Importance of modulus of elasticity In surface repair materials (ACI 364.5T-10)

Description

Concrete repairs may be broadly classified as structural (load-carrying) and nonstructural (protective and/or cosmetic). The load-carrying capacity and stress distribution must be considered in the case of structural repairs where replacement of deteriorated load-bearing concrete is required. The repair material may be subjected to tension, compression, and/or shear forces. Yet, even with equivalent or higher modulus repair material, perma-nent loads will not be shared unless the existing member is unloaded prior to repair. Different approaches may be required for the design and analysis of such repairs. It should be emphasized, however, that a structural repair must also protect the underlying concrete and reinforcing steel from deterioration and corrosion.

The modulus of elasticity is a measure of the stiffness of a material.1,2 A material with a higher modulus of elasticity is more rigid than a lower modulus material, which is more flexible. The modulus of elasticity is the constant of proportionality between the applied stress and the strain within the linear stress-strain range of the material. It corresponds to the slope of the straight line portion of a graph of stress* versus strain.† The term elasticity refers to the reversible character of the dimensional change (as a spring would recover if compressed or stretched). The modulus of elasticity of repair materials is typically measured using ASTM C469 or ASTM C580 test methods. Additional information regarding the modulus of elasticity testing and its significance is included in ACI 546.3R.

The modulus of elasticity of a repair material with respect to that of the substrate partially determines the relative loads resisted by the new repair and the remaining substrate materials. One important consideration for proper material selection is dimensional compatibility between the substrate and repair.3-5 If a material of lower modulus than the substrate is selected, the repair is unable to carry as much load as the original concrete, but it is helpful in reducing stresses induced by volume changes in the material or surrounding concrete. Conversely, if a higher modulus material than the substrate is selected, not only will the repair carry more load relative to the substrate, but the thermal- and shrinkage-induced stresses may also be concentrated in the repair material—a situation that can lead to cracking or debonding of the repair.