Service life prediction of concrete bridge decks repaired with bonded concrete overlays

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 34, January-February 001, pp TECHNICAL REPORTS Service life prediction of concrete bridge decks repaired with bonded concrete overlays J. Paulsson-Tralla Royal Institute of Technology, Dept. of Structural Engineering, SE Stockholm, Sweden Paper received: October 8, 1999; Paper accepted: February 8, 000 A B S T R A C T Service life predictions with respect to chloride initiated corrosion of repaired concrete bridge decks are of major concern in order to develop cost-efficient repair strategies. A service life prediction incorporates a service life criterion, a concrete cover and a method to predict the chloride ingress rate. All three topics are discussed and quantified in this paper on old concrete bridge decks repaired by water jetting and bonded steel fibre reinforced concrete overlays. The service lives for two bridge decks are estimated. The parameters used are based on comprehensive field studies of repaired decks that have been in service between five and ten years after the repairs. The proposed method is capable of taking the distribution of covers and transport coefficients into consideration and any probabilistic level could be used. All calculations can be made by hand and no subjective decisions are needed. The service life with respect to chloride initiated corrosion was found to be more than 100 years for the repaired concrete bridge decks. Bonded concrete overlays constitute a durable repair alternative for deteriorated concrete bridge decks. R É S U M É En ce qui concerne la corrosion due aux chlorures, les prédictions de durée de vie en service, après rénovation, des tabliers de ponts en béton sont de première importance pour développer une stratégie de rénovation financièrement efficace. Une telle prévision est constituée d un critère de durée de vie, d une couverture en béton et d une méthode pour prédire le taux de pénétration des chlorures. Ces trois aspects sont discutés et quantifiés dans cet article sur les vieux tabliers de ponts en béton, réparés par un giclage d eau et une couverture en béton armé avec des fibres d acier. Ensuite, les durées de vie en service de deux tabliers de ponts sont estimées. Les paramètres utilisés sont determinés à partir d une étude d ensemble de tabliers qui ont été en service entre cinq et dix ans après rénovation. La méthode proposée est capable de prendre en considération la distribution de la couverture et les coefficients de transports ; n importe quel niveau de probabilité peut être utilisé. Cette méthode est facile à employer et à comprendre. Tous les calculs peuvent être faits à la main et aucune décision subjective n est nécessaire, ce qui rend la méthode indépendante de la personne qui réalise la prédiction de durée de vie. En ce qui concerne la corrosion due aux chlorures, la durée de vie en service a été estimée à plus de 100 ans pour les tabliers réparés. Fixer une couverture de béton constitue une alternative durable pour la rénovation des tabliers de ponts en béton qui sont détériorés. 1. INTRODUCTION This paper is part of a large investigation of the longterm performance of concrete bridge decks repaired with bonded concrete overlays. For additional information, see [1-5]. The analytical solution, the error-function, to Fick s nd law of diffusion, may be used in order to predict the service life of a repaired concrete bridge deck with respect to chloride initiated corrosion in a de-icing environment [6]. However, the transport mechanism in a deicing environment is not only diffusion but also convection [7]. The surface concentration of chloride ions (boundary condition) can also double in ten days and also be lowered with 50% in another ten days [8]. For more information about the physical and mathematical aspects, see [9-11]. Predictions using the error-function are, to some extent, subjective since the surface concen- Editorial Note The Royal Institute of Technology is a RILEM Titular Member /01 RILEM 34

2 Paulsson-Tralla tration has to be estimated and the diffusion coefficient may vary and the method requires costly specialist skills. There is, therefore, a need for methods that objectively predict the service life reasonably accurately with limited amount of data. A method based on the moving boundary theory [1] is, therefore, proposed and evaluated. The input data is the resistance of the concrete to the ingress of a threshold value and its distribution and the concrete cover and its distribution. The criterion for failure (threshold value of chloride ions) is also needed and discussed. At least three but preferably six (or more) accurate chloride profiles from the overlay are needed. The method is, therefore, simple, cheap and objective in the sense that nothing has to be guessed. Probabilistic theories can be used and the predictions may be fairly insensitive to however perform them. The prediction is also simple and cheap to update by evaluating new chloride profiles. The method incorporates all environmental effects, all chloride transport mechanisms and any layering of the concrete into one single parameter.. SAMPLING Only two of the eight bridges in the larger investigation will be discussed in this paper. One core from the overlay at Teg Bridge and two cores