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山东建筑大学毕业设计外文文献及译文外文文献：Materials and StructuresRILEM201010.1617/s11527-010-9700-yOriginal ArticleImpact of crack width on bond: confined and unconfined rebar DavidW.Law1, DengleiTang2, ThomasK.C.Molyneaux3 and RebeccaGravina3(1)School of the Built Environment, Heriot Watt University, Edinburgh, EH14 4AS, UK(2)VicRoads, Melbourne, VIC, Australia(3)School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, VIC, 3000, AustraliaDavidW.LawEmail: D.W.Lawhw.ac.ukReceived: 14January2010Accepted: 14December2010Published online: 23December2010 AbstractThis paper reports the results of a research project comparing the effect of surface crack width and degree of corrosion on the bond strength of confined and unconfined deformed 12 and 16mm mild steel reinforcing bars. The corrosion was induced by chloride contamination of the concrete and an 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. The results indicated that potential relationship between the crack width and the bond strength. The results also showed an increase in bond strength at the point where initial surface cracking was observed for bars with confining stirrups. No such increase was observed with unconfined specimens.Keywords:bond;corrosion;rebar;cover;crack width;concrete 1 IntroductionThe corrosion of steel reinforcement is a major cause of the deterioration of reinforced concrete structures throughout the world. In uncorroded structures the bond between the steel reinforcement and the concrete ensures that reinforced concrete 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. The deterioration of reinforced concrete is characterized by a general or localized loss of section on the reinforcing bars and the formation of expansive corrosion products. This deterioration can affect structures in a number of ways; the production of expansive products creates tensile stresses within the concrete, which can result in cracking and spalling of the concrete cover. This cracking can lead to accelerated ingress of the aggressive agents causing further corrosion. It can also result in a loss of strength and stiffness of the concrete cover. The corrosion products can also affect the bond strength between the concrete and the reinforcing steel. Finally the corrosion reduces the cross section of the reinforcing steel, which can affect the ductility of the steel and the load bearing capacity, which can ultimately impact upon the serviceability of the structure and the structural capacity 12, 25. Previous research has investigated the impact of corrosion on bond 25, 7, 12, 20, 2325, 27, 29, with a number of models being proposed 4, 6, 9, 10, 18, 19, 24, 29. The majority of this research has focused on the relationship between the level of corrosion (mass loss of steel) or the current density degree (corrosion current applied in accelerated testing) and crack width, or on the relationship between bond strength and level of corrosion. Other research has investigated the mechanical behaviour of corroded steel 1, 11 and the friction characteristics 13. However, little research has focused on the relationship between crack width and bond 23, 26, 28, a parameter that can be measured with relative ease on actual structures. The corrosion of the reinforcing steel results in the formation of iron oxides which occupy a larger volume than that of the parent metal. This expansion creates tensile stresses within the surrounding concrete, eventually leading to cracking of the cover concrete. Once cracking occurs there is a loss of confining force from the concrete. This suggests that the loss of bond capacity could be related to the longitudinal crack width 12. However, the use of confinement within the concrete can counteract this loss of bond capacity to a certain degree. Research to date has primarily involved specimens with confinement. This paper reports a study comparing the loss of bond of specimens with and without confinement. 2 Experimental investigation 2.1 Specimens Beam end specimens 28 were selected for this study. This type of eccentric pullout or beam end type specimen uses a bonded length representative of the anchorage zone of a typical simply supported beam. Specimens of rectangular cross section were cast with a longitudinal reinforcing bar in each corner, Fig.1. An 80mm plastic tube was provided at the bar underneath the transverse reaction to ensure that the bond strength was not enhanced due to a (transverse) compressive force acting on the bar over this length. Fig.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 confined specimens had three sets of 6mm stainless steel stirrups equally spaced from the plastic tube, at 75mm centres. This represents four groups of specimens with a combination of different bar diameter and with/without confinement. The specimens were selected in order to investigate the influence of bar size, confinement and crack width on bond strength. 2.2 Materials The mix design is shown, Table1. The cement was Type I Portland cement, the aggregate was basalt with specific gravity 2.99. The coarse and fine aggregate were prepared in accordance with AS 1141-2000. Mixing was undertaken in accordance with AS 1012.2-1994. Specimens were cured for 28days under wet hessian before testing. Table1Concrete mix designMaterialCementw/cSand10mm washed aggregate7mm washed aggregateSaltSlumpQuantity381kg/m3 0.49517kg/m3 463kg/m3 463kg/m3 18.84kg/m3 14025mmIn order to compare bond strength for the different concrete compressive strengths, Eq.1 is used to normalize bond strength for non-corroded specimens as has been used by other researcher 8. (1)where is the bond strength for grade 40 concrete, exptl is the experimental bond strength and f c is the experimental compressive strength. The tensile strength of the 12 and 16mm steel bars was nominally 500MPa, which equates to a failure load of 56.5 and 100.5kN, respectively. 