DEGRADATION MECHANISMS AND SERVICE LIFE OF CONCRETE SLABS OF COMPOSITE BRIDGES

Transcription

1 DEGRADATION MECHANISMS AND SERVICE LIFE OF CONCRETE SLABS OF COMPOSITE BRIDGES J.- C. DOTREPPE Laboratory of Bridges and Structural Engineering, Civil Engineering, University of Liège, Belgium Durability of Building Materials and Components 8. (1999) Edited by M.A. Lacasse and D.J. Vanier. Institute for Research in Construction, Ottawa ON, K1A 0R6, Canada, pp National Research Council Canada 1999 Abstract Composite steel-concrete constructions are presently widely used, and certainly in the field of composite bridges where they appear quite competitive. Most of the structural problems related to this type of construction are presently solved, concerning particularly the design of the steel girders. Nowadays studies are focused on the durability of the concrete slab, which conditions the performance of these bridges during their full service life. The model commonly accepted for the description of the corrosion process in a reinforced concrete element is presented. The evolution regarding the problem of the influence of the crack width on the durability of concrete is discussed. The factors leading to cracking of the concrete slab are examined, with special attention to thermal and autogenous shrinkage involving early cracking, and the results of a practical example are presented. The most essential requirements regarding durability are mentioned. Concerning reinforcement of the slab the classical solution consists in using passive steel. However, as the slab is cracked, durability will be controlled by the corrosion development, which leads to uncertainty regarding service life. A satisfactory performance during a sufficiently long period can be ensured by prestressing. Several parameters have to be assessed carefully, such as the type of prestressing and the amount of prestress to be introduced in the slab. Keywords : bridges, composite steel-concrete construction, concrete quality, corrosion development, cracking, passive and active reinforcement, serviceability limit states. 1 Introduction During the last decades the use of composite steel-concrete construction in bridge design has shown considerable extension, which can be explained by several developments and innovation techniques (Johnson and Buckby 1988). The material characteristics have been improved for steel and for concrete. A better understanding of

2 the structural behaviour, as well as improvements of detailing and construction methods must be pointed out. Presently the innovations that have to be brought to bridge construction, and more generally to structural design are related to various criterions such as cost, detailing, durability, aesthetics and environmental conditions. In this article, only the problem of durability will be examined and more particularly the durability of the concrete slab, which cannot be ensured easily, as the concrete deck of a bridge is submitted to severe environmental conditions. The slab can be reinforced by passive or active steel. When passive reinforcement only is used, cracks will open and there is some doubt about durability conditions for a very long period. In Belgium large span composite bridges have been built 20 years ago with passive reinforcement in the slab, and up to now very few in situ observations have been made regarding the degradation of the concrete deck. An investigation program will start in spring 1999 to examine the cracking situation in the slab of some of these bridges. Prestressing of the slab is considered as a promising solution to ensure a satisfactory performance. Despite this there are still discussions going on in Belgium and in some European countries on whether or not it is advisable to use prestressing in future composite bridges, since in this case prestressing is introduced for durability and not for stability purposes. The aim of this paper is first to show the importance of calculating precisely the tensile stresses and cracking development by considering the example of a classical continuous bridge. Another important feature is the economical level of prestress to be prescribed to the practitioners and design engineers. A discussion is presented on this matter and a proposal is made with respect to the various load combinations considered presently in the new European codes. 2 Evolution in the design of composite bridges For large span bridges, the tendency is nowadays to use composite structures. Such structures take advantage of both the lightweightness and the high strength of steel components, and of the low cost of the concrete slab. Forty years ago steel structures were used for large span bridges, with plate or box steel girders and an orthotropic steel deck plate. Because the construction of such a deck is complicated and requires skilled labour, and because the cost of manpower increases continuously, this type of bridge has become extremely expensive. Under the pressure of competition, a concrete slab has been progressively substituted for the orthotropic steel deck plate. Developments in composite bridge design have been accompanied by theoretical and experimental research studies (Lebet l988). The results of these studies have been incorporated in national and international recommendations (Eurocode 4 Part 2: Bridges 1997). The design of composite bridges must meet the two classical requirements: verification of ultimate limit states and of serviceability limit states. Regarding serviceability limit states particular attention is paid nowadays to the durability of the

