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Durability simulation of FRP bridge decks subject to weathering
Composites: Part B 51 (2013) 162–168
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Durability simulation of FRP bridge decks subject to weathering Hwai-Chung Wu ⇑, An Yan Advanced Infrastructure Materials Laboratory, Wayne State University, Detroit, MI 48202, USA
a r t i c l e
i n f o
Article history: Received 28 May 2012 Received in revised form 24 September 2012 Accepted 10 March 2013 Available online 22 March 2013 Keywords: A. Layered structures B. Environmental degradation C. Finite element analysis B. Strength
a b s t r a c t Fiber Reinforced Polymer (FRP) composite panels are particularly attractive as bridge decks due to their high strength, low density, and durability, which are of importance in the bridge industry. Although the short term performance of FRP decks is satisfactory, the long-term performance under weather conditions still awaits future testimony and remains a major concern in their use as primary load bearing members. Since the load capacity and structural stiffness of FRP decks deteriorate over time at different rates, it is necessary to develop robust mechanics models to simulate the long-term performance of FRP deck structures subject to the combined effects of mechanical and environmental loading. To this end, a comprehensive mechanics framework has been developed, taking into account the critical deterioration rates of the FRP constituents. Such deterioration relationships were obtained by calibrating the accelerated laboratory durability test data with the in-service field measurements. Simulation results agree well with the 4-year performance data of a FRP-deck road bridge. Long-term validation data is, however, still needed. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction There are about 600,000 bridges in the US transportation system, and 27% of these bridges are structurally deficient or functionally obsolete [1]. The same ASCE report also estimated that it would cost the US $9 billion annually for 20 years to fix the bridges alone. Infrastructure deterioration is not unique in the US but rather is becoming a global issue [2,3]. One of the main causes is the deterioration of the concrete and steel materials used in the construction and repair of these facilities. The corrosion of reinforcing steel in concrete decks leads to cracking of the concrete. Water then migrates through these concrete cracks and further accelerates the corrosion of the steel. Such a snow-ball effect is exacerbated by road salts used to combat winter ice and snow. Furthermore, a conventional concrete deck usually accounts for a major portion of the bridge’s dead load. Reducing the dead load will increase the allowable live load capacity of the bridge without the need of significant repair to the existing super-and-sub structure, thus lengthening its service life. Hence lightweight and durable FRP composites could be excellent alternatives to concrete for decks [4–7]. A great deal of work has been carried out in developing FRP bridge decks. It was estimated that over $30 million has been spent by the federal government in developing FRP composites for infrastructure usage; over half of that has been spent on bridges [8]. However, standard analysis and design procedures
⇑ Corresponding author. Tel.: +1 313 577 0745; fax: +1 313 577 3881. E-mail address: [email protected] (H.-C. Wu). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.03.018
for FRP bridge decks are yet to be developed. Most current designs were carried out by following the specifications provided by the FRP deck manufacturers, which have been justified by proof tests. The American Association of State Highway and Transportation Officials (AASHTO) just published a new document, The Guide Specifications for Design of FRP Pedestrian Bridges (1st Edition), in 2008 [9]. However, the new guide specifications apply to FRP composite bridges that are intended to carry primarily pedestrian and/or bicycle traffic only. Under everyday service conditions, bridge decks are subjected not only to heavy traffic loads but also to a wide range of temperature and moisture changes. In winter, deicing salts are widely used in the Northern US, and their effect must also be considered as part of the service environment. The effect of cold region climate on FRP materials is of special concern since that is this region where concrete deterioration is most often observed. Sub-zero temperatures can cause changes in mechanical properties and create additional microcracks of the FRP materials [10]. In a hot and humid climate, hot temperature, especially coupled with high humidity or chloride concentration in a coastal area has a different but also very detrimental effect on FRP materials than in a cold region. The combination of complex environmental and mechanical loading complicates the durability assessment of FRP composites [11–15]. While the current efforts are making great progress, insufficient understanding of the durability performance of FRP structures under natural weathering and a lack of reliable prediction capabilities are severe obstacles to the wide acceptance of FRP structures by the construction industry. Time-dependent effects manifested by creep rupture under sustained loads, and deterioration due to weathering
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by temperature, UV radiation, and humidity, are important in timedependent reliability of FRP structures during their service life. At present, a comprehensive mechanics framework for durability assessment is largely unavailable. Creep behavior of FRP composites is usually not significant when the composites are subjected to low long term mechanical loads [16]. In bridge deck applications, the daily service loads acting on the decks are very limited, hence creep was not considered in this study. 1.1. FRP decks and weathering effects Available FRP bridge deck systems are typically made of glass fiber and polyester resin or vinyl ester resin matrix. They are often factory-assembled sandwich panels or protruded FRP composite panels that are sized appropriately for shipping to the bridge sites. The panels are then fastened and/or bonded together to the support system at the site using mechanical connections, high performance adhesives, or both. At the material level, FRP composites are typically composed of two major constituents, namely fiber and matrix. The properties and failure modes of a composite therefore depend on the properties of each individual constituent, i.e. fiber, matrix, and their interfaces. The degradation of each constituent over time affects various composite properties to different extents and even changes the order of significance of failure modes since there are quite a few failure modes operative simultaneously at any given time. Each failure mode can be attributed to be matrix-dominated, fiber-dominated, or interface-dominated. The long term performance of a FRP bridge deck may not simply degrade over time; abrupt failure might occur due to a change in the dominant failure mode. As a result, the weather durability of FRP composites remains a major concern in their use as primary load bearing members and structures. Moreover, how load capacity and structural stiffness will change over time present a great challenge to bridge designers when specifying allowable loads and reductions factors. Composite structures for civil infrastructure purposes have only been in service for a relatively short time. There are very few long-term data available to quantify environmental effects such as natural weathering on FRP structural components or systems [17,18]. Instead, most of the available durability studies were based on simulated laboratory testing. Test conditions generally vary greatly, since there are no standard test methods. Freeze and freeze/thaw exposures lead to material level degradation through matrix cracking and fiber–matrix debonding, increased brittleness, and substantial changes in damage mechanisms from those commonly observed under ambient conditions [19–23]. Chu et al. [24] used accelerated test data to predict long-term properties of pultruded E-glass/vinyl ester composites, and the predictions correlated well with their experimental results. Karbhari et al. [25,26] adopted a semi-empirical approach to predict the long-term modulus and strength of unidirectional carbon/vinyl ester composites subject to freeze–thaw cycling. Their study indicated that the analytical predictions are somewhat conservative for the strength and freeze/thaw cycling does not have an appreciable effect on modulus. FRP composites used in bridge applications are normally under sustained loads, however, sustained loading was not considered in the Karbhari’s durability study. It was also shown that freeze–thaw cycling between 4.4 and 17.8 °C alone caused very insignificant changes in flexural strength, storage modulus, and loss factor for the E-glass/vinyl ester specimens conditioned in distilled water and saltwater [27]. Furthermore, Wu et al. [28] concluded that (1) small reductions in modulus were observed after 250 freeze–thaw cycles in water when the specimens were loaded to 25% of their ultimate strain while undergoing freeze/thaw cycles, (2) no deterioration was observed for the specimens prestrained and freeze–thaw cycled in
