Damage of PC tendons in a prestressed concrete structure need to be detected before such damage accumulates to cause a serious failure. That, however, is quite a difficult task because the PC tendons are invisible from outside and the damage location cannot be known beforehand. Phase space analysis based on vibration data is a novel method for damage detection. An earlier study by the authors demonstrated that Change of Phase Space Topology (CPST) was effective in identifying the existence of PC tendon damage. However, CPST from impact hammer test, which is a more practical method, did not show a trend as obvious as that of the free vibration test. As it is known that PC tendon damage affects several modes of vibrations and that the hammer can be used for excitation to the higher modes, it may be possible to improve the capability of impact hammer test by considering CPST separately in different frequency ranges. As in the previous study, the current study conducted experiments on PC tendon damage, but with an increased number of accelerometers for constructing mode shapes and investigated CPST further in different frequency ranges. The results still revealed that CPST was more sensitive to damage than the parameters from modal-based analysis. CPST in different frequency ranges can improve results from impact hammer test and has the capability to identify roughly the damage locations.
Hardened cement pastes (HCP) with different water contents were irradiated with gamma rays under different temperatures and irradiation dose rates. The relationship between the quantity of hydrogen gas produced and the water content as well as the stability of HCP under gamma irradiation was evaluated. It is experimentally confirmed that hydrogen gas was mainly produced from the evaporable water. The G value of the hydrogen production assuming the radiation energy absorbed by the total water composed of chemically bound water (CBW) and evaporable water was ranging from 0.03 to 0.42. The G value of the hydrogen production for CBW was ranging from 0.03 to 0.07, which were an order of magnitude smaller than that of the bulk water (0.45).
Assuming that the radiation energy on evaporable water is used for the formation of hydrogen, it is experimentally confirmed that, in case of low dose rate, the G value tended to converge to a constant value when the evaporable water exceeded a certain value, while, in case of high dose rate, the G value increased as evaporable water increased. However, the G values of all cases grew with increasing evaporable water content and exceeded the G value of the bulk water (0.45). The CBW was not susceptible to gamma irradiation. Only 2 to 3% of the CBW was estimated to be decomposed by 200 MGy of gamma irradiation.
A multi-scale model for significant characteristics of cementitious composite and structural concrete at high temperatures is presented and the experimental verification at micro, meso and macro-scales is conducted. Deterioration of cement hydrates, reduced stiffness of concrete composite and their rehydration are modeled at high temperature of 100 – 1000°C and integrated up to the multi-scale simulation platform. This framework enables us to reproduce some meso-scale chemo-physics such as progressive spalling of concrete cover and exposure of reinforcing bars. The temperature-dependent thermal characteristics of both aggregates and cement matrix are also considered to truck the rising temperature inside RC members. The behavioral simulation of columns, slabs and beams, which are subjected to axial compression and out-of-plane flexural shear, is conducted to be ready for fire.
In order to enhancement accuracy of shear design of reinforced concrete (RC) beams, detail understanding of the shear resistance mechanism is required. This study evaluated the shear resistance mechanism of RC beams based on arch and beam actions by using three dimensional Rigid-Body-Spring-Method (3-D RBSM). Firstly, RC deep and slender beams with and without shear reinforcement failed in shear were tested to measure local behavior. Then, the validity of local behaviors obtained from 3-D RBSM was confirmed by comparing with the test results and the applicability of decoupling of shear resistance mechanism using simulated stress distribution was presented. Moreover, the contributions of arch and beam actions in RC beams until failure stage were investigated numerically by changing the shear reinforcement ratio and shear span to depth ratio and was compared with the current shear design recommendations in JSCE Standard Specification. As a significant finding, the numerical results upon the quantitatively evaluation of shear resistance mechanisms that the shear strength of RC beam could be evaluated without classification of deep beams and slender beams was presented.
The distribution of restraint stresses in bottom-restrained walls is an important information for the efficient crack control of wall-like concrete members. Practical examples are retaining walls, bridge abutment walls or tank walls, for which the results can be used in order to assess the risk and intensity of harmful separating cracks over the wall height.