3 concrete slab. Cracking of the slab is not the only parameter to be considered, but it is one of the most important. The majority of composite bridges have continuous spans. It is common to find bridges with ten or more spans with typical lengths varying between 30 and 90 m. Looking at the classical bending moment diagram for a continuous beam (Fig.1), it can be seen that hogging moments exist near the supports. Therefore tensile stresses should appear in the concrete slab only in these regions. This is not true for composite bridges due to various factors (creep, shrinkage, external temperature variation) that will be examined later on. Therefore tensile stresses and cracks may develop everywhere in the slab, near the supports but also at mid-span. Fig. 1 : Bending moment diagram in a continuous beam 3 Deterioration mechanisms of the concrete slab 3.1 Short considerations on the service life of bridges Ageing and deterioration of bridge structures are unavoidable. Therefore the main task must be to control the types and rates of deterioration. A durable construction implies that this construction does not necessitate important rehabilitation and renovation works. The service life of bridges must be specified. It is presently agreed that this period of time is situated between 50 and 100 years, and close to 100 years. A duration of 50 years is too short and non economical. For most bridges a duration of 100 years seems a little bit too long due to the evolution of the traffic needs, and 80 years is presently considered as an optimum service life. In case of exceptional structures, like the Oresund and Vasco da Gama Bridges, a service life of 100 years or more may be required, due to the difficulty and cost of rehabilitation works. 3.2 Corrosion process in a reinforced concrete element Two dominant factors are involved in most chemical and physical processes influencing the durability of concrete structures: transport within the pores and cracks, and free water. More particularly, in the case of the concrete deck of a bridge the following factors have to be considered: freezing and thawing cycles, effects of de-icing salts, penetration of chemically aggressive agents, alkali-silica reaction (Bulletin d'information N 183 of CEB 1992). It appears therefore that appropriate durability cannot be ensured easily, as the concrete slab is submitted to severe environmental conditions.

4 Corrosion is the most critical degradation process, as it can lead to failure of the element. Therefore the mechanisms of corrosion must be examined carefully. Corrosion is mainly due to carbonation of concrete in relation with penetration of CO 2, and to penetration of chloride ions originating from de-icing salts. Passivation of steel is due to the alkalinity of concrete. In such an environment the corrosion rate is extremely low and steel is therefore protected. The passivity of steel may be destroyed by the carbonation of concrete surrounding the reinforcing bars and by the penetration of chlorides through the pores. For passive or active reinforcement situated near the top or the centre of the concrete slab of a composite bridge, the penetration of the chloride ions is the prevailing action. During the last two decades, designers and researchers have focused on the deterioration processes existing under different types of environment. As a result, rational and scientifically sound models of major deterioration mechanisms, and more particularly of the corrosion process, are available today (Schiessl 1988 ; Andrade et al ; Bulletin d'information N 238 of CEB 1997 ; Liu and Weyers 1998). On the basis of these theoretical and experimental studies, it has been shown that the corrosion mechanisms develop through two different phases as illustrated in Fig.2. In the initiation phase, the metal embedded in concrete remains passive. No deterioration or damage can be seen, but some inherent resistance against the prevailing environment is diminishing or overcome by the aggressive media, e.g. penetration of chlorides or carbonation. These environmental changes lead ultimately to depassivation. In the propagation phase, which begins at the moment of depassivation, corrosion develops, often at an accelerating rate, until a final stage is reached. The corresponding damage to the structure is only visible when this propagation phase has been reached. Fig. 2: Schematic representation of corrosion process Corroded steel develops rust products which expand and cause cracking and deterioration of the concrete cover. A further reduction of the steel area may result in falling concrete fragments. Sudden failure may occur if longitudinal cracking along the bar develops in the region of the bar anchorage. These unacceptable damages usually correspond to the end of the service life of the element (time t 1 in Fig.2).

