Corrosion of reinforced concrete
2.1 Chloride attack4
3. Minimising corrosion6
3.1 Metallic coatings6
3.2 Cathodic protection7
3.3 Other solutions8
List of Figures
2.1 Chloride attack on reinforced concrete…………………………………………....4
2.2 Carbonation attack of RC due to poor concrete cover…………………………….5
3.1 How impressed current cathodic protection stops corrosion ……………………...8
Reinforced concrete (RC) is an essential construction material that is beneficial for strengthening structures. The steel reinforcement provides tensile stress ‘which plain concrete lacks’ (Chudley and Greeno, 2006). However when combined with the compressive strength of concrete, the overall strength of the composite material is increased. A particular concern for the RC is the corrosion of embedded steel, this can have a detrimental effect to the material and influence its structural integrity and durability therefore it is vital that the steel is protected to prevent the occurrence of corrosion. There are various procedures considered when minimising the risk of corrosion and their effectiveness in doing so has been reviewed.
The rate of corrosion in reinforcing steel or rebar is increased significantly when structures are exposed to a ‘highly alkaline environment’ (Tretheway and Chamberlain, 1995) which forms a ‘passive layer’, a ‘thin layer of oxide’ that contributes to a slow rate of corrosion on the steel surface. (Broomfield, 2003) This subsequently leads to the formation of rust due to the ‘carbonation of the concrete cover and penetration of chlorides’ (Martinez and Andrade, 2009). If ‘water, oxygen and carbon dioxide’ are able to attack the concrete this results in a loss of bond between the concrete and the steel (Tretheway and Chamberlain, 1995). Causing ‘high tensile forces’ from the expanded steel bar and eventually the appearance of cracks on the concrete surface (Apostolopoulos and Michalopoulos, 2006). Cracks are apparent ‘along the plane of the rebar’ and ‘around the end rebar’ (Broomfield, 2003) and as corrosion becomes progressive the cracks widen and cause ‘spalling’ of the concrete. (NRMCA, 1995)
2.1 Chloride attack
Chloride attack is prevalant where ‘sea water or de-icing salts’ are present (Cadman, 2012). Typical of ‘marine environments’ (Abossra et. al, 2011) chloride ions can disrupt the passive layer of steel by permeating through ‘sound concrete’ (PCA, 2012) or cracks in the concrete leaving the steel susceptible to corrosion. The presence of chloride ions can have a tendency to accelerate beyond ‘0.4% chloride ions by weight of cement’ (Hinks and Cook, 2003) anything below this level is normally weak enough to penetrate and the alkalinity of the concrete can protect against the attack. .The chloride ions acts as a ‘catalyst’ (Broomfield, 2003) to help speed up the break down the passive layer of the oxide but are normally effective when they are in the form of ‘water-soluble chlorides’ (PCA, 2012). The result of chloride attack is the ‘formation of pits’ (Broomfield, 2003) which form small holes in the steel due to the ‘electrical conductivity of the pore water’ which creates a ‘corrosive current’ causing the ‘dissolution of iron’ therefore the protective oxide layer is ineffective and the steel becomes ‘notched and brittle’ moreover once it expands to promotes ‘bulging or cracking of the concrete’. (Hinks and Cook, 2003)
Figure 2.2 Chloride attack on reinforced concrete
When carbon dioxide and moisture diffuse into the concrete they react with ‘calcium hydroxide’ to form ‘calcium carbonate’ in a process known as carbonation. (PCA, 2012). If the concentration of calcium hydroxide in the concrete pores is high then this...