Concrete Subject Research Articles

Effect of Fibre Reinforced Polymer Wrapping on Mechanical Properties of Concrete Subjected to Fire

Effect of Fibre Reinforced Polymer Wrapping on Mechanical Properties of Concrete Subjected to Fire

Resistance Analysis of Crack Propagation in Concrete Subjected to Hydraulic Pressure.

The KR resistance curve for hydraulic crack propagation in a concrete beam was determined and discussed. A semi-analytical method was introduced to calculate the hydraulic crack propagation in concrete. A series of concrete beams with various hydraulic pressures and initial crack depths were tested, and the hydraulic crack propagation in these beams was calculated. The calculated P-CMOD curves were first verified, and then the calculated KR resistance curve for hydraulic crack propagation was determined. Based on the test results and calculation results, the following conclusions can be drawn: The proposed analysis method can accurately predict the hydraulic crack propagation process in concrete. The KR resistance to hydraulic crack propagation in concrete decreases with the increase in hydraulic pressure but is less influenced by the initial crack depth of the test beams. In addition, the concrete beams collapse immediately under hydraulic fracturing once the KIw curve reaches the KR resistance curve. This indicates that the failure of concrete structures under hydraulic fracturing occurs immediately once the driving force of crack propagation, dominated by the hydraulic pressure in the crack, becomes significant.

PERFORMANCE OF BAMBOO- AND GLASS-FIBRE-REINFORCED CONCRETE SLAB SUBJECTED TO STATIC AND CYCLIC LOADS

In the construction field, mild steel is using as reinforcement in concrete, which can easily corrode and destroy the environment. So, the construction industries are focusing on alternate materials like natural and synthetic fibres for sustainability. In this study bamboo, glass-fibre bars were used as reinforcement and compared with a normal reinforced concrete slab subject to static and cyclic incremental loading conditions. According to this, the experimental studies were carried out on 9 slab elements of size 600 mm × 450 mm with different forms of reinforcement. The slab elements’ deflection, deformation, load-carrying capacity, stress, strain, crack patterns, stiffness and energy absorption were determined and compared. The ultimate load-carrying capacity of elements like conventional, GFRP and bamboo are 28.90 kN, 29.31 kN and 5.50 kN respectively; similarly, deflection 12.70 mm, 13.50 mm and 8.60 mm were found under static load. In the same way, the ultimate load-carrying capacities of the elements are 24.23 kN, 25.00 kN and 6.80 kN under cyclic loading conditions; deflections of 10.10 mm, 13.50 mm and 3.60 mm, respectively, on conventional, GFRP and bamboo elements. From the test results the stress, strain, energy absorptions were calculated and compared and the energy absorption of bamboo was 9.38% less than conventional and GFRP was 22.49 % higher than conventional elements. The study proved that GFRP material has a significant impact than conventional and bamboo elements and the bamboo reinforcement can be used in light-loading conditions.

Evaluation of the physical characteristics of reinforced concrete subject to corrosion using a poro-elastic acoustic model inversion technique applied to ultrasonic measurements

The use of reinforced concrete is foundational to modern infrastructure. Acknowledging this, it is imperative that health monitoring techniques be in place to study corrosion within these structures. By using a non-destructive method for detecting the early formation of cracks within reinforced concrete, the method presented in this paper seeks to improve upon traditional techniques of monitoring corrosion, within reinforced concrete structures. In this paper, the authors present a method to evaluate the physical characteristics of reinforced concrete subject to corrosion using a poro-elastic acoustic model inversion technique applied to a set of ultrasonic measurements, which constitutes a novel approach to the problem of observing the impact of corroding rebars and resulting concrete damage. A non-contact ultrasonic transducer is operated at a carrier frequency of 500 [kHz], with a layer of saltwater separating the sensor from the concrete surface. Following this non-contact measurement collection of the surface and rebar echo responses, a poro-elastic model is used to model the sound propagation, through an adapted version of the Biot-Stoll model. At first, a set of default parameters, obtained from the physical characteristics of the reinforced concrete, are used to match experimental and simulated acoustic signature of the sample. Performing statistical averaging along the corroding rebar within three samples over a period of nearly nine months, a small but monotonous increase in the distance between the concrete surface and the top of the rebar, indicating gradual corrosion of the rebar. Next, a non-linear optimization algorithm is used to optimize the match between measured and simulated echoes. Through the implementation of this model parameter optimization, the root mean square error between measured and simulated responses was reduced by 63.7% for the full signal, and 62.6% for the rebar echo.

