compatibility of ultra high performance concrete as repair material

Concrete Repair Institute 1997; Silfwerbrand 1990; Sprinkel 1997). On the other hand,there are conflicting opinions between engineers about how other factors affect the bond.The moisture condition of the substrate material is one example. It is hard to draw ageneral conclusion about how the saturated concrete substrate helps to improve the bondstrength because it depends on the sorptivity and porosity characteristics of the substratematerial. A large porosity could produce a weak interface zone due to the lack of waterfor proper hydration, but in contrast a low porosity could reduce the mechanical interlockbetween the substrate and overlay materials (Beushausen 2010). Momayez et al. (2005)stated that the bond strength as well depends on the loading scenario. According to hisexperimental results, the measured bond strength decreases with the test method in thenext order: slant shear, bi-surface shear, splitting and pull off.2.3.1 Tests Methods to evaluate Bond StrengthThe actual tests used for assessing the bond strength between concrete substrate andoverlay material are classified into three main categories according to the stressmeasured: tension, pure shear, and a combined state of shear and compression stresses(Espeche and Leon 2011; Momayez et al. 2005).The first group includes all those tests that evaluate the bond strength under tensionstress, being the pull off test (a), splitting tensile test (m,n) and direct tension test (l) themost accepted methods of this first group (Espeche and Leon 2011).The second category covers all those tests that measure the pure shear stress. Torsionbond test (b), the direct shear test (c), the modified vertical shear bond test (f), the pushouttest (j), the bisurface shear test (k), and the guillotine test (g, h, i) fall in this group.The main challenge with the pure shear tests is properly subjecting the interface betweenboth materials to only shear stress without transmitting any bending stress (Espeche andLeon 2011). One example of this is given by Sprinkel (1997) who states that theguillotine should not be considered as a reliable indicator of adhesion strength due that itsresults highly depend on the alignment of the bond line, if it is not perfectly centered, the27