2.3 Experiment methodology Accelerated corrosion has been used by a number of authors to replicate the corrosion of the reinforcing steel happening in the natural environment 2, 3, 5, 6, 10, 18, 20, 24, 27, 28, 30. These have involved experiments using impressed currents or artificial weathering with wet/dry cycles and elevated temperatures to reduce the time until corrosion, while maintaining deterioration mechanisms representative of natural exposure. Studies using impressed currents have used current densities between 100A/cm2 and 500mA/cm2 20. Research has suggested that current densities up to 200A/cm2 result in similar stresses during the early stages of corrosion when compared to 100A/cm2 21. As such an applied current density of 200A/cm2 was selected for this studyrepresentative of the lower end of the spectrum of such current densities adopted in previous research. However, caution should be applied when accelerating the corrosion using impressed current as the acceleration process does not exactly replicate the mechanisms involved in actual structures. In accelerated tests the pits are not allowed to progress naturally, and there may be a more uniform corrosion on the surface. Also the rate of corrosion may impact on the corrosion products, such that different oxidation state products may be formed, which could impact on bond. The steel bars served as the anode and four mild steel metal plates were fixed on the surface to serve as cathodes. Sponges (sprayed with salt water) were placed between the metal plates and concrete to provide an adequate contact, Fig.2. Fig.2Accelerated corrosion systemWhen the required crack width was achieved for a particular bar, the impressed current was discontinued for that bar. The specimen was removed for pullout testing when all four 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 measured at 20mm intervals along the length of the bar, beginning 20mm from the end of the (plastic tube) bond breaker using an optical microscope. The level of accuracy in the measurements was 0.02mm. Measurements of crack width were taken on the surface normal to the bar direction regardless of the actual crack orientation at that location. Bond strength tests were conducted by means of a hand operated hydraulic jack and a custom-built test rig as shown in Fig.3. The loading scheme is illustrated in Fig.4. A plastic tube of length 80mm was provided at the end of the concrete section underneath the transverse reaction to ensure that the bond strength was not enhanced by the reactive (compressive) force (acting normal to the bar). The specimen was positioned so that an axial force was applied to the bar being tested. The restraints were sufficiently rigid to ensure minimal rotation or twisting of the specimen during loading. Fig.3Pull-out test, 16mm bar unconfinedFig.4Schematic of loading. Note: only test bar shown for clarity3 Experimental results and discussion3.1 Visual inspection Following the accelerated corrosion phase each specimen was visually inspected for the location of cracks, mean crack width and maximum crack width (Sect.2.3). While 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 being a dynamic process with cracks propagating at different rates. Thus, while individual bars were disconnected, once the target crack width had been achieved, corrosion and crack propagation continued (to some extent) until all bars had achieved the target crack width and pull out tests conducted. This resulted in a range of data for the maximum and mean crack widths for the pull out tests. The visual inspection of the specimens showed three stages to the cracking process. The initial cracks occurred in a very short period, usually generated within a few days. After that, most cracks grew at a constant rate until they reached 1mm, 34weeks after first cracking. After cracks had reached 1mm they then grew very slowly, with some cracks not increasing at all. For the confined and unconfined specimens the surface cracks tended to occur on the side of the specimens (as opposed to the top or bottom) and to follow the line of the bars. In the case of the unconfined specimens in general these were the only crack while it was common in the cases of confined specimens to observe cracks that were aligned vertically down the sideadjacent to one of the links, Fig.5. Fig.5Typical crack patternsDuring the pull-out testing the most common failure mode for both confined and unconfined was splitting failurewith the initial (pre-test) cracks caused by the corrosion enlarging under load and ultimately leading to the section failing exhibiting spalling of the top corner/edge, Fig.6. However for several of the confined specimens, a second mode of failure also occurred with diagonal (shear like) cracks appearing in the side walls, Fig.7. The appearance of these cracks did not appear to be related to the presence of vertical cracks observed (in specimens with stirrups) during the corrosion phase as reported above. Fig.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 washed in distilled water again. Following the pull-out tests, the corroded bars were cleaned in the same way and weighed again. The corrosion degree was determined using the following equation where G 0 is the initial weight of the steel bar before corrosion, G is the final weight of the steel bar after removal of the post-test corrosion products, g 0 is the weight per unit length of the steel bar (0.888 and 1.58g/mm for 12 and 16mmbars, respectively), l is the embedded bond length. Figures8 and 9 show steel bars with varying degree of corrosion. The majority exhibited visible pitting, similar to that observed on reinforcement in actual structures, Fig.9. However, a small number of others exhibited significant overall section loss, with a more uniform level of corrosion, Fig.8, which may be a function of the acceleration methodology. Fig.8Corroded 12mm bar with approximately 30% mass lossFig.9Corroded 16mm bar with approximately 15% mass loss3.2 Bond stress and crack width Figure10 shows the variation of bond stress with mean crack width for 16mmbars and Fig.11 for the 12mmbars. Figures12 and 13 show the data for the maximum crack width. Fig.