When considering durability of concrete, chloride attack is the most imminent enemy. It is responsible for almost 40% of failure of concrete structures. In the presence of oxygen and water, chloride attack corrodes the steel reducing the strength of the structure drastically.
WHAT IS CHLORIDE ATTACK
Chloride ion (CL-) is formed when the element chlorine gains an electron or when a compound such as hydrogen chloride is dissolved in water. High concentrations of chloride ions in concrete can be very problematic. Due to its electro-chemical nature, chloride ions break down the passive layer of reinforcing steel, without the need to drop the pH levels. Corrosion takes place as the chloride ions meet with the steel and the surrounding passive material to produce a chemical process which forms hydrochloric acid. The hydrochloric acid eats away at the steel reinforcement and thus leads to concrete cracking, spalling, and eventually failure.
There are two main sources of chloride ions; one is from the concrete mix components, and the other from the surrounding environment. The first could come from unwashed aggregates and sand, admixtures, and even from the use of seawater in the concrete processing. The second comes mainly from being exposed to marine environment such as sea salt spray, direct seawater wetting, when concrete is in contact with soils rich with chlorides deposits, or it can come from deicing salts and use of chemicals. It is by the process of diffusion that chloride penetrates the concrete.
DAMAGES ASSOCIATED WITH CHLORIDE ATTACK
The main problem involving the corrosion of the steel is the spalling of the concrete cover. The oxide resulting from the corrosion is very porous and takes up to 10 times the volume of the steel which causes the break-up of the concrete.
There are two types of concrete cracks that can be observed. Horizontal cracks occur in the materials interface due to the oxide formation and leads to loss of the concrete cover. Vertical cracks occur when the tensile strength of the steel is exceeded. Also, if they are large enough, the cracks may accelerate the corrosion process by allowing access of corrosion agents.
If rust stains are observed around cracks and spalls, these are indicative of chloride attack. When inspecting an exposed corroded reinforcement, it will typically show black colored rusting and pitting of the steel where the aggressive hydrochloric acid has eaten away the reinforcing.
TESTING FOR CHLORIDE
There are several laboratory tests used to calculate and understand the penetration and amount of chloride present in the concrete such as ASTM C1202, AASHTO T259, BS EN 1881-124, BS 14629 etc. Penetration of deicing salt or seawater can be assessed by profiling chloride concentration at various positions in the sample. Tests taken at different levels or areas can be helpful in determining if chloride was added in the mix or if the sample was exposed to chloride salts.
PREVENTION OF CHLORIDE ATTACK
For new structures, there are several methods to prevent or reduce chloride attack:
Increase concrete cover (min. 50 mm)
Use epoxy coated rebars
Use stainless steel rebars
Use low water/cement ratio
Apply of anti-carbonation concrete coating
For existing structures suffering from chloride attack the following repair methods can be applied:
Apply of anti-carbonation concrete coating to slow down the corrosion process
Use of corrosion inhibitors
Install a cathodic protection system
In the case of extensive spalling or section loss, a comprehensive concrete repair or a section replacement will be required
SHI, X., XIE, N., FORTUNE, K and GONG, J. (2011) “Durability of steel reinforced concrete in chloride environments: An overview. Construction and Building Materials.” 30, 125-138.
BROOMFIELD, J. P. (1996) “Corrosion of Steel in Concrete: Understanding, Investigation and Repair.” Spon Press.
SHREIR, L. L., JARMAN, R. A. and BURSTEIN, G. T.(1994) Corrosion: Volume 1. 3rd ed. Butter worth-Heinemann.
§The Constructor. “Chloride Attack Concrete Structures – Cause & Prevention”. https://theconstructor.org/concrete/chloride-attack-concrete-structures-cause-prevention/7802/ [viewed on November 9, 2018].
[viewed on November 9, 2018].