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dry air conditions, (3) constant freeze at 17.8 °C resulted in a minor increase in flexural strength and storage modulus over time. It is noted that any accelerated test environment in a laboratory setting is certainly different than the real ‘‘service environment’’. An accelerated test method uses one or several accelerating mechanisms to increase the rate of degradation of a given material in a controlled laboratory setting. The rate of degradation under the accelerated condition must be related to the actual degradation rate under everyday or ‘‘service’’ conditions. Any credible accelerated condition must also produce similar failure mechanisms/sequences with those actually observed in the field. At present, there is no standardized durability test procedure for FRP materials for infrastructure applications. The goal of the on-going project is to simulate the effects of environmental exposure on long-term structural performance of FRP decks. The first objective of this study was to understand the effects of laboratory accelerated test conditions on the residual response of FRP composites that are being used for FRP decks [28,29]. The second objective is to develop acceleration factors that relate the laboratory accelerated test environments to the actual weather environments so that the long-term performance of FRP decks could be predicted in terms of realistic in-service time scale. This is the subject reported in this paper. 2. Mechanics based durability model A FRP deck is composed of several constituent materials of different characteristics. Under weathering conditions, individual material constituents can degrade at different rates. Therefore, a reliable performance simulation of the deck must be able to account for possible changes in the order of significance of failure modes that may occur over time. In this research, a performance driven design approach has been followed to construct the framework within which the durability study of fiber reinforced plastic composite bridge deck is organized [30]. Natural weathering conditions directly influence the composite deck’s performance. Quantifying the changes in various properties of the matrix and composite over time as a result of the environmental exposure is a key aspect to this study. In FE simulations of a FRP deck, the first step begins by defining the geometry, the layup (orientation of plies within the laminate), the environmental conditions, and a pre-straining level. The last factor is needed to reflect on the level of the traffic loads. Using the timedependent material parameters as inputs to the durability finite element model, predictions of weathering effects on the stiffness and load capacity of the FRP decks over time can be achieved. To expedite screening of the weather durability of FRP materials, a laboratory accelerated testing is usually employed. Therefore, it is necessary to establish correlation between the laboratory test environment and the actual field environment. This can be accomplished by measuring the degradation rates of the matrix materials and their unidirectional ply aged under these two conditions separately and by calibrating them with acceleration factors. The higher degradation rates from the laboratory testing can be ‘‘scaled down’’ by an acceleration factor to reflect the actual degradation rates in the field. Using such universal degradation rates of the matrix and its unidirectional ply, the deterioration of the modulus and strength of any FRP laminate of designated stacking sequence and ply orientation as a function of time, can be determined by employing micro-mechanics and the laminate theory. 2.1. Failure analysis of FRP decks FRP composites always experience some degree of delamination, broken fibers and cracked matrix, but any of those is only significant if the damage grows to occupy a large and/or critical area
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of the material so that the structure member starts to lose its integrity. For a composite sandwich structure, three common failure modes include face yielding (or rupturing), face buckling, and core shearing [29]. Core shearing is unlikely to occur in the case of a bridge deck. Instead, delamination between the face skin and the core is commonly observed in laboratory tests or from field inspections. Thus, delamination instead of core shearing was used in this study to determine the ultimate load capacity of FRP decks over time. In addition, a deflection limit was added according to code requirements such as AASHTO [31]. By continuously increasing live loads, the ultimate load capacity of a FRP-deck bridge is reached when any of the four failure criteria is first met, i.e. (1) first ply failure; (2) interface shear failure; (3) local buckling of the web or the skin; (4) displacement limits under service conditions. The four criteria can be categorized into two groups, i.e. long-term ultimate strength limit (Criteria 1–3) and long-term serviceability limit (Criterion 4). A detailed discussion on the formulations of these four criteria can be found elsewhere [29]. 2.2. Validation of short-term FRP deck predictions To simulate the behavior of FRP sandwich decks, an in-house FEA model has been developed [32,33]. The in-house layered model was incorporated into ABAQUS to simulate the behavior of FRPdeck bridges in two studies. In the first study [34], the deflections of a GFRP honeycomb deck bridge were simulated. This bridge, located in St. James, Missouri, had a total span of 8000 mm and a width of 8330 mm, and was test loaded by a truck of 242.17 kN with 76.69 kN, 81.58 kN, and 83.90 kN on the three axles. The deflections predicted by the FEA model agreed well with the field measurements. In the second study [33], the load–displacement response of a sandwich panel of 4460 mm by 1830 mm was simulated. The composite panel consisted of a lightweight core sandwiched between two six-ply skin plates (glass/vinyl ester composite). The load was applied over a 250 mm by 600 mm bearing plate with a 6.4-mm-thick Neoprene pad to simulate a factored wheel load of a HS25 truck. Two linear variable differential transducers (LVDTs) were set up at the bottom of the panel to monitor the displacements at two control points. Very good agreements between the predicted and measured displacements have been observed when the panel was subjected to loadings of 110 kN and 208 kN, respectively. It is noted that all the above simulations were carried out on newly constructed FRP deck structures without considering material deterioration due to weathering. For the purposes of durability simulations, the same FRP sandwich panel of 4460 mm by 1830 mm (as reported in [33]) was used. This panel was only tested once right after its construction and long term durability testing was not conducted on the panel. 3. Simulations of durability performance of FRP decks To simulate the long-term behavior of FRP decks, material constituent laws accounting for both strength and stiffness deterioration over time must be constructed and incorporated into the FE analysis. Such constituent relationships may be established confidently from in situ measurements, which are expensive and time-consuming. Alternatively, accelerated laboratory test results can also be used but the laboratory time scale must be well correlated to the in-service time scale under natural weathering conditions. This aspect will be further discussed in the next section. In addition, attention must be given to considering all possible failure modes. Individual material constituents degrade at different rates likely leading to a change in the order of significance of failure modes over time, since the stiffness and final structural failure is determined by its dominant failure mode at the time of failure.
Therefore initial design based on the failure mode at the time of the new construction would certainly unreliable when the actual failure modes change over time during the service period. 3.1. Material degradation There are two approaches to establishing the degradation relationships of FRP composites subject to natural weathering. The simplest way is to prepare neat resin samples and unidirectional FRP ply samples and place them in an outdoor environment. After exposure at designated time intervals, the samples are retrieved for testing. Therefore the degradation of the material properties could be characterized and the degradation rates could be determined. However, this procedure is time-consuming and the results are specific to the material under evaluation. The second approach involves accelerated laboratory test conditions that are designed to simulate natural environments. Laboratory results, then, must be correlated to the field performance under natural weathering conditions. Consequently, all materials of interest can be readily screened in laboratory for their long-term weather durability. In either way, the deterioration of off-axis plies and their laminates can be determined by micro mechanics and the laminate theory. The second approach was used in this study. After an extensive review on this subject, an accelerated test procedure to simulate the effects of natural weathering on FRP materials has been developed [27,28]. This procedure incorporates the combined effects of temperature, medium, and sustained load into a complete test program. Coupon samples will be preloaded and immersed in a selected medium in a special fixture. Each fixture can hold multiple specimens and is equipped with an adjustable pin that imposes bending strains to the specimens. Several fixtures can be loaded onto a programmable environmental chamber. Then thermal cycling begins. Samples are retrieved for evaluation after predetermined cycle numbers. More details of the durability testing can be found elsewhere [27,35]. For a glass/vinyl ester composite, the degradation rates of various properties of the skin laminate were measured up to 1250 h under the following laboratory conditions, 2-h freeze–thaw cycles (between 4.4 and 17.8 °C) and pre-straining (25% of ultimate strain capacity) in distilled water (hereafter referring to 2H-PS conditions). These conditions were found to be suitable to simulate a cold climate environment [28]. Assuming the same degradation rates beyond 1250 h, the time-dependent stiffness and strength of the same laminate over time were predicted. A detailed description of this approach can be found elsewhere [35]. It was confirmed that the properties of the matrix (vinyl ester) are more sensitive to environmental exposure than those of the composite, and the fiber dominated properties Ex and Ey decrease slowly with increasing the duration of environmental exposure as compared to the matrix dominated property Gxy. After 50,000 h of exposure under the 2HPS condition, the modulus of the matrix decreased by around 25%, while the longitudinal modulus of the laminate, Ex, decreased by only 5% [29]. No appreciable deterioration of the same FRP material was detected after 50,000 h exposure in water at room temperature [29]. These results confirmed again that the material properties of FRP laminates are strongly influenced by freeze–thaw cycles under pre-straining, especially for matrix and matrixdominated properties. Similar to modulus, the effect of cold climate conditions on strength reduction of FRP composites could be determined. The normalized flexural strength-vs.-time curves in various accelerated environmental conditions are plotted in Fig. 1. It is confirmed that freeze–thaw cycles coupled with pre-straining resulted in a much more rapid strength reduction, while the cycle length (2-h vs. 5-h) showed insignificant effect. It was assumed that the matrix domi nated strengths S L (longitudinal compression), ST (transverse com-
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pression), and SLT (in-plane shear) all experienced the same deterioration rates as the flexural strength and such information was used in the finite element analysis of the deck in the following sections. This shortcoming could be improved in the future when such data on strength reduction become available.
