Concrete Bars Research Articles

The Constituent Phases and Micromechanical Properties of Steel Corrosion Layers Generated by Hyperbaric-Oxygen Accelerated Corrosion Test.

Hyperbaric oxygen-accelerated corrosion testing (HOACT) is a newly developed method to study in the labor the corrosion behavior of steel bars in concrete. This work aimed to intensively investigate the mechanical properties and microstructures of HOACT-generated corrosion products by means of nano-indentation tests, Raman micro-spectrometry, and scanning electron microscopy. The local elastic modulus and nanohardness varied over wide ranges of 6.8-75.2 GPa and 0.38-4.44 GPa, respectively. Goethite, lepidocrocite, maghemite, magnetite, and akageneite phases were identified in the corrosion products. Most regions of the rust layer were composed of a complex and heterogeneous mix of different phases, while some regions were composed of maghemite or akageneite only. The relationship between the micromechanical properties and typical microstructural features is finally discussed at the micro-scale level. It was found that the porosity of corrosion products can significantly influence their micromechanical properties.

Effect of Bonding Area on Bond Stress Behavior of GFRP Bars in Concrete

The application of Glass Fiber Reinforced Polymer (GFRP) bars is suitable for concrete structures that are susceptible to corrosion, owing to their corrosion-resistant characteristics. Therefore, it is feasible to reduce the concrete cover on reinforced concrete beams by utilizing GFRP bars. However, this can reduce the bonding strength between GFRP bars and concrete. Therefore, this study aims to investigate the bonding behavior between GFRP bars and concrete as a preliminary test for structural applications. The bond stress behavior between GFRP bars and concrete was analyzed by 18 pull-out tests. The test specimens comprised GFRP bars with three different variations, namely GFRP bars with concrete cover (GFRP-C), GFRP bars without concrete cover (GFRP-E), and GFRP bars with a complete wrapping of GFRP sheet (GFRP-C-Sheet). The bond stress-slip curve, bond strength, and failure pattern were utilized to analyze the effect of each variation. The research results indicate that the bonding stress between GFRP bars and concrete was strongly influenced by the concrete cover, where the bonding strength decreased by 65%. Nevertheless, the utilization of a complete wrapping GFR) sheet resulted in a 26.4% increase in bonding stress. The present study has identified three distinct modes of failure, including pull-out (GFRP-C), concrete crushing (GFRP-E), and GFRP sheet debonding (GFRP-C-Sheet). Doi: 10.28991/CEJ-SP2023-09-010 Full Text: PDF

Durability of bond between carbon/glass hybrid fiber-reinforced polymer (HFRP) bar and concrete in water

Glass-fiber-reinforced polymer (GFRP) bars have been widely used as a reinforcement in concrete, and glass fibers are susceptible to reacting with alkali ions in concrete pores. Thus, carbon fibers are wrapped around GFRP bars to produce carbon/glass hybrid fiber-reinforced polymer (HFRP) bars so as to promote the durability of the internal GFRP bars in concrete. This work studies the durability of the interfacial bond between GFRP bars/HFRP bars and concrete using pullout tests. The specimens are tested after immersion in distilled water for 0, 150, 255, and 544 days. The failure of the GFRP bar–concrete composite is due to damage to the GFRP bar, while the HFRP bar–concrete composite fails because of a combination of the scratching of the HFRP bar and the failure of the concrete. Both HFRP and GFRP bars are scratched as the immersion period extends. Further, the strength of the bond between the HFRP bar and concrete is lower than that of the bond between the GFRP bar and concrete, while the bond stiffness of the HFRP bar–concrete composite is higher than that of the GFRP bar–concrete composite. The bond strength of the HFRP bar–concrete and GFRP bar–concrete composites increases in the early immersion periods but decreases after 544 days of immersion in water. The retention of the bond strength of the HFRP bar–concrete composite is superior to that of the GFRP bar–concrete composite. The carbon fiber coat is proven to improve the durability of the bond between HFRP bars and concrete in water. A formula for predicting the bond strength of HFRP bar–concrete and GFRP bar–concrete composites in water is also developed. Finally, the modeling results demonstrate that the bond strength retention of the HFRP bar–concrete and GFRP bar–concrete composite is 76% and 46% after 50 years of service time, respectively.

