compatibility of ultra high performance concrete as repair material

The different failure modes obtained throughout this experimental work show that theslant shear test method is highly dependent on the joint angle used: the bond capacity andfailure mode can vary significantly from one interface angle to other.As previously discussed, there is controversy between researchers about the sensitivity ofthis test to the degree of roughness of the concrete substrate. The excellent bond betweenUHPC and NSC prevents the development of a conclusive statement about this matterdue to the fact that most of the specimens did not fail in the bond interface. Therefore,most of the bond capacities estimated are a lower bound of the real strength of the bond.However, there is some evidence that suggests the sensitivity of this test to the degree ofroughness of the concrete substrate, such as the study case of specimens with bondinterface of 70° at 8 days, all specimens with brushed surface failed in the concretesubstrate whereas that the most of other specimens still failed in the concrete substrate. Inorder to draw a definitive conclusion in this matter, it would be necessary to designconcrete substrate with higher mechanical strengths to try to obtain more sliding failuremodes (failure along the interface).The evolution of the bond strength has a critical point between the 48 hours and 72 hoursafter casting the UHPC on the concrete substrate. During the second and third day, theUHPC and bond interface strengths exceed that of the NSC. This conclusion is applicableto the specified UHPC used in this research and under ambient conditions. This intervalmight vary with other UHPC formulations or under different curing treatments. Thethreshold of 3 days to acquire a UHPC and bond with higher strengths than that of NSCcoincides with the recommendation given by Lafarge North America (the providercompany of the UHPC material used in this project) which suggests not to apply any loaduntil UHPC has developed a compressive strength of 14500 psi. Notice that thecompressive strength under ambient conditions obtained in this research at 2 days was11688 psi, at 3 days 12298 psi and at 8 days greater than 15286 psi.The COV varies from 2.4% to 23.9% hence, the test can be considered as consistent. Thisrange is similar to the different COV’s obtained by other authors, such as (Santos andJulio 2011), from 2.1% to 38.3%, or Momayez et al. (2005), from 4.7 to 15.8%.90