Performance Of Concrete Structures Research Articles

Evaluation of post-heating flexural behavior of steel fiber-reinforced high-strength concrete beams reinforced with FRP bars: Experimental and analytical results

Despite many advantages provided by fiber-reinforced polymer (FRP) bars, their low resistance against fire has been an important barrier for their widespread application in reinforced concrete (RC) members. Studying the post-fire performance of concrete structures reinforced with FRP bars can be beneficial in understanding the structural safety and estimating the serviceability conditions after a fire incident. Since little information is available in the literature in this regard, especially for high-strength concrete (HSC) and fiber-reinforced concrete (FRC), this research attempted to investigate the flexural behavior of HSC beams reinforced with glass fiber-reinforced polymer (GFRP) bar and steel fibers after exposure to elevated temperatures. The variables under consideration consisted of the applied temperature (20, 250, 400, and 600 °C), volume ratio of steel fibers (0 and 1%), and reinforcement ratio (0.314 and 0.872%). Following the quasi-static four-point flexural test on the heated and non-heated beam specimens, various parameters involving the load-carrying capacity, load-deflection relationship, cracking pattern in terms of the number and width of cracks at service and ultimate loads, and ductility of the beams were examined. It was found that exposing the beams to 250 °C reduced their load-carrying capacity negligibly and exposing them to 400 °C increased their residual load-carrying capacity notably while exposing them to 600 °C led to severe degradation in their flexural capacity. Further, the effectiveness of steel fibers on the behavior of beams with the lower reinforcement ratio was not significant; however, as the reinforcement ratio of the beams and applied temperature increased, steel fibers properly demonstrated their ability in improving the load-carrying capacity and reducing deflection at service load. By evaluating prediction relationships provided by codes and other researchers against the experimental results observed in this research, ACI 440.1R-06 and ACI 440.1R-15 codes were found to be able to predict deflection under service conditions with proper accuracy. Ultimately, for capturing the load-deflection relationship of heated and non-heated beam specimens, an analytical model was developed using the sectional analysis method, which was able to predict the experimental results appropriately.

Analysis of Local Damages Effect on Mechanical Responses of Underwater Shield Tunnel via Field Testing and Numerical Simulation

The investigation of concrete structural performance is crucial to maintain the stability of infrastructure. In order to assess structural stability, this work focuses on the development of an integrated framework to detect damaged conditions in the field and analyze their effect on mechanical performance through nondestructive testing (NDT) technology and numerical models. First, a ground penetrating radar (GPR) and an infrared camera work collaboratively to identify the damaged positions of the concrete structure, with parameters calibrated by laboratory experiments. Then, a finite element model is established to study structural mechanical performance based on field conditions and detected results. In addition, the influenced regions induced by local damage are studied under different boundary conditions. As a case study, the devised method was employed in the Nanjing Yangtze River tunnel for stability assessment and disaster prevention. The detected results of the damaged conditions agree well with the actual conditions in the field. Numerical results show that the circumferential stress component is more significant than that observed longitudinally. The effect of local damage on stress implies a positive correlation with the rise of water pressure, in which the maximum stress response to the variation of water level is 45KPa per meter.

Experimental research on seismic performance of precast cogging high-strength bolt composite joint and influence of its arrangement location

