2014 - Sajna Et Al

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RILEM International workshop on performance-based specification and control of concrete durability11 - 13 June 2014, Zagreb, Croatia

499

INFLUENCE OF BLENDED CEMENT TYPE ON CONCRETECARBONATION, CAPILLARY UPTAKE AND CHLORIDEPENETRATION

A. Šajna, V. Bras and L. Završnik

Slovenian National Building and Civil Engineering Institute, Slovenia

AbstractCarbonation 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 ofresistance to carbonation of concretes prepared with different types of blended cements andmeasurement 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 (exposureclasses 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 inducedcorrosion environment. In such cases performance based determination of carbonation depthis essential.

1. INTRODUCTION

Carbonation of concrete is a chemical reaction between calcium hydroxide in concrete andcarbon dioxide from air, resulting in calcium carbonate. Carbonation allegedly increasesmechanical 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 ofsteel reinforcement. According to the EU standards EN 206-1 [1] and Eurocode 2 [2], theenvironmental 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 nationalappendices to EN 206-1, like the Slovenian SIST 1026 [3], additional requirements orrecommendations for the concrete compositions like cement content and w/c ratio, or concretecharacteristics like compressive strength classes or resistance to penetration of water are givenfor XC1 to XC4 concrete. Extra, in Eurocode 2 the minimum concrete cover depthsdepending on claimed structure service life, exposure class and concrete compressive strengthclass are defined.

In the paper test results of an extensive research program are presented. The test results areanalysed and compared to EN 206-1, Eurocode 2 and some national requirements for durablestructures exposed to carbonation induced corrosion.

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2. EXPERIMENTAL PROGRAM

2.1. Concrete mixtures

In this investigation five different concrete mixtures, with five different types of cementswere prepared and investigated. Basic concrete mixture proportions are given in Table 1. Thefive cement types investigated and the concrete mixture codes are listed in Table 2.

Table 1: Basic concrete mixture proportions

Raw material kg/m3

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 averageacc. to EN 197-1 [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 methodsFor concrete characterisation the following tests were performed: slump (EN 12350-2 [5]),

density of fresh concrete (EN 12350-6 [6]), 28-day compressive strength (EN 12390-3 [7]),resistance to penetration of water under pressure (EN 12390-8 [8]) and density of hardenedconcrete (EN 12390-7 [9]).

Resistance to carbonation was measured according to FprCEN/TS 12390-12:2010 Testing

hardened concrete - Part 10: Determination of the potential carbonation resistance of concrete:accelerated carbonation method [10], although the technical specification was never acceptedfor publication by CEN TC 104 Concrete and related products. During the exposure tocarbonation the temperature was kept at 20° C, relative humidity at 55 % and CO2 concentration at 4 %. The carbonation depth was measured by means of 1 % Phenolphthaleinsolution, which turns colourless in acidic solutions and pink in basic solutions (pH > 8.2).

Capillary water uptake was measured according to EN 13057 [11]. Samples, concretecores, were for 28 days cured according to EN 12390-2. After first series of exposure tocapillary uptake, the samples were exposed to carbonation according to FprCEN/TS 12390-12. After 70 days of exposure to carbonation, the capillary uptake tests were repeated. After

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the capillary uptake test the samples were exposed to carbonation again. After additional 6months of exposure to carbonation the capillary uptake tests were performed for the thirdtime.

Resistance to chloride penetration was performed according to CEN/TS 12390-11 [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 significantdifference was observed.

The porosity and pore size distributions of the investigated samples were determined bymeans of mercury intrusion porosity (MIP). Small blocks, approximately 1 cm3 in size, weredried in an oven for 24 h at 105 °C and analysed on a Micromeritics Autopore IV 9500model. 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 thesame 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 todistinguish between non-carbonated and carbonated region. It has to be stated that only onesample per concrete type was tested which due to small sample size end relativeinhomogeneity of concrete can be scarce.