Bond performance of reinforcing bars in SFRC: Experiments and meso-mechanical model

The bond performance of reinforcing bars in steel fiber-reinforced concrete (SFRC) remains a topic of discussion in the community. The effect of deformed steel fiber types is not fully understood, and a rational mechanics-based model is required to consider this behavior. This study experimentally investigated the bond performance of steel rebars in SFRC containing milled-cut (M) fibers with proven high bridging capacity, namely MCSFRC. The results indicated that M fibers yielded a higher bond strength of rebars compared to hook-end (H), crimped (C), and straight (S) fibers from previous studies. This was attributed to the superior bond resistance of M fibers in the matrix, provided by the rough inner surface, stiff end hook, and longitudinal twist. A meso-mechanical model (rib and fiber scale) was then developed to effectively assess the bond strength of rebars in SFRC, accounting for rib-concrete interaction, as well as the fiber reinforcement differences including fiber–matrix interaction, fiber spatial distribution, content, and dimension. This model was applicable to rebars in SFRC with H, C, S, and M fibers, and the results showed good agreement with 301 tests in the established database. On that basis, a unified local bond-slip model for rebars in SFRC was proposed, which was able to suitably reproduce the entire bond-slip response under pull-out and splitting failures.

Surface-Dependent Interfacial Characteristics of Glass Fiber-Reinforced Polymer Bars in Concrete

Surface-Dependent Interfacial Characteristics of Glass Fiber-Reinforced Polymer Bars in Concrete

Bond of Plain Bars in Early-Age Concrete under Bilateral Pressures

Bond of Plain Bars in Early-Age Concrete under Bilateral Pressures

Bond Stress Distribution and Bond–Slip Model of Deformed Steel Bars in Iron Tailing Sand Recycled Aggregate Concrete

In this study, the bond stress distribution and bond–slip model of steel bars and iron tailing sand recycled aggregate concrete (ITRAC) were investigated using central pullout tests on 33 steel bars and ITRAC bonded specimens. The results show three failure modes for the bonded specimens: splitting, pullout, and splitting–pullout. Compared with the maximum bond strength of nature sand concrete (NAC), the maximum bond strength of the iron tailing concrete and ITRAC specimens increased by 23.12% and 6.08–23.96%, respectively. After adding 1% steel fiber, the maximum and residual bond strengths of ITRAC increased by 40.82% and 129.10%, respectively, compared with those of NAC. The maximum bond strength of ITRAC decreased after the configuration of the stirrups. The bond stress distribution characteristics of the ITRAC specimens resembled those of recycled aggregate concrete (RAC). Generally, two bond stress peaks emerged, and the uniformity of the bond stress distribution improved after adding RAC to the iron tailing sand (ITS). The results of the proposed ITRAC bond–slip constitutive model agreed with the test results.

Bond performance between FRP bars and geopolymer concrete after elevated temperature exposure

The use of geopolymer concrete structure reinforced with fiber reinforced polymer (FRP) bars is an effective approach to solve the problems of high energy consumption, large CO2 emission, and steel corrosion caused by traditional reinforced concrete structures. This paper presents an experimental study on the effect of elevated temperatures on bond performance between FRP bars and geopolymer concrete. The influence factors include exposure temperature, surface treatment of FRP bars, fiber type of FRP bars, geopolymer concrete strength and type of concrete. The test results indicate that the bond strength between FRP bar and geopolymer concrete increases first and then decreases gradually with the increase of exposure temperature. After exposure to 350 °C, the bond strength retention ranges from 56.0% to 83.3%, while after exposure to 400 °C, the residual bond strength retention is not more than 17.7%. The bond strength retention of FRP bars in geopolymer concrete is basically higher than that in ordinary concrete from 25 °C to 350 °C. Moreover, the ribbed FRP bars possess a higher bond strength retention than the sand-coated FRP bars after elevated temperature exposure. With the increase of the geopolymer concrete strength, the high temperature resistance of geopolymer concrete decreases, resulting in a corresponding decreasing in the bond strength retention. In addition, the fiber type of FRP bar has no significant effect on the interfacial bonding properties compared with surface treatment of FRP bar and concrete strength. Finally, the available equations for calculating the development length of FRP bar in geopolymer concrete after elevated temperature exposure was proposed, which may be used for the design of fire prevention and fire safety assessments.

