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Δευτέρα, 03 Αυγούστου 2020

Επεξήγηση της χρήσης των προστατευτικών βαφών στο οπλισμένο σκυρόδεμα και παρουσίαση των σχέσεων που περιγράφουν τον βαθμό προστασίας που επιτυγχάνεται συναρτήσει των βασικών παραμέτρων από τις οποίες εξαρτάται η ενανθράκωση του σκυροδέματος και η ταχύτητα εξέλιξής της (το άρθρο είναι στην Αγγλική).

Selection of Protective Coatings according to ELOT EN 1504-2 Against Concrete Carbonation by Chris A. Rodopoulos, MSc In Materials Science, PhD in Stress Corrosion Fatigue a) Introduction Paint systems or coatings are considered as the ultimate protection system against concrete carbonation and against the subsequent probability of reinforcement corrosion. In this article the author is trying to explain the use of protective paint systems and the equations describing the resulted degree of protection, making reference to fundamental parameters controlling the carbonation process and speed. The article is written in such way as to assist engineers involved with the protection of concrete structures either during the design or rehabilitation phase. Examples referring to the particular environment of Greece are also included to enhance assistance to the reader.Typical structural failure due to indoor carbonation b) Carbonation as a Process The atmosphere contains substantial amounts of carbon dioxide. Yet, gaseous CO2 cannot, react directly with the hydrates of the cement paste. Thus the CO2 gas must first dissolve in water and form carbonate ions that in turn will react with the Ca ions of the pore water solution of the cement paste. The type of carbonate ions depends on the pore solution pH. When CO2 comes into contact with water at neutrality it forms bicarbonate (HCO3-). Inside concrete, the pH is high and as a result the bicarbonate dissociates and forms carbonate ions (CO32-). Thus in the carbonated layer bicarbonate forms but closer to the uncarbonated cement paste this carbonate ions form (due to higher pH) and precipitate into calcium carbonate crystals (CC). Calcium carbonate crystals exist in three crystallographic forms, aragonite (Αραγωνίτης), vaterite (Βατερίτης) and calcite (ασβεστίτης), Figure 1. Calcite and vaterite are commonly found in carbonated concrete, Figure 2. p. 1 / 15 a) Calciteb) Aragonitec) VateriteFigure 1. Photos of calcium carbonate crystals taken from SEM.Figure 2. Calcite crystal in cement paste along with Wollastonite needles. The carbonation process can be described by the following chemical equations, 1. CO2 (g) + H2O = HCO3- (bicarbonate ion) +H+ 2. HCO3 - = CO32- (carbonate ion) + H+ The carbonate ion will react with Ca ions in the pore solution to form, 3. Ca2++ CO32-= CaCO3 This will lead to lower concentration of Ca2+ which in turn will result into the dissolution of primarily calcium hydroxide (CH). Since the solubility of CC is much lower than that of CH, 4. Ca(OH)2 = Ca2+ + 2 OH- (solubility 9.95 x 10-4) 5. Ca2+ + CO32- = CaCO3 (solubility 0.99 x 10 -8) Ca(OH)2 (CH) will dissolve and CaCO3 (CC) will precipitate and the process will continue until all of the CH is consumed.p. 2 / 15 The rate of carbonation depends on the solubility and speed of diffusion. Diffusion is controlled by concentration differences. Thus we must consider the diffusion processes and the effect on the structure of the carbonated layer. In simple terms is a process with inward diffusion of carbon dioxide gas and carbonate ions. Gas diffusion is much faster than ion diffusion. Thus the speed of carbonation depends on the moisture content of concrete. In other words the level that concrete pore system or network is filled with water, Figure 3.Figure 3. Typical concrete porosity. In dry concrete the carbon dioxide can penetrate deeply but there is not enough water for the carbonation reaction. In fully water saturated concrete, only carbonate ions can move and hence carbonation is slow. Thus there is an optimum where the speed of carbonations is at maximal, Figure 4. Carbonation rate has also been found to increase at elevating ambient temperatures. Indoor climate or exposure in warmer regions will usually lead to faster carbonation, given that all the other affecting parameters remain invariable. In contrast, outdoor concrete structures in low temperature regions will exhibit lower carbonation rates.1.1 C20/25Degree of Carbonation Speed1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0102030405060708090100Relative Humidity of Air in Equilibrium to ConcreteFigure 4. Effect of environmental humidity on carbonation rate on a C20/25 class concrete. Perhaps the most intriguing parameters affecting carbonation rate is the porosity of the near surface. The term near surface refers to depths between 0.1-2 mm from the surface, Figure 