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Performance of epoxy-coated prestressing strands at elevated temperatures.

Epoxy-coated prestressing strand (ECS) is a relatively new product designed to reduce the effects of corrosion on steel strands in pre-stressed applications. The strand itself is ordinary seven-wire, low-relaxation strand, coated with an epoxy by a high-temperature powder application method (fusion bonding).[1] The epoxy coating is then impregnated with aluminum oxide grit to improve the strand's bonding characteristics.

Potential users are concerned about how well ECS will perform when exposed to elevated temperatures. High-temperature performance is critical since, during accelerated curing, concrete temperatures near the prestressing strand could range between 52 [degrees] C and 88 [degrees] C.[2] Research indicates that the epoxy coating on the strand begins to soften at temperatures as low as 63 [degrees] C.[3] This softening allows shear strain to occur along the epoxy-strand/epoxy-concrete interface. Significant strand slip has been found at temperatures above 71 [degrees] C.[3-6] Recently published Pre-stressed Concrete Institute (PCI) guidelines recommend that prestress transfer be conducted with the temperature of the concrete near the strand being no greater than 65.5 [degrees] C and falling.[7] This recommendation is intended to prevent prestress loss during stress transfer due to softened epoxy.

As part of its "Investigation of Development Length of Uncoated and Epoxy-Coated Prestressing Strand" project, the Federal Highway Administration recently concluded a series of experiments to determine the temperature at which epoxy softening causes slip, the effect of that slip on the stress in the strands and the transfer length, and the effect of cyclic temperature variations. These findings are summarized below.

Study Specimens

The 10 concentrically prestressed specimens used for this experimentation were rectangular beams, 3.7 meters long, with cross sections ranging from 102 by 102 millimeters (mm) to 229 by 229 mm. The specimens contained either one or four epoxy-coated strands with strand diameters of 9.5, 12.7, or 15.2 mm. A single specimen of 12.7-mm uncoated strand was also tested. The ECS used was grade 270, seven-wire, low-relaxation strand, conforming to ASTM Standard A416.

The strand was stressed with a center hole hydraulic jack and a hand pump. The maximum stress in the strand at transfer was 75 percent of the strand's guaranteed ultimate tensile strength. Thermocouples were placed along the center line of the specimens in the area of the strands to provide internal temperature readings during the testing.

Test Apparatus and Instrumentation

A test oven was constructed out of fire-resistant plywood; it was lined with 50 mm of mineral wool insulation. Ceramic resistance heaters were controlled by adjustable thermostats, and thermocouples monitored the ambient oven temperature. The oven was constructed with a detachable top to allow transfer lengths to be measured before and after the elevated temperature test without moving the specimen. Whittemore gauge points were installed along the concrete surface on both sides of the specimen during casting; these were used to monitor the transfer length for one year after the specimens were cast.

A door at either end of the test oven allowed access to the specimens for placing instrumentation on them while they were in the oven. The strand end slip was measured with linear variable differential transducers designed for elevated temperature environments. The transducers were attached to the ends of the strands and used to measure the slip of the strand relative to the face of the concrete. A measured strand end slip of greater than 0.254 mm on one end of the strand was established as the threshold for defining beam failure due to strand slip failure.

Test Procedures

Four procedures were used to evaluate ECS performance when exposed to elevated temperatures:

* Procedure 1 involved heating the specimens slowly over an eight-hour period. Specimen temperature was increased until end slip measurements exceeded the slip failure threshold. Temperature measurements were taken every 15 minutes.

* In procedure 2, specimens were heated to approximately 66 [degrees] C, then cooled overnight, on four consecutive days.

* Procedure 3 consisted of heating single specimens to above 77 [degrees] C - the temperature that had caused similar specimens to slip - and maintaining that temperature for 48 hours.

* Procedure 4 was identical to procedure 3, except that the temperature was 88 [degrees] C, maintained for three hours.

Strand slip measurements were made during all four procedures.

Findings and Conclusions

The following are the general findings of the experiments:

* ECS in the specimens tested slipped in excess of the 0.254-mm failure threshold at temperatures above 71 [degrees] C. Strand slippage of less than 0.254 mm occurred at temperatures above 63 [degrees] C, indicating some softening of the epoxy at that temperature.

* Cycling at temperatures less than 71 [degrees] C resulted in slippage of more than 0.254 mm in only four cycles. This is further indication that the epoxy coating softens at temperatures of less than 71 [degrees] C.

* Strand slippage resulted in a longer transfer length. This finding may be the result of disruption of the aluminum oxide grit from its original position in the cement matrix. The grit impregnated in the epoxy coating plays a significant role in ECS bonding, and the dislocation of these particles from their original position may affect strands' transfer and development length. Increased transfer lengths resulted from increased strand slip.

This research indicates that temperatures in the vicinity of ECS of greater than 66 [degrees] C will undermine the strand's bonding ability. This may result in a strand stress significantly less than anticipated. Epoxy coating on strand may soften at temperatures as low as 63 [degrees] C and degrade the strand's ability to bond with the surrounding concrete. These temperatures are significantly below the maximum temperatures that may be reached during accelerated curing.

The research also showed that the slippage of strand after concrete curing will result in longer transfer length.

These findings support the PCI guidelines; PCI recommendations should, therefore, be followed in producing prestressed beams using ECS.

For more detailed descriptions of the test procedures and the results of each test, contact Sue Lane (703-285-2111) or Glenn Washer (703-285-2388) at the Turner-Fairbank Highway Research Center in McLean, Va.

References

1. Susan Lane. "Transfer Lengths in Rectangular Prestressed Concrete Concentric Specimens," Public Roads, Vol. 56, No. 2, September 1992.

2. Prestressed Concrete Institute. Manual for Quality Control for Plants and Production of Precast and Prestressed Concrete Products, Section 3.4.2. Pub. No. MNL-116-85, Chicago, 1985.

3. Fernando E. Fagundo. "The Effect of Steam Curing and Sustained External Temperature of the Bond Slip Performance of Epoxy Coated Strand (ECS)," Department of Civil Engineering, University of Florida, Gainesville, Fla., 1991.

4. Fernando E. Fagundo. "Bond Slip of Epoxy Coated Strands (ECS) at Elevated Temperatures," Department of Civil Engineering, University of Florida, Gainesville, Fla., 1991.

5. Philip J. LeClaire. "The Effect of Temperature on the Bond Strength of Epoxy Coated Prestressing Strand," University of Wisconsin-Milwaukee, 1991.

6. T.D. Lin. "Test of Epoxy-Coated Strand at High Temperatures," Construction Technologies Corporation, Skokie, Ill., 1989.

7. Precast/Prestressed Concrete Institute. "Guidelines for the Use of Epoxy-Coated Strand," PCI Journal, Vol. 38, No. 4.

Glenn Washer is a research engineer in the Structures Division in the Office of Engineering and Highway Operations Research and Development at the Turner-Fairbank Highway Research Center in McLean, Va. He has a bachelor's degree in civil engineering from Worcester Polytechnic Institute and is currently participating in the structural engineering graduate program at the University of Maryland. He is a licensed professional engineer in Virginia.
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Author:Washer, Glenn A.
Publication:Public Roads
Date:Jun 22, 1995
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