Evaluation method of cracking resistance of lightweight aggregate concrete

The cracking behavior of lightweight aggregate concrete(LWAC) was investigated by mechanical analysis, SEM and cracking-resistant test where a shrinkage-restrained ring with a clapboard was used. The relationship between the ceramsite type and the cracking resistance of LWAC was built up and compared with that of normal-weight coarse aggregate concrete(NWAC). A new method was proposed to evaluate the cracking resistance of concrete, where the concepts of cracking coefficient ζt(t) and the evaluation index Acr(t) were proposed, and the development of micro-cracks and damage accumulation were recognized. For the concrete with an ascending cracking coefficient curve, the larger Acr(t) is, the lower cracking resistance of concrete is. For the concrete with a descending cracking coefficient curve, the larger Acr(t) is, the stronger the cracking resistance of concrete is. The evaluation results show that in the case of that all the three types of coarse aggregates in concrete are pre-soaked for 24 h, NWAC has the lowest cracking resistance, followed by the LWAC with lower water absorption capacity ceramsite and the LWAC with higher water absorption capacity ceramsite has the strongest cracking resistance. The proposed method has obvious advantages over the cracking age method, because it can evaluate the cracking behavior of concrete even if the concrete has not an observable crack.

Microstructural Damage Evaluation of High Alumina Castables with Addition of Different Dimensions of Functional Aggregates After Several Thermal Shock Cycles

Refractory materials are subjected to mechanical and chemical loads during application,which respectively lead to fracture and corrosion. The understanding of the mechanical fracture and the microstructural modification after progressive thermal shocks is necessary for the suitable design of high performance refractory materials.The fracture process zone can be divided into two areas,where different toughening mechanisms are likely to take place： the crack tip and the following process region called wake region. In this study,the emphasis is laid on the microstructural mechanisms occurring in the wake region of the crack and the crack tip. Therefore,functional aggregates,namely fused eutectic aggregates of Al2O3- ZrO 2- SiO 2and Al2O3- ZrO 2,and andalusite,are added to a model high alumina castable formulation based on tabular alumina. The discrepancy in thermal behavior between aggregates and matrix as well as phase transformation such as mullite formation favor further the nucleation of microcracks that reduce the stress field at the level of the crack tip. Two different grain fractions of tabular alumina（ 0. 2- 0. 6 mm or2. 24- 3. 00 mm） are substituted by these functional aggregates. After sintering at 1 500 ℃ for 6 h,the samplesare quenched up to 10 times in air without pressure from950 ℃ to room temperature. The resulting elastic properties are examined after each thermal shock cycle,as well as the residual bending strength after 0,1,3,5,7and 10 thermal shocks. These results are correlated with Scanning Electron Microscopy analyses after the different thermal shock cycles.