International Concrete Abstracts Portal

This research project was undertaken to determine the effect on the chloride permeability of concretes of increasing amounts of fly ash in low water-cementitious material ratio concrete as compared with comparable high-quality concretes containing combinations of portland cement, silica fume, and ground-granulated blast furnace slag. The test method utilized was the Rapid Determination of the Chloride Permeability of Concrete (AASHTO T-277). Fifteen superplasticized concrete mixtures were evaluated for compressive strength at ages of 28 and 56 days, and for chloride permeability at 56 days. The inclusion of fly ash, silica fume, and ground-granulated blast furnace slag all significantly reduced the chloride permeability of concrete as compared with concrete containing only portland cement. Increasing amounts of fly ash generally showed decreased permeability in the tests conducted.

Long-term exposure test results of reinforced concrete beams are reported. One hundred forty-nine pairs of beams with open cracks were exposed to sea air for 2 to 20 years. The main variables were thickness of concrete cover, type of cement, cement content (water/cement), and crack width. The type of cement has a great influence on the depth of chloride ion penetration. The thickness of concrete cover is the most important factor in the prevention of corrosion of the reinforcing steel. With a thin cover, the crack width has no influence on corrosion of reinforcing steel. Epoxy coating is effective in improving corrosion protection. Measurements of electrical potential on the surface of concrete give valuable information on corrosion activity of reinforcing steel.

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.

This paper addresses the effect of condensed silica fume on the factors controlling the two stages of corrosion of reinforced concrete. The corrosion-resisting capabilities of concretes with and without condensed silica fume were evaluated through impermeability, electrical resistivity, carbonation, chloride penetration, and soaking and stoving tests of reinforced concrete. In the case of concrete containing condensed silica fume, tests were carried out with various amounts of condensed silica fume. The test results show that concrete with condensed silica fume exhibits excellent corrosion resistance. A comparison between concrete containing condensed silica fume and conventional concrete for the same cement content shows the following: Use of condensed silica fume raises impermeability to 4 to 46 times and electrical resistivity to 2 to 9 times: the total charge passed decreases markedly after 6 hr of testing at 70 VDC; the silica fume concrete has a depth of carbonation of only 3.7 mm, while the conventional concrete has a depth of carbonation of 17.8 mm after 28 days of exposure to 20 percent carbon dioxide; and the concentration of chloride ions in the concrete surrounding the reinforcing bar is reduced 9 times after 22 soak-stove cycles in seawater. The durability of silica fume concrete in seawater is extended more than 2.2 times, as inferred from the time of initiation of corrosion. The paper gives a description of the application of concrete containing condensed silica fume to the Lianyun Harbor timber warf.

Studies of slag activation by alkalies have been carried out since 1973 at the Institute of Building and Refractory Materials, Academy of Mining and Metallurgy, in Cracow, Poland. Laboratory tests were followed by production of the activated slag on a large scale. It appeared that the new cementing material composed of the granulated blast furnace slag mixed with an alkaline activator showed high strength and corrosion resistance. The present work deals with the problem of reinforcing steel corrosion in the alkali-activated slag mortar exposed to the attack of concentrated chloride solution. The observations of reinforcement in ordinary portland cement (OPC) mortars, OPC plus silica fume (SF) mortar, or OPC plus limestone flour mortar were carried out simultaneously. The resistance of alkali-activated slag mortar to the attack of a solution of high Cl- concentration was proved previously. The effective, protective action of the alkali-activated slag mortar was confirmed by electrochemical measurements and weight loss determination after 365 days' exposure to a chloride solution. A similar effect was found in the case of silica fume or limestone flour addition to the OPC mortar, but the corrosion of the reinforcement was clearly visible, as shown by corrosion pits in the reference standard OPC mortar samples.