Bond fatigue of epoxy coated reinforcing bars

This paper reports the findings of a study concerning bond fatigue of epoxy coated reinforcing bars in comparison to that of normal mill scale bars and blast cleaned bars. There were two principle objectives of the series of fatigue tests. The first was to determine whether the bond-slip performance of epoxy coated reinforcement is significantly different from that of normal mill scale bars and blast cleaned bars when subjected to numerous cycles of loading in a working stress range. The second was to determine whether the static bond strength of epoxy coated reinforcement is significantly different from that of normal mill scale and blast cleaned bars after being subjected to numerous cycles of loading in a working stress range. The test results indicate no detrimental influence of the epoxy coating in relation to bond fatigue. The static bond strength of epoxy coated bars without cyclic loading was found to be slightly less than that of mill scale bars.

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Résumé On décrit les résultats d'une étude sur l'adhérence en fatigue de fers d'armature revêtus de résine époxy en comparaison avec des fers calaminés et des fers décapés. Le premier objectif de ces séries d'essais de fatigue était de déterminer si la performance adhérence glissement des armatures revêtues d'époxy est notablement différente de celle des fers calaminés décapés soumis à un grand nombre de cycles de charge dans un domaine de contrainte utile. Le second objectif était de déterminer si l'adhérence statique des fers revêtus d'époxy est notablement différente de celle des fers normaux après avoir été soumis à un grand nombre de cycles de chargement dans un domaine de contrainte utile. Les résultats d'essai n'indiquent pas d'influence nuisible du revêtement époxy par rapport à l'adhérence en fatigue. On a établi que l'adhérence statique des fers revêtus d'époxy était légèrement moindre que celle des barres calaminées en l'absence de sollicitations cycliques.

     

Bond behaviour of reinforcing bars in self-compacting concrete: experimental determination by using beam tests

The force transfer between concrete and reinforcement depends on the bond characteristics between the two materials. Bond influences crack widths and the deflections of RC members and also determines anchorage and splice lengths. Self-compacting concrete (SCC) is characterized by a smaller amount of coarse aggregates when compared to conventional vibrated concrete. To investigate whether this change in mix design has a significant influence on the bond behaviour, a series of beam tests (according to RILEM recommendation RC6) was performed. A total of 36 specimens were cast using 3 different concrete types: one conventional vibrated concrete (CVC) and two powder-type SCC’s. The bond stress—slip behaviour of reinforcing bars with diameters ranging from 12 to 40 mm has been recorded. From these tests, it can be concluded that the bond strength of SCC is even higher than it is for CVC for small bar diameters, but the difference becomes smaller for larger bar diameters.     

变形钢筋/超高性能混凝土搭接黏结性能

超高性能混凝土(UHPC)是一种高强度、高韧性和高耐久性的水泥基复合材料。为了研究钢筋/UHPC的搭接黏结性能,进行了21组考虑搭接长度、纤维掺量和配箍率影响的钢筋搭接对拉拔出试验,3组考虑锚固长度影响的钢筋直接拔出锚固试验;试验出现了劈裂拔出破坏和钢筋拉断破坏2种破坏模式;钢筋/UHPC平均黏结强度随钢筋埋置长度的增大而减小,随配箍率的增大而增大;钢纤维掺量的增大,有利于增大对UHPC的约束作用,增加配箍率和适当增大纤维掺量均能减小钢筋/UHPC的临界搭接长度;结合前人的试验结果,拟合得到平均锚固和搭接黏结强度计算公式及临界锚固和搭接长度计算公式,根据混凝土结构设计规范,建立了钢筋/UHPC锚固和搭接长度简化算法,计算结果较为准确。      

Failure of reinforcing concrete steel ribbed bars

In order to analyse the causes of unexpected failures occurred in steel ribbed bars used in the transportation of concrete slabs for maritime works purposes, a detailed study of the material’s properties has been carried out. Full mechanical, compositional, metallographic and fractographic analyses have been performed; complementary measurements of residual stresses by X-ray diffraction techniques and finite element modelling of the in-service conditions were decisive in casting some light on the problem. The combination of maritime environment exposure and high residual stress levels associated to severe bendings could be responsible for the characteristic stress corrosion cracking failures reported in the bars.     

Bond of cement grouted reinforcing bars under constant radial pressure

A ribbed rebar or rock bolt, grouted with Portland cement is the most common type of reinforcement in geomechanical projects such as tunnels, rock slopes and foundations. Due to the frictional nature of the bond slip the normal stress acting on the rebar is the most important parameter controlling the bond capacity of the reinforcement. The more the confining stress, the higher would be the mobilized load bearing capacity of the system. To quantify this effect, series of laboratory tests were designed to study the effect of confining pressure on the bond capacity. A modified triaxial Hoek cell (usually used for testing the strength of rock samples under radial confining pressure) was used to facilitate application of a “constant radial confining pressure” to the grouted sample while pulling the bolt axially through the cement annulus. During test, axial load and displacement of the bolt as well as the radial dilation of the grout was recorded and stored in computer using a data acquisition system. The results show a non-linear relation between the increase of bond capacity and confining pressure. The radial dilation is quantified also as a function of confining pressure. A peak-residual behaviour is also a characteristic of these results which shows the importance of limiting the deformation of rock blocks to avoid entering into the post peak range of the reinforcement with low values of bond.     

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.     

