The Effect of Different Exposure Conditions on the Chloride Diffusion into Concrete in the Persian Gulf Region

Transcription

1 The Effect of Different Exposure Conditions on the Chloride Diffusion into Concrete in the Persian Gulf Region P. Ghods, M. Chini, R. Alizadeh, M. Hoseini, Graduate Research Assistants, Civil Engg. Dept., University of Tehran, Tehran, Iran M. Shekarchi Assistant Professor, Civil Engineering Department, University of Tehran, Tehran, Iran A.A.Ramezanianpour Professor, Civil Engineering Department, Amirkabir University, Tehran, Iran Abstract Concrete structures are increasingly being deteriorated in Persian Gulf region, mainly due to the chloride-induced corrosion of the embedded steel. Severity of this environment in which the average temperature exceeds more than 30 o C and the relative humidity is about % has made Persian Gulf one of the most aggressive environments in the world. Nevertheless, there has not been any specific work to develop a model for the service life design in this area. In this study, necessary experiments were conducted to evaluate various exposure conditions influencing the diffusion of chloride ion into concrete. Thus, 15x15x60 cm concrete prism specimens were fabricated with different silica fume contents and water/binder ratios. After 28 days of curing in saturated calcium hydroxide solution, the specimens were subjected to different exposure conditions in Persian Gulf region for three months. Soil, atmosphere, Submerged, tidal and splash zones were five major exposure conditions investigated in this study. Accordingly, the powder samples for chloride analysis were ground off specimens at the age of 3 month during exposing period, and subsequently their chloride contents at different depths from exposed surface were determined according to ASTM C114. The diffusion coefficients (D c ) and surface chloride contents (C s ) of concrete specimens were determined by nonlinear curve fitting of chloride profiles to Fick s second law of diffusion and the influence of various exposure conditions was quantified to be employed in the service life design and the estimation of the timeto-corrosion-initiation using DuraPGulf, the service life design model for concrete structure in the Persian Gulf region. Keywords:exposure conditions, service life design, diffusion, Persian Gulf, durability, chloride ion Pouria Ghods Construction Materials Institute Department of Civil Engineering University of Tehran Tehran, Iran pghods@ut.ac.it Tel:

2 1.0 Introduction Concrete structures are increasingly being deteriorated in Persian Gulf region, mainly due to the chloride-induced corrosion of the embedded steel. Severity of this environment in which the average temperature exceeds more than 30 0 C and the relative humidity is about % has made Persian Gulf one of the most aggressive environments in the world [1]. Although exposure condition to which concrete structure is subjected in marine environment, plays an important role on the chloride penetration, there is a little experimental investigation in the world in order to quantify its effect on the rate of chloride penetration into concrete [2].Considering the location of structural elements in relation to seawater level, five exposure zones could be introduced including splash, tidal, submerged, soil and atmospheric zone. In each of these zones, the mechanism of chloride penetration into concrete is different and is influenced by environmental conditions such as humidity, temperature, wind and also solar radiation [3]. In this study, necessary experiments are conducted to evaluate the influence of exposure conditions on the diffusion of chloride ion into concrete. 15x15x60 cm concrete prism specimens were fabricated for the assessment of field environment. Concrete specimens were subjected to five alternate exposure conditions in the Persian Gulf region including splash, tidal, submerged, soil and atmospheric zone. Also, it has been assumed that the diffusion mechanism mainly contribute to chloride penetration into concrete. Accordingly, the apparent diffusion coefficients and equilibrium surface chloride contents of concrete specimens are determined by curve fitting of the chloride profiles to Fick s second law of diffusion and the impact of exposure conditions on service life design of reinforced concrete structures was determined by the estimation of the time-to-corrosion-initiation using DuraPGulf, a computer-based finite-element model developed in Construction Materials Institute at the University of Tehran [4]. 2.0 Experimental Procedure 2.1 Mixture Proportions The investigation carried out on 6 concrete mixes containing type II Portland cement and silica fume (SF).Mixture designs are presented in Table 1. A mix ratio of 1:2.1:2.6 by mass was used with water to binder ratios of 0.4 and 0.5. Total cementitious material mass was 400 kg/m 3 and it was kept constant for all concrete mixes. The silica fume replacements were 0, 7.5 and 12.5 % by mass. Also the use of polymer-based superplasticiser in mixes containing silica fume was necessary to achieve desired slump. Chemical analysis of used cement and silica fume is presented in Table 2. Also Table 3 presents the physical properties of the used aggregate. The slump and air content of fresh concrete were measured according to ASTM C143 and ASTM C231 respectively and the results are reported in Table 1. Concrete was cast into 15 cm cube and 15x15x 60 cm prism molds compacted on vibrating table and were used for the measurement of compressive strength and field seawater exposure tests, respectively.

