THE EFFECT OF CEMENT TYPE ON CORROSION RESISTANCE OF REINFORCEMENT

Transcription

1 ISSN: RCEE Research in Civil and Environmental Engineering Research in Civil and Environmental Engineering 014 (04) THE EFFECT OF CEMENT TYPE ON CORROSION RESISTANCE OF REINFORCEMENT Hanifi Binici * Kahramanmaras Sutcu Imam University, Faculty of Engineering, Department of Civil Engineering, Kahramanmaras, Turkey. Keywords Corrosion Reinforcement Steel Grinding A B S T R A C T In this study, an experimental study was carried out to decrease the corrosion resistance of reinforced steels. The composite cement were prepared by using Portland cement clinker, ground granulated blast-furnace slag (GGBFS) and ground basaltic pumice (GBP). Specimens were weighted to determine the corrosion effects in the sense of weight loss. Prepared mortar was sand in 10 cm x 10 cm x 10 cm dimensional cubic structure. Four deformed and plain steel rounded bars were placed into the structure. The obtained concrete blocks were put into natural seawater. After years period, concrete blocks were crushed and steel bars were withdrawn. Surface of the bars are cleaned carefully and put into % HCl solution. After that, corrosion rate was determined by weighting the reinforcement. Corrosion resistance is increased if the rate of additives is increased. Intergrinding and separate grinding plays and important role in corrosion of reinforcement steel. Specimens, prepared by intergrinding, have lower corrosion than separate grinding specimens. There is a close relationship between fineness of cement and reinforcement corrosion. Corrosion rate decreases if the fineness of cement increases. 1 INTRODUCTION Corrosion can be defined as the reaction of a metallic material with its environment, which results in deterioration of the material itself or its properties (Esin, 1981). Shreir also defined that corrosion is the reaction of a metal with its environment, and it is emphasized that this term embraces a number of concepts of which the rate of attack/unit area of the metal surface, the extent of attack in relation to the thickness of the metal and its form are the most significant. (Shreir, 1976). Concrete normally provides to embedded steel a high degree of protection against corrosion. This is because concrete inherently provides * Corresponding author (Phone: + 90 (44) ; Fax: + 90 (44) 80166; hbinici@ksu.edu.tr).

2 a highly alkaline environment for the steel which protects, passivates the steel against corrosion (Verbeck, 1987). The high performance concrete is the one which has better properties such as workability, strength, durability and permeability and usual concretes in two cases; fresh and hardened (Türkmen et al., 00). The corrosion rate of steel material in concrete is extremely low when the metal is in passive state at normal conditions. Chlorine ions around steel, at the passive state, destroy the reinforced concrete. To improve the mechanical properties of concrete and to increase the corrosion resistance of steel embedded in concrete, Portland cement clinker, Basaltic Pumice and granulated blast furnace slag were used. Turkey is rich in natural puzzolan, which are also called "trass" in the cement industry. Almost km of the country is covered by Tertiary-and Quenternary-age volcanic rock, among which tuffs occupy important volumes. Although there are many geological investigations on these volcanic, their potential as natural puzzolan is not well established (Turkmenoglu, 00; Binici et al., 01). It is well know that the incorporation of fly ash and granulated blast furnace slag at normal fineness can reduce the early strength of concrete. Incorporating 0% fly ash or granulated blast furnace slag can significantly increase the compressive strength of concrete after days. However, no big difference in compressive strength was found between themselves. Incorporating 10% fly ash or granulated blast furnace slag can increase the compressive strength of concrete at all ages. The compressive strength is higher than that incorporating fly ash and granulated blast furnace singly, quantitatively similar that of concrete. Thus, the combination of fly ash and granulated blast furnace slag can be used as a strengthening agent of concrete (Kefeng, 1998). In most structures, metallic reinforcement provides static- constructional security. Concrete normally provides excellent corrosion protection for reinforcing steel due to the high ph value of hydrated cement (Mietz, 1996). A number of studies have been conducted on the corrosion of steels in concrete. Baweja et al. (1998), determined lower corrosion current density of concrete containing 5% BFS than without BFS. Also, Huang et al. determined that concrete with 15% and 0% BFS have high corrosion resistance. Similar findings were also reported by Arya and Xu, (1995). Baghabra et al, (199), found that corrosion of the steel rods in concretes decreased with increasing BFS content, except during the first 7 days after casting. The dry-unit weight of concretes decreased with increasing of water-binder ratio and by use of BFS and SF mineral admixtures. The use of 0 % blast furnace slag (BFS) and 10% silica fume (SF) instead of OPC increased the compressive strength of specimens by about 85 on the 8th day and 16 % on the 50th day. The corrosion rate of BFS specimens was lower than those of SF and PC. The corrosion current density increased with the increase of the water-binder ratio and decreased with the increase of the mineral additives percent value of the specimens up to 50% in which the situation may be related to low permeability characteristics of the use of BFS and SF which is very fine compared to PC decreased the corrosion current density of the specimens. In the design of reinforced concretes and hence buildings, it is essential to prevent reinforcement from the corrosion. The type of reinforce has also vital importance. Comparison of deformed and plain steel bars for concrete reinforcement was carried out from the corrosion perspective. Cement cubes were prepared and reinforcement steels, either plain or deformed, were placed to determine the corrosion effects. Firstly, specimens were weighted to determine the corrosion effects in the sense of weight loss. Specimens were exposed in % HCl solution and sustained 1 hour in glass tubes individually. The dried specimens are 18

