Study On The Control Of Concrete Mechanical Properties And Deformation Properties Based On Close Packing Theory

In the construction process of the prestressed concrete continuous rigid frame bridge,the prestressed tension will be carried out at an early age.Therefore,the elastic modulus of concrete needs to increase rapidly.In addition,the shrinkage and creep of concrete will cause excessive deflection and cracking of the prestressed concrete box girder.In this thesis,in order to optimize the mechanical properties and development characteristics of concrete at early ages,and control the development of shrinkage and creep,the concrete mix ratio is optimized based on the close packing theory.Then,the mechanical properties and deformation performance tests are carried out to explore the effect of the optimized control method.Finally,several microscopic experiments are carried out to explain the microscopic mechanism of the experimental phenomenon.This research mainly includes the following contents:(1)Through experiments,the effects of different types of mineral admixtures on the mechanical properties and their growth rates at the early age of cement paste and mortar are explored.The results show that replacing the same amount of cement with 15%slag will not significantly reduce the early-age strength of cement paste and mortar,while 15%fly ash has a negative effect.Therefore,15%S75 slag is used as the concrete mineral admixture.(2)A concrete optimization experiment based on the close packing theory is carried out to explore the influence of mixing ratio of two single-grain coarse aggregates,sand ratio and grout-to-bone ratio on the mechanical properties and development characteristics of concrete at an early age.The test results show that when the mixing ratio of 5～10mm and 10～20mm gravel is 20:80,the sand ratio is 40%,and the grout-to-bone ratio is 32.5:67.5,the mechanical properties and development characteristics of the concrete at early age are the best.(3)An unconstrained shrinkage test is carried out for the concrete before and after the optimization,and the influence of the optimization control method on the shrinkage characteristics of the concrete is explored.The results show that the total shrinkage of the optimized concrete is reduced by 32.8%,and the autogenous shrinkage is reduced by 28.5%.A compressive creep test is carried out to explore the effects of optimized control methods,different environments and different loading ages on the creep characteristics of concrete.The results show that relative humidity has the greatest influence on creep.When the average humidity increases from 62.9%to 92.7%,the creep coefficient decreases by44.5%-50.6%.The influence of the optimized control method is second.After optimization,the creep coefficient is reduced by 12.4%～23.9%.When the loading age is in the range of5-10d,its influence is the smallest,and the creep coefficient changes by about 10%.(4)Based on the CEB-FIP(1990)creep prediction model,considering the influence of elastic modulus maturity on creep characteristics and introducing the coefficientβ_E,the original model was revised according to the compression creep test data.(5)Through XRF,SEM and MIP,the chemical composition of the cementitious material is analyzed,and the micro morphology and pore structure of the concrete sample are observed and analyzed.The analysis shows that the optimized control method improves the compactness of the concrete particle system and reduces the porosity.When the age is 5d,the volume of harmful pores with a pore diameter of 150nm or more is significantly reduced;when the age is 28d,the volume of harmful pores with a pore diameter of 3800nm or less is significantly reduced.Prolonging the curing period significantly reduces the micro-cracks and harmful pore volume in the concrete.