Prediction of the Long-Term Tensile Strength of GFRP Bars in Concrete

The durability of two types of widely used glass fiber reinforced polymer (GFRP) bars, one without coating (G1) and one with slightly surface sand-coating (G2), were studied through accelerated aging. Concrete cylinders reinforced with GFRP bars were immersed in tap water in temperature-controlled tanks. The influence of different exposure temperatures, 20, 40, and 60 °C, and also different exposure times, 30, 60, 90, 120, and 180 days, on the degradation of the two types of GFRP bars was investigated. The tensile strengths of GFRP bars after different exposure times were evaluated with tensile tests, and the variation of the microstructure and elemental compositions of conditioned specimens was evaluated with scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS), respectively. The degradation rate of the tensile strength retentions of two types of GFRP bars decreased with an increase in the exposure time at all exposure temperatures. The tensile strength retentions of the GFRP bars were studied by three commonly used prediction models. Based on the degradation mechanism of fiber-matrix debonding, a new model was proposed. These four models were evaluated with the test results and a new model proposed was suggested as the best model to predict the residual tensile strength of the GFRP bars. The durability parameters of the GFRP bars were discussed. The tensile strength retention tended to converge to a constant value (52%) with the increase of exposure time, which contributes to the determination of the environmental reduction factor in relevant design guides, and the fiber-matrix debonding was found to be the main degradation mechanism due to the surrounding concrete environment. The sand-coating had some effect on the activation energy of the GFRP bars.

Evaluation of the bond-dependent factors for CFRP bars used as structural reinforcement: A critical review

This study analyzes the precious data from the literature on bond strength between concrete and CFRP bars associated with the pullout test. In this regard, an overview of the pullout test and bond stress-slip model, along with a systematic analysis of characteristic bond strength behavior, were reviewed extensively. Over 273 specimens of CFRP bar involved in pullout tests were mined from the experimental works reported by preceding researchers worldwide. Significant findings revealed that the pullout test is a widely used method for determining the bond strength of embedded CFRP bars in concrete. The concrete compressive strength, bar diameter, modulus of elasticity, embedment length and surface treatment all have a major impact on bond performance. Moreover, the development of an analytical model for bond stress-slip is paramount to present the interaction behavior as well as to perform numerical analysis. In addition, bond strength predictions by international guidelines are conservative. However, the bond strength calculated by ACI 440.1R-15 has proven to be more practical across all the statistical measures. In line with this review, recommended research areas are also successfully discussed and proposed for further investigation.

Experimental Study on the Protection of an Asphalt Coating to Reinforcement in Magnesium Oxychloride Cement Concrete

To mitigate the corrosion of steel bar in magnesium oxychloride cement concrete (MOCC), an asphalt coating is used for protection. According to the service environment in the Salt Lake area of western Qinghai, China, a solution immersion test on MOCC was performed. The test results showed that, after soaking for 2160 days, the corrosion current density icorr was 7.88 µA/cm2, and RC was 113.51 Ω for group A and icorr is 0.08 µA/cm2 and RC is 842.57 Ω for group B. The time parameters for groups A and B fell in the high-frequency range of 103–105 Hz and low-frequency range of 10−2 to 10−1 Hz. The relative dynamic moduli of the elasticity evaluation parameters ω1 and ω2 were 0.942 and 0.959 and 0.946 and 0.962 for groups A and B, respectively, at 2160 days, exhibiting a downward trend on the whole. Fourier transform infrared spectroscopy and microscopic testing revealed that the corrosion of the chloride solution on asphalt was mainly the corrosion of chloride in the interior of the asphalt and that the invaded salt provided a channel for the water solution to invade into the interior.

Bond Strength Prediction Model of Ribbed FRP Bars in Concrete

The bond behavior of Fiber-Reinforced Polymer (FRP) bars is a critical factor in the mechanical performance of FRP rebar reinforced concrete structures, but until now, there has been limited research on the effect of rib forming process and geometrical features of ribbed FRP bars on the bond behavior, and no relevant provisions are incorporated into the current main design codes. In the paper, the spirally-glued FRP bar was taken into consideration, and bond test data were collected to investigate the main test parameters on the bond failure modes and bond strength. Greater emphasis was placed on the effect of rib forming process and geometrical features of such ribbed FRP bars on bond behavior. Results shown the macroscopic failure mode was pullout failure. More specifically, the spirally-glued FRP bar mainly exhibited shearing off of FRP bar ribs, and bond strength increased by increasing concrete strength. Spirally-glued FRP bars shown higher bond strength by increasing concrete cover which contributes to better confinement to FRP bars. Bond strength of spirally-glued FRP bars could be improved by increasing relative rib height hrd and FRP bar rib width ratio Fr. Multivariate nonlinear regression was adopted to propose a suitable formula to estimate bond strength. The calculated results by the proposed equation were in good agreement with the test results with greater accuracy than the current design codes. This is because the effect of rib forming process and geometrical features of FRP ribs on bond strength is accurately accounted in the proposed equation.

