Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

Fiber Composite Dowels in Highway Pavements

Max L. Porter, Bradley W. Hughes, and Bruce A. Barnes

M.L. Porter,
Department of Civil and Construction Engineering,
Iowa State University,
Ames, Iowa 50011.

B.W. Hughes, Ch2M Hill, Inc.,
411 E. Wisconsin Avenue, Suite 1600,
Milwaukee, Wisconsin 53202.

B.A. Barnes, Black & Veatch,
11900 E. Cornell Avenue,
Aurora, Colorado 80014.

Fiber composite dowel and reinforcing bars have been investigated at Iowa State University for use in highway pavements and other concrete structures. The nonmetallic composite bars provide a corrosion-resistant alternative to steel dowel and reinforcing bars. Two recently funded research projects supported by the Highway Division of the Iowa Department of Transportation and the Iowa Highway Research Board were conducted in the Structural Engineering Laboratory. Fatigue and static tests were performed on full-scale concrete pavement slabs supported by a simulated subgrade, including a single transverse joint. Single dowels cast in concrete underwent static shear testing using a test method developed previously, and test results from elemental and full-scale tests were compared and related. The behavior of full-scale specimens with both steel and fiber composite dowels placed at test joints was monitored during several million load cycles, which simulated truck traffic at a transverse joint. Performance of the fiber composite dowels indicated that they are at least as effective as steel dowels in resisting degradation of load transfer efficiency under cyclical loading. This paper focuses primarily on the dowel bar results.


This research studied the use of fiber composite (FC) dowel bars as shear load transfer devices in concrete highway pavements. Fiber composite dowels (1) are closer in relative stiffness to concrete than are steel dowels, which reduces damage to the concrete at the dowel-concrete interface and (2) eliminate corrosion because of their resistance to the corrosive agents that attack typical metallic reinforcement.

Dowels placed at the locations of transverse joints in highway pavements act only to transfer shear across the joints. Steel dowel bars are a standard type of load transfer device used by the Iowa Department of Transportation (Iowa DOT), for whom this research was conducted. Because steel dowels are a very rigid member within the concrete, repeated loading of the joints can lead to an "ovalling" of the concrete around the standard steel dowels. This results in a looser joint which can lead to further problems, such as subgrade failure. Because dowels are placed at the locations of shrinkage cracks in highway pavement, corrosion of steel dowels by de-icing salts that leach through cracks is a common concern. Corrosion may cause dowels to bind or lock if they are unable to move longitudinally within the pavement during temperature shrinkage or expansion.

The FC materials studied consisted of E-glass fibers and a thermoset vinyl ester resin molded into the shape of a 1.75-inch-diameter (45 mm) rod. The rod was produced by the pultrusion process, and a smooth exterior was applied for dowel applications.

EXPERIMENTAL AND ANALYTICAL INVESTIGATION

The object of this study was to develop a laboratory test method for the evaluation of highway pavement dowels that approximated actual field conditions, and to compare static and fatigue behaviors of FC dowels to those for steel dowels when used as load transfer devices in transverse joints of highway pavements.

STATIC AND FATIGUE TESTING OF DOWELS

A test method, referred to as a modified Iosipescu shear method, was verified during work by Lorenz (1) for a single dowel specimen in concrete. In this research, a FC dowel and concrete system was tested using the modified Iosipescu shear test shown in Figure 1. The test specimens had outside dimensions of 10 by 10 by 23 inches (254 by 254 by 584 mm) and were formed of concrete with a single dowel embedded in the center.

Strain gages measured longitudinal strain at 1.5 inches (38 mm) from the joint. The instruments provided a means of relating flexure of a dowel within the concrete to the load transferred by the dowel by a linear equation developed for both steel and FC dowels.

