The Meso-Structure Optimization And Its Effect On The Permeability Of Self-Compacting Concrete

Concrete structures can be divided into macro, meso and micro levels. Meso-structureincludes aggregates, cement paste and the interfacial transition zone between them.Self-compacting concrete (SCC) has a large cementitious material consumption, sand ratioand small particle aggregate and other characteristics. Compared with ordinary concrete，itsmeso-structure has changed greatly, resulting in changes in the permeability of concrete. Inthis paper, using concrete chloride ion diffusion coefficient for the characterization ofpermeability, using a mercury intrusion porosimetry technique (MIP), the capillary absorptiontest, the backscattered electron imaging (BSE) and an optical microscope image processingtechnology porosimetry and other test methods, we studied the factors of paste’pore structureand ITZ and its relationship with the permeability of SCC, and proposed a mix designmethods basing on meso-structure optimization, providing a reference for the wideapplication of SCC.This paper studied the effects of water-cement ratio and mineral admixtures for pastepore structure and permeability of SCC. The results show that: the pore size is smaller. In therange of0.33to0.38, when the water-cement ratio is0.34, the thickness of the water film ofcement and other particles is moderate. The interconnected degree of capillary is lower. Themost probable pore size and critical aperture are smaller. Slag can significantly reduce thecapillary pores of SCC, and reduces Cl-diffusion coefficient. But it can not reduce the coarsepores very well. So the most suitable content is within30%. The fly ash can improve theperformance and density of fresh concrete. And it also reduces coarse pores. But it can notreduce the capillary very well. So the most suitable content is within40%. Silica fume canreduce the porosity in the aggregate interface, and silica fume can effectively cut capillary’sconnectivity. But at the same time, the hydration products formed finer gel pore. So the mostsuitable content is within7%. Double mixed slag and fly ash can take advantage of theirrespective strengths. Slap can reduce the capillary very well, and fly ash can reduce the coarsepores very well. Together they improve the pore structure of SCC. When the slag/fly ash ratio is70/10, SCC has a better pore structure.The results show that: in the range of49%to55%, when the sand ratio is51%, thefresh concrete has better mobility and higher density. And the fine aggregate interface is less.The interconnected degree of capillary is also lower. When the surface area of all aggregates3716m2/m3, the most probable pore size and critical aperture are smaller with fewermacroporous content, relatively more large pores and low porosity.the total surface area offine aggregate is linear with the chloride ion diffusion coefficient. And the correlationcoefficient is0.974. So we should use a moderate total aggregate interface surface area (216m2/m3or so). When the fine particle size aggregate (5~10mm) content is between30%and40%, the packing density of aggregate is higher, and the surface porosity on the aggregate islower. The porosity is much less. The interfacial bonding is much better. With the increase ofaggregate size, the porosity increases on the aggregate surface, and the interface is moreobvious, the interfacial bonding is worse. On the fine particle size aggregate and coarseaggregate particle surface, Ca, O and Al element enrich, and there are much hydrationproducts of larger crystals CH and sulphoaluminate. When designing SCC mix, we shouldchoose the appropriate content of intermediate particle aggregates (10~16mm).And then using grey incidence analysis, we study the contribution of all factors on thepermeability of SCC. As the result, the factors affect permeability of SCC in the followingorder: the surface area of10~16mm aggregates, the pore content of greater than1000μm, thepores content of20~50nm, the surface area of coarse aggregates, the surface area of allaggregates, the pores content of100~400μm and the pores content of50~200nm.At last, this paper presents a method for optimizing the design. When the cementitiousmaterial consumption down to448~430kg/m3, when mixed with20%slag and30%fly ash,using a larger content of medium and fine particle aggregates, the performance of mechanicalproperties and chloride ion diffusion coefficient can also be prepared to meet the requirements.At this point, the most probable pore size and critical aperture is40.258nm and6.032nmrespectively, the quantity of macroporous and the diameter does not increase significantly, andthe pore structure does not deteriorate. The porosity on aggrrgate surface reduces, the width of crack is reduced. In the near-surface, there is distributing uniformly CH, C-S-H hydrationproducts. Fly ash and other particles can be distributed in the aggregate surface, filling thepores. So it has a more favorable ITZ.