When materials with differing moduli of elasticity act monolithically, the difference in stiffness may lead to distress under externally applied loads, depending on the induced stress feld with respect to the structure layout. For example, when the external load is essentially perpendicular to the bond line (Fig. 2(a)), as in the case of pavement overlay on a rigid foundation, a difference in modulus of elasticity between the repair mate- rials and the substrate does not signifcantly affect the overall mechanical behavior of the structure. Conversely, in repairs where the service load stresses are parallel to the bond line, the deformation of the lower modulus material causes the load to be transferred to the higher modulus material, which may then become overstressed and fracture (Fig. 2(b)).
When the moduli of elasticity of the repair and of the substrate differ signifcantly, environmental loads (that is, drying shrinkage and thermal deformations) can also generate internal stresses that could cause cracking and, ultimately, debonding to occur. Such a risk exists, for instance, if the modulus of elasticity of the repair materials is very high, promoting excessive shrinkage- and thermal deformation-induced stresses at the bond line. It is important to consider volume change compatibility and mechanical compatibility when selecting repair mate- rials. Refer to ACI 546.3R and ACI 562 for information regarding these considerations. More compliant (lower modulus of elasticity and higher creep) repair materials will reduce stresses due to shrinkage and thermal incompatibility, but may not be acceptable for structural repairs.
Surface repairs should be selected for volume change and mechanical compatibility with the existing concrete substrate to minimize cracking and ensure that the repair and existing structure act monolithically. To achieve repairs that comply with these requirements, the magnitude of the repair materials’ modulus of elasticity with respect to that of the substrate needs to be taken into account. Ideal behavior is obtained when the modulus of elasticity of the repair materials is similar to that of the substrate for structural repairs, whereas a repair mate- rial with a lower modulus of elasticity may be benefcial in the case of nonstructural repairs.ACI PRC-364.5 pdf download.