from Gullmarsplan Bridge were examined after eight years in service. The chloride profiles were obtained by turning down the core (Ø 100 mm). The powder was analysed at 1 mm intervals in the top 10 mm and in mm interval between 10 and 30 mm from the road surface. The total chloride content in the cores from Gullmarsplan Bridge was analysed by the Swedish Cement and Concrete Research Institute [13] and the content in the core from Teg Bridge was analysed by the Dept. of Building Material, Chalmers University [14]. The concrete powder was dissolved with acid. The chloride content was measured with a chloride ion selective electrode and the calcium content was measured by EDTA-titration. The errors in the chloride and calcium detection methods are small, ± 0.5% [13]. The influence on the estimated service life is, therefore, small. The measuring accuracy for the distance from the road surface to the sampling point was ± 1.0 mm. 3. SERVICE LIFE CRITERION (SLC) The SLC used with respect to chloride initiated corrosion is usually a chloride content (threshold value) at the level of the reinforcement (percentage of cement by weight). It is the free chloride ions that are believed to initiate the corrosion process [15] but the separation of free and bound chloride is hard. Anyhow, the acid soluble chloride content is easy to measure and the correlation between the chloride content and the onset of corrosion is fairly clear [16]. 3.1 Factors influencing the service life criteria for a structural member The SLC for bridge decks is discussed from a material, environmental and structural view. The consequences of an eventual failure are hardly ever incorporated in the SLC but this is the basic issue to discuss. The risk could be defined as the probability for an event to occur times the consequence of the event [17]. If the consequences are small, i.e., the risk of damage to man and/or property is low; the SLC could be higher than if the risk of damage is large. One way of dealing with this is to use the coefficient of safety [18]. If the SLC is increased by 50%, the predicted service life will increase by 100%, roughly speaking Function and location of reinforcement Fagerlund [1] concluded that since corrosion (carbonisation) first affects the bond and first at later stages the cross section of the reinforcement, the function of the reinforcement has to be analysed. If corrosion occurs where the reinforcement carries tensile forces, partly loss of bond will only result in increased deflection, and a failure will be announced in advance. If the corrosion instead occurs at the anchorage zone, the loss of bond will cause failure of the element. Different SLC may therefore be needed for the same element Size and location of the damaged area The size and location of the damaged area should be taken into consideration. Paulsson [1] showed that the service life for repaired decks was more than 100 years below solid overlays but it was reduced to 30 to 40 years below deep cracks in the overlays. The cracks were few and easy to survey and the SLC may, therefore, be increased in these zones Type of member Teg Bridge is a continuous steel girder bridge with reinforced concrete bridge decks designed without composite action. The deck is continuous and reinforced in two directions in both the upper and lower part. Loss of moment capacity in one zone of the deck can be compensated by moment capacity in adjacent zones. Membrane action [19] or dome action [0] or arch action [1] may increase the load carrying capacity with a factor of two to three. Bridge decks are often secondary load carrying members and failures will not be progressive but local with little impact on the structure. A higher SLC could, therefore, be used on continuous bridge decks on girders than for primary load carrying members Punching failure Concrete bridge decks may fail due to punching action, see Fig. 1. Many bridge decks have adequate punching capacity even without reinforcement. The punching capacity increases with increased height and if the overlay is a steel fibre reinforced concrete the punching capacity increases even more []. The Swedish Bridge Code [3] allows a bonded overlay to be taken into static considera- 35

3 Materials and Structures/Matériaux et Constructions, Vol. 34, January-February 001 Fig. 1 - Influence of the bonded overlay and eventual damages on the punching capacity. Fig. Bending (positive) moment. The capacity increases due to the increased depth. Fig. 3 - Bending (negative) moment. Crack increases the ingress rate of chloride and water. tion if the member is secondary load-carrying Flexural failure Corrosion in the lower reinforcement can cause flexural failure (positive moment), see Fig.. The attacks will be concentrated to construction joints in the old deck. The loss of bond due to corrosion will be minor since the anodic zones may be small and the cathodic zones large in the old concrete [4]. The deck may therefore not de-laminate. The overlay has also increased the effective depth of the cross section by 40 to 60%. If the effect of the upper reinforcement is taken into consideration the moment capacity may have increased by 100%. Some cross section reduction of the reinforcement may therefore be accepted. Corrosion in the upper reinforcement may cause failure due to negative moment, see Fig. 3. The attack