Chloride penetration into concrete in marine environment Part II: Prediction of long term chloride penetration

Transcription

1 Materials and Structures/Matériau et Constructions, Vol. 32, June 1999, pp SCIENTIFIC REPORTS Chloride penetration into concrete in marine environment Part II: Prediction of long term chloride penetration A. Costa and J. Appleton Departamento de Engenharia Civil, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1096 Lisboa Code, Portugal Paper received: June 22, 1998; Paper accepted: November 23, 1998 A B S T R A C T The development of chloride penetration models is essential for the assessment of the service life of concrete structures eposed to marine environment. Simple models derived from Fick s second law of diffusion are at present the best way to predict the chloride penetration in practical situations. However these models need to be calibrated with eperimental results. This paper presents an eperimental study where the parameters used in the penetration model were calibrated to allow the prediction of long term chloride content in concrete. The results showed that both the concrete cover and concrete quality requirements stated in the present codes need to be increased so that an acceptable service life can be achieved. R É S U M É Le développement de modèles sur la pénétration des chlorures est fondamental pour évaluer la période de service des structures en béton soumises à l environnement marin. Des modèles simples dérivés de la seconde loi de diffusion de Fick sont la meilleure façon d évaluer la pénétration des chlorures dans les constructions. Mais ces modèles doivent être calibrés avec des résultats epérimentau. Cette étude présente les résultats d une recherche epérimentale qui a permis la calibration des paramètres qui sont utilisés dans l estimation à long terme de la pénétration des chlorures dans le béton. Les résultats ont montré que l enrobage et la qualité du béton spécifiés dans les normes actuelles doivent être augmentés pour garantir une durée de service acceptable. 1. INTRODUCTION Service life prediction is becoming one of the present major tasks in the design of concrete structures. The durability design must be based on consistent models that can describe the deterioration mechanisms more accurately. In marine environment, the service life of reinforced concrete structures depends mainly on deterioration due to reinforcement corrosion. Two stages can be considered in this mechanism [1, 2]: the initiation period corresponding to the critical chloride penetration up to the level of reinforcement and the propagation period related to the reinforcement corrosion and its detrimental effects on the structure. The high corrosion rates generally observed in marine environment and the difficulty in modelling this mechanism and its effect on the structures led to the usual consideration of service life as the initiation period. To assess this period, models based on the Fick s second law of diffusion are generally used. According to this methodology the chloride concentration inside concrete is given by the following equation: C t Cs erf (, ) = 1 (1) 2 Dt where C(,t) is the chloride content at depth at time t, D is the diffusion coefficient, Cs is the surface chloride content and erf is the error function. This equation is valid for constant diffusion coefficients and surface chloride contents. However recent eperimental studies [3-7] have shown that these parameters are strongly time dependent. Thus, it is very important to study the relation between such parameters and time for different eposure conditions in order to obtain more precise models. Part I of this paper [8] presents chloride penetration in concrete samples eposed to marine environment. The influence of concrete quality and eposure conditions (micro-environments) on the penetration rate was studied. The diffusion coefficients and surface chloride concentra /99 RILEM 354

