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Investigation of chloride ingress into concrete under very early age exposure conditions
Construction and Building Materials 225 (2019) 801–811
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation of chloride ingress into concrete under very early age exposure conditions Alireza Bagheri ⇑, Abbas Ajam, Hamed Zanganeh Civil Engineering Faculty, K. N. Toosi University of Technology, Tehran, Iran
h i g h l i g h t s Effect of very early age exposure on chloride ingress in concrete was experimentally investigated. Functions describing time variations of diffusion coefficient and surface chloride value were derived. Very early age exposure was found to have considerable effect on service life.
a r t i c l e
i n f o
Article history: Received 14 March 2019 Received in revised form 17 June 2019 Accepted 19 July 2019
Keywords: Chloride ingress Very early age exposure Surface chloride value Service life
a b s t r a c t Some structures such as cast in place reinforced concrete piles and quay walls are exposed to chlorides from the moment of casting. Considering the lack of previous work on such an early age exposure, the current study was carried out. Based on an experimental study, functions describing time variations of diffusion coefficient and surface chloride values for both the normal age and very early age exposures were derived. Using these functions, the effects of very early age exposure on service life was investigated. The results show that very early age exposure considerably increases chloride penetration into concrete, reducing service life by about 34 and 13% for concrete covers of 50 and 90 mm respectively. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Chloride induced corrosion of reinforcement is the major source of deterioration in reinforced concrete structures, particularly in marine structures and bridge decks [1–4]. Considerable research effort has therefore been directed towards chloride ingress into concrete and a vast body of knowledge has been accumulated on the subject [5–8]. Research has led to the development of standardized test methods for appraisal of chloride resistance of concrete mixes [6,9–11]. Numerical and analytical methods have also been developed which make possible the prediction of chloride profiles in concrete structures at various times and facilitate service life prediction of reinforced concrete structures [6,12–15]. An area which has received less research attention is the early age exposure of concrete to chloride environments [16–18]. Some structures such as cast in place reinforced concrete piles and quay walls in coastal areas are subjected to very early age exposure and become in contact with chloride laden environment from the time ⇑ Corresponding author at: 470, Mirdamad Ave. West, 19697 Tehran, Iran. E-mail addresses: [email protected] (A. Bagheri), [email protected] (A. Ajam). https://doi.org/10.1016/j.conbuildmat.2019.07.225 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
of casting. Concrete in such structures is in fact exposed to chlorides before setting and high initial penetration rates are expected. Yeih et al. [19] carried out an experimental study for determining the effect of age of exposure on chloride diffusion coefficient of concrete and mortar specimens. The study was limited to initial exposure times of 7 and 28 days and it was reported that exposure at 7 days resulted in a 50 %increase in diffusion coefficient compared to that of exposure at 28 days. Caballero et al. [17] used the Rapid Chloride Migration (RCM) test method to derive diffusion coefficients of mortar specimens at the ages of 1, 2, 3, 7, 14, 21 and 28 days. They also used the natural diffusion test to determine the apparent diffusion coefficients for various early age exposure intervals, including 1.5–4, 3–8 and 7–16 days. Two different cement types, namely, CEM I (normal Portland cement) and CEM III/B (slag cement which has a lower rate of hydration) were considered. The results showed that diffusion coefficients at the age of 1 day were one and two order of magnitudes higher compared to the 28 day values for the mixes with cement types CEM I and CEM III/B respectively. Although valuable data on early age diffusion coefficients were derived, the experimental work only considered short durations of exposure of up to a few days and the effect of early age exposure on chloride diffusion coefficients at longer exposure
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durations was not experimentally determined by these researchers. Based on numerical modelling considering only the time variation of the diffusion coefficient and assuming the surface chloride value as constant, these researchers reported that the effect of early age exposure on long term chloride ingress into concrete was relatively minor (less than 10%). Chen et al. [16] studied chloride ingress into concrete specimens exposed to sea water at the age of 1 day. The study did not include early age profile determinations and these were only determined at ages of 90, 180 and 360 days. As expected some decrease in chloride diffusion coefficient, was observed with increased duration of exposure. The study also yielded surface chloride values at the ages of 90, 180 and 360 days. The surface chloride values of the specimens which were exposed to chlorides at the age of 1 day were found to be quite high at 90 days and little further increase in in this parameter occurred at 180 and 360 days. This is in contrast to general experience which indicates that with normal ages of exposure such as 28 days, the buildup of surface chloride with time takes place gradually [13,20]. Unfortunately the study by Chen et al. [16] did not include control specimens with normal exposure ages such as 28 days and it was therefore not possible to directly compare chloride ingress due to early age exposure with that of normal age exposure. Considering the limited previous research on early age exposure of concrete to chlorides and particularly lack of previous work on very early age exposure, where concrete is cast against chloride laden environment, the present study was carried out to provide further information in this regard. The study included experimental work for determining the effects of variations in exposure age on diffusion coefficient and surface chloride values. Numerical modeling using experimentally derived time variations of diffusion coefficients and surface chloride values was also carried out to quantify the effects of early age exposure on service life of reinforced concrete. 2. Experimental program Three concrete mixes with w/c ratios of 0.3, 0.4 and 0.5 were considered for the experimental investigation. The methods used for evaluation of chloride resistance of these mixes included the natural diffusion test by immersion according to NT Build 443 [21] and the Rapid chloride migration (RCM) test according to NT Build
492 [10]. For the immersion test, the initial exposure ages to chlorides included immediate exposure after casting and exposure at the ages of 1, 3, 7 and 28 days. The duration of exposure extended up to 180 days so that longer time effects of early age exposure could be studied. For the immersion test the derived diffusion coefficient is considered as the apparent diffusion coefficient which depends on both the age at start of exposure and the duration of exposure. Tests with short exposure interval of 0–1, 0–7, 1–7 and 7–28 days were also considered to provide data with of regards to variation of diffusion coefficient at early ages. Rapid migration tests were carried out for determining instantaneous chloride diffusion coefficients at various ages including 1, 3, 7, 28, 90 and 180 days. The study also included determination of development of electrical resistivity of various mixes with time in order to provide qualitative data with regards to pore connectivity (in early age) particularly before and around the setting of concrete. Time development of compressive strength of various mixes were also determined. Details of the materials, mixes and test methods used are given in the following sections. 2.1. Materials used and mixture properties The cement used for the study was a type II Portland cement conforming to the requirements of ASTM C150 [22]. The compound composition of the cement comprised of 48.1 C3S, 24.3 C2S, 7.6 C3A and 12.0 C4AF. The initial and final setting times of the cement were determined as 105 and 150 min respectively. Water of drinkable quality was used for production of the mixes. The aggregates used for production of mixes comprised of a crushed coarse aggregate with nominal maximum size of 19 mm and specific gravity of 2.58 and a river sand with specific gravity of 2.49. The aggregates conformed to the requirements of ASTM C 33 [23]. Three concrete mixture with w/c ratios of 0.3, 0.4 and 0.5 were considered for the study. Details of these mixes are given in Table 1. The slump value of all mixes were kept in the range 125 ± 25 mm, through the use of required quantities of a polycarboxylate ether type superplasticizer. The specimens for various tests were subjected to wet curing as tests carried out in this study require the specimens be in saturated state prior to conducting the test. Being exposed to chlorides in a saturated state ensures that diffusion is the main mechanism for chloride ingress. 2.2. Tests carried out -Initial and final setting times of concrete mixture was determined according to ASTM C403 [24] on samples prepared by sieving the mixtures on No.4 (4.75 mm) standard sieve. -Compressive strength test was carried out at the ages of 1, 3, 7, 28, 90, 180 days on 100 mm cubic specimens in accordance with BS EN 12390 part 3 [25]. -Electrical resistance of concrete mixture were monitored at the ages of 0.5, 1, 2, 3, 4, 5, 6, 8 and 10 h and 1, 3, 7, 28, 90 and 180 days. The test was conducted according to the procedure described by the Swedish National Testing and Research Institute [26] using an AC current based instrument. For the period before demolding of specimen, the electrical resistance was monitored by two copper plates placed in the two opposite faces of the molds (Fig. 1).
