Optimized Design And Key Performance Of Eco-HDCC With Reclaimed Asphalt Pavement As Aggregate

With the advancement of my country’s"One Belt One Road"initiative,the infrastructure construction industry will continue to flourish.The large-scale and complicated environment of civil engineering put forward higher requirements for concrete.High performance and high functionality are important development directions for concrete materials.Ecological High Ductility Cementitious Composites（Eco-HDCC）have shown great advantages in improving the safety and durability of buildings due to their ultra-high tensile ductility and excellent crack control capabilities.On the one hand,with the gradual shortage of natural high-quality river sand resources,it is unable to meet the huge increasing demand for construction aggregates.On the other hand,my country maintains and maintains asphalt roads every year,producing a large amount of asphalt milling material,and the recycling rate is less than 30%.In order to improve its utilization rate,this paper explored the potential use of asphalt milling material（RAP）to replace river sand（RS）in Eco-HDCC production,and studied the impact of different RAP content and particle size ranges on the key performance of Eco-HDCC.In this process,the effect of different fresh mixing states on the degree of fiber dispersion in the matrix and its influence on the tensile properties of Eco-HDCC were studied.The main research contents are as follows:First,the effect of RAP content on Eco-HDCC fresh mix performance and mechanical properties after hardening was studied.The fluidity of Eco-HDCC doped with RAP is reduced.The reason is that the surface of RAP is covered with a layer of asphalt film,which has a hydrophobic and rough surface and high water absorption.The SEM results show that the interface between RAP and the surrounding cement matrix is well bonded,and there is no significant increase in pores.This phenomenon explains why the compressive strength of Eco-HDCC fully doped with RAP is maintained at about 55 MPa.With the increase of RAP content,the tensile strain capacity of Eco-HDCC is increased by up to 70%,reaching 2.44%.The reasons are as follows.On the one hand,the fracture toughness(₈))and crack tip toughness(₄))of Eco-HDCC matrix decrease with the increase of RAP content.On the other hand,the matrix/fiber interface parameters related to fiber bridging ability,with the increase of RAP content,lower friction stress（τ₀）and slip hardening coefficient（β）are observed,resulting in fiber bridging residual energy(^′_（7）increase.Finally,the increase in the calculated quasi-strain hardening index（PSH）explains the significant increase in the tensile strain capacity of Eco-HDCC doped with RAP.Secondly,Eco-HDCC was prepared with full-mixed asphalt milling material as aggregate,and the effect of cement-based slurry in different freshly mixed states on the uniformity of fiber dispersion in the hardened matrix was studied,and its influence on the macroscopic tensile properties of Eco-HDCC.The results show that increasing the amount of water reducing agent can effectively reduce the plastic viscosity and yield stress of the Eco-HDCC fresh slurry.Thickeners can effectively increase the plastic viscosity of the slurry,but have little effect on the yield stress.Hershel-Bulkley equation is suitable for describing the rheological behavior of Eco-HDCC fresh slurry（without fiber）,and the correlation coefficient reaches 0.999.The Mini-slump expansion degree of the fresh slurry has a high correlation with the yield stress,and the results of Marsh time and plastic viscosity fit well,which can be applied to semi-quantitatively characterize the flow properties of the Eco-HDCC fresh slurry,and guide the laboratory and on-site Trial formulation of construction.The fiber dispersionα₁)has little correlation with the yield stress,but has a higher correlation with the plastic viscosity.Theoretically,the higher the plastic viscosity of the fresh slurry,the lower the yield stress,the higher the fiber dispersion.In fact,under the premise of ensuring the strength of the matrix,in order to obtain a fiber dispersion greater than 0.75,the plastic viscosity of the fresh slurry is 5-11 Pa·s,and the yield stress is between 0-18 Pa.As the degree of fiber dispersion increases,the ultimate tensile strain of Eco-HDCC generally increases exponentially,indicating that the ultimate tensile strain is more sensitive to fiber dispersion.Whenα₁)>0.75,the increase in ultimate tensile strain is more obvious.The ultimate tensile strength has a low correlation with fiber dispersion,which is related to the internal defect size of the matrix itself,but whenα₁)>0.75,the ultimate tensile strength is generally greater than 3.64 MPa.Finally,the influence of RAP with different particle size ranges on the mechanical properties of Eco-HDCC and matrix is explored.As the RAP particle size range increases from0-0.6 mmto 0-2 mm,the overall compressive strength of Eco-HDCC increases slightly.As the maximum particle size of RAP increases,the porosity of Eco-HDCC decreases overall.Except for the RAP-1 group,the pore size distributions of the other groups are relatively close.The elastic modulus and fracture toughness of the matrix decrease with the increase of the RAP particle size range,but the reduction in fracture toughness is small,so the calculated crack tip toughness increases.This phenomenon explains the increase in the RAP particle size The range is not conducive to the reason why the matrix bears the tensile load.With the increase of the RAP particle size range,the initial cracking strength,ultimate tensile strength and ultimate tensile strain generally showed a downward trend.Finally,when the maximum particle size is2mm,the ultimate tensile strain attenuation of Eco-HDCC is 44.3%of the benchmark group RAP-0.6,which is only 1.83%.Similar to the change trend of tensile properties,the initial bending strength,ultimate strength,mid-span deflection,and bending energy consumption all decreased significantly with the increase of the RAP particle size range,and the maximum decrease reached 25.2%,29.7%,46.2%and 61.5%.