UDK 691.328.1:620.19:620.17 ISSN 1580-2949 Professional article/Strokovni članek MTAEC9, 46(5)511(2012) EFFECT OF FLY-ASH AMOUNT AND CEMENT TYPE ON THE CORROSION PERFORMANCE OF THE STEEL EMBEDDED IN CONCRETE UČINEK KOLIČINE LETEČEGA PEPELA IN VRSTE CEMENTA NA KOROZIJO JEKLA V BETONU Ahmet Raif Boga1, liker Bekir Top^u2, Murat Öztürk3 1Afyon Kocatepe University, Faculty of Engineering, Civil Engineering Department, Afyonkarahisar, Turkey 2Eski5ehir Osmangazi University, Faculty of Engineering and Architecture, Civil Engineering Department, 26480 Eskijehir, Turkey 3Sel9uk University, Faculty of Engineering and Architecture, Civil Engineering Department, Konya, Turkey muratozturk@selcuk.edu.tr Prejem rokopisa - received: 2011-10-10; sprejem za objavo - accepted for publication: 2012-03-01 In this study the corrosion performance of the steel embedded in the concrete produced by using three different types of cement (CEM II/B-M (P-L) 32.5 R, CEM I 42.5 R and CEM I 52.5 R) was investigated. 300 kg/m3 and 375 kg/m3 dosages of the cement with (0, 10 and 20) % of fly-ash (FA) replacements of cement were used to produce the concretes. These concretes were cured for 28 d and 180 d. The mechanical properties of the concretes were determined and the corrosion performances of the reinforced-concrete specimens were determined using the impressed voltage test. After the impressed voltage test weight losses occurred because of the corrosion that was determined. The results of this study show that using composite cement and an FA replacement of the cement are useful in combating corrosion. Keywords: concrete, corrosion, mechanical properties, fly ash V tej študiji je bilo preiskovano korozijsko vedenje jekla, vgrajenega v beton, izdelan iz treh vrst cementa (CEM II/B-M (P-L) 32,5 R, CEM I 42,5 R in CEM I 52,5 R). Odmerki 300 kg/m3 in 375 kg/m3 cementa z (0, 10 in 20) % letečega pepela (FA) kot nadomestila za cement, so bili uporabljeni za izdelavo betona. Beton je bil preskušen po 28 d in po 180 d. Določene so bile mehanske lastnosti betona. Korozijske lastnosti armiranega betona so bile določene s pospešenim korozijskim napetostnim preizkusom. Pri pospešenem napetostnem korozijskem preizkusu se je zaradi korozije pojavilo zmanjšanje mase. Rezultati te študije kažejo, da je za zmanjšanje korozije ugodna uporaba kompozita cementa z letečim pepelom, ki nadomesti cement. Ključne besede: beton, korozija, mehanske lastnosti, leteči pepel 1 INTRODUCTION mineral admixtures.5 It is generally recognized that the incorporation of fly ash (FA) in blended cements helps to Corrosion of the steel embedded in concrete plays a protect concrete against the chloride-induced corrosion vital role in the determination of the life and durability of of steel reinforcement by reducing its permeability, the concrete structures.1 The durability of reinforced particularly for chloride-ion transportation, and in- concrete is largely controlled by the capability of the creasing the resistivity of the concrete.6,7 concrete cover to protect the steel reinforcement from In this study, the corrosion performance of the steel corrosion. Chemical protection is provided by the embedded in the concrete produced by using three concrete's high alkalinity and physical protection is different types of cement was investigated. Cements in afforded by the concrete cover acting as a barrier to the 300 kg/m3 and 375 kg/m3 dosages and also (0, 10 and access of aggressive species.2 The corrosion of the steel 20) % FA replacements of cement were used to produce in concrete is retarded by the passivating ferric-oxide the concretes. The mechanical properties of the concretes film (y-FeOOH) formed in the concrete medium (that is were determined. The impressed voltage test was highly alkaline with a pH of around 13).1 Corrosion of performed on the reinforced concrete specimens. In the reinforcing-steel bars is initiated to form an inactive conclusion, the best cement type, FA ratios and curing thin layer that can be broken when immersed in times for the corrosion performance of the steel carbonate, chloride or sulphate solutions.3 Corrosion of embedded in concrete were determined. steel produces rust products that have a volume three to eight times greater than that of