PORTUGUESE APPROACH FOR CONCRETE DURABILITY RELATED WITH REINFORCEMENT CORROSION

Transcription

1 PORTUGUESE APPROACH FOR CONCRETE DURABILITY RELATED WITH REINFORCEMENT CORROSION André Valente Monteiro (1), Manuel Vieira (1) and Arlindo Gonçalves (1) (1) Concrete Division, National Laboratory for Civil Engineering, Lisbon, Portugal Abstract The introduction of the European Standard EN 206-1:2000 Concrete - Part 1: Specification, performance, production and conformity mentions that during the development of the standard it was considered a performance-based approach to the specification of durability. For that purpose, a review of methods for specifying the concrete based on performance and test methods was carried out. However, CEN / TC 104 concluded that these methods are not yet sufficiently developed to be considered in the EN standard, but acknowledged that some members of CEN have gained confidence in testing and local criteria. For this reason, EN allows the continuation and development of such practices valid in the place of use of concrete, as an alternative to the prescriptive approach. This paper presents the existing approaches in Portugal to guarantee the service life design of 50 or 100 years for concrete structures in environments corresponding to different classes of exposure concerning carbonation and chloride-induced corrosion. One of the approaches is prescriptive, where requirements for minimum binder content, minimum cover, minimum strength and maximum water-binder are given for different exposure classes in function of the cement type, and the other is based on probabilistic models using performance properties of concrete. 1. INTRODUCTION As stated in EN 206-1, this standard is intended to be applied in Europe under different climatic and geographical conditions, different levels of protection and under different, well established, regional traditions and experiences. Classes for concrete properties have been introduced to cover these situations. Where such general solutions were not possible, the relevant clauses contain permission for the application of national standards or provisions valid in the place of use of concrete. Thereby, the Portuguese Standard NP EN 206-1:2007 includes in its National Document of Application the following three Technical Specifications as valid national standards regarding the durability of concrete structures: 49

2 LNEC E 461:2004 Concrete. Methodology for avoiding internal expansive reactions ; LNEC E 464:2005 Concrete. Prescriptive methodology for a design working life of 50 and 100 years under environmental exposure ; LNEC E 465:2005 Concrete. Methodology for estimating the concrete performance properties allowing to comply with the design working life of reinforced or pre-stressed concrete structures under environmental exposures XC and XS. The present paper describes the last two specifications concerning only the carbonation and chloride-induced corrosion. The results of a comparative study of these two methodologies, from which a more thorough description can be found in RILEM Proceedings PRO 56 [2], are also quoted. 2. PRESCRIPTIVE METHODOLOGY The Specification LNEC E 464 replaces the Annex F of NP EN and follows a similar deemed to satisfy methodology, allowing the use of cements complying with NP EN other than CEM I, as well as its mixture with additions (which suitability is also established in LNEC E 464). The environment exposure classes used in this specification are those of EN 206-1, but with a broader description. In the Tables 1 and 2, taken from LNEC E 464, the composition limits and the minimum compressive strength class of concrete, for a design working life of 50 years, are presented. Table 1: Limits for the composition and the compressive strength class of the concrete under the action of carbon dioxide, for a design working life of 50 years Type of cement CEM I (Reference); CEM II/A (1) CEM II/B (1) ; CEM III/A (2) ; CEM IV (2) ; CEM V/A (2) Exposure class XC1 XC2 XC3 XC4 XC1 XC2 XC3 XC4 min_c (3) max_w/c (4) min_cc (5) C25/30 C25/30 C30/37 C30/37 C25/30 C25/30 C30/37 C30/37 (6) min_f ck LC25/28 LC25/28 LC30/33 LC30/33 LC25/28 LC25/28 LC30/33 LC30/33 (1) Not applicable to cements II/A-T and II/A-W and to cements II/B-T and II/B-W, respectively; (2) Not applicable to cements with a Portland clinker percentage less than 50%, by mass; (3) Minimum nominal cover (mm); (4) Maximum water/cement ratio; (5) Minimum cement content (kg/m 3 ); (6) Minimum strength class. The limits presented in Tables 1 and 2 shall be implemented together with a minimum nominal cover, also presented in those tables, obtained by adding an allowance for deviation of 10 mm (accepted deviation given by NP ENV ) to the minimum cover values presented in Tables 4.4N and 4.5N of EN : 2004, respective to structural class S4. For the design working life of 100 years, the minimum nominal cover values of Tables 1 and 2 should be increased by 10 mm. 50

