INFLUENCE OF BLENDED CEMENT TYPE ON CONCRETE CARBONATION, CAPILLARY UPTAKE AND CHLORIDE PENETRATION

Transcription

1 INFLUENCE OF BLENDED CEMENT TYPE ON CONCRETE CARBONATION, CAPILLARY UPTAKE AND CHLORIDE PENETRATION A. Šajna, V. Bras and L. Završnik Slovenian National Building and Civil Engineering Institute, Slovenia Abstract Carbonation allegedly increases mechanical strength of concrete on one hand, but it decreases its alkalinity on the other. At a ph below 10 the steel's thin layer of surface passivation dissolves causing corrosion of steel reinforcement. In the paper test results of an extensive research program including measurement of resistance to carbonation of concretes prepared with different types of blended cements and measurement of influence of carbonation on capillary absorption and on resistance to chloride penetration are presented. The test results are analysed and compared to EN 206-1, Eurocode 2 and some national requirements for durable structures exposed to carbonation induced corrosion (exposure classes XC). It is shown that requirements given in the standards do not guarantee the required structural service life when blended cement concrete is used in carbonation induced corrosion environment. In such cases performance based determination of carbonation depth is essential. 1. INTRODUCTION Carbonation of concrete is a chemical reaction between calcium hydroxide in concrete and carbon dioxide from air, resulting in calcium carbonate. Carbonation allegedly increases mechanical strength of concrete on one hand, but it decreases its alkalinity on the other. At a ph below 10 the steel's thin layer of surface passivation dissolves and causing corrosion of steel reinforcement. According to the EU standards EN [1] and Eurocode 2 [2], the environmental conditions in which carbonation induced corrosion is expected during the service life of a reinforced concrete structure are designated as exposure classes XC1 to XC4, depending on the humidity of the environment. In Eurocode 2 and in some national appendices to EN 206-1, like the Slovenian SIST 1026 [3], additional requirements or recommendations for the concrete compositions like cement content and w/c ratio, or concrete characteristics like compressive strength classes or resistance to penetration of water are given for XC1 to XC4 concrete. Extra, in Eurocode 2 the minimum concrete cover depths depending on claimed structure service life, exposure class and concrete compressive strength class are defined. In the paper test results of an extensive research program are presented. The test results are analysed and compared to EN 206-1, Eurocode 2 and some national requirements for durable structures exposed to carbonation induced corrosion. 499

2 2. EXPERIMENTAL PROGRAM 2.1. Concrete mixtures In this investigation five different concrete mixtures, with five different types of cements were prepared and investigated. Basic concrete mixture proportions are given in Table 1. The five cement types investigated and the concrete mixture codes are listed in Table 2. Table 1: Basic concrete mixture proportions Raw material kg/m 3 cement 320 water 170 (w/c=0.53) superplasticizer 0.9 % 2.7 aggregate 1976 Table 2: Cement types investigated and concrete mixture codes Cement type Clinker content in average acc. to EN [4] Mixture code CEM I 42,5 R 97.5 % B1R CEM II/B-M (P-S-L) 42,5 N 72.0 % B2PSL CEM II/B-M (W-L) 42,5 N 72.0 % B2WL CEM IV/B-W 32,5 N 54.5 % B4B CEM V/A (S-V-P) 42,5 N - LH 52.0 % B5A 2.2 Test methods For concrete characterisation the following tests were performed: slump (EN [5]), density of fresh concrete (EN [6]), 28-day compressive strength (EN [7]), resistance to penetration of water under pressure (EN [8]) and density of hardened concrete (EN [9]). Resistance to carbonation was measured according to FprCEN/TS :2010 Testing hardened concrete - Part 10: Determination of the potential carbonation resistance of concrete: accelerated carbonation method [10], although the technical specification was never accepted for publication by CEN TC 104 Concrete and related products. During the exposure to carbonation the temperature was kept at 20 C, relative humidity at 55 % and CO 2 concentration at 4 %. The carbonation depth was measured by means of 1 % Phenolphthalein solution, which turns colourless in acidic solutions and pink in basic solutions (ph > 8.2). Capillary water uptake was measured according to EN [11]. Samples, concrete cores, were for 28 days cured according to EN After first series of exposure to capillary uptake, the samples were exposed to carbonation according to FprCEN/TS After 70 days of exposure to carbonation, the capillary uptake tests were repeated. After 500

