Study On Crack Resistance Of Shrinkage-compensating Concrete With Dual Expansion Sources

Concrete is a kind of hydraulic cementitious material.In the process of hydration and hardening,the concrete will shrink and deform due to its own hydration reaction and the transmission of concrete water medium.Concrete cracking caused by shrinkage is the main cause of engineering diseases.In order to solve the problem of cracking of mass concrete,a certain amount of expansive agent can be added to the concrete,and the expansion source can be generated through the hydration reaction of the expansive agent,which in turn produces the expansion of the concrete volume.Under the constraints of external constraints,the concrete itself will produce a certain precompressive stress,thereby improving the crack resistance of the concrete.At present,the widely used concrete expansive agent in China is UEA expansive agent based on the hydration product ettringite,which has the advantages of early expansion rate and large expansion amount.But there are also shortcomings such as large shrinkage gap in the later period,delayed ettringite reaction,large water requirement and high curing requirements.Due to its relatively slow hydration rate,Mg O expansive agent has the characteristics of delayed expansion,followed by stable hydration products,which can improve the durability of concrete.The UEA expansive agent is mixed with the Mg O expansive agent,and the two are used to make up for each other's shortcomings to obtain an appropriate expansion amount and expansion distribution.Improve the crack resistance of concrete at all ages.In this paper,UEA expansive agent and Mg O expansive agent are blended,and mechanical property tests(compressive,tensile,fracture)and deformation performance tests(restrained expansion rate and dry shrinkage deformation)are carried out.The effect of the mixing ratio of the two expansive agents on the mechanical and deformation properties of concrete was studied.At the same time,an index suitable for the evaluation of the crack resistance of shrinkage-compensating concrete is proposed.This index can be combined with the actual project to optimize the optimal mix ratio of the project.The main conclusions obtained in this article are as follows:1.The 10% UEA expansive agent reduces the compressive strength,tensile strength,ultimate tensile value and elastic modulus of concrete.The incorporation of Mg O expansive agent significantly improved the compressive strength,tensile strength and ultimate tensile value of concrete in the later period.The expansion effect of UEA expansive agent is mainly generated before 14 days.As the amount of Mg O expansive agent increases,the early restrained expansion rate of concrete decreases.From 14 days to 28 days,the UMC4:1 group has the best expansion performance,and then with the continuous hydration reaction of Mg O,the expansion performance of the concrete with more Mg O content is better.The incorporation of Mg O expansive agent can improve the coordination between the strength and deformation of concrete.2.Two types of expansive agents have a certain inhibitory effect on the dry shrinkage of concrete for two reasons: one is that the Aft generated by the hydration reaction of the expansion agent fill the internal pores of the concrete,making the concrete structure more compact and concrete internal moisture evaporation is weakened,which reduces the dry shrinkage value of the concrete.The second is that the hydration reaction of the Mg O expansive agent requires less humidity than the UEA expansive agent,so the Mg O agent can produce a certain expansion in a relatively dry environment.3.Through the P-CMOD curve of the shrinkage-compensating concrete,it is found that the fracture process of the shrinkage-compensating concrete can be divided into three stages: crack generation stage,stable expansion stage and instability expansion stage.The early fracture toughnessand and fracture energy of shrinkagecompensating concrete mixed with 10% UEA expansive agent is lower than that of ordinary concrete,but UEA expansive agent promotes the growth of later fracture toughness and fracture energy.Under the condition of mixed Mg O and UEA expansive agent,the increase of Mg O content leads to lower early fracture toughness and fracture energy of concrete,and the hydration reaction of Mg O promotes the fracture toughness and fracture energy of concrete,but when the amount of Mg O is 5%,the excessive expansion in the later stage can inhibit the improvement of fracture performance.4.An evaluation index for crack resistance of shrinkage-compensating concrete is proposed.The index not only combines the mechanics and deformation properties of concrete,but also takes into account the dry shrinkage of the concrete.The research results show that: when the ratio of UEA to Mg O expansive agent is 2:1,the concrete has the best crack resistance.