Experimental Study On The Packing Density And Its Effects On The Compressive Behavior Of Recycled Aggregate Concrete

The packing density of particles,which is defined as the ratio of the solid volume of the particles to the bulk volume occupied by the particles.Studie s on normal concrete show that: Considering the water in paste/mortar,it is the excess water(the water in excess of that needed to fill up the voids in the particle system)that lubricates the particles.Therefore,a higher packing density of the particle system would at the same water content lead to a higher flowability or at the same flowability requirement allow the use of a lower W/CM ratio to increase the strength and durability.Considering the paste in concrete,it is the excess paste(the paste in excess of that needed to fill up the voids between the aggregate particles)that lubricates the aggregate particles.Therefore,a higher packing density of the aggregate would at the same paste volume lead to a higher workability or at the same workability requirement allow the use of a smaller paste volume to increase the dimensional stability.On the other hand,higher packing density of aggregates means more excess paste to surround and lubricate the aggregates,so that to improve the rheological properties.This MSc theis presents a study on the packing density of double-blended coarse recycled aggregates,blended fine(sand)and coarse aggregates,fresh recycle aggregate concrete.Also,the mechanical properties of recycled aggregate concrete cylinder specimens made from different concrete mixes were tested and discussed.The main contents covered in this thesis includes:(1)Packing density of several double-blended coarse recycle aggregate were measured under dry condition to study the influences of aggregate size and its contents on dry packing density.The test results showed that: With the increasing of one kind of coarse aggregate contents,packing density of different combinations of coarse recycle aggregates are increasing and then declining.It’s more efficient to increase the packing density by using aggregates with significantly different particle sizes.Besides,the maximum of dry packing density tends to appear when the volume ratio of the larger size aggregates and smaller size aggregates are 3:2.(2)Packing density of blended fine(sand)and coarse aggregates were measured under wet condition to study the influences of sand ratio and water/solid ratio on the packing density.The test results showed that: with the sand ratio increasing,the basic water/solid ratio increases.The wet packing density of blended fine and coarse aggregates are higher than that in dry condition,and the filling effect of sands are more effective by the lubrication of water.Furthermore,it’s efficient to increase the packing density of blended fine and coarse aggregates when the volume ratio between the large-size and small-size course aggregates is 3:2,and the optimum sand ratio is about 30%-40%.(3)The packing density of RAC were measured under wet conditio n to explore the effect of cement content on the packing density of RAC,so as to optimize the cement content.The results showed that the packing density of recycled aggregate concrete is highest when the cement content is 20%.Furthurmore,it’s efficient to increase the packing density of recycled aggregate concrete when the volume ratio between the large-size and small-size course aggregates is 3:2 and the sand rate is about 30%-40%.(4)The short-term compressive behavior of different concrete mixes were tested by using standard cylinder specimens.Compressive stress-strain curves,Lateral strain-axial strain curves,compressive stress-volumetric strain curves were obtained.Concrete cylinder strength,elastic modulus,poisson’s ratio,toughness,speci fic toughness were determined.The test results showed that increasing of packing density for recycled aggregate concrete is helpful for improving the strength,the elastic modulus as well the thoughness and specific toughness.