Chloride penetration and electrical resistivity of concretes containing. Nanosilica hydrosols with different specific surface areas

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1 Accepted Manuscript Chloride penetration and electrical resistivity of concretes containing Nanosilica hydrosols with different specific surface areas Hesam Madani, Alireza Bagheri, Tayebeh Parhizkar, Amirmaziar Raisghasemi PII: S (14) DOI: Reference: CECO 2353 To appear in: Cement & Concrete Composites Received Date: 12 October 2012 Revised Date: 6 June 2014 Accepted Date: 17 June 2014 Please cite this article as: Madani, H., Bagheri, A., Parhizkar, T., Raisghasemi, A., Chloride penetration and electrical resistivity of concretes containing Nanosilica hydrosols with different specific surface areas, Cement & Concrete Composites (2014), doi: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Chloride penetration and electrical resistivity of concretes containing Nanosilica hydrosols with different specific surface areas Hesam Madani a,*, Alireza Bagheri b, Tayebeh Parhizkar c, Amirmaziar Raisghasemi d a Assistant professor, Civil Engineering Faculty, Graduate University of Advanced Technology, Haftbagh Exp. Way, Kerman, Iran. b Assistant professor, Civil Engineering Faculty, K.N.Toosi University of Technology, No. 1346, Vali Asr Street, Mirdamad Intersection, Tehran, Iran. c Assistant professor, Department of Concrete technology, Building and Housing Research Center (BHRC), Pas Farhangian Street, Sheikh Fazlollah Exp. way, Tehran, Iran. d Lecturer, Department of Concrete technology, Building and Housing Research Center (BHRC), Pas Farhangian St, Sheikh Fazlollah Exp. way, Tehran, Iran. Abstract In this study, the influence of nanosilica hydrosols with specific surface areas of 100m 2 /g, 200m 2 /g and 300m 2 /g and silica fume with specific surface area of 21m 2 /g on chloride permeability and electrical resisitivity of concrete is investigated. The results indicate that the nanosilicas, especially at higher replacement levels, enhanced the chloride and electrical resistance of concrete at early ages, while silica fume did not have a significant influence on these characteristics at early ages. The results also show that the nanosilicas with lower surface areas had better performance in enhancing the chloride and electrical resistance of concretes compared to the finer ones, especially at 28days and 90days. * Corresponding author: Tel: , Fax: , 1

3 Silica fume achieved a similar performance to the best performing nanosilica at 28days and surpassed it at 90days. Keywords Chloride resistance, Electrical resisitivity, Nanosilica hydrosol, Surface area, Silica fume 1. Introduction Chlorideinduced rebar corrosion is a major concern for reinforced concrete structures, in chlorine environments. In the highly alkaline pore solution of concrete a thin impermeable oxide layer is formed and strongly adhered to the steel surface. This film is stable and makes the steel passive to corrosion. It is well documented that in the presence of sufficient amounts of chloride ions, this protective layer is destroyed and corrosion is initiated [19]. Hence, the risk of chlorideinduced corrosion increases with ease of migration of Cl through the cement paste. The corrosion of steel in concrete essentially is an electrochemical process [15]. The steel surface can be divided into anodically and cathodically acting areas, which are connected by the electrolyte in the form of pore water of concrete. At anodic sites positively charged ferrous ions Fe 2+ pass into solution and the excess free electrons flow through the steel to cathodic sites where they combine with water and oxygen to form hydroxyl ions. The process is completed by electro migration of hydroxyl ions through the electrolyte and combining them with ferrous ions, leading to formation of a corrosion product [14]. Thus, the progress of corrosion is influenced by the resistivity of the concrete between the anode and the cathode [1]. In other words, the electrical resistivity 2

