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 StructuresThe 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
- Exposure of concrete to seawater
- Use of salt to melt the ice
- Presence of chlorides in the substances placed for storage
Prevention of Chloride Attack on Concrete StructuresSeveral 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 StructuresThe 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|
|Category||Acid-soluble (ASTM C 1152)||Water-Soluble (ASTM C 1218)||Water -Soluble (ACI 222.1)|
|Prestressed (Pretensioned or post-tensioned)||0.08 (40%)||0.06 (40%)||0.06 (40%)|
|Non-prestressed Water conditions||0.10 (50%)||0.08 (53%)||0.08 (53%)|
|Non-prestressed, Dry Conditions||0.20 (100%)||0.15 (100%)||0.15 (100%)|