StudyonChlorideIonDiffusionofFlyAshConcreteunder

1、two and three dimensional chloride ingress into fly ash concrete zhang yunsheng1*, sun wei1, chen shudong1, guo fei1, li zongjin2(1. jiangsu key laboratory for construction materials, southeast university, nanjing 211189, china; 2. department of civil engineering, the hong kong university of science

2、 and technology, clear water bay, kowloon, p.r.china)abstract: in this paper, 2d, 3d chloride ion concentration at the edge and corner zones are systematically investigated for fly ash concretes made with different cement replacement percentage by fly ash (0%, 10%, 20%, 40%, 60%), water to binder ra

3、tios (0.3, 0.35, 0.4), curing ages (28d, 90d). an interaction effect caused by 2d and 3d diffusion can obviously be observed through the comparison with 1d testing results. in order to quantify the interaction effect, 2d and 3d diffusion interaction coefficients is proposed in this paper. finally, t

4、he changes of 2d and 3d interaction coefficients with the change in the free chloride ion concentration are given. the above research provide an insight into chloride ion attack on the edge and corner reinforcing bars of concrete structures in the field of civil engineering.keyword: fly ash, concret

5、e, chlorides, two and three dimension ingress1. introductionchlorides penetration through concrete cover is considered to be the primary cause of concrete deterioration that affects the durability of marine concrete structures 1. reinforcement corrosion can be initiated when the critical chloride co

6、ncentration is reached at the steel-concrete interface. due to its critical role in causing corrosion of reinforcing bar, the mechanism of chloride transport into concrete has become becoming a hot topic in the area of concrete structural durability 1-8. chlorides ingress into concrete should be one

7、 complex combination of several transport mechanisms including diffusion, capillary sorption and permeation. for most cases, however, diffusion is recognized to be the governing mechanism for chlorides ingress into concrete. through 40 years of studies, many valuable results on chlorides diffusion i

8、nto concrete have been obtained and successfully applied in the service life prediction of field concrete structures 1-8. however, it can be found that these observations are mostly obtained by studying one dimensional (1d) chlorides diffusion in concrete. as well be known, some important locations

9、of the field concrete structures such as the edges or corners of beams and columns are subjected to the simultaneous attack of two or three dimensional (2d or 3d) chlorides flows. the 2d or 3d diffusion of the edge or corner concrete is quite different from 1d diffusion of the other locations. in pr

10、actical engineering, the diffusion rate and deterioration degree of the edge or corner concrete exposed to 2d or 3d chlorides flows is much larger than that of the other concretes only exposed to 1d chlorides flow 9. thus it is very important to study the 2d and 3d diffusion of concrete exposed to c

11、hloride salt solution. in addition, utilization of fly ash as a cement replacement material has become common practice in marine concrete structure in recent years 9-14. fly ash is used in concrete for economical and environmental reasons. moreover, the fly ash particles reacting with calcium hydrox

12、ide, i.e. pozzanlic reaction, greatly decrease concrete porosity. this leads to a lower chloride diffusivity and higher resistivity, therefore, less susceptible to the ingress of the harmful chlorides, which was verified by some long-term field and laboratory studies of chloride ingress into fly ash

13、 concrete 9-14. whereas, little literatures are available on 2d or 3d chlorides diffusion of fly ash concrete 11-13.for the reason, 2d or 3d chlorides ingress is systematically investigated for fly ash concretes made with different fly ash content (0%, 10%, 20%, 40%, 60%), water to binder ratios (0.

14、3, 0.35, 0.4) and curing ages (28d, 90d). the 2d and 3d diffusion interaction effect is also quantitatively analyzed through a comparison between 1d diffusion and 2d or 3d diffusion. the above research provides an insight into chlorides attack on the edge and corner reinforcing bars of concrete stru

15、ctures.2. materials and methods2.1. materialsa chinese standard graded 42.5 p ii type portland cement with the compressive strength of 47.6 mpa at 28 days is used, similar to astm c150 type ii cement. grade i fly ash, similar to class f fly ash according to astm, are supplied by nantong power plant,

16、 jiangsu province, p.r.china. rive sand with fineness modulus of 2.6 and continuous grade crushed basalt stone with maximum size of 20 mm are used as fine and coarse aggregates. a polycarboxylic-type superplasticizer with water reducing ratio of 25.8% is used, and the dosage is adjusted to keep the

17、slump of fresh concrete mixture in the range of 200220 mm. 2.2 experimental programa total of 7 bathes are made in this study. the details of experimental program are given in table2. batch “fa00i35” to batch “fa60i35” are used to compare the influence of fly ash content (0%, 20%, 40%, 60%). batch “

18、fa40i30”, batch “fa40i35” and batch “fa40i40” is specifically designed to investigate the effect of water-to-binder ratio (0.30, 0.35, 0.40). batch “fa40i40” and batch “fa40i35(90d)”are used to study the effect of curing ages (28days and 90days). the mass ratio of sand-to-(sand+stone) is kept at 0.3

19、85 for all the batches. table2 experimental programbatchesbinder(kg/m3)fly ash content (%)water to binder ratioworkability(mm)strength (mpa) at 28 daysslumpflowabilitycompressiveflexuralfa00i3545800.3520555077.09.85fa20i35458200.3520553072.25.44fa40i35458400.3521058065.54.98fa60i35458600.3522058555.