4 would be a decisive factor in controlling the rate of steel corrosion [1, 5, 7, 10]. Considering the above points, sufficient resistance against chloride ingress and adequate electrical resistivity are essential requirements for durable concretes in aggressive environments. The electrical resistivity and chloride penetration of concrete are fundamentally related to the pore structure of concrete. A denser microstructure could provide higher electrical and chloride resistances for concrete. Pozzolanic admixtures like silica fume could provide a denser and more discontinuous and tortuous microstructure via the pozzolanic reactions. Thus, these materials could significantly enhance the electrical resistivity of concrete and its resistance against chloride penetration. In this regard, the superior performance of silica fume on enhancing these characteristics has been well established [1, 1118]. It is notable that Gjorv [18] has suggested that even if passivity is breached, the electrical resisitivity of concretes containing silica fume may be so high that steel corrosion will not present any practical problem. Structural similarities of amorphous nanosilica materials with silica fume have lead to a considerable research effort in investigating the influence of nanosilicas on the characteristics of cement composites. In this regard, some researches have investigated the influence of nanosilicas on the chloride resistance of concrete. Zhang and Islam [19] and Zhang et al [20] observed that the total passed charges through the fly ash and slag concretes with 2% nanosilica powder (surface area of 200m 2 /g and particle size of 12nm) was lower than that of the corresponding reference fly ash and slag concrete at 28days. However, the difference between the resistances against chloride penetration was not significant between the nanosilica concrete and silica fume concrete 3

5 at similar replacement level. Ozyildirim and Zegetosky [21] reported that incorporation of 3% nanosilica hydrosol (partcle size of 22nm) reduced the passed electrical charge through concrete by about 45% as compared with the companion control concrete at 28days. Chandra and Maiti [22] also observed 72% and 75% lower chloride penetration coefficients of mortars compared to that of control mixture at 3% and 8% replacement levels of cement with a hydrosol (specific surface area of 80m 2 /g) at 28days. The most interesting results are those of Ottersetedt and Greenwood [23], who reported that replacing only 0.2% of cement weight with a nanosilica led to about 50% lower passed charge compared to the reference concrete. In the aforementioned studies, which have investigated the influence of nanosilicas on the properties of cement composites, the nanosilicas used were not of same type and did not have similar surface areas. It is notable that nanosilicas can be produced by various methods, yielding different characteristics. The main types of nanosilicas, which have been used in studies of cement mixtures, include nanosilica hydrosols, pyrogenic nanosilicas, precipitated nanosilicas and nanosilica gels. The nansoilicas can also be produced with different specific surface areas ranging from 50m 2 /g to 1000m 2 /g. Therefore, concentrated studies are required on different types of nanosilicas to characterize their effects on the properties of concrete. The influence of variation in surface areas of nanosilicas on the properties of cement composites should also be considered. This study was thus carried out to evaluate the influence of addition of monodispersed nanosilica hydrosols with different specific surface areas (100, 200 and 300 m 2 /g) and different incorporation levels on chloride and electrical resistivity of concretes. It should 4

6 be mentioned that the nanosilica hydrosols are mainly produced through nucleation and growth of silicic acid in the aqueous medium [2427]. In a previous study, the pozzolanic reactivity of the nanosilica hydrosols in lime pastes and cement pastes were determined through the thermogravimetric studies [26]. It was shown that the nanosilica hydrosols had accelerating influence on the early hydration of cement; however, by progress of hydration and after early ages, less hydration degree of cement was observed compared to the plain paste. It was also observed that hydrosols with finer particles had faster pozzolanic reaction than the coarser ones. The tests performed in this study on the concrete specimens containing the nanosilicas were electrical resistivity test [28], rapid chloride penetration test (RCPT as per ASTM C1202 [29]) and rapid chloride migration test (RCMT as per AASHTO TP 64 [30]). The results were also compared with the results of concretes containing silica fume 2. Experimental program 2.1. Materials and mixture characteristics Three types of monodispersed nanosilica hydrosols with different specific surface areas including Levasil 100/45, Levasil 200/30 and Levasil 300/30 supplied by H.C.Stark Gmbh were used in this study. These products, according to the manufacture s data, have specific surface areas of 100, 200 and 300 m 2 /g, mean particle sizes of 30, 15 and 9 nm and mass concentrations of 45%, 30% and 30%, respectively. The silica fume, used for the study, was supplied by Azna Ferroalloy Company and complied with ASTMC