5. p. 3 / 15 Figure 5. Concrete near surface porosity. The images shows increased porosity at the first 450µm. Near surface porosity is affected by a variety of parameters including curing temperature, W/C ratio, formwork compaction, wind speed, mix design etc. Carbonation gives rise to volume changes. Transformation of CH to calcite gives a volume change of 11 % and to the metastable vaterite 14 %. The volume changes will affect the porosity in the carbonated layer and thus the speed of diffusion. We know that the volume changes do not affect the mechanical stability of the carbonated layer which remains stable and hard. This indicates that, normally, the surplus volume of calcite precipitation mainly fills empty space in the capillary system and thus reduces the porosity of the cement paste. c) The speed of Carbonation Concrete will carbonate whenever carbon dioxide and some water are available. Independently of the local environmental conditions, carbonation should be considered as an inevitable phenomenon. As explained in section b), the speed of carbonation depends on how fast the carbon dioxide and/or the carbonate ions can move into the concrete and react with the cement paste. Even in the case of concrete submerged in water or under water saturated conditions, carbonation will take place but at much slower rate and in potentially another manner. When the capillary pore system of the cement paste is blocked with water, carbon dioxide gas has difficulty to diffuse. Thus, for concrete submerged in water (i.e. damns, water tanks, etc) we have to consider the concentration of carbonate ions in water and not the concentration of CO2 in gaseous form. Similarly in soil applications, like retaining walls, deep foundations, piles, etc, the decay of organic matter may result into a high CO2 concentration, but on the other hand the speed of the diffusion of CO2 gas or carbonate ions in the soil may be slow. Therefore, the question is not whether carbonation will occur? but rather, when carbonation becomes critical? According to Model Specification for Protective Coatings for Concrete, issued by the Government of Hong Kong- Civil Engineering Department in 1994, such limit, xcrit, is considered whenp. 4 / 15 Χcrit≤ Cmin-5mm where Cmin is the minimum concrete cover thickness over the stir-up or Χcrit≤ Caver - 7mm where Caver is the average concrete cover thickness over the stir-up To better understand the two critical limits previously suggested, it is worth examining the following example. Cover thickness and carbonation measurements were performed on several external columns of a 6-storey building in Athens, Greece in 2013, Figure 6.Figure 6. Typical carbonation measurement using the colour indicator procedure. The results are shown in Table 1. Table.1. Measurement matrix CarbonationCoverDepth (mm) 6 8 7 8 6 5 6 8 5 6 6 5 6Thickness (mm) 17 20 22 19 18 19 22 24 20 21 22 23 24As per the Cmin approach, the critical depth of carbonation is Xcrit=17mm-5mm=12mm. Similarly according to Caver approach, Xcrit=20.84 mm-7mm=13.84 mm.p. 5 / 15 The speed of carbonation can be determined either in terms of Equation 1, (Eq.1) where X is the carbonation depth in mm, K is the carbonation rate coefficient in mm year-1/2 and T is the exposure time in years. Similarly the speed of carbonation can be determined in terms of Equation 2, (Eq.2) where X is the carbonation depth in mm, D is the carbonation diffusion coefficient in mm2 / year and T is the exposure time in years. Equations 1 and 2 are obviously related resulting into, (Eq.3) It is worth noting that Eqs.(1,2) are quite simplistic in describing the phenomenon, but at the same time they represent a sound tool in the hands of a practicing engineer. d) Determination of Safe Life Whether corrosion of the reinforcement will initiate as a result of low pore solution pH values of cement pore solution pH ≤9.2-8.6 are considered critical for loss of reinforcement passivity - is not something that engineers around the world should bother with in terms of protecting or performing a durability analysis. In every report, standard, building code once carbonation depth becomes equal or larger than the stir-up cover thickness; carbonation corrosion is considered as being initiated. Hence, in the literature such condition is referred as "corrosion threshold due to carbonation". Based on the above we can now transform the statement Therefore, the question is not whether carbonation will occur? but rather, when carbonation becomes critical? to What shall we do in order to prevent reaching the corrosion threshold due to carbonation? Estimation of the critical corrosion threshold can be made using Eqs (1 or 2) if we know the time of exposure T being Τcurrent-Tinitial. Τcurrent is taken as the date of performing the measurements, in the case of the example 2013 and Tinitial is the date that concrete was first poured. Lets us considered that the construction of the 6-storey building started in 2002. In this case equation 1 becomes,p. 6 / 15 Herein, we have considered the worst case scenario of X being the maximum value of the sample (8mm). Using Eq.