4 Durability: Aging of Composites
4.8 ACCELERATED AGING
where, σ, σa, f, and γ are, respectively, instantaneous strength, maximum applied cyclic stress, and dimensionless functions that do not depend on σ. Other methods of expressing damage due to fatigue include stiffness as the damage metric. Fatigue studies conducted on FRP bonded wood and FRP composite deck panels have been modeled using damage energy concepts by comparing the total energy to the energy loss per fatigue cycle [Natarajan et al. 2005]. Several studies indicate excellent fatigue performance of FRP composites under fatigue [GangaRao et al. 2001;
Odagiri et al. 1997]. FRP bar-reinforced concrete decks have shown excellent fatigue performance similar to those of steel bar-reinforced decks [Kumar and GangaRao 1998].
4.7.8 ULTRAVIOLET (UV) RADIATION
FRP composites exposed to UV radiation undergo photochemical damage near the exposed surface, leading to discoloration and reductions in molecular weight that results in the degradation of composites [Kato et al. 1998]. Long UV exposure durations can lead to resin erosion that may lead to fiber exposure, moisture penetration, and matrix cracking, causing a reduction in the thermomechanical properties of composites [GangaRao et al. 1995]. Carbon fibers are less susceptible to UV damage in comparison to glass or aramid. Strength and stiffness reduction due to UV exposure is greater in thin composites than in thick composites. UV inhibitors are mixed with resins during FRP manufacturing to resist damage caused by UV radiation. External FRP reinforcement bonded to concrete beams is pro-tected from UV radiation with aesthetically pleasing special coatings that contain UV inhibitors.
4.8 ACCELERATED AGING
Information on the durability of field-installed FRP applications is limited and not available for a variety of resin-fiber-process combinations. Typically, long-term strength and stiffness values of FRP reinforcement for concrete applications are extrapolated based on short-term accelerated aging test results. Accelerated aging tests consist of subjecting FRP composites and FRP reinforced or bonded concrete beams to elevated temperatures or freeze-thaw cycling under water, salt, alkaline, or acidic solution immersion. Based on accelerated aging test results, charts are prepared using time-temperature-stress superposition principles (Section 4.8.1 and Section 4.8.3). Using those charts, accelerated aging test data are correlated to
dσ/dN= −( / )1 γ σ σf aγ 1−γ
natural aging results of FRP composites. Natural aging consists of exposing FRP specimens to natural environmental weathering in open areas consisting of some or all elements such as sunlight, rain, snow, freeze-thaw cycling, humidity changes, and temperature variations. The Arrhenius temperature dependence concept described below (Section 4.8.2) is used for correlating tension test data obtained from accelerated aging tests with those from natural aging (weathering). Additional details on accelerated aging methodology are available in the literature and a brief summary of accelerated aging methodology — along with its limitations — is provided in Section 4.8.3 [Litherland et al. 1981; Proctor et al. 1982; Porter and Barnes 1998].
4.8.1 TIME–TEMPERATURE–STRESS SUPERPOSITION PRINCIPLE
A polymer composite material property such as time-dependent stress at one tem-perature can be used to find those properties at another temtem-perature (with certain limitations), which is referred to as the time-temperature-stress superposition prin-ciple. This principle is employed to calibrate naturally aged results of FRP at ambient temperature with accelerated aging results. A procedure employing the above prin-ciple to predict the service life of an FRP composite is described in Section 4.8.3 along with a brief description of the Arrhenius principle.
4.8.2 THE ARRHENIUS PRINCIPLE
The Arrhenius principle states that rate at which chemical degradation occurs is dependent on temperature. This principle is employed to exploit the temperature dependence of polymers subjected to environmental aging consisting of several temperature levels.
where k is the reaction rate constant with respect to a temperature T, A is a “pre-exponential factor,” Ea is the activation energy for the reaction, R is a constant, and T is the temperature in Kelvin.
4.8.3 ACCELERATED AGING METHODOLOGY
The following procedure is used to correlate natural aging to accelerated aging [Litherland et al. 1981; Vijay and GangaRao 1999; Vijay 1999].
Step 1: Consists of subjecting the composite specimens immersed in cement representative pH solution conditioning schemes to 6 or 7 evenly spread different temperature aging from –20°F (low temperature may slow down aging but causes brittle failures) to 180°F (below glass transition temperature).
Step 2: Consists of plotting strength loss curves (which are typically nonlinear curves conforming to some power law, e.g., C = Co + mtn) with respect to an aging
k=Ae− /E RTa
period (number of days). Strength loss is plotted along the vertical axis and the aging period is plotted along the horizontal axis (Figure 4.1).
