Original ArticleImpact of crack width on bond: confined and unconfined rebarDavid w1, Denglei Tang2, Thomas K. C.Molyneaux3 and Rebecca Gravina3(1)School of the Built Environment, Heriot Watt University, Edinburgh, EH144AS, UK(2)VicRoads, Melbourne, VIC,Australia(3)School of Civil, Environmental and Chemical Engineering, RMITUniversity, Melbourne, VIC, 3000, AustraliaDavid W. LawEmail:*************.ukReceived: 14January2010Accepted: 14Decemb er2010Published online: 23December2010 AbstractThis paper reports the results of a research project comp aring the effect of surface crack width and degree of corrosi on on the bond strength of confined and unconfined deforme d 12 and 16mm mild steel reinforcing bars. The corrosion was induced by chloride contamination of the concrete andan applied DC current. The principal parameters investigated were confinement of the reinforcement, the cover depth, bar diameter, degree of corrosion and the surface crack width. T he results indicated that potential relationship between the cra ck width and the bond strength. The results also showed an increase in bond strength at the point where initial surface cr acking was observed for bars with confining stirrups. No suc h increase was observed with unconfined specimens.Keywords:bond;corrosion;rebar;cover;crack width;concrete1 IntroductionThe corrosion of steel reinforcement is a major cause of the deterioration of reinforced concrete structures throughoutthe world. In uncorroded structures the bond between the st eel reinforcement and the concrete ensures that reinforced co ncrete acts in a composite manner. However, when corrosion of the steel occurs this composite performance is adversely affected. This is due to the formation of corrosion products on the steel surface, which affect the bond between the steel and the concrete.2 Experimental investigation2.1 SpecimensFig.1Beam end specimenDeformed rebar of 12 and 16mm diameter with cover of three times bar diameter were investigated. Duplicate sets of confined and unconfined specimens were tested. The con fined specimens had three sets of 6mm stainless steel stirr ups equally spaced from the plastic tube, at 75mm centres.This represents four groups of specimens with a combin ation of different bar diameter and with/without confinement. The specimens were selected in order to investigate the infl uence of bar size, confinement and crack width on bond stre ngth.2.2 MaterialsTable1Concrete mix designMateri al Cement w/c Sand10 mmwashedaggregate7 mmwashedaggregateSalt SlumpQuantit y 381 kg/m30.49517 kg/m3463 kg/m3463 kg/m318.84 kg/m3140 ± 25 mm(1)where is the bond strength for grade 40 concrete, τexptl is the experimental bond strength and f c is the experi mental compressive strength.The tensile strength of the Φ12 and Φ16mm steel ba rs was nominally 500MPa, which equates to a failure load of 56.5 and 100.5kN, respectively.2.3 Experiment methodologyFig.2Accelerated corrosion systemWhen the required crack width was achieved for a parti cular bar, the impressed current was discontinued for that bar. The specimen was removed for pullout testing when all fo ur locations exhibited the target crack width. Average surface crack widths of 0.05, 0.5, 1 and 1.5mm were adopted as the target crack widths. The surface crack width was measu red at 20mm intervals along the length of the bar, beginni ng 20mm from the end of the (plastic tube) bond breaker using an optical microscope. The level of accuracy in the m easurements was ±0.02mm. Measurements of crack width were taken on the surface normal to the bar direction regardl ess of the actual crack orientation at that location.Fig.3Pull-out test, 16mm bar unconfinedFig.4Schematic of loading. Note: only test bar show n for clarity3 Experimental results and discussion3.1 Visual inspectionWhile each specimen had a mean target crack width for each bar, variations in this crack width were observed prior to pull out testing. This is due to corrosion and cracking b eing a dynamic process with cracks propagating at different r ates. Thus, while individual bars were disconnected, once the target crack width had been achieved, corrosion and crack p ropagation continued (to some extent) until all bars had achi eved the target crack width and pull out tests conducted. Thi s resulted in a range of data for the maximum and mean cr ack widths for the pull out tests.Fig.5Typical crack patternsFig.6Longitudinal cracking after pull-outFig.7Diagonal cracking after pull-outThe bars were initially (precasting) cleaned with a 12% hydrochloric acid solution, then washed in distilled water and neutralized by a calcium hydroxide solution before being w ashed in distilled water again. Following the pull-out tests, th e corroded bars were cleaned in the same way and weighed again.The corrosion degree was determined using the followin g equationwhere G 0 is the initial weight of the steel bar before corrosion, G is the final weight of the steel bar after remova l of the post-test corrosion products, g 0 is the weight per u nit length of the steel bar (0.888 and 1.58g/mm for Φ12 