(3), the carbonation diffusion coefficient D is,In the case of engaging the average value of the sample, the carbonation rate coefficient results into,and thePerhaps the reader will raise the question, what is the difference between having a “fair face” concrete and having a concrete over coated with plaster of a particular thickness and paint? The answer is none. Table 1 contains values obtained from examining only the concrete depth. Whether the measurements are the result of (paint+plaster+concrete) or just concrete, they do not affect the parameters K or D. In other words, equations 1 and 2 examine the “end effect”. Another potential questions emerging from the so far analysis could “why shall I calculate Ks' and Ds' for both maximum and average values?” Table 1, in terms of carbonation depth exhibits a probability distribution profile (normal distribution) having a rather small standard deviation (1.10mm). As such the value of 8mm is quite close to the average of 6.30mm with probability of only 6.11% being outside the distribution. Reality however is somehow different. Concrete cracking, surface blow outs, temperature, humidity and so many other factors are parameters playing a major role on standard deviation; Figure 7 shows the effect of a crack.p. 7 / 15 Figure 7. Effect of cracking on carbonation depth.Carbonation Depth (mm)To better understand the relationship between K and Xcrit, visualization of the results is needed, Figure 8. 18 17 16 15 14Cmin(12 mm, 24.8 years) DATE=202713 12 11 10 9 8Xcrit for CminProtection Limit(8 mm, 11 years) DATE =20137 6 5 4 3 2 1K max=2.41 mm year 05101520253035404550-1/255Years of ExposureFigure 8. Visualization of the problem in terms of Kmax. From Figure 8 we can deduct the following conclusions, a) In 2027 being 14 years since our measurements, carbonation depth would be equal to Xcrit and therefore corrosion due to carbonation will materialize (safe life limit). b) The protection limit in terms of remaining protection depth is 4mm. p. 8 / 15 e) Examining ELOT EN 206-1 For over 50 years, engineers have learned that avoidance of corrosion due to carbonation or due to chloride ingress was achieved via concrete cover thickness. Several National Building codes refer specifically, to certain cover limits. The concept of concrete cover thickness is also postulated in EN 206-1, while and not for the first time, see KTS'97 (Κανονισμός Τεχνολογίας Σκυροδέματος), is related to minimum cement content and Water/Cement or Water/Binder ratio. Let us take for example the draft version of ELOT EN 206-1:2000, as shown in Figure 9.Figure 9. Minimum requirements of cover and concrete class against environmental load. Herein we can transform the requirements in terms of K or D using equation 1 and 2. The results from such action are depicted in Table 2. In Table 2, of course we do not have taken into account the effect of cement content or W/C ratio. Such action is not necessarily unsound, since K and D are controlled only by the exposure time and cover thickness in the equations provide by Ceb/FiP (The International Federation for Structural Concrete) in Eurocode. Table.2. Ks and Ds according to ELOT EN 206-1 for the case of carbonation CategoryXC1XC2XC3XC4Min. Cover (mm)25253535W/C0.650.60.550.5Cement (Kgr)280300300320-1/2K (mm year )3.543.544.954.95D (mm2 / year)6.256.2512.2512.25T (years)50505050p. 9 / 15 To better evaluate the minimum requirements as expressed in the draft version of ELOT EN 206-1, it is worth making a certain comparison with realistic data taken from Greece, Fig. 10. Carbonation Rate of Major Cities in Greece 60 Age vs Crete-Hrakleio Age vs Athens Centre Age vs Kalamata Age vs Thessaloniki-Centre Age vs Volos Age vs Mykonos Age vs Kifisia-Athens Age vs Ioannina Age vs LarissaCarbonation Depth (mm)50403020100 01020304050Age in YearsFigure10. Carbonation rate measured from structures at major cities in Greece. Concrete strength in all cases had a minimum strength category of C16/20. To better appreciate the results in Figure 9, we project on top of them the carbonation depth as results from Table 2, Figure 11. Carbonation Rate of Major Cities in Greece Age Age Age Age Age Age Age Age Age60Carbonation Depth (mm)50vs Crete-Hrakleio vs Athens Centre vs Kalamata vs Thessaloniki-Centre vs Volos vs Mykonos vs Kifisia-Athens vs Ioannina vs LarissaK=3.54 mm year-1/2 (XC1, XC2) K=4.95 mm year-1/2 (XC3, XC4)403020100 0102030405060Age in