3. TEST RESULTS

3.1. Concrete characterizationThe concrete characterization test results are presented in Table 3. The highest

compressive strengths exceed the B1R and B2WL mixtures. As expected, due to lowestclinker content, the lowest compressive strength has B5A mixture. The measured water

penetration depths are in accordance with expectations.

Table 3: Characterisation test results

Mixture codeSlump

[mm]

Compressivestrength[MPa]

Density ofhardened concrete

[kg/m3]

Water penetration, max

[mm]B1R 170 51.5 2450 Not determined

B2PSL 160 48.8 2445 11B2WL 170 52.5 2440 15B4B 170 40.8 2435 17B5A 160 38.2 2455 Not determined

3.2. Carbonation depthResistance 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 tosquare root of time. The regression lines and carbonation rates were calculated according toSanjuan [13], assuming the carbonation rate is proportion to square root of time. Thecarbonation rates are listed in Table . In general, the carbonation rates are within theexpectations, lower for higher clinker content concretes and vice versa. Surprisingly similar isthe carbonation rate of B4B and B5A concretes and unexpectedly high is the difference incarbonation rate between B2PSL and B2WL. It has been confirmed that resistance to

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RILEM I

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505

Concretes tested fulfil all Slovenian requirements and recommendations for XC4 exposureclass. They also fulfil requirements and recommendations given in EN 206-1 and the majority

of requirements valid in other CEN members (Table 5). They also achieve the recommendedcompressive strength class of concrete cover given in Eurocode 2, i.e. C30/37, except for B5Amixture.

Although the concrete mixtures tested satisfy all above mentioned recommendations andrequirements the 50 year carbonation depths calculated based on test results surpass theconcrete cover depth prescribed by Eurocode 2. For the most severe wet-dry exposure classXC4 a minimum cover depth of 30 mm is prescribed, where else the calculated carbonationdepths 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 30mm of concrete will initiate at the construction age of app. 45 years for B1R mixture, at 37years for B2PSL mixture, at 20 years for B2WL mixture and at 11 years for B4B and B5Amixtures respectively.

In the investigation presented an accelerated carbonation procedure (i.e. at elevated CO2 concentration) was applied, although the test method (FprCEN/TS 12390-12) was notapproved 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 theresistance of concrete to carbonation. Indeed, two questions remain unanswered: 1) is thecarbonation process under elevated CO2 concentration (4 %) comparable to the one underenvironmental conditions and 2) is the contrasting using Phenolphthalein solution, whichactually 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 semi-carbonated region. The existence of so called semi-carbonated zone was proven byYongsheng [20], too. He also showed that the phase compositions produced in the naturalcarbonation and the accelerated carbonation are comparable, which indicates that bothcarbonation 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 donot guarantee an intended working lifetime, especially when blended cements are used. Therequired minimum concrete cover depth shall be cement type dependent or shall bedetermined based on actual concrete performance. An accelerated, simple and low-budget testmethod 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 Sloveniaand 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 chlorideconcentration tests.

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REFERENCES

[1] EN 206-1:2000: Concrete - Part 1: Specification, performance, production and conformity[2] EN 1992-1-1:2004: 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 206-1[4] EN 197-1:2011: Cement - Part 1: Composition, specifications and conformity criteria for

common cements[5] EN 12350-2:2009: Testing fresh concrete - Part 2: Slump-test[6] EN 12350-6:2009: Testing fresh concrete - Part 6: Density[7] EN 12390-3:2009: Testing hardened concrete - Part 3: Compressive strength of test specimens[8] EN 12390-8:2009: Testing hardened concrete - Part 8: Depth of penetration of water under

pressure[9] EN 12390-7:2009: Testing hardened concrete - Part 7: Density of hardened concrete

[10]

FprCEN/TS 12390-12:2010 Testing hardened concrete - Part 10: Determination of the potentialcarbonation 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 12390-11:2010 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, 171-180[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 59-67[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),

23-28[20] J. Yongsheng, Correlation of concrete carbonation process under natural conditions and high CO2

concentration artificial accelerated climate environment, 1st International Conference onMicrostructure related Durability of Cementitious Composites, 13-15 October 2008, Nanjing,China, 495-505

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