Table 1 Mixture proportions for concrete mixture studied. Mix Designation
W/C
Cement (kg/m3)
Water (kg/m3)
SP (% cement)
Coarse agg. SSD (kg/m3)
Fine agg. SSD (kg/m3)
1 2 3
0.50 0.40 0.30
420.0 420.0 420.0
210.0 168.0 126.0
– 0.47 1.48
813 867 920
813 867 920
Fig. 1. Determination of electrical resistivity a) From the time of casting until demolding b) after demolding.
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A. Bagheri et al. / Construction and Building Materials 225 (2019) 801–811 -Chloride profile determination under natural diffusion was carried out in accordance with NT Build 443 [21] using a solution with NaCl concentration of 16.5%. At the end of exposure periods considered, powder samples at 2 mm depth intervals were taken by grinding of the specimens (Fig. 2). Powder samples were subsequently analyzed according to ASTM C1152 [27] and chloride concentrations as % weight of concrete mass were determined. By fitting the error function solution of Fick’s second law of diffusion to the experimentally derived chloride concentrations at various depths, apparent diffusion coefficient Da and surface chloride concentrations Cs were derived. -The Rapid Chloride Migration (RCM) test was carried out at ages of 1, 3, 7, 28, 90 and 180 days in accordance to NT Build 492 [10] on various mixes studied. The top end of the cylindrical test specimens were exposed to a 0.3 M NaOH solution with the opposite end being exposed to a 10% NaCl solution. A 10–60 V DC external potential was imposed across the specimen for the required time interval (in most cases 24 h). The specimens were then split open, and a silver nitrate solution was sprayed to the split section and the depth of penetration of the chloride ions was determined using a colorimetric method. The non-steady-state migration coefficient Dnssm was then calculated using the average penetration depth of the chloride ions.
3. Results and discussions 3.1. Compressive strength In Table 2 compressive strength values at various ages from 1 to 180 days are presented. The strength values increase with increasing age and the considerable effect of reduced w/c ratio on strength enhancement are evident in the results. 3.2. Concrete electrical resistivity and initial and final setting times The results of electrical resistivity of concrete mixes which were measured from the time of casting up to 180 days are depicted in Fig. 3. As the results show the electrical resistivity of mixes with lower W/C ratios even from the very early ages are higher than those with higher W/C ratio. It is interesting to note that there is an initial reduction in electrical resistivity in the first few hours after casting. The value then becomes stable which lasts for a few hours after which electrical resistivity increases rapidly. The observed
Fig. 3. Variation of electrical resistivity of concrete mixes with age.
trend can be attributed to various ions being released into the pore solution due to initial cement dissolution which increase the electrical conductivity of the pore solution phase. During the dormant period the electrical resistance is almost constant, after which electrical resistance rises due to progress of hydration of cement which results in the development of the microstructure and reduces connectivity of pores in the cement paste phase. The initial setting times of concrete mixes with w/c ratio of 0.3, 0.4 and 0.5 were determined as 5.3, 4.4 and 5.6 h respectively. The corresponding values for the final setting times were 7.7, 5.8 and 7.6 h which show that the rise in electrical resistance has occurred close to or shortly after final setting of concrete mixes. 3.3. Rapid chloride migration test Chloride diffusion coefficients derived from the RCM test at the ages of 1, 3, 7, 28, 90 and 180 days are given in Table 3. The results show high early age values which rapidly decrease with the progress of hydration. The 180 days diffusion coefficients were about an order of magnitude lower than the corresponding values at the age of 1 day. 3.4. Natural diffusion by immersion test By fitting the error function solution to Ficks 2nd law of diffusion to the experimentally determined chloride concentrations at various depths, the apparent diffusion coefficient (Da) and surface chloride value (Cs) for each exposure intervals were derived. Chloride profiles for the mix with w/c ratio of 0.4 and exposure intervals of 0–1, 0–7, 1–7 and 7–28 days are depicted in Fig. 4. In the exposure intervals, the first number shows the age of concrete at the start of exposure to chlorides and the second number shows the age of concrete at the end of exposure interval. For instance exposure interval (0–1) means specimens were exposed to chlorides from the time of casting and exposure continued up to the age of 1 day. Similarly exposure interval of (1–7) days means that specimens were exposed to chlorides at the age of 1 day and exposure continued up to the age of 7 days, i.e., exposure duration of 6 days. The derived values of diffusion coefficients which are presented in Table 4, show very high values at very early ages (0–
Fig. 2. Grinding equipment.
Table 2 The results of compressive strength test. W/C
0.5 0.4 0.3
Compressive Strength (MPa) 1 day
3 day
7 day
28 day
90 day
180 day
9.9 11.7 14.6
24.5 28.6 38.3
34.6 41.6 54.1
48.0 55.4 62.7
52.9 61.6 75.1
57.1 68.1 78.9
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Table 3 Results of the Rapid Chloride Migration (RCM) test. w/c
0.5 0.4 0.3
Diffusion coefficient (1012 m2/s) 1 day
3 day
7 day
28 day
90 day
180 day
132 118 107
47 41 32
34 26 19
21 17 15
19 13 11
16 11 9
1 day interval) of 262 1012 m2/s. This value is about 20 times higher than the corresponding value for the 7–28 days interval. The effect of early age exposure on longer term chloride profiles can be observed by comparing the 180 day chloride profiles for the exposure ages of 0, 3 and 28 days. These profiles for the mix with w/c ratio of 0.4 are presented in Fig. 5 and the corresponding diffusion coefficients are given in Table 4. The results show that very early age exposure has a relatively small influence on later age apparent diffusion coefficients. Exposure at ages of 0 and 3 days result in increases of only 16% and 7% respectively in the apparent diffusion coefficient compared to that with exposure age of 28 days. The effect of w/c ratio on 180 day chloride profiles of mixes exposed to chlorides form the time of casting and 28 days after casting are shown in Fig. 6 and the derived diffusion coefficients are presented in Table 4. The observed trends in the results of the mixes with w/c ratios of 0.5 and 0.3 are in general agreement with those of the mix with w/c ratio of 0.4 and confirm the relatively minor effect of early age exposure on long term diffusion coefficients. The surface chloride values for the mixes studied which were derived from the experimentally determined chloride profiles are given in Table 5. It is interesting to note that although early age of exposure, only had a small effect on longer term diffusion coefficients, the effect on surface chlorides was found to be significant. For the mix with w/c ratio of 0.4 and for exposure ages of 0 and 28 days the 180 day, Cs values expressed as % weight of concrete were determined as 1.96 and 0.83% respectively. This shows a 140% increase in the 180 day surface chloride for the very early age exposure compared to that exposed to chlorides from the age of 28 days. The results for the mixes with w/c ratio of 0.5 and 0.3 show a similar trend to that observed for the mix with w/c of 0.4.
The surface chloride values for the exposure intervals of 0–1 and 0–7 days were 1.29 and 1.73% by mass of concrete respectively which compare with the value of 1.96% for the 0–180 days exposure interval. This shows that by early age exposure, high surface chloride values develop quickly. This fact has to be taken into account in modeling chloride diffusion in concrete exposed to chlorides at early ages. 4. Effect of very early age exposure on service life predication Fick’s 2nd law of diffusion is generally used to describe the nonsteady state diffusion of chlorides into concrete. The equation takes the form [6]:
@C @2C ¼D 2 @t @x
ð1Þ
where, C is the total chloride content at the exposure time t at depth of x from the exposed surface and D is the diffusion coefficient. Provided that both the diffusion coefficient and surface chlorides concentration are constant for the duration of exposure, the analytical solution to Eq. (1) gives the empirical model for chloride diffusion expressed in terms of the error function as [6,7,21]:
x C ðx; t Þ ¼ C i þ ðC s C i Þ 1 erf pffiffiffiffiffiffiffiffiffi 2 Da t
ð2Þ
where, C ðx; t Þ is the chloride concentration (% mass of concrete) at the depth x and the time t, C i is initial chloride concentration (% mass of concrete) before exposure to chloride environment, C s is the surface chloride concentrations (% mass of concrete) on the exposed surface, erf is the error function , x is the depth below the exposed surface (m), t is the exposure time (s), Da is the apparent diffusion coefficient of chlorides (m2/s). However such solutions
Fig. 4. Chloride profiles for the mix with w/c = 0.4 for various intervals of exposure.
