Concrete Bars Research Articles

Experimental Study on the Performance of Lap-Spliced Steel Bars in Ultrahigh-Performance Concrete

Experimental Study on the Performance of Lap-Spliced Steel Bars in Ultrahigh-Performance Concrete

Unification and calibration of steel corrosion models based on long-term natural corrosion

The corrosion of steel bars in concrete has a significant influence on the durability deterioration of reinforced concrete structures. The purpose of this study is to investigate the prediction models of steel corrosion rate and the main influencing factors in the case of natural corrosion. The experimental results are obtained from the long-term natural corrosion induced by accelerated chloride erosion. Then, based on electrochemical corrosion kinetics, Ohm’s law, and potential field theory, numerical models for uniform and local corrosion are established respectively, and compared with the experimental results. The main parameters and influencing factors are deeply analyzed through the numerical models. The main conclusions are as follows: (1) The cell model based on Ohm’s law can be well adapted to uniform corrosion, especially when the resistivity of concrete is the controlling factor. (2) Among the parameters of electrode polarization, the cathode Tafel slope has the greatest influence on the numerical results, followed by the anode equilibrium potential.

Degradation of mechanical properties and microstructure evolution of basalt-carbon based hybrid FRP bars in real seawater and sea-sand concrete

The degradation of basalt fibre-reinforced polymer (BFRP) bars induced by alkaline environments hinders their application in seawater and sea-sand concrete (SWSC). The use of carbon fibres (CFs) to partially replace basalt fibres in the preparation of hybrid-FRP (HFRP) bars is an effective method for enhancing the durability of BFRP-bar. To evaluate the effectiveness of using CFs to replace part of the basalt fibres in preparing HFRP bars to enhance the durability of BFRP-bar in real SWSC. The tensile and interlayer interface properties of HFRP bars with different CFs hybrid contents (VCF, %) and arrangements in the SWSC were tested after exposure to seawater at different temperatures. Digital microscopy, scanning electron microscopy, X-ray computed tomography and matrix digestion analysis were used to investigate the microstructural evolution of HFRP bars and SWSC after conditioning. The results showed that the VCF increasing to 10% or 25% significantly alleviated the interlaminar shear strength (ILSS) and tensile strength (TS) losses. The maximum enhancements in the ILSS and TS retention for the SWSC-embedded BFRP bar were 19.12% and 17.55%, respectively. The hybrid CFs changed the failure model of the BFRP-bar. Compared with HFRP bars with CFs in the core, HFRP bars are more likely to deteriorate when the CFs are dispersed circumferentially. The hybrid CFs improved the environmental reduction coefficients (ERC) of TS and ILSS of the HFRP bars. Compared with the SWSC, the ERC of the HFRP bars in the normal concrete was significantly improved.

Bond responses and anchorage length of GFRP bar in precast recycled aggregate concrete

Precast recycled aggregate concrete (PRAC) employs precast reject-derived recycled aggregates (PRAs) and exhibits mechanical properties on par with or superior to those of natural aggregate concrete (NAC). Nevertheless, the durability of PRAC is hampered by aged mortar. To address this issue, integrating high durability glass fiber reinforced polymer (GFRP) bars into PRAC emerges as a potential solution for constructing enduring structures. Building upon this context, the study focuses on the bond responses between PRAC and GFRP bars, an area with limited existing research. Three main tasks were conducted: Firstly, a comprehensive test program involving 108 pullout samples was conducted to investigate the key variables' effects on bond failure mode and stress-slip relationship. Subsequently, an equation was developed to determine the peak bond strength of recycled aggregate concrete (RAC) based on available test data. Additionally, reliability analyses were performed to discuss the anchorage length of GFRP bar embedded in RAC. The findings are as follows: (a) The PRAC's bond capacity initially increased then decreased with rising PRA content. At full replacement, PRAC's bond strength was 28.3% lower than NAC. However, a two stage mixing approach restored PRAC's bond capacity to NAC levels. (b) Helical wrapping of the GFRP bar consistently exhibited the highest bond capacity, with an average bond strength 51% greater than a sand coated GFRP bar and 21% higher than a GFRP bar treated with both helical wrapping and sand coating. (c) Current codes specifying the critical anchorage length for FRP bar in NAC were often unsafe for RAC. Hence, a new expression, factoring in RA content, type and RAC mixing method, was proposed to determine the suitable anchorage length.