The connection of precast joints is an important factor that affects the seismic and endurance performance of precast concrete structures. Currently, the seismic performance of existing precast joints cannot satisfy the application requirements of complex and harsh environments. This paper proposes a cogging high-strength bolt composite joint (C-HSB joint), and introduces its main components and assembly methods. Two precast beam-column joint (B-C joint) specimens are designed and manufactured using C-HSB joints. The C-HSB joints are arranged outside (SJ2) and inside (SJ3) the plastic hinge area of the beam. For comparison, a B-C joint specimen using a traditional cogging joint (T-C joint) is also designed and manufactured. The T-C joint is arranged outside of the plastic hinge area of the beam (SJ1). Then, quasi-static tests of these three specimens are performed. The seismic performance of the C-HSB and T-C joints are systematically analysed through the following parameters: damage phenomenon, bearing capacity, displacement ductility, rigidity deterioration, energy dissipation, operating state of the cogging seam, and bond-anchorage performance of rebar. Moreover, the influence of the arrangement location and HSB settlement of the C-HSB joint is discussed in detail. The results show that cogging can limit the development of seam cracks and improve the shear capacity of the joint. The HSB can delay the seam cracking, improve the bond-anchorage performance of rebar, and increase the crack resistance and self-resetting capacity of joints. The C-HSB joint can delay the development of seam cracks. The longitudinal rebar of the specimen can utilise more effectively using a C-HSB joint. The location of the C-HSB joint significantly affects the failure mode of the specimen. The internal arrangement (inside the plastic hinge area) of the C-HSB joint will induce the upward motion of the beam plastic hinge and exacerbate the core area damage. The external arrangement (outside the plastic hinge area) will increase the bearing capacity and energy dissipation of the specimens, and the rigidity deterioration will also be slowed. Compared with the C-HSB joint, the T-C joint has a lower displacement ductility, lower energy dissipation, and more rapid rigidity deterioration. The seismic performance of the T-C joint is significantly inferior. The C-HSB joint can be applied to precast concrete structures under complex and harsh environments. The external arrangement (close to the upper boundary of the plastic hinge area) of the C-HSB joint is a better choice.

Viscoelasticity: A new perspective on correlation between concrete creep and damping

Concrete creep and damping are related to reinforced concrete structural static and dynamic performance respectively. Comprehensive investigation of the fundamental behavior of concrete under dynamic and static loading is quite significant to material science of concrete. However, over more than a hundred years since concrete creep was found, it was seldom studied jointly with concrete damping. This paper analyzed the correlation between concrete damping and creep from the perspective of viscoelasticity. Firstly, concrete creep and damping tests under different loading rates were conducted to prove that the damping and creep characteristics of concrete are related to viscoelasticity of concrete. Secondly, in terms of theoretical calculation, a new conversion formula of concrete creep and damping was proposed based on viscoelastic theory. Comparison with experimental results shows that the proposed conversion formula is applicable to concrete.

Performance of structural concrete using Waste-to-Energy (WTE) combined ash

In the U.S., about 27 million metric tons of municipal solid waste are used as fuel in Waste-to-Energy (WTE) power plants, generating about seven million tons of mixed bottom ash and fly ash (combined ash) annually, which are disposed of in landfills after metal separation. This study assessed the effect of using combined ash as a substitute of mined stone aggregates on the mechanical properties and leachability of cement mortar and concrete. The as-received combined ash was separated into three fractions: fine (<2 mm), medium (2–9.5 mm), and coarse (9.5–25 mm). The substitution of up to 100% of stone aggregate by the coarse and medium fractions of combined ash produced concrete with compressive strength exceeding 28 MPa after 28 days of curing. Similar results were obtained when the fine combined ash was used as a sand substitute, at 10 wt%, in mortar. The concrete specimens were subjected to several days of curing and mechanical testing. The results were comparable to the properties of commercial concrete products. The mechanical test results were supplemented by XRD and SEM analysis, and leachability tests by EPA Method 1313 showed that the optimal concrete products effectively immobilized the heavy metals in the combined ash.

Framework to optimise two-dimensional DIC measurements at different orders of accuracy for concrete structures

Despite the widespread application of digital image correlation (DIC) method in concrete structures, there is no standard procedure to systematically optimising the parameters of DIC. A framework is thus proposed in this paper to optimise two-dimensional (2D) measurements with DIC at different orders of accuracy required in concrete structure models. An accuracy analysis method acting as the core of the framework is introduced and illustrated through specific case studies on reinforcement corrosion, concrete crack and seismic performance of concrete structures, as well as an example of a specific subset size. The parameters presented in the case studies and the example can act as a sound reference for selections of parameters in using DIC for concrete structures. The framework can be used as a guideline for structural engineering researchers who use DIC to measure displacement and strain at different orders of accuracy required for concrete structures.

