ࡱ> _a^G <bjbjَ .1/ ]8$qsssI<<4$rXX  zqq  rTqD@}wzc Corrosion Limit State Design for Concrete Bridges Session: Modelling of Concrete Structures Dubravka Bjegovi*, Prof.Dr. Jure Radi*, Prof.Dr. Goran Pu~*, M.Sc. Dunja Mikuli**, Dr. Vedrana Krsti***, M.Sc. * - University of Zagreb, Ka ieva 26, 10000 Zagreb, Croatia ** - Department of Civil and Environmental Eng., Rutgers University, New Jersey, USA *** - Prasons Brinckerhoff, 506 Carnegie Center Blvd., Princeton, New Jersey 08540, USA Keywords: concrete bridge, durability loads, corrosion limit state, durability design criteria Summary Deterioration processes observed on the existing concrete bridges in Croatia suggest that design procedures based on current specifications do not always ensure required service life, mostly because of the underestimated environmental influences. Some of the mechanisms of these influences can be simulated mathematically, so that they can be considered as a special type of loads, and consequently included into reinforced concrete structures design procedure by prescribing relevant durability criteria. The Corrosion Limit State Design procedure consists of three phases. The first phase focuses on analysing durability of the structural elements exposed to environmental loads; the second one refers to structural modifications based on physical laws; and the third phase takes part in prescribing new material and sectional properties. Corrosion limit state procedure is demonstrated on a design example for a prefabricated PC girder. 1. Introduction Serviceability Limit State Design Specifications limit concrete crack width according to environmental influences. Highly demanding crack width criteria for structures in aggressive environment may result in increase of the reinforcement quantity compared to reinforcement quantity obtained according to Ultimate Limit State Design. In some cases, even though the additional reinforcement is provided, wider than allowed cracks may occur. Design model proposed in the paper takes into consideration effect of diffused chloride ions on reinforced concrete as one of the most critical environmental influences, thus offering more realistic design procedure. Basic consequence of corrosion initiated in concrete is reduction of the rebar cross-section. Corrosion Limit State is defined as the moment when rebar cross-section is reduced to predefined minimum value. The paper presents research related to the reinforcement corrosion due to chloride diffusion conducted on superstructure of bridges directly exposed to seawater. Croatian bridges included in the research have superstructure cross-section formed of precast, prestressed girders in composite action with cast-in-place deck slab. From durability point of view, this type of superstructure system is rather sensitive to penetration of aggressive substances, mostly because of large exposed concrete surfaces of relatively thin girders. Basic structural data for seven analyzed bridges are shown in Table 1. Proposed durability design procedures [4] have been already incorporated in the design of the most recently built bridge among those included in the research (Maslenica, 1997). 2. Cross-section analysis according to the exposure to the corrosion Cross-sections formed of precast, prestressed girders in composite action with the cast-in-place deck slab are commonly used for bridges and viaducts with spans of 22 to 35 m. The advantage of such system in terms of its durability is achieved by prefabrication of main PC girders. Table 1. Basic structural data for analyzed bridges on the sea. Name, locationYear of constr.spanno. of gir. in c.s.Girder heightGirder weightlongitudinal continuitympcsmtonesExistingBistrina Ston196531.251.748.0not establishedbridges ibenik bridge196523.341.3526.0elastic contin.above Pag bridge196823.341.3523.5elastic contin.the sea}drelac197322.6541.331.6-Privlaka  Vir19753622.1676.0not establishedKrk bridge198033.531.8635.0not establishedMaslenica19973081.7577.0full flexural continuityTypicalT300-3051.7566.2continuitybridgesT301-3061.754.1slab* Proposed designs for new bridges on the Adriatic Highway [6]. Bridges Bistrina, }drelac and Privlaka are beam bridges. All other bridges are arch bridges, where beam type superstructures above arch were observed. Deterioration problems on these structures started very early. For example, Pag bridge started deteriorating in less than 15 years after its completion. On all of the aforementioned bridges problems due to reinforcement corrosion have been reported (1(, (2(, (3(, (5(. Exposed area plays significant role when determining the influence of the severe marine environment, which includes frequent and sudden air moisture, temperature changes, tide and wave influence. In order to establish a relationship between the exposed surface and quantity of endangered structural concrete in superstructure elements (girders and slab), a parameter V/O called coefficient of exposure is introduced. (V) represents quantity of structural concrete per one linear meter of superstructure, and (O) represents exposed surface of girders and slab per one linear meter of superstructure. Figure 1 shows the coefficient of exposure obtained for some of the analyzed bridges. Quantity of structural concrete per one linear meter of superstructure (V) is divided with exposed surface of girders and slab (O). Chart in Figure 2 shows the relationship between the parameter V/O and the length of the girder. Analysis of damages on existing bridges suggests that V/O parameter should be chosen greater than 5, fulfilling other relevant guidelines considering shape, minimum dimensions and concrete cover. Further investigations including in situ monitoring are required in order to confirm this assumption. Pag arch bridge superstructure: significant damages due to corrosion of reinforcement reported after 15 years in service. Replacement of complete superstructure in progress (1999.) (2(, (5(. Privlaka Vir bridge superstructure: significant damages due to corrosion of reinforcement reported after 20 years in service. Extensive repair works have been proposed (3( Maslenica highway bridge (7(: structural dimensions increased, low permeability concrete designed, min. concrete cover of 5 cm proposed. Structure designed for service life of 100 years. Figure 1. Coefficient of exposure obtained for some of the observed superstructures. Figure 2. Coefficient of exposure vs. span length obtained for bridges listed in Table 1. 