Experimental Study on the Bond Performance of Steel–Basalt Fiber Composite Bars in Concrete

Steel–basalt fiber composite bars (SBFCBs) exhibit excellent strength, elastic modulus, toughness, corrosion resistance, and low cost. Understanding the bond performance between SBFCBs and concrete is important for evaluating the mechanical behavior of SBFCB-reinforced concrete structures. In this study, the bond–slip performance of SBFCB in concrete was experimentally assessed via pullout and beam bond tests. The influence of various factors on the bond performance was discussed, and the bond mechanism between SBFCB and concrete was analyzed. The results indicate that reducing the thread pitch ratio in the SBFCB can reduce slippage at the postpeak bonding stress–slip curve. The bond strength of the SBFCB was significantly improved by sandblasting. Longer bond lengths correspond to smaller average bond strengths in SBFCB. The load corresponding to the maximum bond strength of a specimen with a bond length of 10d was determined as the ultimate load value for the SBFCB. An increase in the SBFCB diameter had a negative impact on the bond strength. Moreover, an increase in the concrete cover thickness was conducive to improving the bond strength; if the thickness of the concrete cover was sufficiently increased, failure occurred owing to SBFCB tension rather than bonding. Based on the test results and the existing bond–slip model, a simple bond–slip constitutive model of the SBFCB embedded in concrete was proposed to simulate the pullout process of the SBFCB. Subsequently, the established bond–slip model can be used to analyze the mechanical performance of SBFCB-reinforced concrete structures.

Calculation of the formation of normal cracks in a reinforced concrete element based on the deformation theory of plasticity of concrete by G.A. Geniev

The authors present a refined method of determining the moment of cracking in reinforced concrete bar constructions using the diagram of deformation of concrete built on the basis of the deformation theory of plasticity by G.A. Geniev in which the stress and strain invariants of concrete are linked by nonlinear dependences. In the resulting defining equations, the hypothesis of flat sections, as well as the premise of reaching the limit values of concrete deformations on the stretched fibers of the cross-section are used. Stresses in concrete are determined through deformation values in accordance with the nonlinear deformation diagram of concrete. On the basis of the assumptions accepted, analytical dependences for determining the moment of cracking in the sections of bending elements with single and double reinforcement have been acquired. The formulas obtained were used in the analysis of various factors influence on crack resistance of bendable reinforced concrete elements. It was found out that the moment of crack formation practically does not change when percentage of reinforcement of longitudinal tensile or compressed reinforcement changes. The most effective method of crack resistance improvement is the increase of concrete strength. The proposed methodology is verified by comparison with experimental results on reinforced concrete prototypes. It is concluded that the use of the diagram of nonlinear deformation of concrete on the basis of the theory of plasticity by G.A. Geniev allows to estimate more strictly the crack resistance of reinforced concrete rod elements.

Local bond-slip behaviour of reinforcing bars in fibre reinforced lightweight aggregate concrete at ambient and elevated temperatures

This paper presents an experimental investigation of the bond-slip behaviour of reinforcing bars in fibre-reinforced lightweight aggregate concrete (FRLWAC) at ambient and elevated temperatures. Sixty-four bond-slip specimens with four target temperature exposures, two concrete covers and four steel fibre contents were tested to failure. The experimental results showed that exposure to elevated temperature shifted failure pattern from more ductile pull-out to brittle splitting modes, and reduced the peak bond strength and toughness of bond-slip behaviour. Scanning electron microscope (SEM) images of the concrete-steel interface showed that thermal mismatch induced microcracks contributed to degradation of bond strength, in addition to temperature induced degradation of concrete materials. The temperature exposure also negated beneficial effect of concrete cover in enhancing bond behaviour. Addition of steel fibre helped in preventing brittle failure mode, enhancing bond strength in specimens with small concrete cover and improving the post-peak bond-slip behaviour at all target temperatures. A simple semi-analytical bond-slip model that considers different failure modes and incorporates fibre-reinforced concrete tensile behaviour from Swedish Code SS 812310 was proposed and validated based on the test results in this study.

Effect of Corrosion on the Bond Behavior of Steel-Reinforced, Alkali-Activated Slag Concrete

Alkali-activated slag concrete (ASC) is regarded as one of the most promising sustainable construction materials for replacing ordinary Portland cement concrete (OPC) due to its comparable strength and outstanding durability in challenging environments. In this study, the corrosion of steel bars embedded in ASC and OPC was studied by means of an electrically accelerated corrosion test of steel bars in concrete. Meanwhile, the bond performance of the corroded steel bars embedded in ASC was tested and compared with corresponding OPC groups. The results showed that ASC and OPC behaved differently in terms of bond deterioration. The high chemical resistance of ASC decreased the corrosion of steel bars and, thus, increased the residue bond strength and the bond stiffness.