Different solutions exist for the determination of these stress distributions, ranging from advanced computational methods over analytical and semi-analytical solutions up to empirical approaches. The aim of the present contribution is twofold. On the one hand, the general applicability as well as commonalities and differences of the investigated solutions were demonstrated by using them for the analysis of a given demonstration example. On the other hand, a para-metric study was carried out in order to assess the dependence of the prediction quality of the applied solutions on changing conditions. Altogether it was found that advanced computational methods and analytical or semi-analytical solutions showed a good agreement for common design tasks. Solutions with empirical modifications, however, were proved to be less satisfying from engineering perspective due to predefined parameters or mechanically inconsistent modifications.
Ana Mafalda Matos, Sandra Nunes, José L. Barroso Aguiar
The primary goal of the present paper is to investigate the influence of cracking on water transport by capillary suction of UHPFRC. Prismatic specimens were firstly loaded under four-point bending up to specific crack open displacement (COD). Target COD, under loading, was varied between 200 and 400 μm, in steps of 50 μm. After unloading, a COD recovery was observed with residual COD ranging between 116-334 µm and 75-248 μm for UHPFRC-1.5% and UHP-FRC-3.0% specimens, respectively. The crack pattern created was characterised (number of cracks and crack width) before capillarity testing. Sorptivity results of cracked UHPFRC-1.5% and UHPFRC-3% specimens remained in the range of 0.024 to 0.044 mg/(mm2.min0.5), which are about 2 to 4 times higher than the sorptivity results of non-cracked UHPFRC specimens. However, the maximum sorptivity observed on cracked UHPFRC is relatively low as compared to typical sorptivity results found in good quality conventional concrete or engineered cementitious composites (ECC).
To estimate the remaining life of existing RC bridge decks damaged by alkali silica reaction (ASR), multi-scale numeri-cal analysis with chemo-hygral model is integrated with visual inspection data at site. First, the applicability of the poro-mechanical models for ASR expansion in the multi-scale frame are examined with the experiments of the real scale RC slabs and the model is validated to bring about fair prediction of the 3D anisotropic expansion and the fatigue life of the slabs. Second, visually inspected cracks on bottom surfaces of RC decks are converted to space-averaged strains, and the magnitude of ASR is estimated from the vertical deformation, based on which the internal pre-stress and the damage fields are re-produced by numerical predictor-corrector cycles, and the remaining life of ASR damaged RC bridge decks is fairly estimated. By conducting sensitivity analyses in terms of ASR-gel volumes and cracks, allowable error range of site inspection data is clarified to meet the requirement of asset management.
The aim of this study is to experimentally investigate the effect of drying on a shear wall, and to clarify the mechanism of the changes in the structural performance due to drying. Two sufficiently hydrated wall specimens are prepared. Then, one is loaded without drying, while the other is tested after sufficient drying until the shrinkage of concrete reaches an equi-librium state. The results show a reduction in the initial stiffness and little change in the ultimate shear strength in the dry specimen, in spite of an increase in the compressive strength. Reproduction numerical analysis using Rigid Body Spring-network Model (RBSM) coupled with a truss network model for moisture transport is conducted, and an ac-ceptable agreement is confirmed in the ultimate strength and the crack patterns. From the numerical results, it is revealed that two factors are balanced in the ultimate shear strength after drying in this experiment: 1) an increase in the com-pressive strength due to aging (material scale), and 2) a strength reduction due to lateral strain, which is evaluated using the formula suggested by Vecchio and Collins (1986) (member scale). This indicates that the wall reinforcement ratio and concrete shrinkage have the influence on the ultimate strength through increasing/decreasing the number of cracks and the crack width.
Darko Tasevski, Miguel Fernández Ruiz, Aurelio Muttoni
This paper investigates the behaviour of concrete failing under high stress levels and subjected to different types of loading. The aim of this investigation is to clarify the development of linear and nonlinear creep strains and how they relate to material damage and eventual failure. This research is supported on the results of a new experimental programme performed on concrete cylinders tested in uniaxial compression under varying strain and stress rates. The results of this programme allow investigating the influence of the loading history on the material response in terms both of its strength and deformation capacity. On this basis, a failure criterion related to the inelastic strain capacity of concrete is defined. Such failure criterion, showing consistent agreement for all types of loading histories, allows calculating in a simple manner the reduction of the strength for a long-term loading situation and also its associated deformation capacity. On that basis, a comprehensive method for predicting failure of concrete under different long-term loading patterns is proposed and validated.