TRB Dowel Study

Bian, Kohler, and Harvey 1

LABORATORY EVALUATION OF FIBER REINFORCED POLYMER DOWEL BARS FOR JOINTED CONCRETE PAVEMENTS

Yi BianDoctoral candidate

Dept. of Civil and Environmental EngineeringUniversity of California, Davis

One Shields Ave.Davis, CA 95616

Tel.(530) 304-5763 Fax (530) [email protected]

Erwin KohlerProject Scientist, Ph.D.

University of California Pavement Research Center, UCPRCDept. of Civil and Environmental Engineering

University of California, DavisOne Shields Ave.Davis, CA 95616

Tel.(530) 754-8699 Fax (530) [email protected]

John T. HarveyAssociate Professor

Department of Civil and Environmental EngineeringUniversity of California, Davis

Engineering III, Room 3153One Shields Avenue

Davis, CA 95616(530) 754-6409 FAX (530) 752-7872

[email protected]

Text: 5010Figures: 10 Tables: 0

Total Word Count: 7510

Submitted on November 15 2006

TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal.


ABSTRACT

Fiber Reinforced Polymer (FRP) dowel bars were subjected to mechanical evaluation inlaboratory conditions. Flexural and shear performance were measured on control and conditioned specimens to determine their adequacy for long-term service in concrete pavements. The conditioning process consisted of submergence in alkali solution to simulate the high pH in concrete, exposure to water to simulate high moisture content, and ultraviolet radiation from sunlight to simulate the sunshine during transporting and handling. Strength, stiffness, and fatigue life are described in detail, and test results including susceptibility to the three conditioning processes on bars from two manufacturers (two types of bars) are discussed. Stiffness was found to be independent of load frequency, but a 10 to 15 percent stiffness reduction was found to occur as temperature changed from 20°C to 40°C. It was also found that when FRP dowels fail in bending fatigue, they fail when their stiffness is reduced to about 50 percent of the original (unconditioned) stiffness.



1.0 INTRODUCTIONFor rigid pavement, load transfer efficiency (LTE) between adjacent slabs is an important factor influencing both pavement ride quality and structural life. Load transfer is the ability of a joint in rigid pavement to transfer load from one slab to the next through shear action (1), which can be provided by aggregate interlock and by load transfer devices such as dowel bars. During a pavement’s early years of service and under warm weather conditions, aggregate interlock can provide high load transfer efficiency, but load transfer will vary throughout the year because of temperature changes. When temperatures are low, the joint opening widens and aggregate interlock is greatly reduced. Over time, traffic load repetitions also reduce aggregate interlock by abrading the faces of the joint.

To provide high and consistent LTE, dowel bars in highway pavements are placed across transverse joints (2, 3). When dowel bars are used, loads near the joint are carried by the two adjacent slabs, and the slab stresses and deflections are smaller than those in an undoweled pavement. As a result, a doweled pavement develops little or no faulting, thus improving ride quality considerably.

Use of steel dowel bars as the load transfer device at transverse joints has been an essential design principle in many U.S. states. Steel dowel bars, however, risk corrosion due to moisture and road de-icing agents. The mechanism of how corroded steel dowels lose their effectiveness is not very clear yet (4). One possible reason that joints with corroded dowel bars undergo decreased LTE could be that the dowels lose volume after the corrosion products are washed away (5). Another problem for the pavement is that the corrosion products may expand and lock the joints. However, if corrosion damage to dowel bars were to be eliminated, the effective life of the dowels could be considerably extended. In view of the corrosion risk for steel dowels, some alternative materials, such as fiber reinforced polymer (FRP), are considered.

FRP is a material that has advantages such as a high strength-to-weight ratio and an excellent resistance to electrochemical corrosion. This material has been used extensively to repair and strengthen reinforced concrete beams and columns in many parts of the world during the past decade (6).

