Audenaert, Katrien

Boel, Veerle

De Schutter, Geert

English

Ghent University Academic Bibliography

   Carbonation of Filler Type Self-compacting Concrete  K. Audenaert, V. Boel, G. De Schutter Magnel Laboratory for Concrete Research, Ghent University,  Ghent, Belgium   1. Introduction  Self-compacting concrete (SCC) is a new kind of concrete that combines a high flowability and a high segregation resistance obtained by a large amount of fine particles and the use of superplasticizers. Due to the presence of the large amount of fine particles, the pore structure is somewhat different from the pore structure of traditional concrete. Because the pore structure is one of the major influencing factors concerning durability, the actual application of SCC might be somewhat risky due to the lack of knowledge concerning the actual durability of the new cementitious material. Significant research programmes have already been carried out concerning the fresh properties of SCC, but only few information is known about the durability of SCC. The durability is not like strength a bulk property but in the first place a surface property. The concrete surface differs from the inner concrete due to the wall effect, sedimentation and segregation. Therefore, the surface layer determines the ease with which fluids and gases can enter into and move through the concrete, thus influencing the service life of the concrete structure.  The concrete cover generally provides protection against corrosion of steel due to its highly alkaline nature. This alkalinity corresponds to a pH value somewhat larger than 12.5 and is provided by the dissolution in the pore water of a very small amount of the solid Ca(OH)2 in the hardened cement gel. The dissolved Ca(OH)2 in the pore water and the solid Ca(OH)2 in the cement gel is converted to CaCO3 due to the reaction with the atmospheric CO2, causing a loss of alkalinity. This process of conversion of calcium hydroxide to calcium carbonate is termed carbonation. Depassivation of the steel occurs if the pH value of the pore water is reduced below 9. If the carbonation front reaches the steel, the latter will depassivate and corrode afterwards in the presence of moisture and oxygen.  In order to study the carbonation behaviour of self-compacting concrete, concrete cubes were made of 16 SCC mixtures and 4 traditional c oncrete mixtures. Four types of cement (portland cement and blast furnace slag cement), three types of filler (fly ash and two types of limestone filler with a different grading curve) and two types of coarse aggregate are used and the influence of the amount of powder (cement and filler material) and the amount of water is studied. These concrete cubes were stored  permanently in a carbonation room at 20°C, 60% R.H. and 10% CO2. At certain times, the carbonation depth is determined with phenolphthalein and the carbonation coefficient is determined. The main influencing factors in the carbonation process are the diffusion of CO2 into the concrete and the amount of hydrated cement reacting with the CO2. These two parameters could be determined based on the concrete composition, degree of hydration and the chemical composition of the cement. In the last part of the paper, the calculated and experimental carbonation coefficient are compared.  2. Experimental programme  2.1 Mixture design  At the Magnel Laboratory for Concrete Research, 16 self-compacting concrete mixtures (SCC) and 4 traditional concrete mixtures (TC) were investigated.   