Steel–concrete bond strength of lightweight self-consolidating concrete

The bond behavior of lightweight self-consolidating concrete (LWSCC) must be understood in order to use this type of high performance concrete in structural members. The objective of this research program is to assess the bond behavior of reinforcing steel bars embedded in LWSCC members. Three different classes of LWSCC mixtures were developed with two different types of lightweight aggregates. In addition, one normal weight SCC (NWSCC) was developed and used as a control mixture. A total of twenty four pullout tests were conducted on deformed reinforcing bars with an embedded length of either 100 or 200 mm and the load-slip responses, failure modes and bond strengths of LWSCC and NWSCC were compared. Based on the results of this study, the bond strength of deformed bars for LWSCCs are found to be less (between 16 and 38%) as compared with NWSCC. Under the conditions of equivalent workability properties and compressive strength, bond slip properties were shown to be significantly influenced by the type of lightweight aggregate used. In this study, the use of expanded shale in the production of LWSCC significantly enhanced the pullout strength when compared with lightweight slag aggregate.     

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.     

Evaluation of bond quality at the interface between steel bar and concrete using the small-dimension break-off test

This paper investigates the feasibility of using the small-dimension break-off test for evaluation of the bond quality at the interface between steel bar and concrete. Experimental studies were performed on bar-type concrete specimens and reinforced concrete beams. Twelve bar-type concrete specimens containing plain and deformed steel bars with different diameters were used to develop the relationship between the break-off moment and the adhesive strength at the steel bar/concrete interface. Subsequently, three reinforced concrete beams containing normal reinforcing bars, epoxy-coated reinforcing bars, and bars smeared with oil to simulate various adhesive conditions at the bar/concrete interface were used to study how the break-off moment and the bond strength were affected by the different adhesive conditions. In addition, two beam specimens containing normal reinforcing bars were vibrated severely on a self-made shaking table shortly after initial setting of concrete to simulate the bond damage in fresh reinforced concrete beams due to unexpected vibration or impact. Experimental results show that the effective break-off moment has a good correlation with the adhesive strength at the interface between steel bar and concrete. The break-off moment increases with an increase in bond strength. It is demonstrated that the small-dimension break-off test is capable of evaluating damage at the steel bar/concrete interface.     

Bond behavior of GFRP and steel bars in ultra-high-performance fiber-reinforced concrete

The bond behavior of glass fiber-reinforced polymer (GFRP) and steel bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated according to embedment length and bar diameter. Post-peak bond stress-slip softening curve of the GFRP bars was obtained, and a wedging effect was quantitatively evaluated. Test results indicated that a normalized bond strength of 5 was applicable for steel bars embedded in UHPFRC, and the development lengths of normal- and high-strength steel bars were determined to be 2 and 2.5 times the bar diameter, respectively. The GFRP bars exhibited approximately 70% lower bond strength than the steel bars, and the bond stress additionally applied by the wedging effect increased almost linearly with respect to the slip. Based on dimensionless bond stress and slip parameters, an appropriate theoretical model for the bond stress and slip relationship of steel bars in UHPFRC was suggested, and it was verified through comparison with the test data.     

Bond of ribbed galvanized reinforcing steel in concrete

The ASTM beam end test (ASTM A944) has been used to compare the bond and slip behaviour of deformed (i.e. ribbed) galvanized, epoxy-coated and black steel bars in concrete. The objective was to determine whether galvanizing adversely affects bond strength. From a series of thirty specimens, the average bond strength of black steel and galvanized steel reinforcement used in these tests has been determined and bond stress has been shown to act uniformly over the embedded bar area. A slip value of approximately 0.4 mm has been confirmed to be associated with bond failure by concrete splitting. The results indicated that while epoxy coating resulted in a significant loss in bond strength of the order of 20% compared to black steel, there is no adverse effect on bond with the use of galvanized steel. Chromate treatment of galvanized bars is deemed unnecessary since there was no evidence of long term reduction in bond due to the possible effects of hydrogen gas evolution resulting from the reaction between zinc and wet concrete.     

