| China’s transportation infrastructure construction develops rapidly.Recycled concrete will be used in a large scale to realize “carbon neutrality and emission peak”.There are extensive distributions of salinized soils in western China and countries along “The Belt and Road”.Concrete in coastal regions in eastern China is also easy to be attacked by sulfates.How to reflect transmission paths and states of sulfate ions accurate in recycled concrete is vital to make reasonable design of recycled concretes and promote their applications in saltenriched regions.On this basis,the process of recycled concrete attacked by sulfate based on adhesion characteristics of attached mortar was discussed systematically by combining microscopic characterization and macroscopic test as well as comparison of experimental study and numerical simulation.Based on laboratory test data,whether attached mortar of recycled aggregate is a key factor influencing recycled concrete properties was discussed by the grey correlation entropy method.Through image recognition method and combining chemical test,some characterization indexes of adhesion characteristics of attached mortar of recycled aggregate were proposed: attached mortar adhesion rate,attached mortar coverage rate and sulfate ion content of attached mortar.Through ball-milling method,micro-heating method and sulfate soaking method,the preparation of recycled aggregates produced by crushing of sulfate attack building structures in saline soil regions was simulated.Adhesion characteristics of the attached mortar were tested and their influences on compressive strength,relative mass and relative dynamic elastic modulus of recycled concrete were investigated systematically.Besides,degradation mechanism of recycled concrete under sulfate attack was disclosed from macroscopic and microscopic aspects by combining SEM and XRD test methods.Results showed that attached mortar of recycled concrete is a key influencing factor of recycled concrete properties.Coverage rate of attached mortar tested by image recognition method has repeatability and reproducibility.Damage coefficient of compressive strength of recycled concrete increases with the reduction of attached mortar adhesion rate,and it presents an inverted V-shaped variation with the increase of sulfate content.The higher coverage rate of attached mortar,the lower damage coefficient of comprehensive strength of the recycled concrete.The three-dimensional recycled concrete is transformed into a two-dimensional visualization model through the Valavan’s formula.The multi-phase division program of recycled aggregate was compiled based on Matlab software platform and a six-phase recycled concrete model with adhesion characteristics of attached mortar of recycled aggregate was built.A porosity test was carried out and the transmission process of sulfates in concrete under influences of porosity was characterized.A diffusion-reaction model of sulfate ions was constructed.According to the tensile stress-strain relationship of ordinary concrete,the swelling stress-strain model of recycled concrete attacked by sulfate was built up.This study discloses the transmission paths of sulfate in recycled concrete based on sixphase computation model by simulating migration of sulfate ions in the recycled concrete.Results demonstrated that compared with five-phase concrete model,the calculation results of sulfate ion concentration of the six-phase concrete model are more accurate.The inhibition of tortuosity on ion transmission and the enhancement of interfacial transition zone(ITZ)on ion transmission influence transmission of sulfate ions in recycled concrete together.With the increase of adhesion rate of attached mortar,the recycled concrete has the poorer sulfate resistance.When the adhesion rate of attached mortar is fixed,sulfate ion concentration in the recycled concrete increases with the increase of attached mortar coverage rate.The higher sulfate ion content in recycled aggregate leads to the longer time consumption for ion reaction.The influences of adhesion characteristics of attached mortar on sulfate ion transmission-reaction in recycled concrete are investigated based on the six-phase recycled concrete model.Relevant influencing laws were disclosed.Based on the sulfate transmission-reaction model in recycled concrete based on adhesion characteristics of attached mortar,the whole process form sulfate transmission-reaction and cracking damages in recycled concrete is simulated by combining the cracking equation.Results showed that with the increase of adhesion rate and coverage rate of attached mortar,the time for recycled concrete to reach the threshold of swelling strain after sulfate attack shortens.When the adhesion rate of attached mortar of recycled aggregate is relatively high and it carries high sulfate concentration,sulfate filling of ITZ reaches the strain threshold.When there’s an external loads act on the recycled concrete structure,cracks extend quickly in ITZ,thus resulting in structural failures. |
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