Deformed Steel Rebars Research Articles

Cyclic loading test on a locally post-tensioned precast concrete beam–column connection

A novel precast concrete beam–column connection locally post-tensioned using arc-shaped prestressing bars was proposed for satisfactory seismic performance and rapid construction. Three full-scale cruciform specimens, including one monolithic reference specimen, were tested under reversal cyclic loadings to evaluate the seismic behaviours. Grade 630 steel rods and high-strength deformed steel rebars were used for the arc-shaped prestressing bars in the precast specimens. The results show that the proposed precast connection presents an acceptable seismic performance and that the structural details should be ameliorated to improve the energy dissipation capacity. The design philosophy of strong column-weak beam is applicable to the new precast system. Finally, a strut-and-tie model was developed to investigate the force transfer mechanism of the novel precast connection.

Bond performance of steel rebar embedded in 80–180 MPa ultra-high-strength concrete

Ultra-high-strength concrete (UHSC) has attracted attention from engineers because of its great capability on reducing the size of reinforced concrete columns. However, the bond performance of steel rebar embedded in UHSC has not been examined enough yet, although it is a fundamental information for structural design. So, this study comprehensively investigates the bond performance of deformed steel rebar embedded in high-strength concrete (HSC), very-high-strength concrete (VHSC), and UHSC with compressive strengths ranging from 80 to 180 MPa. Different bar diameters (12.7, 15.9, and 19.1 mm), embedment lengths (0.5, 1.0, and 1.5 × bar diameter), cover depths (1, 2, and 3 × bar diameter and center), steel fiber volume fractions (0, 0.5, 1.0, and 1.5%), and yield strengths of steel rebar (normal-strength vs. high-strength) were all considered. Test results indicate that the average bond strength increased significantly with an increase in the compressive strength of the concrete and decreased slightly with an increase in the bar diameter. Average bond strengths of steel rebars in HSC, VHSC, and UHSC were all increased by increasing the embedment length. The cover depth marginally influenced the bond strength when pullout failure was generated, and it significantly increased the bond strength when splitting failure occurred. The incorporation of steel fibers most effectively increased the bond strength in the UHSC mixture, compared with the HSC and VHSC mixtures. Lastly, the widely used prediction models for average bond strength were not accurate for concrete with a compressive strength greater than 80 MPa; thus, a new, appropriate model needs to be proposed in the near future.

Experimental investigations on the bond behavior between concrete and FRP reinforcing bars

This study focuses on the experimental characterization of the bond behavior between concrete and Fiber Reinforced Polymer (FRP) reinforcing bars (rebars). Pull-out tests were performed on glass, carbon, and aramid FRP rebars, as well as on deformed steel rebars. The influence of various parameters on the bond behavior was studied, such as the type of fibers, the diameter of the FRP bars and their surface geometry. Scanning-Electron-Microscope (SEM) observations were performed to precisely study the sand coating characteristics of these rebars. A main originality of the proposed approach relied on the instrumentation of pull-out samples using Distributed Optical Fiber Sensing (DOFS) instrumentation. Such a distributed measurement system provided access to the longitudinal strain distribution along the rebar near the rebar-concrete interface, and then made it possible to determine the effective development length of the various types of rebars considered in this study. As the introduction of DOFS instrumentation may be intrusive, its influence on the interface behavior was also discussed.

Bond Behavior of Pretensioned Strand Embedded in Ultra-High-Performance Fiber-Reinforced Concrete

This study aimed to investigate the bond properties of prestressing strands embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC). Toward this end, two types of prestressing strands with diameters of 12.7 and 15.2 mm were considered, along with various concrete cover depths and initial prestressing force magnitudes. The average bond strength of the strands in UHPFRC was estimated by using pullout tests, and the transfer length was evaluated based on a 95% average maximum strain method. Test results indicated that the average bond strength of the pretensioned strand reduced as the diameter of the strand increased, and was between the bond strengths of round and deformed steel rebars. Higher bond strength was also obtained with a lower embedment length. Based on a comparison of p value, the bar diameter and embedment length most significantly influenced the bond strength of strands in UHPFRC, compared to a ratio of cover depth to diameter and initial prestressing force. Pretensioned strands in UHPFRC exhibited much higher bond strength and shorter transfer length compared with strands embedded in ordinary high-strength concrete. Lastly, ACI 318 and AASHTO LRFD codes significantly overestimated the transfer length of the strands embedded in UHPFRC.

Bonding Behavior of Deformed Steel Rebars in Sustainable Concrete Containing both Fine and Coarse Recycled Aggregates.