3 Table 1: Mixture proportions, Slump and Air content. Mix Code Composition W/b Ratio Cement (Kg/m 3 ) Silica Fume (Kg/m 3 ) Water (Kg/m 3 ) Aggregate Slump (Kg/m 3 ) (mm) 1 SF0W2 PC SF0W4 PC SF2W2 PC+7.5% SF SF2W4 PC+7.5% SF SF4W2 PC+12.5%SF SF4W4 PC+12.5%SF Table 2: Cement and silica fume chemical composition Air content (%) SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 K 2 O Na 2 O L.O.I Type II Portland Cement Silica Fume Table 3: Physical properties of the aggregates Aggregate Type Type MSA * Water Specific Fineness (mm) Absorption (%) Gravity Modulus Fine aggregate Crushed limestone Coarse aggregate Crushed limestone * Maximum Size of Aggregate 2.2 Exposure Conditions After 28 days of curing, the four sides of prism specimens were sealed with an epoxy polyurethane coating to ensure that diffusion would occur in only one dimensional of the specimens. Then they were exposed to five different conditions in Persian Gulf region which are schematically shown in Figure 1[5] and are defined by the codes in Table 4. In atmospheric zone, the specimens have never been directly connected with seawater, and in soil condition, the level of underground seawater is such low that the specimens are always in natural humid condition. In other words, they are never immersed in seawater during the exposure time. The area between minimum and maximum height of water tide defines the tidal zone. The above of this zone has been named splash zone in which seawater particles wash out concrete surface. The Persian Gulf seawater contains around 32 gr/lit NaCl and the recorded air temperature during three months of exposure is shown in Table 5 [1]. Table 4: Different exposure conditions code definition Exposure Condition Atmosphere Soil Tidal Submerge Splash Code EC01 EC02 EC03 EC04 EC05

4 Figure 1: Schematically five different exposure conditions in Persian Gulf Table 5: Monthly average temperature Month May June July Temperature( o C) Sampling After three months of exposure in seawater at Persian Gulf, the first 100 mm of each prism specimen was cut with concrete saw (Figure 2) and the cut face of specimen was coated and exposed again in seawater for further studies. Powder samples for chloride analysis were ground off by a Profile Grinder parallel to the exposed surface according to NordTest NT Build 443 method [6] with the accuracy of 0.5 mm at 9 different depths (Figure 3). The first 1mm powder was not included in calculations as it might be affected by actions such as washout. 10 gram samples at each depth were collected to the nearest 0.01g to use in chemical analysis according to ASTM C114 [7] and chloride contents at different depths from exposed surface were determined by potentiometeric titration method.

