There are a number of factors such as strength, durability, thermal coefficient of thermal expansion, permeability, low dry shrinkage, chemical and electrical properties, curing, and cost that must be considered to select the most appropriate, compatible, cost-effective repair materials.
This ensures a successful and durable repair work that provides the designated strength regain. Nonetheless, the availability of materials of relevance, equipment and skilled labor has to be investigated.
11 Factors Affecting the Selection of Repair Materials
Bond and compressive strengths are important properties in almost all repairs and protective works. The strength of the base material and repair materials should be nearly the same or that of repair materials should be slightly higher. This is to achieve a proper and uniform flow of stress and strains through materials.
However, when the strength of both base and repair materials is great, then the repair material would experience premature failure due to non-uniform flow of stresses and strains.
The bond between the underlying concrete surface and repair material should be satisfactory. If there are uncertainties regarding the bond strength, then proper measures can be considered to improve the bond strength such as the use of adhesive, surface interlocking system, mechanical bonding, or combination thereof.
The repair material needs to be durable under exposure conditions to which the defected structure is exposed. It should demonstrate adequate resistance against chemical attack and resistance to any form of energy such as ultra-violate rays and heat.
3. Coefficient of Thermal Expansion
The repair material must have a coefficient of expansion almost same as that of the existing concrete to make sure that undue stresses are not transferred to the bonding interface or the substrate. Thermal incompatibility may cause failure either at the interface or within the material of lower strength, particularly for overlays.
4. Low Drying Shrinkage
Due to the fact that concrete base shrinkage is already completed, it is essential for repair materials to have lowest possible dry shrinkage to save the bond between repair material and the underlying concrete surface.
If this condition is not met, then the repair material bond to the concrete base would be endangered which leads to delamination or shrinkage crack development on surfaces. Shrinkage cracks allow ingression of air and moisture, hence steel bars would face corrosion.
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
The permeability of the repair materials should be low to prevent the penetration of aggressive substances such as carbon dioxide, water, oxygen, and industrial gases and vapors. This is to protect the reinforcement from corrosion.
However, 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.
6. 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.
7. 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.
8. Electrical Properties
Materials that have high electrical resistance tend to isolate repaired areas from concrete having chances of corrosion.
Differentials in electrical potential 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.
9. 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.
10. Curing Requirement
Repair materials with no or minimum curing requirement are highly desirable in order to decrease post-repair care. Repair materials that need high curing effect may suffer from improper curing and consequently designated strength would not be achieved.
11. Cost of Repair Material
It is desirable to employ cost-effective repair material but this measure should be at the expense of the performance properties of the material.