In order to assess the bond behavior of deformed steel rebars in recycled-aggregate concrete (RAC) incorporating both fine and coarse recycled aggregate, pull-out tests were carried out in this study on 16-mm diameter deformed steel rebars embedded concentrically in RAC. The concrete was designed using equivalently mixed proportions of both recycled coarse aggregate and recycled fine aggregate. The tests employed five types of recycled aggregate replacement combinations and three types of rebar placement orientation (i.e., vertical bars and two-tiered and three-tiered horizontal bars). Based on the pull-out test results, the maximum bond strength tended to decrease and the slip at the maximum bond strength increased as the average water absorption of the aggregate increased, irrespective of the rebar orientation or placement location within the concrete member. The pull-out test results for the horizontal steel rebars embedded in RAC indicate that the casting position effect could be determined from the mid-depth of the concrete member, irrespective of the member’s height. The normalized bond versus slip relationship between the deformed rebar and the RAC could be predicted using an empirical model based on regression analysis of the experimental data.

Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

Abstract This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative.

Unified Model of Local Bond between Deformed Steel Rebar and Concrete: Indentation Analogy Theory and Validation

AbstractTwo distinct groups of bond models were developed on the basis of dominant concrete splitting and rebar–concrete interfacial failures, respectively. Their accuracy highly depended on the specific test data sets selected in analysis and validation. In this study, a unified model of local bond is proposed and developed on the basis of the indentation analogy at the rib front of the deformed rebar and on the concrete plastic deformation around the rebar. Unlike existing models, the unified model is completely validated with all sets of test data available in the literature. It can accurately predict the transition mechanism of failure modes from concrete splitting to rebar pullout. The predicted bond strength is in excellent agreement with experimental results as the rib spacing-to-height ratio changes in practical range.

Chemically reactive enamel coating of steel rebar for enhanced durability of reinforced concrete structures

In this paper, the corrosion performances of three types of enamel coating (pure, mixed and double enamel) applied on smooth and deformed steel rebar are compared based on the electrochemical impedance spectroscope (EIS) tests in 3.5 wt.% NaCl solution. The effects of mortar cover, rebar deformation, enamel coating and coating thickness variation on the corrosion resistances of steel rebar are investigated. Mortar cover increases the corrosion resistance of tested specimens. The coated deformed rebar has lower corrosion resistances than the coated smooth rebar due to the variation in coating thickness around rebar ribs. Due to the isolated nature of air bubbles trapped during high temperature firing, pure and double enamel coatings are more corrosion resistant and more sensitive to the coating thickness than the mixed enamel coating with interconnected pores. Therefore, pure and double enamel coatings are recommended for the design of reinforced concrete structures in corrosive environments.

Numerical prediction for bond stress-slip relationship between deformed steel rebar and recycled coarse aggregate concrete

To assess the bond behavior of deformed steel bars in recycled aggregate concretes (RACs), pullout test curves were compared with the curves predicted using the equation proposed by Haraji and Guo. The test curves are well predicted by the numerical manner, which demonstrates that the overall bond stress-slip relationship of deformed steel rebar in RAC could be expressed numerically. For each recycled aggregate replacement level, the value of parameter a for the ascending branch is

Bond behavior of sand-coated deformed glass fiber reinforced polymer rebars

Bond behavior of sand-coated deformed glass fiber reinforced polymer rebars was experimentally investigated in this study. A total of 84 specimens, including 30 beam specimens, 48 Losberg pullout specimens, and six RILEM/CEB/FIP pullout specimens, were constructed and tested under monotonic loading. Two failure modes including pullout failure and splitting failure were observed. The pullout failure occurred when the embedment length of specimens was less than 5 times glass fiber reinforced polymer rebar diameter, while the splitting failure occurred when embedment length was more than 5 times rebar diameter. The specimens with the embedment length of 5 times bar diameter failed either by glass fiber reinforced polymer rebars pulling out or by concrete splitting. The results also showed that bond strength of sand-coated deformed glass fiber reinforced polymer rebars was, on average, 59–127% that of the deformed steel rebars. With higher concrete strength, higher bond strength of sand-coated deformed glass fiber reinforced polymer rebars was obtained. When rebar diameter or embedment length increased, bond strength of sand-coated deformed glass fiber reinforced polymer rebars decreased. Based on these results, formulas for the bond strength and the development length of glass fiber reinforced polymer rebars were proposed, considering the effects of concrete strength, bar diameter, embedment length, and rebar outer surface.

