A STRENGTH DEGRADATION MODELING APPROACH TO LIFE EXPECTANCY OF FRP STRENGTHENED BRIDGES

Transcription

1 A STRENGTH DEGRADATION MODELING APPROACH TO LIFE EXPECTANCY OF FRP STRENGTHENED BRIDGES A. Sawant 1, J.J. Myers 2 1 Graduate Student, Dept. of Civil, Arch. & Envir. Engr., Center for Infrastructure Engineering Studies (CIES); University of Missouri-Rolla; Rolla, Missouri, USA. 2 Associate Professor, Dept. of Civil, Arch. & Envir. Engr., Center for Infrastructure Engineering Studies (CIES); University of Missouri-Rolla; Rolla, Missouri, USA. ABSTRACT The use of fiber reinforced polymers (FRP) in repairing and strengthening bridges has been an active area of research and implementation in recent years. In particular, adhering FRP to the tension face of reinforced concrete (RC) beams has provided an increase in load carrying capacity and extended the service life of the structures. However, the life expectancy of this technology has not yet been fully investigated or documented due to insufficient data. In this paper, the authors present one possible approach from a strength degradation approach using an analytical modeling approach to the life expectancy of FRP strengthened bridges. An actual CFRP field strengthened bridge in Dallas County, Missouri, USA is utilized to demonstrate the life expectancy approach. Keywords: reinforced concrete, strengthening, repair, life expectancy, bond degradation, freeze and thaw etc. 1. INTRODUCTION 1.1 General The repair and strengthening of bridges has become a major problem for civil engineers in the past few decades. Recently, bridge designers have been faced with the challenge to remedy the increasing number of deteriorating bridge decks. According to an article from Concrete International, Concrete bridge decks deteriorate faster than any other bridge components because of direct exposure to the environment, deicing chemicals, and ever increasing traffic loads (Benmokrane et al., 2004). In the United States, 100 billion dollars per year is expended due to corrosion alone. According to National Bridge Inventory it was stated that over 28% bridges are classified as either structurally deficient or functionally obsolete (NBI, 2002).

2 To address this problem, new techniques for the repair and strengthening of bridge girders have evolved. Bonding of fiber reinforced polymers (FRP) to reinforced concrete (RC) members for upgrade provides a high strength-to-weight ratio material that is corrosion resistant, lightweight with nonconductive properties. 1.2 Scope of Work The use of fiber reinforced polymers (FRP) in repairing and strengthening bridges has been researched in recent years. In particular, attaching FRP to the tension face of reinforced concrete beams has provided an increase in stiffness and load capacity of the structure (Meier et al., 1991). One of the most important criteria in the design of FRP strengthening system is the required life expectancy of the system. However, as yet, long-term field performance data does not exist due to the short period since the first CFRP application to a concrete bridge was carried out in Limited research in this area has been conducted & preliminary mathematical models are available to be able to predict the long-term characteristics of fibers, resin, existing concrete & bond between FRP and concrete. Consequently, it is important from both a safety and cost-benefit perspective to develop a methodology or guideline for the life expectancy of FRP strengthened bridges. The primary object of this work is to evaluate the life expectancy of repairing/strengthening of bridge girders with CFRP sheets. The main components of this investigation included a literature review, laboratory testing to determine chloride ingress for the case study bridge girder, and field testing to determine the bond strength between CFRP & existing concrete. This paper will present the current work undertaken to date. As described later in this paper, several assumptions are utilized and noted in the characterization by the authors to present a life expectancy estimate for a case study bridge in Missouri, USA. 1.3 Case study of Bridge in Dallas County MO (P-0962) Bridge P-0962, located in Dallas County, MO, USA is summarized. As depicted in Fig. 1 and 2 the bridge consists of three m (42.5 ft) spans [total length = 38.9 m (127.5 ft)] with a deck width of is 7.2 m (23.75 ft). Each span consists of three reinforced concrete (RC) girders monolithically cast with a 152 mm (6 in.) deck slab and mid-span transverse beam. 2. LITERATURE REVIEW 2.1 General The method of strengthening concrete structures with externally FRP composites has become increasingly popular among researcher & engineers. When compared with more traditional techniques (e.g., bonding of steel plates, post-tensioning etc.) it offers the advantage of protection against corrosion & efficiency of application. Extensive research has shown that externally bonded CFRP laminates improve both the short-term (Triantafillou and Plevris, 1992) and long-term behavior (Shahawy and Beitelman 1999) of concrete girders. Based on the research conducted so far, ACI committee 440 is currently developing design guidelines for external strengthening of concrete structures using fiberreinforced polymer systems. With the exception of a few studies, most of the research conducted on CFRP-strengthened structures has been done in a deterministic manner, and