Bond between deformed reinforcement and normal and high-strength concrete with and without fibers

This paper describes an experimental study that consisted of pullout tests of deformed reinforcing bars in NSC and HSC specimens, with and without hooked-end steel fibers. Two types of test setups were applied, direct and flexural tests, and three bar diameters were tested (8, 12 and 20 mm). The experimental setups were based on standard RILEM pullout (direct) and beam tests, with several modifications. The experimental program included study of the effects of concrete strength and inclusion of steel fibers on the bond strength, as well as the influence of bar geometry and concrete cover. Discussion of the results shows coupling of these effects and proposes an empirical expression that represents this coupling. The results from the current study are also compared with the design bond strengths specified in American and European standards as well as a known model.     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Bar-concrete bond in mixes containing calcium carbide residue,fly ash and recycled concrete aggregate

A mixture of calcium carbide residue and fly ash (CRFA) is an innovative new binder for concrete instead of using ordinary Portland cement (OPC). Therefore, this study aims at investigating the bond interaction between common steel reinforcing bars and the aforementioned concrete. To this end, both CRFA and OPC concretes using crushed limestone and recycled concrete aggregate (RCA) as a coarse aggregate were prepared to investigate the bond strength of smooth and deformed bars by pull-out tests. The bond stress−slip relationships were also identified to determine the effects of CRFA binder and RCA on the bond strength behavior. The results indicate that the values the of bond-slip behavior and bond strengths of steel bar in CRFA concretes are similar to those embedded in OPC concrete. Moreover, the bond strength was significantly affected by RCA and the types of steel bar. Although the concretes had the same compressive strengths, the deformed bar embedded in CRFA concrete with RCA had a lower bond strength than the one with crushed limestone. However, the reduction in bond strength of the CRFA concrete with RCA was still less than that of OPC concrete with RCA. For the CRFA concretes, the bond strengths of the deformed bars were approximately 1.7–3.6 times higher than that of smooth bars.     

Bond behaviour between recycled aggregate concrete and deformed steel bars

The results of thirty pullout tests carried out on 8 and 10 mm diameter deformed steel bars concentrically embedded in recycled aggregate concrete designed using equivalent mix proportions with coarse recycled concrete aggregate (RCA) replacement percentages of 0, 25, 50, 75 and 100 % are reported towards investigation of bond behaviour of RCA concrete. Bond strengths of the natural aggregate concrete and the RCA concrete was found to be comparable, particularly for the 10 mm rebars, and the RCA replacement percentage had an insignificant effect on peak bond stress values. However, for both the bar sizes, when the measured bond strengths were normalized with the respective compressive strengths, then the normalized bond strengths so obtained across all the RCA replacement percentages were higher for the RCA concrete compared to the natural coarse aggregate concrete. Further, higher normalized bond strength values were obtained for the 8 mm rebars compared to the 10 mm bars. An empirical bond stress versus slip relationship between RCA concrete and deformed steel bars has been proposed on the basis of regression analysis of the experimental data and it is conservatively suggested that anchorage lengths of 8 and 10 mm diameter deformed bars in RCA concrete may be taken the same as in natural aggregate concrete.     

Numerical calibration of bond law for GFRP bars embedded in steel fibre-reinforced self-compacting concrete

An experimental program was carried out at the Laboratory of Structural Division of the Civil Engineering Department of the University of Minho (LEST-UM) to investigate the bond behaviour of glass fibre reinforced polymer (GFRP) bars embedded in steel fibre reinforced self-compacting concrete (SFRSCC) for the development of an innovative structural system. Thirty-six pull-out-bending tests were executed to assess the influence of the bond length, concrete cover, bar diameter and surface treatment on the bond of GFRP bars embedded in SFRSCC. This paper reports the results of a numerical study aiming to identify an accurate GFRP–SFRSCC bond–slip law. Thus, the above mentioned pullout bending tests were simulated by using a nonlinear finite element (FE) constitutive model available in FEMIX, a FEM based computer program. The bond–slip relationship adopted for modelling the FE interface that simulates the interaction between bar and concrete is the key nonlinear aspect considered in the FE analyses, but the nonlinear behaviour of SFRSCC due to crack initiation and propagation was also simulated. The evaluation of the values of the relevant parameters defining such a bond–slip relationship was executed by fitting the force versus loaded end slip responses recorded in the experimental tests. Finally, correlations are proposed between the parameters identifying the bond–slip relationship and the relevant geometric and mechanical properties of the tested specimens.     

