International Concrete Abstracts Portal

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.

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.

A comprehensive research program was carried out to evaluate the quality of the outer layer of concrete (the covercrete), in terms of its durability and protective value to the underlying reinforcing bars. Part of the results of that investigation are presented in this paper, namely those dealing with the influence of the concrete grade and cement type on the permeability to oxygen and water absorption properties of the 'covercrete' of concrete specimens subjected to 7 days moist + 21 days dry curing. The results obtained show that the permeability to oxygen is reduced 40 times when the concrete strength is raised from 25 to 85 MPa, whilst the rate of water absorption is reduced by a factor of 4. In addition, the cement type (including Portland and blended cements) seems to play only a secondary role in the quality of the 'covercrete' of concretes of the same grade, especially above 25 MPa.

There is a need for a new anode for use in the impressed-current cathodic protection (CP) of inland concrete piers, which are deteriorating because of salt-induced corrosion of reinforcing bars. A new water-based conductive coating was used recently on the cathodic protection of some concrete piers in Virginia. Further, as a possible means of eliminating the need for regular site visits to inspect and insure that the CP is functioning properly (which is a disadvantage common to existing CP systems), a microprocessor-based data acquisition device that facilitates remote monitoring was tested with the system. This paper describes the design, installation, and performance of the CP system during its first year of operation.

The impressed current-type of cathodic protection was evaluated for controlling of marine concrete structures as follows: To establish criteria of cathodic protection on marine concrete structures, to develop a system that can distribute current uniformly to all reinforcement, and to investigate the over-protection problem that effects the bonding of reinforcement and durability of anode material. The application of the cathodic protection system for the rehabilitation of a harbor concrete structure in Japan was also examined.