5 For the design engineer two possibilities can be envisaged: t o > expected service life: this solution is quite safe, since any depassivation of steel is avoided; t 1 > expected service life: the safety level is not known precisely, as it is difficult to assess the propagation period due to the number of parameters involved. 3.3 Influence of crack widths on durability of concrete Twenty years ago it was admitted that crack widths had a significant effect on the corrosion process. There was then a strong gradation of the admissible crack widths in the existing recommendations. There is now a considerable evolution regarding this problem. The observation of existing constructions and laboratory tests have shown that there is no direct relation between the crack widths and the degree of corrosion provided they remain smaller than 0.4 mm approximately (CEB-FIP Model Code ; Favre et al. 1997). However the existence of cracks, even with a small width (0.1 mm), does influence significantly the corrosion process. The diffusion of chloride ions is ten times more rapid in a cracked than in an uncracked concrete (Schiessl 1988). This means that the initiation period will be approximately ten times longer in an uncracked material compared with a cracked one, provided that in both cases the permeability of the material is low and concrete cover is sufficient. Therefore, in an uncracked concrete, t o will be large enough to prevent the steel from reaching the propagation stage during the service life. If concrete is cracked, t o is small with respect to the service life, and t 1 becomes the critical parameter with much more uncertainty regarding structural safety. 4 Development of cracking in the concrete slab 4.1 Early cracking Cracking before hardening is due to plastic settlement and plastic shrinkage. These two phenomenons may induce important cracking, but preventive measures can be adopted in order to avoid them, and it will not be examined here. Cracking occurring just after hardening is mostly due to two phenomenons: thermal shrinkage and autogenous shrinkage. The behaviour of concrete during hardening is important in elements with large thicknesses like dams and bridge piles. For elements with moderate thicknesses (20-30 cm), the temperature differences between the edges and the centre are small. However, if the displacements of concrete during the heating and cooling phases are restrained by another structural element, self-equilibrated stresses appear and may induce early cracking (Emborg 1989). In composite bridges the steel profile is the restraint element, when connection is ensured between concrete and steel during concrete hardening The tensile stresses related to thermal shrinkage are due to the fact that the modulus of elasticity varies considerably between the heating and the cooling phases. Research performed in this domain has shown that the main parameter to be considered is the ratio between the area of the steel profile and the area of the concrete slab (Emborg 1989).

6 Another factor often neglected is the effect of autogenous shrinkage, i.e. shrinkage due to self-dessication. Since its magnitude is of the order of 10-4 for ordinary concrete (w/c > 0.45), it has been ignored for practical purposes. However concrete with low water-cement ratio is used more and more extensively. In this case autogenous shrinkage increases very rapidly, and it can reach 2 to when w/c < 0.40, which can lead to non-negligible tensile stresses and early cracking (Tazawa and Miyazawa 1993). 4.2 Cracking under service conditions There exists at least four causes of cracking under service conditions : drying shrinkage, creep, external loads, external temperature variations (daily and seasonal). Drying shrinkage has the same effect as thermal and autogenous shrinkage, but it develops for a much longer period. Creep does not create tensile stresses, but as it cannot develop freely due to the steel profile, it induces relaxation of the compressive stresses in the concrete slab. External loads such as truck loading may lead to various types of stresses in the deck according to their position along the bridge (Lebet 1990). External temperature variations (daily and seasonal) induce general thermal effects in hyperstatic structures and self-equilibrated stresses in the concrete slab. The determination of these effects is rather involved (Cope 1987). The temperature distribution on a composite cross-section is slightly different from that on a full concrete cross-section. 4.3 Example of a classical continuous bridge In order to evaluate the tensile stresses and cracking development that may occur in the slab, the example of a classical composite continuous bridge with two spans has been analysed (Fig.3). The stress distribution has been calculated precisely using the computer code SAFIR developed at the University of Liège. Values recommended for external temperature variations have been adopted (Cope 1987). Fig. 3: Classical composite continuous bridge with two spans

7 Table 1: Tensile stresses in the concrete slab under service conditions Position in Top stress (MPa) Bottom stress (MPa) the beam perm perm + var perm perm +var Mid-span Support 1.2 (1) 5.1 (1) 2.5 (2) 7.8 (3) 1.7 (1) 4.1 (1) 2.2 (2) 5.3 (3) (1) : permanent loads + early shrinkage + drying shrinkage (2) : (1) + external temperature variation (3) : (1) + variable loads (rare combination ) + external temperature variation Table 1 shows the maximum tensile stresses under permanent and permanent + variable conditions. It can be seen that tensile stresses are present in the slab during the whole life even at mid-span and that cracking may occur both near supports and at midspan. 5. Control of cracking 5.1 Essential measures A few essential measures are required in order to obtain adequate durability of the concrete slab of a composite bridge: to place a waterproof membrane of good quality. to use a concrete with good mechanical characteristics and very low permeability. to provide sufficient concrete cover. to adopt an appropriate concreting sequence for slabs directly cast in situ on the steel structure. 5.2 Passive reinforcement The classical solution is the use of passive reinforcement. In this case cracks will open and it is impossible to limit their width to values < 0.1 mm. As mentioned previously, it is now acknowledged that crack widths up to 0.4 mm do not influence the corrosion development. However adequate detailing regarding the reinforcing bars should be recommended in order to limit the crack width to 0.15 or 0.2 mm. Nowadays many bridges are subjected to very heavy traffic loads. Crack widths may tend to increase, due to progressive deterioration of bond at the boundaries of the cracks. Additional research studies on this matter and observations on existing bridges should be performed. The limitation of crack width can be reached by appropriate measures: a minimum amount of passive reinforcement. a limitation of the tensile stresses in the rebars. a careful study of the spacing between the bars. a limitation of the diameter of the bars.

