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.
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.
For performance evaluation of existing reinforced concrete members under irradiation conditions, a numerical code called “DEVICE” (Damage EValuation for Irradiated ConcretE), which takes into account the heat, moisture, and radiation transport coupled with cement hydration, is proposed. This code is composed of the established computational cement-based material (CCBM) model and the one-dimensional deterministic transport Sn code “ANISN”. In the proposed model, temperature-dependent irradiation-induced expansion of aggregate minerals and resultant strength deterioration of concrete are introduced. Currently, the knowledge and modeling of irradiation-induced expansion of aggregate mineral is limited only for α-quartz. DEVICE was used for evaluating the strength distribution of the decommissioned plant Japan Power Demonstration Reactor (JPDR). Compressive strength distribution in a concrete biological shielding (CBS) wall of the JPDR was obtained by core sampling, and the compressive loading test results were compared with the calculation results. This comparison proved the practicality potential of DEVICE to predict the concrete strength distribution in a CBS. In addition, concrete strength change and its distribution in a CBS of an anonymous two-loop pressurized water reactor was simulated by DEVICE. The contributing factors for the change in the distribution of concrete strength at the inner surface of the CBS are discussed. Furthermore, the ways of integrity evaluation other than the existing allowable fast neutron fluence method are proposed and discussed as follows: 1) mineral composition-based allowable fast neutron fluence; 2) strength prediction at the inner surface based on the expansion of mineral composition of aggregates and the lower limit curve of the ratio of compressive strength of the specimen after irradiation (Fc) to that of the reference specimen (Fco) as a function of concrete expansion; and 3) direct numerical calculation for seismic performance by considering irradiation-induced volume expansion and degradation of concrete.
For estimating remaining fatigue life of RC bridge decks subjected to traveling wheel-type loads, presented is the data assimilation procedure, i.e., coupled life-span simulation with inspection data at site. Multi-scale analysis with hygro-mechanistic models is used for the platform of data assimilation on which the visual inspection of cracking on the members’ surfaces and the acoustic emission (AE) tomography are numerically integrated. For verification, the wheel running load experiments of slabs were conducted with continuous data acquisition of both crack patterns and the acoustic emission data over the life till failure. Visually inspected cracks are converted to space-averaged strains, based on which the internal strains and damage fields are re-produced by numerical predictor-corrector cycles. The 3D field of elastic wave identified by AE tomography is also converted to the fracture parameter of concrete. Although no information on cracking is available, the proposed assimilation method successfully reproduces most probable internal cracks over the volume of analysis domains, and the remaining life of the deck slabs inspected was successfully estimated.
In 2008, Nuclear and Industrial Safety Agency (NISA) (currently integrated to the Nuclear Regulatory Authority) launched a project to develop a soundness assessment method for concrete members subject to a radiation environment. Presently, the soundness of concrete members subject to radiation is evaluated based on whether the predicted fast neutron fluence and gamma-ray dose values are lower than specific reference values in Japan, which are 1×1020 n/cm2 and 2×105 kGy, respectively. These reference values were determined based on report by Hilsdorf et al. This project begins by reviewing Hilsdorf et al.’s report, and we find that the scientific evidence for the current reference values is weak. We thus conclude that new experimental research is required to assess the current reference values and to propose a new alternative soundness assessment procedure if needed. We quantitatively evaluated the influence of neutrons, gamma-rays, and the resultant heating and drying processes on the strength of concrete as well as their underlying mechanisms. The irradiation experiments confirmed the degradation mechanism of concrete due to neutron irradiation. The main reason for this degradation is the metamictization of rock-forming minerals, which, in turn, leads to aggregate expansion. Due to aggregate expansion, cracks around aggregates form, which reduce the compressive strength and Young’s modulus of concrete. Among the rock-forming minerals, α-quartz is the most sensitive to neutron radiation. 60Co gamma-ray irradiation experiments demonstrated that concrete strength increased as the gamma-ray dose and gamma-ray flux does not have a dose-rate impact on the first radiolysis of evaporable water in cement paste within the present study. The effect of gamma-ray irradiation on the properties of concrete is equivalent to that of heating and drying. Concrete strength alteration due to heating and drying is attributed to the colloidal and porous nature of hardened cement paste and crack formation around the aggregate due to a mismatch in the volume changes of the mortar and aggregate. In addition, a numerical analysis code called DEVICE (Damage EValuation for Irradiated ConcretE) is developed to harness knowledge obtained from concrete samples to predict the distribution of the physical properties in concrete members and their changes over time. From these fundamental studies, we propose a new soundness assessment procedure for concrete members subject to radiation. We also recommend a new radiation-induced strength-degradation reference value of 1×1019 n/cm2 for fast neutron.
Di Qiao, Hikaru Nakamura, Yoshihito Yamamoto, Taito Miura
This paper presents an electro-mechanical model for evaluating the corrosion-induced damage in reinforced concrete under accelerated conditions. The model consists of structural analysis of concrete and the rebar using the Rigid Body Spring Method and corrosion current analysis with the truss networks model. The electric corrosion process is coupled with concrete cracking conditions by relating current efficiency to local crack width; the predicted radius losses of rebars are used to evaluate concrete crack propagation and residual tensile performance of corroded rebars. The model is validated with the results of accelerated corrosion tests using impressed current and a sodium chloride pond on the concrete cover. Good agreement with the test data is obtained with the proposed model, in terms of corrosion degree and profile, concrete crack pattern, and tensile behavior of corroded rebars. The model offers a corroborative tool with the accelerated corrosion technique to study the mechanical behavior of corroded concrete structures.
The aim of this study is to clarify the mechanism of the progressive excessive deformation observed in real underground RC box culverts of about 30 years of age. It was found by the site-inspection, monitoring and the destructive testing that the excessive deflection of top slabs for the culverts, which is almost 10 times the design estimated value, accompanies the out-of-plane shear failure. It is also computationally investigated that the coupling of subsidence of the backfill soil and the combined creep and shrinkage of concrete after cracking is closely associated with the delayed shear failure found in the culvert in service. In order to prove the delayed shear failure under higher sustained loads, the time-dependent shear crack propagation was reproduced in the laboratory test and the computational approach used in this study was examined.
Here we investigate the mechanism by which shrinkage reducing admixture (SRA) affects hardened cement paste (hcp). The first desorption process for hcp is always accompanied by irreversible shrinkage. Initially we demonstrate the well-known mechanism of SRA acting on capillary force using Vycor glass. Additionally, sorption isotherms and length-change isotherms are measured for both saturated hcp as well as hcp aged at 11% RH for two years. SRA was concluded to be present on the surface of the concave meniscus of the pore solution in Vycor glass, and that the inclusion of SRA reduces the surface tension of the pore solution, the equilibrium Kelvin radius, and the shrinkage due to capillary force. However, a comparison of long-term and short-term length-change isotherms and water vapor sorption isotherms of hcp suggests the possibility of partial evaporation of SRA molecules during the 2-year drying process, as well as the presence of immobile SRA in cement paste. Moreover, the immobile SRA is still active, and is found to reduce the amount of water sorption and shrinkage strain. It is thought that the secondary role of the SRA, which is related to immobile SRA in the cement paste, becomes active at room temperature, at below 80％ RH, and only occurs in the irreversible shrinkage component of hcp produced by the initial desorption process.