6000
Failure Load (kN)
3.2. Failure modes
3.2.2. Shear failure Shear stresses at the interfaces between the core and the outer skins may also result in shear failure. The transverse shear stresses can be determined at section integration points (listed as output variables TSHR13 or TSHR23 in ABAQUS). As shown in Fig. 2, the ultimate loads due to shear failure decrease more rapidly over time than those determined by the first ply failure criterion. Since the bonding material between the core and skins is a neat vinyl ester resin, the resin deteriorates much more rapidly over time than does the FRP laminate of the skin.
Normalized flexural strength
3.2.3. Local buckling failure Buckling modes and corresponding failure loads can be obtained by performing finite element buckling analysis. Among several critical factors (eigenvalues), only the first buckling mode is useful for the purpose of bridge deck design [35]. Furthermore, local buckling is predicted to take place right under the wheel load, and is consistent with the observations from several modular FRP bridges [36,37]. The local buckling failure load is found to be much lower than either the first-ply or the shear failure load. This is mainly due to the thin wall sandwich structure of the deck panel. Buckling loads strongly depend on the elastic properties of the FRP constituents. Therefore, stiffness degradations would cause reductions in ultimate load capacity. Fig. 2 also shows the ultimate load capacity due to buckling as a function of environmental expo1 0.8 0.6 0.4
2H-PS 5H-PS 2H 5H
0.2
5000
10000
15000
20000
Time (h) Fig. 1. Flexural strength deterioration in various environmental conditions. 2H and 5H represent 2-h and 5-h freeze/thaw cycles without pre-straining.
"Buckling"
3000 2000
0 0
5000
10000
15000
20000
25000
Time (h) Fig. 2. Various failure load vs. time under the standard 2H-PS condition.
sure time; the ultimate load capacity decreases by about 5% after 10,000 h under the standard freeze/thaw conditions. 3.2.4. Limit of deflection Time-dependent maximum allowable deflection was used as an index of serviceability limit in this FRP deck study. The deflection failure index Eld was calculated as the ratio of the maximum deflection of the deck to the allowable limit of 1/800 of the span length. Usually a FRP deck design is stiffness-controlled instead of strength-controlled. Under the standard accelerated freeze/thaw conditions, the deck was predicted to lose about 6.3% and 15.7% of its initial stiffness after 10,000 and 50,000 h exposure, respectively. The long term displacements at any monitor point on the deck could be then calculated from the FE analysis. For example, the displacements at the two locations near the center line but on the opposite side of the FRP sandwich panel of 4460 mm by 1830 mm were determined to be 2.6 and 3.2 mm during the initial test [33]. The deflection of the deck is shown to increase rapidly within a short time period and then follow a nearly linear increase over a much longer period. The same two locations are predicted to have deflection of 3.0 and 3.8 mm respectively after 50,000 h. Without pre-straining, the deflections remain almost the same under the same 2-h freeze–thaw cycling. This is due to a much higher stiffness reduction when the FRP material is aged under sustained loads. 3.3. Time scale It is difficult to define ‘‘service environment’’ that depends on its geographical location, and is vastly different than the laboratory accelerated testing conditions. Hence it is difficult to predict realistic service life of a FRP-deck bridge based on the deterioration rates of the FRP materials which are determined from laboratory testing. The rates of degradation under everyday or ‘‘service’’ conditions must be related to the degradation rates under the accelerated conditions. Such correlations may be accomplished by an acceleration factor. An acceleration factor therefore correlates the laboratory time scale from standardized accelerated testing to the real service time in the field. Acceleration factors for various natural environments may be determined using the framework outlined in ASTM E632, Standard Practice for Developing Accelerated Tests to Aid in the Prediction of the Service Life of Building Components and Materials. In a simple form, an acceleration factor can be calculated as the ratio of the degradation rate in the accelerated environment to the degradation rate in the service environment:
0 0
Shear
4000
1000
As discussed above, there are four failure indices for the determination of the ultimate load capacity of FRP sandwich decks, i.e., first-ply failure, interlaminar shear failure, local buckling failure, and deflection limit. 3.2.1. First ply failure The load capacity is determined by increasing vehicular wheel loads incrementally until the first ply failure occurs. The ultimate vehicular wheel load is thus time-dependent on the strength deterioration of the deck materials over time. As shown in Fig. 2, the load capacity decreases more rapidly after 10,000-h exposure under the standard 2H-PS (2-h cycle frequency and pre-straining) conditions than during the first 10,000 h. The load capacity under the standard accelerated conditions of 2H-PS loses its strength by 15% and 62% after 10,000 h and 20,000 h exposure, respectively. Compared to the stiffness deterioration, the load capacity decreases much more rapidly, especially after 10,000 h of exposure.
First Ply
5000
R¼
dA=dt dS=dt
ð1Þ
where dA/dt is accelerating degradation rate and dS/dt is in-service degradation rate.
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Degradation rates (dA/dt and dS/dt) in both the accelerated and service environments could be determined from the line regression of the strength or stiffness vs. time plots. When the degradation relationships are not linear with respect to time, other forms of regression might be more appropriate. To construct such correlations between laboratory and field environments, ideally the same FRP materials shall be placed in the field over a sufficient period of time. Periodically a few field samples should be retrieved and tested in laboratory in the same manner as those of the samples exposed to the laboratory conditions. The test results of the field-exposed specimens could then be correlated to the results of the laboratory specimens with confidence. However, composite structures for civil infrastructure purposes have only been in service for a relatively short time. There are very limited long-term material data available to quantify natural weathering effects. In this study, available field monitoring data on displacements from a FRP bridge deck were used to indirectly determine the acceleration factor for this bridge location.
In this study, in situ strain measurements at the location of Strain gage #1 over the entire monitor period of 4 years were selected for calibration purposes to determine the acceleration factor (see Fig. 4). By applying a linear regression analysis, the acceleration factor (R) was found to be 5.4 for the location of this bridge. The differences in the R values due to various accelerated conditions (as reported in Fig. 4) are insignificant. An acceleration factor of 5.4 indicates that the effect of undergoing 1,000 h under the laboratory freeze–thaw conditions of 2-h frequency and pre-straining in water results in the same level of damage after 5400 h outdoor exposure. Therefore, this factor permits a much reliable way of converting laboratory time scale to in-field time scale. It should be noted that the current acceleration factor was determined based on the 4-year’s data available at present and it was assumed valid over the entire life of the bridge. A more precise acceleration factor which may be a function of time is yet to be determined when longer term monitoring data are available. 4. Discussion on long term predictions in real time
3.3.1. Example bridge A FRP two-way web core skew bridge (NY, USA) was used as an example in this study. This bridge deck was constructed by the same FRP materials that were characterized by the accelerated tests at Wayne State University. This bridge was instrumented with 24 conventional strain gauges mounted externally on the bottom face skin of the superstructure [38]. The first proof load test was conducted right before the FRP deck was opened to traffic in October 1998, and established the reference/baseline for future bridge monitoring through similar load testing. The FRP deck was then tested periodically for four years to monitor its in-service durability. For this example bridge, the in situ changes in material properties were not directly measured; such changes could be indirectly estimated by finite element analysis. The FE procedure, described above, was used to simulate the load–displacement behavior and strain values at various locations of the bridge during the first load test. Very good agreements between the FE simulated strains and the field test results have been obtained (as shown in Fig. 3). The first load test was conducted right after the construction of the FRP deck, hence the strain measurements reflected the properties of the virgin FRP composites. The in-service degradation rates could be then determined by reducing the stiffness of the FRP materials until the predicted strains matched those of the field measurements during the subsequent load tests. On the other hand, the degradation rates of the FRP constituents, determined from the laboratory accelerated tests, were input into the FE model to calculate strains at all locations over time. By analyzing both sets of the strain data, the acceleration factor could be determined by Eq. (1).