8 It must be pointed out that bars with small diameters have also some drawbacks. They are more susceptible to corrosion. Furthermore concreting may become more difficult due to the density of steel, which may lead to a diminution of concrete quality. De-icing salts will cause chloride ions to penetrate in concrete. As the slab is cracked, the rate of penetration is very high, depassivation occurs quickly and the durability is controlled by the corrosion development. In these circumstances it is presently not possible to ensure a service life of 80 years. Values situated between 20 and 40 years are sometimes mentioned, but this has to be confirmed by additional research studies. 5.3 Prestressing In order to improve the durability of the slab prestressing can be used. Transversal and longitudinal prestressing have both beneficial effects. In this article only longitudinal prestressing will be considered. Several factors have to be examined carefully. The project must remain economical, as prestressing is introduced for durability purposes only and is not necessary regarding structural behaviour. It is furthermore difficult to calculate the stresses induced by prestressing of the slab. The efficiency of prestressing is reduced by the composite interaction and by the classical time-dependent losses due to creep, shrinkage and relaxation. The type of prestressing must also be examined carefully. Several procedures are used and the various methods have their advantages and drawbacks. 1. Prestressing by jacking supports. In this method the steelwork is raised above its final level before the slab is cast. After curing of the slab the composite section is lowered to its final level. In continuous composite bridges with large spans the required vertical displacement of the steel beams becomes very large and furthermore, with this method, prestressing losses in the concrete slab may be as high as 50 %. 2. Prestressing the slab and steel section (entire cross-section) using cables placed longitudinally in the concrete slab. This method is often used simply to prestress regions near internal supports. Large cracks can occur near anchorages if adequate reinforcement is not provided. Furthermore, if the slab is prestressed over the entire bridge length, losses may approach 50 %. 3. Prestressing the slab only, using cables placed longitudinally in the slab. In this method, the slab is prestressed before being connected with the steelwork. This method is convenient for precast slabs. It is also suitable when the slab is cast in situ if holes are left in the slab around groups of shear connectors during concreting. During prestressing, the slab slips on the upper flange of the steel structure. In this procedure prestressing forces and losses are smaller than those observed when prestressing the entire composite section. However it is difficult to achieve a satisfactory shear connection. 4. Prestressing with external cables using truss action. This method is sometimes used to strengthen existing bridges. In this system cable control and maintenance are facilitated, and replacement and addition of cables may be performed easily. The loss of prestressing due to creep and shrinkage is also minimised.

9 Disadvantages are costly anchorages and cable supports, and application of concentrated forces to the steel profile. It is important to define the amount of prestress to be introduced in the slab in order to ensure sufficient durability for a service life of approximately 80 years. This question has so far not received enough attention. It is examined here with respect to the load combinations now defined in the European recommendations (Eurocode 1 Part ). First of all the initial prestress to be considered has to take into account prestressing losses due to creep and shrinkage. Therefore, for large span bridges, procedures l and 2 mentioned previously should not be recommended, since prestressing losses in the concrete slab are very high. Three combinations of actions for serviceability limit states are defined : quasipermanent, frequent and rare. The quasi-permanent combination for a bridge includes no traffic loads and no external temperature variation. In this case, prestress must be such that under actions existing at any time, i.e. permanent loads including early shrinkage, drying shrinkage and creep, no crack would occur. In this situation cracks will open under variable actions such as traffic loads and variation of external temperature. This type of design can be unsafe, as cracks may be present during rather long periods and heavy traffic loads may cause fatigue effects. Recent research studies have shown that crack widths increase with increasing number of load cycles or with time under sustained loading (Balazs 1993 ; Bulletin d'information N 235 of CEB 1997). Consequently a progressive increase of the crack widths may occur and after a certain time some cracks may remain permanently open. Another combination includes quasi-permanent actions and external temperature variation. As it is not critical, it will not be discussed here. The highest level of prestress is to be applied for the rare combination of actions including external temperature variation. This combination has a return period of more than one year. In this case cracks will open very rarely in limited areas of the concrete deck. This design is suitable for durability purposes, but it is very difficult to introduce such a high level of prestress for technical and economical reasons. In the example described in Fig.2 and Table 1, it has been calculated that a compressive stress of approximately 5.5 MPa should be introduced in the slab in order to fulfil this condition. It is more appropriate to define the level of prestress from the frequent combination of actions including external temperature variation. This combination has a return period of a few days. This appears to be a good compromise between durability and economy. Referring to the same example a compressive stress of approximately 4 MPa should be introduced, which is technically much more acceptable. 6 Conclusions The following conclusions can be drawn from this research study. 1. A few essential measures are required in order to obtain adequate durability of the concrete slab of a composite bridge.