and Φ16mm bars, respectively), l is the embedded bond length.3.2 Bond stress and crack widthFig.10Mean crack width versus bond stress for 16 mm barsFig.11Mean crack width versus bond stress for 12mm barsFig.12Maximum crack width versus bond stress for 16mm barsFig.13Maximum crack width versus bond stress for 12mm barsTable2Best fit parameters, crack width versus bondThere was also a significantly better fit for the unconfin ed specimens than the confined specimens. This is consistent with the observation that in the unconfined specimens the b ond strength will be related to the bond between the bars an d the concrete, which will be affected by the level of corros ion present, which itself will influence the crack width. In c onfined specimens the confining steel will impact upon both the bond and the cracking.3.3 Corrosion degree and bond stressFig.14Bond stress versus corrosion degree, 12mm bars, unconfined specimenSignificantly larger crack widths were observed for the unconfined specimens, compared to the confined specimens with similar levels of corrosion and mass lost. The largest o bserved crack for unconfined specimens was 2.5mm compa red to 1.4mm for the confined specimens. This is as expe cted and is a direct result of the confinement which limits t he degree of cracking.3.4 Effect of confinementThe data is perhaps unexpected as it could be anticipate d that the corrosion products would lead to an increase in b ond due to the increase in internal pressures, caused by the corrosion products increasing the confinement and mechanical interlocking around the bar, coupled with increased roughnes s of the bar resulting in a greater friction between the bar a nd the surrounding concrete. However, these pressures would then relieved by the subsequent cracking of the concrete, w hich would contribute to the decrease in the bond strength a s crack widths increase. A possible hypothesis is that due to the level of cover, three times bar diameter, the effect of c onfinement by the stirrups is reduced, such that it has little impact on the bond stress in uncracked concrete. However, o nce cracking has taken place the confinement does have a b eneficial effect on the bond.It may also be that the compressive strength of the con crete combined with the cover will have an effect on the bo nd stresses for uncorroded specimens. The data presented her e has a cover of three times bar diameter and a strength of 40MPa, other research ranges from 1.5 to four times cover with compressive strengths from 40 to 77MPa.3.5 Comparison of 12 and 16mm rebarThe maximum bond stress for 16mm unconfined bars was measured at 8.06MPa and for the 12mm bars it w as 8.43MPa. These both corresponded to the control speci mens with no corrosion. The unconfined specimens for both the 12 and 16mm bars showed no increase in bond stress due to corrosion. For the confined specimens the maximum bond stress for the control specimens were 7.29MPa for t he 12mm bars and 6.34MPa for the 16mm bars. The maximum bond stress for both sets of confined specimens c orresponded to point of the initial cracking. The maximum b ond stresses were observed at a mean crack width of 0.01mm for the 12mm bars and 0.28mm for the 16m m bars. The corresponding bond stresses were, 8.45 and 7.2 0MPa. Overall the 12mm bars displayed higher bond stresses compared to the 16mm bars at all crack widths. Thi s is attributed to a different failure mode. The 16mm spec imens demonstrate splitting failure while the 12mm bars b ond failure.3.6 Effect of casting positionFig.15Bond stress versus mean crack width for 12mm bars, top and bottom cast positions, confined specime n4 ConclusionsA relationship was observed between crack width and b ond stress. The correlation was better for maximum crack wi dth and bond stress than for mean crack width and bond str ess.Confined bars displayed a higher bond stress at the poin t of initial cracking than where no corrosion had occurred. As crack width increase the bond stress reduced significantly.Unconfined bars displayed a decrease in bond stress at i nitial cracking, followed by a further decrease as cracking in creased.Top cast bars displayed a higher bond stress in specime ns with no corrosion. Once cracking had occurred no variati on between top and bottom cast bars was observed.The 12mm bars displayed higher bond stress values th an 16mm with no corrosion, control specimens, and at sim ilar crack widths.A good correlation was observed between bond stress an d degree of corrosion was observed at low levels of corrosio n (less than 5%). However, at higher levels of corrosion no correlation was discerned.Overall the results indicated a potential relationship betw een the maximum crack width and the bond. Results shown herein should be interpreted with caution as this variation ma y be not only due to variations between accelerated corrosio n and natural corrosion but also due to the complexity of th e cracking mechanism in reality.中文译文：约束和无约束的钢筋对裂缝宽度的阻碍收稿日期：2010年1月14 纳稿日期：2010年12月14日线上发表时刻：2010年1月23日摘要本报告公布了局限约束和自由的变形对粘结强度12、16毫米钢筋的表面腐蚀程度和裂纹阻碍的比较结果。