Corrosion of Steel in Concrete: Some Fundamental Aspects of Concrete with Added Silica

Flyash, which is a waste product of the ferrosilisium industry, has been used as an additive to concrete. This flyash consists of minute spherical silica particles with pozzolanic properties. In the present study steel embedded in concrete (W/C = 0,55 to 0,60) with and without added silica has been exposed to seawater at 20°C for two years. During the exposure period polarisation studies and chloride analysis have been performed. A diffusion model for chloride penetration into the concrete gives an apparent diffusion coefficient of 6 · 10^-9 cm²/s for a concrete with 16% added silica (flyash) compared to 1,5 · 10^-7 cm²/s for the similar concrete without added silica. The corrosion rates calculated from polarisation studies showed that the corrosion of steel in concrete will be insignificant at a total chloride concentration in concrete of less than 0,30% (dry weight of concrete). The corrosion rates with and without added silica were not significantly different at any given total chloride concentration in the concrete. The initial resistivity of a concrete with 8% added silica will be around 2 · 10⁶ ohmcm compared to 4 · 10³ ohmcm for the similar concrete without added silica.

Experiments with pozzolanic materials have increased recently. In Norway a lot of work has been done with a silica dust (flyash), which is a waste product of the ferrosilisium industry. This dust consists of quite small spherical particles (< 0,5 μm) with almost a glass structure (1). This liquid state of silica is rather reactive and together with calcium hydroxide it gives calcium silicate hydrates (1, 2). The silica dust fits well into the interstitial pore spaces of the concrete particles in such a manner as to encircle these larger particles. Thus weak contacts between large particles can be replaced by cementing bonds. The resulting concrete will have few water inclusions and give a good attachment to the rebars. Water penetration, the diffusion of carbondioxide as well as the diffusion of chloride and other anions are also reported to be reduced compared to similar concretes without added silica (1, 2). Carbonatization of the silica added concrete is also considered to be lower than for ordinary Portland cement because of the low penetration of carbondioxide (3). The destructive alkali-silica reaction is also avoided using silica dust because the pozzolanic reaction in this case involves these sodium and potassium compounds (1, 4). Normally the silica added concrete will also have an increased compressive strength compared to concrete based on ordinary Portland cement (1, 2, 5). The strength of the silica added concrete can be further enhanced by the application of dispersive agents. The short term strength can also be increased compared to Portland cement by a suitable choice of added silica and dispersive agents (2). The added silica will also cause an increased cohesive strength for the uncured concrete. This makes the concrete suitable for underwater work as the uncured concrete will not easily be dispersed or separated in water.

The steel reinforcement in concrete is usually passivated owing to the character of the liquid phase in the pores of the concrete (6). The corrosion of steel in concrete is mainly caused by the diffusion of oxygen into the concrete. Physically corrosion may be accellerated due to cracks in the surface and inhomogenities in the concrete. The traditional view has been that the metal remains passivated owing to the high pH (12, 5) of the pore solution in concrete (6). More recent works have discussed the importance of the buffer action afforded by the lime reservoir at the steel/concrete interphase. Thus the lime rich layer which occurs on the rebar steel will be of fundamental significance for the electrochemical reactions on the steel (7). Work on the diffusion of chlorides into standard Portland cement pastes has shown that it is just a question of time before detrimental amounts of chloride reach the embedded steel even for high quality concrete (8). It is established that the diffusion of oxygen in a silica added concrete is about the same as for an ordinary Portland cement based concrete. However, as stated above, the chloride diffusion will be greatly reduced when silica is added. Due to this reduced chloride penetration, the corrosion of steel will be less than for a similar Portland cement based concrete structure. The nature of the change in microstructure caused by the silica addition results in a considerably increased resistivity for the concrete (3). It is generally held that the main reasons for the better corrosion resistance of steel in a silica added concrete are the reduced chloride penetration and the increased resistivity as compared to the standard Portland cement based concrete (2).