7 To utilize silica fume in the mixtures, this material was first mixed with water in mass concentration of 30% and then was dispersed by a high shear mixer for 4 minutes. The cement, used for the study, was a type II Portland cement complying with the requirements of ASTM C150. A polycarboxylic etherbased superplasticizer (Gelenium 110) was employed to achieve the desired workability. The physical and chemical properties of the nanosilicas, silica fume and cement are shown in table 1. A river sand with maximum nominal aggregate size of 4.75 mm was used as fine aggregate. The coarse aggregate used was a crushed gravel with maximum nominal aggregate size of 19mm. The specific gravity for both fine and coarse aggregates was 2550 kg/m 3. Table 1. Chemical composition and physical properties of the cement, silica fume and nanosilicas The concrete mixtures were prepared at a water/binder ratio of 0.45 and cementitious materials content of 395 kg/m 3. The replacement levels of cement by the solid content of nanosilica hydrosols were (i) 3, 5 and 7.5% for L100 (ii) 1, 3 and 5% for L200 and (iii) 1 and 3% for L300. Higher replacement levels were not considered because of the significant increase in superplasticizer demand to achieve similar workability levels for the mixtures. A control concrete without incorporating any pozzolanic material and concrete mixtures with 3, 5 and 7.5 % of cement replaced by silica fume were also made for comparison purposes. Details of mixtures are presented in table 2. It should be noted that the water content of the nanosilica hydrosols, the silica fume slurry and the superplasticizer was considered as part of mix water. 6

8 The mixing procedure used in this study consists of: i) mixing the dry ingredients with a conventional concrete mixer for 1.5 minutes, ii) adding nanosilica hydrosol (or silica fume suspension) and water to the dry ingredients, iii) mixing the mixture for 2 min (adding superplasticizer to the mixture during mixing) and iv) mixing concrete for another 3 minutes. The workability of all concrete mixtures was kept constant in the slump range mm. The difference in water demand of various mixes was accounted for by the use of required amounts of the superplasticer. After casting, the specimens were covered to minimize water evaporation. They were demolded after 24h, and cured in water (CHsaturated) at 23±2 C until testing. Table 2. Mixture proportions and compressive strength results of concrete mixes 2.2. Test methods Compressive strength Compression tests were performed at 1, 3, 7, 28 and 90days in accordance to BS EN At each age, four 100mm cubic concrete specimens were tested for determining compressive strength. The coefficients of variation of compressive strength test results at ages of 1, 3, 7, 28 and 90 days had mean values of 2.9%, 2.4%, 2.6%, 2.6% and 3.2% with maximum values of 4.9%, 5.3%, 4%, 3.5% and 5.6%, respectively Electrical resistivity of the concretes The electrical resistivity was measured using AC current according to the procedure proposed by Swedish national testing and research institute [28] at ages of 7, 28 and 90 days. At 7, 28 and 90 days, the coefficients of variation of electrical resistivity test results 7