the original metal. This 2 GENERAL OBSERVATIONS ABOUT THE generates stress and causes cracking and spalling of the BUILDING STOCK IN TURKEY concrete cover, which further accelerates corrosion.4 Various methods have been applied to protect reinforced According to the studies in various regions of Turkey, steel against corrosion; these methods include variation the average concrete compressive strength is approxi- of the concrete formulation, cathodic protection, surface mately 10 MPa.8,9 Several identified improper practices treatment of the rebar and addition of inhibitors and for fabricating an in-situ concrete caused this signifi- Figure 1: Corrosion of the reinforcement in a damaged column observed after the Van earthquake of 23 October 2011 (Murat Öztürk's archive) Slika 1: Korozija armature v po{kodovanem stebru med potresom 23. 10. 2011 (arhiv Murat Öztürk) cantly low concrete quality. Important factors include the ignorance of the aggregate gradation, the use of unwashed sea sand and/or river sand, and the use of the aggregate sizes that are too large. One of the most negative factors contributing to a low concrete quality is the use of the sea sand or river sand in a concrete Table 1: Chemical and physical properties of the cements and fly ash Tabela 1: Kemijske in fizikalne lastnosti cementov in lete~ega pepela Chemical Composition, % Cement Ty pe FA CEM I 52.5 R CEM I 42.5 R CEM II/B-M (P-L) 32.5 R SiO2 20.47 20.74 30.88 58.25 Al2O3 5.68 5.68 8.01 16.66 Fe2O3 3.08 4.12 3.57 12.91 CaO 62.66 63.70 47.78 1.95 MgO 1.1 1.22 1.30 5.08 Na2O 0.20 0.17 0.12 0.33 K2O 0.75 0.53 1.33 1.37 SO3 2.5 2.29 1.67 0.41 Cl 0.010 0.019 0.011 0.002 Loss of ignition 1.9 1.34 6.20 2.09 Insoluble residue 0.7 0.57 0.27 - Free lime 1.0 1.29 1.31 0.16 Physical Properties Specific gravity 3.17 3.14 2.85 2.34 Specific surface, cm2/g 3700 3450 3580 Compressive Strength, MPa 2 days 27 26 13 - 7 days 41 38 27 - 28 days 59 59 43 - mixture. According to the obtained results, the significantly high chloride content in the sea-sand concrete is an indication of a high tendency for the corrosion of the reinforcement in the reinforced concrete structures. The observations of the building rubbles showed that most rebars had corroded due to the use of the river/sea sand, thereby reducing the efficiency of the longitudinal rebar area and the anchorage (Figure 1). 3 EXPERIMENTAL STUDIES 3.1 Materials used 3.1.1 Cement The experimental studies used the CEM I 42.5 R Portland cement and the CEM II/ B-M (P-L) 32.5 R Portland composite cement, produced by the Eskisehir cement factory according to TS EN 197-1 : 2 000 standards. The CEM II/B-M (P-L) cement contains natural pozzolans and limestone in the ratio of 21-35 % by weight. In addition, the CEM I 52.5 R Portland cement was used. The results of the chemical and physical analyses of these cements, which were provided by the factories, are given in Table 1. 3.1.2 FA In the experiment, the Tungbilek thermal power plant's FA was used. The chemical composition and the physical properties are given in Table 1. Because the SiO2+Al2O3+Fe2O3 content exceeds 70 % and the CaO value is under 10 %, the FA of Tungbilek is of class F (low lime) according to the ASTM C 618 standard. 3.1.3 Aggregates The Osmaneli sand and the Sögüt Zemzemiye crushed-stone aggregates were used. The maximum particle size of the aggregates was 31.5 mm. According to the results of the experiment, the specific gravities of the sand and the crushed stones I and II are 2620, 2710 and 2710 and the unit weights are (1550, 1720 and 1770) kg/m3, respectively. (35, 30 and 35) %, of the sand and the crushed stones I and II, respectively, were used in the grain mixture. 3.1.4 Steel reinforcements and the NaCl solution 14-mm-diameter deformed steel reinforcement was used for the preparation of the reinforced concrete specimens to attempt the corrosion tests. According to TS 708 the minimum yield strength of this steel is 420 MPa and the minimum tensile strength is 500 MPa. In the experimental setup for the corrosion testing, an industrial type of the NaCl salt was used for obtaining the solution. 