ACCELERATED CHLORIDE PENETRATION TEST AS A BASIS FOR SERVICE LIFE PREDICTION MODEL FOR R/C CONSTRUCTIONS

Transcription

1 ACCELERATED CHLORIDE PENETRATION TEST AS A BASIS FOR SERVICE LIFE PREDICTION MODEL FOR R/C CONSTRUCTIONS L. Schueremans 1, D. Van Gemert 1, A. Beeldens 1 ABSTRACT Constructions that are highly exposed to chlorides need to be protected by a protective coating, a water-repellent agent, admixtures in the mix or a thick reinforcement cover. The effectiveness of such measures can be tested by means of accelerated chloride penetration tests. This paper focuses on the chloride penetration test (CPT). The aim is to find a relation between this CPT-time and the expected lifetime of the construction. Reinforced concrete samples are submerged in a NaCl- solution. An external electric potential forces the chlorides to penetrate into the concrete, causing the corrosion process to start. The resulting CPT-time varies between several minutes and about 1000 hours. A probability method is proposed for the interpretation of test results and prediction of service life, in which the chloride diffusion coefficient is a random variable. A relation is then determined between the chloride diffusion process and the CPT-time in the test. This reliability analysis is subsequently illustrated for typical conditions of exposure and protection. The paper will discuss the reliability of the proposed test and prediction method. Keywords: chloride penetration test, chloride-ingress, service life prediction, reliability analysis 1. Katholieke Universiteit Leuven, Belgium

2 INTRODUCTION Chloride-ingress is one of the most important causes of corrosion of the reinforcement in concrete structures such as on-shore constructions in marine environment. Those structures need to be protected. Besides this protection, it is important to be able to evaluate the applied protection and thus the resulting service live of the structure. This paper uses the Chloride Penetration Test (CPT) in combination with an in-field test program to provide the input, necessary to predict the service life of a structure, for which corrosion due to chloride-ingress is the major failure mode. First the Chloride Penetration Test is described, and obtained test results are given. Secondly, an in-field test program, executed on the quay-wall of the container terminal at Zeebrugge Harbor, is presented. Fick s Second Law is used to model the material transport in the concrete due to (forced) diffusion. A time dependent reliability method is described to obtain a value for the service life. Finally, a relation is established between the fictitious diffusion coefficient calculated from the Chloride Penetration Tests and the real diffusion coefficient, as back-calculated from the chloride profiles measured from the in-site test program. Using this relation, service life prediction values can be obtained based on forced chloride penetration tests, as illustrated in the last paragraph. ACCELERATED CHLORIDE PENETRATION TEST Test setup and chemical reactions In the accelerated Chloride Penetration Test, chloride penetration is measured in an electrochemical way [1], [2]. A concrete cylinder with a centroidal placed reinforcement bar is submerged in a 3% NaCl-solution. A constant potential of 3V is applied between the reinforced concrete and a rust-free steel bar. Due to the external electric potential, a forced diffusion of chlorides into the concrete takes place. This is summarized in figure 1. The following geometry has been applied for the reinforced concrete samples [3]: diameter = 50 mm, embedded steel bar: N8 and concrete cover thickness = 20 mm. The concrete reinforcement bar acts as anode in the electric system and will attract the anions OH -, CL -. The steel bar is the cathode of the system and O 2 reduces to OH -. The electrolytical environment is realized by the NaCl-solution and the concrete cover. The electric potential forces the anions to the anode and the cations Na +, H + to the cathode. By this means, the chemical reactions, which appear in the traditional corrosion process, are accelerated. These chemical reactions are summarized in figure 2 [3].

3 Figure 1Test setup - Chloride Penetration Test Figure 2Chemical reactions The current flow between the anode and the cathode is measured. This electric current is a measure of the corrosion velocity. Determining the CPT As the flow of the electric current is plotted as a function of time, three major parts can be determined. In the first part, the current remains constant or even decreases. The decline is caused by the polarisation of the anode. The available OH - -ions strengthen the passivating layer on the concrete, which causes a decrease in the release of free electrons.

4 The second part is characterized by a strong increase in the current flow. This is the moment at which the chloride ions reach the concrete reinforcement bar and at which the corrosion process takes off. Figure 3A typical outcome of the current-time relationship The Chloride Penetration Time (CPT) is defined as the time at which the first chlorides reach the reinforcement bar and is determined as the time that collapse with the intersection of two linear approximations of the inclining and declining part of the current - time function as measured in the CPT-test. This is illustrated in figure 3, representing a typical output of the current - time relationship. The intersection of the two linear approximations in figure 3 results in a CPT of 180 hours. After a while, cracks will appear in the concrete, causing more chlorides to reach the reinforcement bar and further accelerate the corrosion process. Finally, the cylinder will collapse, resulting in most cases in a single vertical crack over the full length of the cylinder. The rust is pressed through this crack, causing the crack to be filled. This partial filling of the crack causes a decrease in the chloride flow and thus of the electric current. This is the third part in the current-time flow [3]. Test results Before immersion in the NaCl-solution, the concrete samples are immersed in water for 7 days. This is done to prohibit an immediate penetration of chlorides to the reinforcement bars due to capillary forces. After immersion in water, the pores are assumed to be filled with water. Thus, chlorides will penetrate into the concrete by (forced) diffusion only. The tests were carried out in preconditioned circumstances, with a temperature of 23 C and relative humidity of 60 %. For a concrete mix, with approximately kg OPC, and a water to cement

