Study On Durability Of Reactive Powder Concrete Under Long-term Sulfate Immersion

With the development of social economy and the demand of production and life,the concrete building structure develops towards the direction of super high-rise and large span.Because of the huge population and the continuous growth of our country,it is difficult for above ground buildings to meet people’s living and travel needs.Domestic and foreign scholars put forward the idea of underground rail transit and underground space construction.Traditional concrete is difficult to meet the requirements of strength and durability of underground buildings and structures.On the basis of ordinary concrete,reactive powder concrete is prepared by removing coarse aggregate,adding steel fiber and reactive admixture according to a certain proportion according to the principle of micro-density accumulation.It has the advantages of high strength,good durability,explosion prevention and good toughness.In addition,concrete buildings in saline-alkali soil environment suffer from sulfate corrosion for a long time,which greatly reduces the service life of concrete buildings.Therefore,It is important to study the mechanical property attenuation of reactive powder concrete under sulfate environment.In this paper,the method of indoor corrosion is used to simulate the process of sulfate attack on concrete buildings that are under the groundwater level for a long time.The main research contents and conclusions are as follows:(1)In the test,group A(clean water),group B(5%Na2SO4),group C(10%Na2SO4),and group D(15%Na2SO4)were set up to corrode the reactive powder concrete specimens in the way of long-term immersion.The appearance record,mass weighing,compressive strength test and flexural strength test of the specimens were carried out in each cycle,and the change rule of the appearance,quality and strength of the specimens with the immersion cycle was analyzed.The results show that with the increase of immersion period,the corrosion and salt crystallization of steel fiber on the surface of the specimen become more and more serious;In the early stage of soaking(0-3),the quality of group A to C decreased,with a change rate range of-0.02%to-0.05%;During the mid soaking period(4-12),the mass of each group of specimens increased.The mass growth rate range of Group A was 0.12%～0.27%,Group B was 0.02%～0.26%,Group C was 0.23%～0.55%,and Group D was 0.03%～0.91%;In the later stage of soaking(13-18),the quality of the specimen decreased,with a maximum change rate of-0.30%in Group A,-0.42%in Group B,-0.58%in Group C,and-0.97%in Group D.(2)The compressive strength test and flexural strength test show that the compressive strength and flexural strength of the reactive powder concrete specimens made with this mixture ratio are between 100-110 MPa and 23-25 MPa,respectively.After 18 cycles of erosion,group D specimens suffered the most severe damage,with compressive strength decreasing from 105.16 MPa to 92.54 MPa and a compressive strength loss rate of-12.00%;The flexural strength decreased from 25.64 MPa to 19.53 MPa,and the rate of loss of flexural strength was-23.83%.The results of the dynamic modulus test also showed that the density of group D specimens was the worst,and the dynamic modulus decreased the fastest.The dynamic modulus changed from 42.4 GPa to 39.9 GPa,with a loss coefficient of-5.90.The BP neural network and flexural strength attenuation prediction model established based on strength test data have well predicted the strength values of each group of specimens in each soaking cycle.The stress-strain curve shows that the peak strain increases with the increase of erosion time,with the peak strain of group D specimens increasing from 0.00406 to 0.00529,with a change rate of 30.30%.(3)The acoustic properties of reactive powder concrete specimens subjected to compression and ring breaking were obtained through acoustic emission tests.The results showed that the process of specimen b value decreasing from large to small represents the appearance of small cracks,which propagate and extend to form large cracks.According to the acoustic emission activity function,it can be seen that the water group specimen has the highest level of acoustic emission activity,and the higher the concentration of sulfate solution,the lower the acoustic emission activity of the specimen.The ultrasonic non-destructive testing test tested the damage layer thickness of each group of specimens during each soaking cycle.The results showed that the damage layer thickness of each group of specimens increased with the increase of soaking cycle.Among them,the damage layer thickness of Group A increased by 1.62mm,Group B increased by 2.22mm,Group C increased by 2.95mm,and Group D increased by 3.12mm;When the erosion time is the same,the thickness of the damaged layer in each group of specimens ranges from large to small:15%Na2SO4 solution group>10%Na2SO4 solution group>5%Na2SO4 solution group>water group.(4)SEM scanning electron microscopy shows that the active powder concrete has lamellar texture structure,and active additives with different particle sizes fill the closed and disconnected holes in the concrete.After sulfate attack,there are needle-bar alum,hexagonal-shaped gypsum,sodium sulfate crystal and other substances in the concrete.