3 the capillary uptake test the samples were exposed to carbonation again. After additional 6 months of exposure to carbonation the capillary uptake tests were performed for the third time. Resistance to chloride penetration was performed according to CEN/TS [12]. Only resistance of non-carbonated concrete has been determined due to long-running testing procedure till now. The resistance of carbonated concrete to chloride penetration is on-going. Profiles of both, water and acid soluble chlorides were determined, but no significant difference was observed. The porosity and pore size distributions of the investigated samples were determined by means of mercury intrusion porosity (MIP). Small blocks, approximately 1 cm 3 in size, were dried in an oven for 24 h at 105 C and analysed on a Micromeritics Autopore IV 9500 model. Samples were analysed in the range of 0 to 414 MPa using solid penetrometers. Samples of non-carbonated and carbonated concrete were taken simultaneously, from the same concrete sample, from non-carbonated and carbonated part of the concrete core. Spraying the splitting surface of the concrete core by Phenolphthalein solution was used to distinguish between non-carbonated and carbonated region. It has to be stated that only one sample per concrete type was tested which due to small sample size end relative inhomogeneity of concrete can be scarce. 3. TEST RESULTS 3.1. Concrete characterization The concrete characterization test results are presented in Table 3. The highest compressive strengths exceed the B1R and B2WL mixtures. As expected, due to lowest clinker content, the lowest compressive strength has B5A mixture. The measured water penetration depths are in accordance with expectations. Table 3: Characterisation test results Slump Compressive Density of Water Mixture code strength hardened concrete penetration, max [mm] [MPa] [kg/m 3 ] [mm] B1R Not determined B2PSL B2WL B4B B5A Not determined 3.2. Carbonation depth Resistance to carbonation test results are presented in Figure 1. The carbonation depths determined after 56, 63, 70, 105 and 133 day exposure to carbonation are given in relation to square root of time. The regression lines and carbonation rates were calculated according to Sanjuan [13], assuming the carbonation rate is proportion to square root of time. The carbonation rates are listed in Table. In general, the carbonation rates are within the expectations, lower for higher clinker content concretes and vice versa. Surprisingly similar is the carbonation rate of B4B and B5A concretes and unexpectedly high is the difference in carbonation rate between B2PSL and B2WL. It has been confirmed that resistance to 501

4 RILEM International workshop on performance-based specificationn and controll of concretee durability a carbonation does not dependd on clinker content admixtures [14][15]. solely, but also on the type of mineral Based on carbonation rates determinedd under accelerated conditions c (4 % of CO 2 ), the carbonation rates under environmental conditions were assumed based onn Sanjuan [13] and carbonation depths in 50 year life-time off a concrete structure were w calculated (Table 4). Table 4: Measured and estimated carbonation rates Concrete mixture B1R B2PSL B2WL B4B B5A Figure 1: Resistance to carbonation test results Carbonation rate under accelerated conditions [mm/ year] Carbonation rate under environmental conditions (estimation) [mm/ year] Estimated carbonation depth in 50 years [mm] 3.3. Capillary water uptake Capillary water uptake test results of non-carbonated and carbonated concrete are presented in Figure 2. An average of 3 samples is given. Capillary water uptake of carbonated concretee is lower for all tested mixtures, except for the B2PSL. For both non-carbonated and carbonated concrete the lowest capillary water uptake was measured for mixture B1R and the highest for B2WL. A significant reduction of capillary water uptake wass observed for B1R and B4B mixtures. As hydration of cement was not stoppedd during this investigation it s possible, that hydration of cement contributed to the reduction of capillary water uptake too. No testss have been performed on B5A mixtures

