| Ultrahigh toughness cementitious composite (UHTCC) is a new type of high performance cementitious composite which was designed by proper selection of fiber, matrix and interface. UHTCC can be used for durability repair of deteriorated concrete structures since it has a pseudo strain hardening property, super ductility and multiple cracking behaviors. The tensile strain capacity of UHTCC is high up to 3-7% and the crack width in the UHTCC usually can be controlled below 100μm under a service condition even though a low fiber volume fraction is used (generally less than 2-3%). A key issue for this repair technology is the behavior of the interfacial bond between the UHTCC overlay and the concrete substrate. Furthermore, combining advantages of textile and UHTCC, a new composite system, textile reinforced UHTCC (TRU), is expected to provide a dual strengthening effect to the original reinforced concrete (RC) beams due to the high strength of the textile reinforcement and the strain-hardening behavior of UHTCC. The research is supported by 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 detailed contents of this thesis are as follows:1. A program of experiments on bond performance was carried out, including direct tension test, bending test, and shear test. Six types of surface roughness, four strengths of concrete substrate, ten types of primer and bonding agent were utilized. Three ages including 28 days,120 days and 366 days were considered. Two types of UHTCC were prepared in two forms:cast on-situ and prefabricated. The results of tests show that all the tension bond strength, bending bond strength and shear bond strength increase with the increasing surface roughness and the compressive strength of concrete substrate. The primers and bonding agents have a strong influence on the interface. It is found that the fly ash, silica fume and the styrene-butadiene latex can greatly improve bond behavior between UHTCC and concrete substrate. And fly ash and silica fume can also improve the long term strength of UHTCC matrix, which cause higher tension strength at 1 year of age compared with other two repair materials, ie. traditional steel fiber reinforced concrete (SFRC) and normal concrete. Besides, the prefabricated UHTCC system exists two interface layers and the bond strength is decided by the strength of weaker interface.2. Four-point bending tests were conducted on UHTCC overlaid concrete beams with a "T" notch. The objective was to evaluate the fracture behavior of the interface between the UHTCC and the concrete substrates. In the test, a row of strain gauges were arranged nearby the bond interface to monitor the occurrence of the initial interfacial cracks, the interfacial crack propagation length, initial fracture toughness. Test results indicate that, UHTCC overlaid concrete beams possess much better performance than conventional SFRC overlaid ones in terms of their global load vs. deflection response and crack distributing characteristics. A much higher energy dissipation capacity of the UHTCC/concrete interface is achieved as compared to the SFRC/concrete interface due to the tensile strain-hardening property of UHTCC. The prefabricated UHTCC overlay system exhibits more desirable behavior particularly in terms of the deformation ability and crack dispersing property of the whole repair system as compared to the cast-in-situ one. The former overlay system seemed to be able to achieve a larger critical crack propagation length along the interface before the failure of the repair system probably due to its doubly layered bonding configuration. The ultimate load of the repaired system tends to increase with the thicknesses of the UHTCC overlays but the mid-span deflection showed an opposite manner. When their thickness is small, the propagation of the interfacial cracks is interrupted by the material failure of overlays, leading to relatively concentrated cracks in the overlay. Hence a certain thickness may be required to provide sufficient damage tolerance of the whole repair system.3. A total of 9 textile reinforced UHTCC (TRU) strengthened reinforced concrete beams (TRUSRC beam) were tested. The tested parameters covered the layers of tensile, grid sizes, the anchorage lengths of the textile reinforced composites and the types of cementitious matrix. It is found that textile reinforced UHTCC is an ideal composite overlay system for the strengthening of RC members. A series of tests confirm that RC beams strengthened with TRU exhibit a significant increase in the flexural capacity and a ductile failure mode, compared to textile reinforced mortar (TRM) overlay system and SFRC overlay system, due to the dual strengthening effects of textile reinforcement and UHTCC as well as the pseudo-plastic behaviour of TRU. The TRU overlay system is found to be able to contribute a much improved serviceability because of the multi-crack behaviour of the UHTCC.4. Based on material constitution models and plane section assumption, further analytical study was required to develop design theories to facilitate the practical implementation of such a new TRU strengthening system. The development processes under flexural load including elastic stage, cracked stage and failure stage were discussed. Calculation formulae of moment and neutral axis height at every stage were presented. The deflection of TRUSRC beam was determined based on conjugate beam method. And the result show that the calculated values of load and deflection agree well with experimental values. Therefore, the TRU strengthening system may have good potential in use for repair of concrete structures.
|
PDF Full Text Request