Bond performance of deformed steel bars in concrete produced with coarse recycled concrete aggregate

Results of an experimental study on the bond performance of deformed steel rebars in concrete made with coarse recycled concrete aggregate (RCA) are presented. The distinctive feature of the study ...

초고성능 콘크리트(UHPC)의 부착특성에 관한 연구

초고성능 콘크리트(ultra high performance concrete, UHPC)는 종래의 보통 콘크리트와 다른 새로운 재료로써 높은 강도와 향상된 인성을 그 특징으로 한다. 이러한 새로운 재료의 활용을 위하여 이 연구에서 초고성능 콘크리트의 부착 성능을 평가하고자 하였다. 수정된 RILEM 방법을 사용하여 초고성능 콘크리트와 이형 철근의 인발실험(pull-out test)을 수행하였으며 보통 콘크리트와 비교하여 5~10배에 달하는 부착강도를 확인하여 기존의 설계 기준에 비하여 현저하게 감소한 정착길이와 피복 두께를 제안하였다. 700 ㎫급 고장력 철근의 실험 결과의 비교로부터 초고성능 콘크리트에서 고강도 철근 활용의 유효성을 확인하였다. 강연선의 응력전달길이 측정실험을 통하여 현재 전달길이 기준이 UHPC의 경우 매우 보수적이라는 것을 확인하였다. 또한 유한요소해석을 통하여 실험 결과를 검증하였다.

The effect of fibers on the variation of bond between steel reinforcement and concrete with casting position

Bond to plain and deformed steel rebars was measured by pull-out testing along the height of wall-shaped specimens. The performances of steel, polypropylene and glass fiber reinforced concrete were compared with non-fiber concrete. The results did not indicate significant difference in the behavior of the concretes with plain bars. The differences in the bond with deformed bar were correlated with the cracking patterns as a function of fiber type and casting position. Steel fiber concrete had the highest bond reduction with concrete depth despite the highest bond strength, which was attributed to the segregation of steel fibers.

STUDY ON TENSION-STIFFENING EFFECT OF ECC

In order to investigate the tension-stiffening effect of reinforced Engineered Cementitious Composite (ECC), which is one of the High Performance Fiber-Reinforced Cementitious Composite (HPFRCC), uniaxial tension test is carried out for the specimens with one deformed steel rebar arranged. Bond analysis between ECC and rebar, in which fiber bridging effect of ECC at crack is considered, is conducted under the assumption of the bond stress - slippage relationship. Analysis results show that the tension-stiffening effect between ECC and steel rebar is relatively small rather than bridging action of ECC. It is also confirmed that bridging stress is smaller than crack strength of ECC as the average and strain capacity of ECC increases due to restriction of crack localization.

Cyclic Behavior of RC Columns Strengthened by FRP and Steel Devices

This paper presents several results of a wide experimental program undertaken to evaluate the seismic performance of RC columns, externally confined with fiber-reinforced polymers (FRPs). Specimens consisted of full scale square columns, 300 mm on each side, reinforced by using both smooth and deformed steel rebars and subjected to a constant axial load and monotonic or cyclic flexure; in particular, two levels of the axial load were considered. Tests were conducted, in displacement control, on both FRP confined and unconfined RC columns: the confinement system was obtained by partially wrapping two or four unidirectional carbon (CFRP) or glass (GFRP) fiber layers around the member; additionally, some tests were performed on members strengthened by using both the external CFRP wrapping and longitudinal steel angles. Experimental results allowed evaluating depending on the level of applied axial load and on the type of steel rebars adopted for reinforcing each column—the benefits introduced by the considered strengthening systems in terms of strength, ductility, and energy dissipation capacity.

Effect of High Temperature on Bond Strength of FRP Rebars

The bond properties of fiber reinforced polymer (FRP) reinforcing bars (rebars) at temperatures ranging from room temperature (20°C) to high temperatures of up to 250°C are discussed in this paper. The bond properties in this temperature range were studied for a number of commercially produced rebars, where different bond “treatments” were applied to FRP rebars. Test results showed a reduction of between 80 and 90% in the bond strength as the temperature increased from 20 to 250°C. In comparison, ordinary deformed steel rebars showed a reduction of only 38% in the same temperature range. In addition, a reduction in the bond stiffness, which was determined from the slope of the ascending branch of the pullout load versus slip curve, was seen as the temperature increased. At elevated temperatures the postpeak bond decrease was gradual as compared with the instantaneous drop at room temperature. Greater sensitivity to high temperatures was seen in FRP rebars, in which the bond relies mainly on the polymer treatment at the surface of the rod.