3 Table 2: Limits for the composition and the compressive strength class of the concrete under the action of chlorides, for a design working life of 50 years Type cement Exposure class XS1/XD1 CEM IV/A (Reference); CEM IV/B; of CEM III/A; CEM III/B; CEM V; CEM I; CEM II/A (1) CEM II/B (1) ; CEM II/A-D XS2/XD2 XS3/XD3 XS1/XD1 XS2/XD2 XS3/XD3 min_c (2) max_w/c (3) min_cc (4) (5) C30/37 C30/37 C35/45 C40/50 C40/50 C50/60 min_f ck LC30/33 LC30/33 LC35/38 LC40/44 LC40/44 LC50/55 (1) Not applicable to cements II-T, II-W, II/B-M and II/B-LL; (2) Minimum nominal cover (mm); (3) Maximum water/cement ratio; (4) Minimum cement content (kg/m 3 ); (5) Minimum strength class. The above limits were established expecting that concrete compressive strength is the most demanding requirement since it is the parameter most effectively controlled on site. A comparative study carried out by TC 104 [1] revealed that LNEC E464 is the most demanding, among the other national standards of European countries, in terms of minimum compressive strength class concerning the XD and XS exposure classes. Five situations are allowed to reduce the minimum nominal cover: a) use of stainless steel, austenitic or ferrite-austenitic, according to EN ; b) use of a protection system on concrete surface (coating) that complies with the requirements of NP EN ; c) use of epoxy resins and coated steels that complies with ASTM A775/A 775M-04a; d) laminar elements, e.g. slabs and walls; e) use of concrete from a strength class equal or greater than those indicated in Table 3. The reduction is of 20 mm in case a) and 5 mm in the other cases. The use of more than one of these reductions should not reduce min_c to less than the one corresponding to S2 or S4 for a working life of 50 or 100 years, respectively. In the cases where stainless steel is used, those classes can either be class 1 or 3. Table 3: Strength classes of the concrete, which make it possible to reduce the concrete cover XC0/XC1 XC2/XC3 XC4 XD1/XD2/XS1 XD3/XS2/XS3 C30/37 C35/45 C40/50 C40/50 C50/60* * When the cement used is CEM I or II/A (except CEM II/A-D) C45/55 C60/75* 2.1 Equivalent performance concept The Specification LNEC E 464 allows the use of compositions and cements (or mixtures of cements and additions) other than those presented in Tables 1 and 2 through the application of the equivalent performance concept (EPC). 51

4 Thereby, the specification establishes limits for the ratios between the properties of the candidate concrete and the properties of a reference concrete, complying with the requirements of Tables 1 and 2, and using the reference cement, both using the same aggregates and the corresponding proportions. The properties to be determined and compared are presented in Table 4. Table 4: Properties and ratio limits for EPC Exposure class Properties to be determined Candidate/Reference ratio XC1; XC2; XC3; XC4 Accelerated carbonation Oxygen permeability Compressive strength 1,3 2,0 1,1 Chloride diffusion coefficient 2,0 XS1/XD1; XS2/XD2; Capillary absorption XS3/XD3 Compressive strength 1,3 1,1 Specific test conditions are defined in LNEC E 464 in order to determine the above properties. Besides the candidate and reference concrete mixes (principal mixes), four secondary mixes, obtained by varying ±5% the binder content of each principal mixture, shall also be included in the comparative tests. The performance of the candidate concrete is then considered equivalent to that of the reference concrete if the average values of the penetrability properties and compressive strength of concrete from the three candidate mixes are not less or not great, respectively, than those of concrete from the three reference mixes and if the individual values from each principal or secondary mix, accordingly, satisfy the limits of Table METHODOLOGY FOR ESTIMATING DESIGN WORKING LIFE The Specification LNEC E 465 embodies the Annex J of NP EN and incorporates a probabilistic methodology presented in RILEM Report 14 [2] using the service-life models developed in Europe during the 1990s. It complies with the general rules presented in EN 1990:2002 regarding the partial factor approach, establishing a safety factor γ that affects the intended working life of the structures, t g, through the following expression: t = γ t = γ t + t t = γ t t, (1) d g ( ) ( ) i p ic g p where t d is the design working life, t i the initiation period and t p the propagation period, according to the Tuttii s model of reinforcement concrete deterioration under the environmental actions XC or XS/XD. This methodology consists basically in determining the propagation period, t p, and the limit value of the relevant concrete property that ensures the initiation period t ic obtained from (1). For the calculation of γ the reliability classes of EN 1990 are considered. Reliability indexes of 1.2, 1.5 and 2.0 for classes RC1, RC2 and RC2 are established, leading to safety factors of 2.0, 2.3 and 2.8, respectively. It is assumed that the 52

