Experimental Research On Bonding Mechanical Performance Between Ultra High Toughness Cementitious Composites (UHTCC) And Existing Concrete

Ultra High Toughness Cementitious Composites (UHTCC), which is a new high performance cementitious composites reinforced with random short fibers, possesses strain-hardening characteristic and outstanding crack controlling capacity under tension. Its tensile strain capacity can be steadily up to 3%, and its maximum crack width can be maintained at a low level (below 0.1mm) until the ultimate tension strain is achieved. Therefore, it is potential to increase bearing capacity of existing structure and prevent kinds of cracks for improving the concrete structure durability by using UHTCC as repair material or exterior protective layer. Many researches on the mechanical properties, composite structure design with steel or concrete and field applications, but limited studies have yet been reported on bonding working performance between UHTCC and existing concrete. Therefore, the bonding performance between UHTCC and existing concrete, flexural behaviors and crack controlling capacity of composite member is researched by experimental studies and theoretical analysis. And this research is part of the Key Program of the National Natural Science Foundation of China (No.50438010) and the Research and Application Programs of Key Technologies for Major Constructions in the South-North Water Transfer Project Construction in China (JGZXJJ2006-13). The main content of this thesis are as follows:(1) 164 cubic specimens and T-shape specimens of UHTCC to existing concrete were test for behavior researching of bonding splitting tension and direct tension. And 164 cubic specimens were test for behavior researching of bonding direct shear. The main influence factors were investigated, such as the existing concrete interfacial roughness, strength, interfacial moisture states and UHTCC pouring directory. The results show that the bonding strength could be enhanced by improving the micro and fine structure of existing concrete interface and increasing the contacting area between UHTCC and concrete. The bonding splitting strength and shear strength reached more than 72.8% and 52.9% of existing concrete corresponding strength.(2) Four-point bending tests were conducted up to failure on concrete control beams and concrete beams strengthened with post-poured UHTCC layer on the tension face. The length, width, and depth of all specimens were kept as 400×100×100mm. The main influence factors on flexural behavior were investigated, such as UHTCC layer thicknesses, existing concrete strength and interfacial roughness. Experimental results indicated that considerable reduction in crack width was observed for composite specimens, as UHTCC layer restricted the cracks in upper concrete and dispersed them into multiple fine cracks effectively. The use of UHTCC layer significantly increased the first crack load and ultimate flexural load. The effect on bearing capacity of UHTCC layer decreased with the concrete strength increasing. The interfacial roughness between UHTCC and concrete had a certain influence on bending behavior. Based on the plane section assumption, analytical equations were derived to predict the flexural capacity of composite beam by using nonlinear analysis theory. Combined with the test results, the minimum critical thickness of UHTCC layer was calculated by energy principle.(3) The flexural behavior and crack controlling capacity of RC beam strengthened with externally poured UHTCC were improved by four-point bending tests. The carrying capacity, crack development and failure mode of the beams are studied, compared with those of the contrast beams. The results indicate that pouring UHTCC material on the bending surface of RC beams properly could improve the flexural performance and toughness of existing structure. Introducing UHTCC material into strengthening dispersed the cracks in upper concrete into multiple fine cracks to prolong the appearance of harm cracks on the bending face. The maximum crack width on the UHTCC layer was below 0.06mm until the reinforcement in the RC beam yielded. End stress of the locally strengthening beams increased accordingly with the increase of strengthening layer stiffness. Planting reinforcing bar was an effective method for improving bonding interfacial state. Based on the above mentioned, the calculation method of minimum critical thickness of UHTCC layer also was proposed. The flexural capacity of composite beam and end cracking load of post-poured layer were calculated by corresponding analytical methods.(4) Two-way simply supported composite slab tests under the action of concentrated load were carried out. The bending surface of the existing reinforced concrete slab was poured on by UHTCC or high strength grade concrete materials to form composite slab. The results showed that UHTCC layer could enhance the overall bearing capacity, limit and disperse the cracks on the bending surface. Different from brittle failure of slab strengthened with concrete, strengthening slab with UHTCC presented an obvious tough failure. And UHTCC layer could exert the strength of rebar by avoiding the stress concentration around the main cracks. Non-linear three-dimensional finite element software was applied, through the modeling of material properties and definition of constraint conditions reasonably, to analysis and forecasting test results, including the load-deflection curve, deflection and strain distribution on the bending surface.