3 weighted six times with one hour intervals. The exact length, diameter and weight are determined after these processes. Prepared mortar was sand in 10x10x10 cm dimensional cubic structure. Four deformed and plain steel rounded bars were placed into the structure. The obtained concrete blocks were put into solution which was composed of 4% NaCl and water. After the specimens tested, concrete blocks were crushed and steel bars were withdrawn. Surface of the bars are cleaned carefully and put into % HCl solution. Initial tests were repeated and all data were presented. MATERIALS AND METHODS This paper presents the results of corrosion resistance studied on basaltic pumice (BP) collected from pyroclastic exposures around Osmaniye region (southern of Turkey. Granulated blast furnace slag (GBFS) samples were obtained from Iskenderun iron and steel work. The studied basaltic pumice has basalt to rhyolictic compositions. They contain glass shards, mineral phases and less amount of volcanic rock. Essential minerals are feldispat, quartz and biotite. Terminology for all specimens is presented in Table 1. The chemical composition and some physical properties of Plain Portland cement (PPC), BP, GBFS and blended cement are presented in Table. Table 1 The terminology and composition of the studied cements Composition (% percentages by weight) Cement Blaine (m /kg) Clinker Gypsum GBFS NP PPC PPC B 1 (Separate grinding) B (Separate grinding) C 1 (Separate grinding) C (Separate grinding) D 1 (Intergrinding ) D (Intergrinding ) E 1 (Intergrinding ) E (Intergrinding ) Specimens 18 Table Chemical, mineralogical and physical characteristics of materials used Oxides (%) SiO Al O Fe O CaO MgO SO LOI (loss on ignition) Clinker PPC NP GGBS Specimens Cement modulus Mineral composition (%) HM SM AM LM C S C S C A C 4 AF Clinker PPC Physical properties of materials Specific gravity Blaine Sieve analysis (%)