Fig. 3. Comparisons of strains at various gage locations between FE simulations and field measurements from the first proof test in 1998.
The calibrated real time-dependent constituent relationships, together with the in-house layered model were input into ABAQUS to simulate the durability performance of the example FRP deck. HYPERMESH was used as the pre- and post- processor. A multi-step static analysis option (available in ABAQUS) was used to determine the failure load for each of the four failure modes. In each load step, stresses and displacements were determined, and then checked against the four failure criteria. If any of the failure indices, normalized load (or displacement) vs. critical failure load (or displacement) under each mode (details can be found in [29]), exceeded 1.0, the associated failure occurred. The load was then increased to the next higher level until another failure took place. The simulation was terminated after all four failures were found. For the new FRP deck, the failure sequences were deflection limit, local buckling, interlaminar shear, and first-ply failure (Fig. 5). Thus, the design of this FRP bridge deck was indeed ‘‘stiffness controlled’’, which was the most common case. The predicted maximum deflection was 3.08 mm which was in good agreement with the 2.75 mm measured by external LVDTs. The maximum allowable deflection was 9.76 mm per AASHTO [39]. After only 4 years of natural weathering, the dominant failure mode of the FRP deck would be changed from deflection limit to shear failure (comparing Figs. 5 and 6), followed by deflection, local buckling, and first-ply failure. The shear index under one standard wheel load increased from 0.15 at the time of new construction to
Fig. 4. Normalized strain vs. time at the location of gage #1 from field measurements and from various accelerated test conditions (e.g. 2H-PS: 2 h cycle length under prestraining load, suffix-S: salt water, -W: distilled water).
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5. Conclusions
5 Deflection index
Failure index
4
First-ply index Interlaminar shear index
3
Local buckling index
2 Failure 1 Service 0 1
3
5
7
9
11
13
15
Wheel loading index (Wheel load/initial wheel load) Fig. 5. Failure index vs. wheel load at the completion of the deck construction.
5 Deflection index
Failure index
4
First-ply index Interlaminar shear index
3
Local buckling index
2 Failure 1 Service 0 1
3
5
7
9
11
13
15
Wheel loading index (Wheel load/initial wheel load)
References
Fig. 6. Failure index vs. wheel load after 4 years of weather aging.
5 Deflection index
4
Failure index
First-ply index Interlaminar shear index
3
Local buckling index 2 Failure 1 Service 0 1
3
5
7
9
11
13
To allow for sufficiently generalized descriptions of service life, credible durability simulations of FRP bridge decks must be created and must be able to predict the combined effects of mechanical loads and service environments. These simulations must be robust and be developed from reliable descriptions of material degradation mechanisms and their interactions, which may include characterizations from accelerated testing to extend the validity of the predictions. Such simulations must be validated over a wide range of conditions, at both the component and structural levels. In this paper, a mechanics based durability model has been formulated to predict the performance of FRP-deck bridge systems under natural weathering conditions, especially in cold climates. The focus was placed on the ability to predict long term structural performance. Material constituent laws accounting for both strength and stiffness deterioration over time were first developed and then incorporated into the FE analysis. Such constituent relationships were established from experimental data obtained from laboratory accelerated freeze/thaw testing and from in-field performance measurements. An acceleration factor correlating laboratory time scale with in-service time scale was proposed and found to be able to predict the changes in stiffness and deflection of a monitored FRP bridge deck over a 4-year period. Nevertheless, more long-term performance data is needed to further validate the proposed durability model, in particular the acceleration factor.
15
Wheel loading index (Wheel load/initial wheel load) Fig. 7. Failure index vs. wheel load after 50 years of weather aging.
0.47 after 4 years, indicating that the probability of shear failure has greatly increased. Partial delamination was actually observed from visual inspection during the 4-year monitoring period [40]. After 50 years in service, it was predicted that the shear index (1.27) already exceeds 1.0 under one HS25 wheel load (Fig. 7), followed by deflection (0.37), buckling (0.22), and first ply failure (0.21). This suggested that the interface layer would deteriorate much faster than the skin and the core of the FRP sandwich materials. In this case, it was predicted that interlaminar shear stress at the interface between the core and the skin would exceed the residual shear strength of the adhesives within 20–30 years, leading to total delamination. Although in-service long term performance data are not available for verification, the current predictions seem to be reasonable. It should also be noted that other environment effects such as UV radiation or high temperatures can be readily incorporated into the existing framework by employing adequate degradation rates in a similar fashion to those due to freeze–thaw cycling.
[1] American Society of Civil Engineers (ASCE), Report card for America’s infrastructure. Reston, VA; 2005. [2] Hammad A, Yan J, Mostofi B. Recent development of bridge management systems in Canada. In: Proc., annual conference (CD-ROM), transportation association of Canada, Saskatoon, Saskatchewan, Canada, October 14-17, 2007; 2005. [3] Ikeda K. Technology trends in stock management of road structures. Sci Technol Trends Q Rev 2007;25:73–96. [4] Zureick AH, Shih B, Munley E. Fiber reinforced polymeric bridge decks. Struct Eng Rev 1995;7(3):257–66. [5] Hayes MD, Ohanehi D, Lesko JJ, Cousins TE, Witcher D. Performance of tube and plate fiberglass composite bridge deck. J Compos Construct 2000;4(2):48–55. [6] Hayes MD, Lesko JJ, Haramis J, Cousins TE, Geomez J, Masarelli P. Laboratory and field testing of composite bridge superstructure. J Compos Construct 2000;4(3):120–8. [7] Dutta PK, Lopez-Anido R, Kwon S-C, Durell GD. Fatigue evaluation of multiple FRP bridge deck systems over existing girders: Phase II Report. US Army Corps of Engineers; 2003. [8] Busel J. Product selection guide: FRP composite products for bridge applications. Market Development Alliance (MDA) of the FRP Composites Industry, New York, NY; 2000. [9] American Association of State Highway and Transportation Officials (AASHTO), Guide specifications for design of FRP pedestrian bridges. First Edition, Washington DC; 2008. [10] Gibson RF. Principles of composite material mechanics. Mc-Graw-Hill Press; 1994. [11] Hollaway LC, Head PR. Advanced polymer composites and polymer in the civil infra structure. Elsevier; 2001. [12] Karbhari VM. Fiber reinforced composite bridge systems-transition from the laboratory to the field. Compos Struct 2004;66(1–4):5–16. [13] Helbling C, Abanilla M, Lee L, Karbhari VM. Issues of variability and durability under synergistic exposure conditions related to advanced polymer composites in the civil infrastructure. Composites Part A 2006;37(8):1102–10. [14] Bank LC. Composites for construction: structural design with FRP materials. NY: John Wiley & Sons; 2006. [15] Dutta PK, Lopez-Anido R, Kwon S-C. Fatigue durability of FRP composite bridge decks at extreme temperatures. Int J Mater Prod Technol 2007;28(1/ 2):198–216. [16] Ascione L, Berardi VP, D’Aponte A. Creep phenomena in FRP materials. Mech Res Commun 2012;43:15–21. [17] Karbhari VM, Chin JW, Hunston D, Benmokrane B, Juska T, Morgan R, et al. Durability gap analysis for fiber-reinforced polymer composites in civil infrastructure. ASCE J Compos Const 2003;7(3):238–47. [18] Shahrooz BM, Neumann AR, Reising RMW. Durability of FRP composite bridge decks – construction and temperature effects. Int J Mater Prod Technol 2007;28(1/2):66–88. [19] Lord HW, Dutta PK. On the design of polymeric composite structures for cold regions applications. J Reinf Plast Compos 1988;7(5):435–58.