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 barsThe data show an initial increase in bond strength for the 12mm specimens with stirrups, followed by a significant decrease in bond, which is in agreement with other authors 12, 15. For the 16mm specimens an increase on the control bond stress was observed for specimens with 0.28 and 0.35mm mean crack widths, however, a decrease in bond stress was observed for at the mean crack width of 0.05mm. The 12mm bars with stirrups displayed an increase in bond stress of approximately 25% from the control values to the maximum bond stress. An increase of approximately 14% was observed for the 16mm specimens. Other researchers 17, 24, 25 have reported enhancements of bond stress of between 10 and 60% due to confinement, slightly higher to that observed in these experiment. However the loading techniques and cover depths have not all been the same. Variations in experimental techniques include a shorter embedded length and a lower cover. The variation on the proposed empirical relationship between bond strength, degree of corrosion, bar size, cover, link details and tensile strength predicted by Rodriguez 24 has been discussed in detail in Tang et al. 28. The analysis demonstrates that there would be an expected enhancement of bond strength due to confinement of approximately 25%corresponding to a change of bond strength of approximately 0.75MPa for the 16mm bars (assessed at a 2% section loss). For the 12mm bars the corresponding effect of confinement is found to be approximately 35% corresponding to a 1.0MPa difference in bond stress. The experimental results (14 and 25%, above) are 6070% of these values. Both sets of data indicate a relationship showing decreasing bond strength with (visible surface) crack width. A regression analysis of the bond strength data reveals a better linear relationship with the maximum crack width as opposed to the mean crack width (excluding the uncracked confined specimens), Table2. Table2Best fit parameters, crack width versus bond strengthUnconfined 12mmConfined 12mmUnconfined 16mmConfined 16mmMean crack widthR 2 0.9200.6370.6720.659Slope (m)3.9973.6532.9998.848Intercept (b) 7.5608.1226.4968.746Maximum crack widthR 2 0.9370.8550.7140.616Slope (m)2.7192.9681.8155.330Intercept (b) 7.8058.4036.7079.636There was also a significantly better fit for the unconfined specimens than the confined specimens. This is consistent with the observation that in the unconfined specimens the bond strength will be related to the bond between the bars and the concrete, which will be affected by the level of corrosion present, which itself will influence the crack width. In confined specimens the confining steel will impact upon both the bond and the cracking. 3.3 Corrosion degree and bond stress It is apparent that (Fig.14) for corrosion degrees less than 5% the bond stress correlated well. However, as the degree of corrosion increased there was no observable correlation at all. This contrasts with the relationship between the observed crack width and bond stress, which gives a reasonable correlation, even as crack widths increase to 2 and 2.5mm. A possible explanation for this variation is that in the initial stages of corrosion virtually all the dissolved iron ions react to form expansive corrosion products. This reaction impacts on both the bond stress and the formation of cracks. However, once cracks have been formed it is possible for the iron ions to be transported along the crack and out of the concrete. As the bond has already been effectively lost at the crack any iron ions dissolving at the crack and being directly transported out of the concrete will cause an increase in the degree of corrosion, but not affect the surface crack width. The location, orientation and chemistry within the crack will control the relationship between bond stress and degree of corrosion, which will vary from specimen to specimen. Hence the large variations in corrosion degree and bond stress for high levels of corrosion. Fig.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 observed crack for unconfined specimens was 2.5mm compared to 1.4mm for the confined specimens. This is as expected and is a direct result of the confinement which limits the degree of cracking. 3.4 Effect of confinement The unconfined specimens for both 16 and 12mm bars did not display the initial increase in bond strength observed for the confined bars. Indeed the unconfined specimens with cracks all displayed a reduced bond stress compared to the control specimens. This is in agreement with other authors 16, 24 findings for cracked specimens. In cracked corroded specimens Fang observed a substantial reduction in bond strength for deformed bars without stirrups, while Rodriguez observed bond strengths of highly corroded cracked specimens without stirrups were close to zero, while highly corroded cracked specimens with stirrups retained bond strengths of between 3 and 4MPa. In uncorroded specimens Chana noted an increase in bond strength due to stirrups of between 10 and 20% 14. However Rodriguez and Fang observed no variation due to the presence of confinement in uncorroded bars. The data is perhaps unexpected as it could be anticipated that the corrosion products would lead to an increase in bond due to the increase in internal pressures, caused by the corrosion products increasing the confinement and mechanical interlocking around the bar, coupled with increased roughness of the bar resulting in a greater friction between the bar and the surrounding concrete. However, these pressures would then relieved by the subsequent cracking of the concrete, which would contribute to the decrease in the bond strength as crack widths increase. A possible hypothesis is that due to the level of cover, three times bar diameter, the effect of confinement by the stirrups is reduced, such that it has little impact on the bond stress in uncracked concrete. However, once cracking has taken place the confinement d
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