What Is Chloride Ion Penetration In Concrete?
What Is Chloride Ion Penetration In Concrete?
Chloride ion penetration in concrete is the process of chloride ions (salt) entering into the concrete, which can lead to corrosion of embedded steel and other reinforcements in the structure.
This happens when chloride ions combine with moisture, air, or carbon dioxide from the atmosphere, forming a solution that migrates through the structure.
The amount of chloride penetration depends on the concentration of chlorides around the concrete surface, the permeability (porosity) of the material, and environmental factors such as temperature and humidity.
When left unchecked, this process can be damaging to reinforced structures by leading to premature failure.
How Does Chloride Penetrate Concrete?
Chloride ions can penetrate concrete through physical and chemical processes. Physically, chloride ions from rain, seawater, or road salts can penetrate concrete directly by moving through air-filled pores and capillary channels in the concrete matrix.
Their penetration rate is mostly determined by porosity and pore size distribution of the material, which can be controlled by composition and production methods.
Chemically, a reaction between calcium hydroxide (Ca(OH)2), a common cementitious material present in hardened concrete, with chlorides can form soluble compounds such as calcium chloride (CaCl2).
This process leads to an increase in chloride concentration at the surface of hardened concrete.
Furthermore, high alkalinity of cementitious materials also accelerates migration of chlorides into the cement matrix due to electrochemical diffusion mechanisms.
How Does Chloride Affect Concrete?
Chloride ions can affect concrete in a number of ways. These ions are known to cause corrosion of the steel rebar that is often used to reinforce concrete, leading to weakened structural integrity of the concrete and increased risk of cracking or spalling.
Chlorides can also accelerate sulfate attack, which leads to further damage and deterioration.
Additionally, chloride ions promote alkali-silica reaction (ASR), which when left unchecked results in significant expansion, ultimately leading to cracking and reduced strength over time.
As such, controlling the amount of chloride present in concrete is essential for its effective use and long-term durability.
What Is The Allowable Chloride Content In Concrete?
The allowable chloride content in concrete is a vital factor to consider when designing, constructing and maintaining concrete structures.
Chlorides are a type of salt which can cause steel reinforcing bars (rebar) within the concrete to corrode, leading to deterioration of the structure, and therefore should be kept below certain limits.
It is recommended that for reinforced concrete structures exposed to chlorides from deicing salts or seawater, total chloride ion content should not exceed 0.15%.
However, for structures that may experience higher levels of chlorides due to harsh conditions such as sea walls or marine environments, limits may be higher depending on additional protective measures taken such as reinforcing bar coatings.
In any case, because of the potential for corrosion caused by high chloride content, it is important to adhere closely to these recommended levels whenever possible.
Does Chloride Corrode Concrete?
Yes, chloride can corrode concrete. When chlorides are present in sufficient concentrations in concrete, they result in a chemical reaction with the calcium hydroxide (CH) and combined water of the hardened cement paste.
This reaction produces calcium chloride and causes corrosion of the steel reinforcement. The corrosive effect is more pronounced when there is high humidity and low alkalinity, resulting in an accelerated corrosion rate ultimately leading to failure of the structure.