20、14.39fa40i30458400.3020553064.95.73fa40i40458400.4021059061.64.72fa40i35(90d)458400.4020560067.05.33note: faxxixxs(90d)faxx represents the fly ash content (xx); i represents grade i fly ash; xx after i represents water-to-binder ratio; (90d) represents 90days of curing ages.2.3 specimen preparationa

21、ccording to the mixture proportions in table2, fresh concrete mixtures are made in a compulsory planetary mixer. then concrete specimens with size of 100mm by 100mm by 100mm are cast, and then placed in room temperature for 24 hours. thereafter, the specimens are demoulded and cured in a condition o

22、f 20 ± 3 and 95% rh for 28 days.2.4 methods2.4.1 2d and 3d chlorides diffusion test2d and 3d chloride diffusion test is similar to the 1d test. the differences between 1d carbonation and 2d or 3d ones are as follows: for 2d chloride diffusion test, the oven-dried concrete specimen is covered wi

23、th epoxy on four faces (two end faces, the forming face, one side face), leaving free two vertical side faces to be exposed to chlorides solution. for 3d chloride diffusion test, three faces of specimen (one end faces, the forming face, one side face), leaving free three vertical faces to be exposed

24、 to chlorides solution (350 g nacl/l) for 1, 3, 6, 9 and 12 months, respectively. three samples for each concrete mixture are made. in order to keep the constant cl- concentration, the nacl solution is refreshed every one week in the first 3 months, then refreshed every 3 weeks in the later 9 months

25、. the temperature was kept constant at 20°c throughout the entire test period. at the end of specified exposure periods, the specimens are taken out, subsequently placed into specially designed fixtures for 2d and 3d sampling that can ensure the powder samples are always drilled along the diago

26、nal face () of the specimen, i.e 2d direction, or along the body diagonal line, i.e 3d direction. the detailed testing method can be seen in literature . 3. results and discussion3.1 effect of fly ash contentfig.2 shows the 1d, 2d and 3d chlorides diffusion development with time of various fly ash c

27、oncrete made with different fly content of 0%, 20%, 40% and 60%, respectively. it can be seen from fig.2 that fly ash content plays an important role in 2d and 3d diffusion of fly ash concrete. similar to 1d diffusion, 2d and 3d cl- diffusion concentration exhibit rapid decreasing trend with an incr

28、ease in the distance from the surface for various fly ash concrete. it seems to be consistent with ficks second diffusion law. compared to the concrete without fly ash, the free chlorides concentration in fly ash concrete is obviously reduced. when fly ash content is less than 40%, fly ash addition

29、obviously reduce the chlorides diffusion, resulting in low free chlorides concentration in various distances excluding the first layer of fly ash concrete compared with the concrete without fly ash, as shown in fig.2. the free chlorides content in the first layer (0-5mm) is easily misjudged due to c

30、arbonation of surface layer concrete. so when anglicizing chlorides diffusion behavior, the data in the surface layer is often discarded. when too much fly ash (60%) is incorporated, the strength is greatly reduced. about 28% strength loss can be observed, as shown in table 2. the porosity is also c

31、onsiderably increased (fig.3). as a result, the chlorides diffusion coefficients show an incremental trend (table3). the chlorides easily transports into the concrete with high volume fly ash. fig.2 effect of fly ash content on 1d, 2d and 3d chlorides diffusionfig.3 effect of fly ash content and w/b

32、 on porosities of concretetable3 chloride diffusion coefficients of various types of fly ash concretes after 12 months of immersionbatchesapparent chloride diffusion coefficients (10-8cm2/s)1d2d3dfa0i351.74.25.2fa20i350.882.94.8fa40i351.82.84.9fa60i352.46.711.9fa40i301.73.14.5fa40i401.14.67.93.2 eff

33、ect of w/bw/b also exhibits great impact on 2d and 3d diffusion of fly ash concrete, as shown in fig.7. it can be seen from fig.7 that when fly ash content is kept constant (40%), cl- diffusion concentration in spite of 1d, 2d or 3d obviously increases with an increase in w/b. it is especially true

34、for w/b ranging from 0.30 to 0.35. when w/b varies from 0.35 to 0.40, an increase in w/b will have little influence. similar trend is also observed in diffusion coefficients of fly ash concrete with different w/b. this may be attributed to an increase in porosity caused by the high w/b (fig.3), resu

35、lting in a rapid cl- diffusion3.3 effect of curing agesit is well known that fly ash possess potential pozzolanic reactivity, which, however, can only be slowly developed through a long period of curing ages (at least 90 days). so long curing ages are of benefit to develop the advantages of fly ash.