10 2. The causes of cracking have been analysed. With the use of concrete with low w/c ratio, early cracking due to thermal shrinkage and autogenous shrinkage should be considered carefully. Cracking under service conditions has also been examined. It has been shown that cracking may exist during the whole service life of the structure, not only on the supports but also at mid-span. 3. If concrete is cracked depassivation of reinforcing steel will arrive rather quickly and durability will be controlled by the corrosion development. This leads to uncertainty regarding service life. 4. The classical solution is the use of passive reinforcement. Despite the recent studies on the influence of crack widths, adequate detailing regarding the reinforcing bars should be recommended, as the crack widths tend to increase due to heavy traffic loads inducing fatigue effects. Additional research studies and observations on existing bridges should be performed. 5. In order to improve the durability of the slab prestressing can be used. The example of a classical composite continuous bridge has shown the various loads and effects that have to be taken into account, and the way of combining them to calculate precisely the tensile stresses appearing in the slab. Full prestressing will lead to very high compressive stresses and is not economical. On the other hand, to allow crack development for all variable loads may be unsafe due to fatigue effects. Therefore on the basis of the load combinations defined presently in the European codes, it is proposed, in this article, that the frequent combination would be the reference for the amount of prestress to be considered. 7 References Andrade, C., Sanjuan, M.A., Recuero, A. and Rio, O. (1993) Calculation of chloride diffusivity in concrete from migration experiments in non-steady state conditions. Cement and Concrete Research, Vol. 24, No 7, pp Balazs, G.L (1993) Cracking analysis based on slips and bond stresses. ACI Materials Journal, Vol.90, No 4, pp CEB (1992) Durable concrete structures. Bulletin d'information No 183, Comité Euro- International du Béton, Lausanne. CEB (1993) CEB-FIP Model Code 1990, Bulletin d'information No 213/214, Comité Euro-International du Béton, Lausanne. CEB (1997) Serviceability models-behaviour and modelling in serviceability limit states including repeated and sustained loads. Bulletin d'information No 235, Comité Euro-International du Béton, Lausanne. CEB (1997) New approach to durability design - An example for carbonation induced corrosion, Bulletin d'information No 238, Comité Euro-International du Béton, Lausanne. Cope, R.J. (1987) Concrete Bridge Engineering: Performance and Advances. Elsevier Applied Science, London. Emborg, M. (1989) Thermal stresses in concrete at early ages. Doctoral Thesis, Lulea University of Technology. Eurocode 1, ENV (1995) Basis of Design and Actions on Structures-Part 1 :

11 Basis of Design. European Committee for Standardisation, Brussels. Eurocode 4, ENV (1997) Design of composite steel and concrete structures Part 2: Bridges. European Committee for Standardisation, Brussels. Favre, R., Jaccoud, J-P., Burdet, O. et Charif, H. (1997) Dimensionnement des structures en béton Aptitude au service et éléments de structure. Nouvelle Edition, Presses Polytechniques et Universitaires Romandes, Lausanne. Johnson, R.P. and Buckby, R.J. (1986) Composite structures of steel and concrete Volume 2: Bridges. Second Edition, Collins Professional and Technical Books, London. Lebet, J.P. (1990) Composite Bridges. Proceedings of the IABSE Short Course on Composite Steel-Concrete, Brussels, pp Liu, Y. and Weyers, R.E. (1998) Modelling the time-to-corrosion cracking in chloride contaminated reinforced concrete structures, ACI Materials Journal, Vol. 95, No 6, pp Schiessl, P. (1988) Corrosion of steel in concrete. RILEM Report, Chapman and Hall, London. Tazawa, E. and Miyazawa, S. (1993) Autogenous shrinkage of concrete and its importance in concrete technology. Proceedings of the Fifth International RILEM Symposium on Creep and Shrinkage of Concrete, E & FN Spon, London, pp