锈胀开裂钢筋混凝土粘结疲劳性能试验研究

重复加载和锈胀开裂均会导致钢筋混凝土粘结性能退化,进而对钢筋混凝土构件力学性能产生不利影响。该文开展了一系列偏心拔出试验,研究了重复加载以及锈胀开裂对粘结滑移性能的耦合影响规律。主要研究变量包括重复加载次数、应力水平以及钢筋锈蚀程度。结果表明:重复加载对非锈蚀试件和锈胀开裂试件粘结强度及峰值滑移没有显著影响,但会导致钢筋和混凝土之间不断累积残余滑移。重复加载后,粘结应力-滑移曲线形态特征与单调加载试件相似。该文还发现表面锈胀裂缝宽度对粘结强度、峰值滑移以及残余滑移的增长规律有明显影响。锈胀开裂会导致钢筋混凝土试件粘结疲劳寿命显著下降。基于试验数据及文献中研究结论,该文建立了重复及单调荷载作用下非锈蚀及锈胀开裂试件的局部粘结应力-滑移本构关系模型,推导得到了粘结疲劳寿命预测模型。     

Rebar pullout testing under dynamic Hopkinson bar induced impulsive loading

The dynamic behavior of the bond slip between a deformed reinforcing bar and plain concrete has been experimentally investigated by employing Hopkinson bar techniques. Pullout tests with various specimen types (unconfined, confined, cast-in-place, post-installed etc.) have been performed. Pullout of the steel rebar and splitting of the concrete cylinder have been the failure modes induced. Test results comprise peak pullout forces and complete bond stress–slip diagrams. They clearly show that the dynamic pullout forces and curves are well above the static ones, and that the pullout work of bond failure is considerably greater for the dynamic impact loading. Confinement, provided by a steel tube, leads to improved bonding; peak loads increase up to 2.5 times. The effects of bond length and concrete strength have also been put into evidence. Finally it has been verified that post-installed rebars, depending upon the particular adhesive employed, can achieve the same bond resistance as the cast-in-place ones.     

型钢高强高性能混凝土梁粘结滑移行为研究

通过12榀不同混凝土强度等级、剪跨比、配箍率的实腹式型钢高强高性能混凝土简支梁试验,得到了各级荷载作用下沿梁长度(即跨度)方向型钢应变、型钢与混凝土界面粘结应力与相对滑移分布规律。结果表明:在界面粘结应力尚未达到局部粘结强度之前,型钢应变和界面滑移近似呈指数变化;此后界面出现粘结软化,且粘结软化区段随荷载的增加逐渐向试件自由端扩散,型钢与混凝土协同工作的能力降低。通过对界面粘结滑移行为与诸影响因素的分析,从机理上揭示了粘结软化的形成与演化过程,分析了局部粘结强度与其主要影响因素的关系,并由试验结果统计回归得出了型钢高强高性能混凝土局部粘结强度的计算公式,进而提出型钢高强高性能混凝土梁局部粘结应力与加载端粘结滑移本构关系的数学模型。最后建立了型钢与混凝土界面粘结滑移分析的理论模型,求解了各级荷载作用下粘结应力的传递规律。研究成果为改进型钢高强高性能混凝土梁设计计算理论和有限元分析提供了试验依据。     

Evaluation of the bond behavior of steel reinforcing bars in recycled fine aggregate concrete