Modeling carbonation depth of recycled aggregate concrete containing chlorinated salts

The durability of recycled concrete is susceptible to degradation from both chloride salt attack and carbonation when exposed in service environment. The depth of carbonation in recycled concrete subject to chloride salt attack was studied here. This investigation assessed the effects of mineral admixture, replacement rate of recycled coarse aggregate, and quality of recycled coarse aggregate on the carbonation depth. The carbonation depth Recycled concrete was studied through accelerated simulated carbonation in the laboratory for 7d, 14d and 28d. Based on the results, the correlation analysis was conducted to investigate the relationship between depth of carbonation, mineral admixture, recycled coarse aggregate replacement rate, recycled coarse aggregate quality and the presence of chloride ions. The objective of this study was mainly to investigate the effect of internal chloride ions on the depth of carbonation of recycled concrete subjected to chloride salt attack. The study referred to previous academic research on carbonation in recycled concrete that had not been affected by chloride salts, and conducted a comparative analysis to identify the carbonation depth pattern in recycled concrete influenced by chloride salt exposure. The study discovered a negative correlation between the replacement rate of mineral admixture and recycled coarse aggregate and the carbonation resistance of concrete. Moreover, enhancing the quality of recycled coarse aggregate positively impacted the carbonation resistance of concrete. An improved model was developed that considers the effect of internal chloride salts on the carbonation depth of recycled concrete, in reference to the empirical prediction model presently available. The model demonstrated a high level of agreement with experimental outcomes, displaying an error range of between −4.32% and 8.27%. It enables an in-depth comprehension of the carbonation behavior of recycled aggregate concrete in an environment exposed to chloride salt attack. Based on the model, the service life of recycled concrete structures can be more accurately forecasted, which can provide a theoretical direction for enhancing the durability of engineering structure.

Behavior and design of functionally graded concrete subject to triple impacts of the shaped charge, projectile penetration and explosion

The present issue is how to improve the concrete structure to resist multiple impacts of the tandem warhead. This study developed an approach to design a functionally graded concrete (FGC) structure with fiber-graded (FG) and centralized high-strength aggregates (CHSA) to address this issue. The enhancement of the FG and CHSA in the concrete structure was verified by static mechanical performance tests and dynamic compressive simulation. The behavior of the FGC structure subject to the tandem warhead was investigated by the experimental and numerical dynamic destructive tests, including three stages: shaped charge, projectile penetration and explosion. There are three types of targets, including ultra-high-performance concrete (UHPC), ceramic aggregate reinforced (CAR) FGC and steel aggregate reinforced (SAR) FGC. The results showed that the SAR-FGC had the best multiple-impact resistance in concrete structures. The blocking effect to impact waves was improved by increasing the wave impedance of the anti-penetration layer. Energy absorption was increased by 7 times through the combined action of functional structures to ameliorate damage distribution. The triple destruction depth of it was reduced by 36% compared to UHPC. According to the Forrestal empirical equation, a penetration depth model was proposed for the FGC with jet damage.

Effectiveness of project-based learning in a reinforced concrete course of civil engineering

The application of Project-Based Learning (PBL) in a Reinforced Concrete course through the development of a real-life civil engineering project from its conception, as a coursework, is a challenged experience for students. As long as the coursework evolves, students develop real ways to think as an engineer and they work in an environment similar to that of daily engineering practice.The effect of the PBL implementation on learning and acquisition of knowledge and skills in a Reinforced Concrete subject from the [University] in [Country] has been analyzed through some evidences such as the partial and final grades, the results of a survey and the perceptions drawn from students’ interviews. As observed, thanks to the application of the PBL strategy, learning and motivation of students has increased, in addition to the final summative assessment, without a significant increase on the workload. This methodology improves not only the acquisition of knowledge and engineering skills, but also transversal skills such as entrepreneurship, leadership, communication, time management, teamwork and other social abilities directly related to those required by the current market for their future professional life.