2 Costa, Appleton Table 1 Values of D 1 and m for the various eposure conditions and concrete mies Eposure Concrete mi D 1 m condition [ m 2 /s] Spray zone C C Tidal zone Atmospheric zone C C Dockyard 20 C C Dockyard 21 C tions were obtained from the chloride profiles using equation (1) and the influence of concrete quality and eposure conditions on these parameters was analysed. In Part II the time dependence of D and Cs for the various marine environment conditions is studied. The relation between these parameters and time was derived from the eperimental results and incorporated in equation (1) to model the long term chloride penetration. The service life achieved for different concrete covers, concrete qualities and eposure conditions was established from the new equations. 2. MATERIALS AND METHODS A detailed description of the materials and methods used in the study was presented in Part I of this paper [8]. 3. RESULTS AND DISCUSSION 3.1 Time dependence of the chloride diffusion coefficients Since the diffusion coefficient varies according to the concrete quality and eposure conditions, the time dependence of this coefficient has to be studied for each particular concrete mi and eposure condition. The relation between the diffusion coefficient and time can be epressed, as proposed by Mangat and Molloy [9], by a power function: D(t) = D 1 t -m (2) where D(t) is the diffusion coefficient after eposure time t, D 1 is the diffusion coefficient at one year, if t is epressed in years, and m is an empirical coefficient. Maage et al. [10] adopted a similar function to model the time dependence of D. The parameters D 1 and m were obtained by applying equation (2) to the eperimental results presented in [8]. This equation was written in a linear form and a linear regression analysis was applied. The values obtained for the different cases are presented in Table 1. Fig. 1 shows the relation between the diffusion coefficient and time for concrete mi eposed to the Fig. 1 Relation between diffusion coefficient and time for mi eposed to the various environmental conditions. 355

3 Materials and Structures/Matériau et Constructions, Vol. 32, June 1999 various environmental conditions. Such relation fits quite well with the eperimental results. A similar behaviour was observed for the other concrete mies. Looking at the values of D 1 in Table 1 it is clear that this parameter is considerably dependent on concrete quality and eposure conditions. The values of m are also significantly dependent on these parameters. For the poorer concrete quality m is higher, which means that the reduction of the diffusion coefficient over time will be higher. For the most severe eposure conditions the values of m are higher which means that the reduction of the diffusion coefficients will also be higher in these cases. This behaviour means that the higher the chloride penetration is, the higher the reduction of D over time will be. 3.2 Time dependence of the surface chloride concentration A paper by Swamy et al. [11] presents the results of a large number of case studies, some of them on long eposure periods. In all eposure conditions, ecept the submerged zone, the surface chloride content presented a Table 2 Values of C 1 and n for the various eposure conditions and concrete mies Eposure Concrete mi C 1 n condition [% wt. of concrete] Spray zone C C Tidal zone Atmospheric zone C C Dockyard 20 C C Dockyard 21 C quasi linear variation with the square root of time, showing that the increase of Cs tends to be attenuated over time. In this study the five-year reference period is further reduced to verify the law of variation of Cs over time. Nevertheless equation (3) seems to epress the eperimental results quite well. Cs(t) = C 1 t n (3) where Cs(t) is the surface chloride content after time t, C 1 is the surface chloride content after one year, if t is epressed in years, and n is an empirical coefficient. Parameters C 1 and n were obtained by applying equation (3) to the eperimental results presented in [8]. Table 2 shows the values of C 1 and n for the various concrete mies and eposure conditions. Fig. 2 shows the relation between Cs and time for concrete mi eposed to the various environmental conditions. As we can see, equation (3) is a good approimation of the eperimental results. The values of C 1 depend more on the eposure conditions than on the concrete mi type. The most severe eposure conditions are those that lead Fig. 2 Relation between surface chloride content and time for mi eposed to the various environmental conditions. 356