This paper describes pullout test results on deformed reinforcing bars in natural and recycled fine aggregate (RFA) concrete. The effects of bar location and RFA grade on bond strength between reinforcing bar and recycled aggregate concrete (RAC) were evaluated through the experimental program. A total of 150 pullout specimens were fabricated for the experiment. Two reinforcing bar orientations were considered with respect to the casting direction; vertical bars and horizontal bars, the latter of which was prepared to evaluate top-bar effect. Considered variables included four RFA replacement ratios (RFArs), two water-absorption grades (RFA-A: 5.83%, RFA-B: 7.95%) of RFA and three reinforcing bar locations (75, 225 and 375 mm height from the bottom of the casting mold). In addition, to evaluate the thermal and aging effect on bond behavior between the reinforcing bar and RFA concrete, some parts of pullout specimens had exposed to rapid freeze–thaw environment or been cured at air during 28 or 730 days. Test results demonstrated that bond strength does not seem to be affected by the RFAr for higher RFA grades (RFA-A), at least up to 60% RFAr. In contrast, the RAC including lower RFA grade (RFA-B) showed clear decreases in bond strength with increasing RFAr, similar to the trend observed for compressive strength. For horizontal pullout specimens, RFA concrete specimens showed higher bond strength gap between top and bottom bars than natural aggregate concrete (NAC) specimens. Bond strengths of the horizontally cast pullout specimens were affected by the flowability of concrete rather than the RFAr or RFA grade. No noticeable degradation occurred during freeze–thaw cycling of the RAC specimens, indicating that the RFA used in this study is appropriate for use in freeze–thaw environments.     

Bond failure performances between near-surface mounted FRP bars and concrete for flexural strengthening concrete structures

Near-surface mounted (NSM) fiber reinforced polymer (FRP) has been established as an effective technique for strengthening concrete member. In preview literatures, bond failure was observed usually in the strengthened beam test for increasing flexural capacity. Bond behavior is of primary importance for the transfer of stress between the concrete and the FRP reinforcement to develop composite action. In this paper, a total of 22 tests were conducted to study the bond failure performance between NSM FRP bars and concrete besides only one test as a comparison. Failure modes, load–deflection curves, strain distribution of FRP bars, and local bond stresses at the FRP-epoxy adhesive interface from the tests were analyzed in detail. Some of the factors expected to affect bond performance were presented, namely: diameter of FRP bars, type to FRP material, concrete compressive strength and bonded length. The test results reported in this paper should be useful for further establishing local bond–slip constitute relationship and further verification of numerical simulation models, in addition to gaining a better understanding of bond failures for flexural strengthening concrete structures with NSM FRP bars.     

微锈蚀钢筋混凝土高温后粘结锚固性能试验研究

为研究高温对锈蚀钢筋混凝土结构粘结锚固性能的影响,对锈蚀试件(锈蚀率为1.08%)与非锈蚀试件先进行高温试验(20℃、200℃、400℃、600℃、800℃),再进行中心拔出试验。试验结果表明:随温度的升高,锈蚀试件与非锈蚀试件粘结强度均呈下降趋势,与非锈蚀试件相比,锈蚀试件在温度不超过400℃时,其粘结强度下降趋势较为平缓。分析了高温作用后混凝土抗压强度、钢筋极限强度、钢筋与混凝土间粘结强度三者之间的关系,并建立了考虑不同温度、不同锈蚀率等因素影响下钢筋与混凝土的粘结-滑移关系式。     

Experimental Investigation of Bond Stress and Deformations in LWAC Ties Reinforced with GFRP Bars

