Papers by Keyword: Residual Strength

Abstract: This research investigates the impact of various environments and material conditions on the residual flexural strength of corroded reinforced concrete beams. Parameters such as corrosion rate (R), potential (E_corr), corrosion current density (I_corr), polarization resistance (R_P), curing age (t), and acid concentration (C_a) from 0.0 M to 0.3 M were used to evaluate the residual strength of the beams. The combination of Python software version 3.11 and IBM SPSS version 25 was used to develop, train, and analyze the necessary variables for the model. The regression model was developed using 64 data points and 15 predictors, resulting in 48 degrees of freedom for the residuals. The Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) values are 897.9 and 932.5, respectively, functioning as metrics for assessing this model's quality of fit in comparison to others. A decreased AIC or BIC indicates a more optimal model fit. The Log-Likelihood value of -432.97 objectively evaluates this model's fit, with higher values indicating a better match. The regression results were further analyzed using polynomial terms to demonstrate the influence of the factors on the residual strength of corroded reinforced concrete beams. The findings indicate that the model sufficiently aligns with the data, as shown by a high R-squared value of 0.946, which indicates that 94.6% of the variability in residual strength is due to the independent variables. The adjusted R- squared score of 0.920 substantiates the model's robustness after accounting for the number of predictors. The investigation revealed that these factors significantly influence the residual strength of the reinforced concrete beams as established by the formulated model.

Abstract: There are concerns towards the vulnerability of geopolymer concrete towards fire. High-temperature conditions instigate physical alterations and chemical reactions in concrete, which progressively breaks down the gel structure of cement. Consequently, the breakdown leads to an increase in tendency of drying shrinkage, changes to colors of aggregates and losses in load-bearing capacity and durability. In the present study, geopolymer concrete samples were exposed to fire at 1000°C at varying heating duration to investigate the effects on mass loss, residual strength and its microstructure properties. Samples with three grades of strength, GEO20, GEO40 and GEO60, were prepared. Six heating durations ranging from 30 to 180 minutes were adopted. Overall, mass losses were less than 3%, ranging from 1.65% as obtained by the low-strength concrete to 2.93% as obtained by the high-strength concrete. For the most part, as heating duration increased, residual strengths decreased, except for when residual strengths of low and medium-strength concrete initially increased at the heating duration of 30–60 minutes, where the exposure to fire facilitated geopolymerization. Analysis of the microstructure reveals that structural integrity of the matrix at high-temperature conditions is adequate. The study investigated the geopolymer concrete is able to resist the exposure to fire and must be seriously considered as an alternative to ordinary-Portland-cement-based concrete for the future of sustainable construction.

Abstract: One of the most important processes of physical deterioration in concrete is the exposure to high temperatures that influences their durability and stability during life service. In addition, if self-compacting concrete is formed by lightweight aggregates, such as EPS, it is necessary to analyze its behaviour after it has been exposed to high temperature conditions. This research shows research that evaluates the effect EPS on the mechanical properties of self-compacting concrete at high temperatures. This study evaluated physical-mechanical properties, including residual compressive strength and thermal conductivity after exposure at 22oC, 150oC, 350oC, 500oC and ISO834. Experimental results showed that the loss of residual compressive strength of the specimens up to 350oC is almost insignificance, but it will be reduced by 49% and 70% when temperatures increase up to 500oC and 700oC respectively. EPS contributes to lightness, thermal insulation, and commitment to the environment in lightweight SCC.

Abstract: Composite repair systems of buried pipeline will be affected by moisture and other factors due to anti-corrosion and construction problems. These environmental factors will reduce the service life of the composite system. In this paper, the performance of composite and interface between composite and steel under the action of water were studied. It was found that the formation of micro-cracks on the surface of composite materials and the hydrolysis of epoxy resin were the important reasons for the Performance degradation. Moreover, the aging properties of composite materials and their interfaces under water immersion were analyzed by residual strength theory, and the life prediction equation of composite materials and interfaces were obtained, which can be useful to the field application of composite repair systems.

Abstract: In the present work, a model describing the fatigue behaviour of composite materials under spectrum loading is presented. The approach is based on a two-parameter model describing the strength degradation kinetics for fibre-reinforced materials subjected to cyclic loadings, both with constant and variable amplitudes. The point of strength of the presented approach is that it allows to consider cyclic loadings as they are, without simplifications or reductions. In particular, the analytical background of the model is based on the statements that govern the composites behaviour and focused on the issues related to this class of materials when subjected to fatigue. Then, the final step is the definition of a damage accumulation rule that goes over Miner’s rule unreliability when applied to fibre-reinforced materials. As a matter of fact, it allows to take into account the effects of different loading histories, underlining the importance of the sequence of loads application in the framework of the presented approach.

