BSt 500s 钢筋抗腐蚀性能研究

1、 本科毕业设计（论文）翻译英文原文名tensile behavior of corroded reinforcing steel bars bst 500s中文译名bst 500s 钢筋抗腐蚀性能研究 班 级姓 名学 号指导教师填表日期英文原文版出处：earthquake engineering and structural dynamics译文成绩： 指导教师签名： 原文：1. introductionsteel bars in reinforced concrete carry mainly tension loads. according to the present day stand

2、ards, e.g. 1, for involving reinforcing steel in concrete structures, certain minimum values for the mechanical properties modulus of elasticity (e), yield stress (rp), ultimate stress (rm) and elongation to failure (fu) of the steel are required. furthermore, the standard sets rm/rp > 

3、1.05 1. with increasing service life of a reinforced concrete structure damage accumulates gradually. nowadays, significant resources are allocated worldwide for the repair and rehabilitation of deteriorating concrete structures. recent reports indicate that the annual repair costs for the reinforce

4、d concrete structures of the network of highways in the usa alone amounts to 20 billion usd 2. the respective repair costs for reinforced concrete bridges in england and wales amount to 615 million gbp 3. yet, although in recent years the problem of the actual residual strength degradation of ageing

5、 reinforced concrete structures has attracted considerable attention, it is far from being fully understood and, even less, resolved. it is worth noting that up to now, little work has been done to account for the effects of corrosion on the mechanical properties of the reinforcing steel bars and he

6、nce on the degradation of the load bearing ability of a reinforced concrete element 4. such effects are the reduction of the effective cross-section of the reinforcing steel, micro and macro cracking of concrete and finally the spalling of the concrete. the underestimation of the corrosion problem a

7、rises from the fact that under normal circumstances, concrete provides protection to the reinforcing steel. physical protection of the reinforcing steel against corrosion is provided by the dense and relatively impermeable structure of concrete. the thin oxide layer covering the reinforcement, durin

8、g concrete hydration, ensures chemical protection. the oxide layer remains stable in the alkaline concrete environment (ph > 13), but begins to deteriorate when the ph of the pore solution drops below 11 5 and 6. the rate of deterioration due to corrosion rises when the ph drops below 9

9、. for corrosion to commence, the oxide film must be broken or depassivated. depassivation may occur if the alkalinity of the pore solution in the concrete pores decreases and/or penetration of the chloride ions takes place. this may be caused by carbonation, especially in the proximity of cracks, or

10、 by water dilution which accompanies cracking 7, 8 and 9. the advancing corrosion results in a reduction of the load carrying cross-section of the bars and an increase in their volume, which may cause cracking of concrete as well as an appreciable decrease on the bond strength between the reinforcin

11、g bars and concrete 10 and 11. the above considerations do not account for the effect of corrosion on the mechanical behavior of the reinforcing steels. most of the available studies on the corrosion of reinforcing steels refer to the metallurgical aspects of corrosion such as the mass loss, the dep

12、th and the density of pitting etc., e.g. 12 and 13. it is worth noting that the corroded steel bars are located in a zone of high tensile or shear stresses 5, 12, 14, 15, 16 and 17. maslechuddin et al. 10 evaluated the effect of atmospheric corrosion on the mechanical properties of steel bars. they

13、concluded than for a period of 16 months of exposure to atmospheric corrosion, rusting had an insignificant effect on the yield and ultimate tensile strength of the steel bars. almusallam 18 evaluated the effect of the degree of corrosion of the steel bars in concrete, expressed as percent mass loss

14、, on their mechanical properties. the results of the study indicated a close relationship between the failure characteristics of steel bars and slabs with corroded reinforcement. a sudden failure of slabs in flexure was observed when the degree of reinforcement corrosion expressed as percent mass lo

15、ss exceeded 13%. the above results on the mechanical behavior of corroded reinforcing steels refer to bst 420s of din 488, (s420s according to the hellenic standards). the above results clearly indicate the need to account for the effects of corrosion on the mechanical properties of the reinforcing

16、steel bst 500s (s500s according to the hellenic standards) which at present is almost exclusively used in reinforced concrete structures. it is worth noting that corrosion damage of the reinforcement, is expected to become more noticeable in new constructions using reinforcing steel s500s, given the

17、 fact that this type of steel exhibits greater mass loss due to corrosion compared to steel classes s400 and s220 19. recall that many reinforced concrete structures are located in coastal areas with an intense corrosive environment. on the other hand, a wide spread use of corrosion-resistant steel

18、reinforcing bars should not be expected as these bars cost about six to nine times more than plain carbon steel reinforcing bars. in the present study, the effects of corrosion on the tensile behavior of reinforcing steel bars class s500s tempcore are investigated. the specimens were pre-corroded us

19、ing laboratory salt spray tests for different exposure times. the dependencies of the degradation of the tensile properties on the corrosion exposure time have been derived. the tensile properties of the corroded material were compared against the requirements set in the standard for involving steel

20、s in reinforced concrete structures. 2. experimental researchthe experiments were conducted for the steel s500s tempcore, which is similar to the bst500s steel of din 488 part 1 20. a stressstrain graph of the uncorroded material is shown in fig. 1. the chemical composition (maximum allowable % in f