4 Materials (kg/m ) (m /kg) Residue on 90 m Residue on 00 m Basaltic pumice and GGBS and Clinker and TS 114 standard requirements for basaltic pumice (NP) and GGBS SiO + Al O + Fe O SO LOI >61 <.5 <10 (HM: Hydraulic Modulus SiO AM: Aluminate Modulus= Al Fe O CaO Al O FeO O, LM: Lime Modulus=.8SiO, SM: Silicate Modulus= 100. CaO 1.1Al O 0. 7 SiO Al O Fe O In normal conditions ph value for concrete is higher than 1 and this value is sufficient to protect concrete from corrosion. But, the environmental effects decrease the ph value and these causes to corrosion in steel-reinforced concrete. The former may occur because of leaching of the alkalies or their reaction with pozzolanic materials in concrete, by carbonation of the paste, or possibily the presence of calcium chloride. The latter may occur through the use of some admixtures or exposure to decaying salts or sea water. Alkali environment in the concrete forms a thin film layer by surrounding the steel. The main reason for corrosion is chlorine ions. The exact mechanism of corrosion is different for different materials and environments, but it can safely say that corrosion is electrochemical in nature. Electrochemical corrosion can not occur unless there is a potential difference between anode and cathode (Fig. 1). Atoms of the metal are oxidized and form ions. In electrochemistry, oxidation is loss of electrons. The atom that lost the electrons become an ion. Fe O ) Fig. 1 Corrosion mechanism of metal in concrete Section reduction in steel will be 0.5mm/year due to corrosion if ph value of medium is between 4 and 10 (Gel, M.K. and Report on Corrosion of Metals in Concrete). From this perspective, for example; a steelreinforced concrete which has a diameter of 10mm and 65 MPa strength can carry 8.6 KN of load. Load carrying capacity of this steel will decrease from year to year due to the corrosion effect. As it is seen from the Fig., approximately 1 years later steel will loose its load carrying capacity by its half. 184

5 Fig. Load carrying capacity of a sample steel This tremendous decrease in carrying capacity is of course due to very low ph value of medium. Therefore it is concluded that the lower ph value of the medium lower the steel life. Properties of plain and deformed steel bars used in experiments are given in Table. Table Physical Properties of Reinforcement Steels Diameter Gauge Length Final Length Yielding Load Fracture load Yield Stress Deformed 10 mm 50 cm 66 cm 00 kg 4900 kg 41 MPa Plain 8 mm 40 cm 56 cm 1800 kg 00 kg 51 MPa EXPERIMENTAL STUDY Cubic shaped concrete specimens, which have sizes of 10 x 10 x 10 cm in size, formed by PPC and blended cement were prepared. Four steel bars were placed at a distance of 1, 1.5,.5 and cm. Specimens were hold in laboratory conditions of 0 o C and 90 5% relatively humidity. No plasticizer or other chemical admixtures were used. Concrete mix was poured into mould, and reinforcement steels, which have sizes of 8 mm in diameter and 60 mm in length, were positioned. Reinforcement steels were sustained in % hydrofluoric acid before moulding. The weights of steels were recorded in 0,001 g accuracy. Concrete mixture was mixed by external vibrator. All concrete samples were first waited 4 hours in laboratory conditions, and then 8 days in the water saturated by lime. After 8th day, samples were subjected in 5% NaCl solution. The effects of exposure were determined by loss in compressive strength. Compressive strength and the weight loss of steel bars, embedded in mortar, is a direct method for the determination of corrosion at 6, 1, 4 and 6 months. Changes in ph values were recorded in period of 0 day intervals. The compressive strength of the PPC and blended cement concrete determined in accordance with ASTM C

6 4 RESULT AND DISCUSSION 4.1 Weight loss of the steel bars Table 4b and Figs.-4, shows the weight loss of the steel bars embedded in PC and blended cement. Fig. Corrosion rates of PC and separately ground blended cement specimen Fig.4 Corrosion rates of PC and interground blended cement specimens Weight loss of steel bars in PPC and blended cement concrete was found greater than that in PPC concrete in NaCl solution. In the case of the interrupted exposure in NaCl solution, the corrosion rate of reinforcement steels were higher than that of PPC specimens, for which it became significant after 4 months of exposure. It has been found that prescribing the thickness alone, without specifying the quality of concrete can be misleading. For determination of concrete quality; adequate reinforcement, sufficient concrete cover and permeability numbers to liquids can be used effectively as indices. 186

7 Specimen PPC1 PPC B 1 B C 1 C D 1 D E 1 E Concrete cover (mm) Table 4 Corrosion rates of specimens Weight loss (10 - ) % Time 6 month 1year years years Average Average Average Average Average Average Average Average Average Average