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cannot be used when time variations of both the diffusion coefficient and surface chloride value are considered [5,6,28–30]. A numerical solution was therefore used in the current study for solving the differential equation of chloride diffusion in concrete. A Crank-Nikolson based finite difference solution was applied and concrete was modeled as a semi-infinite medium taking into account the appropriate initial and boundary conditions. Programing was done in the Matlab environment and the developed program was validated by comparing its results with the well-known Life365 service life prediction software [15]. 4.1. Time variation of the diffusion coefficient The following function has often used for describing variation of diffusion coefficient with time [13,31–33].
Dðt Þ ¼ D0
m t0 t
ð3Þ
where, DðtÞ is the instantaneous concrete diffusion coefficient at time t, D0 is the concrete diffusion coefficient at a reference time t 0 , and m is the aging factor. The apparent diffusion coefficients Da for various time intervals obtained in the current study using the immersion test were given in Table 4. These values are not instantaneous values and correspond to the time domain of the test. Stanish and Thomass [34] presented a methodology for deriving the effective instantaneous time corresponding to the exposure time interval of the immersion test. The method is based on assuming that the apparent diffusion coefficient Da is in-fact an average diffusion coefficient corresponding to the period from the start to the end of the exposure.
Time integration of the function describing instantaneous diffusion coefficient (Eq. (3)), which is in terms of reference time (t 0 ) usually taken as 28 days, the corresponding reference diffusion coefficient D0 , and the unknown aging factor m is then carried out. In this way equivalent instantaneous time is defined in terms of aging factor m and initial and final times of exposure. The method requires at least two sets of experimental data for apparent diffusion coefficient and corresponding exposure times. The method then uses an iterative process beginning by choosing an arbitrary m value and calculating instantaneous time values which in turn allow deriving a new m value. Using the derived m, the process is repeated until m converges to its find value. Using this procedure instantaneous times can be derived which correspond to the diffusion coefficient derived from the immersion tests. This methodology was applied to the results of the immersion tests of the current study and the derived instantaneous values are presented in Fig. 7. Chloride diffusion coefficients obtained by the RCM test (DRCM ), which due to the relatively short duration of the test can be considered as instantaneous values are also given in Fig. 7 for comparison. As seen good correlation exists between the results of the two tests. Based on the immersion test data of Fig. 7, the aging factor (m) for the early age period (less than28 days) and normal age (28 days) were derived as 0.74 and 0.33 respectively. Aging factor m which depends on mix proportions particularly the water/ cement ratio, is an important parameter in service life modeling as it defines time variations of the diffusion coefficient. The reference time t0 was considered as 28 days and D0 value was derived as 13.5 1012 m2/s.
Table 4 Apparent diffusion coefficients derived from the results of the immersion test according to NT Build 443. w/c
Apparent Diffusion coefficient (1012 m2/s) Exposure interval (day)
0.5 0.4 0.3
0–1
0–7
1–7
7–28
0–180
3–180
28–90
28–180
– 262.6 –
– 37.6 –
– 28.49 –
– 13.40 –
16.18 12.26 5.98
– 11.24 –
– 12.36 –
14.19 10.53 4.10
Fig. 5. The effect of age of exposure on 180 days chloride profiles.
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Fig. 6. Effect of w/c ratio on 180 day chloride profiles of mixes with exposure ages of a) 0 days b)28 days.
Table 5 Surface chloride values derived from the results of the immersion test. w/c
Surface chloride concentration (% mass of concrete) Exposure interval (days) 0–1
0–7
1–7
7–28
0–180
3–180
28–90
28–180
0.5 0.4 0.3
– 1.29 –
– 1.73 –
– 1.57 –
– 0.81 –
2.84 1.96 1.44
– 1.83 –
– 0.62 –
1.24 0.83 0.80
4.2. Variations of surface chloride concentrations with time Apart from the diffusion coefficient, the surface chloride Cs is the other parameter needed in modeling chloride ingress into concrete. Unlike the diffusion coefficient which is mainly a material property of concrete, the surface chloride concentration depends on concrete properties and the chloride environment the concrete is exposed to including chloride concentration and exposure conditions [35,36]. Furthermore surface chloride concentration in concrete has a cumulative nature and hence experimental data for
deriving its time variation, will only be valid for the particular exposure age considered. Surface chloride values for concrete increase with duration of exposure. Various researches have proposed functions describing time variation of this parameter [13,37–41]. Yang et. al. [40,41] developed a model based on an equation of the type given below (Eq. (4)), which was reported to provide satisfactory correlations with experimentally determined surface chloride values.
C s ðtÞ ¼ a 1 ebt
ð4Þ
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Fig. 7. Variation of instantaneous chloride diffusion coefficient with time for the mix with w/c = 0.4.
In Eq. (4), t is exposure duration in years and a and b are empirical values. The results of surface chloride values obtained from the immersion tests in the current study for the mix with w/c ratio of 0.4, exposed from the time of casting are plotted in Fig. 8. Eq. (4) was fitted to the experimental data and the function describing the time variations of surface chloride was determined as:
C s ðt Þ ¼ 1:848 1 e434 t
ð5Þ
As can be seen from the Fig. 8, early age exposure has resulted in a rapid rise in surface chloride concentrations. The rapid rise in surface chlorides due to early age exposure was also reported in a study by Chen et al. [16]. They found that for concrete exposed to sea water from the age of 1 day, the surface chloride values peaked at 90 days and no further increase occurred at 6 month and one
year. A further point which needs to be considered in surface chloride evaluation, is the chloride concentration of the solution to which the concrete is exposed. Surface chloride values derived from the immersion test, correspond to a chloride concentration of 10% in the solution, while the chloride level in sea water is generally about 2%. The effect of the level of salinity of water on surface chlorides has been studied by Presuel-Moreno et al. [42] who reported that the surface chloride due to exposure to a solution with chloride concentration of 10% is about twice that exposed to a solution with chloride concentration of 2%. A recent study by Shakouri and Trejo [43] also showed a similar ratio. In deriving the function for describing the time variation of surface chlorides for sea water exposure, a factor of 0.5 was therefore applied to Eq. (5) which was based on the immersion test results, giving the following equation:
C s ðtÞ ¼ 0:924 1 e434 t
Fig. 8. Surface chloride concentrations obtained from the immersion test for exposure from the time of casting.
ð6Þ
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This function which is plotted in Fig. 9 will be used for studying the effect of very early age exposure on service life of reinforced concrete. With regards to normal exposure age of 28 days, Dousti et al. [44] reported measured surface chloride values for a period of up to 3 years. The function for describing time variation of Cs was fitted to the experimental data reported by Dousti et al. [44] for a concrete with 400 kg/m3 of a type II Portland cement with w/c ratio of 0.4 for submerged condition and the result is plotted in Fig. 9. As can be seen, normal age exposure results in a more gradual rise in Cs compared to the very early age exposure. Most researches state that Cs values become constant after a period of time. This time has been reported to be about 5 to 10 years [15,32,41]. For deriving the function describing the time variation of surface chloride values for the case of normal age (28 days)
exposure it was assumed that surface chloride values for both the very early age and normal age exposure (from 28 days) attain same constant value after 10 years. Using the above assumption and also the experimentally determined values of surface chlorides for normal age (28 days) exposure of the current study the derived function takes the form:
C s ðtÞ ¼ 0:919 1 e1:522t
ð7Þ
This function is plotted in Fig. 9 and will be used for service life estimation of concrete with normal age exposure. 4.3. Results of service life prediction The finite difference method was used to determine the chloride profiles in the concrete mix with w/c ratio of 0.4, for the
Fig. 9. Variation of surface chloride with time for the very early age and the normal age (28 day) exposure to chlorides.
Fig. 10. Chloride concentration profiles after 5 years of exposure considering only the time variation of the diffusion coefficient.
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two exposure ages of 0 and 28 days. A chloride concentration of 0.18% by concrete mass was used as the threshold value for initiation of corrosion [45]. To investigate separately the effects of time variations of the diffusion coefficient and the surface chloride values, first an analysis was carried out considering only the time variations of the diffusion coefficient. The surface chloride values for both the very early age and normal age 28 days exposure was considered constant at an arbitrary value of 0.8% of mass of concrete. This value was chosen as it fall within the range reported for site measured surface chloride value of concrete mixes similar to the mix investigated in the current study [13,16,17,20,44]. In Fig. 10 the predicted chloride profiles after 5 years of exposure for the two exposure ages of 0 and 28 day are given.