Application of Electrochemical Method to Evaluate the Corrosion Conditions of Steel Bars

The purpose of this research is to use an electrochemical method to evaluate corrosion conditions of steel bars in concrete. According to the description of ASTM C876, when the corrosion potential measurement values of the steel bar are below -350 mV, there may be a greater than 90 percent chance of corrosion. If the current impedance of the concrete is below 10 kΩ-cm, there is a possibility of corrosion in the steel bar. The test plan is as follows: to use concrete with different concentrations of chloride ions or applying an electric current to the steel bars to accelerate the corrosion reaction. In addition, we plan to use a solution of chloride ions to simulate sea tidal erosion on reinforced concrete structures by spraying. According to the experimental observations, concrete containing chloride ions is very likely to cause internal corrosion of the steel bar. The current impedance of the concrete can be used as the basis for assessing the internal humidity and moisture content of concrete. With an increase in curing time, the concrete's resistance correspondingly increases.

Bond Strength of Reinforcing Steel Bars in Self-Consolidating Concrete

This paper presents an experimental investigation of the bond strength of reinforcing steel bars in tension in self-consolidating concrete (SCC). The effects of the reinforcing bar’s location, orientation, size, and coating on the bond strength with SCC were studied and compared to those with conventionally vibrated concrete (CVC). Several SCC mixtures were developed to cover a wide range of applications/components and material types. The fresh properties of the SCC mixtures were determined to evaluate their filling ability, passing ability and stability. Two hundred and thirty-four pull-out tests of rebars embedded in cubes, wall panels and slabs were conducted. Almost half of the tests were conducted to evaluate the bond with SCC and the other half with CVC. Load–slippage relationships were measured for each test. Pull-out test results were analyzed, and the bond strength was reported in two values: critical strength, which corresponds to slippage of 0.01 in. *0.25 mm); and ultimate strength, which corresponds to the maximum load. The critical strength of SCC and CVC were compared against the ACI 318-19 provisions and comparisons between the ultimate strength of SCC and CVC were conducted. The comparisons indicated that SCC has lower bond strength with vertical rebars than CVC, and a 1.3 development length modification factor is recommended. A similar conclusion applies to epoxy-coated and large diameter rebars. Also, SCC with high slump flow has shown a less top-bar effect than that of CVC.

Bond characteristics of quartz sand modified enamel coated steel bars with concrete

Porcelain enamel, as a coating material, has a glassy and smooth surface that reduces the bond action with surrounding concrete when applied to steel rebar. In this work, quartz sand modified enamel (QSME) coatings are proposed for the first time to improve the bond action with concrete. The bond characteristics of QSME-coated steel bars in concrete are experimentally investigated by pull-out tests, and the effects of the size and content of quartz sand are considered. The microstructure of the steel bar/concrete interface is characterized by using a scanning electron microscope, and the surface profile is analyzed with a 3D scanner. Results show that QSME coating significantly increases the bond strength, bond stiffness, bond ductility, bond toughness, and residual bond strength of plain steel bars, which increase with an increase in the quartz sand content. The bond strength of QSME-coated deformed steel bars decreases compared with that of uncoated rebars due to the reduced rib height. However, QSME coating improves the bond stiffness, bond ductility, bond toughness, and residual bond strength of deformed steel bars. For QSME-coated steel bars, the bond strength linearly increases with an increase in the surface roughness.

FAILURE ANALYSIS OF PRESTRESSED CONCRETE STEEL BAR DURING THE INITIAL DRAWING PROCESS

Reinforced concrete is a common structural material used in the modern era. High tension steel provides strength to the concrete structure to reduce internal tensile forces and eliminate cracking. Failure of reinforcing steel causes the reinforced concrete to not have optimal strength and has the potential to cause failure in the main structure. The present paper describes the investigation of the failure case in the prestressed concrete bar (PC bar) during the manufacture of prestressed or pre-tensioned concrete. The failure occurred in the head section area during the initial drawing process. The chemical composition and tensile test showed the raw material complied to the manufacturer's specification. The fractography and microscopic examination of the failure area showed the characteristics of brittle fracture. Stress concentration in the head section and the tempering process of PC bar after quenching process at an inappropriate temperature caused temper embrittlement to be the cause of failure during the initial drawing process.

