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The effects of hydraulic pressure and crack width on water permeability of penetration crack-induced concrete
Construction and Building Materials 25 (2011) 2576–2583
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The effects of hydraulic pressure and crack width on water permeability of penetration crack-induced concrete Seong-Tae Yi a,⇑, Tae-Yang Hyun b, Jin-Keun Kim c a
Department of Civil and Environmental Engineering, Inha Technical College, 253, Yong Hyun-dong, Nam-gu, Inchon-si 402-752, South Korea Material Division, Hyundai Institute of Construction Technology, 102-4, Mabuk-dong, Gihung-gu, Yongin-si 446-716, South Korea c Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon-si 305-701, South Korea b
a r t i c l e
i n f o
Article history: Received 31 December 2009 Received in revised form 30 September 2010 Accepted 26 November 2010 Available online 24 December 2010 Keywords: Hydraulic pressure Crack width Water permeability Leakage Cracked concrete Autogenous healing
a b s t r a c t Cracks in concrete generally interconnect flow paths and increase the permeability of concrete. The increase of permeability due to gradual crack growth allows more water or aggressive chemical ions to penetrate the concrete and facilitate deterioration. This research aims to study water permeability and how it is affected by hydraulic pressure and crack widths in cracked concrete. Tests were carried out as a function of hydraulic pressure and crack width, using the splitting and reuniting method to manufacture concrete specimens with controlled crack widths. Crack widths were examined using a microscope. The results showed a considerable increase in water transport as crack width and hydraulic pressure increased. But when the crack width was smaller than 50 lm, it had little effect on concrete permeability. Due to autogenous healing, the water flow through such cracks was gradually reduced over time. However, when the crack width was between 50 and 100 lm and hydraulic pressure was greater than 0.025 MPa, concrete permeability increased rapidly. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cracks in concrete members occur for various reasons including drying shrinkage, external loading, etc. Although cracks themselves do not directly affect the serviceability and durability of reinforced concrete (RC) structures, a decrease in structural performance results from repetition and acceleration of a cycle centered on the occurrence of cracks and an increase in their permeability. In the case of water leaking through cracks, not only the serviceability and durability of structures but so their external appearance is damaged. In addition, additional problems resulting from water leakage can be compounded. More specifically, for deep underground floors of a high-rise building, underground structures such as underground railways and tunnels, and water storage facilities such as ocean structures, dams, and filtration plants, when high hydraulic pressure is applied, water leakage problems caused by structural cracking are intensified. Consequently, during and after construction, considerable sums have been spent on repairs related to water drainage [1,2]. Up to now, crack width codes in South Korea have prescribed only exposure conditions and purposes in use of structures. However, rules for water leaks and experimental methods for water ⇑ Corresponding author. Tel.: +82 32 870 2237; fax: +82 32 870 2510. E-mail address: [email protected] (S.-T. Yi). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.11.107
permeability coefficients of crack-induced concrete have not been established. However, since there are limits on stipulating rules that apply to water leakages in structures using crack width only, new rules for water permeability coefficients and water leak contents are urgently required. To endorse such rules, a systematic study on water permeability coefficients of crack-induced concrete under high-water pressure is necessary. Currently, the water permeability coefficient of concrete is obtained indirectly by measuring the diffusion coefficient of chlorine ion or by calculating the passing distance of the water within the specimen after applying water pressure to a concrete specimen. When there is a crack penetrating the concrete, however, most of the leaked water is caused by the crack. Accordingly, in this case, the method for measuring water permeability coefficients should be markedly different from existing methods, which have mainly relied on low-pressure water permeability tests (WPT) [3–7]. However, these test methods have problems taking into account the effects of highly pressurized water. Accordingly, when high-water pressure is applied, it is not easy to directly estimate the characteristics of water penetration coefficients of concrete through a crack. In addition, most research studies for water permeability tests of concrete with existing cracks have induced cracks by using a feedback-controlled splitting test method [3,8]. From this method, however, it is not easy to clarify whether or not the cracks within the specimen develop in an identical pattern.
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In this study, to correctly investigate the water permeability coefficients of penetration crack-induced concrete, the high-water pressure was directly controlled. In addition, identical crack pattern problems within the specimen were solved by splitting the specimen in two and reassembling it afterwards confirming the pattern. Namely, an experimental apparatus and the method for preparing the specimen were newly developed.
The most significant factor influencing autogenous healing is the precipitation of calcium carbonate. It is noted that growth velocity of the precipitation is affected by hydraulic pressure and crack width. However, the influence of concrete proportion and degree of water hardness was found to be minor [10]. The water permeability coefficient through the crack due to autogenous healing progressively decreased and, in the extreme case, the crack closed perfectly, likely preventing further leakage through the crack.
2. Water permeability theory of concrete 3. Permeability experiments 2.1. General 3.1. Mixture proportioning When a crack occurs in concrete, water permeability patterns within the concrete are largely changed. Various factors influence concrete cracks such as drying shrinkage, thermal stress, corrosion of the reinforcing bar, poor quality construction, and external loading. Generally speaking, however, external loads applied to the structure and fluctuations in the tensile stress due to changes in the volume of the concrete are considered major factors. Cracks that are present irregularly at the initial stage are connected by additional cracks, and, finally, become larger penetration cracks. Such cracks increase the water permeability coefficient of concrete from several hundred times to several thousand times and, in such cases, the water permeability coefficient is controlled by a crack [9]. Under normal circumstances, cracks are not simple geometrically and range in size from several microns to several millimeter. Sometimes, only one crack can occur and the water permeability coefficient in such cases is bigger than that of the concrete without cracks. In such cases, the electric influence within the concrete and the effect of adsorption water, which have relatively minor values, may be ignored. 2.2. Darcy flow The basic model of concrete water permeability is illustrated using Darcy flow. When it is assumed that both surfaces of a concrete specimen are in contact with a liquid that can be infinitely supplied, the liquid moves through the concrete due to differences in pressure. The flow of liquid within concrete saturated by the liquid differs from that of unsaturated concrete. However, in the case of leakage through concrete, water leaks under saturated conditions are more important and predominant since the thickness of concrete is thinner compared to the ground and pressure is continuously applied. Eq. (1) shows the water permeability coefficient K (mm/s) with unit pressure and unit time when static pressure is applied.
K¼
Ql Aht
ð1Þ
where Q is the flow rate through the specimen (mm3); l is the thickness of the specimen (mm); A is the cross-sectional area of the concrete specimen (mm2); h is the drop in hydraulic head across the specimen (mm); and t is the time required for a certain amount of water (s). 2.3. Autogenous healing of cracks in concrete Before loads are applied, autogenous healing, which gradually decreases crack widths, including microcracks within concrete, may occur for the following reasons: (a) hydration and swelling of cement paste; (b) precipitation of calcium carbonate crystals; (c) blocking of flow path by water impurities; and (d) blocking of flow path by concrete particles broken from the crack surface due to cracking.