Efficiency of a highway pavement joint is determined by monitoring the relative displacement between the two sides of a joint and load transfer across the joint. The fatigue caused by cyclic loading reduces the joint efficiency in transferring load (2). Indicators of reduced efficiency as the number of load cycles increases are (1) an increase in the relative displacement of the two sides of the joint when loaded and (2) a decrease in the fraction of load transferred across the joint. In this research, a method of laboratory testing was developed that monitored the performance of doweled pavement joints undergoing cyclic loading.

Test specimens used in the fatigue testing of pavement dowels were full-scale concrete slabs with dowels placed in the slabs at a formed joint in the specimens. Each slab was cast in place in the laboratory on top of steel supporting beams and was 12 inches (305 mm) thick, 6 feet (1.83 m) wide, and 12 feet (3.66 m) long. Dowels were placed in the slab at the middle of the thickness with one-half of their length on each side of the joint.

The slab specimen called Slab 2 was formed and cast in place in the laboratory using 1.75-inch-diameter (45 mm) FC dowels at 8 inches (203 mm) center-to-center along the joint. This spacing was determined by computer modeling to be equivalent to using 1.5-inch (38 mm) steel dowels at 12 inches (305 mm), the standard for Iowa DOT highways. The slab referred to as Slab 3 used the standard steel dowel configuration along the joint.

Fatigue Test Setup and Procedure

In the Iowa State University Structural Engineering Laboratory, a dynamic actuator system capable of loading at high frequencies applied cyclical loads to the test slab with a maximum load of 9,000 pounds (40 034 kN). Figure 2 is a diagram of the laboratory testing setup.

Relative displacements at the joint under static loads were measured at each dowel location. Load transfer across the joint by the dowels during static load testing was also monitored and measured. Strain gages were mounted on the steel supporting beams underneath the test specimens, from which strain was measured, and the load applied to each of the beams was calculated. In addition to monitoring load transfer by the individual dowels, strain gages were mounted on the dowels nearest the load application exactly as gages were mounted on the dowels in the elemental specimens discussed above.

The full-scale slab testing procedure involved subjecting the specimen to cyclic loading and, at intervals, stopping to test the slab under static loads equivalent to those during cycling. Data were collected only during the static load tests performed on the slabs. During the cyclic loading of the specimens, load was applied alternately to each side of the joint to simulate truck traffic passing over the joint by the two hydraulic actuators.

The dowel underwent a full range of load transfer during the repeated loading. This subjected the dowel/concrete system to the most extreme fatigue loading conditions to which an actual system would be subjected with the same magnitude of load. A maximum of 2 million load cycles was applied to Slab 2, and ten million cycles were applied to Slab 3.

Results

The critical relative displacement was that which occurred at the maximum applied static load of 9,000 pounds (40 034 N). This displacement, as well as the load transfer across the joint, were indicators of the degradation of joint efficiency, and were observed at each static load test as the number of load cycles increased. The two slabs, one with FC dowels and one with steel dowels, both tended to follow the anticipated trend of degrading efficiency of the joint as the number of applied load cycles increased. Degradation was noted by observing the relative displacements at the joint, load transfer across the joint, and measured strains in the dowel bars.

In terms of relative displacements at the joint, the performance of the test slabs was evaluated by observing plots of the maximum relative displacements due to a 9,000 pound (40 034 N) static load applied to one side of the joint versus the logarithm of the number of applied load cycles at the particular load test. For the slab specimens with both FC and steel dowels, the maximum relative displacements at the joint during the static load tests tended to

increase as the number of cycles increased. A common behavior for both specimens was that the most significant change in the relative displacements occurred during the first 200,000 cycles. The increase in relative displacements during the first 200,000 cycles was roughly equivalent to that occurring beyond that point. Such behavior indicates that the long-term performance of a pavement dowel system should be evaluated only after a large number of load cycles have been applied.

A second method for evaluating the efficiency of pavement joints and dowels is the load transferred across a joint by the dowels. The joint efficiency of the full-scale test slabs, indicated as a percentage of the total applied load that is transferred, is shown in Figure 3 for both Slabs 2 and 3. From the plot of data for Slab 2, the percentage of load transfer appeared to stay rather constant over the 2 million applied load cycles, whereas the Slab 3 results over the same number of cycles indicated a decrease in the percent transferred.

