Selecting Repair / Protection System Materials
The materials for repairs and protective systems are selected based on the following factors:
a) Strength: Bond and compressive strengths are important material properties in almost all repairs and protective works.
b) Coefficient of thermal expansion: The repair material must have a coefficient of expansion almost same as that of the existing concrete. Thermal incompatibility may cause failure either at the interface or within the material of lower strength, particularly for overlays.
c) Drying shrinkage: The repair material must have minimum shrinkage to protect the bond with the underlying concrete surface as the shrinkage of the substrate is already complete. The limits for cement-based repair materials for 28-day and ultimate drying shrinkage are 400 and 1000 millionths, respectively. Shrinkage of cementitious repair materials can be reduced by using mixtures with low w/c, the maximum practical size and volume of course aggregate, shrinkage-reducing admixtures, or using construction procedures that minimize the shrinkage potential. Curing of the materials is very critical especially if the thicknesses are smaller
d) Permeability: If impermeable materials are used for large patches, overlays, or coatings, moisture that rises up through the base concrete can be trapped between the concrete and the impermeable repair material. This moisture can cause failure at the bond or may make freezing and thawing significantly more critical. The repair or protection material must allow breathing of concrete below.
e) Modulus of elasticity: The modulus of elasticity of the repair material must be similar to that of the existing concrete. In nonstructural repairs, a lower modulus of repair material is desirable to help in the relaxation of tensile stresses induced by restrained drying shrinkage. The maximum modulus of elasticity for cement based repair materials is generally taken equal to 24 GPa.
f) Chemical properties: A pH close to 12 (alkaline environment) of repair material is better for corrosion protection to embedded reinforcement. Otherwise, additional protection for the existing reinforcement may be provided by cathodic protection or reinforcement coatings.
g) Electrical properties: Materials that have high electrical resistance tend to isolate repaired areas from concrete having chances of corrosion. Differentials in electrical potential resulting from variations of permeability or chloride content between the repair material and the original concrete may increase corrosion activity around the perimeter of the repair area, resulting in premature failure.
This is commonly referred to as the anodic ring or halo effect.
h) Color and texture properties: For repair of architectural concrete surfaces, color and texture of the repair material must not differ appreciably from the adjacent surface. Trials may be made on the site before beginning actual repair work.