Self-healing engineered materials to prevent corrosion of reinforced concrete

Worldwide 5 tons of steel is lost every second due to corrosion. In case of reinforced concrete structures, corrosion is one of the most common deterioration mechanisms. Especially when cracks are present in the concrete matrix, there is a high risk for chloride induced corrosion. Cracks mainly occur at locations where the concrete is subjected to tensile stresses, which is nearly always at the location of the steel reinforcement. Due to cracking, aggressive substances such as chlorides can rapidly penetrate into the concrete and reach the reinforcing steel. In order to prevent the rapid deterioration of concrete due to cracks, self-healing concrete was developed. One approach to self-healing concrete is the embedment of brittle capsules filled with a liquid healing agent in the concrete matrix. At the moment of crack appearance, the capsules break and the healing agent is released in the crack. Subsequently, the healing agent hardens in the crack and seals it off to prevent aggressive substances to enter through the crack.

Schematic overview of the autonomous healing mechanism in concrete.
Schematic overview of the autonomous healing mechanism in concrete.

 

 In the ISHECO project (Bjorn Van Belleghem, Philip Van den Heede, Kim Van Tittelboom) this self-healing mechanism in concrete is tested as a method for the prevention of reinforcement corrosion. In a first step, chloride diffusion tests are performed on concrete with and without self-healing properties. Therefore concrete specimens are subjected to a high concentration of chlorides after creation of cracks. At different times (7 weeks, 19 weeks and 52 weeks) specimens are removed from the chloride environment and the ingress of chlorides in the cracked area is determined by potentiometric titrations. From the resulting chloride profiles the efficiency of the self-healing mechanism to prevent ingress of chlorides through cracks in concrete can be evaluated.

Concrete specimens subjected to a high concentration chloride solution (left). Layers of concrete ground off to powders for chloride analysis (right).
Concrete specimens subjected to a high concentration chloride solution (left). Layers of concrete ground off to powders for chloride analysis (right).

 

 

Determination of the chloride content in the concrete layers by potentiometric titrations.
Determination of the chloride content in the concrete layers by potentiometric titrations.

 

 Next to the determination of the chlorides in the concrete, the corrosion process of steel reinforcement is investigated by means of electrochemical measurements. Reinforced concrete beams are produced where the anodic and cathodic part of the steel reinforcement are spatially separated, but electrically connected outside the beams. The beams are then subjected to three-point bending to induce a crack and subsequently they are exposed to weekly wet-dry cycles of a realistic chloride solution to simulate a marine environment. Due to the specific design of the beams, current measurements and potentiostatic measurements of the potential of the steel can be performed every week. In this way the corrosion state of the steel reinforcement can be continuously assessed and the effect of the self-healing mechanism on preventing reinforcement corrosion can be investigated.

Reinforced concrete beams cyclically exposed to a realistic sodium chloride solution.
Reinforced concrete beams cyclically exposed to a realistic sodium chloride solution.

 

Electrochemical measurements performed by potentiostats to assess the corrosion behavior of (self-healing) reinforced concrete beams.
Electrochemical measurements performed by potentiostats to assess the corrosion behavior of (self-healing) reinforced concrete beams.