Experimental investigation on the bond of reinforcing bars in high performance concrete under cyclic loading

The bond performance between reinforcing bars and concrete under cyclic loads were experimentally investigated in this paper. In particular, the effects of steel fibers and hybrid fibers (including steel fibers and carbon fibers) on␣the bond performance were analyzed. Based on the experimental results, the deterioration laws of the peak bond stress, the stiffness of the unloading branch, frictional bond resistance and remnant ultimate bond stress under cyclic loads were summarized. Meanwhile, the increasing law of the remnant slip and the effect of the loading order on the deterioration of the bond performance were studied. Then, two damage factors d _u and D _u were introduced, where d _u is a factor that reflects the damage of the peak bond stress under constant controlling displacement and D _u is also a factor that reflects the damage of the remnant ultimate bond stress under changeable controlling displacement. Making use of the two damage factors and other key variables, the bond constitutive relationships between reinforcing bars and concrete under cyclic loads could be expressed by formulas. The computed τ–s relationships agreed well with those obtained by experiments.     

Anchorage capacity of reinforcing bars in autoclaved aerated concrete lintels

This paper is a study of the shear capacity of autoclaved aerated concrete lintels reinforced with unbonded bars. A set of tests was performed to measure the anchorage capacity of the beam ends. Full lintels were also tested to failure under transverse loading. The comparison of the two sets of tests shows that the lintels fail in a brittle mode when the anchorage capacity of the longitudinal bars is reached. The mechanism of anchorage by tie-bearing is described. Two variations of the reinforcement details are tested and discussed.

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Résumé Cet article présente une étude sur la résistance au cisaillement de poutres en béton cellulaire autoclavé en l'absence d'adhésion entre les barres de renforcement et le béton. Une série d'essais a été menée pour mesurer la capacité d'ancrage à l'extrémité des poutres. En complément, une série d'essais de surcharge latérale de poutres a été effectuée. La comparaison des résultats des deux séries montre que la rupture des poutres était de type fragile lorsque la capacité d'ancrage des barres était atteinte. Le mécanisme d'ancrage par pression des ligatures est décrit. Deux variations des types d'armatures ont été testées et les résultats des essais interprétés.

     

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 strength of reinforcing steel embedded in fly ash-based geopolymer concrete

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel.     

Monitoring freshly poured concrete using ultrasonic waves guided through reinforcing bars

Durability and strength of mature concrete can be judged a great deal from its properties when it is freshly poured. This paper demonstrates an ultrasonic in-situ monitoring technique for freshly poured concrete. The solidification and curing of freshly poured concrete is monitored through the propagation of ultrasonic waves in waveguides such as steel reinforcing bars. As concrete solidifies and cures, more wave energy escapes into the surrounding concrete resulting in signal attenuation. RC beam specimens are monitored with carefully selected ultrasonic signal patterns during the first 24 h of setting of concrete. Destructive tests such as bar pull out and compressive strength are also performed at different stages of setting of concrete. The ultrasonic signals have been calibrated for determination of early age concrete properties.     

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.     

Effect of transverse reinforcement on bond strength of reinforcing bars in silica fume concrete

The objectives of the research program were to investigate the effect of transverse reinforcement on the bond-slip characteristics of tension lap splices in high performance silica fume concrete, to study the validity of the upper limit of 70 MPa imposed by the ACI Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of tension lap splices embedded in high strength concrete and confined with transverse reinforcement. Twelve beam specimens were tested. Each beam specimen included two bars in tension, spliced at the center of the span. The beams were designed in way that bars would fail in bond, splitting the concrete conver in the splice region, before reaching the yield splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume and the amount of confinement over the splice region.     

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.     

Variation of strength and stiffness of fibre reinforced polymer reinforcing bars with temperature

This paper presents the results of tensile mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Detailed experimental studies were conducted to determine the strength and stiffness properties of FRP bars at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester bars of 9.5 mm diameter and glass fibre reinforced polyester bars of 9.5 mm and 12.7 mm diameter were considered. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Data from the experiments was used to illustrate the comparative variation of tensile strength and stiffness of different types of FRP reinforcing bars with traditional steel reinforcing bars. Also, results from the strength tests were used to show that temperatures of about 325 °C and 250 °C appear to be critical (in terms of strength) for GFRP and CFRP reinforcing bars, respectively. A case study is presented to illustrate the application of critical temperatures for evaluating the fire performance of FRP-reinforced concrete slabs.     

Bond between steel reinforcement bars and Electric Arc Furnace slag concrete

This paper deals with the bond between steel reinforcement and recycled aggregate concrete, including Electric arc furnace (EAF) slag as full replacement of natural coarse aggregates. Pull-out tests were carried out according to RILEM standard on specimens made with six concrete mixtures, characterized by different w/c ratios and types of aggregates. Plain and ribbed steel reinforcement bars were used to observe the influence of steel roughness. Experimental bond-slip relationships were analyzed, and results show similar bond mechanisms between the reference and EAF concrete specimens. Significant bond strength enhancement is observed in concretes with low w/c ratio, when EAF slag is used as recycled coarse aggregate. Experimental results in terms of bond strength were also compared to analytical predictions, obtained with empirical formulations.     

全玻璃纤维增强树脂筋混凝土电缆排管的抗弯试验

提出了采用全玻璃纤维增强树脂基复合材料（GFRP）筋混凝土电缆排管代替传统的钢筋混凝土电缆排管,该结构形式具有减少能耗的优点。通过对小尺寸和足尺GFRP筋混凝土电缆排管试件进行抗弯性能试验,研究其抗弯能力、变形及破坏特征等。试验结果表明,GFRP筋混凝土电缆排管具有与普通钢筋混凝土梁相似的力学特征,以混凝土开裂为分界点,位移-荷载曲线表现为双线性,排管侧面拉应力分布不均匀,部分区域出现了较高拉应力。提出了GFRP筋混凝土电缆排管的抗弯设计计算方法,理论计算结果与试验测试结果较为吻合。     

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.