Study On The Mechanical Properties And Durability Of Ultra-high Performance Concrete Prepared From Recycled Aggregate And Stone Chips

Ultra High Performance Concrete(UHPC)is a cement-based composite material with ultra-high strength,high toughness and high durability.Ultra-high performance concrete aggregates are made of quartz sand with a particle size of less than 2mm,which reduces internal defects,improves uniformity,and has excellent mechanical properties and durability.The production process of quartz sand resources is complicated,which increases the production cost of UHPC and limits the large-scale promotion and application of UHPC.This paper uses recycled aggregates produced from waste concrete and stone chip aggregates produced from waste stone chips to prepare ecological UHPC,which reduces the cost of UHPC preparation and consumes solid waste.The effects of raw materials,health conditions,mix ratio and other factors on the mechanical properties and durability of ecological UHPC prepared from recycled aggregates and stone chips are studied.After the mix ratio test,the main conclusions obtained are as follows:(1)Using stone chip aggregates,an eco-type UHPC that is inexpensive,environmentally friendly and meets performance requirements is prepared.The mechanical properties of stone chip UHPC have reached the performance level of RPC100 in the national standard.Through the mix ratio test,it is found that as the water-binder ratio increases in the UHPC mix ratio,the mechanical properties gradually decrease and the fluidity gradually increases;with the increase of the aggregate content,the mechanical properties first increase and then decrease,and the fluidity gradually decreases;With the increase of the amount of water reducing agent,the change of mechanical properties is small,and the fluidity gradually increases;with the increase of the amount of steel fiber,the mechanical properties gradually increase and the fluidity decreases.When the aggregate ratio is 1.6,the water-gel ratio is 0.2,the volume of steel fiber is 1.5%,and the content of water reducing agent is 2.1%,the mechanical strength and working performance of stone chips UHPC are the best.The characteristics of the stress-strain curve of stone chips UHPC are similar to those of traditional UHPC.Steel fiber can not only improve the mechanical strength of UHPC,but also effectively improve the toughness and residual compressive strength of UHPC.Thermal curing can effectively improve the compressive strength of stone chips UHPC,but has little effect on the flexural strength.The effect of the four curing conditions on the UHPC of stone chips is from high to low: combined curing>dry heat curing>water bath curing>standard curing.(2)The ecological UHPC made from recycled aggregates has a flexural strength that reaches the RPC100 mechanical grade standard,and the compressive strength exceeds the performance requirements of high-strength concrete.Through the mix ratio test,it is found that in the UHPC mix ratio,as the aggregate ratio increases,the mechanics gradually decreases;with the increase of the steel fiber content,the flexural strength and split tensile strength gradually increase,and the compressive strength changes little;Steam curing can greatly improve the mechanical properties of ecological UHPC.UHPC with 0.6～2.36 mm aggregate has the best mechanical properties.It is recommended that recycled aggregates in this range should be used first in engineering applications.(3)The slow freezing method was used to study the frost resistance in the durability of recycled aggregate UHPC.The test found that after the freeze-thaw cycle,the compressive strength of recycled aggregate UHPC specimens decreased by 21% at the highest and 9% at the lowest,reach the frost resistance mark of concrete D50.Recycled aggregate UHPC has excellent durability and can be used as antifreeze concrete in cold regions.With the increase of aggregate content,the loss rate of compressive strength gradually decreases,and the frost resistance of UHPC gradually improves.The strength loss rate of UHPC specimens after standard curing and steam curing is not much different,and steam curing has a limited effect on the frost resistance of ecological UHPC.The aggregate size of different particle sizes has little effect on the durability of UHPC,and the strength loss rate is about 12%.After freezing and thawing,the splitting tensile strength loss rate of the split specimens is 8.8%～23.3%,and the strength loss rate of the steam curing specimens is relatively low.