Table 1 lists the concrete mixture proportion selected for the permeability test and the 28 days compressive strength cylinder specimens. Ordinary Portland cement (ASTM Type I) was used in all mixtures. Crushed gravel was used as the coarse aggregate and the maximum aggregate size Gmax was 20 mm. Specific gravities of the coarse aggregate and fine aggregate were 2.68 and 2.60, respectively. In addition, the volume ratio was 0.658. The expected average concrete compressive strength was 30 MPa. During the permeability test, the change of the water permeability coefficient of concrete itself by the hydration and the influence of autogenous healing due to hydration and expansion of cement paste occurring on crack surfaces should be reduced at a minimum. Accordingly, all test cylinders were wet-cured in a curing room with 100% relative humidity (RH) at 20 °C more than 90 days until the testing date. 3.2. Details of test specimens When cylindrical specimens are used, there is a merit to easily induce a crack caused by tensile force. In this study, cylindrical specimens were also adopted due to the simplicity of the measurements and ease of comparison with other researchers’ test results in further studies since, for most existing permeability tests, cylindrical specimens have been used. To prepare test specimens, 150 mm diameter 300 mm height cylinders were first made. To eliminate the wall effect, after cutting 25 mm from each of the extreme upper and lower bounds of the 150 mm diameter 300 mm height cylinders to the center direction, the remaining parts were used in the tests. The cylinders selected for the test were of uniform shape and dimensions, that is their circular cross-sections were 150 mm in diameter and 50 mm height. When the penetration crack was induced, a method that applied only a pure tensile force utilizing a wedge was newly suggested. This method called for splitting the 150 mm diameter 50 mm height cylinder in two while applying the load after inserting the wedge into a pre-formed notch on the upper part of the specimen. A crack in the diameter direction was then induced as shown in Fig. 1. During this process, such a crack starts from the wedge and develops to the lower part of the specimen. After that, using a steel bend, these divided parts were fastened using a method for reassembling the cylinder on both crack surfaces. For specimens having slightly larger crack widths, the crack width was adjusted by inserting aluminum foil into the crack surfaces before reuniting of the specimen.
Table 1 Mixture proportion of the concrete. Slump (mm)
W/C (%)
S/a (%)
Air (%)
Unit content (kg/m3) W
C
S
G
SP
94
54
41
4
185
342
727
1012
1.026
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Fig. 1. Crack generation and measurement.
As shown in Fig. 1, basic lines on the front and back surfaces of the specimen were drawn horizontally at the center and 40 mm upper and lower locations vertically from the center. Crack width was measured at the intersecting point of the marked lines and cracks. The crack widths were measured in 10 lm using a microscope with 100 magnification. Since measuring cracks using a magnifier is a very delicate process, errors can easily occur. Accordingly, a great deal of effort was put into reducing measurement error, which can be influenced by subjective factors, by having the same person determine the crack widths of all specimens. Based on the minimum, average, and maximum values of the crack widths, only specimens coming under the allowable limits were used in all tests. For all specimens having a targeted crack width, epoxy was applied to the upper and lower surfaces and side surface except for surfaces where penetration was induced and leakage was likely to occur. After hardening of the epoxy, the surface of the specimens was rendered smooth using a grinder. 3.3. Test variables and test procedure The test variables were hydraulic pressure and crack width. The crack widths were 30, 50, and 100 lm and the hydraulic pressures
applied were 0.01, 0.025, 0.05, 0.1, and 0.2 MPa. To reduce the errors that can happen in the specimen manufacturing process, each experiment was simultaneously performed for three specimens of test conditions. After finishing the tests, an average value of test data, which has the same condition, was calculated since it was more reliable compared to one-point data. The serial capitals of specimens having the same condition were A, B, and C. In the test, specimen nomenclature is as follows:
C ½crack width ðlmÞ ½specimen serial capital P ½hydraulic pressure ðMPaÞ: All specimens were located between plexiglas plates. By fastening screws located on the upper and lower parts of the specimen, cylindrical pipes were kept close to the specimen. Hydraulic pressure was applied to the upper part and a device collecting water that leaked through the specimen was designed to be attached to the lower part. To prevent leakage through the gaps between the cylindrical pipes and the specimen, a rubber ring was used. The permeability test apparatus is shown in Fig. 2. In this paper, the term ‘‘developed device’’ is defined as ‘‘permeability test apparatus’’.
Fig. 2. Overall view of test set-up.
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To reproduce the underground water level of 0–20 m, a hydraulic pressure of 0–0.2 MPa is required since the hydraulic pressure of 0.1 MPa increases as the underground water level of 10 m increases. Since the pressure should be constantly maintained, more than the required pressure should be prepared. The hydraulic pressure applied to the specimen was controlled using a regulator with a capability of 0.5 MPa and the pressure was measured using a manometer. In addition, the setup was composed of applying the target hydraulic pressure to the upper part of the specimen and measuring water flow contents collected through the lower part of the specimen. Finally, the leaked contents with crack length and time were calculated from the leaked water contents based on Darcy’s flow. The test apparatus proposed in this study has the merit of simplicity: the pressure control apparatus is composed of a regulator, filter, and manometer (Fig. 3). A schematic view of the water permeability test apparatus is shown in Fig. 4. As shown in this figure, when hydraulic pressure was applied, pressure was checked using the regulator and then adjusted to a required pressure using the manometer. To remove impurities in the water, a filter was installed between the regulator
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and manometer. As the water passed the crack of the specimen, it was collected in a barrel and went to a reservoir along the pipe line. A weight of water collected in the reservoir was measured to obtain flowing water contents per time. All tests were performed in a constant temperature room at 20 ± 0.5 °C. 3.4. Permeability patterns Since a constant water pressure was applied to the specimen, the inner part of the concrete was saturated; the water permeability coefficient of the concrete can be obtained using Eq. (1). For water permeability tests of the cracked concrete, the leaked water content is the sum of values through both the sound parts of the concrete and the cracks. The leaked water content through the sound concrete over time was not closely measured in the initial testing stage, but gradually approached a constant value over time. The water permeability coefficient of concrete used in this test was measured as a 3.74 1010 mm/s. Since the leakage through concrete inner parts did not occur in the initial stage of testing, it did not influence the initial leakage contents. After approximately 4 days, however, the influence dependent of the applied hydraulic pressure should be considered. The pure water contents leaked through the cracks were obtained by subtracting leaked water contents calculated using the water permeability coefficient of uncracked concrete from leaked water contents measured from the tests. 4. Evaluation of test results 4.1. Results of water permeability test
Fig. 3. Water pressure control system.
Minimum crack width was fixed to 30 lm and only specimens coming under the allowable limits of minimum, average, and maximum values of crack widths determined previously were used in the tests after grouping based on the classification criteria. The classification of crack categories is shown in Table 2. Figs. 5 and 6 show the relationships between the leaked contents with crack length and time (Ke, mm2/s), respectively, with hydraulic pressure and crack width. In these figures, the leaked contents with crack length and time were calculated using the following equation.
Fig. 4. Schematic diagram for water permeability test.
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Table 2 Classification of crack categories.