8 Up to certain degree, the increase of cover thickness increases the corrosion protection of the reinforcement steel. A thick cover has greater mechanical strength against abrasion and impacts. Furthermore, the permeability of the concrete cover depends markedly on its thickness. In addition, the thickness of the cover influences the progress of penetration of carbonation. In the presence of moisture, carbonation converts the calcium hydrate into calcium carbonate thereby reducing the alkalinity of the concrete cover below ph<8 to 9, may create favourable conditions for formation of corrosive cells (Szilard 1987). 5 CONCLUSION It has been found that the main factors influencing the effectiveness of the concrete cover in the corrosion protection of steel are; the alkalinity of the concrete, permeability of the concrete cover, the quality of the concrete and the corrosion environment (Binici et al., 01). In this study, corrosion effects on reinforcement steels were observed within a limited time. Obviously this time is not so enough but gives an idea. Based on the investigations described herein, followings are concluded: The high alkalinity of the concrete cover, which is primarily responsible for the corrosion protection of the steel, is only slightly influenced by the cement of the mix. Maximum corrosion rate is obtained at specimen PPC1 having 1cm concrete cover, while minimum corrosion rate is obtained at specimen C having cm concrete cover Intergrinding and separate grinding plays and important role in corrosion of reinforcement steel. Specimens, prepared by intergrinding, have lower corrosion than separate grinding specimens. There is a close relationship between fineness of cement and reinforcement corrosion. Corrosion rate decreases if the fineness of cement increases. It would appear that by taking proper advantage of all of this knowledge the corrosion of steel in concrete could for all practical purposes be avoided. However, the design of a structure, the concrete used may not be perfectly adequate nor the varying environment to which the concrete is exposed fully appreciated. In general, better the concrete, the less its permeability. This important relationship between concrete quality and its permeability can be used to determine the quality requirements for corrosion protection. References Arya C., Xu Y. (1995). Effect of cement type on chloride binding and corrosion of steel in concrete. Cement and Concrete Research, 5, Baghabra O. S., Amoudi Al., Rasheeduzzafar M.M., Al-Mana A.I. (199). Prediction of long-term corrosion resistance of plain and blended cement concretes. ACI Material Journal, November- December, Baweja D,.Roper H., Sirivivatnanon V. (1998). Chloride-inducet steel corrosion in concrete; part. 1 corrosion rates, corrosion activity and attack areas. ACI Material Journal,

9 Binici H, M. Yasin Durgun, Tamer Rizaoğlu, Murat Koluçolak. (01).p Investigation of durability properties of concrete pipes incorporating blast furnace slag and ground basaltic pumice as fine aggregates, Scientia Iranica. Transactions A: Civil Engineering 19, Binici H, Aksogan O, Durgun M. Y. (01). Corrosion of Basaltic Pumice, Colemanite, Barite and Blast Furnace Slag Coated Rebars in Concretes. Construction and Building Materials, 7, Esin, A. (1981). Properties of Materials for Mechanical Design, Middle East Technical University, Ankara, Turkey. Kefeng, T, Xincheng. P. (1998). Strengthening effect of finely ground fly ash, granulated blast furnace slag, and their combination, Cem. Concr Res, 8, Mietz J, Isecke B. (1996). Monitoring of concrete structures with respect to rebar corrosion. Construction and Building Materials, 10, Shreir, L. L. (1976). Corrosion Control, The Butterworth Group, Second Edition, Newnes-Butterworths, England. Szilard, R., Wallevik, O. (1987). Effectiveness of Concrete Cover in Corrosion Protection of Prestressing Steel, Corrosion of Metals in Concrete, American Concrete Institute, Detroit, Michigan, USA, Türkmen İ, Gavgalı M, Gül R. (00). Influence of mineral admixtures on the mechanical properties and corrosion of steel embedded in high strength concrete. Materials Letters, Turkmenoglu A.G., Tankut A. (00). Use of tuffs from central Turkey as admixture in pozzolanic cements. Cement Concrete Research,, Verbeck, G. J. (1987). Mechanisms of Corrosion of Steel in Concrete, Corrosion of Metals in Concrete, American Concrete Institute, Publication SP-49, USA. 189