FRP dowel bars are being considered as an alternative to steel dowel bars because of their better resistance to corrosion, lower maintenance cost, and longer pavement life. However, the application of FRP is restricted by a lack of material specifications (7). In order to assess FRP dowel bars for broader applications, both lab and field tests are required to study the material’s mechanical properties in detail, especially durability and fatigue properties. In-service dowel bars are subject to a combination of both shear and bending loads from heavy vehicles. To evaluate the performance of FRP dowel bars, the stiffness, strength, and fatigue behavior, both flexural and shear, were tested in this study. Because of the potential changes in FRP dowel bar properties after environmental exposure, the results of different conditioning processes were evaluated.

2.0 LITERATURE REVIEWSteel dowel bars have been used extensively in the U.S. over the last 30 years. However, the exposed conditions at slab joints (drainage, moisture, de-icing chemicals) are highly corrosive to steel, and therefore the lifespan of steel dowel bars may be significantly reduced due to metal corrosion (5). As a result, the benefit of dowels is lost when LTE is reduced due to corrosion.

To eliminate the dowel bar corrosion, corrosion-free FRP dowel bars are being considered as possible replacements for steel dowel bars (8, 10). Related research work has been



conducted to explore the properties of FRP dowel bars. Larralde (8) analyzed the feasibility of utilizing FRP bars in the longitudinal joints of concrete pavements. It was found that the cross-sectional area required for FRP dowel bars was greater than that needed for steel dowel bars. This was because the elasticity modulus of FRP was lower than that of steel. In terms of material cost, FRP bars are approximately 50% more expensive than steel bars (8); however, preventing corrosion can potentially extend the service life of joints and therefore that of the concrete pavement.

Sargand and Cinadr (9) compared the performance of four different dowel bar types: 38-mm diameter steel and glass FRP dowels, and 38-mm high steel and glass FRP I-beams. They also studied how the environment affected dowel bars. It was concluded that the dowel bars with greater stiffness or moment of inertia could better transfer higher loads across joints. For dowels made of the same material, the 38-mm I-beams transferred lower loads than the 38-mm diameter dowels; while for the dowels with same cross-sectional shape, steel bars were more effective than FRP bars. Among the four dowel types, the 1.5-in. diameter steel dowel was the most effective. Under the environmental test, the FRP I-beams experienced much lower bending stresses than the other dowel types. In general, the forces caused by environmental changes were more significant than those caused by dynamic loads.

Eddie et al. (10) presented field and laboratory results on glass FRP dowel bar performance. The study included laboratory testing under static and cyclic loading as well as field application using the falling weight deflectometer (FWD). Three types of 38mm-diameter FRP dowel bars together with the 32mm-diameter epoxy-coated steel dowel bars were tested and the shear strengths of the dowel bars were determined by double shear tests. Testing results indicated that according to the measured LTE and slab vertical deflection, FRP dowel bars offer similar short-term performance compared with steel dowels (both in static and cyclic testing). Also, it was found that the FRP with relatively higher shear strength provided higher load transfer capacity than the FRP with lower shear strength. The author concluded that larger FRP dowel diameter and lower elastic modulus would decrease dowel-concrete bearing stresses and consequently reduce dowel looseness and eventual faulting.

Lab and field studies have been performed at other research institutions (11 - 18). In Iowa, the results of lab and field testing indicate that FRP dowels provide adequate performance. The lab component of the work (11) had looked into material properties, direct shear, elemental fatigue, and pullout forces. One interesting result from the pullout tests is that FRP dowels (and also stainless steel dowels) require about 10% of the force than for the epoxy coated dowels, indicating that these alternative dowels allow for slab expansion and contraction with a lower internal force in the concrete. Full scale lab testing led to conclusions that the spacing between dowel bars has to be reduced to obtain adequate bearing stresses in concrete. However, the field component of the study, which evaluated about 700 dowels in Iowa (13), revealed that all of the dowel types investigated produced adequate load transfer, and faulting and joint opening were similar for all cases; therefore all the joints were operating properly. Porter et al. (14, 15, 16)compared un-aged FRP and steel dowel properties and developed an approximate model for accelerated aging of FRP material that simulates real weather aging. It was found that the accelerated aging solutions of water, lime and salt had little effect on the shear strength behavior of any of the FRP dowel bars.