Effect of NaCl solution wet-dry cycles on pore structure and oxygen diffusivity of axial tensile fatigue damaged concrete

Oxygen diffusion plays an important role in the corrosion process of reinforced concrete structures exposed to marine environment. The diffusion properties of concrete subject to fatigue loads and the performance recovery after periodic drying-wetting cycles have not been studied. In this study, the pore structures and the oxygen diffusivity of concrete with different damage degrees before and after the NaCl solution drying-wetting cycles were measured by nuclear magnetic resonance (NMR). After 28 days of drying-wetting cycle, the concrete porosity decreased and the pore structures became finer. After the drying-wetting cycle, the increase in the fractal dimension of pores between 100 and 1000 nm was the most obvious. After the drying-wetting cycles, the oxygen diffusivity of damaged concrete decreased by 3.3–7.7 times, while the diffusivity of undamaged concrete decreased by nearly 10 times. This reduction effect on the diffusivity was generally smaller as the concrete damage degree increased. The result showed that the oxygen diffusivity was most relevant to the fractal dimension of pores between 100 and 1000 nm.

Multidimensional Transport Experiment and Simulation of Chloride Ions in Concrete Subject to Simulated Dry and Wet Cycles in a Marine Environment.

Chloride ion erosion is an important factor affecting the durability of marine engineering concrete. In particular, concrete structures in wave splash and tidal zones are subjected to dry and wet cycles and multidimensional diffusion of chloride ions. To investigate the intricate diffusion of chloride ions within concrete under these dynamic conditions, we devised a comprehensive experiment. This experiment encompasses multiple dimensions, involving dry and wet cycles, as well as static immersion. The experiment intends to reveal how chloride ions are distributed in the concrete and clarify the changes that occur in its microstructure. Based on Fick's second law, the multidimensional diffusion model of chloride ions in concrete under the dry and wet cycles and static immersion was established by comprehensively considering the effects of chloride ion exposure time, environment temperature, relative humidity, and the action of dry and wet cycles. The results show that, under the same conditions, the chloride content in concrete decreases with the increase in penetration depth but increases with the increase in the chloride diffusion dimension and exposure time. Dry and wet cycles and multidimensional diffusion of chloride ions increase the development of cracks and pores in the concrete structure and generate large quantities of C3A·CaCl2·10H2O, which will exacerbate the chloride ion transport rate and penetration depth of concrete. Under the same exposure time and penetration depth, the chloride ion content in concrete under two-dimensional (2D) and three-dimensional (3D) diffusion under dry and wet cycles was 1.09~4.08 times higher than that under one-dimensional (1D) diffusion. The correlation coefficients between the simulation results of the multidimensional transport model of chloride ions in concrete under multi-factor coupling and the experimental results were all greater than 0.95, and the model can be utilized to predict the distribution of chloride ion concentration in concrete.

Reinforcement mechanism of nano-SiO2 and polypropylene fibers on concrete abrasion resistance subject to dynamic sandy water

Abrasion resistance of concrete is essential for the safety assessment of hydraulic structures subject to sandy water scouring. In this study, ten types of concrete specimens were designed to investigate the effect of nano-SiO2 (NS) and polypropylene fibers (PF) on the abrasion resistance and mechanical properties of concrete. In view of the discreteness of the results, grey theory, Support vector machine (SVR) and artificial neural network (ANN) model were selected to explore the relationship between abrasion resistance and factors involving NS, PF, compressive strength, splitting tensile strength and curing age. The results revealed that the appropriate addition of NS and PF promote the hydration process and remarkably enhanced the performance of concrete. And the grey relation analysis indicated that the optimal content of NS and PF were 3 % and 0.9 kg/m3, respectively. Compared with GM (1, N) and SVR, ANN model benefited from more comprehensive consideration of the effects of various factors to predict abrasion resistance more reliably. The results of grey relation analysis and ANN model showed that there was a close correlation between the mechanical properties, admixtures and the abrasion resistance.