3 the statistical variations associated with the main design variables have been largely ignored. Fig. 1: Bridge P-0962 Fig. 2: Superstructure of the Bridge FRP s have been in use since the 1940 s. Due to the incurring of very heavy financial costs, however, the application of FRP was limited to the aerospace and defense industries. To meet the higher performance challenges of space exploration and air travel in the 60 s and 70 s, fiber materials with higher strength, higher stiffness and lower density (such as boron, aramid and carbon) were commercialized. During the 1970 s, research was channeled to developing ways to improve the cost of high performance FRP s. By the late 1980 s and early 1990 s, the defense industry waned and emphasis was now placed on cost reduction and the continued growth of the FRP industry (Bakis et al. 2002). Although FRP s have had a long history, it is only in recent times that it has won the attention of Civil Engineers as a potential alternative to more conventional structural materials. Throughout the 1990 s, various industries have financed demonstration projects and sponsored research programs on this burgeoning field. As research continues, FRP materials are now finding wider acceptance in the construction industry. 2.2 Mechanism of Corrosion Corrosion, in general terms, means deterioration or destruction of a material due to a reaction with its environment. Corrosion of a metal can be defined as a chemical reaction which returns the metal to compounds that are similar to the minerals from which it was extracted in the first place. Some call metallic corrosion extractive metallurgy in reverse. Nearly all metallic corrosion processes are electrochemical in nature; they involve water that is in the liquid or vapor phases. The chemical reaction of a corroding metal occurring at the metal-liquid interface can be written in a form of anodic and cathodic reactions. An anodic reaction, oxidation, produces electrons. The anodic site is the place where corrosion of metals occurs. The cathodic reaction, called reduction, consumes electrons. The anodic and cathodic reactions for a metal (M) are as follows: Anode: M M e - (1) Cathode: 2H + + 2e H 2 for ph < 7 (2) O 2 + 2H 2 O + 4e - 4OH - for ph > 7 (3) For corrosion to proceed both oxidation and reduction must occur simultaneously and at the same rate. Also, the electrolyte must allow for movement of cations from cathodic sites to the anodic ones and anions in the opposite direction. Finally, the anode and cathode must be electrically connected to allow current flow. 2.3 Bond Behavior between FRP and Concrete In regions where FRP sheets cross a crack, tensile stresses resulting from shear cracking must be transferred to the concrete. Strength capacity is generally limited more by the bond