9 had mean values of 2.8%, 2.4% and 4.4% with maximum values of 3.3%, 4.6% and 5.2%, respectively Rapid chloride permeability tests (RCPT) Rapid chloride permeability test (RCPT) was carried out in accordance with the ASTM C1202 [29]. In the current study, the RCPT has been done on 3 specimens. The RCPT results obtained in this study at ages of 7, 28 and 90 days had mean coefficients of variation of 5.8%, 4.3% and 3.3%, respectively. Among the mixtures, the maximum Coefficients of variation at 7, 28 and 90 days were 10.3%, 8.4% and 5.6%, respectively. In the RCPT test the total electrical charge passing through a 50mm thick concrete specimen during a 6 hour period, under an electrical potential of 60 Volts is determined. The specimen in the RCPT test cell is in contact with a 0.3 molar solution of NaOH on one face and with 3% solution of NaCl on the other face. The electrode in the NaCl solution is connected to the negative pole and the electrode in the NaOH solution is connected to the positive pole of the direct current source providing 60 Volt electrical potential Despite its widespread use, the RCPT test suffers from certain drawbacks. As the RCPT test is in fact a prolonged resistivity test, a main concern has been the rise in temperature of concrete specimens under the high electrical potential applied. This rise in temperature, which is higher in mixes with higher conductivities, has been reported to results in an increase in the passed charge [15, 3132]. The other criticism towards the RCPT test is the possible role of ions other than Cl in conducting the electrical charge 8

10 [1617,31]. In order to avoid such possible influences, it was decided to include the rapid chloride migration test (RCMT) which is not affected by the aforementioned parameters Rapid chloride migration test (RCMT) The RCMT test was performed according to the method described in AASHTO TP64 [30] at the ages of 28days and 90days. For each mixture, 3 specimens have been tested according to the RCMT method to measure their chloride resistivity. At ages of 28 and 90 days, the coefficients of variation of RCMT results had mean values of 4.1% and 5.4% with maximum values of 9.2% and 9.1%, respectively. In this method, the diffusion of chloride ions into 50mm thick concrete disks is accelerated by applying an electrical potential for a duration of 18hours. In this procedure, the magnitude of applied potential is selected based on initial current passed through the specimen. At the end of the test, the average depth of chloride penetration is directly measured by splitting the specimen and spraying 0.1M AgNO 3 on the broken surfaces. The chloride diffusion depth is then divided by the applied voltage and test duration in hours and the RCMT test result is expressed in mm/volt.hour units. 3. Results 3.1 Compressive strength The results of compressive strength of various mixes are presented in table 2. To provide a better comparison, the normalized strength results relative to the control mix are given in fig.1. 9

11 As indicated in table 2 and fig.1, nanosilicas due to their accelerating influence on cement hydration and also their high rate of pozzolanic reactivity have significantly increased early compressive strength of the concretes, especially at the first day. The amount of increase is relatively low at the dosage of 1% and is considerably higher at higher dosages. Silica fume did not have a significant influence on enhancing early age strength of concrete mixes up to 7 days, which shows its lower rate of pozzolanic activity compared to the nanosilicas. With the progress of hydration, the increase in strength due to incorporation of nanosilicas gradually diminishes. For instance, for concrete containing L200 at 3% replacement level the improvement in compressive strength of the concrete compared to the control mix drops from 42% at the first day to about 6% at 28 and 90days. For silica fume, due to progress of pozzolanic reactivity after 7days the compressive strengths were enhanced and surpassed those of the control concrete. The most significant influence of silica fume on improving the compressive strengths was obtained at 28days, where for dosages of 3%, 5% and 7.5% compressive strengths were enhanced by 14%, 17% and 19% over control. These improvements are equal or better than those of nanosilicas. It should be noted that similar to the nanosilica concretes, the improvement compared to the control mix due to silica fume addition at 90days was lower than that of 28days. Fig1. Normalized mean Compressive strengths of the concrete mixtures relative to the control mixture 3.2. The electrical resistivity test results 10