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As shown the difference is fairly small. The chloride concentration reaches the corrosion threshold value at a depth of 48.0 mm for normal age exposure, while for the case with very early age exposure the corresponding depth is 50.5 mm. With the assumption of constant surface chloride value, time to initiation of corrosion for various depth of cover were determined and the results are presented in Fig. 11. As shown in this figure, for a cover depth of 50 mm the time to initiation of corrosion for the exposure ages of 0 and 28 days are 4.6 and 5.4 years respectively indicating a decrease in service life of about 14% due to very early age exposure. The effect becomes even smaller for higher cover depth, amounting to only a 4% reduction in service life at a cover depth of 90 mm.
Fig. 11. Time to initiation of corrosion at various cover depths assuming only time variation of diffusion coefficient.
Fig. 12. Chloride concentration profiles with time variation of both the diffusion coefficient and the surface chloride values.
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Fig. 13. Time to initiation of corrosion at different depths considering time variation of both the diffusion coefficient and surface chloride values.
Considering the time variations of both the diffusion coefficient and the surface chloride value, the 5 year chloride profiles, for the very early age and the normal age exposures are presented in Fig. 12. As can be seen the difference between profiles for the two exposure ages is considerable. The chloride concentration reach the corrosion threshold at depths of 45.5 mm for the exposure age of 28 days. This depth increases to a value of 53.5 mm for the case of exposure immediately after casting. In Fig. 13 time to initiation of corrosion when time variations of both the diffusion coefficient and surface chloride values are taken into account are presented. For a concrete cover of 50 mm, the time to initiation of corrosion for the two exposure age of 0 and 28 day are 3.9 and 5.9 years respectively. This shows a decrease of about 34% in service life due to very early age exposure compared to normal age exposure. For a cover depth of 90 mm, the predicted service life for normal age and very early age exposures are 26.7 and 23.2 years respectively. This shows a reduction in service life of 13% due to very early age exposure. In Fig. 14, effect of age of exposure on service life of reinforced concrete for the mix considered is summarized. As shown, very early age exposure has a significant effect on service life. The effect become less pronounced with increased depth of cover. 5. Conclusion Based on the results of the experimental and numerical investigation of the effects of very early age exposure of concrete to chlorides, the following conclusions can be stated. - Diffusion coefficients DRCM obtained from the Rapid chloride migration test, show that the effect of concrete age on this parameter is substantial. Diffusion coefficients at the age of 1 day were about an order of magnitude higher than those obtained at the age 180 days. - The effect of concrete age on the apparent diffusion coefficient Da, obtained from the natural diffusion test was also found to be substantial. The Da value corresponding to the time interval from immediately after casting to the age of 1 day was about 25 times higher than that for the interval 28–180 days. However with increasing duration of exposure the effect of early
Fig. 14. Effect of age of exposure on service life of reinforced concrete.
age exposure on apparent diffusion coefficient becomes relatively minor. The Da value for the time interval from immediately after casting to 180 days was only about 16% higher than that of the interval 28–180 days. - The effect of early age exposure on the rate of build up of surface chloride value was found to be very considerable. For the concrete exposed to chlorides immediately after casting, the surface chloride value at the age of 180 days was 2.5 times higher than the 180 day surface chloride value of concrete exposed to chlorides at the age of 28 days. - Using experimental results of the natural diffusion tests functions for defining time variations of both the diffusion coefficient and surface chloride values were derived for both the very early age and normal age exposure conditions. - Using a numerical (finite difference) solution to the governing differential equation of chloride diffusion and considering only the time variation of diffusion coefficients and considering the
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surface chloride values as constant, it was found that effect of very early age exposure to chlorides on service life, compared to that with exposure age of 28 days, was relatively minor 14% at cover depth of 50 mm and 4% at a cover depth of 90 mm. However when both the time variations of diffusion coefficients and surface chloride values were considered, the resulting decrease in service life due to very early age exposure becomes considerable 34% at a cover depth of 50 mm and 13% at a cover depth of 90 mm. - Based on the results of the experimental study and the numerical modelling, it appears that effect of very early age exposure on service life of structures subjected to chlorides is considerable and requires more attention in the durability design of such structures.
Declaration of Competing Interest None. References [1] S. Helland, Design for service life: implementation of fib Model Code 2010 rules in the operational code ISO 16204, Struct. Concr. 14 (1) (2013) 10–18. [2] D.P. Bentz, W.S. Guthrie, S.Z. Jones, N.S. Martys, Predicting service life of steel reinforced concrete exposed to chlorides, Concr. Intern. (2014) 55–64. [3] Y. Wang, L. Wu, Y. Wang, Q. Li, Zh. Xiao, Prediction model of long-term chloride diffusion into plain concrete considering the effect of the heterogeneity of materials exposed to marine tidal zone, Constr. Build. Mater. 159 (2018) 297– 315. [4] E.Ø. Hansen, M.R. Iskau, M.T. Hasholt, Chloride Ingress in Concrete with Different Age at Time of First Chloride Exposure, Nordic Concr. Res. 55 (2) (2016). [5] L.F. Yang, Q. Ma, B. Yu, Analytical solution and experimental validation for dual time-dependent chloride diffusion in concrete, Constr. Build. Mater. 161 (2018) 676–686. [6] L. Tang, L.O. Nilsson, P.A.M. Basheer, Resistance of Concrete to Chloride Ingress: Testing and Modelling, CRC Press is an imprint of Taylor & Francis Group, 2012. [7] E. Poulsen, L. Mejlbro, Diffusion of Chloride in Concrete: Theory and Application, CRC Press, 2010. [8] X. Shi, durability of steel reinforced concrete in chloride environments: an overview, Constr. Build. Mater. 3 (2012) 125–138. [9] P. Spiesz, H.J.H. Brouwers, The apparent and effective chloride migration coefficients obtained in migration tests, Cem. Concr. Res. 48 (2013) 116–127. [10] NT Build 492, , Chloride Migration Coefficient from Non-Steady Sate Migration Experiments, NORDTEST, Finland, 1999. [11] AASHTO-T357, Standard Method of Test for Predicting Chloride Penetration of Hydraulic Cement Concrete by the Rapid Migration Procedure, American Association of State Highway and Transportation Officials, Washington DC, 2015. [12] H. Justnes, M.O. Kim, S. Ng, X. Qian, Methodology of calculating required chloride diffusion coefficient for intended service life as function of concrete cover in reinforced marine structures, Cem. Concr. Compos. 73 (2016) 316– 323. [13] J. Kim, W.J. McCarter, B. Suryanto, S. Nanukuttan, P.A.M. Basheer, T.M. Chrisp, Chloride ingress into marine exposed concrete: comparison of empirical and physically based models, Cem. Concr. Res. 72 (2016) 133–145. [14] J.M. Frederiksen, L. Mejlbro, L.O. Nilsson, Complete solutions for chloride ingress into concrete–with focus on time dependent diffusivity and boundary condition Report TVBM-3146, Division of Building Materials, Lund University, Lund, 2008. [15] Life-365 Service Life Prediction Model, user’s manual, Produced by the Life365TM Consortium III, Version 2.2.1, (2014). [16] H.J. Chen, S.S. Huang, C.W. Tang, M.A. Malek, Lee-Woen Ean, Effect of curing environments on strength, porosity and chloride ingress resistance of blast furnace slag cement concretes: a construction site study, Constr. Build. Mater. 35 (2012) 1063–1070.