will be initiated by chloride ions in zones with deep cracks. If reinforcement is placed in the overlay (stainless steel if deicing agents are used frequently), the moment capacity could be increased. It will also decrease the crack widths Moisture variation, moisture state and resistivity The moisture variations were low at the level of the reinforcement in the old concrete [5]. Low moisture variations may increase the threshold value [6] and [7] and decrease the corrosion rate [18]. The relative humidity in outdoors balcony slabs [8] and bridge decks [1] and [4] is rarely less than 85% and that the average is 90%. This is where the corrosion process usually has the maximum rate [9]. The use of the later propagation stage in the service life predictions should therefore not be used. Parts of elements with low resistivity had higher corrosion rates than zones with low resistivity [30]. The reinforcement in the repaired bridge decks is mainly located in the old concrete where the moisture state and the high w/c-ratios (0.5 to 0.65) will cause low resistivity of the concrete and the corrosion rate may therefore be high Oxygen and temperature Corrosion in cracked concrete (w/c-ratios 0.5 and 0.6) subjected to cyclic wetting and drying of water containing 1% NaCl and fresh water is cathodic controlled [4] by the oxygen. If the oxygen supply is low, a higher SLC could be chosen than if the oxygen supply is high. Since the overlays in this paper have large covers and w/c(b)-ratios of 0.33 to 0.36, the corrosion rates for reinforcement in the overlay are expected to be low but high for reinforcement in the old concrete with oxygen supply from the soffit too. Structures located in a cold environment may have both lower ingress rate and higher threshold values [7]. Al-Khaja [31] showed that an increase of the temperature from 0 C to 40 C increased the chloride ingress rate by 100% Presence of defects at the steel-concrete interface and amount of reinforcement Sandberg [7] concluded that defects at the steel-concrete interface lowered the threshold value. The overall quality of the concrete is, therefore, important. The ratio of reinforcement has been found to affect the decrease in the moment capacity of a corroding member [18] and [30]. Members with ratios less than 0.5% was found to be more ductile than members with high ratios even though they lost similar fractions of the reinforcement. Concrete bridge decks usually have smaller ratios than concrete beams and a higher SLC may therefore be used for concrete bridge decks. 3. Proposed service life criteria The SLC used is the acceptable total chloride concentration, see Table 1. The chosen SLC are supported by [16, 3, 11, 3 and 33]. 4. CONCRETE COVER In new bridges the existing cover could be checked before the concrete is cast. For existing structures, the concrete cover has to be estimated with the help of blue 36

4 Paulsson-Tralla prints, regulations and physical measurements. For a waterjetted bridge deck, overlaid with a bonded concrete overlay, the prediction is even harder than for a nonrepaired structure. The uncertainties increase due to the fact that some concrete has been removed and new concrete has been installed. Most of the uncertainties could be avoided if the contractor documented the real height. The height could be estimated by coring. 4.1 Literature study Table 1 SLC for different types of structures Type of member Secondary load Statically SLC % Cl/C Concrete cover carrying member indeterminate Table Height of overlays in the investigation Bridge Min Min Max Mean STD 5% STD of 5 % percentile No. of blue print percentile mean of mean samples mm mm mm mm mm mm mm mm Södertälje (68) 3 83 (83) 15 Mälsund (100) (116) 15 Överboda (56) 3 71 (70) 17 Bjurholm (30) 5 61 (61) 0 Viktoria (51) 6 81 (80) 16 Teg (59) 3 75 (75) 18 Långhals (86) (104) 17 Gullmarsplan (76) (136) 97 1 No information regarding thickness of overlay found in blue prints but 60 mm could be assumed. Data on concrete covers and their distribution on old bridges are scarce in Sweden. An investigation on new bridges [34] concluded that the spread was large. The tolerances were used systematically and not sporadically as intended. The covers were often less than prescribed minus the tolerance. Matsushima et al. [33] concluded that the lack of depth of the concrete cover had a mean value of 4 mm and a STD of 5 mm for slabs. Clark et al. [35, 36] presented results from a survey and the overall result was depressing. The defined variables in Fig. 4 (original concrete cover, depth of the removed concrete and depth of the overlay) are treated as stochastic variables with normal distributions, which is supported by [37]. They are also assumed to be independent and have known STD. The concrete covers in the old deck shown are representative to bridge decks in Sweden [38]. The spread in the removal depth is based on observations in field by the author and in laboratory [39, 40]. The approach is simple but it avoids over-sophistication. The data input is the main source of error. The examples in Fig. 4 show that it is the depth and the spread of the overlay that affect the final cover. If the STD is assumed to be 30 mm, the minimum cover of the repair will be 57 mm in 95 % of the bridge deck. If the STD is decreased to 15 mm the minimum cover will be 79 mm. If the service