Ke ¼
Crack width category (lm)
Wmin
Wave
Wmax
30 50 100
0 20 50
25–35 45–55 90–110
60 80 150
Ql lc ht
where Q represents the leaked contents (mm3); l is the thickness of the specimen (mm); lc is the nominal length (mm); h is the drop in hydraulic head (mm); and t is the time (s). In this paper, the nominal length is not the length of specimen’s thickness direction
1
C30AP0.05 C30BP0.05 C30CP0.05 C50AP0.05 C50BP0.05 C50CP0.05 C100AP0.05 C100BP0.05
1
C30AP0.01 C30BP0.01 C50AP0.01 C50BP0.01 C50CP0.01 C50DP0.01 C100AP0.01 C100BP0.01 C100CP0.01
1E-3
0.01
2
0.01
0.1
Ke (mm /s)
0.1
Ke (mm2/s)
ð2Þ
1E-4
1E-3
1E-4
1E-5
1E-5 0
50
100
150
200
250
300
350
400
450
0
50
100
150
Time (hour)
(a) hydraulic pressure 0.01 MPa
C30AP0.025 C30BP0.025 C50AP0.025 C50BP0.025 C100AP0.025 C100BP0.025
300
350
400
450
C30AP0.1 C30BP0.1 C30CP0.1 C50AP0.1 C50BP0.1 C50CP0.1 C100AP0.1 C100BP0.1
1
0.1
Ke (mm /s)
0.1
0.01
2
2
250
(c) hydraulic pressure 0.05 MPa
1
Ke (mm /s)
200
Time (hour)
1E-3
1E-4
0.01
1E-3
1E-4
1E-5
1E-5 0
50
100
150
200
250
300
350
400
0
450
50
100
150
200
250
300
350
400
450
Time (hour)
Time (hour)
(b) hydraulic pressure 0.025 MPa
(d) hydraulic pressure 0.1 MPa
1
C30AP0.2 C30BP0.2 C30CP0.2 C50AP0.2 C50BP0.2 C100AP0.2 C100BP0.2
2
Ke (mm /s)
0.1
0.01
1E-3
1E-4
1E-5 0
50
100
150
200
250
300
350
400
450
Time (hour)
(e) hydraulic pressure 0.2 MPa Fig. 5. Permeability with hydraulic pressure. (a) Hydraulic pressure 0.01 MPa, (b) hydraulic pressure 0.025 MPa, (c) hydraulic pressure 0.05 MPa, (d) hydraulic pressure 0.1 MPa, (e) hydraulic pressure 0.2 MPa.
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2
Ke (mm /s)
0.1
0.01
1E-3
1E-4
C50AP0.01 C50BP0.01 C50CP0.01 C50DP0.01 C50AP0.025 C50BP0.025 C50AP0.05 C50BP0.05 C50CP0.05 C50AP0.1 C50BP0.1 C50CP0.1 C50AP0.2 C50BP0.2
1
0.1
0.01
2
1
Ke (mm /s)
C30AP0.01 C30BP0.01 C30AP0.025 C30BP0.025 C30AP0.05 C30BP0.05 C30CP0.05 C30AP0.1 C30BP0.1 C30CP0.1 C30AP0.2 C30BP0.2 C30CP0.2
1E-3
1E-4
1E-5
1E-5 0
50
100
150
200
250
300
350
400
0
450
50
100
150
200
250
300
Time (hour)
Time (hour)
(a) crack width 30 µm
(b) crack width 50 µ m
350
400
450
1
C100AP0.01 C100BP0.01 C100CP0.01 C100AP0.025 C100BP0.025 C100AP0.05 C100BP0.05 C100AP0.1 C100BP0.1 C100AP0.2 C100BP0.2
2
Ke (mm /s)
0.1
0.01
1E-3
1E-4
1E-5 0
50
100
150
200
250
300
350
400
450
Time (hour)
(c) crack width 100 µm Fig. 6. Permeability with crack width. (a) Crack width 30 lm, (b) crack width 50 lm, (c) crack width 100 lm.
but the crack length of radial direction of the surface, which is perpendicular and subjected to the water pressure. Although the total length including zigzag-typed crack lengths is longer than 150 mm, each 10 mm from outer part to center direction of the specimen with diameter 150 mm was fastened to the experimental device. The permeability was happened through the circular section composed of remained and inner length 130 mm. When the test results were summarized, accordingly, the nominal length 130 mm as a value of lc was used. For all specimens, it is noted that Ke decreases as the time increases due to autogenous healing. Since the tests were performed using the long-term (i.e., more than 90 days) aged concrete specimens, the influence of cement hydration was reduced to a minimum; therefore, its main causes are attributed to chemical closure by the occurrence of carbonic acid calcium and physical blocking by impurities in the water and concrete fragments within the crack surfaces.
4.2. Relationships between the leaked contents and time with hydraulic pressure and crack width In Fig. 5, when the pressure is 0.01 MPa, the final Ke values of all crack widths are similar. However, when the pressure is 0.025 MPa or greater and the crack width is 100 lm, the Ke values are particularly high. In Fig. 6, when the crack widths are 30 and 50 lm, the decreasing patterns and converging values of Ke with time regardless of the pressure are similar. In the case of 100 lm crack width,
however, the patterns and values with time are changed. In addition, the time taken to become a constant value after the decreasing of Ke values becomes longer as crack width and hydraulic pressure decrease. Table 3 shows the Ke values with hydraulic pressure and crack width at an initial stage (i.e., 30 min). From this table, it is noted that the Ke values increase as the crack width increases regardless of the pressure. When the crack width is 30 lm, the difference of Ke values with pressure is not significant and the average value is 4.22 103 mm2/s. In the cases of the 50 lm and 100 lm crack widths, the average values are 8.79 103 and 165 103 mm2/ s, respectively. From these findings, we can conclude that when the crack width is increased 1.7 times from 30 lm to 50 lm, the initial Ke values increase approximately 2.1 times. However, when the crack width is increased 2.0 times from 50 lm to 100 lm, the initial Ke values increase approximately 18.8 times. Table 4 shows the Ke values with hydraulic pressures and crack widths summarized at stabilized and final stages. Similarly with the initial Ke values, the final Ke values increase as the crack width increases. For the final Ke values compared to the initial Ke values in Table 3 Initial Ke values (103).
30 lm 50 lm 100 lm
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
Average
2.31 9.60 147
3.62 3.91 271
3.18 3.64 208
11.3 8.56 154
2.39 21.8 113
4.22 8.79 165
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Table 4 Final Ke values (105).
30 lm 50 lm 100 lm
Table 6 Ratio of final Ke values to initial Ke values (unit: %).
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
20.0 36.5 69.1
8.09 4.02 13,000
4.74 4.50 14,100
1.08 3.84 11,700
2.25 3.68 8910
Table 3, however, the effect of crack width is minor and they show smaller values due to autogenous healing. When the crack widths are 30 and 50 lm, the final Ke values with hydraulic pressure are similar. For a crack width of 100 lm, however, the final Ke value increases approximately 190 times when the pressure increases from 0.01 MPa to 0.025 MPa. When the crack widths are 30 lm and 50 lm and pressures are between 0.025 and 0.2 MPa, the final Ke value is approximately 4.0 105. In addition, when the crack widths are 30 lm and 50 lm and the pressure is 0.01 MPa, the final Ke value is approximately 7 times greater than the average value. This is considered a testing error and the reason why, when the final Ke value converses a constant value, to reduce elapsed time, the test was stopped. In particular, for the case of 0.01 MPa, it is possible to reduce further the final Ke values. Table 5 shows the ratios of Ke values just after 50 h to initial Ke values. The ratios of specimens having a crack width of 30 lm and pressures of 0.01 and 0.025 MPa are reduced to approximately 8% and, when pressures are 0.05, 0.1, and 0.2 MPa, the values are decreased to 3% at the same crack width. Specimens with a crack width of 50 lm also show a pattern similar to the case of 30 lm. Namely, when the crack widths are 30 and 50 lm, the ratios are more decreased as the pressure increases. In the case of cracks with a width of 100 lm, when the pressure is only 0.01 MPa, the ratio is greatly decreased to 2%. However, the ratios corresponding to pressures greater than 0.01 MPa showed relatively lower decreased values (i.e., 55–85%). Table 6 shows the ratios of final Ke values to initial Ke values. When the crack widths are 30 and 50 lm and pressures are 0.025 MPa or more, the ratios are decreased to approximately 1%. This is because the cracks have closed due to autogenous healing and, as a result, the leaked contents through all crack widths decreased regardless of applied pressures. When the crack width is 100 lm and pressure is 0.01 MPa, the ratio decreased greatly to one smaller than 1%. When the pressures are 0.025 MPa or more, however, the decrease of ratios is reduced to approximately 47–79%. From this phenomenon, it is noted that, when crack width is more or less increased, there is almost no decrease in leakage due to autogenous healing under the highly-pressurized leakage condition. The main reason for this phenomenon is assumed to be a disappearance of chemical healing caused by fast water permeability or by the reduction of physical closure due to the larger size of the crack widths.
