Summary. 1. Introduction

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1 Application of a life-time management method on existing concrete structures of 25 scholastic facilities by probabilistic estimation of the residual service life Tiziano TERUZZI Ph Head of LTS Laboratory ept. Construction and Environment University of Applied Sciences of Southern Switzerland PO Box Canobbio (CH) tiziano.teruzzi@dct.supsi.ch Ezio CAONI Ph Head of Research ivision ept. Construction and Environment University of Applied Sciences of Southern Switzerland PO Box Canobbio (CH) ezio.cadoni@dct.supsi.ch He received his Ph in experimental physics from Polytechnic of Zurich in His research interests include durability and artificial weathering of buildings materials. He is head of LTS laboratory and professor of applied physics at the University of Applied Sciences of Southern Switzerland He received his Ph in structural engineering from Polytechnic of Turin in His research interests include high strain-rate technology, experimental mechanics, nondestructive testing and concrete. He is professor of structural mechanics at the University of Applied Sciences of Southern Switzerland Summary The paper presents a probabilistic method for the estimation of the residual service life of existing concrete structures subjected to carbonation induced corrosion of the reinforcement. The method was developed and is being applied within the framework of a building management research project addressing maintenance planning of scholastic facilities owned by the Government of Canton Ticino (Switzerland). The results of the application of the method to the concrete façades of an existing sample object are presented and discussed with regard to the actual condition of the façades and with regard to the results given by methods developed for durability design. 1. Introduction uring the last years, investments in the maintenance and rehabilitation of existing structures, as well as in the extension of their operating life, are reaching and overcoming those related to the construction of new ones: this fact implies the development of new competencies, new professional figures and a new political vision of preservation of constructed assets. For these reasons maintenance of buildings has become a topic of major importance, especially in the European countries, because of the generalized ageing of the existing structures and of the necessity to preserve the constructed assets in terms of both efficiency and productivity. The management of ageing buildings requires, on the one hand, tools that should be relatively simple, efficient and reliable and, on the other hand, a global approach. This is the reason why in the research projects in progress at the epartment of Construction and Environment of the University of Applied Sciences of Southern Switzerland, condition assessment is performed on the basis of the analysis of a broad set of elements including, among others, structural elements, technical plants and saving and rational use of energy. For the outer concrete structural elements and façades under investigation, condition assessment data are normally used to calculate the residual service life by means of probabilistic methods. 11

2 Surface degradation due to carbonation induced cracking and spalling of the concrete cover is the principal cause of damage in the buildings examined. The results of in-situ, non-destructive inspections and of laboratory tests performed on concrete cores taken from the objects under investigation allow the obtaining of the distribution functions of the thickness of the concrete cover and of the time-dependent carbonation depth, which are necessary for the probabilistic evaluation of service life using performance principles. The method uses the carbonation process as a degradation model. The parameters defining its precise timedependence have been determined through accelerated carbonation tests performed by exposing noncarbonated cores to a carbon dioxide rich atmosphere and through the measurement of the carbonation depth at the time of the investigation by means of an alkalinity indicator. 