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[20] Dutta PK, Hui D. Low temperature and freeze-thaw durability of thick composites. Composite Part B: Eng 1996;27(3–4):371–9. [21] Dutta PK. Structural fiber composite materials for cold regions. J Cold Reg Eng 1998;3(2):124–32. [22] Karbhari VM, Pope G. Impact and flexure properties of glass/vinyl ester composites in cold regions. J Cold Reg Eng, ASCE 1994;8(1):1–20. [23] Karbhari VM, Rivera J, Dutta PK. Effect of short-term freeze-thaw cycling on composite confined concrete. J Compos Const, ASCE 2000;4(4):191–7. [24] Chu W, Karbhari VM, Wu L. Durability evaluation of moderate temperature cured E-glass/vinylester systems. Compos Struct 2004;66:367–76. [25] Karbhari VM. Response of FRP confined concrete exposed to freeze-thaw regimes. ASCE J Compos Construct 2002;6(1):35–40. [26] Karbhari VM, Rivera J, Zhang J. Low-temperature hygrothermal degradation of ambient cured E-glass/vinyl ester composites. J Appl Polym Sci 2002;86:2255–60. [27] Wu HC, Fu G, Gibson RF, Warnemuende K, Yan A, Anumandla V. Durability testing of FRP material in low temperature weathering condition. In: Proc. 21st annual technical conference (CD-ROM), American Society of Composites, Dearborn, MI; 2006. [28] Wu HC, Fu G, Gibson RF, Yan A, Warnemuende K, Anumandla V. Durability of FRP composite bridge deck materials under freeze-thaw and low temperature conditions. J Bridge Eng 2006;11(4):443–51. [29] Wu HC, Yan A. Time dependent strength deterioration of FRP bridge deck under freeze/thaw conditions. Composites Part B 2011;42(5):1226–32. [30] Wu HC, Yan A, Fu G, Gibson RF. Mechanics based durability modeling of FRP bridge deck. In: CD Proc. Composites 2006, St. Louis, MO; 2006.
[31] American Association of State Highway and Transportation Officials (AASHTO), Load resistance and factor design: Bridge design specifications. Third Edition, Washington DC; 2003. [32] Wu HC, Mu B, Warnemuende K. Failure analysis of FRP sandwich bus panels by finite element method. Composites Part B 2003;34(2):51–8. [33] Mu B, Wu HC, Yan A, Warnemuende K, Fu G, Gibson RF, et al. FEA of complex bridge system with FRP composite deck. J Compos Const 2006;10(1):79–86. [34] Wu, H.C., Fu, G., Yan, A., Warnemuende, K., and Gibson, R.F., Gobal-local FE analysis of FRP sandwich deck. In: Proc. of 4nd International Conference on Advanced Composite Materials in Bridges and Structures (CD-ROM), Calgary, Canada; 2004. [35] Yan, A., Durability of glass fiber/vinyl ester composites as bridge deck subject to low temperature weathering conditions. Ph. D. Dissertation, Department of Civil & Environmental Engineering, Wayne State University, Detroit, MI; 2005. [36] Aref AJ. A novel fiber reinforced composite bridge structural system. Ph.D thesis, University of Illinois at Urbana-Champaign, 1997. [37] Nanni Antonio. Laboratory and field testing on FRP composite decks for the city of St. James. Final Report to MODOT; 2002. [38] Alampalli S. Design, fabrication, construction, and testing of an FRP superstructure. Report FHWA/NY/SR-00/134; 2000. [39] American Association of State Highway and Transportation Officials (AASHTO), Load resistance and factor design: Standard specifications for highway bridges. Washington DC; 1996. [40] Alampalli S. In-service performance of an FRP superstructure. Report FHWA/ NY/SR-04/141; 2004.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Durability simulation of FRP bridge decks subject to weathering Hwai-Chung Wu ⇑, An Yan Advanced Infrastructure Materials Laboratory, Wayne State University, Detroit, MI 48202, USA
a r t i c l e
i n f o
Article history: Received 28 May 2012 Received in revised form 24 September 2012 Accepted 10 March 2013 Available online 22 March 2013 Keywords: A. Layered structures B. Environmental degradation C. Finite element analysis B. Strength
a b s t r a c t Fiber Reinforced Polymer (FRP) composite panels are particularly attractive as bridge decks due to their high strength, low density, and durability, which are of importance in the bridge industry. Although the short term performance of FRP decks is satisfactory, the long-term performance under weather conditions still awaits future testimony and remains a major concern in their use as primary load bearing members. Since the load capacity and structural stiffness of FRP decks deteriorate over time at different rates, it is necessary to develop robust mechanics models to simulate the long-term performance of FRP deck structures subject to the combined effects of mechanical and environmental loading. To this end, a comprehensive mechanics framework has been developed, taking into account the critical deterioration rates of the FRP constituents. Such deterioration relationships were obtained by calibrating the accelerated laboratory durability test data with the in-service field measurements. Simulation results agree well with the 4-year performance data of a FRP-deck road bridge. Long-term validation data is, however, still needed. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction There are about 600,000 bridges in the US transportation system, and 27% of these bridges are structurally deficient or functionally obsolete [1]. The same ASCE report also estimated that it would cost the US $9 billion annually for 20 years to fix the bridges alone. Infrastructure deterioration is not unique in the US but rather is becoming a global issue [2,3]. One of the main causes is the deterioration of the concrete and steel materials used in the construction and repair of these facilities. The corrosion of reinforcing steel in concrete decks leads to cracking of the concrete. Water then migrates through these concrete cracks and further accelerates the corrosion of the steel. Such a snow-ball effect is exacerbated by road salts used to combat winter ice and snow. Furthermore, a conventional concrete deck usually accounts for a major portion of the bridge’s dead load. Reducing the dead load will increase the allowable live load capacity of the bridge without the need of significant repair to the existing super-and-sub structure, thus lengthening its service life. Hence lightweight and durable FRP composites could be excellent alternatives to concrete for decks [4–7]. A great deal of work has been carried out in developing FRP bridge decks. It was estimated that over $30 million has been spent by the federal government in developing FRP composites for infrastructure usage; over half of that has been spent on bridges [8]. However, standard analysis and design procedures
⇑ Corresponding author. Tel.: +1 313 577 0745; fax: +1 313 577 3881. E-mail address: [email protected] (H.-C. Wu). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.03.018
for FRP bridge decks are yet to be developed. Most current designs were carried out by following the specifications provided by the FRP deck manufacturers, which have been justified by proof tests. The American Association of State Highway and Transportation Officials (AASHTO) just published a new document, The Guide Specifications for Design of FRP Pedestrian Bridges (1st Edition), in 2008 [9]. However, the new guide specifications apply to FRP composite bridges that are intended to carry primarily pedestrian and/or bicycle traffic only. Under everyday service conditions, bridge decks are subjected not only to heavy traffic loads but also to a wide range of temperature and moisture changes. In winter, deicing salts are widely used in the Northern US, and their effect must also be considered as part of the service environment. The effect of cold region climate on FRP materials is of special concern since that is this region where concrete deterioration is most often observed. Sub-zero temperatures can cause changes in mechanical properties and create additional microcracks of the FRP materials [10]. In a hot and humid climate, hot temperature, especially coupled with high humidity or chloride concentration in a coastal area has a different but also very detrimental effect on FRP materials than in a cold region. The combination of complex environmental and mechanical loading complicates the durability assessment of FRP composites [11–15]. While the current efforts are making great progress, insufficient understanding of the durability performance of FRP structures under natural weathering and a lack of reliable prediction capabilities are severe obstacles to the wide acceptance of FRP structures by the construction industry. Time-dependent effects manifested by creep rupture under sustained loads, and deterioration due to weathering
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by temperature, UV radiation, and humidity, are important in timedependent reliability of FRP structures during their service life. At present, a comprehensive mechanics framework for durability assessment is largely unavailable. Creep behavior of FRP composites is usually not significant when the composites are subjected to low long term mechanical loads [16]. In bridge deck applications, the daily service loads acting on the decks are very limited, hence creep was not considered in this study. 