Little research has been done to investigate FRP dowels’ behavior under flexural fatigue and shear fatigue loadings with different frequencies and temperatures (frequency sweep). Such testing results are important to estimate the FRP dowels’ material properties after being subject



to repeated traffic load and temperature cycles. These properties are essential to predict FRP dowels long-term field behavior. In addition to frequency and temperature, FRP materials may be affected by other environmental factors such as alkaline attack, water damage and ultraviolet radiation. To date, very limited testing data is available on FRP dowel bars long-term durability properties after being exposed to different environmental conditions. Such testing results areimportant to determine whether FRP dowel bars can be applied in a wide range of environmental conditions.

3.0 EXPERIMENTAL SET UPThe study consisted of evaluating the flexural and shear properties of glass FRP dowel bars. The flexural tests were performed following the ASTM D6272-00 procedure and using loading fixtures made according to ASTM D4476-97. The four-point loading configuration is shown in FIGURE 1(a). The flexural fatigue tests were run under load control, and the middle span displacement values were recorded during the tests. The shear tests were performed following ASTM D4475–02 using the same loading fixtures but in a different arrangement. The shear-bending load configuration is shown in FIGURE 1(b).

The testing took place at the University of California, Davis using an MTS loading unit with an environmental chamber. The fatigue tests were performed with a load frequency of 6 Hz, while the stiffness tests were carried out at frequencies of 2, 6 and 10 Hz. The majority of the testing was conducted at a temperature of 20°C, but stiffness testing at 5°C and 40°C was also performed to capture the effect of pavement temperature on the FRP dowels. Operating difficulties prevented the use of more extreme temperatures, as desired from pavement temperature data obtained from different cities in California (Daggett, Los Angeles, Sacramento, San Francisco, and Arcata) (19). These field-measured temperatures range from 1°C to 45°C, taken at mid-depth from hourly records.

The tested dowels were 38-mm diameter (1.5-in) and 457-mm (18-in) long bars obtained from two manufacturers, and are referred to as Type A and Type B. The two bar types differ from each other slightly on the materials and fabrication process. Both have similar fraction content of E-type glass fiber (typically 70 percent for Type A and 72-73 percent for Type B). Additionally, the matrix for Type A bars is polyester resin while for Type B bars the matrix is epoxy.



(a)

(b)FIGURE 1 Load configurations for (a) flexural tests and (b) shear tests.

3.1 Conditioning of SpecimensThe flexural and shear tests were conducted using control as well as conditioned FRP specimens. The conditioning process consisted in exposing the specimens to (1) alkaline solution, (2) water, and (3) ultraviolet radiation.



FRP dowel bars, especially the glass FRP dowel bars, could be highly sensitive to alkaline attack (20). Alkalis can directly attack the silica in the glass. When a glass surface is exposed to alkali solution, the surface will dissolve. This reaction process successively exposes a fresh glass surface to alkali solution and eventually the whole glass mass is dissolved. Since the concrete pore solution is highly alkaline, with a pH level about 12.5 to 13.5 (21), it is important to understand if the fibers are adequately protected by the matrix from such attack.

Water is another factor which may affect the material degradation of FRP dowels. The water absorbed by polymers can soften the polymer structure and lead to FRP stiffness loss (22).Also, moisture can weaken the fiber-matrix interfacial bond. It is suspected that moisture can even penetrate the dowels and this may result in fiber degradation.

Finally, it is possible that FRP materials are prone to the degradation caused by solar ultraviolet (UV) radiation (23). Although dowel bars will not be exposed to UV rays once they are in the concrete, they can be exposed to sunlight during transporting and handling. UVradiation has wavelength between 290 nm (nanometer) and 400 nm, and most FRP materials have bond dissociation energy on this wavelength range (24). Under UV radiation, the bond dissociation between fiber and matrix will be initiated and chemical changes may be induced in the matrix material in the presence of oxygen. The interface degradation can result in stress concentrations and lead to the initiation of rupture at a stress level which is much lower than the failure stress for an original FRP material.

