ENVIRONMENTAL ZONATION FOR DURABILITY ASSESSMENT AND DESIGN OF REINFORCED CONCRETE STRUCTURES IN CHINA

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1 ENVIRONMENTAL ZONATION FOR DURABILITY ASSESSMENT AND DESIGN OF REINFORCED CONCRETE STRUCTURES IN CHINA Qinghua Huang (1), Ning Xu (1), Xinglin Gu (1), Xianyan Jin (2), Weiping Zhang (1) and Nanguo Jin (2) (1) Department of Building Engineering, Tongji University, China (2) Department of Civil Engineering, Zhejiang University, China Abstrat Surrounding environmental ations are important fators whih ause orrosion damages in reinfored onrete strutures. With the appropriate environmental zonation douments, the durability and servie life of an existing reinfored onrete struture in any region of China an be assessed easily, and the durability design of a new reinfored onrete struture an be arried out. Based on the environmental ondition harateristis of engineering strutures in China, a new method of environmental zonation for onrete strutures was proposed. Firstly, environmental ations of onrete strutures were lassified into different ategories. Furthermore, zone division and grade determination for different kind of environmental ations were arried out on the map of our ountry based on the durability damage predition models for onrete strutures. Finally, the zonation results were further adjusted to take into aount other environmental fators whih were not onsidered in the original environmental zonation. Zonation of arbonation environment was taken as an example to illustrate the methodology of the environmental zonation for reinfored onrete strutures in China. 1. INTRODUCTION The servie environments of onrete strutures are omplex due to the fat that it has a vast territory, ompliated topography and hangeable limate in China. Many environmental fators suh as CO 2, SO 2 in the air, salt spray in the oean air, hloride in the seawater, sulfate in the ground water, as well as freezing and thawing yles in the north of China an lead to the deterioration of strutural durability, whih has made serious eonomi loss. The existing disease investigations and mehanism studies show that the durability damage harateristis of onrete strutures are distint in different areas. Based on the environmental onditions and their influene degrees on the durability of onrete strutures, the wide region of our ountry an be divided into different zones, in whih reasonable durability design an be made aording to the different environmental onditions. 735

2 Environmental zonation of engineering strutures has been paid more attention by many sholars in reent years. On the basis of the analysis of environmental limate, Kuwait University divided the Arabian Peninsula into two zones, that is, the hotdry zone and the hothumid zone [1]. The zonation distinguished the environmental influene on the durability of onrete strutures. The design ondition of environment was determined aording to limate zones and environmental exposure grades before the onrete durability design in the durability design ode of Australia and Afria [2, 3]. In China, Zhejiang University [4] and Xi an University of Arhiteture and Tehnology [5] have begun to study environmental zonation for onrete strutures. A zonation method based on life predition was presented in [4]. However, the method was ompliated and further studies need to be arried out. At present, environmental ations of strutures are desribed by the environmental ategories and grades, and the influene degrees on the durability of strutures are distinguished in the form of tables in existing design odes and assessment riterions [69]. The grade lassifiation method is onvenient to inquire and aord with engineering ustom. However, it has many defiienies: 1) the environmental ondition desription is fuzzy and it is hard for engineers to determine the environmental ondition degree in detail; 2) many environmental degrees must be determined by speifi environmental parameters, whih is diffiult for strutural engineers to ollet. Above problems an be solved by an appropriate environmental zonation whih is defined as follows: based on the environmental onditions and their influene degrees on the durability of onrete strutures, some region was divided into different zones, where the influene of orresponding environmental onditions on onrete strutures an be distinguished. A new method of environmental zonation for durability design and assessment of onrete strutures are going to be proposed in this paper, whih an provide useful guidane not only for the design of new onrete strutures but also for the assessment of existing onrete strutures. 2. METHODOLOGY OF ENVIRONMENTAL ZONATION The work on environmental zonation in China is ompliated and diffiult whih is beause of the following reasons: 1) there are many kinds of damages of durability for onrete strutures, suh as onrete arbonation, hloride penetration, freezing and thawing, and so on; 2) every kind of damage of durability is affeted by many environmental fators; 3) different kinds of damages of durability for onrete strutures may be interative; 4) limate in China is omplex and loal limate may disaord with globe limate; 5) different members in a struture may have the different environmental onditions; 6) partial environmental fators are diffiult to obtain and predit, suh as the distribution of CO 2 in the air affeted signifiantly by environmental poliy. Therefore, the following method of environmental zonation was adopted: 1) environmental ations of onrete strutures were lassified into different ategories; 2) the damage predition models for onrete durability were established after the analysis on the influene fators in every environmental ategory; 3) the ation values and distribution of environmental fators were investigated and analyzed; 4) the damage predition values for durability over a period were alulated in the different zones based on the damage predition models; 5) zonation and grade determination of any environmental ation on the map were jointly arried out on the basis of the damage predition values; 6) by using the form of tables 736