2. The framework research project The government of Canton Ticino holds and administers a building asset which comprises more than 8 objects among scholastic and administrative buildings, for an estimated global value of about 2.4 billions of euro. The management of this building asset requires an investment of 1 M each year only for maintenance. In order to increase the efficiency of maintenance and upgrade planning of its building asset, the owner, in collaboration with the University of Applied Sciences of Southern Switzerland, has promoted and initiated a development project of an information system for the documentation of conditions and new demands from users as well as from (inter-)national or regional standards and legislation. Among the capital goals of the research project the following can be put into evidence: to assess the condition of a set of 25 scholastic facilities and to provide elements necessary for the compilation of an information system comprising building user s manual, maintenance manual and inspection/maintenance program. The 25 scholastic facilities were selected so to build a representative sample both in terms of structural typology and of climatic exposure. In fact, the selected facilities are distributed uniformly throughout the territory of Canton Ticino and are located both in alpine and in sub-tropical climate zones in strongly urbanized and rural regions. As far as the structural typology is concerned, the following types were chosen: prefabricated, lightweight, metallic framework, heavy concrete structure, lightweight and heavy brickwork structure ( historic building). Assessment is based on visual inspection, condition appraisal and diagnostics of as much as 5 different elements, like for example: external infrastructures, roofs, floors, façades, structural elements, technical plants. Findings are presented in a report containing a description of degradation and intervention proposals. The analysis of the degradation and the building dimensional coefficients form the basis for an estimation of the building renovation costs. This allows the facility manager to plan the financial requirements on a span of few years, thus allowing him to leave the unpleasant system of breakdown intervention and to pass over to a programmed or preventive maintenance system. Within one of the project sub-tasks, the residual service life of the outer concrete structural elements and façades was evaluated by means of probabilistic methods. Attention was focussed on surface degradation processes due to carbonation induced cracking of the concrete cover. 3. Experimental methods for service life estimation of reinforced concrete structures subjected to carbonation 3.1 eterministic methods The main mechanism governing degradation of concrete façades is corrosion of the reinforcing bars. Failure is assumed to occur when carbonation depth exceeds the thickness of the concrete cover C, > C, since under this condition the reinforcement is depassivated and corrosion is initiated. The degradation model used to describe the carbonation progress into the concrete is generally assumed to be given by the following expression [1].5 ( t) Kt = (1) where is the deterministic carbonation depth at time t and K is the carbonation rate factor. Alternative degradation models are proposed by different authors in [1-4] and in the references therein. The carbonation rate factor is assumed to depend on strength and concrete composition as follows: 12

3 ( f ) b K cenvcaira ck + 8 = (2) where c env is an environmental coefficient, c air is a coefficient taking into account concrete porosity, f ck is the concrete cubic compressive strength, a and b are parameters depending on the binder used for concrete preparation. Numerical values for the different coefficients and parameters entering into equation (2) are given in [1]. Service life is calculated by substituting the carbonation depth with the concrete cover C in equation (1). The expression for the service life t L can thus be written as t 2 C L = (3) K For a not air entrained, Portland cement concrete characterized by a cubic compressive strength f ck of 4 MPa placed in a structure exposed to rain and having a 2 mm concrete cover, equations (2) and (3) give a carbonation rate factor of about 1.2 mm/year.5 and a service life of over 25 years. 3.2 Probabilistic methods Probabilistic methods assume that carbonation depth and thickness of the concrete cover are stochastic quantities. In this case, the failure condition discussed in the previous section is replaced by the probability of its occurrence. Service life is defined as the time at which the probability of failure reaches a maximum allowable value. Mathematically the condition defining service life is expressed as P{failure} = P{(t L ) > C} = P max (4) If the carbonation depth and the thickness of concrete cover are assumed to be both normally distributed, equation (4) can be written as [5, 6] µ ( t ) µ P = { failure } = P{ t } L C L) > C = Φ Pmax ( 2 2 t (5) σ ( L) + σc where Φ is the cumulative standard normal distribution. The task of calculating the probability of failure can be virtually reduced to a degradation problem by substituting the stochastic variable C with a deterministic threshold value C cr. This value can be chosen as the thickness of the concrete cover over which a fraction F of the reinforcing bars lies. Assuming that the thickness of the concrete cover is normally distributed, the threshold value C cr can be determined by the following expression ( F) σc + C 1 C cr = Φ µ where Φ -1 is the inverse of the cumulative standard normal distribution Φ, µ C is the mean thickness of the concrete cover and σ C its standard deviation. In our work we have chosen F =.3, so that expression (2) can be written as C cr. 525σC + µ C (7) The time-dependence of the distribution characterizing the carbonation depth, which we have assumed to be normally distributed, is determined firstly by assessing its distribution at time t by means of measurements on cores taken from the structure under investigation and, subsequently, by calculating the distribution at some later (or previous) time t by means of a degradation model describing the progress of carbonation into the concrete structure. We have assumed that carbonation proceeds with time according to the following model: p ( t ) = Kt µ (8) where µ is the mean carbonation depth, t is the age of the structure, p is an exponent determining the shape of the power-law time-dependence and K = µ (t )/(t ) p is the carbonation rate factor. The exponent (6) 13

4 p tipically lies in the range.15 < p <.5 [6]. In this work, the value of p has been determined through accelerated carbonation tests performed by exposing non-carbonated cores to a carbon dioxide rich atmosphere (see next section). The procedure for determining the time-dependent distribution of carbonation depth, which is assumed to be normally distributed, is shown schematically by the following flux p µ (t) = Kt ( (t ), (t ),..., (t )) ( µ (t ),s (t )) ( µ (t),s (t)) 1 2 N Once the time-dependence of the distribution characterizing the carbonation depth is known, service life can be determined by solving equation (4) for t L given a value of P max. Equation (4) can in this case be written as µ ( t ) C P = { failure } = P{ t } { } L cr L) > Ccr = 1 P ( t L) < Ccr = Φ Pmax ( σ t (1) ( L) In our work, for the maximum allowable probability of failure we have chosen P max = An example of application of the service life estimation method In this section we present the results of the application of the previously described probabilistic method for service life estimation to the concrete façades of a sample scholastic facility [7]. The facility is located in the northern part of Canton Ticino, where environmental conditions are characterized by an alpine climate. Conservation conditions of the concrete façades were assessed by means of visual inspection and of laboratory tests performed on cores taken from the façades (see figure 1). Among the concrete properties which were determined are bulk density, compressive strength, modulus of elasticity and carbonation depth. Bulk density, compressive strength and modulus of elasticity were measured in accordance with the testing procedures defined by the Swiss Standard SIA 162/1, while carbonation depth was determined by means of an alkalinity indicator (phenolphthalein). For each concrete property the corresponding mean value and standard deviation were calculated. The thickness of the concrete cover was measured non-destructively with a reinforcing bar locator. For each façade the number and the spatial distribution of measurement points were chosen so to obtain a statistically representative set of data. For each data set the mean value µ C and the standard deviation s C of the concrete cover were calculated. (9) Fig. 1 Eastern view of the gymnasium (left) and western view of the main building. The exponent p defining the shape of the power-law time-dependence used as degradation model (see equation (8)) has been determined through accelerated carbonation tests performed by exposing noncarbonated concrete cores to a carbon dioxide rich atmosphere according to the testing procedure described in [8]. Conditions inside the testing chamber were such that 1 day of accelerated carbonation is equivalent to about 6.9 years of natural carbonation. Testing duration was as long as 16 days. Results show 14

5 that the degradation model, as given by equation (8), well approximates with p =.5 the progress of carbonation with time. The scholastic facility is located in the village of Corzoneso, in the Blenio valley, at about 58 m above sealevel. It includes two buildings which house the classrooms (main building) and the gymnasium, respectively. The main body was built in 1979, while the gymnasium was built in The main building is a threestorey structure characterized by façades of in situ cast in place concrete (eastern and western façades) and by façades mainly consisting of prefabricated concrete elements (northern and southern façades). The façades of the gymnasium mainly consist of in situ cast in place concrete (figure 1). All façades are exposed to rain. Table 1 Values of service life and of the input quantities used for its calculation. façade m f [MPa] t [y] cond. (m s ) [mm] K [mm/y.5 ] C cr [mm] t