3.2 Conditioning ProcessesA set of specimens was submerged in an alkali solution for three months before being tested. They were kept in covered plastic tanks, undisturbed, and at room temperature inside the laboratory. The alkali solution, which was made to simulate a Portland cement paste, is composed of 0.32 mole/L KOH, 0.17 mole/L NaOH, and 0.07 mole/L Ca(OH)2 in distilled water (25). The pH level for the alkali solution was 13.5. A second set of specimens was submerged in water for three months, in covered plastic tanks, to observe how the moisture affects FRP mechanical properties. The third set of bars was exposed to direct sunshine for 2 months (July and August), and the total measured accumulated radiation density was 4,580 J/cm2. The dowels were turned over every two days to make sure both sides were exposed to almost equivalent amount of sunshine radiation.

4.0 TEST RESULTS

4.1 Flexural StiffnessFlexural Stiffness tests were carried out on two replicate specimens, at a temperature of 20°C and using loading frequencies of 2, 6, and 10 Hz. FIGURE 2 shows the average stiffness from the three test frequencies for control and conditioned bars. The Coefficient of Variation (COV) for all tests was less than 2 percent. The first observation is that Type B bars are approximately 20 percent stiffer than Type A bars. The second observation is that flexural stiffness in Type A barswas practically unaffected by the conditioning processes, while stiffness in Type B bars decreased 4 percent for water conditioned and 6 percent for UV conditioned specimens.



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Type A Type B

Stif

fnes

s (G

Pa)

Control Alkali conditioned Water conditioned UV conditioned

FIGURE 2 Flexural stiffness for control and conditioned bars.

The stiffness of control bars and of all three types of conditioned bars was also tested at 5°C and 40°C to evaluate thermal susceptibility and to compare to the stiffness at 20°C. FIGURE 3 presents stiffness at these three temperatures and the three loading frequencies evaluated in this study. Changing the loading frequency in the range of 2 to 10 Hz had no effect on dowel stiffness. The importance of testing at different frequencies relates to the effect of traffic speed over the doweled joint. As for most polymer materials, the stiffness of FRP decreases with higher temperature. FIGURE 4 better illustrates this point, showing that for both dowel bar types the stiffness (average for all frequencies) at 40°C is lower than at 20°C. The reduction is about 9 percent for Type A bars and 15 percent for Type B bars. The stiffness reduction as the temperature changes from 5 to 20°C is not as pronounced, within 2 percent for both bar types. The anomaly in alkali conditioned Type B bar is attributed to the susceptibility of this bar type to this particular conditioning process, as explained later.



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FIGURE 3. Stiffness at different test temperature for three loading frequencies



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0 5 10 15 20 25 30 35 40 45Temperature (C)

Stif

fnes

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Type A

FIGURE 4 Stiffness versus test temperature.

To estimate the impact of a reduction in stiffness, the maximum stress in the concrete in the slab with different dowel bar stiffness was estimated using finite element software EverFE (26). The analysis showed that the maximum concrete stress is mostly unaffected when dowel stiffness changes 10 – 20%. Therefore, the change in dowel bar stiffness that can be expected within the range of normal operating temperatures for concrete pavements should not significantly impair the pavement’s performance.

4.2 Flexural StrengthFlexural strength results for control and conditioned bars are summarized in FIGURE 5. The COV for all tests was between 1 percent and 4 percent, except for Type B alkali conditioned bars, for which the COV was 19 percent. The failure modes were bottom fiber rupture in middle span. The flexural strength of control (un-conditioned) bars was found to be approximately 80 percent higher for Type B than Type A bars. The results also show that the alkali solution exposure can significantly reduce the strength of Type B bars. The other conditioning process did not affect the Type B bars. Type A bars were not affected by any of the conditioning processes.



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Type A Type B

Flex

ural

str

egth

(M

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FIGURE 5 Results of flexural strength in control and conditioned bars.

The sudden change in Type B alkali conditioned bars strength deserves more explanation. At every step of the experiment a random selection process was followed to determine which conditioned specimens would be subjected to each test (strength, stiffness, or fatigue). Two replicates of Type A and Type B bars were tested. After a discrepancy was noted on the alkali conditioned Type B bars, two more specimens were tested. Out of the four Type B alkali conditioned bars tested for strength, one bar had no initial crack while the other three bars presented initial cracks of the type shown in FIGURE 6. These initial cracks were visible from one or both ends of the bars, and could be observed before any load application. This damage due to conditioning resulted in highly variable strength data (COV = 19 percent) for Type B bars. As shown in FIGURE 5 the average strength of alkali conditioned Type B bars is 40 percent lower than control bars of the same Type.