5 ratio that equals: w/c = 0.5, the obtained CPT-values are listed in table 1. Table 1: Measured CPT-values [3] Sample CPT-time [h] Mean value Standard Deviation / (cov [%]) /(21%) Relation between the accelerated penetration test and Fick s Second Law With the present state of knowledge, it is virtually impossible to introduce a mathematical model accounting for all the variables in the corrosion process. To estimate the service life of a given concrete element, many assumptions need to be made [4,5,6]. To model the chloride transport process in a porous material, it is assumed that Fick s generalized Second Law applies, although it is a simplified representation of reality. Fick s generalized Second Law describes the transport of chlorides in the concrete due to diffusion and an electric field. The diffusion process is only valid in saturated conditions. In the test setup, a preconditioning of 7 days of immersion in water must fulfil this requirement. When the pores are empty, capillary forces drag the outside solution into the concrete, bringing salts along. Other assumptions are made that are not valid for concrete. In the derivation of Fick s Second Law it is assumed that the porous material is homogeneous and that the medium is non-reactive and non adsorptive, which are not valid for concrete [4]. Because of the external electric field, chloride transport is significantly influenced by the external applied electric force. Despite the differences between the assumptions on which Fick s law is based and reality, it provides the only way available to model chloride diffusion into concrete. Remark that a similar analysis can be performed on more accurate models, as they are available. Fick generalized Second Law for one-dimensional chloride transport into concrete due to diffusion and an external electric field, reads: (1) in which the concrete is assumed to be a homogeneous, isotropic material. In eq. 1 C is the chloride concentration at a distance x from the surface, at time t, the time

6 form the first exposure to chloride, D is the diffusion coefficient, R is the universal gas constant, T the temperature, z the valence of the ions, F is Faraday s constant and E the magnitude of the applied electric field. The problem is that even for the cylindrical samples that have been tested, an analytical solution for this differential equation is not available. To be able to link the results of the chloride penetration test with Fick s Second Law, use is made of a fictitious diffusion coefficient. Despite the electric field, it is assumed that the behaviour can be described using Fick s Second Law for mass transport due to diffusion only, using a fictitious diffusion coefficient D fic. Rewriting of the generalized Second Law leads to: (2) In this, D fic is the fictitious diffusion coefficient, replacing the complex behaviour of the combined effect of diffusion and the external electric field. In the case where no electric potential is applied, D fic converges to the real diffusion coefficient D. Then, Fick s Second Law reads: (3) When assumed that no reaction occurs between the concrete and the chlorides, an explicit solution for this differential equation can be obtained, using the following boundary conditions: - C(x,t=0)=C 0 ; 0<x<4 (the initial chloride concentration in the concrete mix) and - C(x=0,t)=C S ; 0<t<4 (the chloride concentration loading at the surface), (4) in which C i (x,t) is the amount of chlorides after time t at a distance x from the concrete surface. To account for the electric field, a diffusion correction factor is introduced in the following manner: (5)

7 SERVICE LIFE PREDICTION, USING A TIME DEPENDENT RELIABILITY METHOD Using the Diffusion coefficient, based on measured chloride penetration values A reliability analysis provides a means to evaluate the probability of failure of a component. The term component describes a structure or a structural element whose limit state is defined in terms of a single, continuous function known as the limit state function [7]. In the present problem, where only the diffusion coefficient D is considered to be random, the limit state function g(d) can be written as: (6) where C T is the threshold chloride concentration and C(D) is the chloride concentration at a distance x from the exposed surface at time t, depending on the value of the diffusion coefficient D. The function g(d) - the limit state function - is positive only if the concrete element is in a safe state, i.e. the chloride concentration at the reinforcement (at a distance x from the concrete surface) is less than the threshold concentration. Having the probability density function of the diffusion coefficient D, the probability that the chloride concentration C T is exceeded can be expressed as: (7) where F C (C T ) is the cumulative distribution function of C, and F D (D T ) is the cumulative distribution function of D, which are related to each other by the one-byone relationship between C and D. The threshold diffusion coefficient D T can be obtained by the inverse relation of Fick s Second Law (eq. 5) D T =F -1 (C T ). Assuming a lognormal distribution for the diffusion coefficient, the probability of failure is consequently obtained as: (8) where 8 D and > D are the parameters of the lognormal distribution and M(D) is the standard normal cumulative distribution function. Linking the fictitious diffusion coefficient, using a time correction factor To be able to predict the service life, based on a forced chloride penetration test, the fictitious diffusion coefficient (D fic ) needs to be calibrated with values of measured