5 working life is lognormal distributed with a coefficient of variation of 50%. The Serviceability Limit State concerning the durability is defined as the beginning of cracking of concrete due to the reinforcement corrosion. LNEC E 465 divides class XC4 in two regions: the dry region, located at the south of Tagus River; and the wet region, located at the north of Tagus River and excluding the Douro River region near Spain. In Table 5 is presented the relative humidity and soaking time of concrete assumed for each environmental exposure class. Table 5: Relative humidity and the soaking time of the concrete for each exposure class Exposure class Relative humidity TdM** XC1 (dry/always wet) Dry environment: 60% Wet environment: 100% * XC2 (wet, rarely dry) 90% 0.8 XC3 (moderate humidity) 70% 0.1 XC4 (cyclic wet and dry) Dry region: 80% Wet region: 80% XS1 (air with sea salts) 80% 0.6 XS2 (permanent immersion) 100% 1* XS3 (tidal and splash zone) 100% 1 * Absence of oxygen for the corrosion process ; ** Yearly average number of days N with rainfall equal to or higher than 1 mm (TdM=N/365) For the estimation of the initiation period due to carbonation, two models are presented. One of the models is based on 1 st Fick s diffusion law assuming a stationary CO 2 flow and constant concentration of 0.7x10-3 kg/m 3 in the atmosphere (similar to DURACRETE [3]), R C = 2 R 3 t i k 0 k 1 k 2 t t 0 ic 2n, (2) where R C65 (kg.year/m 5 ) is the carbonation resistance of concrete with 65% of relative humidity, R (mm) is the concrete cover, k 1 and n are factors that consider the influence of the relative humidity and drying/soaking over the time, k 0 is a factor of value 3 when the tests satisfy the conditions presented in LNEC Specification LNEC E 391 Concrete. Determination of resistance to carbonation, t 0 = 1 year (reference period) and k 2 is a factor that considers the influence of curing conditions, assuming the value 1 for standard curing conditions and 0.25 when a formwork of controlled permeability is used with a curing period of 3 days. The other model is based on the CEN TC 104 model of air permeability [4], 53

6 R c k60 =, p (3) ( a k2 ) tic m where k 60 (m) is the coefficient of air permeability obtained through the CEMBUREAU method described in LNEC Specification LNEC E 392 Concrete. Determination of permeability to oxygen on a specimen with 28 days of age and in equilibrium with RH=60%, m is a factor that relates the coefficient of air permeability of the cover concrete with k 60 ; a = 150, c (kg/m 3 ) is the calcium oxide content of the hydrated cement matrix of concrete (depends on the type of cement used and exposure class), p is an exponent that depends on the relative humidity of concrete and, therefore, on the exposure class, and k 2 is a factor that takes into account the influence of curing conditions, assuming the value 1 for standard curing conditions and 0.5 when a formwork of controlled permeability is used with curing period of 3 days. For the estimation of the initiation period due to chlorides the specification presents the following model based on 2 nd Fick s diffusion law (similar to DURACRETE model [3]): D 0 = k 4 2 n R t ic C S C R t 2 t ic erf C 0 S, where D 0 (m 2 /s) is the potential diffusion coefficient determined in laboratory in accordance with LNEC Specification LNEC E 463 Concrete. Determination of diffusion coefficient of chlorides from non-steady-state migration test (similar to CTH method) with the concrete at the reference age of 28 days, R (mm) the concrete cover, k a factor that takes into account the influence of relative humidity of the environment, temperature and concrete cure conditions, C S is the chloride concentration, in % of the binder mass, on the concrete surface, assumed as constant (depends on the exposure class, water/binder ratio, temperature of concrete, distance of the coast line and on the depth of concrete inside the water), C R (%) is the threshold chloride concentration, in % of the binder mass, that causes the depassivation of the reinforcement (Table 6), t 0 = 1 year (reference age) and n is a factor that takes into account the decrease in the chloride ingress over the years (Table 7). Table 6: Chloride concentration, C R (% of cement mass) Water/cement XS1; XS2 XS3 w/c < w/c w/c > The model to estimate the propagation period is based on Faraday s law and on the experimental expression to estimate the reinforcement radius reduction that leads to concrete cracking with no differentiation between reinforced and prestressed concrete [5]. The model is given by the following expression: (4) t p φ0 = k (5) α Icorr 54