3.2 Mix proportions of the concrete Three different types of cement, CEM II/B-M(P-L) 32.5R (CII-3), CEM I 42.5R (CI-4) and CEM I 52.5R (CI-5), were used in the concrete mixtures. By using 300 Table 2: Mix proportions of the concrete for 1 m3 Tabela 2: Delež sestavin v 1 m3 betona Cement Type Dosage FA % Cement Sand Crushed Stone I Crushed Stone II Water FA CII-3 375 0 375 611 541 632 188 - 10 337.5 608 539 629 188 37.5 20 300 605 537 626 188 75 300 0 300 669 593 692 150 - 10 270 667 591 690 150 30 20 240 665 590 688 150 60 CI-4 375 0 375 622 553 645 188 - 10 337.5 618 550 642 188 37.5 20 300 614 547 638 188 75 300 0 300 678 603 704 150 - 10 270 675 601 701 150 30 20 240 672 598 698 150 60 CI-5 375 0 375 623 554 646 188 - 10 337.5 619 551 643 188 37.5 20 300 615 548 639 188 75 300 0 300 679 604 705 150 - 10 270 676 602 702 150 30 20 240 673 599 699 150 60 *Superplasticiser (SP) was used as 0.4 % by mass of binder (cement and FA) kg/m3 and 375 kg/m3 dosages of each cement, the concrete specimens were also produced as reference specimens containing 10 % and 20 % of FA. These specimens were cured for two different periods: 28 d and 180 d. Thus, 36 series of the concrete mixture were produced. The mix proportions of the concrete are given in Table 2. 3.3 Specimen preparation and testing 3.3.1 Compressive test The compressive strength test was carried out on the specimens with the cube dimensions of 150 mm x 150 mm x 150 mm. The specimens were demoulded after 24 h and immersed in water at (20 ± 2) °C. The compressive strength tests were made after 28 d and 180 d of curing. 3.3.2 Splitting tensile test The splitting tensile test was carried out for the cylinder specimens with the dimensions of ^150 mm x 300 mm. The specimens were demoulded after 24 hours and immersed in water at (20 ± 2) °C. The splitting tensile tests were carried out after 28 d of curing. 3.3.3 Tests for the ultrasonic pulse velocity and the modulus of dynamic elasticity The tests of measuring the ultrasonic pulse velocity and the modulus of dynamic elasticity were carried out for the specimens that were prepared for the splitting tensile tests. 3.3.4 Impressed voltage test The setup for this test included a DC power source, a test specimen and a plastic dish containing a 4 %-NaCl solution, two steel plates and a data logger. The impressed-voltage test setup is shown in Figure 2. The reinforced concrete specimens for the accelerated-corrosion tests were the cylinder specimens with the dimensions of ^150 mm x 300 mm, in which a 14-mm-diameter steel reinforcement was centrally embedded. A 250-mm part of the steel reinforcement was embedded into each concrete cylinder. The specimens were demoulded after 24 h and immersed in water at (20 ± 2) °C. The impressed voltage tests were carried out after 28 d and 180 d of curing. The steel reinforcement (the working electrode) of the reinforced concrete specimen was connected to the positive terminal and the steel plates (the counter electrodes) were connected to the negative terminal of the DC power source applying 30 V of fixed stress to the system. A similar impressed-voltage test setup has been reported by other researchers.7,10 Every five minutes the corrosion current of every reservoir was saved using a data logger and the corrosion current-time figures were drawn using the impressed voltage test system. Figure 2: Setup for the impressed voltage test Slika 2: Skica pospešenega napetostnega preizkusa 3.3.5 Weight loss method The method proposed in the ASTM G1-03 standard was used for the determination of the weight loss of a steel bar.11 After the impressed voltage tests, steel bars were demounted from the reinforced concrete specimens and the weight losses were found by cleaning these steel bars with a Clarke solution, which contained 1000 mL HCL, 24 g SbaOs and 71.3 g SnCla • 2H2O. 4 RESULTS AND DISCUSSION 4.1 Compressive strength Changes in the compressive strengths due to the amount of FA are shown in Figures 3 and 4. According to the figures for the 28 d and 180 d compressive strengths, the strength results of the concrete produced with the CI-5 cement are higher than those of the others. With the increase in the FA amount in the specimens, the compressive strengths of the specimens cured for 28 d decreased because the mineral admixtures like FA cannot Figure 3: Relationship between the compressive strength and the FA amount in 