8 diffusion coefficients in which no external electric potential is applied. Therefore, values were used from an in-field test program as executed on a quay-wall of the container-terminal at Zeebrugge Harbor (Belgium)[8,9]. After 3 years of actual marine exposure, cores were taken from the reinforced concrete quay-wall and the chloride concentration was determined at different depths, figure 4. Figure 4: Water-soluble chloride concentration at different depths The major part of the quay wall was treated with a water-repellent agent. Figure 4 summarizes the results of the chloride concentrations that were obtained for the small part that was not treated. Distinction is made between the cores taken in the tidal zone and cores above the tidal. The water-soluble chloride concentration was measured at the following depths: 0-9 mm, mm, mm, according to the Belgian Standard NBN B15-257, by means of wet chemical analysis. These equal the amount of free chlorides and a great deal of the chlorides, bound under the form of the Salt of Friedel, which dissolve in the water during extraction [9]. These water soluble chlorides are in fact the real danger for corrosion of the reinforcement [8]. The relevant material properties and chloride concentrations are listed in table 2. A full report can be found elsewhere [6,8,9,10]. With the chloride profiles of the concrete samples exposed during 3 years, the probability density function of the diffusion coefficient D can be back-calculated using the inverse relation of Fick s Second Law (eq. 5). Following boundary conditions were used: (9)

9 Table 2: Material properties and water-soluble chloride concentration Core no. N [Vol%] Dry density [kg/m 3 ] amount of cement [kg/m 3 ] %Cl - /cem at x=4.5 mm %Cl - /cem at x=15.5 mm %Cl - /cem at x=50 mm A A A A (10) The obtained probability distribution for the diffusion coefficient is summarized in table 3. Subsequently are mentioned: the type of distribution, its mean value, standard deviation and coefficient of variation (cov). With the computed parameters of the diffusion coefficient, the service life can be predicted using the reliability analysis as outlined before. The reliability analysis is performed for chloride threshold values that equal 0.4 %Cl - /cem and 0.7 %Cl - /cem. Figure 5 shows the probability of failure as a function of time, based on equation 8 [10,11]. The concrete cover for this analysis was 120 mm. Table 3: Distribution parameters for the experimental determined diffusion coefficient Parameter Distribution Type Mean Value :(D)[cm 2 /s] Standard Deviation F(D) [cm 2 /s] / (cov(d) [%]) D Lognormal 9.64x x10-8 / (72.5) D fic Lognormal 2.59x x10-6 / (22,2) corr /

10 Figure 5: Probability of failure due to chloride-ingress as a function of time The service life prediction value is arbitrarily defined as the moment at which the threshold concentration has reached the reinforcement bar with a probability of failure that equals 0.5 [10]. As shown in figure 5, this is 7.6 and 12.6 years for the two threshold values respectively. Based on the CPT-tests, a value for the fictitious diffusion coefficient can be backcalculated, based on the test results as outlined in table 1, using equation 2, 4 and following boundary conditions: (11) This results in a mean value that equals: :(D)=2.59x10-6 cm 2 /s and F(D)=0.57x10-6 cm 2 /s. These results are summarized in table 3. Using the assumption that both concrete mixes are similar, the diffusion correction factor can be calculated, using eq. 5. The result is given in the last row of table 3.

11 SERVICE LIFE PREDICTION VALUES BASED ON THE CPT-TIME Using equation 8, the service life prediction values can be calculated for different values of the CPT-time, as they can be obtained from a forced chloride penetration test. These values can be calculated for different boundary conditions representing different exposure and protection conditions. For purpose of illustration, the service life predictions values are calculated for a concrete cover of 5 cm, figure 6. The x- axis represents the measured CPT-time values, the y-axis the corresponding SLPvalues. Figure 6: SLP-values based on measured CPT-times CONCLUSIONS This paper provides a method to calculate service life prediction values, based on experimental determined CPT-times. To this end, four elements were combined, namely: - the results of forced chloride penetration tests, - the results of chloride profiles as measured after 3 years of exposure in a real marine environment, - Fick s Second Law for chloride transport into concrete due to (forced) diffusion and - a time dependent reliability analysis, in which the in-field measurements provide the necessary values to calibrate the model. REFERENCES 1. D. De Buck, J. De Keyser, Corrosiesnelheid van wapeningsstaven in beton,k.u.leuven, thesis, 1998, 120 p. 2. J. Cantens, J. Polet, Repair systems for concrete: Polymer cement mortars for chloride protection, K.U.Leuven, thesis, 1992, 131 p.

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