Seismic design and experimental testing of precast tolerance concrete shear wall subject to cyclic load

A precast tolerance concrete shear wall (PTCW) characterized by proper-length steel connectors is proposed to adjust the tolerance and improve the joint failure model of the precast wall. The design method for this wall is also presented in this study and verified by test results. Four full-scale walls, including two cast-in-situ walls (RCWs) and two precast walls (PCWs) with axial compression ratios of 0.2 and 0.3, respectively, were tested under lateral loading. The seismic behavior of the panels, including the failure modes, hysteretic behavior, strength envelope curves, stiffness degradation, energy dissipation, and steel strains, was analyzed and discussed herein. The test results showed that all four walls were in a flexural-shear failure mode, and the PCWs showed better performance in terms of hysteretic behavior, strength envelope, stiffness, and energy dissipation, particularly under an axial compression ratio of 0.3. Furthermore, the peak load of PCW1 (axial compression ratio is 0.2) and PCW2 (axial compression ratio is 0.3) increased by 5.93% and 15.51%, and the yield load increased by 12.1% and 11.39% compared with those of RCW1 (axial compression ratio of 0.2) and RCW2 (axial compression ratio of 0.3), respectively. The equivalent viscous damping coefficients of all four panels were higher than 0.05. The lateral bearing capacity of the walls increased with an increase in the axial compression ratio. Moreover, no visible damage or slippage was observed when the PCWs were demolished after testing. The main nonlinear strain was concentrated on the vertical bar and not on the connectors, indicating that the plastic hinge position was transferred from the joint interface to the wall panel. The results indicated that the use of steel connectors to strengthen the joints can achieve the seismic design concept of strong joints and weak members.

A Micromechanical-Based Semi-Empirical Model for Predicting the Compressive Strength Degradation of Concrete under External Sulfate Attack.

As one of the most harmful ions in the environment, sulfate could cause the deformation and material deterioration of concrete structures. Models that accurately describe the whole chemo-transport-mechanical process of an external sulfate attack (ESA) require substantial computational work and contain complex parameters. This paper proposes a semi-empirical model based on micromechanical theory for predicting the compressive strength degradation of concrete under an ESA with basic properties of the undamaged material and limited computational effort. A simplified exponential function is developed for the total amount of the invading sulfate, and a second-order equation governs the chemical reaction. A micromechanical model is implemented to solve the mechanical response caused by an ESA. The model is able to describe the compressive stress-strain behavior of concrete subject to uniaxial loading in good agreement with the experimental results. For the case of a sulfate-attacked material, the relationship between compressive strength and expansion is calculated and validated by the test results. Finally, the deterioration process of compressive strength is predicted with the test results of deformation.

МОДЕЛЪТ SEEM – СЪЩНОСТ И ПРИЛОЖЕНИЕ В СРЕДНОТО УЧИЛИЩЕ

This article shares both the passion for novel educational models and quality teaching. The aim is twofold – to present the Sustainable Educational & Emotional Model (SEEM) as a universal frame to manage the modern educational challenges in post-COVID-19 times. Further, an example is given at the secondary educational level with a focus on mathematics. SEEM – the essence: SEEM is a result of more than 20 years of experience and research in higher education. SEEM is an integrative model that respects the fundamental principles of positive education, utilizing the potential of technologies. SEEM builds on the specific context (culture and policy), content (the concrete subject or topic), methodologies, or just instruments as well as the actors (educators and students). The model is based on the three SEEM pillars or principles: 1) interactive teaching sustained by the values of freedom, holism and respect; 2) technology application – the pandemic crisis has strengthened this pillar; 3) positive emotions (jokes and humor are promoted). SEEM – secondary school application: SEEM has been extensively applied in higher education. Here, the model is used in a secondary educational context allowing proving its versatility and simplicity. The SEEM model is introduced to summarise and synthesize knowledge on the topic of Similarities. The SEEM pedagogical framework was combined with a diagnostic model for the key competencies assessment. Conclusion: SEEM enables us to rethink the classical teaching and assessment approaches in secondary education. An illustration from the realm of mathematics exemplifies this fact. SEEM aspires to establish a comprehensive model, operating on the principle that optimal learning outcomes can be achieved if students find enjoyment in the educational process.

