There are various direct tensile tests to estimate tensile stress-strain curve of concrete. However, in direct tensile test, bending moment which is hard to erase generates strain gradient at cross section of specimen. Present direct tensile test estimates stress in the way of dividing resultant force by the area of cross section. If strain gradient occurs, this calculation method can t estimate accurate stress-strain curve. Inverse analysis using bending test apparatus is proposed which can estimate tensile stress-strain curve from force-deformation curve measured at loading point. However, this inverse analysis has some problem. For example, reproducing detail test condition is required. I propose method to estimate tensile stress-strain curve in direct tensile test under large strain gradient. This method is characterized by measuring system and equations. The measuring system is as follows. Indentation marks are established at the both narrow side of tabular test piece to stabilize fracture location. I define cross section between these two indentation marks as estimation section. Strain measuring apparatuses are established at the unilateral long side to measure strain distribution in estimation section. Give tensile force to tabular test piece and measure resultant force vertical to estimation section. The equations are based on inverse estimation method and can calculate stress corresponding to strain from the time series data of resultant force and strain distribution in estimation section by linking the time series data. I certificate that this method can estimate accurate stress-strain curve under strain gradient by executing numerical analysis. This method gives variance to the shape of test piece around the estimation section. So test equipment can be simplified.

 This study conducted several loading tests by using bridge RC slabs used for 49 years in order to evaluate the structural performance of RC slabs affected by both frost damage and ASR. As the results, it was found that layered cracks occurred between top surface and lower rebars reduce the stiffness and fatigue durability of RC slabs. Also, especially when layered cracks spread throughout the slab, the progress of fatigue damage became rapid. Further, it was indicated that the conventional deflection-based soundness evaluation method, which was proposed for evaluating the soundness of RC slabs subjected mainly to fatigue, seems not be applied to evaluate the soundness of RC slabs affected by frost damage-ASR combined action.

 For prestressed concrete structures, it is important to apply and maintain the required tensile force of the prestressing cable. During the construction and in the service period, in order to directly evaluate the distribution of tensioning force along the whole length of a prestressing cable, a new measuring method by using the optical fiber sensor has been developed. In this study, measurement performance tests were conducted, and it was verified that the optical fiber sensor can be integrated with the PC-strand and the distribution of tensioning force can be measured accurately. Furthermore, this new method was applied to a post-tensioned prestressed concrete viaduct. It was confirmed that, the distribution of tensioning force can be measured even in the actual structure, and it can be directly confirmed that the installed prestressing force exceeded the designed value, along the whole length of the cable. This method can be effectively used for the maintenance of PC structures, in addition to the control of prestressing.