YearsFigure 11. Comparison between the data from Figure 10 and the projected carbonation curves as defined in ELOT EN 206-1. p. 10 / 15 Close examination reveals that application of ELOT EN 206-1 provides insufficient protection over the defined period of 50 years with first indication of failure taking place at 17 years. At T≥25 years, being half the designed life of the structure, it is obvious that over 50% of the sample will experience corrosion by carbonation. Similar conclusions have been drawn in several other works. At this point it is clear that the above analysis is under the assumption of full engagement of the cover thickness. Obviously if the recommendation of Xcrit is introduced deficiencies in protection will further increase. Whether Equation 1 reliably estimates the speed of carbonation and whether other models can provided more accurate predictions is not the case examined in this work. In this work we examine requirements and equations belonging in the EU Building Code. Whether sample data depicted in Figure 10 are indicative for comparison to a newly used minimum class of C20/25, is again something of academic dispute. In brief, someone can claim that the Table shown in Figure 9 is under laboratory conditions having no influence of parameters like quality of construction, compaction errors, curing parameters, formwork, concrete cover variations, etc. f) Examining ELOT EN 1504-2 - Paint Systems - Case A - Without Paint Degradation In paragraph d) we concluded that in 2027 carbonation depth would be equal to Xcrit while the remaining protection depth is 4mm. In order to select a particular paint system (surface protecting coatings according to EN1504-2) as protection medium, it is necessary to first calculate the equivalent concrete thickness, Sc,(4) where Sc Xo D Tm To Xm= Equivalent concrete thickness (mm) = carbonation depth prior to the application of the coating (mm) = carbonation diffusion coefficient (mm2 /year) = Protection period (years) = Time of exposure prior to the application of the coating (years) = Maximum permitted carbonation depth (Xcrit) after the application of the coating at the end of the protection period (mm).In the case of the example, Eq 4. results into,p. 11 / 15 The equivalent concrete thickness is transformed into Equivalent Air Layer Thickness, Sd,CO2 being the thickness of a static layer of air that has the same carbonation resistance as the building material of thickness t expressed in meters. Transformation of Sc into Sd,CO2 is made according to, (5) considering that the carbon dioxide equivalent resistance of concrete µ=400, the Equivalent Air Layer Thickness for Sc=18.27mm is, or 7.3 m The minimum requirement of Sd,CO2 according to EN 1504-2 is 50 m. Of course most coating manufacturers produce Sd,CO2 in the region 200-400 m. In US, UK and Australia, the parameter Sd,CO2 is designated by the letter R. A snapshot from the Model Specification for Protective Coatings for Concrete is shown in Figure 12.Figure.12. Recommended values of Equivalent Air Layer Thickness according to Model Specification for Protective Coatings for Concrete. g) Examining ELOT EN 1504-2 - Paint Systems against ELOT EN 206-1 Equation 4 can also be used in order to calculate the Equivalent Air Layer Thickness of concrete cover as this is defined according to ELOT EN 206-1. For example in the case of XC1 and considering Xcrit=Cmin-5mm,or The results for all carbonation categories are shown in Table 3.p. 12 / 15 Table.3. Results of ELOT EN 206-1 in terms of Equivalent Air Layer Thickness for Xcrit=Cmin-5mm CategoryXC1XC2XC3XC4Min. Cover (mm)25253535W/C0.650.60.550.5Cement (Kgr)280300300320K (mm year-1/2)3.543.544.954.95D (mm2 / year)6.256.2512.2512.25Sd,CO2 (m)2.202.208.438.43T (years)50505050It is easily seen that under the limitation of Xcrit and the values of D as suggested by ELOT EN 206-1, the durability requirements of the standard are below the limit of >50m set by EN 1504-2. ELOT EN 206-1 becomes partially valid only in the case of Xcrit=Cmin, Table 4. Table.4. Results of ELOT EN 206-1 in terms of Equivalent Air Layer Thickness for Xcrit=Cmin CategoryXC1XC2XC3XC4Min. Cover (mm)25253535W/C0.650.60.550.5Cement (Kgr)280300300320K (mm year-1/2)3.543.544.954.952D (mm / year)6.256.2512.2512.25Sd,CO2 (m)-0.23-0.234.804.80T (years)50505050Hence, for ELOT EN 206-1 can be claimed that only for the categories XC1, XC2 provides marginal protection. Categories XC3 and XC4 do fail below the requirements of ELOT EN 1504-2. Of course under the principles of ENV 1990-part 0 limitation that carbonation is under the minimum reliability index of β=3.8 such marginal protection is not accepted. i) Examining ELOT EN 1504-2 - Paint Systems - Case B - With Performance life Limitation In almost every case, the coating