and allow RCC to fully complete with conventional concrete and asphalt in the marketplace. The Bureau of Reclamation, Denver, Colorado, has been investigating the FT durability of RCC through laboratory testing, outdoor exposure testing and recent field demonstrations. The goal of the program is to reduce the need for conventional concrete facing and expand the applications of RCC where FT durability is a requirement, such as in the facing of embankment dams. This paper summarizes the results of testing performed to date in the laboratory and in the field.

Silica fume and/or fly ash concretes were produced to determine the long-term effects of these pozzolans on chloride ingress, electrical resistivity, microstructure, and the subsequent effect on corrosion of embedded steel reinforcement. Initial results of these experiments showed that there was a good relationship between the 28-day compressive strength, rapid chloride permeability, and electrical resistivity. In this paper the long-term corrosion performance, electrical resistivity, chloride ingress, and microstructure are examined after 3 years of partial ponding in 3 percent NaCL. These results are compared to the initial concrete properties to determine how well early measurements of the properties relate to long-term corrosion resistance and chloride ingress. Also, long-term changes in the concrete microstructures are documented. The results show that concrete resistivity and rapid chloride permeability measurements are better indicators of corrosion resistance than concrete strength. In general, increasing silica fume and/or fly ash contents and/or reducing water-cementitious ratios improves corrosion resistance. However, for mixes with over 20 percent pozzolans by mass of cement, some carbonation has occurred next to cracks where the reinforcement exited (the nonsubmerged sections of the samples). The significance of these findings is discussed.

Exposure tests of silica fume concrete with embedded steel bars were carried out in marine environments such as the Inland Sea of Japan, the Pacific Ocean, and the Sea of Japan, in the Kansai district. The effects of water-to-cementitious material ratio, silica fume content, chloride ion content in mixing water, and concrete cover on the chloride corrosion of reinforcing steel were studied by measuring half-cell potential, electric resistance, pH value, depth of carbonation, pore volume, and chloride ion content. When tap water was used as the mixing water and concrete cover was 10 mm, the longitudinal crack due to chloride corrosion was observed in silica fume concrete specimens in about 3 years. Chloride ion penetration into silica fume concrete was much lower in comparison with concrete without silica fume, however, chloride at the region 2 cm from the concrete surface was high enough for embedded steel to corrode. When concrete cover was 25 mm, no longitudinal crack was observed in silica fume concrete specimens until about 3 years. It is necessary to keep sufficient concrete cover, even in silica fume concrete. Chloride corrosion in concrete was accelerated by using silica fume when saline solution was used as the mixing water.

Discusses the utilization of silica fume concrete admixture to prevent reinforcing steel corrosion. The mechanism of steel corrosion in salt-impregnated concrete is described, along with laboratory test date showing how ordinary concrete's corrosion-prone characteristics are altered by the use of silica fume. The mineral admixture significantly lowers the concrete permeability to prevent chloride ingress to the reinforcing steel level, while simultaneously increasing the concrete's electrical resistance to corrosion currents. Test data from the FHWA 90 day Chloride Ponding Test indicates a 98 percent reduction in chloride penetration. The AASHTO T277 rapid chloride permeability test shows a 10-times impermeability and 25-times resistivity improvement with the use of 12 percent silica fume. T he Time-to-Corrosion FHWA/NCHRP 244 slab test is scaled-down steel-reinforced deck, from which macrocell corrosion current, AC resistance, half-cell potential, and chloride absorption are measured. Zero corrosion current was measured after NaCl was ponded in alternate soak/dry cycles for 48 weeks. The second phase test program evaluated the corrosion performance of full-sized concrete bridge sections, including beams, columns, piles, and bridge deck panels. The test members were subjected to environments simulating salt water and deicing agents for 370 days. Test results show that silica fume admixture prevents salt-induced corrosion of steel a reinforcing bar and tensioning strands.