Performance Of Corroded Lightweight Aggregate Concrete Bending Element In Chloride Environment

Porous lightweight aggregate significantly impacts the transport properties and fracture properties of lightweight aggregate concrete(LWAC).The durability variation of LWAC elements in the chloride environment is complex,and the performance degradation of which at the service stage needs to be studied.Hence,the materials components and the mechanical properties of LWAC should be considered to investigate the effect of mesoscopic components on the durability degradation of LWAC structures and elements,as well as to establish the durability analysis model,which is crucial for the durability design of LWAC structures.To investigate the experimental programs on fifteen beam-type bond-slip specimens and twenty-three four-point bending beams were completed.The mesoscopic models for transport and mechanical properties of LWAC were established,by which the transport properties and degradation mechanism of mechanical performance of LWAC elements in the chloride environment was revealed.The analysis model of structural LWAC elements was developed.The limitations of design provisions and durability evaluation method were proposed for the LWAC structures subjected to transport and corrosion phases.The main research includes:1.Chloride diffusion and corrosion-induced cracking behavior of LWAC.The mesoscale numerical model of chloride ion diffusion in LWAC and the mechanical properties of LWAC were established.Fourteen beams made of LWAC were subjected to accelerated corrosion tests.The effects of porous aggregate on diffusion and corrosion-induced cracking behavior were investigated.The limitations of LWAC structures exposed to different chloride environments were proposed,and the prediction model of corrosion-induced behavior of LWAC was developed.It was concluded that the water-to-binder ratio is the most important influencing factor to the diffusion coefficient,and the effect of lightweight aggregate on the diffusion coefficient becomes more obvious with the increase of the water-binder ratio.The decrease of aggregate strength makes the cracking process of LWAC more serious and could lead to internal damage to the LWAC.The increase in the water-to-binder ratio reduced the critical corrosion ratio and the slope of the corrosion ratio-surface crack width curve.Simultaneous corrosion of two steel bars led to cracks between bars,possibly resulting in spalling of the concrete cover.The corrosion of corner steel bars may cause the overall spalling of concrete.2.Experimental study on bending performance of corroded LWAC beam.Twenty-three beams made of LWAC were tested by the four-point loading system,fourteen of which were subjected to accelerated electrical corrosion previously to the bending tests.The effects of aggregate size,aggregate volume fraction,and reinforcement corrosion on the cracking of LWAC cover and bending performance of LWAC beam were investigated.It was found that smaller gradation of lightweight aggregates reduced the cracking moment of the LWAC beam,while the ultimate deflection and ductile can be increased.The ultimate moment and the ductile factor of corroded specimens were decreased by 3% ～15% and 4% ～ 37%,respectively,and the corrosion of reinforcement reduced the number of transverse cracks on the surface of LWAC beams and increased the spacing between cracks.Cross-section strains showed nonlinear distribution due to the corrosion of rebar.The predictions of transverse crack width of corroded LWAC beams in existing codes were lower than test results,and the calculated values of short-term stiffness of non-corroded specimens in the Chinese code is30% higher than the test values on average,while the predicted value of corroded specimens is relatively conservative.3.Deterioration of bond performance and bond-slip model between corroded rebar and LWAC.Fifteen beam anchorage specimens made of LWAC were tested for the bond-slip behavior,of which eleven specimens were subjected to accelerated electrical corrosion previously to the loading.Bond-slip model considering the effects of stirrups and corrosion was developed for LWAC.It was proposed that the reduction in the spacing of stirrups and increase in the thickness of the concrete cover improved the bond strength of the specimens and mitigated the degradation of bond strength due to corrosion.The established bond-slip model reflects the interaction between corroded rebar and LWAC,and it can be used for the prediction of bond-slip behavior.4.Stochastic model of LWAC elements considering reinforcement corrosion.The stochastic method for modeling the LWAC was proposed based on the mesoscopic components of the LWAC,and calculations for crucial model parameters were developed.The effects of longitudinal reinforcement corrosion ratio on the failure mode,bearing capacity,and moment-deflection curve of lightweight aggregate concrete beams was included to modify the multiscale model.The results show that the increase of relative scale parameters will improve the model error,which is recommended to be 3.5 ～ 7.5.The size of the spatial sub-element should be close to the maximum aggregate size.The random aggregate distribution field model can accurately simulate the mechanical properties of LWAC members.The ultimate moment of the LWAC beam obviously decreased with the corrosion ratio of rebar exceeding 10%.5.Research on parameters of the durability design for LWAC structures under the chloride environment.The significance of influencing factors was analyzed based on the MIV-neural network method.The design-oriented parameters for cracking and crack propagation stages were analyzed in combination with each durability stage of the structure and the current code.By combining previous achievements,limitations on the design parameter of LWAC flexural members in chloride environment were discussed,and the durability evaluation method for LWAC bending elements based on surface crack width was developed.In this dissertation,the effect of porous lightweight aggregate on the transport properties and mechanical properties of LWAC elements was determined.The mesoscale analysis model for chloride ion diffusion in LWAC and mechanical properties of LWAC was established,and the random aggregate distribution field model was developed.The suggestions for the design and the evaluation process of LWAC bending members subjected to the chloride environment were made.The research provides a theoretical reference for the promotion and application of LWAC and structural durability design.