Silica Fume Reduces Permeability
In many situations, the durability of concrete is directly related to its permeability. The contribution of silica fume is to reduce the permeability of the concrete. By reducing the permeability, the time is extended to prevent any aggressive chemical to get into the concrete where it can do its damage. Here are a few examples of how reducing permeability is used in actual structures.
Chloride damage to reinforcing steel:
Corrosion of reinforcing steel is the most significant and costly cause of concrete deterioration. It doesn’t matter whether the chloride comes from the ocean or from deicing salts, the results are the same. Silica-fume concrete is used widely in applications where the concrete is exposed to salt from any source. The reduced permeability of this concrete can result in many years of extended life for a structure.
Corrosion damage in concrete is a multi-step process as follows:
The chloride ions slowly work into the concrete to reach the level of the reinforcing steel. Once a certain amount of chlorides, called the threshold amount, reach the steel, corrosion begins.
As the iron ions are removed from the reinforcing steel, they go through several stages of oxidation or rusting. The volume of the iron increases with each stage.
As the amount of rust increases, rust stains will be seen on the surface of the concrete. Next, cracking will be seen. Finally, the cracking will result in delaminations and spalling of concrete over the reinforcing steel.
Sulfate attack:
While the chemistry of the portland cement used plays a role in sulfate attack, it has been shown that the water-cementitio us materials ratio (w/cm) is also a critical factor. Reducing the w/cm effectively reduces the permeability of the concrete. Adding silica fume will further reduce the permeability resulting in further delays of any adverse reactions.
Acid or other chemical attack:
The overall resistance of silica-fume concrete to attack by an aggressive chemical is not significantly different from that of conventional portland cement concrete. However, the reduced permeability of silica-fume concrete may extend the life of a concrete structure or extend the time between repairs simply by slowing down the rate of the attack. If protection against a particular chemical is required on a project, we strongly urge testing to include exposure of specimens of varying silica fume contents to the particular chemical.
The reduction in permeability is not the only contribution of silica fume to durability. There is ample evidence that silica fume, when used alone or in conjunction with a suitable fly ash, can reduce or eliminate the potential for alkali-aggregate reaction when reactive aggregates are used. Again, testing will be required to determine the appropriate amount and types of cementitious materials to be used for each particular application.
Finally, the higher strength of silica-fume concrete will add additional abrasion resistance. For concrete made with a particular aggregate, the higher the compressive strength, the higher the abrasion resistance. High-strength silica-fume concrete has been used in applications such as trash transfer stations and stilling basins in major dams.
Advantages in Various Applications
Another way to look at the use of silica fume is from the viewpoint of constructability. Here, we are actually stepping across the boundary between effects on fresh and hardened concrete to take advantage of all aspects of the performance of silica-fume concrete to make construction easier or, in some cases, even possible. Here are a few examples:
One-pass finishing: In this case, we are taking advantage of the lack of bleeding in silica-fume concrete to complete finishing of flatwork in a single, continuous operation. The owner will get a better surface and the contractor will be able to complete the finishing in a shorter time using fewer finishers.
Reduced heat of hydration: Although silica fume contributes about the same amount of heat of hydration as does portland cement on a pound-for-pound basis, its strength contribution is much greater on the same basis. Therefore, by balancing portland cement and silica-fume in a mixture, heat of hydration can be reduced while strength is maintained.
More and more state DOTs are using combinations of silica fume and fly ash to reduce the heat of hydration for concrete used in bridge decks. Reducing the total amount of cementitious material will reduce heat and help prevent early-age cracking.
Use of three cementitious materials:There is an ever-increasing emphasis on using more waste materials such as fly ash and slag in concrete. However, the early age strength of concrete may suffer as a result. Adding small amounts of silica fume can offset this reduction in early strength. Usually, using combinations of three cementitious materials will reduce the cost of concrete. Mixtures containing three cementitious materials are referred to as “ternary mixtures”.
Shotcrete:Silica-fume shotcrete is being widely used, in both the wet and dry processes and with and without steel fibers. The cohesive nature of this shotcrete allows for many applications that would have been difficult, uneconomical, or impossible to accomplish without the silica fume.