30 lm 50 lm 100 lm
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
8.6 3.8 0.5
2.2 1.0 47.9
1.5 1.2 67.7
0.1 0.4 75.8
1.0 0.2 78.8
pressure increases from 0.01 MPa to 0.025 MPa. In addition, the leaked contents are also fatally increased. Accordingly, when a penetration crack forms in a structure exposed to a hydraulic pressure of less than 0.01 MPa, 100 lm can be considered an allowable crack width. However, when the penetration crack occurs for a structure exposed to 0.025 MPa or greater pressure, the allowable crack width should be reduced to 50 lm. To be more precise, the allowable crack width corresponding to the hydraulic pressure of 0.025 MPa or greater should be between 50 and 100 lm. To solve this problem, the maximum allowable crack width will be clarified in further experimental study. Namely, the allowable crack width of structures exposed to pressures greater than 0.025 MPa will be determined based on the applied hydraulic pressure. 4.4. Verification of carbonation To accurately evaluate the cause of autogenous healing, a 1% phenolphthalein–alcohol liquid was sprayed on the fractured
(a) crack surface
4.3. Allowable crack widths based on leaked contents According to Table 4, when the hydraulic pressure does not exceed 0.01 MPa and the crack width ranges between 30 and 100 lm, the Ke value can be controlled as the smaller value. If the hydraulic pressure is 0.025 MPa or more and the crack width is 30–50 lm, the Ke value is small. However, when the crack width is 100 lm, the Ke value rapidly increases approximately 190 times when the Table 5 Ratio of Ke values just after 50 h to initial Ke values (unit: %).
30 lm 50 lm 100 lm
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
18.2 8.1 2.3
7.6 7.8 55.2
4.1 3.2 70.1
2.9 3.2 82.3
3.1 3.2 84.4
(b) surface perpendicular to crack surface Fig. 7. Verification of carbonation progress. (a) Crack surface, (b) surface perpendicular to crack surface.
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specimen surface after splitting the specimen along the crack surface. During autogenous healing, since the formation reaction of calcium carbonate is basically equal to the carbonated process, the question of whether calcium carbonate on the crack surface was created or not was confirmed using the phenolphthalein–alcohol liquid. As shown in Fig. 7, when the phenolphthalein–alcohol liquid was sprayed, the concrete on the crack surface seemed to be colorless. However, the concrete perpendicular to the crack surface looked red. This is because, since calcium carbonate is generated on the crack surface, the concrete in the neighborhood of the crack surface was carbonated. As a result, it was noted that the decrease of the water permeability coefficient with time is caused by autogenous healing due to the formation of calcium carbonate. 5. Conclusion To quantitatively evaluate the effects of hydraulic pressure and crack width on the water permeability of penetration crackinduced concrete, a study was performed. When there is a crack penetrating the concrete, most of the leaked water is caused by the crack. Accordingly, the method for measuring water permeability coefficients should be markedly different from existing methods, which have mainly relied on low-pressure water permeability tests. In this paper, the method for preparing the specimen and experimental apparatus were newly developed. Finally, it was noted that the allowable crack widths for structures having a leakage problem would be defined based not only on durability considerations by the corrosion of reinforcing bars, as suggested for existing standards, but also taking into account crack width and exposed hydraulic pressure conditions. From the test results, the following conclusions were drawn: (1) Regardless of crack width and hydraulic pressure, the leaked contents Ke with crack length and time decreased as the time increased. This appears to be due to autogenous healing by physical and chemical closure within a crack surface. (2) When the crack widths were 30 and 50 lm, the Ke value showed a relatively rapid decrease as the hydraulic pressure increased. When the pressure increased from 0.01 MPa to 0.025 MPa and the crack width was 100 lm, the Ke value rapidly increased approximately 190 times.
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(3) Due to the influence of autogenous healing, the final Ke value was smaller than the initial Ke value and increased as crack width increased. However, the influence of crack widths on the final Ke value compared to the initial Ke value was small. (4) The decrease of the water permeability coefficient with time is caused by autogenous healing due to the formation of calcium carbonate. 6. Further experiments In this study, when hydraulic pressures were greater than 0.01 MPa and crack widths ranged between 50 and 100 lm, water permeability rapidly increased. Accordingly, to systematically evaluate inter-grade results including a critical value, the authors plan to perform additional studies. Acknowledgments This study has been a part of a research project supported by Korea Ministry of Education, Science and Technology (MEST) via the research group for control of crack in concrete. The authors wish to express their gratitude for the financial support that made this study possible. References [1] Oh SK. A view on water-leakage and countermeasure of concrete structure. Mag Korea Concr Inst 2002;14(6):14–9 [in Korean]. [2] Hyun TY, Kim JY Kim JK. Permeability of cracked concrete as a function of hydraulic pressure and crack width. J Korea Concr Inst 2008;20(3):291–8 [in Korean]. [3] Wang K, Jansen DC, Shah SP, Karr AF. Permeability study of cracked concrete. Cem Concr Res 1997;27(3):381–93. [4] Aldea CM, Shah SP, Karr A. Effect of cracking on water and chloride permeability of concrete. J Mater Civ Eng 1999;11(3):181–7. [5] Aldea CM, Song WJ, Popovics JS, Shah SP. Extent of healing of cracked normal strength concrete. J Mater Civ Eng 2000;12(1):92–6. [6] Kwon SJ, Song HW, Park CK, Byun KJ. A study on permeability characteristics of carbonated concrete considering micropore structure. KSCE J Civ Eng 2005;25(3A):577–83 [in Korean]. [7] Ludirdja D, Berger RL, Young JF. Simple method for measuring water permeability of concrete. ACI Mater J 1989;86(5):433–9. [8] Rapoport J, Aldea CM, Shah SP, Ankenman B, Karr A. Permeability of cracked steel fiber-reinforced concrete. J Mater Civ Eng Tech Notes 2002:355–8. [9] Korea Concrete Institute. Advanced concrete engineering. Kimoondang, Seoul; 1992. p. 477–95 [in Korean]. [10] Edvardsen C. Water permeability and autogenous healing of cracks in concrete. ACI Mater J 1999;96(4):448–55.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
The effects of hydraulic pressure and crack width on water permeability of penetration crack-induced concrete Seong-Tae Yi a,⇑, Tae-Yang Hyun b, Jin-Keun Kim c a
Department of Civil and Environmental Engineering, Inha Technical College, 253, Yong Hyun-dong, Nam-gu, Inchon-si 402-752, South Korea Material Division, Hyundai Institute of Construction Technology, 102-4, Mabuk-dong, Gihung-gu, Yongin-si 446-716, South Korea c Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon-si 305-701, South Korea b
a r t i c l e
i n f o
Article history: Received 31 December 2009 Received in revised form 30 September 2010 Accepted 26 November 2010 Available online 24 December 2010 Keywords: Hydraulic pressure Crack width Water permeability Leakage Cracked concrete Autogenous healing