7 where 0. 1 ( R / φ f )/ ( φ 2) k 0 cd 0 / = is the reduction in reinforced crosssection given in percentage, R (mm) the concrete cover, φ 0 (mm) the reinforcement diameter, f cd the tensile splitting strength of concrete ( MPa for carbonation induced corrosion and MPa for chloride-induced corrosion), I corr (μa/cm 2 ) the corrosion intensity and α = 2 or 10 for carbonation or chloride-induced corrosion, respectively. To estimate I corr, Table 8 may be used. Table 7: Ageing factor, n Exposure class n CEM I/II* CEM III/IV XS XS XS * Except CEM II-W, II-T, II/B-L and II/B-LL Table 8: Corrosion levels for different exposure classes I corr (μa/cm 2 ) Corrosion level Exposure classes < 0.1 negligible XC1; XC3; XS low XC2; XC4 (1) moderate XC4 (2) ; XS1 >1 high XS3 (1) dry regions; (2) wet regions The concrete performance properties used in the models shall be determined at least in 3 different batches (3 specimens each) and the average value should be equal or greater than the minimum required by the specification. However, the maximum deviation obtained in each batch shall not be greater than 50%. 4. COMPARISON BETWEEN METHODOLOGIES Tests carried out by LNEC (National Laboratory for Civil Engineering) on standard specimens [6] with concrete complying with the prescriptive requirements of LNEC E 464 revealed that, in general, and disregarding the propagation period, the prescriptive methodology also complies with the LNEC E 465 requirements concerning carbonationinduced corrosion. For chloride-induced corrosion, the opposite seems to happen for concrete with CEM I or CEM II/A-L. Recent studies [7] performed on concrete exposed to urban and marine environments during 5 years have shown a good agreement between the predicted and measured carbonation depths, suggesting, however, that the models overestimate carbonation depths for concrete with CEM I 42.5 R and concrete under marine environments, and the opposite for 55

8 concrete with CEM IV and concrete under urban environments. Regarding chloride penetration depths, the results will be published soon. 5. FINAL REMARKS The performance test methods are not yet sufficiently developed to be considered in the EN standard. That is why the Portuguese Standard NP EN 206-1:2007 includes in its National Document of Application references to Technical Specifications as valid national standards regarding the durability of concrete structures. These Specifications are the normative documents for the existing approaches in Portugal to evaluate if the service life design of 50 or 100 years for concrete structures is satisfied in environments corresponding to different classes of exposure. One of the approaches, presented in the Specification LNEC E 464, is prescriptive, establishing requirements for minimum binder content, minimum cover, minimum strength and maximum water-binder for different exposure classes, covering the use of a broad range of cement types and additions. The other approach (Specification LNEC E 465) is based on probabilistic models. Meanwhile, tests carried out on concrete with greater times of exposure are needed to properly assess the models presented in Specification LNEC E 465 and proceed with its update. REFERENCES [1] CEN TC 104 CEN/TR 15868:2009 Survey of national requirements used in conjunction with EN 206-1:2000, CEN, [2] RILEM Report 14 (Report of RILEM Technical Committee 130-CSL): Durability Design of Concrete Structures. Edited by A. Sarja and E. Vesicari [3] DURACRETE. Report BE /TG4/C. Brite EuRam Project, Quantification of the environmental parameters in the carbonation and chloride ingress model, March [4] Parrot, J, Design for avoiding damage due to carbonation-induced corrosion, Paper N62, CEN TC104/TG1/WG1/PANEL 1, June [5] Santiago, J., Basagoiti, O., Macías, J., Arenas, J., La corrosion de las armaduras y la vida residual de las estructuras de hormigón, Seminário sobre inspecção e reparação de estruturas de betão armado com corrosão, LNEC, July, [6] Gonçalves, A., Ribeiro, B., Ferreira, E., The new LNEC specifications on reinforced concrete durability, in Integral Service Life Modelling of Concrete Structures, Proceedings of the Int. RILEM Workshop, Guimarães, November 2007 (Edited by R. M. Ferreira, J. Gulikers and C. Andrade, 2007). [7] Ribeiro, S., Ribeiro, A., Gonçalves, A.: Resistance of concrete to carbonation. Predicted and measured values in natural exposure, article (already accepted) submitted to ConcreteLife'09: 2nd International RILEM Workshop on Concrete Durability and Service Life Planning. Haifa, Israel, 7-9 September