Fracture Mechanics Based Fatigue Life Analysis of Rc Bridge Slab Repair by Fiber Cementitious Materials

In this thesis, an analytical tool for predicting fatigue performances of RC bridge slabs isdeveloped based on the concept of fracture mechanics. The degradation of bridging stress in thenormal direction of concrete cracks is considered as a primary mechanism for the propagation ofboth flexural cracks and shear cracks that induce the failure of RC slabs. The analysis of RC slabsrepaired with several kinds of cementitious materials is conducted and the extension of fatiguelife of those repaired slabs is predicted by means of the proposed analysis model.The development of the fatigue analysis model of RC slabs is assigned in three developmentlevels: microstructure level, mesostructure level, and macrostructure level.In microstructure level, the material model so called “Bridging Stress Degradation Relation”is proposed for representing the stress degradation characteristics of fiber cementitious materialsunder fatigue loading. The determination of bridging stress degradation relation of EngineeredCementitious Composite (ECC) is proposed by both micromechanics based approach andexperimental based approach. The micromechanics based model which considers fiber bridgingcharacteristics is developed based on the assumption that rupture failure takes place under fatigueloading. The uniaxial tensile fatigue test of ECC is carried out in order to determine the bridgingstress degradation relation and to investigate multiple cracking characteristics of ECC. It is shownthat the bridging stress degradation relation of ECC determined by the micromechanics basedmodel agrees well with that obtained in the experiment.The fatigue crack growth due to the bridging stress degradation is considered as themechanism linking between microstructure level and mesostructure level. In mesostructure levelor intermediate member level, the fatigue analysis scheme based on the concept of fatigue crackgrowth is developed, and by introducing the bridging stress degradation relation proposed in themicrostructure level, flexural fatigue properties of ECC beams can be predicted. Four pointbending tests of ECC are conducted in order to verify the analysis concept. The comparisondemonstrated that the analysis model can be used for estimating the flexural fatigue properties ofECC beams, such as S-N relationship and the evolution of midspan deflection, when the bridgingstress degradation relation is employed in the analysis.Four point bending tests of several kinds of repair materials, such as polymer cement mortarand fiber reinforced shotcrete, are also conducted in order to investigate the flexural fatigueproperties. The comparison of the flexural fatigue properties between those materials is useful forthe material selection in RC slab repairs.In macrostructure level, a three-dimensional FEM analysis of RC slabs under a fatiguemoving load is developed based on the same fatigue crack growth analysis scheme. The analysisiii is performed for predicting fatigue performances of RC slabs under traffic load condition, such asdeflection evolution, S-N relation, and crack pattern.The analysis model consists of smeared crack elements representing concrete and rodelements representing steel reinforcement. Concrete cracks are allowed to initiate in threeperpendicular directions in smeared crack elements according to the principal stress crackingcriteria. Each crack alignment performs independent crack opening and closing under loadrepetitions. Under fatigue, the hysteresis bridging stress relation with the cycle-dependentdegradation is considered for each crack alignment. The structural performances of RC slab canbe analyzed with given geometries and boundary conditions.The analysis of RC slabs with different loading conditions shows that the fatigue life of theslab under moving wheel load exhibits shorter fatigue life than that under fixed pulsating load.Moreover, the comparison of midspan deflection evolutions between the two loading casesimplies that the slab under moving load exhibits more distributed crack than the slab under fixedload.Finally, the analysis of ECC repaired RC slabs and normal concrete repaired RC slabs areconducted for both underlay and overlay repair cases. The improvement in fatigue life anddeflection capacity of all repaired RC slabs compared with the original RC slab can bedemonstrated by the fatigue analysis. Both overlay and underlay repaired slabs by ECC exhibitmore improved life than slabs repaired by normal concrete.For further application, the comparison of extended life between different repair materials andthe relation between extended life and thickness for each repair materials are useful in theselection of a suitable repair for each slab case. It is possible to use the proposed analysis for thedevelopment of repair design proposal.