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[17] J. Caballero, R.B. Polder, G. Leegwater, A.L. Fraaij, Chloride penetration into cementitious mortar at early age, Heron 57 (3) (2012) 53–168. [18] S.W. Tang, Z.J. Li, E. Chen, H.Y. Shao, Non-steady state migration of chloride ions in cement pastes at early age, RSC Adv. 4 (2014) 48582–48589. [19] W. Yeih, R. Huang, J. Chang, A study of chloride diffusion properties of concrete at early age, J. Marine Sci. Technol. 2 (1) (1994) 61–67. [20] B. Dimitrios, L. Tang, P. Utgenannt, Chloride Ingress in Concrete Exposed to Marine Environment -Field Data Up to 20 Years’ Exposure, CBI betonginstitut, 2014. Final draft rapport. [21] NT Build 443, Concrete Hardened: Accelerated Chloride Penetration, NORDTEST, Finland, 1995. [22] ASTM C150, Standard Specification for Portland Cement, ASTM International, Philadelphia, PA, 2017. [23] ASTM C33, Standard Specification for Concrete Aggregates, ASTM International, Philadelphia, PA, 2017. [24] ASTM C403, Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance, ASTM International, Philadelphia, PA, 2017. [25] BS EN 12390-Part 3, Testing hardened concrete Part 3: Compressive strength of test Specimens, BSI-British Standard Institute, 2009. [26] L. Tang, Guidelines for Practical use of Methods for Testing the Resistance of Concrete to Chloride Ingress, Chlortest, Report D23, SP Swedish National Testing and Research Institute, 2005. [27] ASTM C1152, Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete, ASTM International, Philadelphia, PA, 2017. [28] A. Petcherdchoo, Time dependent models of apparent diffusion coefficient and surface chloride for chloride transport in fly ash concrete, Constr. Build. Mater. 38 (2013) 497–507. [29] A. Petcherdchoo, Closed-form solutions for bilinear surface chloride functions applied to concrete exposed to deicing salts, Cem. Concr. Res. 102 (Suppl. C) (2017) 136–148. [30] L. Wu, W. Li, X. Yu, Time-dependent chloride penetration in concrete in marine environments, Constr. Build. Mater. 152 (2017) 406–413. [31] K.A. Riding, M.D.A. Thomas, K.J. Folliard, Apparent diffusivity model for concrete containing supplementary cementitious materials, ACI Mater. J. 110 (6) (2013) 705–714. [32] L. Tang, P. Utgenannt, Chloride ingress and corrosion from the Swedish field exposures under the marine environment, Workshop proceeding from a Nordic mini-seminar, Hirtshals- Denmark, 12–14. NOVEMBER (2008). [33] M.R. Nokken, A. Boddy, R.D. Hooton, M.D.A. Thomas, Time dependent diffusion in concrete, Cem. Concr. Res. 36 (1) (2006) 200–207. [34] K. Stanish, M. Thomas, The use of bulk diffusion tests to establish timedependent concrete chloride diffusion coefficients, Cem. Concr. Res. 33 (2003) 55–62. [35] Y. Zhang, M. Zhang, Transport properties in unsaturated cement-based materials – A review, Constr. Build. Mater. 72 (2014) 367–379. [36] A. Petcherdchoo, Closed-form solutions for modeling chloride transport in unsaturated concrete under wet-dry cycles of chloride attack, Constr. Build. Mater. 176 (2018) 638–651. [37] M.K. Kassir, M. Ghosn, Chloride-induced corrosion of reinforced concrete bridge decks, Cem. Concr. Res. 32 (1) (2002) 139–143. [38] S.L. Amey, D.A. Johnson, M.A. Miltenberger, H. Farzam, Predicting the service life of concrete marine structures: an environmental methodology, ACI Struct. J. 95 (2) (1998) 205–214. [39] A. Costa, J. Appleton, Chloride penetration into concrete in marine environment-part I: main parameters affecting chloride penetration, Mater. Struct. 32 (1999) 252–259. [40] L.F. Yang, R. Cai, B. Yu, Investigation of computational model for surface chloride concentration of concrete in marine atmosphere zone, Ocean Eng. 138 (2017) 105–111. [41] L.F. Yang, R. Cai, B. Yu, Modeling of environmental action for submerged marine concrete in terms of surface chloride concentration, Struct. Concr. 19 (2018) 1512–1520. [42] F. Presuel-Moreno, W. Arias, V. Echevarria, S. Shill, Y.Y. Wu, Diffusion vs. Concentration of Chloride Ions in Concrete, report No. BDK79-977-03, Center for Marine Materials, Florida Atlantic University, 2014. [43] M. Shakouri, D. Trejo, A study of the factors affecting the surface chloride maximum phenomenon in submerged concrete samples, Cem. Concr. Compos. 94 (2018) 181–190. [44] A. Dousti, R. Rashetnia, M. Shekarchi, Effect of different exposure conditions on chloride diffusion in marine environment, in: Second International Conference on Microstructural-related Durability of Cementitious Composites, 11-13 April 2012, Amsterdam, The Netherlands, 2012. [45] U. Angst, B. Elsener, C.K. Larsen, Ø. Vennesland, Critical chloride content in reinforced concrete d a review, Cem. Concr. Res. 39 (12) (2009) 1122–1138.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation of chloride ingress into concrete under very early age exposure conditions Alireza Bagheri ⇑, Abbas Ajam, Hamed Zanganeh Civil Engineering Faculty, K. N. Toosi University of Technology, Tehran, Iran
h i g h l i g h t s Effect of very early age exposure on chloride ingress in concrete was experimentally investigated. Functions describing time variations of diffusion coefficient and surface chloride value were derived. Very early age exposure was found to have considerable effect on service life.
a r t i c l e
i n f o
Article history: Received 14 March 2019 Received in revised form 17 June 2019 Accepted 19 July 2019
Keywords: Chloride ingress Very early age exposure Surface chloride value Service life
a b s t r a c t Some structures such as cast in place reinforced concrete piles and quay walls are exposed to chlorides from the moment of casting. Considering the lack of previous work on such an early age exposure, the current study was carried out. Based on an experimental study, functions describing time variations of diffusion coefficient and surface chloride values for both the normal age and very early age exposures were derived. Using these functions, the effects of very early age exposure on service life was investigated. The results show that very early age exposure considerably increases chloride penetration into concrete, reducing service life by about 34 and 13% for concrete covers of 50 and 90 mm respectively. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Chloride induced corrosion of reinforcement is the major source of deterioration in reinforced concrete structures, particularly in marine structures and bridge decks [1–4]. Considerable research effort has therefore been directed towards chloride ingress into concrete and a vast body of knowledge has been accumulated on the subject [5–8]. Research has led to the development of standardized test methods for appraisal of chloride resistance of concrete mixes [6,9–11]. Numerical and analytical methods have also been developed which make possible the prediction of chloride profiles in concrete structures at various times and facilitate service life prediction of reinforced concrete structures [6,12–15]. An area which has received less research attention is the early age exposure of concrete to chloride environments [16–18]. Some structures such as cast in place reinforced concrete piles and quay walls in coastal areas are subjected to very early age exposure and become in contact with chloride laden environment from the time ⇑ Corresponding author at: 470, Mirdamad Ave. West, 19697 Tehran, Iran. E-mail addresses: [email protected] (A. Bagheri), [email protected] (A. Ajam). https://doi.org/10.1016/j.conbuildmat.2019.07.225 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
of casting. Concrete in such structures is in fact exposed to chlorides before setting and high initial penetration rates are expected. Yeih et al. [19] carried out an experimental study for determining the effect of age of exposure on chloride diffusion coefficient of concrete and mortar specimens. The study was limited to initial exposure times of 7 and 28 days and it was reported that exposure at 7 days resulted in a 50 %increase in diffusion coefficient compared to that of exposure at 28 days. Caballero et al. [17] used the Rapid Chloride Migration (RCM) test method to derive diffusion coefficients of mortar specimens at the ages of 1, 2, 3, 7, 14, 21 and 28 days. They also used the natural diffusion test to determine the apparent diffusion coefficients for various early age exposure intervals, including 1.5–4, 3–8 and 7–16 days. Two different cement types, namely, CEM I (normal Portland cement) and CEM III/B (slag cement which has a lower rate of hydration) were considered. The results showed that diffusion coefficients at the age of 1 day were one and two order of magnitudes higher compared to the 28 day values for the mixes with cement types CEM I and CEM III/B respectively. Although valuable data on early age diffusion coefficients were derived, the experimental work only considered short durations of exposure of up to a few days and the effect of early age exposure on chloride diffusion coefficients at longer exposure
