土木工程外文文献

1、精选优质文档-倾情为你奉上精选优质文档-倾情为你奉上专心-专注-专业专心-专注-专业精选优质文档-倾情为你奉上专心-专注-专业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,

2、 Heriot Watt University, Edinburgh(2)VicRoads, Melbourne, VIC, Australia(3)School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, VIC, 3000, AustraliaDavidW.LawEmail: Received: 14January2010Accepted: 14December2010Published online: 23December2010 Abstract This paper rep

3、orts 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.

4、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 st

5、rength at the point where initial surface cracking was observed for bars with confining stirrups. No such increase was observed with unconfined specimens. KeywordsBond-Corrosion-Rebar-Cover-Crack width-Concrete 1 Introduction The corrosion of steel reinforcement is a major cause of the deterioration

6、 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

7、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.

8、 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 co

9、rrosion. 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

10、the steel and the load bearing capacity, which can ultimately impact upon the serviceability of the structure and the structural capacity , . Previous research has investigated the impact of corrosion on bond , , , , , , , with a number of models being proposed , , , , , , , . The majority of this r

11、esearch 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 mecha

12、nical behaviour of corroded steel , and the friction characteristics . However, little research has focused on the relationship between crack width and bond , , , a parameter that can be measured with relative ease on actual structures. The corrosion of the reinforcing steel results in the formation

13、 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

14、 loss of bond capacity could be related to the longitudinal crack width . 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 lo

15、ss of bond of specimens with and without confinement. 2 Experimental investigation 2.1 Specimens Beam end specimens 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. Spe

16、cimens of rectangular cross section were cast with a longitudinal reinforcing bar in each corner, Fig. 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

17、 length. Fig.1Beam end specimen Deformed 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

18、, 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, T

19、able. 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. Ta

20、ble1Concrete mix design MaterialCementw/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. is used to normalize bond strength for non-corroded

21、 specimens as has been used by other researcher . (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

22、 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 , , , , , , , , , , . These have involved experiments using impressed currents or artificial

23、 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 . Research has suggested that curre

24、nt densities up to 200A/cm2 result in similar stresses during the early stages of corrosion when compared to 100A/cm2 . 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. Ho

25、wever, 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

26、 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 (spray

27、ed with salt water) were placed between the metal plates and concrete to provide an adequate contact, Fig. Fig.2Accelerated corrosion system When the required crack width was achieved for a particular bar, the impressed current was discontinued for that bar. The specimen was removed for pullout test

28、ing 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 bre

29、aker 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 hydrau

30、lic jack and a custom-built test rig as shown in Fig. The loading scheme is illustrated in Fig. 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 (acti

31、ng 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 unconfined Fig.4Schematic of loading. Note: only t

32、est bar shown for clarity 3 Experimental results and discussion 3.1 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.). While each specimen had a mean target crack width for eac

33、h 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 p

34、ropagation 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. Th

35、e 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 confin

36、ed 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 obs

37、erve cracks that were aligned vertically down the sideadjacent to one of the links, Fig. Fig.5Typical crack patterns During 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 und

38、er load and ultimately leading to the section failing exhibiting spalling of the top corner/edge, Fig. 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. The appearance of these cracks did not appe

39、ar 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-out Fig.7Diagonal cracking after pull-out The bars were initially (precasting) cleaned with a 12% hydrochloric acid solution,

40、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 t

41、he 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. Figures and show st

42、eel bars with varying degree of corrosion. The majority exhibited visible pitting, similar to that observed on reinforcement in actual structures, Fig. However, a small number of others exhibited significant overall section loss, with a more uniform level of corrosion, Fig., which may be a function

43、of the acceleration methodology. Fig.8Corroded 12mm bar with approximately 30% mass loss Fig.9Corroded 16mm bar with approximately 15% mass loss 3.2 Bond stress and crack width Figure shows the variation of bond stress with mean crack width for 16mmbars and Fig. for the 12mmbars. Figures and show th

44、e data for the maximum crack width. Fig.10Mean crack width versus bond stress for 16 mm bars Fig.11Mean crack width versus bond stress for 12mm bars Fig.12Maximum crack width versus bond stress for 16mm bars Fig.13Maximum crack width versus bond stress for 12mm bars The data show an initial increase

45、 in bond strength for the 12mm specimens with stirrups, followed by a significant decrease in bond, which is in agreement with other authors , . 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 bon

46、d 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 , , have repo

47、rted 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

48、variation on the proposed empirical relationship between bond strength, degree of corrosion, bar size, cover, link details and tensile strength predicted by Rodriguez has been discussed in detail in Tang et al. . The analysis demonstrates that there would be an expected enhancement of bond strength

49、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

50、. 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 a

51、s opposed to the mean crack width (excluding the uncracked confined specimens), Table. Table2Best fit parameters, crack width versus bond strength Unconfined 12mmConfined 12mmUnconfined 16mmConfined 16mmMean crack widthR 2 0.9200.6370.6720.659Slope (m)3.9973.6532.9998.848Intercept (b) 7.5608.1226.49

52、68.746Maximum 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 streng

53、th 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

54、is apparent that (Fig.) 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 correl

55、ation, 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. Howev

56、er, 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 t

57、he 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 stres

58、s for high levels of corrosion. Fig.14Bond stress versus corrosion degree, 12mm bars, unconfined specimen Significantly 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 un

59、confined 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 stre

60、ngth 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 , findings for cracked specimens. In cracked corroded specimens Fang observed a substantial reduction in bon