Study On Expansion And Mechanical Performance Of MgO Shrinkage Compensating Concrete Under Different Constraints

The cracking problem of concrete is the main factor affecting its durability and service life.Adding expansive agent to concrete can effectively improve the shrinkage and cracking of concrete,but the expansion caused by expansive agent must be beneficial under the constraint conditions,and the constraint conditions of concrete in practical application are very complex.At present,there are few studies on shrinkage compensating concrete under different constraints,which is not conducive to the scientific use of expansion agent.In this paper,MgO expansive agent is taken as the research object,and the expansion performance,mechanical properties and microstructure of shrinkage compensating concrete with fixed content（48kg/m³）of MgO expansive agent under strength constraint,one-way constraint and three-way constraint are systematically studied.The main conclusions are as follows:（1）The strength of MgO shrinkage compensating concrete itself will affect the development of its expansion.The lower the strength,the greater the expansion.When the content of MgO expansive agent is the same,the expansion amount is related to the water-binder ratio and the strength grade of concrete itself.The greater the water-binder ratio,the greater the hydration degree of MgO expansive agent,and the more expansion source Mg（OH）₂generated.However,the increase of water-binder ratio will lead to the decrease of concrete strength,and the limiting effect of its own strength on expansion will be reduced.The superposition of double effects will eventually lead to the lower strength grade of concrete and the greater expansion.The limited expansion rate of C30 MgO shrinkage compensating concrete at each age is greater than that of C45 MgO shrinkage compensating concrete.The difference between the two reaches a maximum of 0.60×10^-4 at 42d,which is 1.16 times that of the latter,and then gradually decreases.The expansion time window of C30 and C45 MgO shrinkage compensating concrete can be divided into rapid period,medium-speed period and stable period according to the expansion rate and expansion amount.Among them,0～40d is the rapid period,and the expansion develops rapidly.The expansion amount reaches about 70%of the total expansion amount at 40d,then the expansion growth slows down and enters the medium-speed period.The expansion amount reaches about 95%of the total expansion amount at 90d,and enters the stable period after 90d,and the expansion tends to be stable.（2）The reinforcement ratio will significantly affect the expansion development of MgO shrinkage compensating concrete.The larger the reinforcement ratio,the slower the expansion rate and the lower the expansion amount.The double constraints of reinforcement ratio and strength make the expansion amount of concrete with higher strength grade decrease more.Compared with the reference group with a reinforcement ratio of 0.8%,when the reinforcement ratio of concrete is 1.6%and 3.2%,the stable expansion rate（240d）of C30 MgO shrinkage compensating concrete is reduced by 28.6%and 46.2%respectively,and the stable expansion rate（240d）of C45 MgO shrinkage compensating concrete is reduced by 34.7%and 54.6%respectively.When the reinforcement ratio is 0.8%～3.2%,the expansion stress of0.9MPa～2.0MPa can be generated by adding MgO expansive agent to C30 and C45 concrete,which can offset the tensile stress caused by dry shrinkage and temperature shrinkage,and delay or inhibit cracking.（3）The three-dimensional constraint will significantly affect the volume deformation of MgO shrinkage compensating concrete,and the expansion will develop in the direction of smaller constraint force and larger constraint area.The axial and circumferential deformation of the specimen developed rapidly from the beginning of the test to 10 days,and remained stable after 10 days.The axial deformation of the specimen with large constraint force was shrinkage,and the circumferential deformation of the specimen with small constraint force was expansion.The difference between circumferential strain and axial strain increases with the increase of age,but it shows expansion in the overall volume deformation.Under the three-dimensional constraint,the expansion force of MgO expansive agent is constrained by external constraints,and the contribution to expansion is small.More expansion force is used to fill and compact pores,which makes the pore structure more optimized.The less harmful pores,harmful pores and more harmful pores above 20nm are reduced by 37.1%,39.1%and 64.5%respectively.（4）The equivalent substitution of MgO expansive agent for part of cement will reduce the compressive strength of concrete,and the higher the strength grade of concrete,the greater the decrease of compressive strength.The decrease is more significant at 7d,about 10%～18%,and the decrease is not significant at 28d,about 3%～10%.In addition,compared with unconstrained concrete,the compressive strength of shrinkage compensating concrete under constraint conditions will be significantly improved,increasing by 12%～18%at 7d and about 6%at 28d.The substitution of MgO expansive agent for cement leads to the decrease of strength,which can be basically offset by the increase of strength under constraint conditions,so that the strength of MgO shrinkage compensating concrete under constraint conditions is basically the same as that of the blank group.The constraint condition makes the expansion force of MgO expansive agent limited by the test mode to fill the internal pores of the hardened slurry,which reduces the total pore volume,especially for the reduction of harmful pores and harmful pores above 50nm,which can improve the compressive strength of the specimen,which is consistent with the strength test results.