Shear Behavior Of Large-scale High-strength Lightweight Aggregate Concrete Deep Flexural Members

High-strength lightweight aggregate concrete(HSLWAC)deep flexural members with complicated shear failure mechanisms,exhibiting obvious brittle feature,were significant affected by the size effect.The serviceability performance of this kind of members was rarely studied,and whether it is size-dependence remain need to be determined.Therefore,research on the shear behavior of large-scale HSLWAC deep flexural members can provide reliable shear design methods for practical engineering and establish reasonable evaluation system for its serviceability performance.It is also of great theoretical value for promoting the application of HSLWAC and solving the shear problems existed in basic theory of reinforced concrete structures.In this paper,an experimental study of fifteen large-scale HSLWAC deep flexural members subjected to monotonic loading was carried out that focused on the serviceability performance,the shear bearing capacity as well as the size effect mechanism of this kind of members.The major contributions of the work presented in this thesis are listed as follows:1.HSLWAC with compressive strength of 40~60 MPa was prepared by using high-strength expanded shale aggregates.In order to investigate its macro-mechanical properties,the elastic modulus and the uniaxial stress-strain relationship were analyzed and the corresponding calculation models were proposed.Combining with the fracture mechanics-based size effect law proposed by Ba?ant,the conversion relationships of compressive strength for lightweight aggregate concrete(LWAC)with different size and shapes were established to lay a theoretical foundation for subsequent research.2.According to the experimental investigation on the fifteen HSLWAC deep flexural members with beam depths varied from 500 mm to 1400 mm,the failure process,failure modes,load-midspan deflection curves,characteristic loads,reinforcement strain distribution and inclined crack propagation of specimens were studied.Then the effect of different shear span-effective depth ratio,beam section height as well as support or loading plate widths on the shear bearing capacity were analyzed.Test results showed that the main failure mode of specimens was shear-compression failure,but it gradually turned into diagonal splitting failure as the shear span-effective depth ratio increased and were independent of various section sizes.The normalized ultimate shear stresses of specimens exhibit significant size-dependence and were decreased approximately 37.1% when beam height increased from 500 mm to 1400 mm.However,the effect of beam section height on the normalized cracking shear stresses can be neglected.Increasing the loading and support plate widths can obviously widen the diagonal compressive region and approximately improve the normalized ultimate shear stresses of 20.4%.Instead,increasing the shear span-effective depth ratio leads to an appreciable reduction in both the normalized cracking and ultimate shear stresses.3.In order to understand the serviceability performance of HSLWAC deep flexural members,the propagation of diagonal cracks was investigated and the influence of size effect on specimen behavior at each loading phase was analyzed.Simultaneously,an equation for the diagonal cracking load was proposed and then the macroscopic relationship between diagonal crack widths and load level was built.Additionally,the applicability of the limit value for the diagonal crack widths adopted by the current codes was discussed.Results show that the predicted diagonal cracking loads obtained from the proposed model have a good agreement with the test results,but the cracking resistance of LWAC is inferior to that of normal weight concrete.The maximum diagonal crack widths of deep flexural members were significantly affected by the shear span-effective depth ratio and the bearing plate widths,while the contribution of increasing the beam section height to the maximum diagonal crack widths varied with the shear span-effective depth ratio.The size effect still plays a significant role in the shear bearing capacity of deep flexural members at the serviceability limit state,but its influencing trend varies with the beam section height,especially noted that the size effect are more apparent for beams with cross section height less than 800 mm.Instead,the variation of beam section height has no impact on the propagation of diagonal crack widths at each load level.4.Considering that the size effect were induced from two aspects,namely,the aggregate interlock and the energy release at diagonal cracks,the modified compression field theory(MCFT),Tan-Tang model,Tan-Cheng model and crack band theory(CBT)were introduced to predict the shear bearing capacity of HSLWAC deep flexural members,and the predictions of these models were compared to the test results as well as the calculation results obtained from the current code provisions.Comparison results indicate that,if it doesn’t consider the strength reduction caused by the use of LWAC,the Chinese code lack of reasonable evaluation of the size effect,and the Canadian code need to revise the reduction coefficient of the concrete softening.Whereas the predictions obtained from the American code and European code are agree well with the test results,and the accuracy of these two codes is hardly size-dependence.According to the brittle feature of lightweight aggregates,the maximum aggregate size should be equal to zero when using MCFT to calculate the shear strength of HSLWAC deep flexural members.By modifying the elastic modulus of LWAC,the predictions of CBT show a better agreement with the test results,indicates that this modified model can reasonably consider the size effect.Although the Tan-Cheng model introduced the size effect reduction coefficients,its calculation results are conservative.Therefore,the Tan-Cheng model should ignore the reduction that considering the size effect.5.Based on the test results presented in this thesis,some improvements were suggested to the design method presented in Chinese code.Simultaneously,a strut-and tie model-based(STM-based)design method was proposed for HSLWAC deep flexural members by considering the size effect and the properties of LWAC,which can provide guidance for the engineering practice.Through the experimental study and theoretical analysis,the serviceability performance quantification of deep flexural members was determined and the shear models that considering the size effect were established.Finally,the STM-based design method for HSLWAC deep flexural members was proposed.All the aforementioned contributions provide important theoretical reference for the application and shear design of LWAC structures.