The Fracture And Bond Performances Of Fiber Reinforced And Rehabilitated Concrete

Fiber reinforced concrete (FRC), which is randomly mixed into short-cut thin fibers in the concrete matrix, is a composite with higher toughness. With the wide use of FRC, the improvement of design method for FRC structures and the establishment of reasonable fracture criterions for FRC are of greatly regarded by many scholars. Although there are a few of researches on the fracture performance of FRC at home and abroad, these are not systematic and the conclusions on the fracture properties of FRC are inconsistent because of its complexity, especially for the properties of hybrid fiber reinforced concrete (HFRC) that has too many influence factors.Fiber reinforced polymer (FRP) composite has been extensively used in the rehabilitant of concrete structure in civil engineering because of its perfect mechanical behavior. The multitudinous engineering practices and the experimental researches indicate that the local de-bonding of FRP from concrete, which is a key influence factor on the successful of rehabilitant, is a major failure mode in FRP rehabilitated concrete structures. The single and double face shear testing methods are widely used in the experimental research on the bond behaviors of FRP-to-concrete at present. Although these testing methods may reflect the bond properties between FRP and concrete to a certain extent, the FRP stress condition is inconsistent with the actual in FRP rehabilitated concrete structures.Adding lower volume fraction of steel fiber in concrete, to improve the properties of concrete matrix, is essentially improved the cracking resistant of the concrete matrix, which can also improve the bond performance between concrete and FRP. How to evaluate this effect and how to establish the computational model that consistent with the calculation of bond performance between FRP and steel fiber reinforced concrete (SFRC) are not rectified by experimental studies.Based on the fracture mechanics theories, the fracture performances of SFRC and steel fiber reinforced high-strength concrete (SFHSC) and polypropylene fiber reinforced high-strength concrete (PPHSC) and hybrid fiber (steel and polypropylene fiber) reinforced high-strength concrete (HFHSC) are studied in this dissertation. Through three points bending test on notched concrete beams rehabilitated by FRP (also known as modified beam test), the bond behaviors of FRP- to- concrete and FRP-to- SFRC are studied. The following works are included: 1. Through the three points bending (TPB) test on 72 notched SFRC and concrete beams with the size of 100mm×100mm×515mrn, the effect of steel fiber volume fraction (ρ_f) and steel fiber types upon the fracture properties of SFRC, such as critical stress intensity factor (K_IC), fracture energy (G_F), critical crack mouth opening displacement (CMOD_C), critical crack tip opening displacement (CTOD_C) as well as rotating factor (r), have been studied. The testing results show that: steel fiber can significantly improve the fracture properties of concrete; as the increase ofρ_f the fracture parameters of SFRC increase in a various degree; in comparison with the milled steel fiber and waved steel fiber, bowed steel fiber can significantly improve the fractural properties of SFRC. The crack propagation of SFRC beams and their control concrete beams rotate around an image point, and the ratio of CTOD/CMOD and r tend to be stable when the CMOD achieve to a certain value; r tend to decrease asρ_f increase but steel fiber type has no remarkable influence on the rotating factor. The rotating factor of concrete and SFRC are 1.001 and 1.1234 respectively. Based on the stastic analyses of experimental data, the formulas and modified formulas for calculating the fracture parameters of SFRC, which link up to those of concrete, are proposed.2. Through the TPB test on 144 notched SFHSC and high-sreength concrete (HSC) beams with the size of 100mm×515mm, the effect ofρ_f notch depth and steel fiber types upon the fracture properties of SFHSC, such as K_IC, G_F, CMOD_C, CTOD_C and r, have been studied. The influence of test method and the strength of concrete matrix on the fracture properties of FRC are discussed. The testing results show that: steel fiber can significantly improve the fracture properties of HSC; as the increase ofρ_f the fracture parameters of SFHSC increase remarkably, especially for the fracture energy; in comparison with the milled and waved steel fiber, bowed steel fiber can significantly improve the fractural properties of SFHSC; the strength of concrete matrix influence the enhancement of steel fiber, but test method has no remarkable influence on the test results. The r of milled steel fiber reinforced HSC tend to be stable correspond to one notch depth, and have no direct relationship withρ_f and increase appreciably with the increase of notch depth. The r of milled steel fiber reinforced HSC is 0.5013 in the scope of steel fiber volume fraction tested. Based on the statistic analyses of the experimental data, the formulas and modified formulas for calculating the fracture parameters of steel fiber reinforced concrete with different compressive strength, which