Table 1: Mixture Composition         CEM I 42.5 R [kg/m³] CEM I 52.5 [kg/m³] CEM III A 42.5 LA [kg/m³] CEM I 52.5 HSR [kg/m³] limestone filler S [kg/m³] limestone filler P2 [kg/m³] fly ash [kg/m³] water [kg/m³] sand 0/5 [kg/m³] gravel 4/14 [kg/m³] Limestone gravel 2/14 [kg/m³] W/C [-] C/P [-] compr. strength [MPa] SCC1 360     240  165 853 698  0.46 0.60 57.3 SCC2  360    240  165 853 698  0.46 0.60 68.0 SCC3   360   240  165 853 698  0.46 0.60 66.1 SCC4    360  240  165 853 698  0.46 0.60 70.1 SCC5 300     300  165 853 698  0.55 0.50 46.5 SCC6 400     200  165 853 698  0.41 0.67 64.2 SCC7 450     150  165 853 698  0.37 0.75 68.7 SCC8 360    240   165 853 698  0.46 0.60 56.9 SCC9 360      240 165 853 698  0.46 0.60 66.2 SCC10 300     200  137 923 755  0.46 0.60 60.1 SCC11 400     300  192 782 640  0.48 0.57 55.9 SCC12 450     350  220 712 583  0.49 0.56 50.9 SCC13 360     240  144 865 707  0.40 0.60 68.7 SCC14 360     240  198 835 683  0.55 0.60 46.6 SCC15 360     240  216 825 675  0.60 0.60 40.3 SCC16 360     240  165 816  734 0.46 0.60 74.7 TC1 360       165 640 1225  0.46 1.00 48.6 TC2   360     165 640 1225  0.46 1.00 49.7 TC3    360    165 640 1225  0.46 1.00 50.2 TC4 400       165 640 1220  0.41 1.00 53.7    In the first 9 mixtures a constant amount of cementitious materials (cement and filler) is considered: 600kg/m³, as well as a cons tant amount of water, sand and gravel, respectively 165k/m³, 853 kg/m³ and 698 kg/m³. Four types of cement are used (Portland cement CEM I 42.5 R, CEM I 52.5, CEM I 52.5 HSR and blast furnace slag cement CEM III A 42.5 LA), three types of filler (fly ash and two types of limestone filler BETOCARB P2 and Superfine S, the last one having a finer grading). In the next three mixtures, the amount of powder is varied (500 kg/m³, 700 kg/m³ and 800 kg/m³). In the following three mixtures, the amount of water is varied (144 kg/m³, 198 kg/m³ and 216 kg/m³). In SCC16 crushed limestone gravel was used instead of river gravel.  For the traditional c oncrete mixtures, the type of cement is varied in the first three mixtures with the same water/cement ratio as for SCC1. For TC4 a lower water/cement ratio is used.   The amount of superplasticizer, which is a modified polycarboxylether, was determined in order to obtain a sui table flowability without segregation. Also the flowing time in the V-funnel was measured (values between 5s and 10s), air content (values between 1% and 3%) and the U-box requiring self-levelling. In table 1 the mixture composition is given together with the compressive strength at 28 days measured on concrete cubes with side 150mm, the water/cement ratio (W/C) and the cement/powder ratio (C/P).  2.2 Test method  From the mixes described above, cubes 100 x 100 x 100mm³ were made. These concrete cubes were stored in a climate chamber at 20°C ± 2 °C and more than 90% R.H.. At the age of 28 days, an epoxy coating was applied to all surfaces, except on the surface exposed to CO2. The concrete cubes were stored permanently in a carbonation room at 20°C, 60% R.H. and 10 vol.% CO2. At regular times (8, 12, 16, 20, 24 and 28 weeks) the carbonation depth was examined experimentally. At each time, a slice with a thickness of 1 cm was sawn from each specimen. This slice was sprayed with a phenolphthalein solution in order to determine the carbonated zone. This pH indicator changes from colourless to purple as the pH rises from 8.3 to 10.0. The test indicates the depth to which the calciumhydroxide has been depleted. After the sawing of the slices, the remaining concrete specimen was covered again with the carbondioxide resisting epoxy coating and the treatment was continued.         2.3 Experimental results  The carbonation of concrete can be considered as a Fickian problem because it is a diffusion controlled process. This process is governed by the equation:        tAx =                                    (1)  in which x is the carbonation depth (mm), t is the exposure time (year) and A a constant depending on the diffusion resistance of the material (mm/Öyear). In literature, most of the models are following this law, although some other models can be found.  