4 between the FRP and the concrete than by the tensile capacity of the laminate. Bond may be developed by any of four mechanisms of adhesion: mechanical interlocking, diffusion, electronic, and absorption, with mechanical interlocking by far the most prevalent mechanism of adhesion (Karbhari et al., 1996). Mechanical interlocking occurs because of the linking of uneven surfaces concrete and epoxy wherein epoxy occupies voids on the concrete surface. A roughened concrete surface, free from loose material is essential in providing good mechanical interlocking. Karbhari et al. (1996) give five possible modes of failure between the FRP and concrete substrate: peeling within the concrete substrate, interfacial failure between the concrete and adhesive surfaces, cohesive failure within the adhesive, failure between the adhesive and concrete surfaces and finally failure that alternates between the two surfaces. Most common failure modes in the literature for bonded specimens include shearing of concrete beneath the glue line and FRP failure after development of its full tensile capacity. The typical assumption of perfect bond between the FRP and concrete surface was found to be initially justified in the linear elastic range of the concrete (Lee et al., 1999). After the concrete cracks, there is a load level at which the main at the crack reaches a peak and begins to decrease abruptly (Bizindavyi and Neale, 1999). At the same time the shear stress in the region adjacent to the crack begins to increase. The decrease in shear stress near the crack is attributed to distribute cracking in that region, resulting in shear stresses being increased to the adjacent region. This finding was similar to that of Maeda et al. (1997) who found that, in the initial stages of loading, strain is limited to a region within the effective bond length. Once delamination progresses in this area, the area of active bonding is shifted further away from the crack. This phenomenon progresses until the entire sheet is debonded, often rapidly. This failure mechanism is reported by Täljsten (1997) to be governed by tensile strain in the concrete, in tests of bonded steel and CFRP plates. The concrete begins to fracture of about 0.8 to 1.0 mm (0.3 to 0.4 in.) as measured on the plate above the area of debonding. 3. ANALYTICAL MODELING 3.1 Corrosion of Reinforcement The appearance of premature and unexpected corrosion damage in reinforced concrete structures, which at the time of construction were considered of almost unlimited duration, led to the introduction of the concepts of durability and service life in the 1970s. The service life of the structure can be defined as the period of time in which it is able to comply with the given requirements of safety, stability, serviceability and function, without requiring extraordinary costs of maintenance and repair. In this model only chloride induced corrosion was taken into consideration. Carbonation induced corrosion was not considered because the concentration carbon is very low in the rural environment ( 0.03%) as compare to the urban environment ( 0.1%). The case study bridge is located in a rural area in central Missouri. When corrosion is caused by chloride ingress, the service life is usually assumed to be equal to the initiation time: t 1 = t i. The period of propagation, which may be of short duration, is traditionally not taken into account because of the uncertainty with regard to the consequences of localized corrosion. The initiation time (t i ) may be calculated as a function of: the chloride transport properties of concrete (usually the apparent diffusion

5 coefficient), the surface chloride content dictated by the environment, the thickness of concrete cover and the critical chloride content determining the onset of corrosion. The arrival of the critical chloride content at the steel at depth x at a time t is calculated using Fick s second law of diffusion. Using this type of calculation, it is possible to find values for D app (assumed constant), which can be use to obtain a particular service life as a function of thickness of the concrete cover and the critical chloride content, assuming a fixed chloride surface content. The time to initiation of reinforcement of corrosion was determined using the following: 2 D 1 d 1 2 c c 2 1 cr T ( o ) erf I (4) 4D c c c ( i o ) T 1 = Time for Initiation of Concrete (d 1 -D 1 /2) 2 = Clear cover to Reinforcement = 44.5 mm (1.75 in.) D c = Chloride Diffusion coefficient = m 2 /sec erf = Error function varies between 0 to 1, Assume 1 to be on safer side C cr = Critical Chloride Concentration (from chloride testing) = 0.034, as a % of cement weight C o =Surface OR equilibrium chloride concentration depends upon relative humidity = 1.0, as a % of cement weight C i = Initial chloride concentration = 0 From the above parameters, an initiation to corrosion time of T i =17 years was obtained. Fig. 3 illustrates the resulting corrosion degradation model for the Dallas County Bridge. It may be noted that one trend line represents the steel loss without consideration for the effects of FRP strengthening on corrosion rate while another includes the effects with FRP. D t D t I corr = D i (T Ti) I corr, where, T > Ti (i.e. more than 17 years); = diameter of bar at a particular year; D i = initial diameter of bar = Corrosion rate = 500μm/y. A (t) = n (D (t)) 2 ) π/4 Note: is the coefficient of corrosion. A(t)/A(o) FRP Strengthening, Year 46 Corrosion initiation, Year No. of Years Corrosion Rate With FRP Corrosion Rate without FRP Fig. 3: Reinforcement area A(t) as function of time. 3.2 Bond degradation between FRP and Substrate Concrete % capacity of Beam Bond Degradation Rate Utilized in Life Expectancy Model Bond Degradation Based on Laboratory Data No of years Fig. 4: Reduction in load carrying capacity due to freeze-thaw cycles. The author s modeling for bond degradation presented in Fig. 4 was based on correlating laboratory results conducted at Queen s University and published by Bisby and Green