36、 in order to investigate the effect of curing ages on the carbonation of fly ash concrete, two curing ages-28d and 90d are employed in this study. fig.4 shows 1d, 2d and 3d chloride diffusion development with time of fly ash concrete (fa40i35) cured for 28 days and 90 days, respectively. it can be s

37、een that the free chloride ion concentration, in spite of 1d, 2d or 3d, of fly ash concrete cured for 90 days is lower than that of fly ash concrete cured for 28 days. fig.4 effect of curing ages on 1d, 2d and 3d chlorides diffusion4 interaction effect of 2d and 3d chloride diffusion 4.1 comparison

38、between 1d and 2d or 3d diffusion the effect of dimensions on chloride diffusion of fly ash concrete is shown in fig.5. as can be seen, free chloride concentration at the same distance of the concrete with 40% or 60% fly ash follows the order: 3d >2d >1d. this means that more attention should

39、be paid on the chloride ingress of the edge and corner concrete, when we predict the service life of concrete construction exposed to the marine environment. according to the above studies, the steel bars embedded in these locations exposed to 2d or 3d chloride flow will firstly be corrode due to a

40、rapid penetration rate, which will become an important steel corrosion resource. fig.5 effect of dimensions on chloride diffusion2d diffusion3d diffusionfig.6 2d and 3d chloride diffusion profiles of concrete4.2 diffusion interaction coefficienttheoretically, if there doesnt exists diffusion interac

41、tion effect, 2d cl- concentration of the edge concrete at the same distance should be astimes larger as 1d cl- concentration, 3d cl- concentration of the corner concrete should betimes larger than 1d concentration. however, the experiment results show 2d and 3d cl- concentration are much larger than

42、 the theoretical one (1d cl- concentration), as can be seen in fig.5. in addition, 2d and 3d chlorides diffusion profile doesnt exhibit orthogonal angle outline, but a round one, as shown in fig.6. all these indicate an existence of obvious dimension interaction effect, resulting in rapid chlorides

43、transportation in 2d and 3d diffusion zone. in order to quantify the interaction effect caused by the diffusion dimensions, an interaction coefficient is defined as follows: (1)where, -interaction coefficient, -2d and 3d theoretical diffusion coefficients, -2d and 3d experimental diffusion coefficie

44、nts.according to the definition of interaction coefficient, denotes an accelerated factor that embodies 2d and 3d chloride diffusion accelerated rate exceeding 1d diffusion. fig.7 illustrates the relationship between interaction coefficient () and immersion ages (t). it can be observed that slightly

45、 fluctuates around one constant value with the development of immersion ages, ash shown in fig.7. by least square regression, is obtained : k2d=2.01，k3d=2.27. fig.7 relationship between interaction coefficient and immersion ages5. conclusions(1) compared to 1d diffusion, 2d and 3d chlorides diffusio

46、n of fly ash concrete exhibits much larger diffusion rate. this will lead to more serious corrosion in the edge or corner locations of concrete structures exposed to chlorides environment. thus, the steel bars embedded in these locations will be prone to suffer from chlorides attack and initiate cor

47、rosion in the first place.(2) fly ash content, w/b and curing ages have great impact on 2d and 3d chlorides diffusion of fly ash concrete. 2d and 3d chloride diffusion will increase with an increase in fly ash content. a reduction in w/b is helpful to enhance the resistance to diffusion attack. in a

48、ddition, long curing ages can ensure fly ash concrete possesses better resistance to chloride penetration than short curing ages.(3) interaction coefficient of fly ash concrete is obtained: k2d=2.01，k3d=2.27reference1 mehta pk. durabilitycritical issues for the future j. concr. intern., 1997,19(7):

49、2733.2 darmawan ms. pitting corrosion model for reinforced concrete structures in a chloride environment j. mag. concr. res., 2010, 62(2): 91-1013 liang mt., huang r., feng sa. service life prediction of pier for the existing reinforced concete bridges in chloride-laden environment j. j. marine sci. tech.-taiwan, 2009, 17(4): 312-3194 suwito c