3 and haraters, the zonation results were further adjusted to take into aount the other environmental fators whih were not onsidered in the original environmental zonation. 3. CLASSIFICATION OF ENVIRONMENTAL ACTIONS Based on Durability design and onstrution guidane for onrete strutures (CCES ) in China, environmental ations an be lassified into different ategories whih are shown in Table 1. Table 1: Environmental ategory Category C E A B D Name Atmospheri environment in whih steel bars may orrode aused by onrete arbonation Freezing and thawing environment Environmental fators Environmental zonation Grade lassifiation Air temperature Relative humidity Temperature redution rate Freezing and thawing yle Costal and oean environment in whih steel bars may orrode aused by penetration of hloride CO 2 onentration Drywet yle Dry or moist for a long time Moist state Whether ontat with brine Ca Salt spray zone Air temperature Height from the sea Distane from the sea Drywet yle Cb Tidal or splash zone Air temperature Chloride ontents in seawater C Ea Eb Submerged zone Environment in whih steel bars may orrode aused by deiing salt Water temperature Chloride ontents in seawater Partly submerged Other orrosion environment aused by hemial substanes Chemial orrosion environment in soil and water Polluted atmospheri environment E Salt rystal environment 4. DAMAGE PREDICTION MODEL FOR CARBONATION IN CONCRETE STRUCTURES In order to develop environmental zonation, it is neessary to establish all the durability damage predition models in different environment ategories shown in Table 1. In this paper, onrete arbonation is taken as an example to illustrate the method of the environmental zonation for durability assessment and design of reinfored onrete strutures. Carbonation of onrete will destroy the protetive layer around the reinforement steel bars and then ause the orrosion of steel bars. The proess and the veloity of arbonation are related with two main fators, one is the external environmental ations and the other is interior material harateristi against environmental ations. The main external environmental ations whih affet onrete arbonation are environmental temperature, environmental relative humidity, CO 2 onentration and so on. 737

4 4.1 Temperature and arbonation of onrete The veloity of arbonation is almost proportional to the external temperature (Fig.1). 30 Carbonation depth /mm RH=40% RH=50% RH=60% RH=70% RH=80% Veloity of arbonation Temperature / Relative humidity / % Fig.1 Relationship between external temperature and arbonation depth[10] Fig.2 Relationship between external relative humidity and veloity of arbonation[11] 4.2 Relative humidity and arbonation of onrete Relative humidity affets the pore saturation of onrete: it affet not only the diffusion of CO 2, but also the veloity of hemial reation in solution or on the surfae phases of solid and liquid. Fig.2 shows the influene of CO 2 onentration on the veloity of arbonation. 4.3 CO 2 onentration and arbonation of onrete CO 2 onentration is one of the main environmental fators whih dominates the veloity of arbonation. High CO 2 onentration will aelerate the arbonation of onrete. The relationship between CO 2 onentration and the veloity of arbonation is desribed as Fig.3. Carbonation depth /mm CO 2 /% Fig.3 Influene of CO 2 onentration on the veloity of arbonation[12] 4.4 Predition model of arbonation Many arbonation models were established over past 30 years based on the theoretial derivation or the experimental analysis. In order to onsider the influene of environmental temperature and relative humidity, a new method to predit the arbonation of onrete is proposed based on the model developed by Zhang et al [13]. 738