L [y] main build., W (CR) (14.9 ± 6.4 ) main build., E SP (23.3 ± 11. ) gym, W OK (14. ± 2.6 ) gym, S OK (15. ± 8.2 ) > 1 gym, S OK (15. ± 8.2 ) gym, E (CR) (16.3 ± 2.5 ) gym, N OK (12. ± 4. ) OK: good conditions CR: cracking of concrete cover SP: spalling (y = year) The values of the mean carbonation depth listed in the fifth column of table 1 show that carbonation rate K is not influenced much by the orientation of the façades. The higher carbonation rate recorded for the eastern façade of the main building is coherent with the corresponding values of the compressive strength, which is rather low. The carbonation rate factor is significantly higher than that which can be calculated with equation (2) for a f ck = 4 MPa, air entrained, Portland cement concrete in a structure exposed to rain. Equation (2) thus does not allow to estimate correctly the rate with which carbonation progresses. Fig. 2 Failure probability vs. time (thick curve) for the upper part of the southern façade of the gym. The pdf of carbonation depth at time t (bell-shaped curve) and the cdf of concrete cover thickness are also shown (thin continuous curve). 15

6 The values of service life presented in table 1 were determined, in a first step, by calculating the timedependence of the failure probability as given by equation (1) and subsequently by searching for which time t L the obtained curve becomes equal to the maximum allowable probability P max. The time-dependent failure probability for the upper part of the southern façade of the gymnasium is plotted in figure 2 (thick curve, C cr = 22.4 mm). In the graph, failure probability curves for other values of C cr are also plotted. The results for service life show rather good agreement with the findings of visual inspection. The high variability of the values of service life is due to variations of both the threshold value C cr and the parameters characterizing the distribution function of the carbonation depth. 5. Conclusions A probabilistic method for the estimation of service life of existing concrete structures subjected to carbonation induced corrosion of the reinforcement was presented. The method requires the distribution functions of concrete cover and of carbonation depth as input quantities. These are obtained by means of non-destructive inspection techniques and of laboratory tests performed on cores taken from the structure under investigation. The application of the method to the concrete façades of a sample objects gives, for the chosen set of parameters defining the failure condition, service lives that are coherent with the damage conditions recorded by visual inspection, thus proving the practicality of the method. The knowledge of the service lives of concrete structures represents a powerful tool that the facility manager has at his disposal for maintenance planning and for determining the most convenient maintenance actions in terms of scheduling and of investment costs. The possibility of applying the proposed method on newly constructed buildings as a way of checking whether the actual service life corresponds to the required service life should be further investigated. 6. References [1] Sarja, A. and Vesikari, E. (ed.), urability design of concrete structures, RILEM Report 14, Chapman & Hall, London, 1996, 161 pp. [2] Hed, G., Service life estimations of building components, Royal Institute of Technology, Centre for Built Environment, Sweden, Report No. TR28, [3] Marigliotta, R., Aït-Mokhtar, A., Rougeau, P. and umargue, P., Concrete carbonation, a prediciting methodology of the front advance in Life prediction and aging management of concrete structures, Proc. 1 st intern. RILEM Workshop (PRO16), Cannes (F), 2, [4] Hunkeler, F. and Hermann, K., Karbonatisierung von Betonen, Cementbulletin, 67 (12), [5] Kottegoda, N.T. and Rosso, R., Statistics, probability, and reliability for civil and environmental engineers, McGraw-Hill International Editions, Civil Engineering Series, [6] Müller, H.S., Günter, M. and Hilsdorf, H.K., Instandsetzung historisch bedeutender Beton- und Stahlbetonbauwerke, Beton- und Stahlbetonbau, 95 (6), 2, [7] Teruzzi, T., Cadoni, E. and Frigeri, G. Evaluation of the residual service life of existing concrete structures: a valuable tool for maintenance planning in Life prediction and aging management of concrete structures, Proc. 2 nd intern. RILEM Workshop (PRO29), Paris (F), 23, pp [8] Piguet, A., Schnellverfahren zur Beurteilung der Betonkarbonatisierung, Cementbulletin, 56 (8), Acknowledgements Thanks are due to the Government of Canton Ticino for the financial support given to the project and to the staff of the Experimental and Technical Laboratory of the University of Applied Sciences of Southern Switzerland for its precious collaboration in the execution of the in-situ and of the laboratory tests. 16