5 RILEM International workshop on performance-based specificationn and controll of concretee durability a Figure 2: Capillary water uptake test results 3.4. Chloride penetration Water soluble chloride penetration test results (chloride contents by mass of concrete) are given in Figure 3. Unexpectedly similar chloride concentration profiles were measured for all mixtures, except for the B2WL mixture with significantly lower chloride concentrations. No tests have been performed on B5A mixtures. Figure 3: Chloride penetration of non-carbona ated concrete 3.5. Mercury intrusion porosimetry Hg porosity testt results are presented in Figure 4. In the case of B1R both total porosityy and the median pore diameter were reduced by the carbonation process; where else for blended cement concretes they were increased. It has to be stated that only one sample per each concretee type was tested and that the inhomogeneity could have an influence on the rest results. 503

6 RILEM International workshop on performance-based specificationn and controll of concretee durability a Figure 4: Total porosity and median poree diameter of non-carbonated (n) and carbonated (c) concrete 4. DISCUSSION AND CONCLUS SIONS Carbonation of blended cements has not been thoroughly investigated yet, although carbonation is the most common chemical processs of hardened concrete. Still, carbonation unambiguously causes the reduction of ph value, turning the passive reinforcement environment into a corrosion-inducible structure life-time reducing processs by two European standards, Eurocode 2 and EN In EN for carbonation induced corrosion exposuree classes one (Pourbaix [16]). Carbonation induced corrosion is recognized as a concrete XC1 to XC4 are foreseen. In Eurocode 2 minimumm cover depths and compressive strength classes for exposure classes XC1 to XC4 are given. For the most m severee wet-dry exposure class XC4 a minimum cover depth of 300 mm and concrete class C 30/37 is prescribed for 50 year design life-time of the concrete structure. On the other hand requirements and recommendations for XC4 concretee mixtures are given in EN and its national appendences s of some European countries (CEN/TR [17]). Some of them are summarized in Table 5. In addition, in Slovenia the penetration of water under pressure measured according to EN shall not exceed 30 mmm for XC4 concrete. In regards with cement types, recommendation of national CEN members can also be found in CEN/TR Cement CEM I is permitted by EN and alll CEN members for exposure class XC4 and for an intended working lifetime of at least 50 years. Cements CEM II/B-M are permitted in 7 countries, no guidance are available in other countries. Cements CEM IV/B and CEM V/A are permitted in 6 countries and not permitted p inn one. Table 5: Requirements and recommendations for XC4 concrete mixtures (w/c) max reference [/] EN CEN/TR 15868, max 0.60 CEN/TR 15868, median 0.55 CEN/TR 15868, min 0.45 SIST Min. cement content [kg/m3] Min. compressive strength, cube [MPa] / 504