Imaging concrete cracks using Nonlinear Coda Wave Interferometry (INCWI)

Crack imaging in highly heterogeneous materials, such as concrete, plays a great role in structural health monitoring and non-destructive evaluation and testing. Numerous works have shown that Nonlinear Coda Wave Interferometry is a robust tool to assess the overall level of nonlinearity caused by micro or macrocracks in heterogeneous media. This work presents a novel qualitative method for Imaging using NCWI (INCWI) based on the combination of Nonlinear Coda Wave Interferometry (NCWI) and a spatial averaging method. INCWI is used herein to locate cracks generated in a concrete beam subject to three-point bending. Measurements are performed after each loading level, in both pre-peak and post-peak phases, once the beam has been unloaded. The imaging algorithm is described and applied to each stage of beam damage. In the post-peak phase, crack size varies from 10cm to 15cm in length and from 25μm to 200μm in width. Also, the INCWI results are compared with acoustic emission measurements and microscopic observations.

Systematic studies on behaviors of ultra-high performance concrete subject to freezing and thawing cycles combining DIC technology

Freezing and thawing alternation is one harsh weather condition for the concrete structures serving in seasonal frozen areas. As an emerging construction material, the frost-resistance of ultra-high performance concrete (UHPC) is highly necessary to be understood for propelling its practical application. In this study, the frost-resistance of UHPC was systematically evaluated based on the mechanical properties and the cracking behaviors. The fast freezing and thawing (FT) cycles were chosen as the experiment condition, along with the water-immersion condition for comparison. A combination of destructive mechanical and non-destructive resonance frequency test was used to characterize the behaviors. The results showed that, after undergoing 300 FT cycles, the tensile initial cracking strength, the tensile strength, the flexural strength, and the compressive strength exhibited a certain of degradations. The corresponding effects of the water-immersion action were more positive. The tensile strain capacity under the FT condition almost kept constant, whereas that under the water-immersion condition decreased. The two indices (including strain and strength) could reasonably reflect the magnitude of tensile strain. The digital image correlation (DIC) analysis unveiled that there may exist an additional plastic deformation in the tensile full-field deformation, apart from the elastic deformation and crack opening. Compared with the water-immersion action, the FT action resulted in a less zigzag cracking path, shorter cracking time, and larger crack opening under flexural loading. In addition, the variations of dynamic modulus of elasticity and re-hydration reaction degree under the FT condition were not coincident with those of mechanical properties. The findings could provide some valuable references for estimating the bearing capacity of the UHPC structures served in practical engineering.

Assessment of cracking risk of concrete due to restrained strain based on zero-stress temperature and cracking temperature

Cracking of high performance concrete due to restrained volume change has been a rising concern in the last few decades. The present study investigated this topic from a different point of view based on two novel feature temperatures, namely, the zero-stress temperature (Tz) and the cracking temperature (Tx). In this study, the evolutions of Tz and Tx over time, as well as the gradients of Tz and Tx inside a concrete wall were demonstrated. Based on the modeled Tz results, parts of the concrete wall subject to compression and tension were identified, and the corresponding transition time when compressive stress turned into tensile stress was predicted. According to the determined Tx results, the zones of the concrete wall with high cracking risk were described, and the appearance time of cracking was approximated. For the concrete wall investigated, the influences of the drying shrinkage gradients inside the cross-section of the wall on Tz, Tx, and the cracking risk of concrete were assessed quantitatively. Besides, the influences of the proportional height of the concrete wall above the base on the degree of restraint, Tx, and cracking risk were also studied. In the final analysis, a comprehensive yet practical program for evaluating the cracking risk of concrete induced by restrained strain was built based on the proposed Tz and Tx models. Some research gaps in the determination of Tz and Tx that require further research were also identified and analyzed.

Spatial variability of explosive blast loading and its effect on damage risks to reinforced concrete buildings

Explosive blast loading is subject to inherent variabilities due to complex interactions between chemical reactions, air particles, fireball and shockwave. Explosive field trials have quantified this variability, and described herein is the probabilistic characterisation of spatial variability of airblast for pressure and impulse. In other words, pressure–time measurements from adjacent (close) gauges are not identical, but correlated based on the distance between adjacent gauges. The degree of correlation is moderate if distance between adjacent gauges is small (i.e., angle between gauges is small where each gauge has the same radial standoff from the charge), and reduces to a weak correlation for gauges spaced much further apart. The effect of spatial variability of airblast on damage and collapse risks is investigated for a 10 storey reinforced concrete building subject to a vehicle borne improvised explosive device (VBIED) attack, and the accidental detonation of explosive ordnance (EO). The structural reliability analysis considered load redistribution for failed columns, dead and live loading variabilities, and blast loading and resistance model errors. It was found that allowing for the spatial variability of airblast decreased progressive collapse probabilities by up to 65% when compared to the case of fully correlated airblast. Designing against progressive collapse using either a threat dependent design or the Alternate Path method reduced damage risks by over 90%, and in many cases by over 99%.