Bond behaviour between additional aluminium alloy ribs anchored CFRP bar and ultra-high ductile concrete

This study investigated the impact of incorporating additional aluminium ribs (ARs) on the bond behaviour of carbon fiber reinforced polymer (CFRP) bars in ultra-high ductile concrete (UHDC). The research conducted pull-out tests on 48 specimens with various parametric variables, including the number and location of ARs, the type of concrete material, and the depth of CFRP ribs. The outcomes revealed that the use of ARs significantly enhanced the bond performance of the CFRP bars in UHDC. The ultimate and residual bond strengths of both CFRP bars and AR anchored CFRP bars in UHDC surpassed those in ordinary concrete. The ARs also improved the rigidity of the bond-slip curve. Additionally, the bond performance of shallow-rib CFRP bars was substantially improved and comparable to that of deep-rib bars. The wave propagation method and wavelet packet analysis were employed to monitor the bond-slip process and to quantitatively analyze the degree of interface damage. The results suggested that increasing the extrusion length and number of ARs can improve the bond performance of the interface. Therefore, incorporating ARs can be considered as an effective approach to enhance the anchorage performance of FRP-reinforced concrete structures.

Enhancement of bond characteristics between sand-coated GFRP bar and normal weight and light-weight concrete using an innovative anchor

To prevent the concrete bar from slipping, a common approach is to ensure an adequate development length or bend it upwards. However, in the case of fiber reinforcement polymer (FRP) bars, on-site bending is not feasible. Therefore, employing a mechanical end-anchor offers an alternative solution to enhance the bond in reinforced concrete. This study investigates the influence of end-anchor shape on ribbed sand-coated glass fiber-reinforced polymer (GFRP) bars, utilizing four different types of mechanical end-anchors. Additionally, the effects of concrete type and compressive strength are evaluated by considering normal-weight concrete (NC) and light-weight concrete (LC) with compressive strengths ranging from 26 MPa to 38 MPa. The study involves the preparation and testing of 60 specimens using a direct pull-out test. The findings demonstrate that the utilization of the proposed mechanical end-anchors enhances the developed tensile stress of the GFRP bar by 10% to 50%, consequently significantly increasing the ultimate capacity of the reinforced concrete section. Furthermore, optimizing stress distribution and improving the bond behavior between the bar and concrete can be achieved by increasing the number of end-anchors and adjusting the distance between them, while maintaining the total length and diameter.

Beyond time: Enhancing corrosion resistance of geopolymer concrete and BFRP bars in seawater

To improve the durability of Basalt fiber reinforced polymer (BFRP) bars reinforced geopolymer concrete (GPC), it is important to study the time-dependent variation of the corrosion resistance ability of GPC and BFRP in a seawater environment. This paper presents an experimental investigation to study the time-dependent mechanical properties and durability of BFRP bars and geopolymer materials synthesized by granulated blast furnace slag (GGBFS), fly ash, and silica fume. The resulting GPC and Portland cement (PC) concrete were exposed to artificial seawater. The mechanical properties of GPC were evaluated by analyzing and comparing the volume expansion and strength loss rates of GPC and PC concrete in an artificial seawater environment. The corrosion resistance of geopolymer (GP) mortar and PC mortar was evaluated by studying the migration ability and pore structure in corrosive ions attack (Cl−, SO42−, Mg2+) in artificial seawater. Moreover, the time-dependent tensile strength of BFRP was comparatively investigated by immersing in different solutions (tap water, artificial seawater, and alkaline simulated seawater). In addition, the dual interface transition zones (ITZs) characteristics of BFRP reinforced GPC under artificial seawater were also investigated by SEM and BSE tests. The results showed that the volume expansion rate and strength loss rate of GPC decreased by 77.6% and 8.7%, respectively, after 360 days of seawater corrosion compared with PC concrete. This enabled the development of a time-dependent strength model of GPC in marine environments. The coefficient of ions diffusion in GP mortar is much lower than that of PC mortar, and GP mortar shows excellent resistance to ion migration. In addition, the effect of seawater corrosion on the tensile strength of BFRP bars increases with the increase of bars' diameter, and the ultimate strengths of BFRP bars with diameters of 6 mm and 8 mm were 695 MPa and 663 MPa, respectively. The tensile strength degradation model of BFRP bars in geopolymer concrete under seawater corrosion was established. After 360 days of seawater immersion, the average porosity of the ITZ between geopolymer and aggregates, and the average porosity of the ITZ between geopolymer and BFRP bars increased insignificantly compared to that of PC concrete. This research can provide a theoretical basis for the service life prediction of BFRP reinforced geopolymer concrete within marine environments.