12 The electrical resistivity results at 7, 28 and 90days are presented in figures 24. Fig.2 shows that at 7days low levels of cement replacement by nanosilicas (1 and 3 percent) have only had slight effect in increasing the electrical resistivity of concrete, however at 5 and 7.5 percent replacement levels considerable improvement in electrical resistivity was observed. For instance, at the dosage of 5%, L200 and L100 resulted in 50% and 28% increase in concrete resistivity compared to the control mix, respectively. As expected, silica fume did not have a considerable effect on concrete resistivity at 7days. The results of electrical resistivity at 28days (fig.3) show that the nanosilica with lower surface area has resulted in higher electrical resistivity of concrete at 28days compared to the ones with higher specific surface areas. For instance, it was observed that compared to the control mixture, 3% replacement of cement with L300 and L200 led to less than 25% increase in electrical resistance, while the corresponding value for L1003% was 60%. At 28days, higher replacement level of nanosilicas led to higher electrical resitivity of concretes. At a dosage of 5%, the mixtures with L200 and L100 had electrical resistivities of 88 Ω.m and 112 Ω.m; respectively, corresponding to increase of 100% and 150% over control. 7.5% replacement of cement with L100 led to electrical resistivity of about 153Ω.m, which corresponds to about 250% enhancement in electrical resistivity. Although silica fume did not have significant influence on electrical resistivity of concretes at 7days, it performed much better at 28days. Silica fume outperformed L300 and L200 at similar replacement levels, however its performance was somewhat lower than L100 at 5 and 7.5 % replacement levels. 11

13 At the age of 90days, significant increase in electrical resistivity of concretes containing nanosilicas or silica fume, particularly at higher dosages was observed. The performance of nanosilica with the lowest surface area (L100) was significantly better than the finer nanosilicas. It is interesting to note that at the age of 90days, silica fume has outperformed L100 at all levels of replacement. At 7.5% replacement level, silica fume resulted in an increase of about 320% over control while the corresponding value for L100 was 250%. Fig 2. Electrical resistivity values of concretes containing the nanosilicas and SF at 7days Fig 3. Electrical resistivity values of concretes containing the nanosilicas and SF at 28days Fig 4. Electrical resistivity values of concretes containing the nanosilicas and SF at 90days 3.3. Rapid chloride permeability test (RCPT) results As shown in fig.5, low levels of utilization of nanosilicas (up to 3%) has only had a limited effect in reducing the chloride permeability of concrete at the age of 7 days. However, at dosages of 5% and higher these materials significantly increased the chloride resistance of concrete. A 5 percent replacement of cement with L100 or L200 led to 35% and 41% reduction of passed charge compared to the control mixture, respectively, while the corresponding value for the mixture containing 7.5% of L100 was 60%. Silica fume did not have a significant effect in reducing the chloride resistance of concrete at 7days. Fig5. Passed charge of concretes in the RCPT test at 7days 12

14 As shown in fig.6, at age of 28days, at a dosage of 3% the L300 and L200 nanosilicas had a relatively small effect in reducing the passed charge through concrete. However, the L100 nanosilica, which had the lowest specific surface area, performed better and at this dosage significantly reduced the passed charge. Chloride resistance of concrete mixes increased with increased dosage of nanosilicas. Silica fume showed good performance at 28 days and resulted in similar or slightly lower reductions in passed charge compared to the mix with L100 at equal dosage. Fig 6. Passed charge through concrete mixes in the RCPT test at 28days As shown in fig.7, at the age of 90days the superior performance of the nanosilicas with lower specific surface area compared to the finer ones is quite evident. At a dosage of 3%, the reductions in the passed charge compared to the control mix for the mixes with L300, L200 and L100 were 22%, 34% and 50%, respectively. The improvement in chloride resistance of concrete mixes increased with increased dosage of nanosilicas. Silica fume had the best performance among all the mixes at the age of 90days and could even outperform L100 nanosilica at all replacement levels. The improvements in the RCPT test results due to incorporation of nanosilicas or silica fume is attributed to a more tortuous, discontinuous and constrictive microstructure through pozzolanic reaction of these materials. Fig 7. Passed charge through concrete mixes in the RCPT test at 90days 13

