土木工程外文文獻及翻譯

1、-. z.外文文獻：Materials and StructuresRILEM201010.1617/s11527-010-9700-yOriginal ArticleImpact of crack width on bond: confined and unconfined rebar DavidW.Law1, DengleiTang2, ThomasK.C.Molyneau*3 and RebeccaGravina3(1)School of the Built Environment, Heriot Watt University, Edinburgh, EH14 4AS, UK(2)Vi

2、cRoads, Melbourne, VIC, Australia(3)School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, VIC, 3000, AustraliaDavidW.LawEmail: Received: 14January2010Accepted: 14December2010Published online: 23December2010AbstractThis paper reports the results of a research project pa

3、ring 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 c

4、onfinement 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 cr

5、acking 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

6、 the world. In uncorroded structures the bond between the steel reinforcement and the concrete ensures that reinforced concrete acts in a posite manner. However, when corrosion of the steel occurs this posite performance is adversely affected. This is due to the formation of corrosion products on th

7、e 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 e*pansive corrosion products. This deterioration can affect structures in a numb

8、er of ways; the production of e*pansive 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 a

9、nd 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

10、ultimately impact upon the serviceability of the structure and the structural capacity HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR12#CR12 12, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR25#CR25 25. Previous research has investi

11、gated the impact of corrosion on bond HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR2#CR2 2 HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR5#CR5 5, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR7#CR7

12、 7, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR12#CR12 12, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR20#CR20 20, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR23#CR23 23 HYPERLINK springer.li

13、./content/u754755ru8h1h068/fullte*t.html l CR25#CR25 25, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR27#CR27 27, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR29#CR29 29, with a number of models being proposed HYPERL

14、INK ./content/u754755ru8h1h068/fullte*t.html l CR4#CR4 4, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR6#CR6 6, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR9#CR9 9, HYPERLINK ./co

15、ntent/u754755ru8h1h068/fullte*t.html l CR10#CR10 10, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR18#CR18 18, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR19#CR19 19, HYPERLINK ./content/u754755ru8h1h068/f

16、ullte*t.html l CR24#CR24 24, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR29#CR29 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 accel

17、erated 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 HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR1#CR1 1, HYPERLINK springer.lib.tsinghua.ed

18、u./content/u754755ru8h1h068/fullte*t.html l CR11#CR11 11 and the friction characteristics HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR13#CR13 13. However, little research has focused on the relationship between crack width and bond HYPERLINK springer.lib.tsinghua.

19、edu./content/u754755ru8h1h068/fullte*t.html l CR23#CR23 23, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR26#CR26 26, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR28#CR28 28, a parameter that can be measured with relative ease on a

20、ctual structures. The corrosion of the reinforcing steel results in the formation of iron o*ides which occupy a larger volume than that of the parent metal. This e*pansion creates tensile stresses within the surrounding concrete, eventually leading to cracking of the cover concrete. Once cracking oc

21、curs 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 HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR12#CR12 12. However, the use of confinement within the concrete can

22、 counteract this loss of bond capacity to a certain degree. Research to date has primarily involved specimens with confinement. This paper reports a study paring the loss of bond of specimens with and without confinement. 2 E*perimental investigation 2.1 Specimens Beam end specimens HYPERLINK spring

23、./content/u754755ru8h1h068/fullte*t.html l CR28#CR28 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

24、cast with a longitudinal reinforcing bar in each corner, Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig1#Fig1 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 (transve

25、rse) pressive 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

26、 stirrups equally spaced from the plastic tube, at 75mm centres. This represents four groups of specimens with a bination 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 strengt

27、h. 2.2 Materials The mi* design is shown, Table HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Tab1#Tab1 1. 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 114

28、1-2000. Mi*ing was undertaken in accordance with AS 1012.2-1994. Specimens were cured for 28days under wet hessian before testing.Table1Concrete mi* designMaterialCementw/cSand10mm washed aggregate7mm washed aggregateSaltSlumpQuantity381kg/m30.49517kg/m3463kg/m3463kg/m318.84kg/m314025mmIn order to p

29、are bond strength for the different concrete pressive strengths, Eq. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Equ1#Equ1 1 is used to normalize bond strength for non-corroded specimens as has been used by other researcher HYPERLINK ./conte

30、nt/u754755ru8h1h068/fullte*t.html l CR8#CR8 8.(1)where is the bond strength for grade 40 concrete, e*ptl is the e*perimental bond strength and f c is the e*perimental pressive strength. The tensile strength of the 12 and 16mm steel bars was nominally 500MPa, which equates to a failure load of 56.5 a