Impact of CO2 curing on the microhardness and strength of 0.35 w/c cement paste: Comparative study of internal/surface layers

The aim of this paper is to investigate the effect of carbonation on the microhardness and mechanical-related properties of carbonated and uncarbonated layers of cement pastes. The microhardness and mechanical property changes were examined 1 day after demolding and then 3, 7, 14, and 28 days with further CO2 and water curing. CO2 treatment yielded substantial impacts on the microstructure and densification of the surface layer of the paste by consuming calcium hydroxide and forming calcium carbonates. The maximum carbonation depth of 5 mm was recorded at 28 days, and the microhardness of the surface layer was significantly enhanced compared to the internal layer as well as uncarbonated paste samples. However, despite improved surface hardness, the overall compressive strength of carbonated sample was lower than that of water-cured samples, probably due to the non-preconditioning (to remove excess water for carbonation) of the carbonated samples and better hydration in the internal layer of the samples cured in water. In addition, the thin denser surface layer of the carbonated samples could decrease the cement hydration process. However, the denser and lower surface water absorption relation to CO2 treatment could facilitate long-term durability performance of concrete structures.

Exploratory study on bond behavior of plain and sand coated stainless steel rebars in concrete

The performance of concrete structures depends largely on the bond strength between the steel rebar and surrounding concrete. The design equations currently available in the literature for concrete structures reinforced with deformed mild steel (MS) rebar may not be valid for smooth stainless steel (SS) rebar embedded in concrete. In this study, the bond behavior of SS rebar embedded in concrete is investigated using the push-out test method. The compressive bond behavior of 202 specimens is evaluated, and the bond stress–slip relationships for SS rebar are developed. The effects of concrete compressive strength, rebar diameter, embedment length, concrete cover, and rebar surface condition on bond strength are also examined, and a regression model is developed using the experimental results. The results show that an increase in rebar diameter, larger embedment length, and lower compressive strength all serve to reduce the average maximum bond strength of SS rebar. On the other hand, an increase in clear cover significantly improves the bond strength. Moreover, the test results show that surface treatment of SS rebar has less impact on the bond strength compared to other influencing factors.

Probabilistic impacts of corrosion on structural failure and performance limits of reinforced concrete beams

Concrete structures in corrosive conditions have different types of uncertainties in the corrosion phases of their lifetime, which should be considered in the reliability analysis with an appropriate probabilistic model. Although corrosion is an inevitable phenomenon, it can be delayed, and the maintenance costs can be minimized by choosing the suitable methods and using the results of life cycle cost analyses. In this paper, reliability analyses were performed on the performance of concrete structures in two limit states of serviceability and structural capacity. Then, the flexural capacity obtained from modeling by the finite element method was considered as a random variable in the calculation of capacity reduction probability in the corroded beams with different corrosion percentages; this was done by defining the gamma process. Furthermore, the costs caused by the failure in the life cycle of reinforced concrete beams were evaluated and optimized using renewal process analysis in the capacity limit state. In another section, the lifetime of reinforced concrete structures subject to corrosion was investigated in the serviceability limit state, and using the performance-based destruction model, the penetration of aggressive agents of chloride ions into the structure was modeled theoretically by explaining the diffusion laws. In the next stage, the corrosion of bars produced a considerable volume of iron oxide, and the increase in stress in the concrete around the bars caused by iron oxide resulted in the cracking of cover. Probabilities were presented in an algorithmic form with the Monte Carlo method to predict the beginning time of corrosion and cracking propagation until failure. Afterward, the cost caused by failure was calculated using a performance-based life cycle cost management (LCCM) method.