Mechanical behavior of multiscale hybrid fiber reinforced recycled aggregate concrete subject to uniaxial compression

This paper presents a multiscale hybrid fiber reinforcing strategy that can remarkably compensate for the adverse effects of recycled coarse aggregate (RA) on the performance of concrete. The hybrid fibers including macro steel fiber (SF), micro polypropylene fiber (PPF), and cellulose nanofiber (CNF) were utilized for crack arresting at multi-levels and improving the mechanical properties of RA concrete (RAC). This work aims to investigate the mechanical performance of multiscale hybrid fiber reinforced RAC (MFRRAC) under uniaxial compression and unveil the multiscale hybrid fiber reinforcement mechanism. A total of 34 groups of prismatic specimens were tested, with an emphasis on the effects of RA replacement rate and fiber parameters on the failure mode, compressive strength, deformation ability, and stress-strain curve. The results showed that the incorporation of multiscale hybrid fibers has a marked impact on the compressive properties of RAC. Specifically, the addition of multiscale hybrid fibers could significantly alter the failure pattern of RAC. In addition, compared with RAC without fibers, the axial compressive strength and peak strain of MFRRAC could be increased by up to 39.34% and 24.85%, respectively. Furthermore, the hybrid fiber reinforcement mechanism on the mechanical enhancement in a physical-chemical manner was revealed, in particular, multiscale crack resistance and hybrid fiber synergetic effect. Finally, a stress-strain model was developed with RA replacement rates and fiber parameters taken into account, and the predictions can correlate well with available experimental results with wide applicability. This research work lays a foundation for the accurate design and analysis of MFRRAC structures.

A visual condition assessment of a reinforced concrete railway bridge subject to alkali silica reaction (ASR) deterioration in Johannesburg

In the 1940’s, a reinforced concrete (RC) railway bridge located in central Johannesburg was constructed using Witwatersrand Quartzite, at the time, unknown to be an alkali-silica reactive aggregate. The bridge currently displays signs of distress in the form of severe map-cracking. This study presents preliminary results of a series of tests that have been conducted to characterise the severity of alkali silica reaction (ASR) deterioration in the bridge. Twelve concrete cores were extracted for testing from various locations of the railway bridge, which displayed varying degrees of distress. A visual assessment was conducted globally on the entire structure as well as locally (using modified damage rating index (DRI) approach) on the extracted cores. The findings showed that there was extensive map-cracking present on all thirteen railway bridge elements examined, revealing signs of distress in the bridge. Even elements that were sheltered from exposure to direct rain, such as the underside of three arch surfaces, also exhibited extensive map-cracking. Furthermore, the modified DRI method was a useful technique to assess and compare the relative extent of damage of concrete taken from various locations of a RC structure, using local images of the cylindrical surface of the concrete cores.Graphical

Synergistic effects of sustained loading and sulfate attack on the damage of UHPC based on lightweight aggregate

This paper presents an experimental study on the damage progress of lightweight ultra-high performance concrete (LUHPC) subject to sulfate attack under flexural loading. Two different loading environments (natural curing and 15% Na2SO4 solution immersion), which contain three different stress levels and both tensile and compressive regions, are applied in this test. The research results indicate that the damage of LUHPC was increased with the increasing stress level and loading age, which was manifested by the increase of sulfate ion content in the surface layer and the decline of the overall mechanical properties of LUHPC. The lightweight aggregate avoids the expansion damage of LUHPC by absorbing the surrounding excess sulfate ions. In the tensile region, loading accelerate the sulfate attack of the LUHPC, while in the compressive region, loading improve resistance of LUHPC against sulfate attack and this effect is more obvious with the increase of stress levels. Compared with the loading group, the mechanical properties of the LUHPC under loading-sulfate attack decreased less, and the microstructure was denser, indicating a significant synergistic effect between sustained loading and sulfate attack.