Chloride Attack on Concrete Structures is one of the most important phenomena we consider when we deal with the durability of concrete.
Among all sources of failure of concrete structures, the chloride attack accounts 40% contribution. The main effect of chloride attack is the corrosion of reinforcement that induces the strength of the structure drastically.
Causes of Chloride Attack on Concrete Structures
The attack of chloride on concrete structures can be happened either from inside of the concrete or through the ingress of chloride from outside to the inside of concrete structures.
The chlorides exist in concrete during the casting process due to the following reasons:
- Use of seawater for the concrete mixing process
- Use of calcium chloride as an additive to increase the setting time
- Use of aggregates that contained chlorides which were not washed for mixing
- Aggregates with chloride content more than the limit stated in the specification
The chlorides enter the concrete from the exterior environment to concrete interior due to the following reasons:
- Exposure of concrete to seawater
- Use of salt to melt the ice
- Presence of chlorides in the substances placed for storage
Comparing both the means of chlorides, the chances of exterior chloride action are high. Most of the offshore structures are subjected to extreme chloride attacks. This induces reinforcement corrosion of structures.
In reality, the action of chloride in inducing corrosion of reinforcement is more serious than any other reasons. One may understand that Sulphates attack the concrete whereas the chloride attacks steel reinforcements.
A protective oxide film is present on the surface of the steel reinforcement due to the concrete alkalinity. This layer is called passivity. The process of carbonation will affect this protective passivity layer. This layer can also be affected by the presence of chlorides in water or in oxygen. The reinforcement corrosion process is shown in figure-1.
Prevention of Chloride Attack on Concrete Structures
Several methods are available in order to prevent the effect of chlorides on concrete structures. Some of them are:
- Increasing the cover over the reinforcement bar. This is the simplest way to prevent chloride attack. Studies have shown that an increase in cover by one inch can increase the life period of the structure by double.
- The rate of deterioration of the reinforcement under extreme conditions of chlorides can be prevented by having a rebar coated by epoxy, having cathodic protection or by use of stainless steel-clad rebar.
- Another important way is to decrease the chloride ion ingress into the concrete by decreasing the permeability of the concrete. This will decrease the durability and the time, cost of expensive repairs.
Chloride Content Limit in Concrete Structures
The chloride content limit is the amount of chloride that must be present in concrete along with oxygen and moisture in order to facilitate corrosion. Table-1 below shows the ACI 318-95 code limits of water soluble chlorides. These are limits that must be met when designing the mix proportions.
Table.1: Water -Soluble Chloride -ion Limits in ACI 318-95
|Type of Member||Maximum Water-soluble chloride ion(Cl–) content in concrete, percent by weight of cement||Percent of Water-soluble chloride corrosion threshold (0.15% by weight of cement)|
|Reinforced Concrete exposed to chloride in service||0.15||100|
|Reinforced concrete that will be dry or protected from moisture in service||1||666|
|Other reinforced concrete construction||0.30||200|
Table.2: Maximum Chloride content in concrete in percent by weight of cement as per ACI Committee 222
|Category||Acid-soluble (ASTM C 1152)||Water-Soluble|
(ASTM C 1218)
|Prestressed (Pretensioned or post-tensioned)||0.08|
|Non-prestressed, Dry Conditions||0.20|
The amount of chloride required for initiating corrosion is partly dependent on the pH value of the pore water in concrete. At a pH value, less than 11.5 corrosion may occur without the presence of chloride. At a pH value greater than 11.5 a good amount of chloride is required.