Preparation Of Ultra High Performance Concrete With Untreated Sea Sand And Its Performance Study

The chloride ion content of untreated sea sand is very high,and this will accelerate the corrosion of steel bars,it must be desalinated before used safely.However,the shortage of fresh water resources in coastal areas and the cost of desalinated sea sand are too high,therefore,how to use untreated sea sand safely has become a question that engineers need to think about.Utilizing the characteristics of ultra-high performance concrete(UHPC)with excellent durability,this paper proposes to use untreated sea sand for the preparation of UHPC,to achieve the safe use of untreated sea sand,and to study the mechanical and durability properties of sea sand UHPC by sand mortar and concrete experiments,the main conclusions are as follows:(1)Considering the mechanical properties and working performance,the recommended mix proportion of UHPC is Cement: Viscosity reducer: Silica fume: Sea sand: Water: Defoamer=0.7:0.15:0.15:1.0:0.16:0.08%?(2)When the volume content of steel fiber does not exceed 1.5%,the compressive strength and flexural strength of sea-sand UHPC increase significantly with the increase of the volume content of steel fibers,and the effect of steel fiber on strength significantly decreases when continuing to increase the content.Considering economical and mechanical properties,it is recommended that the steel fiber volume is 1.5%.(3)When the volume content of steel fiber is 1.5%,the PVA fiber mixed with the volume content of less than 0.75% has little effect on strength,but the fluidity is significantly decreased.(4)The sea-sand UHPC treated with 90°C heat-retaining showed a significant decrease of 7d compressive strength compared with 4d compressive stren gth,while 70°C thermal curing showed no decrease.Compared with steam curing at 70°C for 1d,the compressive strengths of steam curing at 70°C for 2d and 3d increased by 11.4% and 16.7%,and the flexural strength increased by 18.2% and 1.8%.Considering t he cost and mechanical properties,the heat curing regime is set to steam curing at 70°C for 2 days.(5)The mechanical properties and durability of sea sand UHPC can fully meet the UHPC performance requirements of GB/T 31387-2015?Reactive Powder Concrete?.(6)For sea sand UHPC,the chloride binding degree is lower when the total chloride ion concentration of the matrix is higher.When mixed with seawater,metakaolin can significantly improve the chloride binding degree of the matrix.When not mixed with seawater,metakaolin is not harmful to chloride binding.Compared with standard curing,70 °C steam curing will reduce the chloride binding of matrix.(7)Using the half-cell potential to judge the corrosion of the internal steel bars of UHPC may lead to misjudgment.After 40 cycles of leaching and drying,the half-cell potential of the quartz sand UHPC is always maintained at a positive value greater than-200 mV.According to evaluation method of the hydraulic standard,the corrosion probability is less than 10%,but after break the specimens,6 steel rods have 2 roots corrosion,the probability of corrosion is 33%.The potential of the sea sand UHPC without metakaolin is always a positive value greater than-260 mV,but the potential of the UHPC is significantly reduced when mixed with metakaolin,and the lowest potential can reach-668 mV.All the rebars within the sea sand UHPC are not rusted,for sea sand UHPC with metakaolin,half-cell potential should not be used to judge the corrosion of steel bars.