5 Firstly, an amendable item kt was onsidered in the model, in whih k is a onstant. The fat that the veloity of arbonation is proportional to the temperature is refleted in the model. The revised model an be expressed by Eq.(1). 1.1 w/ 0.34 X = kt ( 1 RH ) CCO t (1) 2 where, X is the arbonation depth of onrete (mm); t is arbonation time (d); k is a influene oeffiient of environmental temperature; T is ambient temperature ( C); RH is ambient relative humidity and it is suggested that RH 55% by Zhang et al.[13]; w/ is water ement ratio; is the ement ontent (kg/m 3 ); C is the environmental CO 2 onentration. Seondly, the relative humidity to the arbonation of onrete is taken into aount. By assuming the simple relationship between the arbonation depth and the relative humidity shown in Fig.4 based on the results in Fig.2, we an get Eq.(2). X 1.1 w/ 0.34 kt ( 1 RH ) CCO t, RH 55% 2 = w/ kt RH CCO t, 0 < RH < 55% 2 CO 2 (2) Carbonation depth /mm Relative humidity Fig.4 Relationship between arbonation depth and relative humidity By using the experimental results shown in Table 2[14], the value of k in Eq.(2) an be alibrated. The alibration result is 102 k =. Table 2: Carbonation depth of Portland ement onrete over 10 years(mm) Time /a Speimen 1 Speimen 3 Speimen 7 Speimen 9 Speimen 10 Note T = RH = 70% C CO = 0.034%

6 5. INVESTIGATION AND ANALYSIS OF ENVIRONMENTAL FACTORS 5.1 Temperature and relative humidity Aording to the harateristi of temperature and relative humidity, the annual average temperature and relative humidity are regarded as representative values in Eq.(2). Based on the data supplied by the China Meteorologial Data Sharing Servie System ( annual average temperature and relative humidity in 210 ities of China over the past 30 years ( ) were obtained. The annual average temperature and relative humidity in other areas of China were alulated by the interpolation method of inverse distane weighted by using ArGIS software. As results, the distribution maps of annual average temperature and relative humidity are desribed in Fig.5 and Fig Fig.5 Distribution of annual average Fig.6 Distribution of annual average temperature in China ( C) relative humidity in China (%) CO 2 onentration Generally speaking, CO 2 onentration in the air is hangeable and diffiult to obtain. The distribution map of CO 2 onentration is hard to make. Referring to the CO 2 onentration value (annual average value, ppm) tested in Waliguan, a globe benhmark observatory, and onsidering the fat that CO 2 onentration may have the rising tendeny, the environmental grade of CO 2 onentration an be lassified into Grade I and Grade II (Table 3). Table 3: Grade lassifiation of CO 2 onentration in China Grade Annual average onentration (ppm) Desriptions I <400 Unpolluted environment of ountry and forest II >400 and <500 Polluted environment of ity and industry 6. ZONATION OF CARBONATION ENVIRONMENT FOR CONCRETE STRUCTURES A benhmark onrete is speified in alulation for damage predition value of all environmental ategories. Portland ement is used and the parameters are: w/ = 0.45, 3 = 400kg/m. 740

7 Aording to the distribution regularity of different environmental ation, environmental ations are taken into aount with two kinds: one is related with the distrit distribution, whih will be onsidered in environmental zonation maps, the other is not related with the distrit distribution, whih will be onsidered in the adjustment of rating for environmental ations in the form of tables and texts. Fators for two kinds of environmental ations are shown in Table 1. Environmental zonation of onrete arbonation is arried out by the following steps: (1) Define the rating of environment ations as in Table 4. (2) Take 210 ities investigated before in China as alulation points of onrete arbonation. (3) Take the annual average temperature and relative humidity of the past 30 years ( ) as the environmental representative values for the predition model. (4) Calulate the arbonation depth over 50 years for eah of alulation point assuming that the value of CO 2 onentration is 0.04%. (5) Based on the predition depth of arbonation in 210 ities, alulate the arbonation depth of other areas in China over 50 years by the interpolation method of inverse distane weighted. (6) Aording to the indexes in Table 5, divide the inland of China into 3 different environmental zones on the basis of the predition depth of arbonation. Table 4: Rating of environmental ations Class Severity Class Severity 1 Negletable 5 Very severe 2 Mild 6 More severe than lass 5 3 Moderate 7 More severe than lass 6 4 Severe 8 More severe than lass7 Table 5: Zonation indexes of arbonation environment for onrete strutures Rating X range(mm) Severity Remarks A1 0 X 15 Negletable Zonation map A2 15<X 25 mild Zonation map A3 25<X 35 Moderate Zonation map A4 35<X 45 Severe Grade adjusting A5 45<X 55 Very severe Grade adjusting Environmental zonation map of arbonation is shown in Fig.7. The different zones show the different influenes of loal environmental ations on onrete arbonation. For example, zone A1 represents that the predition depth of benhmark onrete in the zone is from 0 to 15 millimeters in 50 years only when the zone s loal limate is similar to the globe limate and the zone is hardly polluted. Furthermore, the rating an be adjusted aording to CO 2 onentration and other environmental fators, whih are not onsidered in the zonation map. The adjustment method of rating is shown in Table