6 (2002) to historic ambient conditions in Dallas County, Missouri, USA. Bisby and Green observed an approximate reduction in load carrying capacity of externally CFRP laminated beams around 5% after 50 freeze and thaw cycles. After 200 freeze-thaw (FT) cycles, a reduction in the load carrying capacity was observed at around 10%. These losses were attributed to bond degradation. In their laboratory test, one freeze-thaw cycle was considered from -15ºC to 20ºC (5ºF to 70ºF). Data collected from the National Oceanic & Atmospheric Administration (NOAA) website for more than 30 years at the bridge site was used to determine the average number of freeze-thaw cycles at the location of the case study bridge to develop the bond degradation model. 200 FT cycles were equivalent to 40 years of service exposure based on analyzing site weather data. 3.3 Combined effect of above on strength degradation To obtain a combined effect the following approach was used. First, the nominal moment capacity of the girder with existing reinforcement was calculated. Then, the corrosion model was applied to the girder. This corrosion model was applied before strengthening of the beam with external CFRP. Once the strengthening of the beam was completed, the capacity of the beam increased to satisfy the new design requirements. After strengthening the combined effect of bond degradation and reinforcement corrosion was taken into consideration. A number of studies have indicated that FRP strengthening significantly slows the corrosion process; therefore, the corrosion rate was adjusted accordingly after FRP strengthening (see Fig. 3). Within this approach, CFRP material degradation was not considered by the authors as most studies have indicated that degradation of the carbon material itself is minimal for the exposure conditions of the bridge. Should another FRP material be used, such as glass, the material degradation would be an important consideration to incorporate in the degradation modeling. In the following section a case study for the Dallas County Bridge is detailed with the above approach Case study of Bridge in Dallas County MO (P-0962) The following presents the characteristics of the Case Study Bridge and approach undertaken to determine the ultimate capacity of a typical exterior girder with the implementation of degradation considerations for corrosion and bond degradation. To determine the ultimate strength of the girder prior to FRP strengthening, the ACI generalized approach with one modification was used. A correction factor, ψ cor, was introduced to consider the loss of steel due to corrosion based upon the model presented in Fig. 3. Eq. 5 was used to determine the moment capacity of the section. a Mn = ψ corasfy d 2 (5) Af a where, a = ; c = 0.85f b β s y ' c eff After FRP strengthening, the ACI 440.2R-02 generalized approach was used with modifications to consider bond degradation and steel loss. For loss of steel due to corrosion after FRP strengthening the authors assumed a steel loss rate of 0.65g/day (0.023 oz/day), based on a laboratory study conducted at University of Missouri Rolla by Bae et al. (2005). The moment capacity expression is detailed below that was used to determine the moment capacity of the section considering both corrosion and bond degradation: 1

7 1c 1c M β β n ψcorasfy d = + ψbψfafffe h 2 2 Notations: A s = area of non-prestressed steel reinf., mm 2 A f = area of FRP reinf., mm 2 f c = compressive strength of the concrete, MPa b eff = effective width of compression block, mm a = depth of compr. block, mm c = dist. from extreme compr. fiber to n.a., mm d = dist. from extreme compr. fiber to cgs, mm (6) f y = specified steel yield strength, MPa β 1 = ratio of depth of the equivalent rectangular stress block to the depth of the n.a. ψ cor = steel loss factor based on Fig. 3 ψ b = bond reduction factor based on Fig. 4 ψ f = ACI FRP strength reduction factor The loading and corresponding moments were determined using AASHTO LRFD (1998) which was determined to be 1513 kn-m (1115 ft-kips) including impact and load factors as illustrated in Fig. 5. This approach indicates that the life expectancy of the bridge would degrade to the required moment capacity in Year 66. If the safety factors are not considered within the AASHTO nominal required moment capacity, the life expectancy would be projected based on this process to Year 173. Moment Capacity (ft-kip) Bridge built in 1955 FRP Retrofit, Year 46 Corrosion Initiation, Year 17 Life Expectency with LF, Year 66 Life Expectency without LF, Year 173 Nominal Required Moment Capacity for Load Posting Removal In-situ field evaluation would be required to determine exact steel loss (corrosion levels) at critical moment locations to more precisely determine the residual strength level in girder Age of Bridge (Year) Conversion: 1 ft-kip. = kn-m Fig 5: Moment capacity model considering combined effects of steel corrosion and bond strength reduction. 4. CONCLUSIONS The above presented model is one possible approach in determining the life expectancy of a CFRP strengthened bridge structure. The above work is based on assumed corrosion behavior based on an available corrosion model as well as a bond degradation model based on research performed on bond degradation between CFRP and concrete. The authors have assumed the laboratory bond testing conditions may be representative of field conditions

