Preparation And Characterization Of High Strength Wet-mixed Sprayed Concrete

More and more applications of sprayed concrete in practical engineering are still in the early stages of development.Most of them are in a compressive strength grade of C30 or below,but the research of high strength sprayed concrete has not been carried out.This paper mainly focuses on the high strength of shotcrete to give a guidance of technology research and application.The shootability and mechanical performance of high strength shotcrete are studied and the relationship between them is analyzed,it is recognized that strength is largely dependent on the performance of shootability.w/b,sand to total aggregate ratio,paste volume content,steel fiber and air pressure are researched to evaluate the influence on the shootability parameters build-up thickness and rebound rate,as well as the development discipline of compressive strength and tensile splitting strength under the different circumstances.The long term performance of high strength shotcrete is tested by dry shrinkage,and durability is valued by water permeability resistant test.The features of high strength shotcrete are obtained by the comparison between cast concrete and shotcrete.At the same time,microscopic testing methods of MIP,XRD and SEM are used to test the pore structure characteristic parameters,types of hydration products and morphology.It is correlated to macroscopic test results to give the impact.It is proved that the concrete can achieve high strength on the basis of good shootability performance,the best performed proportion SFC(silica fume concrete,total binder is 500 kg/m~3)concrete compressive strength level can reach above 60 MPa,and the tensile strength can reach about 5 MPa.The better the spraying property,the higher the strength.The larger influence on the spraying property is the volume content of the slurry.The dry shrinkage performance and durability performance are consistent with the results of shootability and strength.The dry shrinkage and permeability resistance of SFC sprayed concrete indicate the best.On the contrary,the dry shrinkage and permeability resistance of HVF(fly ash concrete,total binder is 660 kg/m~3)sprayed concrete with poor spray properties are the largest and the worst.The MIP test results show that the total porosity of HVF is the largest,the permeability of concrete is poor due to the large number of connected voids,and the shrinkage is greater due to the large number of capillary pores.The fine pore rate of SFC is smaller and the spray performance is better,so the shrinkage is reduced.XRD test results show that the hydration products with different proportions have certain differences.SEM observation shows that the relative porosity of the matrix is consistent with the test results of the MIP.The concrete matrix with better placement quality contains a large number of C-S-H gels.The poor quality of the shootability results in a large number of needle-like ettringite that are not effectively bonded to matrix,forming a lot of pores,which affects the performance of the concrete.