1.1. FRP decks and weathering effects Available FRP bridge deck systems are typically made of glass fiber and polyester resin or vinyl ester resin matrix. They are often factory-assembled sandwich panels or protruded FRP composite panels that are sized appropriately for shipping to the bridge sites. The panels are then fastened and/or bonded together to the support system at the site using mechanical connections, high performance adhesives, or both. At the material level, FRP composites are typically composed of two major constituents, namely fiber and matrix. The properties and failure modes of a composite therefore depend on the properties of each individual constituent, i.e. fiber, matrix, and their interfaces. The degradation of each constituent over time affects various composite properties to different extents and even changes the order of significance of failure modes since there are quite a few failure modes operative simultaneously at any given time. Each failure mode can be attributed to be matrix-dominated, fiber-dominated, or interface-dominated. The long term performance of a FRP bridge deck may not simply degrade over time; abrupt failure might occur due to a change in the dominant failure mode. As a result, the weather durability of FRP composites remains a major concern in their use as primary load bearing members and structures. Moreover, how load capacity and structural stiffness will change over time present a great challenge to bridge designers when specifying allowable loads and reductions factors. Composite structures for civil infrastructure purposes have only been in service for a relatively short time. There are very few long-term data available to quantify environmental effects such as natural weathering on FRP structural components or systems [17,18]. Instead, most of the available durability studies were based on simulated laboratory testing. Test conditions generally vary greatly, since there are no standard test methods. Freeze and freeze/thaw exposures lead to material level degradation through matrix cracking and fiber–matrix debonding, increased brittleness, and substantial changes in damage mechanisms from those commonly observed under ambient conditions [19–23]. Chu et al. [24] used accelerated test data to predict long-term properties of pultruded E-glass/vinyl ester composites, and the predictions correlated well with their experimental results. Karbhari et al. [25,26] adopted a semi-empirical approach to predict the long-term modulus and strength of unidirectional carbon/vinyl ester composites subject to freeze–thaw cycling. Their study indicated that the analytical predictions are somewhat conservative for the strength and freeze/thaw cycling does not have an appreciable effect on modulus. FRP composites used in bridge applications are normally under sustained loads, however, sustained loading was not considered in the Karbhari’s durability study. It was also shown that freeze–thaw cycling between 4.4 and 17.8 °C alone caused very insignificant changes in flexural strength, storage modulus, and loss factor for the E-glass/vinyl ester specimens conditioned in distilled water and saltwater [27]. Furthermore, Wu et al. [28] concluded that (1) small reductions in modulus were observed after 250 freeze–thaw cycles in water when the specimens were loaded to 25% of their ultimate strain while undergoing freeze/thaw cycles, (2) no deterioration was observed for the specimens prestrained and freeze–thaw cycled in
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dry air conditions, (3) constant freeze at 17.8 °C resulted in a minor increase in flexural strength and storage modulus over time. It is noted that any accelerated test environment in a laboratory setting is certainly different than the real ‘‘service environment’’. An accelerated test method uses one or several accelerating mechanisms to increase the rate of degradation of a given material in a controlled laboratory setting. The rate of degradation under the accelerated condition must be related to the actual degradation rate under everyday or ‘‘service’’ conditions. Any credible accelerated condition must also produce similar failure mechanisms/sequences with those actually observed in the field. At present, there is no standardized durability test procedure for FRP materials for infrastructure applications. The goal of the on-going project is to simulate the effects of environmental exposure on long-term structural performance of FRP decks. The first objective of this study was to understand the effects of laboratory accelerated test conditions on the residual response of FRP composites that are being used for FRP decks [28,29]. The second objective is to develop acceleration factors that relate the laboratory accelerated test environments to the actual weather environments so that the long-term performance of FRP decks could be predicted in terms of realistic in-service time scale. This is the subject reported in this paper. 2. Mechanics based durability model A FRP deck is composed of several constituent materials of different characteristics. Under weathering conditions, individual material constituents can degrade at different rates. Therefore, a reliable performance simulation of the deck must be able to account for possible changes in the order of significance of failure modes that may occur over time. In this research, a performance driven design approach has been followed to construct the framework within which the durability study of fiber reinforced plastic composite bridge deck is organized [30]. Natural weathering conditions directly influence the composite deck’s performance. Quantifying the changes in various properties of the matrix and composite over time as a result of the environmental exposure is a key aspect to this study. In FE simulations of a FRP deck, the first step begins by defining the geometry, the layup (orientation of plies within the laminate), the environmental conditions, and a pre-straining level. The last factor is needed to reflect on the level of the traffic loads. Using the timedependent material parameters as inputs to the durability finite element model, predictions of weathering effects on the stiffness and load capacity of the FRP decks over time can be achieved. To expedite screening of the weather durability of FRP materials, a laboratory accelerated testing is usually employed. Therefore, it is necessary to establish correlation between the laboratory test environment and the actual field environment. This can be accomplished by measuring the degradation rates of the matrix materials and their unidirectional ply aged under these two conditions separately and by calibrating them with acceleration factors. The higher degradation rates from the laboratory testing can be ‘‘scaled down’’ by an acceleration factor to reflect the actual degradation rates in the field. Using such universal degradation rates of the matrix and its unidirectional ply, the deterioration of the modulus and strength of any FRP laminate of designated stacking sequence and ply orientation as a function of time, can be determined by employing micro-mechanics and the laminate theory. 2.1. Failure analysis of FRP decks FRP composites always experience some degree of delamination, broken fibers and cracked matrix, but any of those is only significant if the damage grows to occupy a large and/or critical area
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of the material so that the structure member starts to lose its integrity. For a composite sandwich structure, three common failure modes include face yielding (or rupturing), face buckling, and core shearing [29]. Core shearing is unlikely to occur in the case of a bridge deck. Instead, delamination between the face skin and the core is commonly observed in laboratory tests or from field inspections. Thus, delamination instead of core shearing was used in this study to determine the ultimate load capacity of FRP decks over time. In addition, a deflection limit was added according to code requirements such as AASHTO [31]. By continuously increasing live loads, the ultimate load capacity of a FRP-deck bridge is reached when any of the four failure criteria is first met, i.e. (1) first ply failure; (2) interface shear failure; (3) local buckling of the web or the skin; (4) displacement limits under service conditions. The four criteria can be categorized into two groups, i.e. long-term ultimate strength limit (Criteria 1–3) and long-term serviceability limit (Criterion 4). A detailed discussion on the formulations of these four criteria can be found elsewhere [29]. 2.2. Validation of short-term FRP deck predictions To simulate the behavior of FRP sandwich decks, an in-house FEA model has been developed [32,33]. The in-house layered model was incorporated into ABAQUS to simulate the behavior of FRPdeck bridges in two studies. In the first study [34], the deflections of a GFRP honeycomb deck bridge were simulated. This bridge, located in St. James, Missouri, had a total span of 8000 mm and a width of 8330 mm, and was test loaded by a truck of 242.17 kN with 76.69 kN, 81.58 kN, and 83.90 kN on the three axles. The deflections predicted by the FEA model agreed well with the field measurements. In the second study [33], the load–displacement response of a sandwich panel of 4460 mm by 1830 mm was simulated. The composite panel consisted of a lightweight core sandwiched between two six-ply skin plates (glass/vinyl ester composite). The load was applied over a 250 mm by 600 mm bearing plate with a 6.4-mm-thick Neoprene pad to simulate a factored wheel load of a HS25 truck. Two linear variable differential transducers (LVDTs) were set up at the bottom of the panel to monitor the displacements at two control points. Very good agreements between the predicted and measured displacements have been observed when the panel was subjected to loadings of 110 kN and 208 kN, respectively. It is noted that all the above simulations were carried out on newly constructed FRP deck structures without considering material deterioration due to weathering. For the purposes of durability simulations, the same FRP sandwich panel of 4460 mm by 1830 mm (as reported in [33]) was used. This panel was only tested once right after its construction and long term durability testing was not conducted on the panel. 3. Simulations of durability performance of FRP decks To simulate the long-term behavior of FRP decks, material constituent laws accounting for both strength and stiffness deterioration over time must be constructed and incorporated into the FE analysis. Such constituent relationships may be established confidently from in situ measurements, which are expensive and time-consuming. Alternatively, accelerated laboratory test results can also be used but the laboratory time scale must be well correlated to the in-service time scale under natural weathering conditions. This aspect will be further discussed in the next section. In addition, attention must be given to considering all possible failure modes. Individual material constituents degrade at different rates likely leading to a change in the order of significance of failure modes over time, since the stiffness and final structural failure is determined by its dominant failure mode at the time of failure.