After sawing a slice of the concrete cube, the carbonation depth is determined at 10 points and the mean value is calculated. For each composition, at least three concrete cubes are made in order to obtain a mean carbonation depth. With the carbonation depths at several times, and with equation (1), a regression value for the parameter A can be determined. The resulting A-values are given in table 2 together with the number of cubes tested for each mixture. Table 2: Values for A (experimental conditions) and number of test specimens N  A [mm/Öyear] N SCC1 14.4 5 SCC2 10.9 5 SCC3 23.3 3 SCC4 14.6 5 SCC5 23.0 5 SCC6 11.0 5 SCC7 7.5 8 SCC8 14.1 5 SCC9 14.2 5 SCC10 11.1 6 SCC11 16.6 6 SCC12 17.6 3 SCC13 12.0 6 SCC14 31.2 6 SCC15 32.2 3 SCC16 7.5 3 TC1 11.5 5 TC2 25.7 3 TC3 3.7 5 TC4 4.2 5   From these results, the following observations can be made, taking SCC1 as reference concrete: · cement type: a cement type with a higher strength (SCC2), resulting in a higher concrete strength (using the same W/C-ratio), leads to a lower carbonation depth.  A blast furnace slag cement (SCC3) gives a higher penetration depth than Portland cement (SCC1). Also for traditional concrete, with TC1 as reference, the use of blast furnace slag cement in TC2 leads to an increase of the penetration depth.  High sulphate resistant cement (SCC4) has a lower C3A content and a higher C2S and C3S content, leading to an increased amount of Ca(OH)2 and CSH and a lower penetration depth. This effect is more pronounced for traditional concrete (TC3 compared to TC1) than for self-compacting concrete (SCC4 compared to SCC1). · W/C ratio: with a constant powder and water content and a varying amount of cement, the water cement ratio is varied (0.37 for SCC7, 0.41 for SCC6, 0.46 for SCC1 and 0.55 for SCC5). In this way, the compressive strength is also influenced (respectively 46, 56, 64 and 69 N/mm²). A lower W/C-ratio and a higher strength result in a lower carbonation depth.  Moreover, by the increased amount of cement, the amount of Ca(OH)2 and CSH will increase, resulting in a higher amount of bounded CO2 and a lower penetration depth. · type of filler: the use of a limestone filler with a finer grading does not change the carbonation depth (SCC8 versus SCC1).  The use of fly ash (SCC9) gives a higher strength and a denser pore structure, yielding a lower penetration depth. But, in the same time, part of the Ca(OH)2 is consumed by the hydration of the fly ash, leading to a higher penetration depth. The result, in this case, is that the carbonation coefficient remains almost constant.  · amount of powder: increasing the amount of powder with a constant C/P ratio and a constant W/C ratio, leads to an increase in the amount of paste. This increase in paste creates an increase in the total porosity, leading to a higher carbonation coefficient.  · amount of water: increasing the amount of water (increasing the W/C ratio and decreasing the strength) results in a higher penetration depth (SCC13: 144 kg/m³, SCC1: 165 kg/m³, SCC14: 198 kg/m³ and SCC15: 216 kg/m³). · type of coarse aggregate: by replacing the river gravel by crushed aggregate, the bonding between the paste and the aggregate will be better. This leads to a decrease of the carbonation depth. The test result shows a stronger decrease than what could be expected. · TC in comparison with SCC: TC gi ves lower values for the penetration depth than the corresponding SCC (TC1 - SCC1; TC3 - SCC4 and TC4 - SCC6), although the traditional concrete has a lower compressive strength. This is not the case for TC2 – SCC3.   3.0 Modelling  3.1 Introduction  From the Fickian law and the work of Schröder [1,2], the carbonation coefficient could be written as:  )cc(aD2A 21 -=               (2)  with D the diffusion coefficient of the penetrating CO2 into the concrete (m²/s), a the amount of carbonatable material (mol CO2/m³ concrete) and c1-c2 the concentration difference between the outside environment and the uncarbonated concrete (mol/m³).  In literature [2,3,4,5], the diffusion of a gas, e.g. CO2, through concrete is often written in function of the capillary porosity 㱠 (-). In this paper, the following relation, given in equation (3) is used. The power 1.8 is also used in [2,3,4].   