土木工程外文文献.docx

1、土木工程外文文献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)VicR

2、oads, 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 Abstract This paper reports the results of a rese

3、arch 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 i

4、nvestigated 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

5、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 of reinforced concrete st

6、ructures 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

7、 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 af

8、fect 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 resul

9、t 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 bea

10、ring 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

11、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

12、 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 result

13、s 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

14、 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 st

15、udy 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 sim

16、ply 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 ac

17、ting on the bar over this 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 spa

18、ced 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 T

19、he 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

20、hessian before testing. Table1Concrete 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.1 is used to normalize bon

21、d 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

22、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 expe

23、riments 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

24、 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

25、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 na

26、turally, 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 o

27、n 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 system When the required crack width was achieved for a particular bar, the impressed current was discontinued for

28、 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, beginn

29、ing 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 tes

30、ts 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 wa

31、s 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, 16m

32、m bar unconfined Fig.4Schematic of loading. Note: only test 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.2.3). While each specimen had a mean target crack width for each b