5 Cl - Exposed Surface 10 cm Coated Surfaces Figure 2: Typical concrete prism specimen and preparing the slice for powder sample Powder samples at nine depths from exposed surface Exposed Surface Figure 3: Preparing powder samples by grinding the exposed surface 3. Test Results and Discussion 3.1 Hardened Concrete Properties The physical and mechanical properties of hardened concrete specimens are given in Table 6. As it was expected, the compressive strength increases with both decrease in water to binder ratio and the use of silica fume. 3.2 Chloride Profiles The chloride penetration rate as a function of depth from the concrete surface and time can reasonably be represented by Fick s second law of diffusion according to the following expression [8]: 2 δ C δ C = D (1) 2 δ t C δ x The solution for this differential equation is: C ( x, t ) = C 1 erf 2 s x D C t where x is distance from concrete surface (m); t is time (s); D C is diffusion coefficient (m 2 /s); C s is equilibrium chloride concentration on concrete surface; C(x,t) is chloride concentration at the depth of x from the surface at time t; and erf is error function. (2)

6 Table 6: Properties of hardened concrete Code Density Comp. Strength (MPa) (Kg/m 3 ) 7-day 28-day SF0W SF0W SF2W SF2W SF4W SF4W Using a computer statistical analysis program, the regression was carried out on the experimental data and by curve fitting of Fick s second law of diffusion, the values of D C and C s were determined. The average values obtained from top and bottom of specimens are given in Table 7. Also, the values of D C and C s are plotted against exposure conditions in Figures 4 and 5, respectively. The results shown in Figure 4 indicate that by improving the quality of concrete microstructure, the chloride diffusion coefficient decreases. In other words, both replacement of silica fume and reduction of water to binder ratio decrease the chloride diffusion coefficient. Besides the chloride diffusion generally decreases from tidal, splash, submerged, soil to atmospheric zone. Similar conclusion was stated by Ghalibafian et al. in 2003 [9] and also by T. Luping in 1998[10]. The results of surface chloride concentration given in Figure 5 show that surface chloride contents of specimens increase respectively in soil, atmosphere, tidal, submerged and splash zone. The wetting and drying cycles result in large amount of surface chloride in splash and tidal zones. Alzahrani et al. also stated same reason to describe the high amount accumulation of chloride on the surface of concrete [11]. However, in submerged zone the direct contact of seawater with concrete surface leads to almost high surface chloride content rather than atmospheric and soil conditions. It must be notified that wind and precipitation contribute to bring chloride ion to atmosphere and soil zones.

7 Table 7: Values of D C and C s after 3 months of exposure ZONE CODE Cs Dcx10-12 EC01-SF0W EC01-SF0W EC01-SF2W EC01-SF2W EC01-SF4W EC01-SF4W Mean EC02-SF0W EC02-SF0W EC02-SF2W EC02-SF2W EC02-SF4W EC02-SF4W Mean EC03-SF0W EC03-SF0W EC03-SF2W EC03-SF2W EC03-SF4W EC03-SF4W Mean EC04-SF0W EC04-SF0W EC04-SF2W EC04-SF2W EC04-SF4W EC04-SF4W Mean EC05-SF0W EC05-SF0W EC05-SF2W EC05-SF2W EC05-SF4W EC05-SF4W Mean Atmosphere Soil Tidal Submerged Splash

8 SF0W4 SF2W4 SF2W2 SF0W2 SF4W4 SF4W2 Dc x10^(-12) m2/s Atmosphere Soil Tidal Submerge Splash Figure 4: Values of diffusion coefficient at different exposure conditions SF0W4 SF2W4 SF2W2 SF0W2 SF4W4 SF4W2 Cs (%W concrete) Atmosphere Soil Tidal Submerge Splash Figure 5: Values of surface chloride at different exposure conditions