Abstract: Corrosion is a common problem in offshore oil and gas wells, especially in thermal recovery wells, which will greatly affect the residual strength of casing strings. Field data show that the corrosion pattern of casing is mainly pitting, local corrosion and uniform corrosion, of which pitting is the most common. For this reason, based on the engineering practice of thermal recovery well casing corrosion, the dynamic changes of wellbore temperature and pressure during the gas production process of thermal recovery wells are considered, and the numerical model of casing corrosion under transient temperature and pressure coupling is established. The influence of corrosion morphology and corrosion depth on the residual strength of the casing is analyzed. The transient stress variation law of the corroded casing is studied. The results show that: (1) Under the same corrosion depth, both pitting and uniform corrosion will reduce the residual internal pressure and external extrusion strength of the casing string, and the degree of influence is similar. The effect of uniform corrosion on the residual tensile strength of the casing string is larger than pitting and local corrosion; (2) the residual strength of the casing decreases linearly with the increase of the corrosion depth of the casing; (3) the stress of the corrosion part of the casing shows a significant dynamic change, in the initial stage of steam injection, casing stress rises rapidly at the corrosion position, and then the rate at which the casing stress rises is slowed, which tends to cause fatigue damage to the casing. The research results have reference significance for the assessment of the service status of offshore thermal recovery well casings.

Abstract: The fire behavior of concrete filled hollow steel sections has been studied extensively in various countries. Almost all essential parameters influencing their resistance have been identified: section shape and dimensions, concrete filling, reinforcement ratio, steel tube thickness, column slenderness, thermal and mechanical properties of steel and concrete, and even the contact problem at the steel-concrete interface. Most of these works were done under standard fire conditions (ISO), which are represented by a continuously increasing temperature over time. It is thus not really a curve reflecting a natural fire which includes not only a heating phase but also a cooling phase during which the temperature of the fire is decreasing back to ambient temperature.In this paper, the behavior of axially loaded concrete filled square hollow section columns subjected to natural fire conditions has been studied. The main objectives of this study are: first, to demonstrate the phenomenon of delayed collapse of this type of columns during or after the cooling phase of a fire, and then study the influence of certain determinant parameters, such as section size, tube thickness, reinforcement ratio, concrete cover and column length.The results show that delayed failures occur for massive sections, small values of the thickness of the steel tube and for the low-slendernes.

Abstract: Sandwich composite materials are widely used in various applications, due to their advanced flexibility in responding to special design requirements. This paper presents the evaluation of thick sandwiches, aimed to be imbedded in platforms of a green energy unit, accommodating storage water tanks. The evaluation of the damage tolerance was made having in view previous studies on similar materials and covered assessment of results obtained during low velocity impact tests and post-impact tests, aimed to establish the residual mechanical performance. Ways to increase the damage tolerance, by diminishing the invasive effect of low velocity impact, were also explored.

Abstract: An efficient implementation of lightweight design is the use of continuous carbon fiber reinforced plastics (CFRP) due to their outstanding specific mechanical properties. Embedded metal elements, so-called inserts, can be used to join metal-based attachments to structural CFRP parts in the context of multi-material design. They differ from other mechanical fasteners and have distinctive benefits. In particular, drilling of the components to be joined can be avoided and, depending on the preforming, fiber continuity can be maintained using such elements. Thus, no local bearing stress is anticipated. Previous work published by the authors [1] dealt with a systematic research of the influence of different types of stresses on the load bearing capacity of welded inserts. This contribution aims at the investigation of the performance of shape-optimized inserts under the same types of loading to compare with the results of the welded inserts serving as a reference. For that purpose, the respective load bearing capacities were evaluated after preinduced damages from impact tests and thermal cycling. In addition, dynamic high-speed tensile tests (pull-out) were conducted under different loading velocities. It is shown that the load bearing capacities increased up to 19% for high velocities (250 mm/s) in comparison to quasi-static loading conditions (1.5 mm/min) showing an obvious strain rate dependency of the CFRP. Quasi-static residual strength measurements under tensile loading identified the influence of the respective preinduced damages of the insert. Influence of the thermal loading condition was evaluated by placing the specimens in a climate chamber and exposing it to various numbers of temperature cycles from-40 °C to +80 °C with a duration time of 1.5 hours each. Here, it turned out that already 10 temperature cycles decreased the quasi-static load bearing capacity up to 31%. According to DIN EN 6038 the specimens were loaded with different impact energies and the residual strength were measured carrying out pull-out tests. It could be shown that the damage tolerance is significantly lower for the shape-optimized insert due to failure-critical delamination. The optimized insert also endured lower impact energies and the influence on the performance was higher.

Abstract: This paper is focused on the research of material characteristics of high performance concrete reinforced with a combination of steel and hybrid fibers exposed to the extreme temperatures. In the performed experiments it was examined several types of mixtures (HPFRC, UHPFRC) exposed to the extreme temperatures up to 200-1200 °C. Outside residual parameters of each examined mixtures (tensile bending strength, compression strength, fracture parameters) was investigated the dependence of porosity of the matrix, sample damage and chemical analysis of samples exposed to extreme temperatures, to the resulting mechanical parameters. Part of the initial results of the research described base material and physical properties of the examined mixes and shows the effect of high temperatures on these properties. The results presented in the current paper are the basis for further research and the preparation of numerical models for the design of HPC exposed to extreme temperatures.