a b s t r a c t Cracks in concrete generally interconnect flow paths and increase the permeability of concrete. The increase of permeability due to gradual crack growth allows more water or aggressive chemical ions to penetrate the concrete and facilitate deterioration. This research aims to study water permeability and how it is affected by hydraulic pressure and crack widths in cracked concrete. Tests were carried out as a function of hydraulic pressure and crack width, using the splitting and reuniting method to manufacture concrete specimens with controlled crack widths. Crack widths were examined using a microscope. The results showed a considerable increase in water transport as crack width and hydraulic pressure increased. But when the crack width was smaller than 50 lm, it had little effect on concrete permeability. Due to autogenous healing, the water flow through such cracks was gradually reduced over time. However, when the crack width was between 50 and 100 lm and hydraulic pressure was greater than 0.025 MPa, concrete permeability increased rapidly. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cracks in concrete members occur for various reasons including drying shrinkage, external loading, etc. Although cracks themselves do not directly affect the serviceability and durability of reinforced concrete (RC) structures, a decrease in structural performance results from repetition and acceleration of a cycle centered on the occurrence of cracks and an increase in their permeability. In the case of water leaking through cracks, not only the serviceability and durability of structures but so their external appearance is damaged. In addition, additional problems resulting from water leakage can be compounded. More specifically, for deep underground floors of a high-rise building, underground structures such as underground railways and tunnels, and water storage facilities such as ocean structures, dams, and filtration plants, when high hydraulic pressure is applied, water leakage problems caused by structural cracking are intensified. Consequently, during and after construction, considerable sums have been spent on repairs related to water drainage [1,2]. Up to now, crack width codes in South Korea have prescribed only exposure conditions and purposes in use of structures. However, rules for water leaks and experimental methods for water ⇑ Corresponding author. Tel.: +82 32 870 2237; fax: +82 32 870 2510. E-mail address: [email protected] (S.-T. Yi). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.11.107
permeability coefficients of crack-induced concrete have not been established. However, since there are limits on stipulating rules that apply to water leakages in structures using crack width only, new rules for water permeability coefficients and water leak contents are urgently required. To endorse such rules, a systematic study on water permeability coefficients of crack-induced concrete under high-water pressure is necessary. Currently, the water permeability coefficient of concrete is obtained indirectly by measuring the diffusion coefficient of chlorine ion or by calculating the passing distance of the water within the specimen after applying water pressure to a concrete specimen. When there is a crack penetrating the concrete, however, most of the leaked water is caused by the crack. Accordingly, in this case, the method for measuring water permeability coefficients should be markedly different from existing methods, which have mainly relied on low-pressure water permeability tests (WPT) [3–7]. However, these test methods have problems taking into account the effects of highly pressurized water. Accordingly, when high-water pressure is applied, it is not easy to directly estimate the characteristics of water penetration coefficients of concrete through a crack. In addition, most research studies for water permeability tests of concrete with existing cracks have induced cracks by using a feedback-controlled splitting test method [3,8]. From this method, however, it is not easy to clarify whether or not the cracks within the specimen develop in an identical pattern.
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In this study, to correctly investigate the water permeability coefficients of penetration crack-induced concrete, the high-water pressure was directly controlled. In addition, identical crack pattern problems within the specimen were solved by splitting the specimen in two and reassembling it afterwards confirming the pattern. Namely, an experimental apparatus and the method for preparing the specimen were newly developed.
The most significant factor influencing autogenous healing is the precipitation of calcium carbonate. It is noted that growth velocity of the precipitation is affected by hydraulic pressure and crack width. However, the influence of concrete proportion and degree of water hardness was found to be minor [10]. The water permeability coefficient through the crack due to autogenous healing progressively decreased and, in the extreme case, the crack closed perfectly, likely preventing further leakage through the crack.
2. Water permeability theory of concrete 3. Permeability experiments 2.1. General 3.1. Mixture proportioning When a crack occurs in concrete, water permeability patterns within the concrete are largely changed. Various factors influence concrete cracks such as drying shrinkage, thermal stress, corrosion of the reinforcing bar, poor quality construction, and external loading. Generally speaking, however, external loads applied to the structure and fluctuations in the tensile stress due to changes in the volume of the concrete are considered major factors. Cracks that are present irregularly at the initial stage are connected by additional cracks, and, finally, become larger penetration cracks. Such cracks increase the water permeability coefficient of concrete from several hundred times to several thousand times and, in such cases, the water permeability coefficient is controlled by a crack [9]. Under normal circumstances, cracks are not simple geometrically and range in size from several microns to several millimeter. Sometimes, only one crack can occur and the water permeability coefficient in such cases is bigger than that of the concrete without cracks. In such cases, the electric influence within the concrete and the effect of adsorption water, which have relatively minor values, may be ignored. 2.2. Darcy flow The basic model of concrete water permeability is illustrated using Darcy flow. When it is assumed that both surfaces of a concrete specimen are in contact with a liquid that can be infinitely supplied, the liquid moves through the concrete due to differences in pressure. The flow of liquid within concrete saturated by the liquid differs from that of unsaturated concrete. However, in the case of leakage through concrete, water leaks under saturated conditions are more important and predominant since the thickness of concrete is thinner compared to the ground and pressure is continuously applied. Eq. (1) shows the water permeability coefficient K (mm/s) with unit pressure and unit time when static pressure is applied.
K¼
Ql Aht
ð1Þ
where Q is the flow rate through the specimen (mm3); l is the thickness of the specimen (mm); A is the cross-sectional area of the concrete specimen (mm2); h is the drop in hydraulic head across the specimen (mm); and t is the time required for a certain amount of water (s). 2.3. Autogenous healing of cracks in concrete Before loads are applied, autogenous healing, which gradually decreases crack widths, including microcracks within concrete, may occur for the following reasons: (a) hydration and swelling of cement paste; (b) precipitation of calcium carbonate crystals; (c) blocking of flow path by water impurities; and (d) blocking of flow path by concrete particles broken from the crack surface due to cracking.