Therefore initial design based on the failure mode at the time of the new construction would certainly unreliable when the actual failure modes change over time during the service period. 3.1. Material degradation There are two approaches to establishing the degradation relationships of FRP composites subject to natural weathering. The simplest way is to prepare neat resin samples and unidirectional FRP ply samples and place them in an outdoor environment. After exposure at designated time intervals, the samples are retrieved for testing. Therefore the degradation of the material properties could be characterized and the degradation rates could be determined. However, this procedure is time-consuming and the results are specific to the material under evaluation. The second approach involves accelerated laboratory test conditions that are designed to simulate natural environments. Laboratory results, then, must be correlated to the field performance under natural weathering conditions. Consequently, all materials of interest can be readily screened in laboratory for their long-term weather durability. In either way, the deterioration of off-axis plies and their laminates can be determined by micro mechanics and the laminate theory. The second approach was used in this study. After an extensive review on this subject, an accelerated test procedure to simulate the effects of natural weathering on FRP materials has been developed [27,28]. This procedure incorporates the combined effects of temperature, medium, and sustained load into a complete test program. Coupon samples will be preloaded and immersed in a selected medium in a special fixture. Each fixture can hold multiple specimens and is equipped with an adjustable pin that imposes bending strains to the specimens. Several fixtures can be loaded onto a programmable environmental chamber. Then thermal cycling begins. Samples are retrieved for evaluation after predetermined cycle numbers. More details of the durability testing can be found elsewhere [27,35]. For a glass/vinyl ester composite, the degradation rates of various properties of the skin laminate were measured up to 1250 h under the following laboratory conditions, 2-h freeze–thaw cycles (between 4.4 and 17.8 °C) and pre-straining (25% of ultimate strain capacity) in distilled water (hereafter referring to 2H-PS conditions). These conditions were found to be suitable to simulate a cold climate environment [28]. Assuming the same degradation rates beyond 1250 h, the time-dependent stiffness and strength of the same laminate over time were predicted. A detailed description of this approach can be found elsewhere [35]. It was confirmed that the properties of the matrix (vinyl ester) are more sensitive to environmental exposure than those of the composite, and the fiber dominated properties Ex and Ey decrease slowly with increasing the duration of environmental exposure as compared to the matrix dominated property Gxy. After 50,000 h of exposure under the 2HPS condition, the modulus of the matrix decreased by around 25%, while the longitudinal modulus of the laminate, Ex, decreased by only 5% [29]. No appreciable deterioration of the same FRP material was detected after 50,000 h exposure in water at room temperature [29]. These results confirmed again that the material properties of FRP laminates are strongly influenced by freeze–thaw cycles under pre-straining, especially for matrix and matrixdominated properties. Similar to modulus, the effect of cold climate conditions on strength reduction of FRP composites could be determined. The normalized flexural strength-vs.-time curves in various accelerated environmental conditions are plotted in Fig. 1. It is confirmed that freeze–thaw cycles coupled with pre-straining resulted in a much more rapid strength reduction, while the cycle length (2-h vs. 5-h) showed insignificant effect. It was assumed that the matrix domi nated strengths S L (longitudinal compression), ST (transverse com-
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pression), and SLT (in-plane shear) all experienced the same deterioration rates as the flexural strength and such information was used in the finite element analysis of the deck in the following sections. This shortcoming could be improved in the future when such data on strength reduction become available.
6000
Failure Load (kN)
3.2. Failure modes
3.2.2. Shear failure Shear stresses at the interfaces between the core and the outer skins may also result in shear failure. The transverse shear stresses can be determined at section integration points (listed as output variables TSHR13 or TSHR23 in ABAQUS). As shown in Fig. 2, the ultimate loads due to shear failure decrease more rapidly over time than those determined by the first ply failure criterion. Since the bonding material between the core and skins is a neat vinyl ester resin, the resin deteriorates much more rapidly over time than does the FRP laminate of the skin.
Normalized flexural strength
3.2.3. Local buckling failure Buckling modes and corresponding failure loads can be obtained by performing finite element buckling analysis. Among several critical factors (eigenvalues), only the first buckling mode is useful for the purpose of bridge deck design [35]. Furthermore, local buckling is predicted to take place right under the wheel load, and is consistent with the observations from several modular FRP bridges [36,37]. The local buckling failure load is found to be much lower than either the first-ply or the shear failure load. This is mainly due to the thin wall sandwich structure of the deck panel. Buckling loads strongly depend on the elastic properties of the FRP constituents. Therefore, stiffness degradations would cause reductions in ultimate load capacity. Fig. 2 also shows the ultimate load capacity due to buckling as a function of environmental expo1 0.8 0.6 0.4
2H-PS 5H-PS 2H 5H
0.2
5000
10000
15000
20000
Time (h) Fig. 1. Flexural strength deterioration in various environmental conditions. 2H and 5H represent 2-h and 5-h freeze/thaw cycles without pre-straining.
"Buckling"
3000 2000
0 0
5000
10000
15000
20000
25000
Time (h) Fig. 2. Various failure load vs. time under the standard 2H-PS condition.
sure time; the ultimate load capacity decreases by about 5% after 10,000 h under the standard freeze/thaw conditions. 3.2.4. Limit of deflection Time-dependent maximum allowable deflection was used as an index of serviceability limit in this FRP deck study. The deflection failure index Eld was calculated as the ratio of the maximum deflection of the deck to the allowable limit of 1/800 of the span length. Usually a FRP deck design is stiffness-controlled instead of strength-controlled. Under the standard accelerated freeze/thaw conditions, the deck was predicted to lose about 6.3% and 15.7% of its initial stiffness after 10,000 and 50,000 h exposure, respectively. The long term displacements at any monitor point on the deck could be then calculated from the FE analysis. For example, the displacements at the two locations near the center line but on the opposite side of the FRP sandwich panel of 4460 mm by 1830 mm were determined to be 2.6 and 3.2 mm during the initial test [33]. The deflection of the deck is shown to increase rapidly within a short time period and then follow a nearly linear increase over a much longer period. The same two locations are predicted to have deflection of 3.0 and 3.8 mm respectively after 50,000 h. Without pre-straining, the deflections remain almost the same under the same 2-h freeze–thaw cycling. This is due to a much higher stiffness reduction when the FRP material is aged under sustained loads. 3.3. Time scale It is difficult to define ‘‘service environment’’ that depends on its geographical location, and is vastly different than the laboratory accelerated testing conditions. Hence it is difficult to predict realistic service life of a FRP-deck bridge based on the deterioration rates of the FRP materials which are determined from laboratory testing. The rates of degradation under everyday or ‘‘service’’ conditions must be related to the degradation rates under the accelerated conditions. Such correlations may be accomplished by an acceleration factor. An acceleration factor therefore correlates the laboratory time scale from standardized accelerated testing to the real service time in the field. Acceleration factors for various natural environments may be determined using the framework outlined in ASTM E632, Standard Practice for Developing Accelerated Tests to Aid in the Prediction of the Service Life of Building Components and Materials. In a simple form, an acceleration factor can be calculated as the ratio of the degradation rate in the accelerated environment to the degradation rate in the service environment:
0 0
Shear
4000
1000
As discussed above, there are four failure indices for the determination of the ultimate load capacity of FRP sandwich decks, i.e., first-ply failure, interlaminar shear failure, local buckling failure, and deflection limit. 3.2.1. First ply failure The load capacity is determined by increasing vehicular wheel loads incrementally until the first ply failure occurs. The ultimate vehicular wheel load is thus time-dependent on the strength deterioration of the deck materials over time. As shown in Fig. 2, the load capacity decreases more rapidly after 10,000-h exposure under the standard 2H-PS (2-h cycle frequency and pre-straining) conditions than during the first 10,000 h. The load capacity under the standard accelerated conditions of 2H-PS loses its strength by 15% and 62% after 10,000 h and 20,000 h exposure, respectively. Compared to the stiffness deterioration, the load capacity decreases much more rapidly, especially after 10,000 h of exposure.