4 Costa, Appleton to the highest values of C 1. The values of n present the opposite variation, i. e. the worst eposure conditions lead to the lowest values of n. This means that the surface chloride content presents higher initial values for the most severe eposure conditions but its increase over time is attenuated. 3.3 Long term prediction of chloride penetration Since both the diffusion coefficient and the surface chloride content show a significant time dependence, such dependence should be considered in the long term prediction of chloride penetration. This can be done substituting equations (2) and (3) into equation (1), which leads to: n C t C t erf (, ) = 1 1 (4) m Dt This equation is a combination of theoretical and empirical approaches and should be considered as an empirical one. Replacing for each case the values of D 1, m, C 1 and n presented in Tables 1 and 2 in equation (4) we obtain for each eposure condition and concrete mi the equations presented in Table 3, in which is epressed in mm, t in years and D 1 in mm 2 /year. Fig. 3 shows the comparison between the chloride profile obtained from equation (4) and the eperimental results for concrete mi eposed to the spray zone for two eposure periods - after 18 and after 36 months. As we can see, equation (4) represents the variation of chloride concentration in depth over time quite well. This equation can thus be used to estimate the long term penetration of the critical chloride content, and allows to assess the service life: Table 3 Equations to model the chloride penetration for the various eposure conditions and concrete mies Eposure Concrete Equation condition mi Spray zone Tidal zone Atmospheric zone Dockyard 20 C2 C3 C2 C3 C2 C C, t. t erf ( ) = t 051. C, t. t erf ( ) = t 048. C, t. t erf ( ) = t 037. C, t. t erf ( ) = t 054. C, t. t erf ( ) = t 069. C, t. t erf ( ) = t 059. C, t. t erf ( ) = t 047. C, t. t erf ( ) = t 054. C, t. t erf ( ) = t 059. C, t. t erf ( ) = t 033. Dockyard 21 C3 C, t. t erf ( ) = t m C cr = 2 D t erf 1 Ct cr n 1 (5) where cr represents the penetration depth of the critical chloride content C cr. The values of C cr depends on various parameters (composition of concrete, type of cement, cover, eposure condition, etc.) and shall be established for each situation [12]. A reference value generally assumed in Europe is 4% by weight of cement [13]. For this value and for the studied con- Fig. 3 Comparison between the predicted profiles and the eperimental results, after 18 and 36 months, for mi. 357

5 Materials and Structures/Matériau et Constructions, Vol. 32, June 1999 Table 4 Minimum concrete covers for a 50 years period of service life Eposure condition Concrete mi Minimum cover [mm] 73 Spray zone C2 59 C3 51 Tidal zone 81 Atmospheric 52 zone C2 46 C Dockyard 20 C2 53 C3 46 Dockyard 21 C3 56 Fig. 4 Time evolution of the critical chloride penetration for the various concrete mies and eposure conditions. crete mies the value of C cr referred to the weight of concrete for mies types, C2 and C3 is 05, 07 and 08%, respectively. From equation (5) and considering the diffusion coefficient and surface chloride content in Tables 1 and 2, the evolution of the critical chloride penetration for the various concrete mies and eposure conditions can be established, as shown in Fig. 4, where A is the spray zone, B the tidal zone, C the atmospheric zone, D the dockyard 20 and E the dockyard 21. Analysis of these figures allows the following conclusions: in the first ages rapid penetration of C cr takes place, mainly for the most severe eposure conditions and worst concrete quality; after this initial period the penetration rate is substantially reduced. Thus the surface concrete (2-4 cm) is rapidly contaminated, even if the concrete quality is good. This justifies the use of large covers (5-7 cm) in marine environment. The general idea that the penetration of the critical chloride content is function of the square root of time is not correct. In fact, from equation (1) it is possible to obtain cr = K (t) and then to conclude that doubling the concrete cover would increase four times the eposure period required for the critical chloride content to reach the reinforcement level. The study presented in this paper shows that this relation is considerably higher, particularly in the most severe eposure conditions. For instance, for concrete types, C2 and C3 eposed in the spray zone, increasing the cover from 30 to 60 mm will increase the initiation period of 8, 6 and 7 times, respectively. This is due to the reduction of the diffusion coefficient over time. The results of this study will help to establish the requirements which ensure an adequate durability of reinforced concrete structures in marine environment. In general, those requirements refer to the concrete mi composition and to the minimum cover. The European prestandard ENV [14] (Eurocode for concrete structures) proposes a single eposure class for marine environment and a minimum cover of 40 mm. This study shows that it is necessary to differentiate the various eposure conditions in marine environment as well as the concrete cover and mi composition requirements. Fig. 4 shows that a cover of 40 mm is not enough to guarantee a service life of 50 years ecept in the atmospheric eposure conditions and for good concrete quality. Table 4 presents the values of the minimum cover for the various concrete mies and eposure conditions required to guaranteeing an initiation period of 50 years. This period can be used as the service life if the propagation phase is neglected. The concrete cover values in Table 4 are much higher than those presented usually in codes. The need to increase the concrete cover requirements defined in the present codes, particularly in the most severe conditions (tidal, splash and spray zones) of marine environment, is clear. In order to use this model to predict chloride penetration in a given structure, you need to know parame- 358