9 Table 8: The required covers of reinforcement calculated by DuraPGulf Exposure Condition Code Cs D C (m 2 /s) Cover (mm) Atmosphere EC01-SF4W E Soil EC02-SF4W E Tidal EC03-SF4W E Submerge EC04-SF4W E Splash EC05-SF4W E Service Life Design To investigate the influence of various parameters on time-to-corrosion-initiation of concrete structures, a finite element model developed at Construction Materials Institute (CMI) was used which has been presented as a computer program named DuraPGulf [4]. The values of D C and C s shown in Table 7 were used as input parameters. A concrete slab was assumed as structural element. The time of exposure was at the age of 28 days and the temperature profile was according to that of site location in Persian Gulf. Concrete mixture properties were given the same as what was used in the experimental work. The required service life based on corrosion initiation time is 30 years. The chloride threshold value used in the model was 0.05% by the weight of concrete. The required covers of reinforcement calculated by DuraPGulf are presented in Table 8 in various exposure conditions for one mixture proportion containing 12.5% Silica Fume and 0.4 water to binder ratio. Considering these results, it is concluded that exposure condition plays an important role in the service life design of concrete structures that should be concerned as a main input parameter in models. Additionally, in tidal zone the highest cover thickness is required in comparison with other zones. 4. Conclusions This study was conducted to investigate the effect of five exposure conditions on the diffusion of chloride ions into concrete. The following conclusions can be drawn based on the test results: a) The diffusion coefficient of chloride ions into concrete in various exposure conditions follows the order of: D c (tidal) > D c (splash) > D c (submerged) > D c (soil) > D c (atmosphere) b) The surface chloride content of concrete in different exposure conditions follows the order of: C S (splash) > C S (submerged) > C S (tidal) > C S (atmosphere) > C S (soil) c) According to the calculated cover by DuraPGulf model based on our experimental data, the tidal zone is the most severe condition in marine environment from durability point of view. d) Exposure condition is one of the most important factors that must be considered in service life design models for concrete structures.

10 Further studies are going to be conducted at the Construction Materials Institute to evaluate other factors affecting the chloride diffusion into concrete in the Persian Gulf region in order to complete the service life design model for this region. 5. Acknowledgement The authors would like to acknowledge the support by BARCO Company and technical contribution of Associate Professor Mehdi Ghalibafian, Director of Construction Materials Laboratory. The experimental work of this research has been financially supported by Management and Planning Organization (MPO) and Construction Materials Institute (CMI) at the University of Tehran. 6. References 1. Ghoddousi, P., Ganjian, E., Parhizgar, T., Ramezanianpou A.A.,, Concrete Technology in the Environmental Conditions of Persian Gulf, BHRC Publication, No. B 283, Spring Lindvall, A., Environmental Actions and Response-Reinforced Concrete, Structures Exposed in Road and Marine Environment., Publication P-11, Department of Building Materials, Chalmers University of Technology, SE-412, 96 Goteborg. 3. Comite Euro-International du Beton (CEB), Bulletin 238, "New Approach to Durability Design., May Shekarchi, M., et al., Introducing DuraPGulf, new software for durability design of concrete structures in Persian Gulf region, International conference on coasts, ports and marine structures (ICOPMAS), Tehran, Iran, November Mehta, P.K., Concrete in the Marine Environment, Elsevier Applied Science, NordTest NT Build 443, Concrete, Hardened: Accelerated Chloride Penetration, ESPOO, Finland, ASTM C114, Standard Test Method for Chemical Analysis of Hydraulic Cement, ASTM, West Conshohocken, PA, J. Crank, The Mathematics of Diffusion, 2 nd ed., Oxford Press, London, Ghalibafian, M.,Zare, A., Shekarchizadeh, M., and Tadayon, M., Chloride Penetration Testing of Silica Fume Concretes under Persian Gulf Conditions, 6th CANMET/ACI International Conference on Durability of Concrete, 2003, Thessaloniki, Greece 10. T. Luping., A. Anderscn, Chloride Ingress Data from Five Years Field Exposure in a Swedish Marine Environment., Publication P, Dept. of Building Materials, Chalmers University of Technology, Goteborg, Sweden. 11. Al-zahrani, M.M., Abdul-Hamid, and al-tayyib, Use of Polypropylene Fibers to Enhance Deterioration Rresistance of Concrete Surface Skin Subjected to Cyclic Wet/Dry Seawater Exposure,ACI Materials Journal 87(4) (1990) 363.