Synergic Action Of Lightweight Aggregate-Matrix On Performance Of Concrete

A lighter, less water-permeable and higher earthquake-resistant concrete can beproduced by using light weight aggregate (LWA). Meanwhile, the autogenous shrinkageof high performance concrete can be mitigated by the internal curing effect induced bysaturated LWA. However, the interaction between LWA and concrete matrix in LWAconcrete is obviously different from that in concrete made of ordinary aggregatebecause of the lower density, lower mechanical property and high water-absorbingcapacity of LWA, leading to adverse influences on the workability, homogeneity,strength development and volume stability of concrete. Therefore, a study ofwater-transfer, mechanical property coordination and density coordination betweenLWA and concrete matrix based on the interaction of the two is of great importancewhen making good use of the advantages of LWA on concrete. The influence ofwater-transfer, strength coordination and density coordination between LWA andconcrete matrix on the performance of concrete was studied in this work and sometechniques were proposed to tackle with the adverse influences.Firstly, the water-transfer action between LWA and concrete matrix, as well as theworkability of concrete mixture was studied under normal and extra-pressure. LWA andcement paste of LWA-concrete were separated by using pressure bleeding instrumentand the water-transfer action between LWA and concrete matrix was illustrated by thealternation of water content of LWA and cement paste. The volume variance of concretemixture under pressure was measured as well. It is shown that more water transfersfrom concrete matrix to LWA under pressure and concrete volume has been remarkablyreduced as well, but the variations are less noticeable when LWA is pre-saturated. It isalso shown that concrete workability is remarkably reduced when extra-pressure isapplied.The influence of water-transfer action between LWA and concrete matrix on thevolume stability of concrete during hydration and hardening processes was studied. Andthe shrinkage-reducing effects of saturated-LWA and expanding agent were investigatedunder the unique and combined usage at a low water-to-binder ratio. Furthermore, theshrinkage-reducing mechanism was analyzed through concrete internal humiditymeasurement and XRD spectrum investigation. It reveals that no good drying shrinkage-reducing effect is seen when the saturated-LWA and expanding agent is solelyused if no increase in drying shrinkage is shown, however, the shrinkage-reducing effectis remarkably improved when the two are incorporated together, which leads to a higherinternal humidity of concrete and a greater hydration degree of expanding agent.Secondly, the coordination effects of mechanical property, as well as bulk densityof LWA and concrete matrix was studied. Concrete matrixes with various mechanicalproperties were made by altering water-to-cement ratio, and then they were mixed withLWAs of various mechanical properties. Results show that it is not practicable toimprove the mechanical property of concrete by increasing the mechanical property ofconcrete matrix when LWA of low mechanical property is used. It also shows that themechanical property of concrete can be significantly improved by optimizing thecompatibility of coarse and fine expanded shale under a comparable density when aconstant water-to-cement ratio is applied. Effect of density coordination of LWA andconcrete matrix on the compressive strength of concrete was also studied and it revealsthat a greater variance of bulk density of LWA and concrete matrix results in a moresignificant reduction of compressive strength resulting from segregation of concretecomponents and the strength loss is more significant in the vertical direction than that inthe lateral direction.Considering a greater water-transfer action between LWA and concrete matrix inthe ultra-light LWA concrete and greater variances of the strength and bulk density ofthe two will induce greater adverse side effects on the workability and mechanicalproperty of ultra-light LWA concrete, a new technical way was proposed to produceultra-light LWA concrete, in which a high water-to-cement ratio was used byconsidering the synergic actions of water-transfer, mechanical property and bulk densityof LWA and concrete matrix. Various types of LWA were used to prepare concrete.Variations of the slump loss, segregation of LWA, relationship between strength anddensity of the concrete with the water-to-cement ratio were investigated. The shrinkageand water absorption of the concrete with higher water-to-cement ratio were tested. It isshown that increasing the water-cement ratio reduces the segregation of lightweightaggregate and slump loss of the concrete; when W/C is0.75, a lightweight aggregateconcrete with bulk density of less than900kg/m~3and compressive strength of14MPa isobtained by using a coarse expanded shale with apparent density of738kg/m~3andresulting fine aggregate by crushing. The shrinkage and water absorption of lightweight aggregate concrete, prepared by using higher W/C, are within an acceptable range. It issuggested that the lighter the lightweight aggregate, the higher the W/C, the less thesegregation of the aggregate and slump loss of the concrete, and the higher the retainedstrength. It has been proved that there are great technical and economic advantages ofutilizing ultra-light LWA concrete on floor insulation works in practical engineeringapplications.It is concluded from the results that the advantages of LWA on concrete can bemore effectively achieved by considering both the coordination actions of water-transfer,strength and bulk density between LWA and concrete matrix and their influences on theperformance of concrete, rather than a mere focus on one certain property. Moreover,the disadvantages can be mitigated as well.