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durations was not experimentally determined by these researchers. Based on numerical modelling considering only the time variation of the diffusion coefficient and assuming the surface chloride value as constant, these researchers reported that the effect of early age exposure on long term chloride ingress into concrete was relatively minor (less than 10%). Chen et al. [16] studied chloride ingress into concrete specimens exposed to sea water at the age of 1 day. The study did not include early age profile determinations and these were only determined at ages of 90, 180 and 360 days. As expected some decrease in chloride diffusion coefficient, was observed with increased duration of exposure. The study also yielded surface chloride values at the ages of 90, 180 and 360 days. The surface chloride values of the specimens which were exposed to chlorides at the age of 1 day were found to be quite high at 90 days and little further increase in in this parameter occurred at 180 and 360 days. This is in contrast to general experience which indicates that with normal ages of exposure such as 28 days, the buildup of surface chloride with time takes place gradually [13,20]. Unfortunately the study by Chen et al. [16] did not include control specimens with normal exposure ages such as 28 days and it was therefore not possible to directly compare chloride ingress due to early age exposure with that of normal age exposure. Considering the limited previous research on early age exposure of concrete to chlorides and particularly lack of previous work on very early age exposure, where concrete is cast against chloride laden environment, the present study was carried out to provide further information in this regard. The study included experimental work for determining the effects of variations in exposure age on diffusion coefficient and surface chloride values. Numerical modeling using experimentally derived time variations of diffusion coefficients and surface chloride values was also carried out to quantify the effects of early age exposure on service life of reinforced concrete. 2. Experimental program Three concrete mixes with w/c ratios of 0.3, 0.4 and 0.5 were considered for the experimental investigation. The methods used for evaluation of chloride resistance of these mixes included the natural diffusion test by immersion according to NT Build 443 [21] and the Rapid chloride migration (RCM) test according to NT Build
492 [10]. For the immersion test, the initial exposure ages to chlorides included immediate exposure after casting and exposure at the ages of 1, 3, 7 and 28 days. The duration of exposure extended up to 180 days so that longer time effects of early age exposure could be studied. For the immersion test the derived diffusion coefficient is considered as the apparent diffusion coefficient which depends on both the age at start of exposure and the duration of exposure. Tests with short exposure interval of 0–1, 0–7, 1–7 and 7–28 days were also considered to provide data with of regards to variation of diffusion coefficient at early ages. Rapid migration tests were carried out for determining instantaneous chloride diffusion coefficients at various ages including 1, 3, 7, 28, 90 and 180 days. The study also included determination of development of electrical resistivity of various mixes with time in order to provide qualitative data with regards to pore connectivity (in early age) particularly before and around the setting of concrete. Time development of compressive strength of various mixes were also determined. Details of the materials, mixes and test methods used are given in the following sections. 2.1. Materials used and mixture properties The cement used for the study was a type II Portland cement conforming to the requirements of ASTM C150 [22]. The compound composition of the cement comprised of 48.1 C3S, 24.3 C2S, 7.6 C3A and 12.0 C4AF. The initial and final setting times of the cement were determined as 105 and 150 min respectively. Water of drinkable quality was used for production of the mixes. The aggregates used for production of mixes comprised of a crushed coarse aggregate with nominal maximum size of 19 mm and specific gravity of 2.58 and a river sand with specific gravity of 2.49. The aggregates conformed to the requirements of ASTM C 33 [23]. Three concrete mixture with w/c ratios of 0.3, 0.4 and 0.5 were considered for the study. Details of these mixes are given in Table 1. The slump value of all mixes were kept in the range 125 ± 25 mm, through the use of required quantities of a polycarboxylate ether type superplasticizer. The specimens for various tests were subjected to wet curing as tests carried out in this study require the specimens be in saturated state prior to conducting the test. Being exposed to chlorides in a saturated state ensures that diffusion is the main mechanism for chloride ingress. 2.2. Tests carried out -Initial and final setting times of concrete mixture was determined according to ASTM C403 [24] on samples prepared by sieving the mixtures on No.4 (4.75 mm) standard sieve. -Compressive strength test was carried out at the ages of 1, 3, 7, 28, 90, 180 days on 100 mm cubic specimens in accordance with BS EN 12390 part 3 [25]. -Electrical resistance of concrete mixture were monitored at the ages of 0.5, 1, 2, 3, 4, 5, 6, 8 and 10 h and 1, 3, 7, 28, 90 and 180 days. The test was conducted according to the procedure described by the Swedish National Testing and Research Institute [26] using an AC current based instrument. For the period before demolding of specimen, the electrical resistance was monitored by two copper plates placed in the two opposite faces of the molds (Fig. 1).
Table 1 Mixture proportions for concrete mixture studied. Mix Designation
W/C
Cement (kg/m3)
Water (kg/m3)
SP (% cement)
Coarse agg. SSD (kg/m3)
Fine agg. SSD (kg/m3)
1 2 3
0.50 0.40 0.30
420.0 420.0 420.0
210.0 168.0 126.0
– 0.47 1.48
813 867 920
813 867 920
Fig. 1. Determination of electrical resistivity a) From the time of casting until demolding b) after demolding.
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A. Bagheri et al. / Construction and Building Materials 225 (2019) 801–811 -Chloride profile determination under natural diffusion was carried out in accordance with NT Build 443 [21] using a solution with NaCl concentration of 16.5%. At the end of exposure periods considered, powder samples at 2 mm depth intervals were taken by grinding of the specimens (Fig. 2). Powder samples were subsequently analyzed according to ASTM C1152 [27] and chloride concentrations as % weight of concrete mass were determined. By fitting the error function solution of Fick’s second law of diffusion to the experimentally derived chloride concentrations at various depths, apparent diffusion coefficient Da and surface chloride concentrations Cs were derived. -The Rapid Chloride Migration (RCM) test was carried out at ages of 1, 3, 7, 28, 90 and 180 days in accordance to NT Build 492 [10] on various mixes studied. The top end of the cylindrical test specimens were exposed to a 0.3 M NaOH solution with the opposite end being exposed to a 10% NaCl solution. A 10–60 V DC external potential was imposed across the specimen for the required time interval (in most cases 24 h). The specimens were then split open, and a silver nitrate solution was sprayed to the split section and the depth of penetration of the chloride ions was determined using a colorimetric method. The non-steady-state migration coefficient Dnssm was then calculated using the average penetration depth of the chloride ions.
3. Results and discussions 3.1. Compressive strength In Table 2 compressive strength values at various ages from 1 to 180 days are presented. The strength values increase with increasing age and the considerable effect of reduced w/c ratio on strength enhancement are evident in the results. 3.2. Concrete electrical resistivity and initial and final setting times The results of electrical resistivity of concrete mixes which were measured from the time of casting up to 180 days are depicted in Fig. 3. As the results show the electrical resistivity of mixes with lower W/C ratios even from the very early ages are higher than those with higher W/C ratio. It is interesting to note that there is an initial reduction in electrical resistivity in the first few hours after casting. The value then becomes stable which lasts for a few hours after which electrical resistivity increases rapidly. The observed
Fig. 3. Variation of electrical resistivity of concrete mixes with age.
trend can be attributed to various ions being released into the pore solution due to initial cement dissolution which increase the electrical conductivity of the pore solution phase. During the dormant period the electrical resistance is almost constant, after which electrical resistance rises due to progress of hydration of cement which results in the development of the microstructure and reduces connectivity of pores in the cement paste phase. The initial setting times of concrete mixes with w/c ratio of 0.3, 0.4 and 0.5 were determined as 5.3, 4.4 and 5.6 h respectively. The corresponding values for the final setting times were 7.7, 5.8 and 7.6 h which show that the rise in electrical resistance has occurred close to or shortly after final setting of concrete mixes. 3.3. Rapid chloride migration test Chloride diffusion coefficients derived from the RCM test at the ages of 1, 3, 7, 28, 90 and 180 days are given in Table 3. The results show high early age values which rapidly decrease with the progress of hydration. The 180 days diffusion coefficients were about an order of magnitude lower than the corresponding values at the age of 1 day. 3.4. Natural diffusion by immersion test By fitting the error function solution to Ficks 2nd law of diffusion to the experimentally determined chloride concentrations at various depths, the apparent diffusion coefficient (Da) and surface chloride value (Cs) for each exposure intervals were derived. Chloride profiles for the mix with w/c ratio of 0.4 and exposure intervals of 0–1, 0–7, 1–7 and 7–28 days are depicted in Fig. 4. In the exposure intervals, the first number shows the age of concrete at the start of exposure to chlorides and the second number shows the age of concrete at the end of exposure interval. For instance exposure interval (0–1) means specimens were exposed to chlorides from the time of casting and exposure continued up to the age of 1 day. Similarly exposure interval of (1–7) days means that specimens were exposed to chlorides at the age of 1 day and exposure continued up to the age of 7 days, i.e., exposure duration of 6 days. The derived values of diffusion coefficients which are presented in Table 4, show very high values at very early ages (0–
Fig. 2. Grinding equipment.