link up to those of concrete, are proposed. Computational results show that modified formulas can safely predict the fracture parameters of steel fiber reinforced concrete through those of concrete.3. Through the TPB test on 44 notched PPHSC and HSC beams with the size of 100mm×100mm×515mm, the effect of polypropylene fiber dosage (W_f) upon the fracture properties of PPHSC, such as K_IC, G_F, CMOD_C and CTOD_C, have been studied. The testing results show that: in the scope of W_f tested, polypropylene fiber has no remarkable influence on K_IC, but increases the G_F and has no remarlable influence on the CMOD_C and CTOD_C of HSC; the K_IC and its increment ratio of PPHSC have no direct relationship with W_f. As increase of W_f, the G_F and its increment ratio of PPHSC display a good increase tendency. The CMOD_C and CTOD_C decrease as increase of W_f, but the increment ratios of CMOD_C and CTOD_C have no direct correlation with W_f. Testing method has varying degree influence on the fracture parameters of PPHSC, especially for G_F. Polypropylene fiber mainly improves the behavior of HSC at the post-cracking zone. Based on the analyses of experimental data, the formulas for calculating the fracture parameters of PPHSC are proposed.4. Through the TPB test on 55 notched HFHSC and HSC beams with the size of 100mm×100mm×515mm, the fracture properties of HFHSC and the synergistic effect of steel and polypropylene fiber have been studied. The testing results show that: in the scope of testing fiber content, the K_IC and its increment ratio of HFHSC have no direct relationship with W_f, but the G_F, CMOD_C and CTOD_C and their increment ratio of HFHSC display a good increase tendency as increase of W_f. The fracture parameters of HFHSC increase with the increase ofρ_f. Steel fiber and polypropylene fiber have displayed a positive synergy effect on the fracture properties of HFHSC, especially for fracture energy of HFHSC. Steel fiber plays a dominate role in the improvement of fracture properties of HFHSC, but the improvement effect of polypropylene on the fracture properties of HFHSC is limited.5. Through TPB test on 90 notched concrete beams with the size of 100mm×100mm×515mm and rehabilitated by externally bonded FRP, the influences of laminate thickness, width, length of bonded carbon fiber reinforced polymer (CFRP) sheet and compressive strength of concrete on maximum load bearing capacity, deflection according to the maximum load, the ultimate bond strength between CFRP and concrete, critical bond length of CFRP-to-concrete and stress distribution of CFRP are studied. The influence of laminate thickness of glass fiber reinforced polymer (GFRP) sheet on the load bearing capacity of rehabilitated beam, ultimate bond strength of GFRP-to-concrete and stress distribution of GFRP are also analyzed. The bond behaviors of hybrid fiber (CFRP and GFRP) reinforced polymer (HFRP) sheet to concrete is also researched. The results show that: the load bearing capacity of beam increase as increase of FRP thickness; the deflection of the rehabilitated beam, which is closely relate to the characteristic of FRP, decrease as the thickness of CFRP increase, but increase with the increase of thickness of GFRP; the stiffness, bond width and bond length of FRP are all influence the ultimate bond strength of FRP-to-concrete, but this enhancement dose not varying linearly with the thickness of FRP; There is no evidence show the bond behavior of CFRP-to-concrete relate to the compressive strength of concrete. The existence of critical bond length of FRP-to-concrete, above which no further improvement in bond behavior of FRP-to-concrete occurs, is also verified in this study and the length is closely relate to the stiffness of FRP. Based on the experimental data, formulas for calculating the ultimate bond strength and critical bond length of different FRP-to-concrete, which are also evaluated by the predecessors' experimental results, are proposed, and the calculated results are agreement well with the results measured.6. Through TPB test on 80 notched SFRC and concrete beams with the size of 100mm×100mm×515mrn rehabilitated by externally bonded CFRP, the influences ofρ_f and bond length of CFRP on the bond performances of CFRP-to-SFRC and CFRP-to-concrete are studied. The results show that: The addition of steel fiber can improve the bond behavior of CFRP-to-concrete to a certain degree. The load bearing capacity of rehabilitated SFRC beams and its corresponding deflection and bond strength of CFRP-to-SFRC and maximum stress level of CFRP increase with the increase ofρ_f. There also exist critical bond length between FRP and SFRC, after which no more improvement of bond behavior of FRP-to-SFRC is observed. The addition of steel fiber to concrete matrix can increase the critical bond length of CFRP-to-SFRC and the value of critical bond length of CFRP-to-SFRC increase with the increase ofρ_f and can also improve the distribution of stress of CFRP. Based on the experimental data, formulas for calculating the ultimate bond strength and critical bond length of CFRP-to-SFRC, which link up to those of CFRP-to-concrete, are proposed, and the calculated results are agreement well with the results measured.