manufacturer provides time limitations regarding the performance life of its product. Performance life is defined by several parameters that in one or the other way degrade the paint below the 50m limit of the Equivalent Air Layer Thickness. Typical values of Performance Life usually found in Product Data Sheets (PDS) range from 10 to15 years. To better appreciate the 50m limit of the Equivalent Air Layer Thickness set by ELOT EN 1504-2, we considered that we apply a paint being just at the limit of 50m. In other words Sc=125mm. The depth of carbonation 10 years after the first application of a paint following ELOT EN 1504-2 with Sc=125mm is given by, p. 13 / 15 (6) where (7) Te= is the time between the first application of the paint and today (Performance life), i.e. 10 years. In the case of the example, Eq.(7) results into,The depth of carbonation 10 years after the application of the paint is,In other words just by using the absolute minimum requirement of ELOT EN 1504-2, the actual increment of carbonation depth after another 10 years of exposure is a mere 0.46mm. In order to calculate the Sc for the second application (another 10 years), we once again make use of Eq.(4),orwhich is below the 50m limit.Repetition of the above calculations per 10 year increment, can be performed to the end of the 50 years of design life. k) Examining ELOT EN 1504-2 - Paint Systems - Case B - With Performance life Limitation and Water Vapor Permeability Limitation The number of coating applications according to ELOT EN 1504-2 is only limited by the water vapor permeability limitation of Sd,H2O<5m. The value is related to the Dry Film Thickness. It is imperative that the manufacturer defines the maximum coating dry thickness to prevent reduction of breathability. Such limitation is quite critical when evaluating the performance characteristics of the paint. Since re-application of the paint increases the total dry film thickness, it is possible during application No. 3 being for example after 30 years to increase Sd,H2O over the limit of 5m. In this case removal of previous paint coatings is required. Hence, the two simultaneous limits being Sd,H2O<5m and Sd,C2O>50m represent perhaps the most vital ratio to perform a quality evaluation of the pool of paints under investigation. To better appreciate such ratio is worth bringing into the equation the cost of scaffolding required for a single application of paint.p. 14 / 15 Concussions Perhaps the only thing that shall remain in the mind of the reader is that a) ELOT EN 206-1 on its minimum limits is not applicable for the environmental load of Greek cities and the committee responsible for producing the National Annex shall pay particular attention, b) ELOT EN 206-1 on its minimum limits contradicts against the required reliability index of the Eurocode c) ELOT EN 206-1 contradicts against the minimum requirements being set by ELOT EN 1504-2 d) Even the minimum requirements of ELOT EN 1504-2 are designed in order to provide a reliable and performance based protection against carbonation. Acknowledgement The author would like to thank the participating students of the 2014 class attending the Continuous Professional Development Programme on Principles of Protection, Rehabilitation and Structural Upgrade according to ELOT EN 1504, TUV Academy, for being the driving force behind this article. References 1.Soroca I, Concrete in Hot Environments, E& FN Spon publishers, 1993.2.Ali, A., Dunster), A., Durability of reinforced concrete -effects of concrete composition and curing on carbonation under different exposue conditions. BRE-report, Garston UK 1998.3.Currie, R. J., Carbonation depth in structural-quality concrete, BRE report, Garston, UK 19864.Parrot, L.J., A reveiw of carbonation in reinforced concrete, A review carried out by C&CA under a BRE contract. July 1987,5.Tuutti, K., Corrosion of steel in concrete. CBI research 4:82 CBI, Stockholm, Sweden 1982.6.C. Rodopoulos, Evaluation of Commercial Protecting Coatings against concrete carbonation, Report 1547-2013, 2013.7.Stepkowska E. T, Pérez-Rodríguez L. J, Sayagués M. J, Martínez-Blanes J. M, Calcite, vaterite and aragonite forming on cement hydration from liquid and gaseous phase, Journal of Thermal Analysis and Calorimetry, 73(1), 247-269, 2003.8.Model Specification for Protective Coatings for Concrete, issued by the Government of Hong Kong, Civil Engineering Department,1994.9.CEB-FIP: Durable of Concrete Structures, Design Guide, T. Thelford, London, 1992.10. CEB-FIP: Eurocode, 2000. 11. Marques P, Chastre C and Nunes A, Carbonation service life modelling of RC structures for concrete with Portland and blended cements, Cement & Concrete Composites 37, 171–184, 2013. p. 15 / 15
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