First Ply
5000
R¼
dA=dt dS=dt
ð1Þ
where dA/dt is accelerating degradation rate and dS/dt is in-service degradation rate.
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Degradation rates (dA/dt and dS/dt) in both the accelerated and service environments could be determined from the line regression of the strength or stiffness vs. time plots. When the degradation relationships are not linear with respect to time, other forms of regression might be more appropriate. To construct such correlations between laboratory and field environments, ideally the same FRP materials shall be placed in the field over a sufficient period of time. Periodically a few field samples should be retrieved and tested in laboratory in the same manner as those of the samples exposed to the laboratory conditions. The test results of the field-exposed specimens could then be correlated to the results of the laboratory specimens with confidence. However, composite structures for civil infrastructure purposes have only been in service for a relatively short time. There are very limited long-term material data available to quantify natural weathering effects. In this study, available field monitoring data on displacements from a FRP bridge deck were used to indirectly determine the acceleration factor for this bridge location.
In this study, in situ strain measurements at the location of Strain gage #1 over the entire monitor period of 4 years were selected for calibration purposes to determine the acceleration factor (see Fig. 4). By applying a linear regression analysis, the acceleration factor (R) was found to be 5.4 for the location of this bridge. The differences in the R values due to various accelerated conditions (as reported in Fig. 4) are insignificant. An acceleration factor of 5.4 indicates that the effect of undergoing 1,000 h under the laboratory freeze–thaw conditions of 2-h frequency and pre-straining in water results in the same level of damage after 5400 h outdoor exposure. Therefore, this factor permits a much reliable way of converting laboratory time scale to in-field time scale. It should be noted that the current acceleration factor was determined based on the 4-year’s data available at present and it was assumed valid over the entire life of the bridge. A more precise acceleration factor which may be a function of time is yet to be determined when longer term monitoring data are available. 4. Discussion on long term predictions in real time
3.3.1. Example bridge A FRP two-way web core skew bridge (NY, USA) was used as an example in this study. This bridge deck was constructed by the same FRP materials that were characterized by the accelerated tests at Wayne State University. This bridge was instrumented with 24 conventional strain gauges mounted externally on the bottom face skin of the superstructure [38]. The first proof load test was conducted right before the FRP deck was opened to traffic in October 1998, and established the reference/baseline for future bridge monitoring through similar load testing. The FRP deck was then tested periodically for four years to monitor its in-service durability. For this example bridge, the in situ changes in material properties were not directly measured; such changes could be indirectly estimated by finite element analysis. The FE procedure, described above, was used to simulate the load–displacement behavior and strain values at various locations of the bridge during the first load test. Very good agreements between the FE simulated strains and the field test results have been obtained (as shown in Fig. 3). The first load test was conducted right after the construction of the FRP deck, hence the strain measurements reflected the properties of the virgin FRP composites. The in-service degradation rates could be then determined by reducing the stiffness of the FRP materials until the predicted strains matched those of the field measurements during the subsequent load tests. On the other hand, the degradation rates of the FRP constituents, determined from the laboratory accelerated tests, were input into the FE model to calculate strains at all locations over time. By analyzing both sets of the strain data, the acceleration factor could be determined by Eq. (1).
Fig. 3. Comparisons of strains at various gage locations between FE simulations and field measurements from the first proof test in 1998.
The calibrated real time-dependent constituent relationships, together with the in-house layered model were input into ABAQUS to simulate the durability performance of the example FRP deck. HYPERMESH was used as the pre- and post- processor. A multi-step static analysis option (available in ABAQUS) was used to determine the failure load for each of the four failure modes. In each load step, stresses and displacements were determined, and then checked against the four failure criteria. If any of the failure indices, normalized load (or displacement) vs. critical failure load (or displacement) under each mode (details can be found in [29]), exceeded 1.0, the associated failure occurred. The load was then increased to the next higher level until another failure took place. The simulation was terminated after all four failures were found. For the new FRP deck, the failure sequences were deflection limit, local buckling, interlaminar shear, and first-ply failure (Fig. 5). Thus, the design of this FRP bridge deck was indeed ‘‘stiffness controlled’’, which was the most common case. The predicted maximum deflection was 3.08 mm which was in good agreement with the 2.75 mm measured by external LVDTs. The maximum allowable deflection was 9.76 mm per AASHTO [39]. After only 4 years of natural weathering, the dominant failure mode of the FRP deck would be changed from deflection limit to shear failure (comparing Figs. 5 and 6), followed by deflection, local buckling, and first-ply failure. The shear index under one standard wheel load increased from 0.15 at the time of new construction to
Fig. 4. Normalized strain vs. time at the location of gage #1 from field measurements and from various accelerated test conditions (e.g. 2H-PS: 2 h cycle length under prestraining load, suffix-S: salt water, -W: distilled water).
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5. Conclusions
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Wheel loading index (Wheel load/initial wheel load) Fig. 5. Failure index vs. wheel load at the completion of the deck construction.
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References
Fig. 6. Failure index vs. wheel load after 4 years of weather aging.
5 Deflection index
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To allow for sufficiently generalized descriptions of service life, credible durability simulations of FRP bridge decks must be created and must be able to predict the combined effects of mechanical loads and service environments. These simulations must be robust and be developed from reliable descriptions of material degradation mechanisms and their interactions, which may include characterizations from accelerated testing to extend the validity of the predictions. Such simulations must be validated over a wide range of conditions, at both the component and structural levels. In this paper, a mechanics based durability model has been formulated to predict the performance of FRP-deck bridge systems under natural weathering conditions, especially in cold climates. The focus was placed on the ability to predict long term structural performance. Material constituent laws accounting for both strength and stiffness deterioration over time were first developed and then incorporated into the FE analysis. Such constituent relationships were established from experimental data obtained from laboratory accelerated freeze/thaw testing and from in-field performance measurements. An acceleration factor correlating laboratory time scale with in-service time scale was proposed and found to be able to predict the changes in stiffness and deflection of a monitored FRP bridge deck over a 4-year period. Nevertheless, more long-term performance data is needed to further validate the proposed durability model, in particular the acceleration factor.
15
Wheel loading index (Wheel load/initial wheel load) Fig. 7. Failure index vs. wheel load after 50 years of weather aging.
0.47 after 4 years, indicating that the probability of shear failure has greatly increased. Partial delamination was actually observed from visual inspection during the 4-year monitoring period [40]. After 50 years in service, it was predicted that the shear index (1.27) already exceeds 1.0 under one HS25 wheel load (Fig. 7), followed by deflection (0.37), buckling (0.22), and first ply failure (0.21). This suggested that the interface layer would deteriorate much faster than the skin and the core of the FRP sandwich materials. In this case, it was predicted that interlaminar shear stress at the interface between the core and the skin would exceed the residual shear strength of the adhesives within 20–30 years, leading to total delamination. Although in-service long term performance data are not available for verification, the current predictions seem to be reasonable. It should also be noted that other environment effects such as UV radiation or high temperatures can be readily incorporated into the existing framework by employing adequate degradation rates in a similar fashion to those due to freeze–thaw cycling.
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