FIGURE 6 Damage on type B bars after exposure to alkaline solution.

Water conditioned bars presented no initial cracks, and the results indicate that the water conditioning process does not considerably lower the FRP strength. No signs of possible UV-induced degradation, such as surface gloss loss, surface resin flaking, pitting, and cracking, were found through visual check. From testing results, the UV radiation does not have a significant influence on FRP bar strength.

4.3 Flexural FatigueThe results of fatigue testing on control bars are presented in FIGURE 7. Five and four control specimens for Type A and Type B bars, respectively, were tested at various stress ratios ranging from 0.4 to 0.8. During the fatigue loading cycles a minimum load of 6 kN was applied to keep the bars in contact with the supporting fixtures. Fatigue life can be well predicted by a base-10 logarithmic function. The results for the two types of bars were combined to obtain the following flexural fatigue life for FRP dowel bars.

n = 10(12.75 – 12.74×S), R2 = 0.9475Where n is the number of load cycles to failure and S is the stress/strength ratio. For

Type A bars the point at S = 0.4 was retained in the regression, even though it deviates from the linear trend that can be derived from the other data points. No replicate fatigue test was conducted because of the long time required to complete testing at low stress/strength ratios.

Conditioned specimens were also tested for flexural fatigue and only Type B bars exposed to alkali solution and Type B bars exposed to UV radiation presented a significant reduction in fatigue life, as shown in FIGURE 7.

Crack



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Stress ratio, S

Flex

ural

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igue

Lif

e, n

(cy

cles

)

Type A - control Type B - controlType A - alkali Type B - alkaliType A - water Type B - waterType A - UV Type B - UVType A regression Type B regressionCombined Types A & B

FIGURE 7 Flexural fatigue life for control and conditioned bars.

4.4 Reduction in Elastic Modulus with Repeated LoadingThe dowel bar elastic modulus was continually calculated during the fatigue testing as the number of cycles increased. The change in elastic modulus throughout the testing for control and conditioned bars is shown in FIGURE 8(a).



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0 1 2 3 4 5 6Load cycle (millions)

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stic

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lulu

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Type A - control, S=0.54 Type B - control, S=0.45Type A - control, S=0.58 Type B - control, S=0.50Type A - alkali, S=0.49 Type B - alkali, S=0.49Type A - water, S=0.57 Type B - water, S=0.48Type A - UV, S=0.56 Type B - UV, S=0.49

(a)

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Type A - control, S=0.54 Type B - control, S=0.45Type A - control, S=0.58 Type B - control, S=0.50Type A - alkali, S=0.49 Type B - alkali, S=0.49Type A - water, S=0.57 Type B - water, S=0.48Type A - UV, S=0.56 Type B - UV, S=0.49

Control Bars

Conditioned Bars

(b)FIGURE 8 Elastic modulus vs. load cycles (a) control and conditioned bars (b) normalized.



The rate of the change of the modulus is greater at the beginning of the fatigue test. Specimens failed, in bending, when the stiffness dropped to approximately half of the initial values. To compare the elastic modulus against loading repetition on several specimens, the “normalized stiffness” against “normalized cycle” for all types of dowel bars is shown in FIGURE 8(b). Normalized stiffness is the ratio to the initial stiffness, while normalized cycle is the ratio to the bar’s fatigue life (27). As mentioned, it appears that the stiffness for the control bars (whose curve falls below the dashed line in FIGURE 8b), had reduced to roughly one half of the initial stiffness by the time of failure. For conditioned bars the normalized stiffness at failure is higher than those of control bars, but that is because of the lower initial stiffness of conditioned bars (conditioning damage). All tested specimens tend to fail when the stiffness drop to a specific range, around 15 – 20 GPa (FIGURE 8a), which is approximately half of the original control (un-damaged) bar stiffness.