Bond behaviour of high-strength steel rebars in normal (NSC) and ultra-high performance concrete (UHPC)

Bond is the interaction and force transfer between rebar and concrete, which directly affects the performance of concrete structures. On the other hand, innovations in the production of ultra-high performance concrete (UHPC) and its unique advantages, such as high strength, low permeability, and high resistance to chemical attacks, have increased the interest in using it in the building industry. While most of the recommended relationships in the standards are based on normal strength concrete (NSC), it is necessary to know more about UHPC behaviour. In this study, 60 cubic concrete specimens reinforced with a rebar in its centre and three bond lengths (2, 4, and 6 times the diameter) were prepared using UHPC and NSC and two types of rebars (high-strength and normal strength). A pull-out test was performed on all specimens. The results show that the concrete strength, the ratio of concrete cover to the diameter (c/d), bond length, rebar yield strength and geometry of the rebar play an important role in determining the failure mode. The use of a high-strength rebar (AIV) in comparison with normal strength (AIII) rebars, by increasing the possibility of applying a higher bond tension to concrete, provides the opportunity to use more concrete capacity. UHPC reduces the embedded length of rebars by increasing bond strength 5-fold relative to NSC. In NSC, increasing the bond length increases the maximum bond stress, while in UHPC, for increasing bond length, the maximum bond stress decreases. New relationships have been proposed to predict bond-slip behaviour.

Coupled application of innovative electromagnetic sensors and digital image correlation technique to monitor corrosion process of reinforced bars in concrete

Corrosion monitoring of reinforcement is of great importance in evaluating the durability performance of concrete structures, especially under harsh environments. This paper presents a creative approach that combine using innovative electromagnetic monitoring apparatus (EMMA) and digital image correlation (DIC) technique in order to monitor the reinforcement corrosion inside concrete. The new-type electromagnetic sensor is capable of evaluating the corrosion degree of reinforced bars in concrete, meanwhile, DIC technique is always used to detect the tiny corrosion-triggered deformation on concrete surface. The result shows that joint use of these two techniques can monitor the entire process from corrosion initiation to further corrosion-induced strain variation on the surface of concrete. Besides, concrete beam with higher vertical load level leads to greater bending movement, which will accelerate the corrosion behavior and limit the horizontal strain. Moreover, the characteristics of DIC technique and strain gauges are compared respectively.

Novel experimental and numerical investigations on bonding behaviour of crack interface in smart self-healing concrete

There are crack interfaces between self-healing agent and cement matrix in smart encapsulation-based self-healing concrete, whose mechanical properties significantly affects the load capacity recovery of crack-healed concrete. In this study, both experimental and numerical investigations were conducted on the crack-healed concrete under uniaxial tension to investigate the interface bonding behaviours and the self-healing agent distribution on the crack surface. The results show that the bonding behaviour of the crack interface depends on the content of healing agent and mechanical properties of the crack surface. However, it is still difficult to accurately understand their effects on the bonding behaviour by experimental investigation due to the high brittleness of the crack interface and the discrepancy of self-healing concrete. Therefore, based on the experimental results, a novel numerical model of the interface between self-healing agent and cement matrix was developed to investigate effects of aggregates, pores and interface properties on the bonding behaviour of crack interface by the cohesive surface technique (CS). Parametric analysis was also performed on the bonding behaviours and a method was proposed for assessing the load capacity of crack-healed concrete. Based on the experimental and numerical investigations on the healing agent-concrete crack interface in the smart encapsulation-based self-healing concrete, this novel numericla methods can be used to assess the recovery efficiency and performance of smart self-healing concrete structure.