Table 1 lists the concrete mixture proportion selected for the permeability test and the 28 days compressive strength cylinder specimens. Ordinary Portland cement (ASTM Type I) was used in all mixtures. Crushed gravel was used as the coarse aggregate and the maximum aggregate size Gmax was 20 mm. Specific gravities of the coarse aggregate and fine aggregate were 2.68 and 2.60, respectively. In addition, the volume ratio was 0.658. The expected average concrete compressive strength was 30 MPa. During the permeability test, the change of the water permeability coefficient of concrete itself by the hydration and the influence of autogenous healing due to hydration and expansion of cement paste occurring on crack surfaces should be reduced at a minimum. Accordingly, all test cylinders were wet-cured in a curing room with 100% relative humidity (RH) at 20 °C more than 90 days until the testing date. 3.2. Details of test specimens When cylindrical specimens are used, there is a merit to easily induce a crack caused by tensile force. In this study, cylindrical specimens were also adopted due to the simplicity of the measurements and ease of comparison with other researchers’ test results in further studies since, for most existing permeability tests, cylindrical specimens have been used. To prepare test specimens, 150 mm diameter 300 mm height cylinders were first made. To eliminate the wall effect, after cutting 25 mm from each of the extreme upper and lower bounds of the 150 mm diameter 300 mm height cylinders to the center direction, the remaining parts were used in the tests. The cylinders selected for the test were of uniform shape and dimensions, that is their circular cross-sections were 150 mm in diameter and 50 mm height. When the penetration crack was induced, a method that applied only a pure tensile force utilizing a wedge was newly suggested. This method called for splitting the 150 mm diameter 50 mm height cylinder in two while applying the load after inserting the wedge into a pre-formed notch on the upper part of the specimen. A crack in the diameter direction was then induced as shown in Fig. 1. During this process, such a crack starts from the wedge and develops to the lower part of the specimen. After that, using a steel bend, these divided parts were fastened using a method for reassembling the cylinder on both crack surfaces. For specimens having slightly larger crack widths, the crack width was adjusted by inserting aluminum foil into the crack surfaces before reuniting of the specimen.
Table 1 Mixture proportion of the concrete. Slump (mm)
W/C (%)
S/a (%)
Air (%)
Unit content (kg/m3) W
C
S
G
SP
94
54
41
4
185
342
727
1012
1.026
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Fig. 1. Crack generation and measurement.
As shown in Fig. 1, basic lines on the front and back surfaces of the specimen were drawn horizontally at the center and 40 mm upper and lower locations vertically from the center. Crack width was measured at the intersecting point of the marked lines and cracks. The crack widths were measured in 10 lm using a microscope with 100 magnification. Since measuring cracks using a magnifier is a very delicate process, errors can easily occur. Accordingly, a great deal of effort was put into reducing measurement error, which can be influenced by subjective factors, by having the same person determine the crack widths of all specimens. Based on the minimum, average, and maximum values of the crack widths, only specimens coming under the allowable limits were used in all tests. For all specimens having a targeted crack width, epoxy was applied to the upper and lower surfaces and side surface except for surfaces where penetration was induced and leakage was likely to occur. After hardening of the epoxy, the surface of the specimens was rendered smooth using a grinder. 3.3. Test variables and test procedure The test variables were hydraulic pressure and crack width. The crack widths were 30, 50, and 100 lm and the hydraulic pressures
applied were 0.01, 0.025, 0.05, 0.1, and 0.2 MPa. To reduce the errors that can happen in the specimen manufacturing process, each experiment was simultaneously performed for three specimens of test conditions. After finishing the tests, an average value of test data, which has the same condition, was calculated since it was more reliable compared to one-point data. The serial capitals of specimens having the same condition were A, B, and C. In the test, specimen nomenclature is as follows:
C ½crack width ðlmÞ ½specimen serial capital P ½hydraulic pressure ðMPaÞ: All specimens were located between plexiglas plates. By fastening screws located on the upper and lower parts of the specimen, cylindrical pipes were kept close to the specimen. Hydraulic pressure was applied to the upper part and a device collecting water that leaked through the specimen was designed to be attached to the lower part. To prevent leakage through the gaps between the cylindrical pipes and the specimen, a rubber ring was used. The permeability test apparatus is shown in Fig. 2. In this paper, the term ‘‘developed device’’ is defined as ‘‘permeability test apparatus’’.
Fig. 2. Overall view of test set-up.
S.-T. Yi et al. / Construction and Building Materials 25 (2011) 2576–2583
To reproduce the underground water level of 0–20 m, a hydraulic pressure of 0–0.2 MPa is required since the hydraulic pressure of 0.1 MPa increases as the underground water level of 10 m increases. Since the pressure should be constantly maintained, more than the required pressure should be prepared. The hydraulic pressure applied to the specimen was controlled using a regulator with a capability of 0.5 MPa and the pressure was measured using a manometer. In addition, the setup was composed of applying the target hydraulic pressure to the upper part of the specimen and measuring water flow contents collected through the lower part of the specimen. Finally, the leaked contents with crack length and time were calculated from the leaked water contents based on Darcy’s flow. The test apparatus proposed in this study has the merit of simplicity: the pressure control apparatus is composed of a regulator, filter, and manometer (Fig. 3). A schematic view of the water permeability test apparatus is shown in Fig. 4. As shown in this figure, when hydraulic pressure was applied, pressure was checked using the regulator and then adjusted to a required pressure using the manometer. To remove impurities in the water, a filter was installed between the regulator
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and manometer. As the water passed the crack of the specimen, it was collected in a barrel and went to a reservoir along the pipe line. A weight of water collected in the reservoir was measured to obtain flowing water contents per time. All tests were performed in a constant temperature room at 20 ± 0.5 °C. 3.4. Permeability patterns Since a constant water pressure was applied to the specimen, the inner part of the concrete was saturated; the water permeability coefficient of the concrete can be obtained using Eq. (1). For water permeability tests of the cracked concrete, the leaked water content is the sum of values through both the sound parts of the concrete and the cracks. The leaked water content through the sound concrete over time was not closely measured in the initial testing stage, but gradually approached a constant value over time. The water permeability coefficient of concrete used in this test was measured as a 3.74 1010 mm/s. Since the leakage through concrete inner parts did not occur in the initial stage of testing, it did not influence the initial leakage contents. After approximately 4 days, however, the influence dependent of the applied hydraulic pressure should be considered. The pure water contents leaked through the cracks were obtained by subtracting leaked water contents calculated using the water permeability coefficient of uncracked concrete from leaked water contents measured from the tests. 4. Evaluation of test results 4.1. Results of water permeability test
Fig. 3. Water pressure control system.
Minimum crack width was fixed to 30 lm and only specimens coming under the allowable limits of minimum, average, and maximum values of crack widths determined previously were used in the tests after grouping based on the classification criteria. The classification of crack categories is shown in Table 2. Figs. 5 and 6 show the relationships between the leaked contents with crack length and time (Ke, mm2/s), respectively, with hydraulic pressure and crack width. In these figures, the leaked contents with crack length and time were calculated using the following equation.
Fig. 4. Schematic diagram for water permeability test.
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Table 2 Classification of crack categories.
