Microstructure and properties of interfacial transition zone in SCC: Fid-Bau Portal

Microstructure and properties of interfacial transition zone in SCC

Zhu, Wengzhong / Bartos, Peter J.M.

The interfacial transition zone (ITZ) between cement paste and aggregates or reinforcement is an area of particular interest, its association with and significant influence on engineering properties and durability of concrete or cementitious composites being widely recognised. The formation of the ITZ in concrete is due, to a large extent, to a wall effect that prevents effective filling of the space adjacent to the aggregate/reinforcement by cement or other particles, as well as to internal bleeding or settlement in the fresh mix. Considering the different characteristics in particle packing (due to the increase of fine fillers) and the highly flowable and vibration-free placing nature of the SCC, there are general concerns that the ITZs around steel bars and coarse aggregate in structural SCC could be radically different from those in conventional vibrated concrete. An improved nano/micro-indentation method was successfully used to study properties of ITZ around coarse aggregate in practical SCC and conventional reference concretes. For the concrete mixes studied, the width of the ITZ around coarse aggregate was approximately 30 micron to 40 micron for both SCC and conventional REF mixes. The width of ITZ was slightly narrower above coarse aggregate than below it for all the mixes. The elastic modulus of the ITZ below a coarse aggregate was significantly lower than that of ITZ above the aggregate for the conventional REF mixes. The results of micro-indentation tests and microstructural observations appeared to indicate that the ITZ around coarse aggregate was denser and significantly more uniform in SCC than in conventional vibrated concrete.