21、inal product) of the alloy s500s is: c, 0.24%; p, 0.055%; s, 0.055%; n, 0.013% 21. (13k) fig. 1. stressstrain graph of uncorroded bst 500s alloy. the material was produced by a greek industry by using the tempcore method (hot rolling followed by quenching and self tempering) and was delivered i

22、n the form of ribbed bars. the nominal diameter of the bars was 8 mm (Ø8). from the bars, tensile specimens of 230 mm length were cut. the gauge length was 120 mm according to the specification din 488 part 3 22. prior to the tensile tests, the specimens were pre-corroded using a

23、ccelerated laboratory corrosion tests in salt spray environment. 2.1. salt spray testingsalt spray (fog) tests were conducted according to the astm b117-94 specification 23. for the tests, a special apparatus, model sf 450 made by cand w. specialist equipment ltd. was used. the salt solution was pre

24、pared by dissolving 5 parts by mass of sodium chloride (nacl) into 95 parts of distilled water. the ph of the salt spray solution was such that when dissolved at 35 °c, the solution was in the ph range from 6.5 to 7.2. the ph measurements were made at 25 °c. the temperature in th

25、e zone of the reinforcement material exposed inside the salt spray chamber was maintained at 35 °c + 1.11.7 °c. when exposure was completed, the specimens were washed with clean running water to remove any salt deposits from their surfaces, and then were dried. in addit

26、ion, a number of steel bars of the same length were exposed to the salt spray for 1, 2 and 4 days to monitor the corrosion damage evolution. 2.2. mechanical testing procedurethe pre-corroded specimens were subjected to tensile tests. all mechanical tests are summarized in table 1. table 1. tensile t

27、ests for s500s Ø8 tempcore steel test seriestest series descriptioncorrosion exposure prior to tensile testnumber of tests conducted1tensile tests on non-corroded control specimensnone42tensile tests on corroded specimenssalt spray corrosion for 10 days33tensile tests on corroded specimenssalt

28、spray corrosion for 20 days34tensile tests on corroded specimenssalt spray corrosion for 30 days35tensile tests on corroded specimenssalt spray corrosion for 40 days36tensile tests on corroded specimenssalt spray corrosion for 60 days37tensile tests on corroded specimenssalt spray corrosion for 90 d

29、ays3the performed tensile tests aim to provide information on: 1. the gradual deterioration of the mechanical properties of the s500s tempcore steel reinforcement during salt spray corrosion;2. whether the exposure of the specimens to salt spray might degrade their tensile property values such that

30、they do no longer meet the limits set by the hellenic standards for using steel in reinforced concrete structures, e.g. 1 and 24.the tensile tests were performed according to the din 488 specification 22. for the tests a servo-hydraulic mts 250 kn machine was used. the deformation rate was 2 mm

31、/min. the tensile properties: yield stress rp, ultimate stress rm, elongation to fracture fu and energy density w0 were evaluated. the energy density is calculated from the area under the true stresstrue strain curve. in the present work, the energy density has been evaluated from the engineering st

32、ressengineering strain curves as(1)as an engineering approximation. 3. results and discussionas expected, corrosion damage increases with increasing exposure time to salt spray. the exposure of the specimens to the salt spray environment causes the production of an oxide layer which covers the speci

33、men and increases in thickness with increasing exposure time of the specimen. removal of the oxide layer by using a bristle brush according to the astm g1-90 25 specification has shown extensive pitting of the specimens already after 10 days of exposure to salt spray. the stereoscopic image of a spe

34、cimen after exposure to salt spray for 10 days is shown in fig. 2. it is compared against the image of the uncorroded material. it was observed that the corrosion attack started at the rib roots and advanced towards the area between the ribs. the indentations of the corrosion attack left on the spec

35、imen surface after removal of the oxide layer increase in dimensions and depth with increasing duration of the exposure. (84k) fig. 2. stereoscopic images (×35) of (a) uncorroded specimen and (b) specimen exposed to salt spray corrosion for 10 days. the production of the oxide layer is ass

36、ociated to an appreciable loss of the specimens mass. the dependency of the obtained mass loss on the salt spray duration is displayed in fig. 3. the derived dependency may be fitted by the weibull function(2)the determined weibull values c1 to c4 are given in table 2. as it can be seen for salt spr

37、ay duration of 90 days the mass loss of the corroded specimen is about 35% of the mass of the uncorroded specimen. it is worth noting that the involved salt spray test is an accelerated corrosion test which is performed at the laboratory. although the salt spray test environment, to some extent, sim

38、ulates qualitatively the natural corrosion in coastal environment, it is much more aggressive and causes a very severe corrosion attack in a short time. currently, there is no direct correlation between the accelerated laboratory salt spray test and the natural corrosion of reinforcing steels such a

39、s to assess a realistic duration for the accelerated laboratory salt spray tests. fig. 4 shows a photograph taken from a building constructed in 1978 at a coastal site in greece. the corroded reinforcing bars indicated a severe mass loss. the mass loss of the corroded bars shown in fig. 4 was as hig

