Chloride Penetration - Remedial Measures
An engineer may employ several different methods to prevent or remedy corrosion induced by chloride intrusion. The simplest way to reduce corrosion is to increase the cover over rebar. Adding just an extra inch of concrete cover could double the life of a structure. Another way to prevent chloride intrusion is to reduce the permeability of concrete. Since chlorides are usually introduced in solution, reduction of water permeability will reduce chloride ion permeability. Concrete mixtures with a low water/cement ratio will produce concrete with lower permeability. Reducing the water-cement ratio seems like a simple method for reducing permeability, but in order to produce workable concrete, water-reducing admixtures are necessary. These admixtures increase the workability of concrete without additional water. Water-reducing admixtures can decrease water content for a given slump by 5 – 10%, while superplasticizers may reduce water content by 15 – 30%.
Addition of pozzolans may also decrease the permeability of concrete. Silica fume, fly ash and ground granulated blast furnace slag in concrete will significantly reduce the water permeability of concrete. These pozzolans react with the calcium hydroxide produced by the reaction of water and cement to produce additional C-S-H, which occupies more of the pore spaces. This aspect is important at the interface of the aggregates with the cement paste. Normally, the aggregate-paste interface enjoys the highest permeability. This area is referred to as the transition zone, where a porosity gradient exists. Permeability in the transition zone may increase by a factor of ten. Silica fume addition creates direct contact between aggregate and paste reducing or eliminating the effect of the transition zone on permeability.
Improved curing is also a simple method for reducing permeability. Hydration of concrete is most effective at high relative humidity. Proper curing will increase the amount of calcium silica hydrate (C-S-H) produced by the reaction of cement and water, filling and segmenting pores. Proper curing can be achieved by monitoring temperatures and using one of the methods recommended by the Portland Cement Association for providing adequate water for curing concrete. These methods include immersing concrete in water, misting with water, and applying wet coverings to the concrete surface.
In addition to admixtures that affect permeability, chemical admixtures known as corrosion inhibiting admixtures are added to the mix in order to stymie the corrosion reaction. These admixtures are often nitrite based, and act as either an electron or proton acceptor. They have an affinity for the cathodic or anodic sites on the rebar, and essentially cover the sites so that corrosion does not occur.
Other modifications can be made to concrete, either during mixing, or after hardening to prevent the ingress of chlorides. Latex modified concrete, internally sealed concrete, and surface coatings are several ways of preventing chlorides from entering concrete.
- Epoxy Coated Rebar – Epoxy coated rebar is widely available, and accepted by ACI 318-95. The layer of epoxy prevents water and oxygen from coming in contact with the steel. When using epoxy-coated rebar, extra care must be taken in laying and tying bars. If cracks or holes occur, exposing the steel, the steel will corrode at higher than normal rate.
- Stainless Steel-Clad Rebar – Stainless steel clad rebar works similarly to epoxy-coated rebar. The external surface of the steel is coated with a layer of stainless steel, which will not corrode. This material is not widely used, but is effective in preventing corrosion. One should exercise care so that the coating does not crack when the bars are bent, for the same reason as when using epoxy-coated rebar.
- Cathodic Protection – Cathodic protection systems are recommended for structures with severe exposure to chlorides. Cathodic protection electrically connects rebar in existing structures to a metal anode (usually zinc or magnesium metal), while applying a voltage to the system. A car battery is usually sufficient for supplying the required voltage. This method essentially reverses the corrosion reaction causing the sacrificial anode to corrode, preventing corrosion in the rebar. This method is very effective in preventing corrosion when properly maintained. Proper maintenance includes replacing the battery and anode from time to time.