Research On Brittleness Mechanism And Modification Of High Strength Lightweight Concrete

Concrete is the most widely used construction material and its intrinsic disadvantages are heavy dead weight and prominent brittleness. The improvement in brittleness of concrete with lightweight and high strength is of the most efficient routes to increase the use effectiveness, reduce the dead load of structures and conserve construction resource and energy, it is also the main target for theoretical research and application development of concrete materials.Under the support of NSFC (National Natural Science Foudation of China) grant "The brittleness characteristics and toughening technology of high strength light weight aggregate concrete (LWAC)" (50272045). The fractural mechanical behavior of LWAC is numerically modeled and experimentally verified by mesomechanics and the brittleness mechanism is explored. Based on this, the toughening method of LWAC is put forward and the mix design is systematically studied, from which, key parameters are obtained and the research outcome is applied to some projects.The primary research carried out and results obtained are as follows:Based on the composition and structure of LWAC, Monte Carlo Method is employed to put forward the random aggregate distribution model of LWAC; the model of finite element method (FEM) is constructed based on the geometry and grading of lightweight aggregate(LWA), from which, the relationship between meso unit dimension and fracture property of concrete, and the meshing of FEM elements; The failure criteria under various stress levels are analyzed, and the influence of LWA-cement interface on the fractural mechanical behavior; The meso non-linear FEM of the fractural mechanical behavior of LWAC is put forward and the model for numerical analysis is constructed.Quasi-static fracture process of LWAC is numerically modeled by meso non-linear finite element model; the fracture behavior of LWAC is investigated by uniaxial extension, compression, three-point flexural experiments and the corresponding stress-strain curves and the stress distribution within concrete are obtained. The crack generation, propagation within LWA is clarified and monitored in situ from the start of loading to total failure, from which, the brittle failure mechanism is demystified.Based on the above results, the toughening method by winding the crack propagation path and toughening the interface is put forward, and the replacement of LWA by high strength, high flexibility material to prepare high strength concrete is put forward. The effect of replacement ratio, conventional mix design method on mechanical property, density and toughening index of concrete is investigated and the optimized parameters are obtained. Results indicate that with a replacement ratio of above 30%, the ductility index of lightweight concrete is increased by 40%. The combination of polymer impregnation and interface toughening to improve the LWA-cement paste interface is put forward and the brittleness of concrete is greatly improved.The effect of w/b, LWA volume and the composition of cementitious materials on strength, density and toughening of concrete is studied and the mix design method by optimizing strength, density and brittleness is mastered; the effect of fibre, polymer toughening on the toughening of lightweight concrete.Concrete of high strength, toughness and light weight is developed by the integration of the above techniques and its application key technologies are summarized. It has been successfully applied to several projects of the paving of steel box bridge and prestressed concrete bridges with noticeable economic and social benefits.