15 3.4. Rapid chloride migration test (RCMT) results The rapid chloride migration test results at the age of 28days are presented in fig.8. Like the RCPT results, the RCMT tests show that nanosilicas with lower surface areas have better performance in enhancing chloride resistivity of concretes at 28days. Silica fume concretes outperformed mixes containing nanosilicas with surface areas of 200m 2 /g and 300m 2 /g (L200 and L300) and had similar performance to the nanosilica with specific surface area of 100m 2 /g (L100) at all replacement levels. As seen in fig.9, at the age of 90 days, the nanosilicas with lower specific surface area outperformed the finer nanosilicas. With the progress of pozzolanic reaction of silica fume, this material had the best performance at all levels of replacement compared to the nanosilicas. Fig 8. RCMT coefficient of concretes at 28days Fig 9. RCMT coefficient of concretes at 90days 4. Discussion The compressive strength of concretes containing nanosilicas was improved significantly at early ages, which is attributed to the high rate of pozzolanic reactivity of these materials and higher rate of cement hydration at early ages compared to the plain mixture. It should be noted that accelerating influence of Nanosilicas on early hydration of cement and their high rate of pozzolanic reactivity at early ages have been reported by various researches [26,3336]. Silica fume concretes which didn t have significant pozzolanic reactivity at early ages had similar compressive strengths to the control concrete up to 7days. 14

16 With the progress of hydration, the initial gains due to acceleration of cement hydration by nanosilicas are lost. Furthermore, due to completion of significant proportion of pozzolanic reaction of nanosilicas at early ages and also the reduction in total amount of hydration reaction of cement in presence of nanosilicas [26], their influence on improving the compressive strength of concretes gradually diminishes and similar strengths compared to the control mixture were obtained at 90 days. As silica fume had significant pozzolanic reactivity after 7days, the concretes incorporating this material had higher compressive strengths compared to the control mixture at 28days. However, at 90 days due to reduction in total hydration of cement in presence of this material [26], the magnitude of improvements compared to the control mix becomes lower. It is well established that pozzolanic reaction could provide a more discontinuous and tortuous porosity [4,9,3739]. Thus, pozzolanic materials could be highly beneficial in improving the chloride and electrical resistances of concrete. The results of this study also indicate that at early ages, fast pozzolanic reactivity of nanosilicas led to higher electrical and chloride resistances of concretes compared to the control mixture, at 7 days, whereas silica fume concretes, which didn t have significant reactivity at early ages, did not improve the durability characteristics at 7days. This study reveals that the nanosilicas with lower surface areas outperformed those with finer particles in enhancing the chloride and electrical resistance of concretes, at 28 and 90 days. The observed trends could be due to the following mechanisms: i The microstructure of cement composites at early ages is not as dense as the microstructure which is formed after early ages. At early ages, the pore structure is not formed well and consists of a high volume of pores which are larger than those formed 15

17 after early ages [4,4041]. Fine nanosilicas like those investigated in this study have high rates of pozzolanic reacticity at very early ages [26, 42], thus these materials are more effective in improving the early microstructure. In contrast to fine nanosilicas, silica fume and coarse nanosilicas have high amounts of pozzolanic reactivity after early ages, therefore these materials improve the microstructure which is well formed and is denser than that formed at early ages. Therefore, they are more effective in improving later age microstructure compared to the finer nanosilicas. ii It has been established that parameters that accelerate early hydration of cement pastes, such as higher curing temperature or use of accelerating chemical admixtures, have an unfavorable effect in the later age properties compared to pastes with slower rates of hydration reaction [4,43]. This is mainly attributed to less uniform and more porous microstructure. The accelerating effect of nanosilicas particularly those with finer particles could have a similar unfavorable effect on later time properties. For instance, it has been shown that the use of nanosilicas and even the microsilica, despite its initial acceleration, has resulted in a lower degree of cement hydration at later ages [26]. This hindering effect appears earlier for finer nanosilicas. 5. Conclusions The influence of nanosilica hydrosols with different surface areas of 100, 200 and 300 m 2 /g and various replacement levels on the rate of strength development and chloride resistance of concrete mixes was investigated. The results indicate that: The nanosilica hydrosols resulted in improved compressive strengths compared to the plain concrete, particularly at early ages. This appears to be due to accelerating effect of 16

