The aim of this work is to present a strut-and-tie model for design of reinforced concrete pile caps. The model considers both failure by crushing of the compressed struts and by yielding of the tie reinforcement. Unlike some traditional models, crushing of the compressed concrete is not checked at the section in direct contact with the column base (column/pile cap interface). In this work, crushing of concrete is verified in a section at a certain depth inside the pile cap. Thus, this verification is replaced by determining the height of the nodal zone at the top of the pile cap required not to cause crushing of the struts. An iterative algorithm is used for this purpose. Comparison with a large number of experimental results available in the literature demonstrates the effectiveness of the proposed model for the design of concrete pile caps. Numerical examples of practical use of the model are also presented.
In the context of life extension of nuclear reactor buildings while ensuring safety and regulatory requirements, a numerical strategy is proposed to compute and forecast the air leakage rate evolution of inner containments in double-wall reactor buildings under standard long-term operation. In order to reduce the numerical cost of such complex computations, the proposed strategy is based on a coarse but adapted mesh together with a chained weakly-coupled thermo-hydro-mechanical modeling. The total leakage rate is then computed with a specially-designed tridimensional finite element based on a non-linear thermal analogy.
This methodology is used to model the behavior of the VeRCoRs mock-up, a simplified nuclear reactor building at scale 1:3, built and monitored by EDF. Results obtained until the first pre-operational pressurization test have been discussed in a dedicated benchmark organized by EDF. The proposed methodology provides delayed strains and leakage results in good agreement with available experimental data. A blind prolongation until the first decennial test of the mock-up is presented and analyzed.
Freeze-thaw cycle is one of the major damage factors of concrete patch repair. Not only the material itself but also the adhesive interface is damaged under freeze-thaw cycles (FTC). Air-entraining agent has long been used to increase the freeze-thaw resistance of concrete materials. However, the effect of air-entraining agent on the adhesive interface has not been explored. The degradation mechanism and failure mode of concrete repair system under FTC has not been studied, either.
In this study, three kinds of substrate concrete were casted and repaired by two kinds of ordinary Portland cement mortars and one kind of polymer-modified cement mortar (PCM), respectively. With up to 150 FTC, splitting tensile strength and failure modes of composite specimens were experimented. Results showed that air-entraining agent in the repairing mortar greatly influenced adhesive tensile strength under FTC. The water cement ratio and air-entraining agent of substrate concrete insignificantly affected the adhesive interface, but affects failure mode. The adhesive tensile strength of PCM-repaired composite specimens decreased faster than that of ordinary Portland cement mortar-repaired composite specimens although PCM itself showed stronger freeze-thaw resistance than ordinary mortar.
The X-ray CT method was applied to evaluate micro-damage – namely, deterioration of concrete specimens under cyclic uniaxial loading tests. The μ-focus X-ray CT scanner was used. Concrete cylinders with a diameter of 50 mm and a length of 100 mm were prepared as specimens. Cyclic uniaxial loading tests were conducted on the specimens. The X-ray CT images of the specimens were taken at the loading level 0, 60, 70, 80 and 90 percent of their uniaxial compressive strength. For three dimensional CT images of the specimen, the Three Dimensional Medial Axis Analysis (3DMA) method was applied to evaluate porosity and width, length, persistence of cracks which represent deterioration of the specimen. As the results, slight micro-fracturing occurs until a loading level R = 60%, and starts to increase at R of 60% to 70%. The cracking occurs in a part of the mortar near boundary between aggregate and mortar, and the crack links to another crack with increasing loading level. The degree of cracking varies according to the position dependent on the final apparent fracture surface of the specimen. It is concluded that the X-ray CT method with 3DMA is effective for not only evaluating the state of micro-cracking within the specimen, but also estimating the process of deterioration of the specimen with an increasing loading level.
This study set out to investigate the effect of gel migration through micro-pores and cracks on concrete expansion caused by alkali–silica reaction (ASR). First, sensitivity analyses with permeation of produced silica gel were conducted by using a computational scheme of multi-scale poro-mechanics. Then, silica gel movements were found to substantially affect the ASR expansion of concrete with the possibility of its scale effect being rooted in the drugging force when the silica gel is in motion. With accelerated ASR tests of mortar, the predicted scale effects were experimentally verified and the permeation of gel was inversely identified in terms of multi-scale mechanics. Here, the effect of alkali ion leaching on the scale-dependency was also discussed in view of chemo-physics as well as mechanics of produced gel migration. The measured characteristic scale-dependency in the experiment can be also simulated by nonlinear analysis of smeared cracks coupled with gel generation, migration and alkali ion leaching.
Medium volume blast furnace slag concrete has been highly expected to reduce carbon dioxide emission in concrete production, while it was found that this concrete was prone to shrinkage cracking specifically under hot summer climates. To improve this negative performance, this study focused on the utilization of trace additives and water curing. The trace additives were gypsum and calcium carbonate, with which binder is called low shrinkage BFS. Furthermore, to reduce prominent autogenous shrinkage, initial water curing was investigated. As a result, combination of low shrinkage BFS and water curing was found to significantly strengthen shrinkage cracking resistance, which was expressed by cracking age resulted in restrained shrinkage cracking tests. Furthermore, curing experiment showed that water curing may be substituted by water covered curing on element surface in construction site as a simple and economical measure to supply water to concrete at initial stage of hardening.
The main feature of a prestressed concrete containment vessel (hereinafter referred to as “PCCV”) is that unbonded tendons, which are not bonded to the structure, are used because the tension of the tendon needs to be measured in periodical in-service inspection (hereinafter referred to as “ISI”) to ensure that the performance of the power plant is maintained during the service life. This report describes the results of measurement of the tension of the tendon, which has been performed as an activity to maintain the PCCV in Ohi Nuclear Power Plant (hereafter referred to as Ohi “NPP”) Units 3 and 4 of Kansai Electric Power in the past twenty some years since the construction of the PCCV, the changes in the standards and guidelines for the maintenance of the PCCV in Japan, as well as the discussion of non-destructive test-based methods for estimating the strength of high-strength concrete used for the PCCV and the development of a hydraulic shim-type load cell-based system for the measurement of the tension of the tendon, taking into account the ease of non-destructive testing, etc.