The structures' durability is an engineering concern for a long time but has been increased in the last years. Lightweight aggregate concrete (LWAC) combined with glass fibre reinforced polymer bars allows to create structures with high performance in terms of durability. The glass fibre reinforced polymer (GFRP) bars have different ribs from those of steel bars, and consequently, its bond to concrete is affected. Moreover, the Young's modulus of GFRP is much below compared with that of steel, and this influences significantly the behaviour of structural elements reinforced with this material. This paper presents an experimental study focused on bond between LWAC and reinforcing bars of GFRP. Thirty‐six pull‐out tests were carried out using steel and GFRP bars. These reinforcements were combined with three types of concrete, all with the same design density 1900 kg m^?3 but with different values of compressive strength: 35, 55 and 70 MPa. Furthermore, 12 reinforced ties were tested, combining different types of bars (steel and GFRP), two different diameters (12 and 16 mm) and the three types of LWAC. Based on experimental results, several relations were established to understand the behaviour of LWAC structures reinforced with GFRP bars, mainly in the serviceability conditions. These results point out that ties deformation and crack width are very affected by the reduced Young's modulus of GFRP: deformations and crack width of ties reinforced with GFRP are significantly higher, approximately three times greater, compared with those of ties reinforced with steel. The tension stiffening effect was also analysed in detail, and it was found that it is slightly influenced by the concrete compressive strength but is highly dependent of the Young's modulus of the reinforcing material.     

Bond over direct support of deformed rebars in normal and high strength concrete with and without fibers

This paper describes results of the second stage of a research on the bonding of deformed bars in normal and high strength concrete with and without fibers. The current study focused on the effect of confinement conditions, which are present over a direct support of a reinforced concrete beam, and compares these results with data obtained in the first stage of the research. The experimental work consisted of pullout tests, which were based on the standard RILEM flexural bond test (??beam test??) of deformed steel bars with diameters of 12 and 20?mm. Results revealed an increase in the normalized bond strength due to support conditions. An unstable post-peak slip of the tested rebar was observed when located over the support, whereas bars bonded near mid-span exhibited controlled bond?Cslip throughout the test. Up to peak load, however, bond?Cslip relations exhibited higher energy absorption over the support than near mid-span. Inclusion of fibers had only a minor effect on bond strength but had a significant influence on pre-peak energy absorption. A previously developed model of bond strength shows good agreement with current results when applying an increase factor due to the support effect.     

Effects of thaumasite on bond strength of reinforcement in concrete

The conditions necessary for the formation of thaumasite are well known and much work is in progress to identify concrete mixes resistant to thaumasite form of sulfate attack (TSA). However, there have been no data to indicate how TSA affects the nature and strength of the bond between reinforcement steel and concrete and hence the load capacity of reinforced concrete elements.

During works to repair and strengthen the thaumasite-affected Tredington–Ashchurch Overbridge in Gloucestershire, sections of column were removed and placed in storage. These column sections presented an opportunity to perform pullout tests on full size TSA-affected structural elements and unaffected control specimens from the same structure. In total 63 pullout tests were performed on plain round reinforcement bars embedded in two unaffected and four TSA-affected reinforced concrete elements. The sections were also characterised in terms of estimated in situ cube strength and depth of softened zone.

A statistical analysis of the experimental results indicates that the bond of the plain round reinforcement bars in the unaffected concrete exceeded that of the plain round reinforcement bars in the TSA-affected concrete. TSA reduced the mean experimental bond coefficient by 24% for corner bars and 10% for other bars, representing an average reduction in mean experimental bond coefficient of 15% for all bars.     

高强钢丝绳网片-聚合物砂浆加固RC板抗爆性能试验研究

为了研究高强钢丝绳网片-聚合物砂浆对钢筋混凝土(RC)板的抗爆加固效果,对5块加固RC板和1块未加固RC板进行了野外现场爆炸试验,研究了砂浆强度、钢丝绳间距、钢丝绳预应力和界面增设销钉等因素对试件的破坏形态、裂缝分布及发展、跨中位移、钢筋应变等影响规律,并对爆炸试验后的试件进行了剩余承载力试验和爆炸损伤评估。研究表明:高强钢丝绳网片-聚合物砂浆加固能显著提高RC板的抗爆性能,相比于未加固板,加固板的裂缝宽度,板底跨中的峰值位移、残余位移和钢筋应变均大幅减小;加固后,构件剩余承载力大幅增加,其损伤程度大为降低。