Influence of silica fume on the interfacial bond between aggregate and matrix in near-surface layer of concrete

The interfacial transition zone (ITZ) as the weakest position in concrete is always paid much attention. This paper presents the results of an investigation on the interfacial bond between aggregate and matrix in the near surface layer of concrete at the macro- and micro-level. Specimens with different silica fume additions (0%, 6%, 9% and 12% by mass of cement) and cement dosages (400 and 450 kg/m3) were prepared by removing certain near-surface mortar and making coarse aggregates exposed. The interfacial bond properties were evaluated by the pull-out test and microhardness test. It was found that ITZs around the near-surface-zone aggregate were influenced not only by the Wall Effect and the accumulation of microbleeding water under aggregate, but also by the near-surface weakness zone effect. The additional silica fume can successively enhance the interfacial bond strength, decrease the thickness of the near-surface weakness zone and improve ITZs in the near-surface layer of concrete.