4.5 Shear StiffnessStiffnesses evaluated with the shear configuration were found to be very similar, as expected, to the results obtained in the flexural configuration (FIGURE 2), and therefore are not presented again.

4.6 Shear strengthThe shear strength results for control bars are summarized in FIGURE 9. The bars failed by fiber rupture in shear. Similar to the flexural testing results, average shear strength of Type B bars is approximately 85 percent higher than Type A bars. As mentioned, some of Type B bars presented fine end cracks after being submerged in alkali solution. The two specimens tested for shear strength test did not have visible initial cracks. From these testing results, it seems that for Type A bars and Type B bars without visible initial cracks, the alkali solution exposure does not influence shear strength.



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Shea

r St

reng

th (

MPa

) .


FIGURE 9 Shear strength.

4.7 Shear FatigueSimilar to the flexural fatigue, shear fatigue curves were fit to data from control bars of both types (FIGURE 10). Type A bars showed greater shear fatigue life than Type B bars. No combined regression was carried out because of the difference in performance. The conditioning process affected in general Type B bars to a greater degree than Type A bars.



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0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Stress ratio, S

Shea

r fa

tigu

e lif

e, n

(cy

cles

) .

Type A - control Type B - controlType A - alkali Type B - alkaliType A - water Type B - waterType A - UV Type B - UVType A regression Type B regression

FIGURE 10 Shear fatigue life for control and conditioned bars.

5.0 SUMMARY AND CONCLUSIONSFiber Reinforced Polymer (FRP) dowel bars from two manufacturers were subjected to mechanical evaluation in laboratory conditions. Flexural and shear performance was measured on control and conditioned specimens. The conditioning process consisted of submergence in alkali solution, water, and ultra violet exposure. Although the number of specimens tested might not be sufficient to draw categorical conclusions, there are several important observations:• Stiffness for FRP bars is not influenced by applied loading frequency in the range of 2 to 10

Hz. However, stiffness is affected by testing temperature as expected: bars exhibit approximately 10 percent (Type A) to 15 percent (Type B) lower stiffness at 40°C that they do at 20°C.

• Flexural stiffness for Type A and Type B bars was found to be about 37 and 45 GPa, respectively. Flexural strength for Type A was approximately 500 MPa while for Type B bars it was near 900 MPa. Strength from the shear test for Type A bars was about 300 MPa and it was 600 MPa for Type B bars. As it can be seen, Type B bars are roughly 20 percent stiffer, 80 percent stronger in bending, and 100 percent stronger in shear than Type A bars. These results are for control bars not subjected to conditioning of any kind.

• Test performed on conditioned specimens revealed that strength of Type B bars might be reduced by the high pH environment within the concrete slabs. After exposure to an alkali solution for three months, many specimens presented fine cracks at the ends, which increased considerably the variability of flexural strength results. Stiffness and shear strength results



for Type B bars were not affected by the alkali conditioning, but the small number of specimens tested prevents a definitive conclusion.

• Water and UV conditioning had no effect on the measured properties of the dowel bars. Type A bars were not affected by any of the three conditioning types.

• Flexural fatigue tests revealed that both types of bars offer similar fatigue performance, and the number of cycles versus stress ratio can be well defined by a logarithmic function. Fatigue in the shear configuration can also be described by logarithmic functions, but the results showed that a greater number of shear cycles can be expected from Type A bars than from Type B bars. Alkali conditioning seems to reduce flexural fatigue life of Type B bars, while UV conditioning reduces their fatigue in shear. At low stress ratio (0.3 to 0.4), both types of FRP bars will likely survive more than 100 millions of wheel load repetitions.

• Analysis of stiffness reduction during flexural fatigue tests indicated that FRP dowels fail in fatigue when the stiffness is reduced to about 50 percent of the original (unconditioned) stiffness.

6.0 ACKNOWLEDGEMENTSThis material describes research activities requested and sponsored by the California Department of Transportation (Caltrans), Division of Research and Innovation. Caltrans sponsorship is gratefully acknowledged. The authors want to thank dowel manufacturers for the samples donated. The contents of this publication reflect the views of the authors and do not necessarily reflect the official views or policies of the State of California or the Federal Highway Administration.

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TRB Dowel Study

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