Early-Age Bond Behavior of Deformed Bars in Concrete Subjected to Bilateral Pressures

In RC structures during construction, the bond behavior of deformed bars in concrete is governed by the age-related concrete strength, the local confining pressure, the bar diameter, and the concrete cover thickness. In this paper, an experimental investigation was performed to quantify the influences of the curing age and the bilateral pressure on the bond behavior of deformed bars in early-age concrete with 204 pull-out specimens for different concrete strengths, bar diameters, and relative concrete cover thicknesses. It was shown that the failure mode was associated greatly with the age-related concrete strength, the concrete cover thickness, and the pressure level, and that the bond strength increased with the pressure level and curing age, whereas the corresponding slip decreased with the curing age. Based on the regression analysis, calculation models were developed to predict the ultimate bond stress and the corresponding slip. A bond–slip model is proposed to describe the bond stress–slip relationship of deformed bars in early-age concrete under bilateral pressures, and the model was verified with the test results.

Study of the mechanism of inhibiting alkali-aggregate reaction and self-healing in concrete materials

Concrete is one of the most widely used materials in the world; however, several pathological manifestations can be associated with the design, execution, and selection of its constituent materials. Cracks, one of these pathological manifestations, are caused by the alkali-aggregate reaction (AAR). This is a chemical phenomenon in which silica, which is present in aggregate, reacts with alkali ions of the cement paste to produce an expansive silica-alkali gel. Herein, the clogging mechanism of the pathological cracks, which form during AAR, was investigated using a reference mortar trace, in order to select innocuous and reactive aggregates. Expansion in the mortar mix was examined using these two types of aggregates: (i) a potentially reactive aggregate, which can promote the formation of pathological cracks during AAR, and (ii) an innocuous aggregate to establish a reference trace without any expansive reaction. Subsequently, a concrete mix was characterized and reactivities of its constituents evaluated. In addition, fine and coarse granulometry analyses were performed. The concrete mix was dosed with the mortar mix, which comprised potentially reactive aggregate, and the bars were molded for the expansion tests. The crystalline mineral additive was added in these concretes to observe the clogging of the cracks. Comparative evaluation of the concrete mix, with and without reactive aggregate, suggested the occurrence of clogging because of the autogenous healing caused by chemical reactions, resulting in a reduction in the expansion of the concrete bars, with crystalline additive, at day 56.

Shrinkage, compressive and bond strengths of alkali activated/cement powder for alternative coating applications

This paper reports the properties of a novel coating material produced from alkali-activated/cement powder (AACP), including setting time, strength development, drying and autogenous shrinkages, and bond strength of one-part AACP paste-coated reinforcing steel bars in concrete. The AACP paste was prepared by activating a mixture of dry geopolymer powder (GPP), Portland cement (PC), and silica fume (SF) with tap water and 2 M sodium hydroxide solution. The effect of PC replacement, sodium silicate-to-sodium hydroxide solution (SS-to-SH) ratio, and pre-curing temperature in the production of GPP on the properties of AACP paste were also investigated. Test results showed that the incorporation of FA and PC activated with an SS-to-SH ratio of 2.0 for the production of GPP decreased their setting time of fresh one-part AACP pastes, whereas their strength development marginally increased. Furthermore, the increasing PC content and SS-to-SH ratio had an adverse effect on drying and autogenous shrinkages. However, the pre-curing temperature had a marginal effect on their shrinkages. In addition, one-part AACP paste-coated reinforcing steel bars positively affected the bond strength of reinforced concrete, especially for the GPP produced by FA without PC. The increased reaction products at the contact zone could assist in improving their bond strength. It can be recommended that the use of FA without PC activated with a low SS-to-SH ratio in the production of GPP would be beneficial in improving shrinkage and bond strength while providing high equivalent substrate stiffness (ks).