18 these materials on cement hydration and also their rapid pozzolanic reactivity. Silica fume due to its lower specific surface area did not significantly influence concrete strength up to 7days. However, with the progress of pozzolanic reaction its performance at 28 and 90 days was similar or superior to that of the nanosilicas. With regards to chloride resistance and electrical resistivity, the incorporation of nanosilicas resulted in improved performance compared to the control mix at 7 days and the improvements were more significant for higher replacement levels. The use of L200 and L100 at 5% dosage resulted in 50% and 28% increase in electrical resistivity of concrete, respectively. The corresponding improvements in the RCPT test results were 41% and 34%. Silica fume did not result in a considerable improvement in chloride resistance and electrical resistance of concrete up to 7days. With the progress of hydration the improvements in chloride and electrical resistance due to use of nanosilicas became more significant, particularly at higher replacement levels. It is interesting to note that at the ages of 28 and 90 days, the nanosilicas with lower specific surface performed better than the finer ones. At the age of 28days, a 5% dosage of L200 and L100 nanosilicas resulted in 100% and 150% increase in electrical resistivity of concrete. The results of the RCPT and RCMT tests also show a similar trend. At the age of 28days, silica fume performed better than the finer nanosilicas; however, compared to the coarsest nanosilica studied; ie L100, its performance was equal or slightly lower. At the age of 90days, silica fume clearly outperformed all the nanosilicas. From the study, it appears that the use of finer nanosilicas does not result in any significant advantage in strength or durability over the coarser ones, and in fact, at 28 and 90 days the coarser nanosilicas perform better. 17

19 Compared to silica fume, the use of nanosilicas only improved the rate of development of properties up to 7days. At the age of 28days any advantages in strength or durability are slight and at 90days nanosilicas are outperformed by silica fume, particularly in the durability characteristics. Acknowledgments Support from the building and housing research center (BHRC) is gratefully acknowledged. 6. Refrences: 1. Neville A. Chloride attack of reinforced concrete: an overview. Mater Struc 1995; 28: Raupach M. Chlorideinduced macrocell corrosion of steel in concretetheoritical backgroung and practical consequences. Constr Build Mater 1996; 10: Schiessl P. Corrosion of steel in concrete. New York: Chapman and Hall, Taylor HFW. Cement Chemistry, second ed. London: Thomas Telford, Mehta PK, Monteiro PJM. Concrete, Microstructure, Properties and Materials. New York: Mac GrawHill, Elsener B. Macrocell corrosion of steel in concreteimplications for corrosion monitoring. Cem Concr Compos 2002; 24: Tuutti K. Corrosion of steel in concrete, CBI forskning research, 04. Stockholm: Swidish cement and concrete research institute,

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34 Properties Cement Silica fume Levasil 100/45 Levasil 200/30 Levasil 300/30 Silica (SiO 2 ) Iron oxide (Fe 2 O 3 ) Alumina (Al 2 O 3 ) Calcium oxide (CaO) Magnesium oxide (MgO) Sulfur trioxide (SO 3 ) Sodium oxide (Na 2 O) Potassium oxide (K 2 O) Loss on ignition <1 <2.5 <1 Moisture content Surface area(m 2 /g) Particle size* (nm) ph Abbreviation SF L100 L200 L300 *According to the producer s data. 33

35 Mix. Cement Nanosilica* Microsilica Fine aggregate Coarse aggregate Water** SP Compressive strength (MPa) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (l/m 3 ) (kg/m 3 ) 1day 3days 7days 28days 90days Control L3001% L3003% L2001% L2003% L2005% L1003% L1005% L1007.5% SF3% SF5% SF7.5% * Solid content ** Including water in nanosilica hydrosols, silica fume slurry and superplasticizer 34

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