Mechanical Properties Of Self-compacting Lightweight Concrete

Self-compacting lightweight concrete (SCLC) is a new performance concrete on the basis of the development of self-compacting concrete and lightweight concrete, with the advantages of light self-weight, high filling capacity and no vibration requirement during mixing. As a new construction material, the thermal, bond and fracture behaviors have not been yet thoroughly studied. In order to ensure the safe application of SCLC in practical engineering structures, the thermal, bond and fracture behaviors of SCLC are studied by conducting the experiment of thermal, pull-out and three-point bending beam SCLC specimens. The main contents and conclusions are as follows:(1) The thermal test of SCLC with strength grade of C40and C50was carried out to explore the causes of spalling. Through the comparison of the mass loss, ultrasonic pulves velocity and mechanical properties of SCLC before and after high temperature, the chemical phase transformation, the damage and mechanical properties of SCLC under high temperature were studied. The thermal behaviors of normal concrete, lightweight concrete and self-compacting concrete with identical strength to SCLC were also compared for a futher explanation on the causes of the thermal behaviour of SCLC subjected to high temperature. Results show that SCLC is prone to spall in high temperature due to the high moisture content of lightweight aggregate and homogeneity and dense matrix in concrete. However, SCLC maintans better residual mechanical properties than normal concrete, which indicates the superiority in terms of strength regain in high temperature.(2) Accroding to the analysis of spalling of SCLC, PPFs was incorporated into SCLC to improve the thermal behaviour of SCLC. PPFs is proved to be an effective way to improve the thermal behaviours of SCLC by reducing the potential of spalling and improving the residual mechanical properties.(3) The bond behaviour of SCLC with deformed bars under lateral pressure was studied. The effects of the orientation and magnitity of the lateral pressure, concrete compressive strength, ratio of concrete cover depth to bar diameter, type of lightweight aggregate on the bond characteristics of SCLC with deformed bars were also evaluated. The experimental results on69series of pull-out specimens showed that, the failure mode of SCLC specimen transferrs from splitting to pull-out as the lateral pressure increases. The variation of bond strength depends on the failure modes and pattern of lateral pressure. Under uniaxial lateral pressure, the bond strength is generally an increasing fuction of the lateral pressure in mixed splitting/pull-out failure, but remains constant in pull-out failure. However, the bond strength increases monotonically irrespective of the failure mode. The numerical regressions of bond strength, slip at the ultimate bond stress, and the ratio between residual and ultimate bond stress were obtained and validated through the experimental results.(4) The mathematical model of bond-slip relationship of deformed bars in SCLC is proposed. The bond-slip curves of deformed bars in SCLC subjected to lateral pressure are reasonably predicted and agree well with experimental resluts.(5) An analytical methond to determine the crack resistance curve of SCLC in mode I fracture is developed based on the cohesive crack model. Accroding to crack propagation criterion, the crack extends when the difference of the stress intensity factor caused by external force and cohesive force equals to initial cracking toughness. The calculated load-displacement curves of eight series of three-point bending beam with different initial crack length to depth ratio and specimen size by crack propagation criterion agree reasonably with experimental results, which indicates the feasibility of the criterion. Thus, the crack resistance of concrete can be expressed by the sum of the stress intensity factor by cohesive force and initiation cracking toughness, or the the stress intensity factor by external force. KR-curves and fracture process zone (FPZ) of the SCLC three-point bending beam of different initial crack length to depth raio were analyzed. The results show that KR-curve is independent of intimal crack length to depth raio. FPZ length increases with crack length, and then decreases after FPZ is fully developed. KR is shown to be insensitive to the shape of strain softening relationship of concrete. The shape of strain softening relationship exhibits insignificant effect on the results.