Design And Performance Characterization Of Ultra-high Strength Lightweight Aggregate Concrete Based On Homogeneous Structure

Lightweight aggregate concrete(LWAC)is a mixture of cementitious materials,lightweight coarse aggregate/lightweight sand(or ordinary sand)and water,which the apparent density is less than 1950kg/m~3.Due to its low dead load(lightweight),excellent thermal insulation and fire resistance,it has broad application prospects in projects such as bridges external wall decoration,lightweight structures,and protective structures.However,the“lightweight”of LWAC is always accompanied by a significant deterioration in mechanical properties,because of the low strength of lightweight aggregate.As a result,this is a critical issure that limits the application of LWAC in structural engineering.Previous studies have been shown that the mechanical properties of LWAC could be enhanced by selecting high-strength lightweight aggregates and constructing core-shell lightweight aggregates(effectively improved the strength of lightweight aggregates).Nevertheless,the performance of the preferred or processed lightweight aggregates still has a significant difference with natural aggregates(sand and stone),resulting in a limited increase in the strength of LWAC.Consequently,it is not possible to fundamentally unify the light weight and high strength of LWAC.It is an effective way to enhance LWAC including reducing the particle size of light aggregates,improving the structural homogeneity,and building a stress-dispersed structure.Unfortunately,the material composition of LWAC prevents the incorporation of large quantities of small particle size lightweight aggregate.The ultra-high performance concrete(UHPC)is a highly homogeneous cementitious material constructed by eliminating coarse aggregates.Due to the ultra-low water-to-cement ratio(w/c)of UHPC,a large amount of cement only plays a physical filling role,and micron-sized fillers are frequently used instead of cement.Moreover,the maximum size of aggregate in conventional UHPC materials is less than 1mm,which provides for the massive introduction of small particle size lightweight aggregates.Therefore,this study uses an UHPC matrix to build LWAC with a highly homogeneous structure by incorporating abundant small particle size lightweight aggregates,and systematically evaluates its macroscopic properties.Furthermore,peridynamics(PD)simulations and fracture mechanics theory are employed to investigate the mechanism of structural homogeneity enhancement of concrete materials on macroscopic properties.Hence,the theoretical foundation and technical support are provided for the design and preparation of high strength/ultra-high strength LWAC.The main conclusions are as follows:(1)The modified Andreasen&Andersen(MAA)model was applied to design the high homogeneous matrix UHPC based on the theory of close packing of particles.Then,a mathematical model between the macroscopic properties(density and strength)of LWAC and lightweight aggregates/filler admixture is developed and solved by response surface methodology(RSM)to determine the dosage of lightweight aggregates/filler admixture.Finally,the LWAC mixture proportion design is achieved.(2)The homogeneity of the LWAC is evaluated by using computerized tomography and with the aid of an image processing system.The results show that the dispersion coefficients of the lightweight aggregates in the LWAC are 0.9450 and 0.9377respectively.The results demonstrate the feasibility of preparing homogeneous structures with a combination of UHPC matrix and small particle size lightweight aggregates.(3)The macro performance of LWAC is characterized as follows:the apparent densities of LWAC1 and LWAC2 are 1670 kg/m~3 and 1800 kg/m~3,respectively;the 28d compressive strengths are 121.9 MPa and 124.4 MPa,respectively;the 28d chloride ion penetration resistance depths of both groups of LWAC is 0mm and the electric fluxes is less than 50C.The above results indicate that the LWAC based on homogeneous elevated design has ultra-high mechanical properties and excellent durability.(4)Peridynamics(PD)simulations and fracture mechanics theory are used to investigate the mechanism by which structural homogeneity enhancement(reduction in the particle size of the lightweight aggregate)acts on the compressive strength of concrete.The results show that cracks sprout from the lightweight aggregate in LWAC,but the increased homogeneity reduces the stress concentration in the lightweight aggregate concrete.This leads to a switch from brittle damage to ductile damage with multiple cracks in the concrete,enabling the preparation of ultra-high strength LWAC.