Monitoring Corrosion of Steel Bar in Concrete Using Ultrasonic Wave

The problem of steel corrosion in reinforced concrete structure is very serious. The purpose of this paper is to monitor steel corrosion by ultrasonic guided wave. Ultrasonic guided wave monitoring is a kind of dynamic nondestructive testing technology, which has the advantages of real-time, on-line and continuous. In this paper, an ultrasonic in-situ monitoring technique of steel bar corrosion based on embedded piezoelectric ceramic sensor is introduced. Based on this, ultrasonic guided wave was used to monitor mortar samples with different strengths of uniform corrosion, and scanning electron microscopy was used to observe the interface between reinforcement and concrete at different corrosion stages. The results show that the variation trend of ultrasonic guided wave amplitude can reflect the degree of corrosion. Mortar with different strength will affect the time and trend of corrosion. The scanning electron microscopy (SEM) method shows that the changing process of the interface between reinforcement and concrete is consistent with the trend of ultrasonic guided wave.

The Impact of using Rice Husks Ash, Seawater and Sea Sand on Corrosion of Reinforcing Bars in Concrete

The global increase in infrastructure development has led to potential shortages of river sand and tap water, particularly in coastal areas, posing challenges for concrete production. To address this issue, numerous studies have been conducted on the use of seawater and sea sand as sustainable alternatives. Rice Husks Ash (RHA), a fine powder derived from the combustion process of agricultural husks, has emerged as potential eco-friendly solution due to its excellent pozzolanic properties, making it a viable candidate for partially replacing cement in concrete. This substitution enhances concrete durability and strength as well as reduces the risk of corrosion in harsh environmental conditions. Therefore, this study examined impact of seawater for mixing, sea sand and substitution ratio of RHA on corrosion of reinforcing bars. The specimen used in the experiment had a thickness of 150 mm, with rectangular areas of 400 x 400 mm and plain steel bars with a diameter of 10 mm. Corrosion levels and concrete quality were evaluated using the half-cell potential (HCP) method and hammer test, respectively. The results showed that all specimens mixed with seawater, river sand, and different substitution ratios of RHA (0%, 5%, 10%, and 15%) exhibited corrosion, as confirmed by the HCP method. However, only the specimen mixed with tap water, sea sand, and an addition of 5% RHA demonstrated effective resistance to corrosion, comparable to normal concrete.

Calculation of Reinforced Concrete Rod Structures of Bearing Frame Systems of Multi-storey Buildings

In the context of limited land resources and their ever-increasing cost, cities cannot expand immensely in breadth, they increase, and will grow upwards due to the growth in the number of storeys of residential, civil and public buildings. With an increase in the number of storeys, buildings become complex and responsible engineering structures. Thousands of people are simultaneously in a residential and administrative high-rise building, the life and well-being of one or the other openly depend on the knowledge and skills of designers and builders. In this regard, cutting a multi-storey frame frame into prefabricated elements has a significant impact on the technical and economic performance of the building. The mass, geometric dimensions, configuration of prefabricated elements, the manufacturability of their manufacture and installation, as well as the number of assembly units and their connections, the composition of the latter but their functional purpose depend on it. In calculations, the most important phase is the construction of calculation models for individual elements and carrier systems as a whole, at the same time, the calculation model must be elementary and freely implementable using computer technology. At the present stage, calculation methods that take into account cracking and nonlinear compliance of joints to the operation of frames are not sufficiently developed, and the effect of the effect of unloading the middle zone of sections on the feature of the stress-strain state of sections has not been determined even for the stage of work before cracking, due to the unsolved problem of opening the internal static indefinability. The relevance of this article is the formation of a discrete calculation model in the form of the method of concentrated deformations and analytical, using it to calculate reinforced concrete bar structures of the load-bearing systems of multi-storey buildings.