Transport properties of surface layers from full‐scale reinforced concrete member

AbstractThe durability performance of concrete structure is intimately related to the mass transport properties for the surface layers of concrete members. However, due to several effects, such as segregation, jamming effect and so on, the quality of the surface layer for reinforced member might be obviously different from their core material and material prepared and cured in the laboratory (labcrete). This study aims at understanding the mass transport properties of surface layers from reinforced concrete member, and clarifying the differences among them and those for accompanied labcrete. Specimens prepared from both a full‐scale reinforced beam and labcrete are subjected to experimental investigations on their gas permeability, capillary sorptivity, chloride diffusivity, and chloride migration coefficients. It is found that, for the same specimen, the gas permeability is approximately linear to capillary sorptivity. While the chloride diffusivity is smaller than the rapid migration coefficient. The durability potential of surface layers for the reinforced beam is inferior to that of the inner core material. While the latter is further obviously worse than that of labcrete. In engineering practice, special attention should be paid to ensure the quality of surface layer. When making quantification or prediction for the durability potential of concrete structure, it is more appropriate to use specimens from surface layers of field concrete members rather than those from labcrete.

A critical review on the effect of organic acids on cement-based materials

Ingression of acids into concrete structure arrests its long-term durability by the progressive degradation of the highly alkaline cementitious materials. Acid attack results in the decalcification of hydration products leading to higher inherent porosity and both micro-macro cracks, providing a path for the passage of aggressive ions into the interior of concrete. Amplification of agriculture and food industries such as the breeding, dairy or sugar industries produce large quantities of effluents that are the major sources of organic acids. These effluents have complex acidic compositions leading to serious degradation of structures made for their production, collection, storage and treatment. New binder systems that are capable of proposing extensive long term durability in aggressive acidic environment and also being a sustainable option need to be evolved. From earlier studies, application of various binder systems containing materials such as fly ash, ground granulated blast furnace slag, silica fume, bagasse ash etc. has an influence on the hydration products, micro-structural and macro-structural changes in cement-based materials when subjected to aggressive acidic media, thus enhancing the performance and durability of concrete structures. This paper outlines the basic mechanism of organic acid attack and highlights the influence of various binder systems on the alteration of degradation kinetics when exposed to various organic acids.

The fractal characteristics of C80 high performance concrete pore structure subject to high temperatures

The pore structure of C80 high-performance concrete (HPC) without and with 0.2% polypropylene fiber (PPHPC) was tested by mercury intrusion porosimetry, combining with fractal theory, the high temperature, PP fiber, pore parameters and fractal dimension was discussed. Results showed that porosity, the most probable pore size, the volume intermediate pore size and the threshold pore size all increase with the rising of temperature, the deterioration of the internal pore structure becomes serious. The pore growth rate of HPC is higher than PPHPC, and the porosity of HPC is about the same as PPHPC at 600°C. With temperature gradually increased, the fractal dimension of HPC increases first and then decreases, and the PPHPC shows a decreasing trend as a whole. The incorporation of PP fibers improves the internal connectivity of materials, reduces the deterioration of micro-structure after high temperature.

Bond behavior between basalt fiber-reinforced polymer bars and concrete under cyclic loading

Concrete structures reinforced with fiber-reinforced polymer (FRP) bars are frequently subjected to cyclic loadings. The bond behavior between FRP bars and concrete under cyclic loading is important in evaluating the seismic and fatigue performance of concrete structures reinforced with FRP bars. Although the bond stress-slip relationship between basalt FRP (BFRP) bars and concrete under static loading has been investigated, investigations on the bond behavior between BFRP bars and concrete under cyclic loading remain lacking. The present study presented experimental investigations on the bond behavior between BFRP bars and concrete under cyclic loading using a pull-out test. Test results showed that: (1) the bond stress-slip relationship between BFRP bars and concrete revealed the behavior at different phases: the elastic phase, the cracking propagation phase, the cracking closure phase, and the friction phase; (2) the bond strength between BFRP bars and concrete under cyclic loading decreased with the increase of number of cycles; (3) the slip corresponding to bond strength between BFRP bars and concrete under cyclic loading increased with the increase of number of cycles; (4) the hysteretic curve area between BFRP bars and concrete under cyclic loading decreased with the increase of number of cycles; (5) several prediction models for bond strength, slip corresponding to bond strength and hysteretic curve area between BFRP bars and concrete under cyclic loading were proposed considering number of cycles; (6) the residual bond strength between BFRP bars and concrete decreased while the slip corresponding to residual bond strength increased after cyclic loading.