Table 2 The results of compressive strength test. W/C
0.5 0.4 0.3
Compressive Strength (MPa) 1 day
3 day
7 day
28 day
90 day
180 day
9.9 11.7 14.6
24.5 28.6 38.3
34.6 41.6 54.1
48.0 55.4 62.7
52.9 61.6 75.1
57.1 68.1 78.9
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Table 3 Results of the Rapid Chloride Migration (RCM) test. w/c
0.5 0.4 0.3
Diffusion coefficient (1012 m2/s) 1 day
3 day
7 day
28 day
90 day
180 day
132 118 107
47 41 32
34 26 19
21 17 15
19 13 11
16 11 9
1 day interval) of 262 1012 m2/s. This value is about 20 times higher than the corresponding value for the 7–28 days interval. The effect of early age exposure on longer term chloride profiles can be observed by comparing the 180 day chloride profiles for the exposure ages of 0, 3 and 28 days. These profiles for the mix with w/c ratio of 0.4 are presented in Fig. 5 and the corresponding diffusion coefficients are given in Table 4. The results show that very early age exposure has a relatively small influence on later age apparent diffusion coefficients. Exposure at ages of 0 and 3 days result in increases of only 16% and 7% respectively in the apparent diffusion coefficient compared to that with exposure age of 28 days. The effect of w/c ratio on 180 day chloride profiles of mixes exposed to chlorides form the time of casting and 28 days after casting are shown in Fig. 6 and the derived diffusion coefficients are presented in Table 4. The observed trends in the results of the mixes with w/c ratios of 0.5 and 0.3 are in general agreement with those of the mix with w/c ratio of 0.4 and confirm the relatively minor effect of early age exposure on long term diffusion coefficients. The surface chloride values for the mixes studied which were derived from the experimentally determined chloride profiles are given in Table 5. It is interesting to note that although early age of exposure, only had a small effect on longer term diffusion coefficients, the effect on surface chlorides was found to be significant. For the mix with w/c ratio of 0.4 and for exposure ages of 0 and 28 days the 180 day, Cs values expressed as % weight of concrete were determined as 1.96 and 0.83% respectively. This shows a 140% increase in the 180 day surface chloride for the very early age exposure compared to that exposed to chlorides from the age of 28 days. The results for the mixes with w/c ratio of 0.5 and 0.3 show a similar trend to that observed for the mix with w/c of 0.4.
The surface chloride values for the exposure intervals of 0–1 and 0–7 days were 1.29 and 1.73% by mass of concrete respectively which compare with the value of 1.96% for the 0–180 days exposure interval. This shows that by early age exposure, high surface chloride values develop quickly. This fact has to be taken into account in modeling chloride diffusion in concrete exposed to chlorides at early ages. 4. Effect of very early age exposure on service life predication Fick’s 2nd law of diffusion is generally used to describe the nonsteady state diffusion of chlorides into concrete. The equation takes the form [6]:
@C @2C ¼D 2 @t @x
ð1Þ
where, C is the total chloride content at the exposure time t at depth of x from the exposed surface and D is the diffusion coefficient. Provided that both the diffusion coefficient and surface chlorides concentration are constant for the duration of exposure, the analytical solution to Eq. (1) gives the empirical model for chloride diffusion expressed in terms of the error function as [6,7,21]:
x C ðx; t Þ ¼ C i þ ðC s C i Þ 1 erf pffiffiffiffiffiffiffiffiffi 2 Da t
ð2Þ
where, C ðx; t Þ is the chloride concentration (% mass of concrete) at the depth x and the time t, C i is initial chloride concentration (% mass of concrete) before exposure to chloride environment, C s is the surface chloride concentrations (% mass of concrete) on the exposed surface, erf is the error function , x is the depth below the exposed surface (m), t is the exposure time (s), Da is the apparent diffusion coefficient of chlorides (m2/s). However such solutions
Fig. 4. Chloride profiles for the mix with w/c = 0.4 for various intervals of exposure.
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cannot be used when time variations of both the diffusion coefficient and surface chloride value are considered [5,6,28–30]. A numerical solution was therefore used in the current study for solving the differential equation of chloride diffusion in concrete. A Crank-Nikolson based finite difference solution was applied and concrete was modeled as a semi-infinite medium taking into account the appropriate initial and boundary conditions. Programing was done in the Matlab environment and the developed program was validated by comparing its results with the well-known Life365 service life prediction software [15]. 4.1. Time variation of the diffusion coefficient The following function has often used for describing variation of diffusion coefficient with time [13,31–33].
Dðt Þ ¼ D0
m t0 t
ð3Þ
where, DðtÞ is the instantaneous concrete diffusion coefficient at time t, D0 is the concrete diffusion coefficient at a reference time t 0 , and m is the aging factor. The apparent diffusion coefficients Da for various time intervals obtained in the current study using the immersion test were given in Table 4. These values are not instantaneous values and correspond to the time domain of the test. Stanish and Thomass [34] presented a methodology for deriving the effective instantaneous time corresponding to the exposure time interval of the immersion test. The method is based on assuming that the apparent diffusion coefficient Da is in-fact an average diffusion coefficient corresponding to the period from the start to the end of the exposure.
Time integration of the function describing instantaneous diffusion coefficient (Eq. (3)), which is in terms of reference time (t 0 ) usually taken as 28 days, the corresponding reference diffusion coefficient D0 , and the unknown aging factor m is then carried out. In this way equivalent instantaneous time is defined in terms of aging factor m and initial and final times of exposure. The method requires at least two sets of experimental data for apparent diffusion coefficient and corresponding exposure times. The method then uses an iterative process beginning by choosing an arbitrary m value and calculating instantaneous time values which in turn allow deriving a new m value. Using the derived m, the process is repeated until m converges to its find value. Using this procedure instantaneous times can be derived which correspond to the diffusion coefficient derived from the immersion tests. This methodology was applied to the results of the immersion tests of the current study and the derived instantaneous values are presented in Fig. 7. Chloride diffusion coefficients obtained by the RCM test (DRCM ), which due to the relatively short duration of the test can be considered as instantaneous values are also given in Fig. 7 for comparison. As seen good correlation exists between the results of the two tests. Based on the immersion test data of Fig. 7, the aging factor (m) for the early age period (less than28 days) and normal age (28 days) were derived as 0.74 and 0.33 respectively. Aging factor m which depends on mix proportions particularly the water/ cement ratio, is an important parameter in service life modeling as it defines time variations of the diffusion coefficient. The reference time t0 was considered as 28 days and D0 value was derived as 13.5 1012 m2/s.
Table 4 Apparent diffusion coefficients derived from the results of the immersion test according to NT Build 443. w/c
Apparent Diffusion coefficient (1012 m2/s) Exposure interval (day)
0.5 0.4 0.3
0–1
0–7
1–7
7–28
0–180
3–180
28–90
28–180
– 262.6 –
– 37.6 –
– 28.49 –
– 13.40 –
16.18 12.26 5.98
– 11.24 –
– 12.36 –
14.19 10.53 4.10
Fig. 5. The effect of age of exposure on 180 days chloride profiles.
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Fig. 6. Effect of w/c ratio on 180 day chloride profiles of mixes with exposure ages of a) 0 days b)28 days.
Table 5 Surface chloride values derived from the results of the immersion test. w/c
Surface chloride concentration (% mass of concrete) Exposure interval (days) 0–1
0–7
1–7
7–28
0–180
3–180
28–90
28–180
0.5 0.4 0.3
– 1.29 –
– 1.73 –
– 1.57 –
– 0.81 –
2.84 1.96 1.44
– 1.83 –
– 0.62 –
1.24 0.83 0.80
4.2. Variations of surface chloride concentrations with time Apart from the diffusion coefficient, the surface chloride Cs is the other parameter needed in modeling chloride ingress into concrete. Unlike the diffusion coefficient which is mainly a material property of concrete, the surface chloride concentration depends on concrete properties and the chloride environment the concrete is exposed to including chloride concentration and exposure conditions [35,36]. Furthermore surface chloride concentration in concrete has a cumulative nature and hence experimental data for
deriving its time variation, will only be valid for the particular exposure age considered. Surface chloride values for concrete increase with duration of exposure. Various researches have proposed functions describing time variation of this parameter [13,37–41]. Yang et. al. [40,41] developed a model based on an equation of the type given below (Eq. (4)), which was reported to provide satisfactory correlations with experimentally determined surface chloride values.