3. Corrosion Limit State Design According to the corrosion limit state it is necessary to prove that the calculated service life tc is higher or at least equal to the designed life tp: to + t1 = tc > tp /1/ where: to is period of initiation of reinforcement corrosion in concrete t1 is period of propagation of reinforcement corrosion in concrete. Example The complete calculation procedure according to the corrosion limit state criterion is given in Reference (4(. An example will be presented in order to expose influence of technological parameters on service life calculated from the corrosion limit state criterion. Design procedure will be demonstrated for prefabricated PC girders used for T300 and T301 bridges (Table 1). The analysis is conducted for four water/cement ratios: 0.6; 0.5; 0.45; and 0.40 accounting following assumptions: - concrete cover c = 5.0 cm, - initial chloride ion concentration Co (t=0) = 0, - coefficient of diffusion D01 = 0.3 for cement with slag, - critical chloride ion concentration C(c, to) = Ccr = 0.4. For the four different w/c factors and constant other parameters, initiation time t0, propagation time t1 and service life tc are shown in Figure 3 and Table 2, Columns 1-7. Figure 3. Analysis of chlor ions diffusion process to the period to Table 2. Calculated parameters of corrosion limit state design. 12345678 w/cc (cm)D Cl- (cm2/s)Observation Time (years)Time to (years)t1 (years)tc (years)New D Cl- for tp = 100 years (cm2/s)0.6055.14 E-850236.738.7<1.00 E-90.5051.69 E-850636.742.71.35 E-90.4551.18 E-8501136.747.71.68 E-90.4059.36 E 9503336.769.73.50 E 9As it can be seen in Table 2, the required service life of 100 years can not be achieved with the above mentioned parameters. In order to fulfill the designed service life requirement it is necessary to change the concrete quality parameters by improving the chloride diffusion coefficient as it is given in Figure 4 and Column 8 of Table 2. Fig.4 C-D-c-t nomograms for recalculation the concrete quality requirements. In order to assure the required service life of the precast PC girders, it is necessary to define additional requirements on the concrete quality: Air entrainment depending on the aggregate grain size Water/cement ratio < 0.4 Cement type appropriate for exposure conditions with min. 30% of slag Soluble chlorides < 0.15% for RC and <0.06% for PC elements Rapid Chloride Permeability < 1000 coulombs or Chloride diffusion coefficient < 3.5E-13 (m2/s) Gas Permeability < 10E-18 (m2) ISAT < 0.20 after 10 min. (ml/m2/s) It is not easy to fulfill demanding quality requirements for structural concrete. Our suggestion is to monitor the performance of the required properties on several structures during the concreting phase as a part of the quality control procedures. That way valuable data could be collected and used for future predictions of in situ conditions. It is our opinion that by implementing adequate quality control procedures it could be possible to achieve aforementioned requirements on concrete quality for precast PC girders. 5. Conclusion Coefficient of exposure is introduced as a parameter that affects the cross-section design of the structures exposed to the unfavorable environmental conditions. The coefficient can be used to minimize negative influence of durability loads such as chloride ions. Once the cross section is defined taking into consideration the exposure coefficient, a corrosion limit state design procedure can be used to prescribe quality requirements. The corrosion limit state design procedure is implemented on the service life calculation for precast PC bridge girders exposed to the chlorine ion diffusion. The method assumes certain concrete quality parameters necessary for achievement of the 100-year life span of the structure, and gives resulting corrosion initiation and propagation times. Based on the achieved results necessary improvements in the concrete quality can be proposed in order to meet the requirement on the service life. References: 1. Radi, J.: Bridge Durability Parameters, Post-Congress Report, 13th Congress IABSE, Helsinki 1988., p. 63 - 68 2. `imuni, }.; Pu~, G.: Testing and Dynamic Analysis of the Damaged Pag Bridge, Proceedings, Vol. 1, International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), Berlin 1995., p. 475 - 485; 3. Ukrainczyk, V., Banjad, I., Ukrainczyk, B., Halle, R.; Rational assessment of concrete corrosion in marine environment case study, Croatian National Report, XIII FIP Congress, Amsterdam 1998., p. 149. 160. 4. Bjegovi, D., Krsti. V., Mikuli D., Radi, J., andrli, V.; New Approach in the Ultimate Life Calculation for Cracked Concrete, IABSE Symposium  Extending the Lifespan of structures , San Francisco 1995., Volume 73/2, p. 1259.  1264. 5. `ram,S.: On the reparation of the bridge between mainland and the island Pag, Collana di Ingegneria Strutturale  No. 5, Manutenzione, riparazione e durabilita delle strutture in cemento armato, Udine 1986., p. 315.-344. 6. Radi,J.; `avor,Z.; Pu~,G.; Typical Bridges for New Croatian Highways, FIB Symposium, Prague 1999.  poster, (presented on this Symposium) 7. andrli,V.; Radi,J.; `avor,Z.; Design and Construction of the Maslenica Highway bridge, FIB Symposium, Prague 1999. (presented on this Symposium)  PAGE 6 h~*>Nlxg!) S-abcdtu~ VXx,x׹׭׭׭׭׭׭׭B*CJOJQJhmH nH B*OJQJhmH nH 5B*CJOJQJhmH nH 5B*CJOJQJhmH nH 5B*OJQJhmH nH  OJQJmH CJ 5OJQJ5CJmH OJQJ>hjl*Nxg !) DER$dddhjl*Nxg !) 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