D ~ 㱠1.8               (3)  For the determination of the amount of carbonatable material , different approaches are given in li terature [5,6,7]. In this paper, the approach of Papadakis et al.  [7] will be followed. This approach is starting from the chemical composition of the cement as will be explained in section 3.3.   3.2 Calculation of capillary porosity  In literature, it is  written that the transport properties of concrete are mainly determined by capillary pores [8,9]. Therefore, the model of Powers et al. [10] is used to determine the capillary porosity.    ( )( )hC3194.0W1hC1319.0hC23.0hC23.0185.01hC72.028.0WhC185.0waterboundedwatergelwaterporescapillarywaterfreeporescapillaryVwcwwcwccap-r+r-=r-÷÷øöççèær+-r-r+r=--+=+= (4)  with Vcap the volume of capillary pores [m³], C the amount of cement [kg], W the amount of water [kg], h the degree of hydration [-], 㰐c and 㰐w the mass density of respectively cement and water [kg/m³].     aggcwfillersandaggregatecoarsecementwaterconcreteFSACWVVVVVVrrr++++=++++=          (5)     concretecapVVporositycapillary =              (6)          with Vconcrete the volume [m³], A the amount of coarse aggregate [kg], S the amount of sand [kg], F the amount of filler [kg] and 㰐agg the mass density of aggregate [kg/m³]. The parameters W, C, A, S and F are known from the mixture proportions. For the mass densities, a value of 1000 kg/m³ is used for water, 2625 kg/m³ for the aggregates, sand and filler and 3115 kg/m³ for portland cement.  The test specimens are stored until the testing age in a climate room at 20°C ± 2 °C and at least 90% R.H. for at least 28 days. This means that the degree of hydration will not strongly differ from the ultimate degree of hydration, which could be determined by the Mill formula:  CWCWhultim+=194.0031.1              (7)   3.3 Calculation of amount of carbonatable material  Papadakis et al. [7] propose that a is equal to the total molar concentration of CaO in the carbonatable constituents of the concrete:  [ ] [ ] [ ] [ ]SC2SC3CSH3)OH(Caa 232 +++=            (8)   These molar concentrations should be determined at the moment the carbonation starts and should be expressed in mol/m³. The amount of C3S and C2S was determined for the Portland cements and is given in table 3, together with the chemical composition of the cements. By using the ultimate degree of hydration, the amount of CSH and Ca(OH)2 could be determined based on the chemical hydration reactions.  For the mixtures with blast furnace slag cement and with fly ash, this approach could not be used. Those mixtures will not be modelled (SCC3, SCC9 and TC2).          Table 3: Chemical composition of cement   CEM I 42.5 R CEM I 52.5 CEM I 52.5 HSR SiO2 (%) 19.6 20.3 20.8 Al2O3 (%) 5.0 4.5 3.6 Fe2O3 (%) 3.0 2.3 3.9 CaO (total) (%) 61.5 64.0 64.2 MgO (%) 0.8 2.2 2.4 SO3 (%) 3.3 3.3 2.7 Na2O (%) 0.4 0.2 0.2 K2O (%) 0.9 0.9 0.5 C3S (%) 58.2 59.0 60.6 C2S (%) 12.7 12.6 16.6 C3A (%) 8.2 8.0 2.7 C4AF (%) 9.1 9.4 13.1   3.4 Modelling results  Because D is proportional to the capillary porosity (eq. 3) and c1-c2 is a constant, the following relation is obtained from equation 2:  A ~a8.1j                (9)   In table 4, the values of the capillary porosity 㱠, the amount of carbonatable material a and a8.1j  are given.  In figure 1, the experimental value of the carbonation coefficient A is plotted in function of a8.1j  for each mixture. A good correlation (R² = 0.88) is obtained. The mixtures TC3 and TC4 are not giving a good correlation. This could be explained by the large scatter in the results for traditional concrete (the values for TC2 being very high and for TC3 and TC4 very low, see table 2). This large scatter, as explained in [2], is mainly due to the vibration with a needle of the test specimens. These were concrete cubes with side 100mm. The vibration could have caused some segregation and a more porous trowelled face. It was this face that was exposed to CO2.        