life is assumed to be proportional to the square root of the cover, it would increase by approximately 90% ((79/57) ) due to less spread. The heights of the overlays for the bridges in the investigation are shown in Table. The 5% percentiles were obtained assuming a normal distribution and the fact that the STD was known. The values in the parenthesis were obtained with the t-distribution, which should be used if the STD is not known but the influence is small. The normal distribution was generally a fair approximation for all eight bridges. In Fig. 5, the histogram of the relative and the cumulative frequencies are shown for the depth of the overlay at Teg Bridge. The spread was small and the prescribed minimum thickness was achieved. The depth of Continuous bridge deck on girders Yes Yes 0,6 5 % percentile of mean Concrete flat slab No Yes 0,4 5 % percentile of mean Simply supported concrete slab No No 0,3 5 % percentile of mean 4. Estimation of concrete covers Fig. 4 - Definition and example of the variables used. 37

5 Materials and Structures/Matériaux et Constructions, Vol. 34, January-February 001 Fig. 5 - Relative and cumulative frequency for the overlay depth on Teg Bridge. All properties of concrete change with time and since different parts of the concrete overlay are exposed to different environments, the properties change differently in time. The third and fourth conditions are, therefore, not fulfilled. The binding of chloride ions is dependent on the chloride concentration. The fifth condition is, therefore, not fulfilled. Considering the two paragraphs above, the moving boundary theory seems to be unsuitable. Nevertheless, the simplicity of the method makes it attractive to use especially if chloride profiles obtained at different occasions from the same structure are available. Fig. 6 Chloride profiles from Gullmarsplan Bridge and Teg Bridge. the overlay should be as even as possible to achieve a cost efficient repair [41]. 5. ESTIMATION OF SERVICE LIFE WITH THE MOVING BOUNDARY THEORY 5.1 Examples In Example 1 the service live for Teg Bridge is predicted. The prediction is based on one accurate chloride profile obtained from Teg Bridge, see Fig. 6, where also two profiles from Gullmarsplan Bridge (GPL) are shown. The core from Teg Bridge was taken in late July after eight years in service and the outward transport of chloride ions during the spring and early summer is clear. The first core from Gullmarsplan Bridge was taken in early May and the second in mid October. The cores were drilled 300 mm from each other. The outward transport during spring, summer and early autumn is clear to at least 15 mm from the surface. The service life can be estimated by a fast, simple and cheap method if the chloride threshold value is assumed to be transported into the concrete as a moving boundary. Several conditions have to be fulfilled in order to apply the moving boundary theory [1]: 1. The binding of the penetrating substance occur instantly.. No counter-diffusion of material that may bind the penetrating substance exists. 3. The diffusion coefficient is constant in time and space domain. 4. The moisture state in the overlay is constant in time and space domain. 5. The penetrating substance is bound at the front. The first condition is not fulfilled since only a part of the chloride ions is bound. The second condition is neither fulfilled since counter diffusion of calcium hydroxide, potassium ions and sulphate ions may occur and they may interact with the chloride ions. Example 1 x= k t k = x = Distance from the surface to the threshold value, m k = Coefficient, m/year0.5 t = Exposure time, year Chloride profile at Umeå Bridge % Cl/C, x = 0.01 m, t = 8 years, CC 0.6% = m (5% percentile of mean values) 06 k.% year = = m/year. SL 06.% 8year = CC k x t 06.% 8 06.% year = 74, 10 CC 0.6% = Concrete cover using a SLC of 0.6% Cl/C at the 5% percentile of mean values, m 3 = 100 years 38

6 Paulsson-Tralla Table 3 - Data from Sandberg [7] Type of binder w/c- Exposure t x k 1 CC 1 ratio 0.6 % Cl/-front 80 years year m m/year 0.5 m Sulphate Resistant Cement (SRC) 0.4 Submerged k 0.6% 8 year = Coefficient using a SLC of 0.6% Cl/C and the distance to the front after 8 years, m/year 0.5 SL 0.6% 8 year = Service life using a SLC of 0.6% Cl/C and k using the distance to the front after 8 years, year. The service life for Umeå Bridge is more or less similar to the results using the error-function [1], which indicates that the two methods may give similar results. However, the method used above is a lot simpler and, in the author s opinion, a lot safer to use, since neither the surface concentration nor the diffusion coefficient has to be estimated. All calculations can also be made by hand and the money saved by this could instead be used to take more samples Submerged SRC and 5 % silica fume 0.4 Submerged Ordinary Portland Cement (OPC) 0.4 Air OPC 0.4 Splash OPC 0.4 Submerged Calculated covers needed for 80 years service life using data from [7] Influence of exposure duration Sandberg [7] presented chloride profiles evaluated after different exposure duration. Samples were kept in the seawater, in the splash zone and in the air. The author used the results [7] and the proposed method to evaluate how k changes in time. The spread is small for the submerged samples, which indicates that the