7 Concretes tested fulfil all Slovenian requirements and recommendations for XC4 exposure class. They also fulfil requirements and recommendations given in EN and the majority of requirements valid in other CEN members (Table 5). They also achieve the recommended compressive strength class of concrete cover given in Eurocode 2, i.e. C30/37, except for B5A mixture. Although the concrete mixtures tested satisfy all above mentioned recommendations and requirements the 50 year carbonation depths calculated based on test results surpass the concrete cover depth prescribed by Eurocode 2. For the most severe wet-dry exposure class XC4 a minimum cover depth of 30 mm is prescribed, where else the calculated carbonation depths reach from 32 mm for the B1R concrete up to 65 mm for the B4B and the B5A. Taking in consideration Tutti s model [18], the corrosion of reinforcement being cover by 30 mm of concrete will initiate at the construction age of app. 45 years for B1R mixture, at 37 years for B2PSL mixture, at 20 years for B2WL mixture and at 11 years for B4B and B5A mixtures respectively. In the investigation presented an accelerated carbonation procedure (i.e. at elevated CO 2 concentration) was applied, although the test method (FprCEN/TS ) was not approved by the CEN TC 104 members. We found this method not complicated to perform, not to time consuming and not expensive, but still delivering valuable information about the resistance of concrete to carbonation. Indeed, two questions remain unanswered: 1) is the carbonation process under elevated CO 2 concentration (4 %) comparable to the one under environmental conditions and 2) is the contrasting using Phenolphthalein solution, which actually is an indicator for ph value, an adequate method for detecting the carbonation depth. As reported by Parrot [19] no sharp border between carbonated and non-carbonated concrete can be observed in concrete. He calls region with ph value from 8.2 to 12.6 the semicarbonated region. The existence of so called semi-carbonated zone was proven by Yongsheng [20], too. He also showed that the phase compositions produced in the natural carbonation and the accelerated carbonation are comparable, which indicates that both carbonation processes are the same for both environments. Based on the test results and analyses presented in this paper it can be concluded that present requirements and recommendations given in international and national standards do not guarantee an intended working lifetime, especially when blended cements are used. The required minimum concrete cover depth shall be cement type dependent or shall be determined based on actual concrete performance. An accelerated, simple and low-budget test method for the evaluation of resistance of concrete to carbonation shall be agreed among experts (RILEM, CEN, etc). ACKNOWLEDGEMENTS The authors would like to gratefully acknowledge generous support of Lafarge Slovenia and CEN TC 104 Concrete and related products members for supplying valuable information. Special thank go to ZAG co-worker Mrs Mojca Škerl for performing the chloride concentration tests. 505

8 REFERENCES [1] EN 206-1:2000: Concrete - Part 1: Specification, performance, production and conformity [2] EN :2004: Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings [3] SIST 1026:2008: Concrete - Part 1: Specification, performance, production and conformity Rules for the implementation of SIST EN [4] EN 197-1:2011: Cement - Part 1: Composition, specifications and conformity criteria for common cements [5] EN :2009: Testing fresh concrete - Part 2: Slump-test [6] EN :2009: Testing fresh concrete - Part 6: Density [7] EN :2009: Testing hardened concrete - Part 3: Compressive strength of test specimens [8] EN :2009: Testing hardened concrete - Part 8: Depth of penetration of water under pressure [9] EN :2009: Testing hardened concrete - Part 7: Density of hardened concrete [10] FprCEN/TS :2010 Testing hardened concrete - Part 10: Determination of the potential carbonation resistance of concrete: accelerated carbonation method [11] EN 13057:2002 Products and systems for the protection and repair of concrete structures - Test methods - Determination of resistance of capillary absorption [12] CEN/TS :2010 Testing hardened concrete - Part 11: Determination of the chloride resistance of concrete, unidirectional diffusion [13] M.A. Sanjuan, C.Andrade, M.Cheyrezy, Concrete carbonation tests in natural and accelerated conditions, Advances in Concrete Research, 2003, 15, No. 4, October, [14] C. Andrade, R. Buják, Effects of some mineral additions to Portland cement on reinforcement corrosion, Cement and Concrete Research, Volume 53, November 2013, Pages [15] A B. Ribeiro, A. Machado, A. Goncalves, M. Salta, A contribution to the development of performance-related design methods, 2nd International RILEM Workshop on Life Prediction and Aging [16] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, HOUSTON, USA, 1974 [17] CEN/TR 15868:2009 Survey of national requirements used in conjunction with EN 206-1:2000 [18] Tutti, K., Corrosion of Steel in Concrete, Swedish Cement and Concrete Research Institute, Report No. 4-82, 1982 [19] A. Parrot A Study of Carbonated-induce Corrosion, Magazine of Concrete Research, 1994 (46), [20] J. Yongsheng, Correlation of concrete carbonation process under natural conditions and high CO2 concentration artificial accelerated climate environment, 1 st International Conference on Microstructure related Durability of Cementitious Composites, October 2008, Nanjing, China,