Study On Influential Factors Of Shrinkage And Cracking Of High-strength Concrete At Early Ages And Anti-cracking Measures

Concrete structure is the most common structural style form in today’s society.With the widespread use of high-strength concrete,which brings convenience to life,the early cracking of high-strength concrete also requires more attention to the safety of the structure.After the cracks are generated,the integrity of the structure will be damaged,and harmful substances will be more easily penetrated.The corrosion of the reinforcing steel will be accelerated,the life of the structure will be reduced,and the appearance of the building will be damaged.In addition,the repair of cracking damage is also very complicated,and the maintenance cost is also very expensive.Therefore,this study uses a temperature stress testing machine to study the deformation and cracking behavior of concrete at early age,and uses the stress relaxation coefficient to establish a constraint stress prediction model.The research content of the thesis mainly includes the following parts:1.Investigate the early deformation of concrete with different water-cement ratio,slag and microencapsulated phase change material content.Using the thermal expansion coefficient formula and the temperature history of the specimen itself,the total deformation is studied separately by separating into temperature deformation and autogenous shrinkage.As the watercement ratio decreases,the temperature shrinkage and autogenous shrinkage will increase,thereby increasing the shrinkage values of total deformation of the concrete.Compared with the sample with water-cement ratio of 0.49,the final shrinkage of samples with water-cement ratio of 0.36,0.28 and 0.2 increased by 66.8%,86.3% and 395.6%,respectively.Slag will extend the reaction time of concrete,and reduce the shrinkage deformation speed.The effect on the autogenous shrinkage will first increase and then decrease with the increase of the amount of slag.The final shrinkage values of total deformation changed by-10%,41%,and-40% respectively when the amounts of slag were 10%,20% and 30%.and the autogenous shrinkage had a very great impact on the total deformation of slag concrete.The microencapsulated phase change material reduced the temperature rise value and temperature drop value of the concrete,reduced the temperature deformation of the concrete,and slowed down the autogenous shrinkage of the concrete.The overall deformation of the concrete was reduced.When the water-cement ratio is 0.2 and the phase change material is added to the 5% specimen,the water-cement ratio is 0.2 and the phase change material is added to the 10% specimen.The shrinkage deformation is reduced by 39.4% and 19.4%,respectively.The effect of 10% microencapsulated phase change material on the early deformation of concrete is better than adding 10% slag.2.Research on the early restraint stress of concrete by using the full restraint mode.As the water-cement ratio decreases,the maximum compressive stress gradually decreases,and the maximum compressive stress changes from 1.47 MPa to 0.55 MPa.The sample with watercement ratio of 0.49 has a maximum expansion compressive stress of 1.47 MPa,which counteract most of the restrained tensile stress,and has a minimum cracking stress of 2.02 MPa.The tensile stress growth rate of the sample with water-cement ratio of 0.2 is 0.056 MPa / h,which is greater than the tensile stress growth rate of 0.042 MPa / h of sample with watercement ratio of 0.36 and the tensile stress growth rate of 0.03 MPa/h of sample with watercement ratio of 0.28,so the cracking stress of the sample with water-cement ratio of 0.2 is less than that of the samples with water-cement ratio of 0.36 and 0.28.3.From the test data,it can be known that the slag mainly slows the growth rate of the tensile stress of the concrete,increases the net tensile time,and prolongs the cracking time by prolonging the hydration reaction,which affects the early cracking behavior of the concrete.Adding microencapsulated phase change material can reduce the temperature change value of the concrete,reduce the thermal stress of the concrete.Meantime by reducing the autogenous shrinkage of the concrete,microencapsulated phase change material slow down the growth rate of the tensile stress of the concrete,and delay the cracking time of the concrete.The two materials have different mechanisms of action,but they show similar laws.4.The tensile strength of concrete was measured at 3,7,and 28 days during the split tensile test.The development of the early tensile strength of the concrete was calculated.The cracking stress of the concrete was compared with the tensile strength.The test specimens which watercement ratios were 0.49 and 0.2 have higher cracking sensitivity.The addition of slag and microencapsulated phase change material will reduce the cracking sensitivity of concrete.Therefore,slag and microencapsulated phase change material can be used as crack prevention measures.5.Using the measured early free deformation and constrained stress of concrete,combined with the concrete elastic modulus,establish a formula for concrete stress relaxation coefficient.Combining concrete autogenous model,temperature deformation model,elastic modulus model and stress relaxation coefficient formula,a model for early restraint stress prediction of concrete is established and verified.