compatibility of ultra high performance concrete as repair material

(a) Molds to cast slant shearspecimens.Figure 3.9 Slant shear molds(b) Molds after applying aconcrete surface retarder toobtain a rough surface.(c) NSC substrate forslant shear samplesafter demolding.Normal concrete was poured in the free corner of the moulds. A steel bar was used toimprove the compaction of the concrete during its placing and a vibration of 5-10 secondswas applied once the mold was totally filled. After casting, all concrete substratesunderwent a 24 hour moist cure, according to ASTM C31/C31M. Afterwards, thesamples were demoulded and cured in a lime water tank for 28 days. Figure 3.5 showsslant shear substrates after demoulding.The surface treatments for the slant shear test were different to those of the splittingtensile test due to the fact that the smooth, brushed and chipped surfaces obtained for theindirect tensile test had similar profile. It was decided to work only with the brushedsurface because it is the most representative. A new surface was introduced in order toobtain a higher contrast with respect the other surfaces (brushed, sandblasted andgrooved).For obtaining the rough surface, a concrete retarder was applied to the wooden inserts inorder to delay the set of the concrete surface, facilitating the creation of exposedaggregate surface. The concrete retarder Formula F provided by The Euclid ChemicalCompany was used. This product does not affect the mechanical properties of theconcrete and offers the advantage that is applied directly to forms instead of applying toconcrete surfaces. This retarder was applied around 24 hours before casting concrete.Several layers had to be applied. More information about the concrete retarder used is54