Nevertheless, lowering the repair material stiffness is very desirable in any nonstructural repairs. The factors aflecting the modulus of elasticity of cement-based materials are related to compressive strength and density. Hence, factors that affect strength such as cenhent content, w/c,n, aggregate size, type and grading, curing conditions, and age at the time of testing should similarly influence modulus. The modulus of elasticity of cement- based materials can be reduced with a higher w/c,n and aggregate with a lower modulus of elasticity.
Cracks exist in a composite repair system for reasons other than service loads. Some of these are caused not only by the drying shrinkage of the repair material and the difference in the coefficients of thermal expansion from the substrate material, but also to the differences in the elastic moduli of the repair material and substrate concrete.
The compatibility in elastic modull, therefore, becomes. in cases of structural repairs, an important factor because incompatibility may lead to considerable stress concentration when widely differential volume changes of the repair material in relation to the concrete substrate occur. Because, iii such situations, the interfaclal bond region (transition zone) is the weak link in the repair system. cracks will tend to form in this region. In certain cases where bond strength is high, cracks will occur in the matrix of the material, which has the highest modulus of elasticity. When external load is perpendicular to the bond line, as in the case of repaired pave- ment, differences in modulus of elasticity regarding the varying stiffness between repair materials and concrete substrate is normally not problematic Vaysburd et al. 2014).
Conversely, in repairs where the service load is parallel to the bond line, differences In modulus of elasticity may cause load transler to the high-modulus material if the other materials yield under the stress. if the load transfer is beyond the load-bearing capacity of the higher-modulus material, it will fracture and damage the structure
Creep—Relaxation through tensile creep reduces the stresses induced in a repair system by restrained drying shrinkage, thereby enhancing the crack resistance of repairs. Test results show tensile creep capacity is dependent on material composition, often more so than shrinkage (Pigeon and Bissonnette 1999). Because a reduction In paste content reduces shrinkage and appears to increase tensile creep, a proper repair mixture should have the lowest practical cement content. Also, in general, creep of cement-based materials is Inversely proportional to the modulus of elasticity and compressive strength; therefore, high-strength, high-modulus materials are generally not desirable for nonstruct ural concrete repairs.ACI 364.16T pdf download.