In order to determine the portion of the total load transferred by each dowel, strain data from full-scale testing of both FC and steel dowels were applied to the equations developed from elemental testing discussed earlier. For both Slabs 2 and 3, measured strains in the dowels at the maximum load of 9,000 pounds (40 034 N) applied to the slab were substituted into the equations developed from the elemental tests.

Load transfer values calculated for both types of dowels demonstrated the behavior of the dowels with instrumentation in the full-scale specimens before cyclic loading was applied. In the full-scale specimen using 1.75-inch (45 mm) FC dowels spaced at 8 inches (203 mm), the average calculated values of load transfer for the center three dowels was 938 lbs (4172 N). In both cases any additional load transferred across the joint was distributed to the remaining six FC dowels or four steel dowels.

CONCLUSIONS

The joints using FC dowels studied in this research performed better than joints using standard steel dowels when both were subjected to conditions that simulated actual highway pavement use, including cyclic loading. The load transfer efficiency of 1.75-inch (45 mm) FC dowels spaced at 8 inches (203 mm) in a full-scale pavement slab was nearly constant (approximately 44.5% load transfer) through 2 million applied load cycles of 9,000 pounds (40 034 N). The load transfer efficiency of 1.5-inch (38 mm) steel dowels spaced at 12 inches (305 mm) in a full-scale pavement slab decreased (approximately from 43.5% to 41.0% load transfer) over the first 2 million load cycles. For both FC and steel dowels, relative displacements due to a 9,000 pound (40 034 N) static-applied, interval-monitored load at pavement joints increased with the log of the number of load cycles applied. These results indicated that the FC dowels spaced at 8 inches (203 mm) provided a more efficient system initially, and a system that did not degrade as rapidly with repeated loads as the steel dowels spaced at 12 inches (305 mm).

This work shows that there is a favorable behavior of FC dowel bars in fatigue and static strength. Therefore, further work on FC dowel bars is justified and necessary. Further fatigue testing of FC dowels, possibly in elemental dowel specimens, to study more closely the performance of dowels and their interaction with concrete should be considered.

ACKNOWLEDGMENTS

The authors would like to thank the personnel at the Iowa DOT who provided support through their experience and knowledge toward the project, namely Brian McWaters and Vernon Marks. Additional consultation and input from several individuals, including: Phil Catsman of Corrosion Proof Products, and Dr. Daniel Adams of Iowa State University, is also acknowledged by the authors. Additional thanks go to Douglas L. Wood, the Materials Analysis and Research Laboratory, and the many laboratory assistants involved in the research at Iowa State University.

REFERENCES
  1. E.A. Lorenz. Accelerated Aging of Fiber Composite Bars and Dowels. M.S. thesis, Iowa State University, Ames, Iowa, 1993.
  2. L.W. Teller and H.D. Cashell. Performance of Doweled Joints under Repetitive Loading. Highway Research Board, Bulletin No. 217, 1958, p. 849.
  3. M.L. Porter, B.W. Hughes, B.A. Barnes, and K.P. Viswanath. NonCorrosive Tie Reinforcing and Dowel Bars for Highway Pavement Slabs. Submitted to Highway Division of the Iowa Department of Transportation and Iowa Highway Research Board, Project No. HR 343, November 1993.
  4. B.W. Hughes. Experimental Evaluation of Non-Metallic Dowel Bars in Highway Pavements. M.S. thesis, Iowa State University, Ames, Iowa, 1993.

The research described herein (3) was conducted at the Iowa State University Structural Engineering Laboratories with the support of the Engineering Research Institute and sponsored by the Iowa DOT and the Iowa Highway Research Board. Additional experimental and analytical results are presented in Mr. Hughes's M.S. thesis (4).

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