CORROSION IN REINFORCED CONCRETE
CORROSION MAY CAUSE CATASTROPHIC DAMAGE
Expensive and difficult to repair, corrosion is the most damaging and dangerous event occurring in concrete.
STEEL REBAR DEGRADATION
Any building will eventually show signs of rust in its concrete structure. It's just a matter of time and location that will determine the speed in which corrosion will form.
DAMAGE TO RESIDENTIAL BUILDINGS
Damage may start with foundations exposed to high concentrations of moisture which will accelerate the steel rebar corrosion process and decay of the concrete.
Usually, the most exposed elements deteriorate first – but the underlying corrosion is unseen. Active corrosion in the steel beneath the surface may take 5 to 15 years to initiate cracks in the concrete, but much of the corroded reinforcement is not visible.
SPALLED CONCRETE EXPOSES CORROSION
Degradation of reinforcing steel and the subsequent weakening of the concrete occurs from the inside and may be unseen for many years. It is often referred to as “concrete cancer.”
MARINE EXPOSED CONCRETE
Constant contact with water accelerates the process of corrosion within and makes concrete peal and spall from the expansion of corroded steel rebar.
COMMON CAUSES OF CONCRETE CORROSION
When carbonation, chlorides and other aggressive agents penetrate concrete, they initiate corrosion that produces cracking, spalling and weakening of the concrete infrastructure
Carbonation is the result of carbon dioxide (CO2) dissolving in the concrete pore fluid and reacting with calcium from calcium hydroxide and calcium silicate hydrate to form calcite (CaCO3). Within a relatively short space of time the surface of fresh concrete will have reacted with CO2 from the air. Gradually, the process penetrates deeper into the concrete and after a year it may reach a depth of 1 mm for dense concrete of low permeability, or up to 5 mm for more porous and permeable concrete, depending on the water-to-cement ratio
Chloride (salt attack)
Chlorides, usually from seaside splash or wind, migrate into the porous concrete over time, causing corrosion when the concentration of chlorides reaches critical levels at the reinforcement. In addition, older structures may have used calcium chloride as a concrete “set accelerator” at the time of construction, resulting in serious corrosion issues
As reinforcing rods rust, the volume of rust product can increase up to six times that of the original steel, thus increasing pressure on the surrounding material, and slowly cracking the concrete. Over the course of years, the cracks eventually appear on the surface and concrete starts to flake off or spall
CORROSION MECHANISMS OF REINFORCING STEEL
In new concrete, alkaline (high pH) conditions form a passive film on the surface of the steel rebar rods, thus preventing or minimizing corrosion initially. But eventually, a pH reduction caused by carbonation or by ingress of chlorides (salt) causes the passive film to degrade, allowing the reinforcement to corrode in the presence of oxygen and moisture.
When this occurs, a voltage differential of approximately 0.5 V is set up between the corroding (anodic) sites and the passive (cathodic) sites, resulting in a corrosion cell where electrons move through the steel from anode to cathode. The rate of the reaction is largely determined by the resistance or resistivity of the concrete. Acid forms at the anodic (corroding) site, which reduces the pH and promotes corrosion of the steel.