Ke ¼
Crack width category (lm)
Wmin
Wave
Wmax
30 50 100
0 20 50
25–35 45–55 90–110
60 80 150
Ql lc ht
where Q represents the leaked contents (mm3); l is the thickness of the specimen (mm); lc is the nominal length (mm); h is the drop in hydraulic head (mm); and t is the time (s). In this paper, the nominal length is not the length of specimen’s thickness direction
1
C30AP0.05 C30BP0.05 C30CP0.05 C50AP0.05 C50BP0.05 C50CP0.05 C100AP0.05 C100BP0.05
1
C30AP0.01 C30BP0.01 C50AP0.01 C50BP0.01 C50CP0.01 C50DP0.01 C100AP0.01 C100BP0.01 C100CP0.01
1E-3
0.01
2
0.01
0.1
Ke (mm /s)
0.1
Ke (mm2/s)
ð2Þ
1E-4
1E-3
1E-4
1E-5
1E-5 0
50
100
150
200
250
300
350
400
450
0
50
100
150
Time (hour)
(a) hydraulic pressure 0.01 MPa
C30AP0.025 C30BP0.025 C50AP0.025 C50BP0.025 C100AP0.025 C100BP0.025
300
350
400
450
C30AP0.1 C30BP0.1 C30CP0.1 C50AP0.1 C50BP0.1 C50CP0.1 C100AP0.1 C100BP0.1
1
0.1
Ke (mm /s)
0.1
0.01
2
2
250
(c) hydraulic pressure 0.05 MPa
1
Ke (mm /s)
200
Time (hour)
1E-3
1E-4
0.01
1E-3
1E-4
1E-5
1E-5 0
50
100
150
200
250
300
350
400
0
450
50
100
150
200
250
300
350
400
450
Time (hour)
Time (hour)
(b) hydraulic pressure 0.025 MPa
(d) hydraulic pressure 0.1 MPa
1
C30AP0.2 C30BP0.2 C30CP0.2 C50AP0.2 C50BP0.2 C100AP0.2 C100BP0.2
2
Ke (mm /s)
0.1
0.01
1E-3
1E-4
1E-5 0
50
100
150
200
250
300
350
400
450
Time (hour)
(e) hydraulic pressure 0.2 MPa Fig. 5. Permeability with hydraulic pressure. (a) Hydraulic pressure 0.01 MPa, (b) hydraulic pressure 0.025 MPa, (c) hydraulic pressure 0.05 MPa, (d) hydraulic pressure 0.1 MPa, (e) hydraulic pressure 0.2 MPa.
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2
Ke (mm /s)
0.1
0.01
1E-3
1E-4
C50AP0.01 C50BP0.01 C50CP0.01 C50DP0.01 C50AP0.025 C50BP0.025 C50AP0.05 C50BP0.05 C50CP0.05 C50AP0.1 C50BP0.1 C50CP0.1 C50AP0.2 C50BP0.2
1
0.1
0.01
2
1
Ke (mm /s)
C30AP0.01 C30BP0.01 C30AP0.025 C30BP0.025 C30AP0.05 C30BP0.05 C30CP0.05 C30AP0.1 C30BP0.1 C30CP0.1 C30AP0.2 C30BP0.2 C30CP0.2
1E-3
1E-4
1E-5
1E-5 0
50
100
150
200
250
300
350
400
0
450
50
100
150
200
250
300
Time (hour)
Time (hour)
(a) crack width 30 µm
(b) crack width 50 µ m
350
400
450
1
C100AP0.01 C100BP0.01 C100CP0.01 C100AP0.025 C100BP0.025 C100AP0.05 C100BP0.05 C100AP0.1 C100BP0.1 C100AP0.2 C100BP0.2
2
Ke (mm /s)
0.1
0.01
1E-3
1E-4
1E-5 0
50
100
150
200
250
300
350
400
450
Time (hour)
(c) crack width 100 µm Fig. 6. Permeability with crack width. (a) Crack width 30 lm, (b) crack width 50 lm, (c) crack width 100 lm.
but the crack length of radial direction of the surface, which is perpendicular and subjected to the water pressure. Although the total length including zigzag-typed crack lengths is longer than 150 mm, each 10 mm from outer part to center direction of the specimen with diameter 150 mm was fastened to the experimental device. The permeability was happened through the circular section composed of remained and inner length 130 mm. When the test results were summarized, accordingly, the nominal length 130 mm as a value of lc was used. For all specimens, it is noted that Ke decreases as the time increases due to autogenous healing. Since the tests were performed using the long-term (i.e., more than 90 days) aged concrete specimens, the influence of cement hydration was reduced to a minimum; therefore, its main causes are attributed to chemical closure by the occurrence of carbonic acid calcium and physical blocking by impurities in the water and concrete fragments within the crack surfaces.
4.2. Relationships between the leaked contents and time with hydraulic pressure and crack width In Fig. 5, when the pressure is 0.01 MPa, the final Ke values of all crack widths are similar. However, when the pressure is 0.025 MPa or greater and the crack width is 100 lm, the Ke values are particularly high. In Fig. 6, when the crack widths are 30 and 50 lm, the decreasing patterns and converging values of Ke with time regardless of the pressure are similar. In the case of 100 lm crack width,
however, the patterns and values with time are changed. In addition, the time taken to become a constant value after the decreasing of Ke values becomes longer as crack width and hydraulic pressure decrease. Table 3 shows the Ke values with hydraulic pressure and crack width at an initial stage (i.e., 30 min). From this table, it is noted that the Ke values increase as the crack width increases regardless of the pressure. When the crack width is 30 lm, the difference of Ke values with pressure is not significant and the average value is 4.22 103 mm2/s. In the cases of the 50 lm and 100 lm crack widths, the average values are 8.79 103 and 165 103 mm2/ s, respectively. From these findings, we can conclude that when the crack width is increased 1.7 times from 30 lm to 50 lm, the initial Ke values increase approximately 2.1 times. However, when the crack width is increased 2.0 times from 50 lm to 100 lm, the initial Ke values increase approximately 18.8 times. Table 4 shows the Ke values with hydraulic pressures and crack widths summarized at stabilized and final stages. Similarly with the initial Ke values, the final Ke values increase as the crack width increases. For the final Ke values compared to the initial Ke values in Table 3 Initial Ke values (103).
30 lm 50 lm 100 lm
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
Average
2.31 9.60 147
3.62 3.91 271
3.18 3.64 208
11.3 8.56 154
2.39 21.8 113
4.22 8.79 165
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Table 4 Final Ke values (105).
30 lm 50 lm 100 lm
Table 6 Ratio of final Ke values to initial Ke values (unit: %).
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
20.0 36.5 69.1
8.09 4.02 13,000
4.74 4.50 14,100
1.08 3.84 11,700
2.25 3.68 8910
Table 3, however, the effect of crack width is minor and they show smaller values due to autogenous healing. When the crack widths are 30 and 50 lm, the final Ke values with hydraulic pressure are similar. For a crack width of 100 lm, however, the final Ke value increases approximately 190 times when the pressure increases from 0.01 MPa to 0.025 MPa. When the crack widths are 30 lm and 50 lm and pressures are between 0.025 and 0.2 MPa, the final Ke value is approximately 4.0 105. In addition, when the crack widths are 30 lm and 50 lm and the pressure is 0.01 MPa, the final Ke value is approximately 7 times greater than the average value. This is considered a testing error and the reason why, when the final Ke value converses a constant value, to reduce elapsed time, the test was stopped. In particular, for the case of 0.01 MPa, it is possible to reduce further the final Ke values. Table 5 shows the ratios of Ke values just after 50 h to initial Ke values. The ratios of specimens having a crack width of 30 lm and pressures of 0.01 and 0.025 MPa are reduced to approximately 8% and, when pressures are 0.05, 0.1, and 0.2 MPa, the values are decreased to 3% at the same crack width. Specimens with a crack width of 50 lm also show a pattern similar to the case of 30 lm. Namely, when the crack widths are 30 and 50 lm, the ratios are more decreased as the pressure increases. In the case of cracks with a width of 100 lm, when the pressure is only 0.01 MPa, the ratio is greatly decreased to 2%. However, the ratios corresponding to pressures greater than 0.01 MPa showed relatively lower decreased values (i.e., 55–85%). Table 6 shows the ratios of final Ke values to initial Ke values. When the crack widths are 30 and 50 lm and pressures are 0.025 MPa or more, the ratios are decreased to approximately 1%. This is because the cracks have closed due to autogenous healing and, as a result, the leaked contents through all crack widths decreased regardless of applied pressures. When the crack width is 100 lm and pressure is 0.01 MPa, the ratio decreased greatly to one smaller than 1%. When the pressures are 0.025 MPa or more, however, the decrease of ratios is reduced to approximately 47–79%. From this phenomenon, it is noted that, when crack width is more or less increased, there is almost no decrease in leakage due to autogenous healing under the highly-pressurized leakage condition. The main reason for this phenomenon is assumed to be a disappearance of chemical healing caused by fast water permeability or by the reduction of physical closure due to the larger size of the crack widths.