Bond performance of spliced GFRP bars in pre-damaged concrete beams retrofitted with CFRP and UHPC

Fiber-reinforced polymer (FRP) reinforced concrete structures are inevitably subjected to extreme loads. The retrofitting of damaged FRP-reinforced concrete structures will become an important issue, especially with the gradual application of FRP bars in concrete structures. For structures that can be retrofitted, the structural performance is significantly affected by the bond of FRP bars in pre-damaged concrete. For this reason, this paper investigated the effectiveness of retrofit methods of carbon FRP (CFRP) sheet and/or ultra-high performance concrete (UHPC) at restoring the bond strength of glass FRP (GFRP) bars in pre-damaged concrete, and twenty spliced GFRP-reinforced concrete beams were tested under four-point loading. The beams were pre-loaded until bond failure occurred, and then retrofitted and reloaded to assess the restoration of the bond strength. The effects of retrofit methods, spacing of confining stirrups, GFRP bar diameter, and pre-loading type were investigated. The bond strength of the 20 mm and 28 mm diameter GFRP bars was increased by 4~138% and 4~234% after retrofitting with CFRP sheet and/or UHPC, respectively. The modified flexural strength model and modified effective moment of inertia addressing the adverse effect of partial fracture of glass fibers and concrete cracks predicted well the load-carrying capacity and deflection of the retrofitted specimens showing flexural failure, respectively.

Effects of Bamboo Leaf Ash on Alkali-Silica Reaction in Concrete

The construction industry is generally faced with so many challenges of which deterioration in concrete structures caused by Alkali-silica reaction (ASR) is one of the pressing challenges. This reaction induces expansion in concrete, resulting in its eventual cracking and subsequent failure. Research direction is being geared towards obtaining properties of pozzolanic concrete of recently discovered different biogenic pozzolans such as bamboo leaf ash (BLA). BLA has been proven to be acceptable in terms of compressive strength and some other properties but few researches have been performed on the impacts of ASR on BLA concrete structures. This research work focuses on investigating the properties of BLA through X-ray diffraction and fluorescence analyses, and its effectiveness in resisting or eliminating ASR that may be present in concrete. Tests were performed on concrete bars soaked in NaOH at a temperature of 80 oC to determine the possible reactivity of aggregates to ASR. In addition, workability and the compressive strengths of BLA concrete at different percentage levels were determined after curing for 7, 28 and 56 days. The findings of the research show that BLA improves the workability of fresh concrete, however, it causes a decline in the compressive strength of concrete when compared with the strength of conventional concrete. Also, BLA has no detrimental effect on the linear expansion of concrete. This study recommends that a 5% partial replacement of cement with BLA will give effective performance when used in areas where strength is not the major priority. Alkali-silica reaction, bamboo leaf ash, compressive strength, concrete, construction, linear expansion, pozzolans

Bond Behavior of Plain Bars in Concrete under Reversed Cyclic Loading.

Plain bars with a diameter of 10 mm are widely used in reinforced concrete buildings, and the bond behavior between the bars and concrete has an essential effect on the seismic performance of concrete structures. Thus, to assess the safety of old buildings and repaired buildings with normal concrete, it was necessary to further investigate the bond performance of the plain bars in the concrete. The bonding tests under monotonic and reversed cyclic loading were carried out on the specimens reinforced with plain bars, and the influences of concrete grade and embedment length on the bond behavior were taken into consideration. The results indicate the maximum bond stress under reversed cyclic loading is less than that under monotonic loading, and this is the same for corresponding slip for the same test parameters. The concrete compressive strength positively affects the maximum bond stress, whereas the embedment length has a negative effect. Based on the elasticity analysis and test data fitting, the expressions of bond stress at characteristic points on the bond stress-slip curves were carried out. Consequently, the bond stress-slip model was established, which could be applied to calculate the bond stress-slip relationships under monotonic and reversed cyclic loading. By comparison between the test curves and proposed model, a good agreement is observed, which indicates that the proposed model can be used to predict the bond stress-slip curve of plain bars in concrete.