Step 3: Consists of plotting the curves in Step 2 for an Arrhenius-type relation-ship, i.e., A = Ao exp (–ΔE/RT). The log (time to reach a particular strength value, i.e., 90, 80 ksi) is plotted along the vertical axis and the inverse of temperature (°K) is plotted along the horizontal axis (Figure 4.2).
FIGURE 4.1 Strength retention of aged FRP at different temperatures.
FIGURE 4.2 An Arrhenius plot for temperature- and time-dependent strength retention.
Aging duration (days)
1 10 100 1000
50 100
Conditioned FRP strength (ksi)
160°F 130°F 70°F 20°F
−20°F 100°F
1/T(×103) °K−1
Time to reach selected FRP strength value (days)
3 3.25 3.5 3.75
100
1 10
1000 40 ksi
50 ksi
60 ksi
70 ksi
80 ksi
90 ksi
Step 4: Involves normalizing the curves in Step 3 into a single curve by plotting the logarithm of the time (for a given strength loss) at different aging temperatures (T = 273 + t0, selected in Step 1) along the vertical axis (relative to the time at some REFERENCE temperature), against the inverse of temperature along the horizontal axis (Figure 4.3).
The normalization procedure is as follows:
• Select a REFERENCE temperature, e.g., 70°F.
• Plot the logarithm of the ratio of the time taken for the composite strength to reduce to a given value at T = 273 + t0 (pick all the temperatures individually as selected in Step 1) relative to the time to reduce to that value at 70°F (reference temperature) versus the inverse of the absolute temperature corresponding to t0 (where the time is read from the fitted curves plotted as per Step 2).
Step 5: A normalized Arrhenius plot gives one overall picture of the relative acceleration of strength or stiffness loss at different temperatures. From the known time-scale shift (i.e., plot of Step 4), changes expected over a long period under lower service temperatures is predicted by considering following calibration.
• Strength loss data from naturally weathered samples (Figure 4.4)
• Using the mean annual temperature and other factors (i.e., moisture, freeze-thaw, and pH level) as a basis for calibration
Litherland, Oakley, and Proctor [1981] have correlated their accelerated aging data of glass fibers with natural weathering samples of about 10 years. In their tests, FIGURE 4.3 Normalized time displacement curve (Arrhenius plot) relative to a reference temperature.
1/T(×103) °K−1
Time shift relative to time at a reference temperature
3 3.25 3.5 3.75
10
0.1 1 100
the media surrounding the glass was cement representative, so as to correlate natural and accelerated aging. Some of the factors to be considered before using Litherland, Oakley, and Proctor’s method described above are:
• The mean annual temperature is taken as the sole criteria for determining the accelerating factors. The identical mean annual temperature at differ-ent locations does not necessarily account for the geographical variations in magnitude and distribution of temperature, humidity, and precipitation throughout the year.
• The correlation of natural and accelerated weathering is carried out on samples without stress.
• Present-day manufacturing methods and durable resins offer a better degree of protection against water, salt, or alkaline attack, thus taking more time to reduce the strength to a selected value under identical aging conditions considered by Litherland et al. [1981]. In effect, the shift of the time-scale factor is necessary while interpreting Litherland’s data.
A study by Vijay and GangaRao [1999b] correlated accelerated and natural weathering on GFRP bars. Calibration charts developed for the nonstressed GFRP bars show that one day of chamber conditioning (accelerated) in their study was equivalent to 34 days of natural weathering at Morgantown, West Virginia, or 36 days of typical U.K. weather. These calibrations were developed similar to the accelerated aging results of Litherland, Oakley, and Proctor [1981] on glass-rein-forced composites. Chamber weathering (freeze-thaw between 12.2° to 120.2°F or –11° to 49°C) of 30 months in alkaline conditioning (pH = 13) carried out in this study corresponds to natural weathering of 1020 months (85 years). However, under FIGURE 4.4 Normalized time displacement curve (Arrhenius plot) with natural weathering data.
1/T(×103) °K−1 Time shift relative to time at a reference temperature (days)
3 3.25 3.5 3.75
100
0.1 1 10
Naturally weathered FRP Accelerated-aged FRP
a sustained stress of 20%, the natural weathering of GFRP bars was equivalent to 704 months (58.67 years) instead of 1020 months. Concrete cover was found to provide a beneficial effect of slowing down the aging duration (time to reach a particular strength loss value) of FRP bars embedded in cracked concrete beams.