8 under similar FT cycles even though many of the field FT condition gradients may occur over a less severe temperature-time cycle. It is believed that this assumption was conservative, but much more work needs to be done to correlate lab durability studies to field conditions and address some of the assumptions and limitations in this work. Additional modeling considerations on a variety of other factors are required to refine the initial work undertaken in this study. Reference: 1. Benmokrane, B., El-Salakawy, E.F., Desgagné, G., and Lackey, T., (2004), "Building a New Generation of Concrete Bridge Decks using FRP Bars," Concrete International, the Magazine of ACI, Vol. 26, No. 8, August, pp National Bridge Inventory, (2002), Federal Highway Administration, Washington D.C., Meier, U. and Kaiser, H., (1991), Strengthening of Structures with CFRP Laminates, Advanced Composite Materials in Civil Engineering Structures, Proceedings of the Specialty Conference Advanced Composites Materials in Civil Engineering Structures Las Vegas, Nevada, eds. S. L. Iyer, and R. Sen, American Society of Civil Engineers, pp Triantafillou, T.C. and Plevris, N., (1992), Strengthening of RC Beams with Epoxy-Bonded Fiber-Composite Materials, Materials and Structures Journal, May, V. 25, pp Shahawy, M., and Beitelman, E.T., (1999), Static and Fatigue Performance of RC Beams Strengthened with CFRP Laminates. ASCE Journal of Structural Engineering, Vol. 125, No. 6, pp Bakis, C.E.; Bank, L.C.; Brown, V.L.; Cosenza, E.; Davalos, J.F.; Lesko, J.J.; Machida, A.; Rizkalla, S.H.; and Triantafillou, T.C., (2002), Fiber-Reinforced Polymer Composites for Construction State-of-the-Art Review, ASCE Journal of Composites for Construction, V. 6, No. 2, pp Karbhari, V. M. and Engineer, M., (1996), Investigation of Bond Between Concrete and Composites: Use of a Peeling Test, Journal of Reinforced Plastics and Composites, V. 15, pp Lee, Y. J.; Boothby, T. E.; Bakis, C. E.; and Nanni, A., (1999), Slip Modulus of FRP sheets Bonded to Concrete, Journal of Composites for Construction, ASCE, V. 3, No. 4, pp Bizindavyi, L., and Neale, K. W., (1999), "Transfer Lengths and Bond Strengths for Composites Bonded to Concrete," J. Compos. Constr., 3(4), , Maeda, T.; Asano, Y.; Sato, Y.; Ueda, T.; and Kakuta, Y. (1997), A Study on Bond Mechanism of Carbon Fiber Sheet, Non-Metallic (FRP) Reinforcement for Concrete Structures, Japan Concrete Institute, Japan, V. 1, pp Täljsten, B. (1997), Defining anchor lengths of steel and CFRP plates bonded to concrete," International Journal of Adhesion and Adhesives, V. 17, No. 4, pp ACI 440.2R-02, (2002), Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, American Concrete Institute, Farmington Hills, MI, Bisby, L.A. and Green M.F. (2002), Resistance to Freezing and Thawing of Fiber-Reinforced Polymer Concrete Bond, ACI Structural Journal, March/April ACI , (2005), Building Code Requirements for Structural Concrete and Commentary, American Concrete Institute, Farmington Hills, MI, Bae, S., Belarbi, A., Myers, J.J., (2005), Performance of Corrosion-Damaged RC Columns Repaired by CFRP Sheets, American Concrete Institute Special Publication-230, FRPRC7- Editors C. Shield, J. Busel, S. Walkup, D. Gremel, November 2005, pp AASHTO, (1998), LRFD Bridge Design Specifications, American Association of State Highway and Transportation Officials, Washington, D.C., 1998.