Experimental Study On Durability Of Shrinkage Compensating Concrete With Double Expansion Sources

Most of the hydraulic concrete belongs to mass concrete,which is easy to crack because of its internal serious hydration temperature rise.At the same time,the environment of hydraulic concrete is complex and abominable.The long-term damage caused by adverse environmental factors is often not considered in the design,which leads to the failure of the service life of hydraulic concrete structure to meet the design requirements.At present,it is an economic and effective method to improve the durability of concrete by adding expansion agent into concrete to prepare shrinkage-compensating concrete.Most of the existing researches focus on the influence of single expansive agent on the durability of concrete.However,adding a kind of expansive agent can only improve the durability of concrete in a certain period.In this paper,based on two kinds of expansive agents with different expansion period:U-type expansive agent?UEA?and magnesium oxide expansive agent?MEA?,four different UEA:MEA mix proportions?4:1,2:1,1:1,and 1:0?were subjected to prepare shrinkage-compensating concrete to rapid freezing and thawing cycling tests,carbonation tests and impermeability tests.The test results were compared with the ordinary concrete?OC?to explore the optimal compound ratio.At the same time,the microstructures and products of different concrete were analyzed by SEM and XRD.The main work and achievements are as follows:?1?The frost resistance of the concrete was derived through the measurement and calculation of the relative dynamic modulus of elasticity?RDME?and the mass loss rate?MLR?,and the freezing-and thawing-cycle microstructures and products of concretes with different expansive agents were analyzed using scanning electron microscopy?SEM?.The results show that RDME of concrete decreases gradually with the increase of freeze-thaw cycles,and MLR increases gradually;RDME of concrete with double expansion agent is higher than that of concrete with single expansion agent and ordinary concrete,and MLR is lower than that of concrete with single expansion agent and ordinary concrete.After 150 cycles,the concrete with a UEA:MEA mix proportion of 2:1 had the best frost resistance,with RDME and MLR values 17.35%higher and 2.1%lower,respectively,than that of an expansive-agent-free concrete.?2?The carbonation test was carried out to study the carbonation resistance of different concrete,the carbonation depth was used to measure the carbonation resistance of concrete,and the change of minerals in concrete was analyzed based on XRD test.With the development of carbonation age,the carbonation depth of concrete increases slowly at first,then rapidly,and finally tends to be gentle.The carbonation depth of concrete with double expansive agent is the lowest,followed by concrete with single expansive agent,and that of ordinary concrete is the highest.XRD test shows that when the ratio of UEA and MEA is 2:1,the corresponding peak value of Ca CO₃ is lower,and the corresponding peak value of Ca?OH?₂ is higher,which indicates that this group of concrete is less affected by carbonation than other groups and has better anti carbonation ability.?3?The impermeability of different concrete is measured by the seepage height studied,which is based on the impermeability test.The results show that the seepage height of different concrete decreases rapidly at first and then tends to slow down with the increase of seepage time.When the ratio of UEA and MEA is 2:1,the concrete seepage height is the lowest,which indicates that this type of concrete has better impermeability.?4?The test results can provide a basis for the durability design of hydraulic concrete structure.It is suggested that the composite ratio of UEA and MEA is 2:1 to prepare compensating shrinkage concrete in practical engineering,so as to improve the durability of hydraulic concrete and extend the service life of infrastructure.