Table 4: Calculation of a8.1j    㱠 (%) a (mol CO2/m³ concrete) √( 㱠  1.8/a) SCC 1 7.33 2311 0.125 SCC 2 6.72 2648 0.108 SCC 4 6.73 2566 0.110 SCC 5 7.33 2311 0.125 SCC 7 8.51 2032 0.152 SCC 8 6.61 2496 0.109 SCC 9 5.76 2688 0.093 SCC 10 6.16 1925 0.117 SCC 11 8.78 2603 0.138 SCC 12 10.07 2968 0.147 SCC 13 5.63 2214 0.101 SCC 14 10.03 2439 0.161 SCC 15 11.51 2503 0.180 SCC 16 7.33 2311 0.125 TC 1 7.11 2311 0.122 TC 3 6.53 2566 0.107 TC 4 6.41 2496 0.106  051015202530350.08 0.1 0.12 0.14 0.16 0.18 0.2(f 1.8/a)0.5A [mm/year0.5]SCCTC  Figure 1: Modelling of the carbonation coefficient A       4. Conclusions  In this article, the carbonation of 16 self-compacting concrete mixtures and of 4 traditional concrete mixtures is discussed. The selection of the mixtures is made in order to consider some important parameters like the type of cement (Portland cement of two different strength classes, blast furnace slag cement and a high sulphate resisting cement), type of filler (fly ash and limestone filler with two different grading curves), the cement/powder and water/cement ratio, the amount of water, the amount of powder (cement and filler) and the type of coarse aggregate. The results of these tests are discussed and a model is proposed. The following conclusions could be made: - the carbonation of concrete is influenced by the diffusion of CO2 through the concrete and by the amount of carbonatable material present in the concrete. The diffusion coefficient is mainly determined by the capillary porosity. The amount of carbonatable material depends on the type of cement, the cement content and the degree of hydration - The type of cement is a main influencing factor, both for traditional and self-compacting concrete. - The use of limestone filler with a finer grading curve does almost not influence the carbonation behaviour. - The use of fly ash as filler material decreases the porosity and decreases the amount of carbonatable material. Both effects are in equilibrium in the investigated mixture, leading to a very comparable carbonation coefficient regarding the reference mixture. - The decrease of the capillary porosity, obtained by e.g. a lower W/C ratio, a higher C/P ratio, a lower water content,..., decreases the carbonation rate.                  5. References  [1]  Schröder F., Smolczyk H., Grade K., Vinkeloe R., Roth R. (1967), Einfluss von Luftkohlensäure und Feuchtigkeit auf die Beschaffenheit des Betons als Korrosionsschutz für Stahleinlagen, DAfStb, Heft 182 [2]  Audenaert K. (2006), Transportmechanismen in zelfverdichtend beton in relatie met carbonatatie en chloridepenetratie, PhD, Ghent University, 370p [3]  Papadakis V., Vayenas C., Fardis M. (1991), Physical and chemical characteristics affecting the durability of concrete, ACI Materials Journal, Vol. 8, No. 2, p 186-195 [4]  Papadakis V., Vayenas G., Fardis M. (1991), Fundamental modelling and experimental investigation of concrete carbonation , ACI Materials Journal, Vol. 88, No. 4, p 363-373 [5] CEB 238 (1997), New approach to durability design – an example method for carbonation induced corrosion, Switzerland, 138p [6] Schiessl P. (1976), Zur Frage der zulässigen Rissbreite und erforderlichen Betondeckung im Stahlbetonbau unter besonderer Berücksichtigung der Karbonatieringstiefe des Betons, DAfStb, Heft 255 [7] Papadakis V. (2000), Effect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress , Cement and Concrete Research, Vol. 30, p 291-299 [8] CEB-FIB Model Code 1990 (1991), Design code, Thomas Telford, 437p [9] Marsh B., Day R., Bonner D. (1985), Pore structure characteristics affecting the permeability of cement paste containing fly ash, Cement and Concrete Research, Vol. 15, No.6, p 1027-1038 [10] Powers T., Brownyard T. (1946-1947), Studies of the physical properties of hardened cement paste (nine parts), Journal of the American Concrete Institute, Vol. 43           

Carbonation of filler type self-compacting concrete

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Carbonation of filler type self-compacting concrete

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Ghent University Academic Bibliography