ACI 364.5T-10 Importance of modulus of e lastIcIty In surface repaIr m aterIals.
When materials with differing moduli 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 (left view in Fig. 1), as in the case of pavement overlay on a highly rigid foundation, a difference in modulus of elasticity between the repair material 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 (right view in Fig. 1).
When the moduli of the repair and of the substrate differ signifcantly, environmental loads (that is, drying shrinkage and thermal deformations) can also generate internal stresses such that cracking and, ultimately, debonding could occur. Moreover, shrinkage or thermal expansion and contraction can cause distress if the modulus of the repair material is too high to permit deformation without excessive stress at the bond line. It is important to consider shrinkage compatibility and thermal compatibility (coeffcient of thermal expansion) when selecting repair materials. Refer to ACI 546.3R for information regarding these parameters. More compliant (lower modulus) repair material will reduce stresses due to shrinkage and thermal incompatibility, but may not be acceptable for structural repairs.
Summary Surface repairs must be dimensionally and mechanically compatible 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 material modulus of elasticity with respect to that of the substrate needs to be taken into account.
Referenced standards and reports The standards and reports listed below were the latest editions at the time this document was prepared. Because these documents are revised frequently, the reader is advised to contact the proper sponsoring group if it is desired to refer to the latest version.ACI 364.5T pdf download.