Design And Preparation Of Low Shrinkage Ultra-high Performance Concrete And Study On Shrinkage Reduction Mechanism

Ultra-high performance concrete(UHPC)has excellent properties such as ultra-high strength,high toughness,high impermeability,high corrosion resistance and so on.Therefore,which has a wide application prospect.UHPC has a very low water-cement ratio,a large amount of cementitious materials and a small particle size,which results in more serious shrinkage deformation than ordinary concrete.In practical engineering,shrinkage cracking of concrete poses a threat to the safety and durability of structure.In this paper,low shrinkage UHPC was prepared from three systems of UHPC,paste,mortar and concrete,based on expansion compensation and aggregate restraint shrinkage control technology.The effects of two shrinkage control technologies on the workability and mechanical properties of UHPC were studied,and the effects of two shrinkage control technologies on the whole process and staged shrinkage change of UHPC were fully evaluated.The hydration process,composition,morphology and distribution of hydration products and pore structure of low shrinkage UHPC were investigated by combining various micro-analysis techniques.The mechanism of shrinkage reduction of UHPC system by two kinds of shrinkage control techniques was explained.Based on the research in this paper,the main conclusions are as follows:(1)UHPC paste system-expansion compensation shrinkage control technologyCa-EA and Mg-EA can reduce the flow expansion of UHPC.The initial setting time of UHPC mixed with Ca-EA is shorter.The addition of expansion agent can improve the compressive and flexural strength of UHPC system.The effect of Ca-EA on the mechanical properties of UHPC paste is more obvious.The shrinkage reduction effect of Ca-EA is mainly the expansion phenomenon occurring within 24 hours after initial setting;the shrinkage reduction stage of Mg-EA is mainly concentrated in the later period of the system,and the effect is stable and slow.The shrinkage reduction mechanism of Ca-EA is mainly to promote the hydration process of UHPC system,generate thermal expansion phenomenon by hydration exothermic,produce expansive substances and crystallization pressure,while the shrinkage reduction mechanism of Mg-EA is to optimize pore structure and reduce capillary pore pressure.Both types of expansive agents can effectively reduce the long-term shrinkage,wholeprocess shrinkage and drying shrinkage of UHPC paste.The shrinkage reduction effect of Ca-EA on UHPC is better than that of Mg-EA,which is consistent with the effect of expansive agent on the early autogenous shrinkage of UHPC.In dry environment,Ca-EA can promote the formation of ettringite in UHPC,which reduces the long-term shrinkage of UHPC to a certain extent.The total porosity of UHPC system with expansion agent decreased,which is greater with Ca-EA.(2)UHPC mortar system-fine aggregate shrinkage suppression technologyThe order of flow expansion of UHPC with different fine aggregates is aeolian sand > manufactured sand > natural river sand > recycled sand.The effects of different fine aggregates on the mechanical properties of UHPC are arranged according to the improvement effect: in compressive strength,recycled sand > manufactured sand > natural river sand > aeolian sand;in flexural strength,recycled sand > natural river sand > manufactured sand > aeolian sand.According to the study of fluidity and mechanical properties,recycled sand can be used in UHPC.manufactured sand and aeolian sand can reduce mechanical properties,but it can basically meet the requirements of high performance concrete.The autogenous shrinkage of UHPC can be significantly reduced by adding fine aggregate.The autogenous shrinkage of UHPC mortar system decreases with the increase of mortar-binder ratio,but the effect is not obvious.Compared with natural river sand UHPC mortar,the shrinkage reduction effect of recycled sand is the most obvious,the autogenous shrinkage value of recycled sand decreases by 39.4%,while the autogenous shrinkage value of UHPC with manufactured sand and aeolian sand increases by about 35%.The obvious shrinkage reduction effect of recycled sand is due to the existence of pore in the recycled sand particles,water absorption in the early forming stage,water retention and release during the hydration process of paste,which can effectively regulate the internal relative humidity and hydration process of the system.The second reason is that the interface transition zone between recycled sand and slurry is well bonded and has no obvious defects,which can effectively inhibit the shrinkage.Natural river sand can effectively inhibit shrinkage,but it has little effect on hydration process.There are defects in the interface transition zone between manufactured sand and aeolian sand system,and the restraint shrinkage effect is poor,so the self-shrinkage is larger.The long-term shrinkage,drying shrinkage and total shrinkage of UHPC mortar with different fine aggregates are basically consistent with the autogenous shrinkage.The dry shrinkage value of recycled sand system accounts for 44% of the total shrinkage value,and that of the other three sand system accounts for 34% of the total shrinkage value.The recycled sand can effectively reduce the early autogenous shrinkage of UHPC,but the change of dry shrinkage is larger than that of the other three fine aggregates.(3)UHPC concrete system-coarse aggregate constrained shrinkage technologyThe flow expansion of UHPC with coarse aggregate decreases with the increase of volume fraction of coarse aggregate.The workability of UHPC decreases significantly when the volume fraction of coarse aggregate is more than 20%.Compared with the UHPC system without coarse aggregate,the compressive strength of UHPC with coarse aggregate decreases.When the volume of coarse aggregate is 20%,the compressive strength is the best.The early autogenous shrinkage of UHPC with coarse aggregate tends to decrease with the increase of the volume fraction of coarse aggregate.Within 24 hours after initial setting,the autogenous shrinkage of UHPC with coarse aggregate tends to decrease rapidly.From 24 hours to 48 hours,the autogenous shrinkage of UHPC with coarse aggregate shows a plateau period,accompanied by a small micro-expansion phenomenon.After that,the autogenous shrinkage deformation of UHPC system occurs again.When the volume fraction of coarse aggregate is 30%,there is no micro-expansion.The long-term shrinkage of UHPC becomes slower with the increase of the volume fraction of coarse aggregate,and decreases gradually.Compared with UHPC mortar system,coarse aggregate can effectively reduce the drying shrinkage of UHPC system by about 30%.When the volume content of coarse aggregate is 20%,the proportion of drying shrinkage to the total shrinkage is the smallest.The total porosity can indicate the compactness of the specimens to a certain extent,but it can be seen from the pore size distribution that the small hole has the greatest influence on the shrinkage deformation.Considering the workability,compressive strength,autogenous shrinkage and drying shrinkage of UHPC with different volume fraction of coarse aggregate,it is found that when the volume fraction of coarse aggregate is 20%,the performance of UHPC is excellent,so the optimum volume fraction of UHPC with low shrinkage is 20%.