31、nd 100.5kN, respectively. 2.3 E*periment methodology Accelerated corrosion has been used by a number of authors to replicate the corrosion of the reinforcing steel happening in the natural environment HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR2#CR2 2, HYPERLINK

32、./content/u754755ru8h1h068/fullte*t.html l CR3#CR3 3, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR5#CR5 5, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR6#CR6 6, HYPERLINK ./conten

33、t/u754755ru8h1h068/fullte*t.html l CR10#CR10 10, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR18#CR18 18, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR20#CR20 20, HYPERLINK ./content/u754755ru8h1h068/fullt

34、e*t.html l CR24#CR24 24, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR27#CR27 27, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR28#CR28 28, HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR30#CR30 30.

35、 These have involved e*periments 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 e*posure. Studies using impressed currents have used current densi

36、ties between 100A/cm2 and 500mA/cm2 HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l CR20#CR20 20. Research has suggested that current densities up to 200A/cm2 result in similar stresses during the early stages of corrosion when pared to 100A/cm2 HYPERLINK springer.lib.t

37、./content/u754755ru8h1h068/fullte*t.html l CR21#CR21 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 t

38、he corrosion using impressed current as the acceleration process does not e*actly 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 impa

39、ct on the corrosion products, such that different o*idation state products may be formed, which could impact on bond. The steel bars served as the anode and four mild steel metal plates were fi*ed on the surface to serve as cathodes. Sponges (sprayed with salt water) were placed between the metal pl

40、ates and concrete to provide an adequate contact, Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig2#Fig2 2. Fig.2Accelerated corrosion systemWhen the required crack width was achieved for a particular bar, the impressed current was discontinued for that bar. The

41、 specimen was removed for pullout testing when all four locations e*hibited 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

42、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 conduc

43、ted by means of a hand operated hydraulic jack and a custom-built test rig as shown in Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig3#Fig3 3. The loading scheme is illustrated in Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html

44、 l Fig4#Fig4 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 (pressive) force (acting normal to the bar). The specimen was positioned so that an a*ial force was appli

45、ed 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 E*perimental results and discussion3.1 Visual inspection

46、Following the accelerated corrosion phase each specimen was visually inspected for the location of cracks, mean crack width and ma*imum crack width (Sect. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Sec5#Sec5 2.3). While each specimen had a mean target crack width f

47、or 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 c

48、rack propagation continued (to some e*tent) until all bars had achieved the target crack width and pull out tests conducted. This resulted in a range of data for the ma*imum and mean crack widths for the pull out tests. The visual inspection of the specimens showed three stages to the cracking proce

49、ss. 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

50、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 mon in the cases of confined specimens to

51、observe cracks that were aligned vertically down the sideadjacent to one of the links, Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig5#Fig5 5. Fig.5Typical crack patternsDuring the pull-out testing the most mon failure mode for both confined and unconfined was

52、 splitting failurewith the initial (pre-test) cracks caused by the corrosion enlarging under load and ultimately leading to the section failing e*hibiting spalling of the top corner/edge, Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig6#Fig6 6. However for seve

53、ral of the confined specimens, a second mode of failure also occurred with diagonal (shear like) cracks appearing in the side walls, Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig7#Fig7 7. The appearance of these cracks did not appear to be related to the pres

54、ence 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

55、 and neutralized by a calcium hydro*ide 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 equationwhere G 0 is the initial weight of the steel

56、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 HYPERLINK ./conte

57、nt/u754755ru8h1h068/fullte*t.html l Fig8#Fig8 8 and HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig9#Fig9 9 show steel bars with varying degree of corrosion. The majority e*hibited visible pitting, similar to that observed on reinforcement in actual structures, Fig.

58、 HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig9#Fig9 9. However, a small number of others e*hibited significant overall section loss, with a more uniform level of corrosion, Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig8#Fi

59、g8 8, which may be a function of the acceleration methodology. Fig.8Corroded 12mm bar with appro*imately 30% mass lossFig.9Corroded 16mm bar with appro*imately 15% mass loss3.2 Bond stress and crack width Figure HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig10#Fig1

60、0 10 shows the variation of bond stress with mean crack width for 16mmbars and Fig. HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig11#Fig11 11 for the 12mmbars. Figures HYPERLINK ./content/u754755ru8h1h068/fullte*t.html l Fig12#Fig12 12 and