Structural, daylighting, and energy performance of perforated concrete shell structures

In this work, a computational interdisciplinary design approach is used to integrate assessment of the structural and environmental performance of perforated concrete shell structures. Design parameters that co-exist in these disciplines and relevant performance criteria are identified. Questions result: how do these design parameters affect performance? What is the tradeoff between performance in each? Computer-aided design tools are used for form generation and performance assessment. Statistical analyses are used to study the sensitivity of performance to each parameter. Finally, the perforation ratio is found to be the most significant parameter affecting both disciplines; a value ≤ 10% to 20% is recommended for shell structures when translucent glazing is installed without shades in a Boston-type climate.

Stress–strain–temperature relationship for concrete

When concrete structures are subjected to load and temperature simultaneously, it is essential to take into account the coupled effects between stress and expansion. However, due to incomplete understanding, such coupled effects have only been incorporated into current Eurocode 2 (EC2) stress–strain curves by means of empirical correlations. These empirical correlations at different target temperatures are presented in tables that do not allow to clearly identify the correlation chosen to obtain the specific values. A further limitation of these tables is that the relationships cannot be used to evaluate the performance of concrete structures during the cooling phase. In this paper, a physically-based model of the coupled effects between stress and expansion is used to define the strain corresponding to the compressive strength, and thus to develop a simple formulation for stress–strain–temperature relationship of concrete. The results are then compared with the EC2 stress–strain–temperature table. The expression of stress–strain–temperature relationship developed in this paper successfully agrees with the stress–strain curves of concrete in EC2 used for the heating phase. More importantly, the proposed stress–strain–temperature relationship can also be applicable for design purposes of concrete structures during the cooling phase.

Axial compressive behavior of partially CFRP confined seawater sea-sand concrete in circular columns – Part II: A new analysis-oriented model

For the comprehensive analysis of the mechanical performance of concrete structures wrapped with partial FRP strips, the development of an accurate and versatile axial stress-strain model is of significant importance. No research, however, has been conducted to present a satisfactory analysis-oriented stress-strain model for partially FRP confined concrete. To this end, this paper firstly investigates the performance of several existing analysis-oriented models in assessing the FRP partially wrapped columns. The results show that all selected models cannot provide reasonable predictions for the test specimens, especially for the specimens possessing strain-softening stress-strain relationships. Therefore, the dilation properties and the peak axial strength expressions of active-confinement base models were both revised based on the test observations and a new analysis-oriented model was proposed for partially FRP confined concrete. Finally, a large database of CFRP partially wrapped columns was assembled through an extensive literature review to evaluate the accuracy of models. By comparing the test results with the predicted calculations, the proposed model can provide accurate and reasonable predictions both for the ultimate conditions and the complete axial stress-strain relationships, which were obviously superior to the other existing models.

Influence of Longitudinal Reinforcement Ratio on Shear Strength of RC Slender Beam Under Different Loading Rates

. A primary current focus in concrete shear design is how to ensure the shear resistance of the concrete structures to withstand the possible adverse creep effects during the service life. All shear-carrying actions are supposed to depreciate due to the enhanced critical shear cracks under sustained loads. However, only a few studies that evaluated the performance of concrete structures failing in shear due to long-term loading. As the longitudinal tensile reinforcement strain influences the shear strength of RC beam, the shear resistance of the beam under sustained load may also be affected by the amount of longitudinal reinforcement. The present study aims at investigating the influence of flexural reinforcement ratio on the shear creep failures of reinforced concrete (RC) beams without stirrups by non-linear finite element analysis. In this study, the numerical model of RC beams with reinforcement ratios varying from 0.4% to 3.2% was evaluated under different loading rates. A loading rate of 1000 and 10000 times slower than the static loading rate were adopted to reveal the creep effects of RC beams.