C s ðtÞ ¼ a 1 ebt
ð4Þ
A. Bagheri et al. / Construction and Building Materials 225 (2019) 801–811
807
Fig. 7. Variation of instantaneous chloride diffusion coefficient with time for the mix with w/c = 0.4.
In Eq. (4), t is exposure duration in years and a and b are empirical values. The results of surface chloride values obtained from the immersion tests in the current study for the mix with w/c ratio of 0.4, exposed from the time of casting are plotted in Fig. 8. Eq. (4) was fitted to the experimental data and the function describing the time variations of surface chloride was determined as:
C s ðt Þ ¼ 1:848 1 e434 t
ð5Þ
As can be seen from the Fig. 8, early age exposure has resulted in a rapid rise in surface chloride concentrations. The rapid rise in surface chlorides due to early age exposure was also reported in a study by Chen et al. [16]. They found that for concrete exposed to sea water from the age of 1 day, the surface chloride values peaked at 90 days and no further increase occurred at 6 month and one
year. A further point which needs to be considered in surface chloride evaluation, is the chloride concentration of the solution to which the concrete is exposed. Surface chloride values derived from the immersion test, correspond to a chloride concentration of 10% in the solution, while the chloride level in sea water is generally about 2%. The effect of the level of salinity of water on surface chlorides has been studied by Presuel-Moreno et al. [42] who reported that the surface chloride due to exposure to a solution with chloride concentration of 10% is about twice that exposed to a solution with chloride concentration of 2%. A recent study by Shakouri and Trejo [43] also showed a similar ratio. In deriving the function for describing the time variation of surface chlorides for sea water exposure, a factor of 0.5 was therefore applied to Eq. (5) which was based on the immersion test results, giving the following equation:
C s ðtÞ ¼ 0:924 1 e434 t
Fig. 8. Surface chloride concentrations obtained from the immersion test for exposure from the time of casting.
ð6Þ
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This function which is plotted in Fig. 9 will be used for studying the effect of very early age exposure on service life of reinforced concrete. With regards to normal exposure age of 28 days, Dousti et al. [44] reported measured surface chloride values for a period of up to 3 years. The function for describing time variation of Cs was fitted to the experimental data reported by Dousti et al. [44] for a concrete with 400 kg/m3 of a type II Portland cement with w/c ratio of 0.4 for submerged condition and the result is plotted in Fig. 9. As can be seen, normal age exposure results in a more gradual rise in Cs compared to the very early age exposure. Most researches state that Cs values become constant after a period of time. This time has been reported to be about 5 to 10 years [15,32,41]. For deriving the function describing the time variation of surface chloride values for the case of normal age (28 days)
exposure it was assumed that surface chloride values for both the very early age and normal age exposure (from 28 days) attain same constant value after 10 years. Using the above assumption and also the experimentally determined values of surface chlorides for normal age (28 days) exposure of the current study the derived function takes the form:
C s ðtÞ ¼ 0:919 1 e1:522t
ð7Þ
This function is plotted in Fig. 9 and will be used for service life estimation of concrete with normal age exposure. 4.3. Results of service life prediction The finite difference method was used to determine the chloride profiles in the concrete mix with w/c ratio of 0.4, for the
Fig. 9. Variation of surface chloride with time for the very early age and the normal age (28 day) exposure to chlorides.
Fig. 10. Chloride concentration profiles after 5 years of exposure considering only the time variation of the diffusion coefficient.
A. Bagheri et al. / Construction and Building Materials 225 (2019) 801–811
two exposure ages of 0 and 28 days. A chloride concentration of 0.18% by concrete mass was used as the threshold value for initiation of corrosion [45]. To investigate separately the effects of time variations of the diffusion coefficient and the surface chloride values, first an analysis was carried out considering only the time variations of the diffusion coefficient. The surface chloride values for both the very early age and normal age 28 days exposure was considered constant at an arbitrary value of 0.8% of mass of concrete. This value was chosen as it fall within the range reported for site measured surface chloride value of concrete mixes similar to the mix investigated in the current study [13,16,17,20,44]. In Fig. 10 the predicted chloride profiles after 5 years of exposure for the two exposure ages of 0 and 28 day are given.
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As shown the difference is fairly small. The chloride concentration reaches the corrosion threshold value at a depth of 48.0 mm for normal age exposure, while for the case with very early age exposure the corresponding depth is 50.5 mm. With the assumption of constant surface chloride value, time to initiation of corrosion for various depth of cover were determined and the results are presented in Fig. 11. As shown in this figure, for a cover depth of 50 mm the time to initiation of corrosion for the exposure ages of 0 and 28 days are 4.6 and 5.4 years respectively indicating a decrease in service life of about 14% due to very early age exposure. The effect becomes even smaller for higher cover depth, amounting to only a 4% reduction in service life at a cover depth of 90 mm.
Fig. 11. Time to initiation of corrosion at various cover depths assuming only time variation of diffusion coefficient.
Fig. 12. Chloride concentration profiles with time variation of both the diffusion coefficient and the surface chloride values.
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Fig. 13. Time to initiation of corrosion at different depths considering time variation of both the diffusion coefficient and surface chloride values.
Considering the time variations of both the diffusion coefficient and the surface chloride value, the 5 year chloride profiles, for the very early age and the normal age exposures are presented in Fig. 12. As can be seen the difference between profiles for the two exposure ages is considerable. The chloride concentration reach the corrosion threshold at depths of 45.5 mm for the exposure age of 28 days. This depth increases to a value of 53.5 mm for the case of exposure immediately after casting. In Fig. 13 time to initiation of corrosion when time variations of both the diffusion coefficient and surface chloride values are taken into account are presented. For a concrete cover of 50 mm, the time to initiation of corrosion for the two exposure age of 0 and 28 day are 3.9 and 5.9 years respectively. This shows a decrease of about 34% in service life due to very early age exposure compared to normal age exposure. For a cover depth of 90 mm, the predicted service life for normal age and very early age exposures are 26.7 and 23.2 years respectively. This shows a reduction in service life of 13% due to very early age exposure. In Fig. 14, effect of age of exposure on service life of reinforced concrete for the mix considered is summarized. As shown, very early age exposure has a significant effect on service life. The effect become less pronounced with increased depth of cover. 5. Conclusion Based on the results of the experimental and numerical investigation of the effects of very early age exposure of concrete to chlorides, the following conclusions can be stated. - Diffusion coefficients DRCM obtained from the Rapid chloride migration test, show that the effect of concrete age on this parameter is substantial. Diffusion coefficients at the age of 1 day were about an order of magnitude higher than those obtained at the age 180 days. - The effect of concrete age on the apparent diffusion coefficient Da, obtained from the natural diffusion test was also found to be substantial. The Da value corresponding to the time interval from immediately after casting to the age of 1 day was about 25 times higher than that for the interval 28–180 days. However with increasing duration of exposure the effect of early
Fig. 14. Effect of age of exposure on service life of reinforced concrete.
age exposure on apparent diffusion coefficient becomes relatively minor. The Da value for the time interval from immediately after casting to 180 days was only about 16% higher than that of the interval 28–180 days. - The effect of early age exposure on the rate of build up of surface chloride value was found to be very considerable. For the concrete exposed to chlorides immediately after casting, the surface chloride value at the age of 180 days was 2.5 times higher than the 180 day surface chloride value of concrete exposed to chlorides at the age of 28 days. - Using experimental results of the natural diffusion tests functions for defining time variations of both the diffusion coefficient and surface chloride values were derived for both the very early age and normal age exposure conditions. - Using a numerical (finite difference) solution to the governing differential equation of chloride diffusion and considering only the time variation of diffusion coefficients and considering the
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surface chloride values as constant, it was found that effect of very early age exposure to chlorides on service life, compared to that with exposure age of 28 days, was relatively minor 14% at cover depth of 50 mm and 4% at a cover depth of 90 mm. However when both the time variations of diffusion coefficients and surface chloride values were considered, the resulting decrease in service life due to very early age exposure becomes considerable 34% at a cover depth of 50 mm and 13% at a cover depth of 90 mm. - Based on the results of the experimental study and the numerical modelling, it appears that effect of very early age exposure on service life of structures subjected to chlorides is considerable and requires more attention in the durability design of such structures.
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