method may predict the ingress rate of the 0.6 Cl/C front accurately. The spread for the samples located in the air on and in the splash zone is larger than for the submerged samples, probably due to variations in the microclimate [7]. The spread seems to decrease with age. 5.3 Effect of, k, and the spread of the concrete cover Troive [41] estimated the service life of concrete structures using a density function of measured concrete covers and the error-function. The cover was assumed to be normal distributed with known STD. The coefficient, k, was assumed to be deterministic and the service life was estimated with the same type of equation as the author used. However, Troive [4] derived a density function for the service life where also k was incorporated. f SL sl ( ) = k sl π σ e k sl+ µ σ cosh k sl µ σ Fig. 7 - Density functions of the service life for Teg (Umeå) Bridge. f SL (sl) = Density function of the service life sl = Service life, year k = Coefficient, m/year 0.5 σ = Standard deviation of concrete cover, m µ = Mean value of concrete cover, m. The density functions for Teg Bridge, using the STD of the single values of the covers are shown in Fig. 7. The coefficient, k, was varied between and m/year 0.5 using a step of m/year 0.5. The positive effect of decreasing k is clear. The service lives, using the 5% percentile of both the single values and the mean value of the concrete cover, are shown in Fig. 8. The results using both Troive s method and the proposed method are shown. The spread of the service life decreases considerably using the small STD of the mean value and the service 39

7 Materials and Structures/Matériaux et Constructions, Vol. 34, January-February 001 Fig. 8 - Service life as function of k for Teg Bridge. life, therefore, increases. The effect of the spread was more or less the same as in Examples and 3 where the method proposed by the author was used. The observation indicates that the simple method proposed by the author could be used instead of the more advanced presented by Troive [4]. The differences in the estimated service lives were small, see Fig Distribution of k The coefficient k is a stochastic variable. However, the density functions of k are unknown. Pentti [37] conducted an investigation where the carbonation depths for elements stored outdoors were measured. The transport coefficient was normal distributed. Tables 4 and 5 show an example, based on a simulated investigation after eight years, where the distributions of k and the cover are used. Six cores were drilled and the concrete covers and the depth to the 0.6 % fronts, x, were measured and k was calculated. The cover and k were assumed to be normal distributed with known STD. The characteristic values of the cover and k could then be established. The author has used the 5% and the 10% percentiles for the concrete cover and the 95% and the 90% percentiles for k. The service life for the concrete bridge deck was then calculated, see Table 5, using three different SLC depending on the type of member as previously discussed. The calculated service lives differ considerably and the service life was 8% less for a primary load carrying indeterminate member than for a secondary load carrying one. The example clearly shows that the method is simple and fast to use. Any risk level can be chosen by the bridge keeping authority and compared with the cost (service life), which is important in order to provide cost efficient structures [43]. The service life prediction can later be updated. The new measurements of the concrete cover can then be added to the old investigation and the information base can be extended. The example also shows that the distribution of the coefficient affects the predicted service life and that it can easily be incorporated in the service life prediction. Table 4 - Result from a simulated field investigation Sample CC x k m m m/year Mean STD Characteristic 5% / 95% Characteristic 10% / 90% Table 5 - Service lives of different types of concrete members Type of member CC 5% CC 10% k 95% k 90% SL m m m/year 0.5 m/year 0.5 year Secondary load carrying and indeterminate Primary load carrying and indeterminate Primary load carrying and indeterminate Primary load carrying and determinate CONCLUSIONS 1. There is a need for probabilistic methods to predict the service life of concrete structures.. The SLC can and need to be differentiated for different types of structures. 3. The depth of the overlays can be considered to be normal distributed. 4. The method proposed and used by the author is easy to use, cost effective and flexible. 5. The distribution of the used parameters can be incorporated and any SLC and risk level can be used and the type of structure can easily be taken into consideration. 6. Since the service life can be predicted, the annual cost can be estimated. The cost versus risk can, therefore, be compared for different strategies. 7. The predicted service lives using the proposed method is close to the service life predicted with the error-function. However, no expert is needed for the calculations. ACKNOWLEDGEMENTS Appreciation is expressed to the Swedish National Road Administration for providing funding for this project and to Assoc. Prof. Johan Silfwerbrand for support and advice throughout the project.

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