Effect Of Light Aggregate On Chloride Penetration Resistance Of UHPC And Its Mechanism

The concrete structures in the eastern coastal and western saline-alkali areas of China usually have poor durability and short service life,mainly due to the existence of a large number of aggressive ions in these areas,such as Mg²⁺,SO₄^2-,Cl^-,etc,among which Cl^-is the main factor leading to the corrosion of concrete reinforcement,greatly shortening the service life of concrete.Ultra-high performance concrete（UHPC）is a kind of concrete designed based on the principle of dense skeleton packing.It has ultra-high mechanical strength and excellent durability,and can effectively resist the erosion of aggressive ions in the environment.It has a broad application prospect in bridges,highways,national defense and other engineering fields.However,UHPC also has some shortcomings such as high self-weight and large shrinkage,which result in its application being limited to a certain extent.In this paper,relying on the general project of National Natural Science Foundation of China"Preparation and Microstructure Formation Mechanism of Lightweight Low Shrinkage Ultra-high Performance Concrete"（51878003）,a kind of lightweight UHPC was prepared by replacing quartz sand with lightweight aggregate.The influence of particle size range,pre-wetting degree and replacing proportion of light aggregate on the chloride ion permeability of UHPC was studied.Meanwhile,the interfacial transition zone（ITZ）and pore structure characteristics of lightweight UHPC were studied by SEM-EDS,low field NMR and other microscopic characterization methods,and the influence mechanism of particle size range,pre-wetting degree and replacing proportion of lightweight aggregate on the chloride ion permeability of UHPC was analyzed.Finally,the transport characteristics of chloride ions in lightweight UHPC were studied by means of simulation at mesoscale.The main research achievements of this paper are as follows:Based on the lightweight and internal curing behavior of lightweight aggregate,UHPC with compressive strength greater than 100MPa,flexural strength greater than18MPa and apparent density less than 2100kg/m~3was prepared,and the influence of particle size range,pre-wetting degree and replacing proportion of lightweight aggregate on its physical and mechanical properties was investigated.With the decrease of the particle size range of lightweight aggregate,the working performance of UHPC mixture showed a trend of first improving and then weakening,while the compressive and flexural strength of UHPC gradually increased.With the increase of pre-wetting degree of lightweight aggregate,the working performance of UHPC mixture is improved,the slump loss of UHPC mixture is reduced,but the mechanical properties are reduced.When the replacing proportion of lightweight aggregate increased,the working performance of UHPC mixture became worse and mechanical properties also decreased,but the dead weight of UHPC decreased significantly.The influence of particle size range,pre-wetting degree and replacing proportion of lightweight aggregate on the chloride ion permeability of UHPC and its mechanism were revealed.It is found that reducing particle size of lightweight aggregate can effectively reduce the width of the ITZ,improve the mechanical strength of the ITZ,effectively refine the pore structure,reduce the total porosity,reduce the rapid chloride ion transport channel,and improve the chloride ion permeability of UHPC.Lightweight aggregate pre-water absorption has a great influence on UHPC.With the increase of pre-wetting degree for lightweight aggregate,the width of the ITZ of UHPC gradually increases,and the strength gradually decreases.At the same time,the porosity of UHPC increases,and the number of harmful pores increases,providing a transport channel for chloride ions,which is not conducive to the resistance of UHPC to chloride ion invasion.Compared with UHPC prepared by quartz sand,the incorporation of lightweight aggregate will degrade the internal structure of UHPC to a certain extent,which is mainly reflected in increasing width and decreasing strength of the ITZ,and the coarsening of the UHPC pore structure and the increase of porosity,which provides a favorable transport channel for the water and chloride ions,so that the chloride ion impermeability of UHPC will deteriorate.However,the chloride penetration resistance of UHPC prepared with quartz sand completely replaced by lightweight aggregate is still excellent,and the electric flux and chloride diffusion coefficient are as lower as 140.9C and 0.19×10^-12m~2/s,respectively.The global model and random aggregate model of UHPC were established based on MATLAB,and the transport process of chloride ion in lightweight aggregate UHPC was simulated by COMSOL software.Found that the extent of lightweight aggregate particle size decreases,and the chloride ion migration rate in UHPC decreases,and the reason is that lightweight aggregate particle size range increased internal UHPC sinuosity is small,the transmission path of chloride ion,chloride ion concentration and dilution,effectively reduce the ITZ area at the same time,reduce the fast transfer of chloride ion channel;With the increase of pre-wetting of lightweight aggregate,the chloride ion in UHPC transfer rate increases,the reason is that open pores are dominated in lightweight aggregate,prewetting water in lightweight aggregate are released when mixing,the release of prewetting water cause increasing water-binder ratio of cement paste around the aggregate.ITZ with high porosity provides fast channels for chloride ion transfer.With the increasing content of lightweight aggregate,the migration rate of chloride ions in UHPC increases gradually,because the lightweight aggregate replaces the quartz sand and reduces the internal tortuosity of UHPC,thus speeding up the transfer rate of chloride ions in UHPC.