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Material Selection for Concrete Repair Works

We need to carefully select parameters that matter for concrete repair strategy, or else it will affect the serviceability.

1. Modulus of Elasticity and Strength of Repair Material
   The modulus of elasticity of the repair material affects the resultant flexural stiffness of the repair members, the tensile stress present within the repair material, and the debonding stress at the interface when differential movement occurs between the repair material and substance. A higher difference in modulus of elasticity between the repair material and substrate may adversely affect the stress distribution within the repaired composite material’s cross-section and may lead to considerable stress concentration.
   Therefore, the repair material selected should have a similar modulus of elasticity as the substrate has. Thus considering the strength of material alone seems less fact, an overemphasis on strength may cause repairs to experience cracking arising from drying shrinkage, creep, and heat of hydration.
2. Coefficient of Thermal Expansion of Repair Material
   Tensile stresses in the repair material caused by changes in the temperature of the surrounding environment are proportional to the differences in the coefficient of thermal expansion and the change in temperature. Therefore, the repair material selected should have a similar coefficient of thermal expansion as the substrate has.
3. The Thickness of Repair
   The internal stresses within the repair material and substrate are affected not only by differential movements but also by the relative thickness. A thinner repair layer is more easily cracks or debonded by the higher tensile stress, which occurs in the repair material. For most available repair materials, there seemed to be an optimum thickness of repair material, which results in the lowest tense stress occurring within the repair material for a given amount of differential movement between the repair material and substrate. This optimum value is affected by the ratio of the modulus of elasticity of the repair material to the substrate and thickness of repair material to the substrate.
4. Shrinkage and Creep of Repair Material
   Differential shrinkage of the repair material and substrate is another important consideration of a durable repair. The most common damages in concrete between substrate and repair materials and are proportional to the differential shrinkage. Therefore, the repair material selected should have fixed shrinkage properties that are as low as possible. For the repair material under tension, creep may mitigate against the tensile stresses caused by differential shrinkage. However, for the repair material under compression, creep may decrease the compressive stress within the repair material and aggravate the compressive stress in the substrate caused by the different shrinkage. The creep of the repair material should be controlled based on the state of stress that the repair material will be subjected to service.

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