28-day specimens Slika 3: Odvisnost med tlačno trdnostjo in deležem letečega pepela (FA) v vzorcu po 28 dneh fully complete the pozzolanic reactions.12 If Figure 4 is examined, it can be seen that the strength values increase as the dosage increases. Looking at Figure 4, by using 10 % of FA instead of the CI-5 and CI-4 cements in the 375 dosed specimens, the compressive strength of the specimens increases respectively by 4.5 % and 15.66 % according to the control specimens and it decreases at the ratio of 6.42 % in the specimens produced with CII-3. If 20 % of FA is used in the specimens, the compressive strength decreases at the ratio of 6.06 % and 12.17 % for the specimen series produced with the CI-5 and CII-3 cements, but increases at the ratio of 5.43 % for the specimen series produced with the CI-4 cement. If Figures 3 and 4 are compared to each other, it is seen that the compressive strength increases as the curing time increases. 4.2 Splitting tensile strength According to a general assessment from Figure 5, the splitting tensile strengths of the specimens produced with the CI-4 and CI-5 cements are very similar to each other. The minimum strengths are found with the specimens produced with the CII-3 cement. As the amount of FA increases in the specimens produced with CII-3, the splitting tensile strength decreases but it increases in the specimens produced with the other cements. According to Figure 5, the splitting tensile strength is seen to increase as the dosage of the cement is increased. 4.3 Tests for the ultrasonic pulse velocity and modulus of dynamic elasticity The ultrasonic pulse velocity and modulus of dynamic elasticity test results are shown in Table 3. Considering the ultrasonic pulse velocity of the 375 kg/m3 and 300 kg/m3 dosage specimens, it can be seen that the ultrasonic pulse velocity of all the specimens produced with CII-3 are fine. The ultrasonic pulse velocity results of the specimens produced with CI-4 and CI-5 are seen to be better than those of CII-3. Depending on the Figure 4: Relationship between the compressive strength and the FA amount in 180-day specimens Slika 4: Odvisnost med tlačno trdnostjo in deležem letečega pepela (FA) v vzorcu po 180 dneh Figure 5: Relationship between the splitting tensile strength and the FA amount Slika 5: Odvisnost med porušno natezno trdnostjo in deležem letečega pepela (FA) ultrasonic pulse velocity values, the best results are seen to be in the specimens produced with the CI-5 cement and without any FA. According to the assessment in Table 3, it can be seen that the modulus of dynamic elasticity decreases as the amount of FA increases. The minimum £din values are for the specimens produced using the CII-3 cement with the dosages of 300 kg/m3 and 375 kg/m3. As seen in Table 3, for the other series, the Edin values are similar. Table 3: Test results for ultrasonic pulse velocity and modulus of dynamic elasticity Tabela 3: Rezultati preizkusov za hitrost ultrazvo~nega impulza in modul dinami~ne elasti~nosti Cement Type FA/% Ultrasonic pulse velocity /(km/s) Edin/GPa 375 Dosage 300 Dosage 375 Dosage 300 Dosage CII-3 0 4.35 4.52 45.7 49.4 10 4.46 4.46 47.9 48.0 20 4.29 4.40 43.6 46.3 CI-4 0 4.62 4.62 52.3 53.1 10 4.63 4.50 52.3 49.9 20 4.62 4.60 51.3 51.5 CI-5 0 4.66 4.63 53.5 52.8 10 4.56 4.56 50.5 50.5 20 4.65 4.55 52.1 50.5 longer DOT. With respect to DOTs, it is observed that the best results are obtained for the specimens produced with CII-3 and 20 % FA cured for 28 d, and the specimens produced with CI-5 and 20 % FA cured for 180 d. A comparison between the 300 kg/m3 and 375 kg/m3 dosage specimen results shows that the DOT extends as the cement dosage increases. Generally, if the cement dosage increases, the compressive strength of the concrete increases as well. There is a general relationship between the compressive strength of the concrete and the permeability.12 According to Table 4, the DOT is extended even if the compressive strengths of the concretes produced with the CII-3 cement are low. Actually, the concretes produced with the CII-3 cement that have low compressive strengths are damaged in a shorter time than the concretes produced with the CI-4 and CI-5 cements, because the concretes produced with CII-3 are more