Bond performance of the additional aluminum alloy ribs anchored CFRP bars in concrete under sustained loading

The novel additional aluminum alloy ribs (ARs) anchorage is a convenient and effective solution to improve the bond of FRP bars in concrete. Although the bond behavior between the ARs anchored FRP bars and concrete under the short-term loading has been recently investigated, its long-term behavior under sustained loading has yet been assessed. This paper aims to study the long-term bond behavior of both the unanchored and ARs anchored CFRP bars in concrete under sustained loading for a period of 380 days. 12 pull-out specimens were tested under sustained loading considering parameters including the bond length, sustained loading level, and the type of FRP bars. Results showed that the ARs anchored specimens effectively reduced the long-term slip by as much as 48.7%-52.5% compared to that of the referenced unanchored specimens due to an improved bond mechanism. In addition, an analytical model was proposed to predict the long-term end slip for both the unanchored and ARs anchored FRP bars in concrete.

Effect of Marble Waste Powder and Silica Fume on the Bond Behavior of Corroded Reinforcing Bar Embedded in Concrete

Bond behavior is the mutual effect and the force transmission between concrete and reinforcing bar that directly influence the behavior of reinforced concrete structures. Considering the silica fume (SF) and marble waste powder (MWP) could enhance the concrete matrix and improve its adherence, the bond-slip behavior of corroded reinforcing bar in concrete containing SF and MWP as ordinary portland cement (OPC) is examined in this work. Three of 16 concrete mixes were selected for this investigation: one control mix, a second mix containing 10% SF and 5% MWP as OPC replacements, and the third containing 10% SF and 20% MWP as OPC replacements. The pullout test was applied to the specimens with 0%, 2%, 5%, and 10% corroded reinforcement. According to the results of specimens with uncorroded reinforcement, replacing 10% of cement by SF and 5% or 20% by MWP, the bond strength between concrete and reinforcing bar increased by 27% and 13%, respectively, compared to that of the control specimens. Also, the samples containing SF and MWP showed higher corrosion resistance and their bond strength’s loss due to the corrosion of reinforcing bars was less than the control samples. The results also indicate that bond stress at low corrosion (2%) levels is partially increased because of the friction resulting from the accumulation of corrosion products on reinforcement’s surface. However, for higher corrosion levels (5% and 10%), the bond strength decreases. By comparing the two diameters of reinforcing bars (12 and 16 mm), it was recognized that the reduction of bond strength due to the corrosion is more severe in the reinforcements with greater diameters.

Flexural performance of 3D-printed composite beams with ECC and recycled fine aggregate concrete: Experimental and numerical analysis

To overcome the difficulties of placing steel bars in 3D-printed concrete (3DPC), a novel reinforcement method combining engineered cementitious composites (ECC) and recycled fine aggregate concrete (RFAC) was proposed. Experiments on the flexural performance of 3D-printed composite beams with various ECC heights (15, 30, and 45 mm) and shear-span ratios (2.0, 2.5, and 3.0) were carried out. The test results indicated that the beams failed by bending, and the load-bearing capacities with ECC heights of 30 and 45 mm are roughly equivalent to those of the reinforced concrete beams with the longitudinal reinforcement ratios of 0.2%. Subsequently, a finite element model considering printed interfaces was established. The effects of interfacial bond strength, ECC height, and ECC strength on flexural performance were explored through a parametric study. The load-bearing capacity improvement by the increase of ECC height and strength may be limited by the bond strength of the printed interfaces.

Cyclic bond deterioration of deformed steel bar in well-confined concrete

The cyclic bond deterioration mechanism of deformed steel bars embedded in concrete is critical for establishing the hysteretic bond stress-slip relationship. For a deformed steel bar, well-confined in concrete, the bond failure model under monotonic loading is governed by pullout, the bond deterioration trend under cyclic loading is remains unclear, especially in the case of applied lateral pressure. In this paper, the effect of uniaxial lateral pressure on the bond performance under different loading histories was studied. The test results indicate that the variation in bond parameters under varied lateral pressure is negligible. Therefore, the monotonic and reversed bond stress-slip constitutive relationship of a deformed bar failed by pullout, without lateral pressure confinement, can be used for simulating the bond response under different uniaxial lateral pressures. Cubic concrete specimens with small openings were used to quantitatively analyze the failure process under reversed loading through digital imaging correlation (DIC). The bond stress was observed to be nearly uniformly distributed along the interface at the initial loading stage. After increasing the pull-out or push-in loading to 75 %–85 % of the peak bond strength, we observed minute shear cracks between the interfaces. Subsequently, the length and width of the cracks were then gradually developed with the controlling slip and number of cycles, leading to reduced mechanical interaction and friction between the shear cracks.