The concrete repair contractor can choose from a wide array of specialty and conventional repair materials. This large selection gives the contractor greater opportunity to match material properties with job demands, but it can also increase his chance of selecting an inappropriate material. No matter how carefully a repair is made, using the wrong repair material will likely lead to early repair failure. Some of the material properties that should be considered when selecting a repair material are discussed in this article.
DIMENSIONAL STABILITY
One of the primary requirements for a successful repair is to provide good bond between the new material and the underlying concrete or substrate. Bond failure between new and old concrete isn't usually caused by the incapability of new concrete to adhere to a properly prepared substrate. It's usually caused by shrinkage. Shrinkage of cementitious repair materials can be reduced by using mixtures with very low water content or by using construction procedures that minimize shrinkage potential. Another way to combat shrinkage problems is to use repair products that actually expand in volume once they are mixed.
COEFFICIENT OF THERMAL EXPANSION
All materials expand and contract with changes in temperature. For a given change in temperature, the amount of expansion or contraction depends on the coefficient of thermal expansion for the material. The coefficient of thermal expansion is the change in length over a unit length divided by the temperature change.
MODULUS OF ELASTICITY
The modulus of elasticity of a material is a measure of its stiffness. High modulus materials don't deform under load as much as do low modulus materials. When materials with widely differing moduli are in contact with each other, the lower modulus material will tend to yield or bulge under load.
PERMEABILITY
Permeability refers to the capability of a material to transmit liquids or vapors. Good quality concrete is relatively impermeable to liquids but freely transmits vapors. If impermeable materials are used for large patches, overlays or coatings, moisture vapor that passes up through the base concrete can be entrapped between the concrete and topping. Entrapped moisture can cause a failure either at the bond line or within the weaker of the two materials.