6 Costa, Appleton ters n and m. These parameters depend on the eposure conditions and on the type and quality of concrete. Since maintenance of concrete structures eposed to aggressive environments involves regular inspections, m and n can be estimated by using the chloride profiles taken at different eposure periods. For planned and eisting structures where there is no information about chloride penetration a rough estimate of the referred parameters can be made taking into account the values presented in Tables 1 and CONCLUSIONS The eperimental results show that the diffusion coefficients and the surface chloride content have a strong time dependence. The consideration of such time dependence in the prediction of long term chloride penetration in concrete is fundamental. The eperimental results and the modelling showed that in the first ages a very quick chloride penetration occurs followed by a significant reduction of the penetration rate and thus the surface concrete is quickly contaminated even for high performance concrete. The results also show that small increases of concrete cover leads to a significant increase in the service life. Small covers (< 40 mm) are not appropriate for any eposure condition in marine environment. The analysis of the present code rules shows that it is necessary to differentiate the various eposure conditions in marine environment and increase the concrete cover in order to achieve an acceptable service life. REFERENCES [1] Tuutti, K., Corrosion of steel in concrete, (Swedish Cement and Concrete Research Institute, Stockolom, 1982). [2] Browne, R. D., Mechanisms of corrosion of steel in concrete in relation to design, inspection, and repair of offshore and coastal structures, in ACI SP-65, Proceedings of the International Conference on Performance of Concrete in Marine Environment, St. Andrews by-the-sea, Canada, 1980, [3] Buenfeld, N. R. and Newman, J. B., Eamination of the three methods for studying ion diffusion in cement pastes, mortars and concrete, Mater. Struct. 20 (1987) 3-1 [4] Mangat, P. S. and Gurusamy, K., Chloride diffusion in steel fibre reinforced concrete, Cem. Concr. Res. 17 (3) (1987) [5] Mangat, P. S. and Molloy, B. T., Factors influencing chlorideinduced corrosion of reinforcement in concrete, Mater. Struct. 25 (1992) [6] Mustafa, M. A. and Yusof, K. M., Atmospheric chloride penetration into concrete in semi-tropical marine environment, Cem. Concr. Res. 24 (4) (1994) [7] Tumidjaski, P. J. and Chan, G. W., Boltzamann-Matano analysis of chloride diffusion into blended cement concrete, ASCE J. of Mater. in Civil Eng. 8 (4) (1996) [8] Costa, A. J. and Appleton, J. A., Chloride penetration in marine environment: Part I - Main parameters affecting the penetration, Mater. Struct. 218 (1999) [9] Mangat, P. S. and Molloy, B. T., Prediction of long term chloride concentration in concrete, Ibid. 27 (1994) [10] Maage, M., Helland, S., Poulsen, E., Vennesland, O. and Carlsen, J. E., Service life prediction of eisting structures eposed to marine environment, ACI Mater. J. 93 (6) (1996) [11] Swamy, R. N., Hamada, H. and Laiw, J. C., A critical evaluation of chloride penetration into concrete in marine environment, in Corrosion and Corrosion Protection of Steel in Concrete, Proceedings of an International Conference, University of Sheffield, England, Jul. 1994, [12] Comité Euro-International du Béton - CEB, Durable Concrete Structures, CEB Design Guide, Bulletin d Information No. 182, 2nd Edition, [13] ENV 206 (1990): Concrete. Performance, production, placing and compliance criteria. [14] ENV (1991): Eurocode 2. Design of concrete structures. Part I - General rules and rules for buildings. 359