30 lm 50 lm 100 lm
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
8.6 3.8 0.5
2.2 1.0 47.9
1.5 1.2 67.7
0.1 0.4 75.8
1.0 0.2 78.8
pressure increases from 0.01 MPa to 0.025 MPa. In addition, the leaked contents are also fatally increased. Accordingly, when a penetration crack forms in a structure exposed to a hydraulic pressure of less than 0.01 MPa, 100 lm can be considered an allowable crack width. However, when the penetration crack occurs for a structure exposed to 0.025 MPa or greater pressure, the allowable crack width should be reduced to 50 lm. To be more precise, the allowable crack width corresponding to the hydraulic pressure of 0.025 MPa or greater should be between 50 and 100 lm. To solve this problem, the maximum allowable crack width will be clarified in further experimental study. Namely, the allowable crack width of structures exposed to pressures greater than 0.025 MPa will be determined based on the applied hydraulic pressure. 4.4. Verification of carbonation To accurately evaluate the cause of autogenous healing, a 1% phenolphthalein–alcohol liquid was sprayed on the fractured
(a) crack surface
4.3. Allowable crack widths based on leaked contents According to Table 4, when the hydraulic pressure does not exceed 0.01 MPa and the crack width ranges between 30 and 100 lm, the Ke value can be controlled as the smaller value. If the hydraulic pressure is 0.025 MPa or more and the crack width is 30–50 lm, the Ke value is small. However, when the crack width is 100 lm, the Ke value rapidly increases approximately 190 times when the Table 5 Ratio of Ke values just after 50 h to initial Ke values (unit: %).
30 lm 50 lm 100 lm
0.01 MPa
0.025 MPa
0.05 MPa
0.1 MPa
0.2 MPa
18.2 8.1 2.3
7.6 7.8 55.2
4.1 3.2 70.1
2.9 3.2 82.3
3.1 3.2 84.4
(b) surface perpendicular to crack surface Fig. 7. Verification of carbonation progress. (a) Crack surface, (b) surface perpendicular to crack surface.
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specimen surface after splitting the specimen along the crack surface. During autogenous healing, since the formation reaction of calcium carbonate is basically equal to the carbonated process, the question of whether calcium carbonate on the crack surface was created or not was confirmed using the phenolphthalein–alcohol liquid. As shown in Fig. 7, when the phenolphthalein–alcohol liquid was sprayed, the concrete on the crack surface seemed to be colorless. However, the concrete perpendicular to the crack surface looked red. This is because, since calcium carbonate is generated on the crack surface, the concrete in the neighborhood of the crack surface was carbonated. As a result, it was noted that the decrease of the water permeability coefficient with time is caused by autogenous healing due to the formation of calcium carbonate. 5. Conclusion To quantitatively evaluate the effects of hydraulic pressure and crack width on the water permeability of penetration crackinduced concrete, a study was performed. When there is a crack penetrating the concrete, most of the leaked water is caused by the crack. Accordingly, the method for measuring water permeability coefficients should be markedly different from existing methods, which have mainly relied on low-pressure water permeability tests. In this paper, the method for preparing the specimen and experimental apparatus were newly developed. Finally, it was noted that the allowable crack widths for structures having a leakage problem would be defined based not only on durability considerations by the corrosion of reinforcing bars, as suggested for existing standards, but also taking into account crack width and exposed hydraulic pressure conditions. From the test results, the following conclusions were drawn: (1) Regardless of crack width and hydraulic pressure, the leaked contents Ke with crack length and time decreased as the time increased. This appears to be due to autogenous healing by physical and chemical closure within a crack surface. (2) When the crack widths were 30 and 50 lm, the Ke value showed a relatively rapid decrease as the hydraulic pressure increased. When the pressure increased from 0.01 MPa to 0.025 MPa and the crack width was 100 lm, the Ke value rapidly increased approximately 190 times.
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(3) Due to the influence of autogenous healing, the final Ke value was smaller than the initial Ke value and increased as crack width increased. However, the influence of crack widths on the final Ke value compared to the initial Ke value was small. (4) The decrease of the water permeability coefficient with time is caused by autogenous healing due to the formation of calcium carbonate. 6. Further experiments In this study, when hydraulic pressures were greater than 0.01 MPa and crack widths ranged between 50 and 100 lm, water permeability rapidly increased. Accordingly, to systematically evaluate inter-grade results including a critical value, the authors plan to perform additional studies. Acknowledgments This study has been a part of a research project supported by Korea Ministry of Education, Science and Technology (MEST) via the research group for control of crack in concrete. The authors wish to express their gratitude for the financial support that made this study possible. References [1] Oh SK. A view on water-leakage and countermeasure of concrete structure. Mag Korea Concr Inst 2002;14(6):14–9 [in Korean]. [2] Hyun TY, Kim JY Kim JK. Permeability of cracked concrete as a function of hydraulic pressure and crack width. J Korea Concr Inst 2008;20(3):291–8 [in Korean]. [3] Wang K, Jansen DC, Shah SP, Karr AF. Permeability study of cracked concrete. Cem Concr Res 1997;27(3):381–93. [4] Aldea CM, Shah SP, Karr A. Effect of cracking on water and chloride permeability of concrete. J Mater Civ Eng 1999;11(3):181–7. [5] Aldea CM, Song WJ, Popovics JS, Shah SP. Extent of healing of cracked normal strength concrete. J Mater Civ Eng 2000;12(1):92–6. [6] Kwon SJ, Song HW, Park CK, Byun KJ. A study on permeability characteristics of carbonated concrete considering micropore structure. KSCE J Civ Eng 2005;25(3A):577–83 [in Korean]. [7] Ludirdja D, Berger RL, Young JF. Simple method for measuring water permeability of concrete. ACI Mater J 1989;86(5):433–9. [8] Rapoport J, Aldea CM, Shah SP, Ankenman B, Karr A. Permeability of cracked steel fiber-reinforced concrete. J Mater Civ Eng Tech Notes 2002:355–8. [9] Korea Concrete Institute. Advanced concrete engineering. Kimoondang, Seoul; 1992. p. 477–95 [in Korean]. [10] Edvardsen C. Water permeability and autogenous healing of cracks in concrete. ACI Mater J 1999;96(4):448–55.