porous than the other concretes, as can be understood from the compressive strengths. Table 4: Compressive strengths and damage occurrence times (DOTs) of the 300 - (375) dosage specimens Tabela 4: Tla~ne trdnosti in ~asi do pojava napak (DOT) vzorcev z odmerkom 300 - (375) 4.4 I-^^^essed voltage test After the impressed voltage tests, steel in the reinforced concrete corroded and the specimens were damaged. The damaged specimens are shown in Figure 6. Since the volume of the corrosion products (rust) are 2.5-6 times greater than the volume of the steel used in the concrete, these corrosion products lead to higher internal tensile stresses in the hardened concrete. Being exposed to these stresses, the hardened concrete cracks and splits off.12 The compressive strength results and damage occurrence times (DOTs) of the accelerated-corrosion specimens are shown in Table 4. As seen in Table 4, the DOT ranges between 251 h and 394 h. According to Table 4, as is the case with the 300 kg/m3 dosage specimens, an increase in the FA amount leads to a Cement Type FA/ % Compressive Strength, MPa 300 (375) Damage Occurrence Time Hour - 300 (375) 28 days 180 days 28 days 180 days CII-3 0 36.7 (37.2) 38.9 (45.2) 291 (325) 368 (308) 10 33.8 (34.7) 41.6 (42.3) 311 (323) 381 (324) 20 31.2 (32.0) 38.3 (39.7) 325 (366) 394 (373) CI-4 0 46.5 (47.5) 48.2 (47.9) 279 (287) 303 (306) 10 43.7 (44.4) 49.2 (55.4) 310 (315) 324 (326) 20 42.6 (42.2) 48.1 (50.5) 319 (326) 327 (329) CI-5 0 53.6 (55.2) 64.7 (64.4) 251 (300) 304 (315) 10 50.7 (54.0) 63.4 (67.3) 268 (316) 343 (368) 20 49.8 (52.8) 53.6 (60.5) 287 (292) 376 (389) Figure 6: Damaged specimens after the impressed voltage test Slika 6: Poškodovani vzorci po pospešenem napetostnem preizkusu However, it is known from the previous studies that fly ash, ground granulated blast furnace slag, silica fume and pozzolans bind chloride ions; thus, the chloride permeability decreases.13-16 The concretes produced with the CII-3 cement are highly porous and permeable due to a high percentage of FA and natural pozzolans that do not pozzolanicly react with water and lime. Therefore, these FA and pozzolan particles react with chloride ions and bind them. In this way, the chloride permeabilities of these concretes are lower than those of the control ones. Figure 7 shows that the corrosion-time curves of the 28 d cured concretes were produced with a 300 kg/m3 dosage of the CII-3, CI-4 and CI-5 cements, and 10 % and 20 % FA replacements of the cement. According to the assessment in Figure 7, for the concretes produced with the CII-3, CI-4 and CI-5 cements, the curves 2, 3, 5, 6, 8 and 9 show that an increase in the FA amount in the concrete, used as a replacement for the cement, reduces the corrosion currents. According to these results the use Figure 7: Variation of the corrosion current according to time (a 300-kg/m3 dosage and 28-day specimens) Slika 7: Spreminjanje korozijskega toka glede na ~as (odmerek 300 kg/m3 in vzorec po 28 dneh) of FA is beneficial. It has been observed that FA binds chloride ions.1516 As a result, the corrosion currents decrease as seen in Figure 7. Figure 8 shows the current-time relations for the corrosion of the concretes that are cured for 28 d and produced by using FA as the 10 % and 20 % replacements of the cement weight with a 375 kg/m3 dosage of the CII-3, CI-4 and CI-5 cements. The series produced by using the CII-3 cement gives better test results. However, the best test results are achieved with the concretes produced with a 20 % FA replacement of the CI-4 cement. It is seen that the corrosion current of the specimens is reduced by using the FA replacement for a 375 kg/m3 dosage of the CI-5 cement with the given ratios. By using FA, the corrosion currents are reduced, but better results are achieved than with the other concrete specimens. According to Figure 8, the corrosion currents increase with time and it is seen that some series have sudden increases. The corrosion current-time relations are shown in Figure 9 for the concretes that are cured for 180 d and produced by using FA as the 10 % and 20 % replacements of the cement weight for a 300 kg/m3 dosage of the CII-3, CI-4 and CI-5 cements. Looking at Figure 9, with the concretes produced with the CI-4 