8 A 1 A 2 A 1 A 2 A 3 A 1 A 2 A 3 Fig.7 Zonation map of arbonation environment for onrete strutures Table 6: Adjustment of rating based on environmental zonation of arbonation Item Adjustment of rating (1) 1 (2) +1 (3) +1 (4) +2 Environmental ondition Dry environment not suffering from rain or indoor dry environment Meeting one of the onditions: The grade of CO 2 onentration is Grade II Drywet yle ours frequently The grade of CO 2 onentration is Grade I, and meet one of the following onditions: Outdoor environment suffering from rain Longtime moist environment Meeting simultaneously the following two onditions: The grade of CO 2 onentration is Grade II Examples Indoor onrete members in the environment of longtime dry, normal temperature and low humidity Members in the plae where CO 2 disharged diretly in a ement industrial zone Members suffered from spray of tail gas of ar Members in the zone where water level is hangeable in potable water Outdoor members suffering from rain in frequent rain zones Members above and lose to the surfae of water for a long time Members suffering from spraying diretly aused by higher CO 2 onentration in ement industrial zone. Drywet yle ours frequently Drywet yle ours frequently Notes: (a) If no steel bars are existed in onrete, the rating of arbonation environment an be regarded as adjustment A1. (b) +1 denotes the ation rating inrease one degree in the result of arbonation environment zonation. For example, if the former rating is A2, it beomes A3 when inrease one degree. () The rating of arbonation environment is regarded as A1 when the rating is lower than A1 after the adjustment. 742

9 7. CONCLUSIONS Based on the investigation and analysis of environment ations, a new predition model for arbonation of onrete is presented by modifying the model of Zhang et al. Aording to the predition model, zonation of arbonation environment for onrete strutures is developed quantitatively. With the zonation map, arbonation indued durability problems for onrete strutures in any inland region of China an be easily solved, whih indiates that the zonation methodology used in the paper is appropriate. For the zonation of other environment ations, further researh work needs to be done in the future. ACKNOWLEDGEMENTS The finanial support from the HiTeh Researh and Development Program of China (863 program. Grant No. is 2006AA04Z415) is gratefully appreiated by the authors. REFERENCES [1] Haque, M.N., AlKhaiat, H., and John, B. Climati zonesa prelude to designing durable onrete strutures in the Arabian Gulf. Building and Environment.42 (2007) [2] AS3600, Australian Standard for Conrete Strutures. Standards Assoiation of Australia, 2001,27pages. [3] Haque, M.N. Afrian onrete odedesign for durability. 2 nd Afrian Conrete Code Symposium, University of Stellenbosh, Western Cape, South Afria.2006,115. [4] LÜ, Q.F. Fundamental Researh on Durability Environmental Zonation Standard(DEZS) of Conrete Strutures. Ph.D. Thesis, Hang Zhou: Zhejiang University, 2007,1143. [5] Wang, Y. Durability Environment Division for Conrete Struture. Master Thesis, Xi an: Xi an Univerity of Arhiteture and Tehnology, 2007,156. [6] BS 81101:1997, Strutural use of onretepart1:code of pratie for design and onstrution. British Standards Institution, 1997, 16pages. [7] ACI 318R08, Buildings Code Requirements for Strutural Conrete and Commentary. Amerian Conrete Institute, 2008,56pages. [8] GB , Code for design of onrete strutures. China Arhiteture&Building Press, 2002,14 pages. [9] CCES012004, Durability design and onstrution guidane for onrete strutures. China Arhiteture&Building Press, 2004,3pages. [10] Zhang, H.Y., Ba, D.D., and Wang, Z.Z. A model for foreasting arbonization depth of onrete. Engineering Journal of Wuhan University.35 (5) (2006) [11] Feng, N.Q. Pratial Conrete Completeditionary, Beijing: Beijing Siene Tehnology Press, 2001,222. [12] Ji, Y.S., Zhao, G.S., and Fan, Z.S. A study on similarity of onrete arbonization. Journal of Huaihai Institute of Tehnology.11 (3) (2002) [13] Zhang, Y., Jiang L.X., and Zhang W.P., et al. Durability of onrete strutures, Shanghai: Shanghai Siene Tehnology Press, 2003,5354. [14] Zhang, L.M., Jiang, W.H. A study on arbonation of onrete in natural ondition and its orrelation with artifiial aelerated arbonation. J. Xi an Inst. of Metall & Cons. Eng.22(3)(1990)