40、h as 18% which corresponds to an exposure of 44.5 days according to the fitting curve in fig. 3. the corrosion measured for the mentioned case appeared rather frequently during an extensive investigation on the integrity of older constructions at coastal sites in greece. even though the above result

41、s are by far not sufficient for establishing exact correlations between laboratory salt spray tests and natural corrosion, they clearly indicate that laboratory salt spray exposures for 40 days and longer are realistic for simulating the natural corrosion damage of steel bars which might accumulate

42、during the service time of reinforced concrete structures at coastal sites. by assuming a uniform production of the oxide layer around the specimen and hence a uniform mass loss, the results of fig. 3 can be exploited to calculate the reduction of the nominal specimen diameter with increasing durati

43、on of the salt spray test. the reduced diameter dr is calculated as(3)where a is the measured mass loss in percent and d is the nominal diameter of the uncorroded specimens (8 mm).the reduced values for the nominal specimen diameter are given in table 3. the reduction specimen diameter with inc

44、reasing salt spray exposure time is displayed in fig. 5. the results in fig. 5 were fitted using eq. (2). the weibull values c1 to c4 for fig. 5 are given in table 2. (18k) fig. 3. effect of the duration of corrosion exposure on mass loss. table 2. weibull values mass lossdiameter reductionyiel

45、d stress reductionultimate stress reductionenergy densityelongation to failurec166.091196.48612282.22252373.991435.19016.69354c2100.240198.01075559.44776649.74026122.6928218.81317c31.709341.730421.658251.641611.547741.60975c47.195497.22599.770719.418323.858453.94068(98k) fig. 4. photograph take

46、n from building constructed in 1978. table 3. values of reduced specimen diameter exposure to salt spray corrosion environment0102030406090diameter (mm)87.987.837.647.296.976.62(18k) fig. 5. reduction of specimens diameter with increasing duration of corrosion exposure. it is essential to notic

47、e that the strength calculation of steel reinforced concrete structures according to the standards, e.g. 24, occurs by using an engineering stress estimated by assuming the cross-sectional area as(4)with d being the nominal diameter of the bars. for the bars of the present study, the nominal diamete

48、r was 8 mm. according to the valid standards, there is no special consideration for the reduction of the nominal diameter of the reinforcing steel, even when evaluating the strength of an older reinforced concrete structure indicating a severe corrosion damage of the reinforcing bars as shown i

49、n fig. 4. displayed in fig. 6 and fig. 7 are the apparent values of the engineering yield stress and ultimate stress over the duration of salt spray exposure by neglecting the reduction of the cross-section of the corroded specimens. in fig. 6 and fig. 7, the above values are referred to as rpapp an

50、d rmapp, respectively. the results have been fitted using the weibull function of eq. (2). the weibull constants c1 to c4 for fig. 6 and fig. 7 are given in table 2. as shown in the figures, the apparent values of rm and rp drop below the limits of rm = 550 mpa for ultimate stress and

51、 rp = 500 mpa for the yield stress, which are set by the standards 21 for involving reinforcing steels, after 32 and 27 days exposure to salt spray, respectively. yet, the obtained degradation of the apparent strength values of the material reflects not only the effect of corrosion on

52、 the mechanical properties of the material but also a stress increase due to the reduction of the specimens cross-section. the effect of corrosion on the tensile strength properties of the reinforcing steel can be assessed when removing from the surface of the bars the corrosion products and also us

53、ing in the calculation of the engineering stress the reduced nominal specimen diameters which are given in table 3. the measured strength values are given in table 4 and in the following will be referred to as rpeff and rmeff, respectively. for the uncorroded material, the effective strength values

54、have been calculated by using the true cross-sectional area: 22(5)where g is the weight and l is the length of the specimen, whereas for the apparent strength values, the cross-sectional area was calculated by using eq. (4). the dependencies of the effective engineering yield and ultimate stress on

55、the duration of salt spray exposure are displayed in fig. 6 and fig. 7 as well. as it can be seen in the figures, the corrosion attack causes a moderate tensile strength reduction which increases with increasing duration of the corrosion exposure, even though for the calculation, the reduced nominal

56、 specimen diameters have been used. this result is consistent to the observation of the performed corrosion characterization. it should be remembered that the indentations of the corrosion attack that remained on the specimen surface after removal of the oxide layer and hence, the associated notch e

57、ffects during tensile loading were found to increase in dimensions and depth with increasing duration of the exposure. furthermore, as corrosion attacks the surface of the bars, the specimen cross-section which is reduced by corrosion damage refers to material rich in high strength martensite. the e

58、xperimentally observed reduction of yield and ultimate stress with increasing time of corrosion exposure can be fitted by the set of equations:rp(t)=a1+b1t+b2t2(6)andrm(t)=a2+b3t+b4t2.(7)the constants a1, b1, b2, a2, b3 and b4 in eqs. (6) and (7) were derived to 596.19291, 2.59222, 0.00563, 695.67537, 2.92755 and 0.01375, respectively. (23k) fig. 6. effect of the duration of corrosion exposure on yield strength. (23k) fig. 7. effect of the duration of corrosion exposure o