and CI-5 cements, and without Figure 9: Variation of the corrosion current according to time (a 300-kg/m3 dosage and 180-day specimens) Slika 9: Spreminjanje korozijskega toka glede na ~as (odmerek 300 kg/m3 in vzorec po 180 dneh) any FA, they are seen to have a corrosion current above 0.1 A. All the other specimens have a corrosion current under 0.1 A at the beginning. The concretes cured for 180 days and produced with a 300-kg/m3 dosage of the CI-4 cement depending on their FA ratio (0 %, 10 % and 20 %) have (0.12, 0.08 and 0.04) A corrosion currents in turn. However, the same series that were cured for 28 d have corrosion currents of (0.18, 0.12 and 0.09) A in turn. According to these results an increase in the curing time reduces the corrosion currents. The corrosion current-time relations are shown in Figure 10 for the concretes cured for 180 d and produced by using FA as the 10 % and 20 % replacements of the cement weight for the 375 kg/m3 dosage of the CII-3, CI-4 and CI-5 cements. With respect to Figure 10, the best results are achieved for the concretes produced with a 20 % FA replacement of the weight of the CII-3 cement. 4.5 Weighs loss method The steel reinforcements cleaned with a Clarke solution are shown in Figure 11. The weight losses of the steel reinforcements in various concrete mixtures that were exposed to corrosion are shown in Figures 12 and 13. According to a general assessment of these figures, Figure 8: Variation of the corrosion current according to time (a 375-kg/m3 dosage and 28-day specimens) Slika 8: Spreminjanje korozijskega toka glede na ~as (odmerek 375 kg/m3 in vzorec po 28 dneh) Figure 10: Variation of the corrosion current according to time (a 375-kg/m3 dosage and 180-day specimens) Slika 10: Spreminjanje korozijskega toka glede na ~as (odmerek 375 kg/m3 in vzorec po 180 dneh) Figure 11: Steel reinforcements cleaned with a Clarke solution Slika 11: Z raztopino Clarke o~i{~ena jeklena armatura Figure 12: Variation of weight loss by mixture type for 28-day specimens Slika 12: Razlike v izgubi mase glede na vrsto me{anice v vzorcih po 28 dneh the weight loss is seen to decrease as the amount of FA, being the concrete replacement of the cement, is increased. Increasing the dosage reduces the weight loss in some series, but increases it in others. As it is seen in Figure 12, the maximum weight loss is observed with the concretes that did not have any FA and were produced with CI-4. According to Figure 13, the weight Figure 13: Variation of weight loss by mixture type for 180-day specimens Slika 13: Razlike v izgubi mase glede na vrsto me{anice v vzorcih po 180 dneh losses decrease as the amount of FA increases in the concretes. According to the increase in the curing time, by comparing Figures 12 and 13, in some series the weight loss of the reinforcements is seen to increase and in some series it decreases. 5 CONCLUSIONS After the tests it was observed that the mechanical properties of the concretes and the corrosion performances of the steel embedded in the concrete changed with different cement types, dosages, FA amounts used as the replacements for the cement and the curing time. • The compressive strength of the specimens increased after an increase in the curing time and the cement dosage. The maximum compressive strengths were observed with the concretes that were produced with the CI-5 cement. • The splitting tensile test results of the specimens that were produced with the CI-4 and CI-5 cements were very similar. The minimum strengths were observed with the specimens that were produced with CII-3. • According to a general assessment of the results, an increase in the dosage, the curing time and the FA amount used to replace the cement caused an increase in the DOT of the reinforced concrete specimens. • With respect to the assessment of the DOTs, the most positive results were observed with the specimens that were produced with the CII-3 cement. Compatible results were observed with the specimens that were produced with the other cements and the 20 % FA amount. • Consequently, the corrosion of the steel embedded in the concrete depends significantly on the cement type, the cement dosage and the curing time. 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