Ozyildirim and Sharp 1 PRESTRESSED CONCRETE PILES WITH CORROSION-FREE CARBON FIBER COMPOSITE CABLE

Transcription

1 Ozyildirim and Sharp 0 PRESTRESSED CONCRETE PILES WITH CORROSION-FREE CARBON FIBER COMPOSITE CABLE Celik Ozyildirim, Ph.D., P.E., Corresponding Author Virginia Center for Transportation Innovation and Research 0 Edgemont Road Charlottesville, VA 0 Tel: () -, Fax: () -0; celik@vdot.virginia.gov Stephen R. Sharp, Ph.D., P.E. Virginia Center for Transportation Innovation and Research 0 Edgemont Road Charlottesville, VA 0 Tel: () -, Fax: () -0, Stephen.Sharp@vdot.virginia.gov Word Count:, + Tables + Figures =,0 Submission Date: July, 0

2 Ozyildirim and Sharp ABSTRACT This project investigated carbon fiber composite cable (CFCC) as a replacement for traditional steel strands in bridge piles. The Virginia Department of Transportation (VDOT) placed CFCC in the piles of the two bents of the Nimmo Parkway Bridge over the West Neck Creek in Virginia Beach. Both the strands and the spirals had CFCC. These piles are considered to be a corrosion-free option compared to the traditional piles. In the beginning, two test piles were cast, instrumented, and driven at each of the two bents. Since the fabrication and driving operation were successful, the remaining piles were cast and driven. VDOT now has the ability to implement the use of a corrosion-free strand in prestressed elements where corrosion is a concern, such as those exposed to brackish water, saltwater, or deicing salts. Keywords: Carbon fiber composite cable, CFCC, CFRP, Reinforcement, Concrete, Corrosion

3 Ozyildirim and Sharp 0 0 INTRODUCTION The Virginia Department of Transportation (VDOT) now uses steel reinforcement that is alloyed for corrosion- resistance in bridge deck construction, which ensures that the steel itself has inherent corrosion resistance rather than relying on a barrier such as an epoxy coating or crackfree low-permeability concrete. This change demonstrated that one of the primary factors in selecting reinforcing materials should not be initial costs (). Instead, the important factor is the up-front cost plus the future cost associated with deck maintenance operations (often-calculated using life-cycle cost analysis); especially when minor changes in material costs could greatly reduce maintenance costs (). VDOT s efforts have addressed conventional reinforcement in bridge structures; however, the use of corrosion-resistant prestressed strands in bridge elements has not been addressed. Strands are used in relatively small quantities in bridge structures but are subjected to greater stress when compared to traditional deck reinforcement. Corrosion is more critical in strands that are under high-stress conditions as compared to traditional reinforcing steel bars. Wires can fracture even though section loss attributable to corrosion is small because of the higher stress in each wire and the stress intensity in the area of corrosion. Then, as corrosion progresses in different wires in an area and more individual wires fracture, the remaining wires in the strand can become overloaded and an unexpected rapid failure of the structural steel strand can result. Corrosion-related damage to prestressed or post-tensioned strands has been observed in the field. An example of a post-tensioned high-strength steel strand that exhibited section loss attributable to corrosion is shown in Figure. Further, when corrosion is left unchecked, complete loss of steel continuity can occur. Examples of failed external post-tensioned tendon and broken strands in a prestressed beam are also shown in Figure. These examples have created uncertainty for transportation agencies because of concern that damage might be occurring in other similar bridge elements. Clearly, with a design life of to0 years being sought, it is important that the high-strength strands also exhibit high corrosion resistance just like the corrosion- resistant reinforcement in the decks. Repairs to elements with strands are costly and difficult since they are generally loadcarrying members under the deck. In many cases, traffic must be interrupted, causing inconvenience and safety concerns related to work zones. In many structures, the costs of traffic control and repairs can easily approach a large percentage of building a new structure. FIGURE Corrosive attack on structural strand showing section loss in steel strand (left), failed posttensioned tendon (middle), and corroding strands in beams because of leaking joints (right).

4 Ozyildirim and Sharp 0 0 As a progression of the corrosion-resistant reinforcement studies, research at the Virginia Center for Transportation Innovation and Research (VCTIR) focused on the high-strength strand materials. Work by others with high-strength carbon fiber composite cable (CFCC) in prestressed elements has shown promising results (-). Corrosion-free (i.e., CFCC) strand in bridge piles is expected to result in large savings by eliminating the widespread costly corrosion problem in prestressed elements, such as those exposed to brackish water, saltwater, or deicing salts. Some concern exists because CFCC typically has a higher initial cost than traditional steel strand. However, CFCC can be cost-effective compared to traditional steel because of its corrosion-free performance (). PURPOSE AND SCOPE The purpose of this study was to validate that CFCC in prestressed elements can be technically and cost-effectively employed for bridge structures at high risk for corrosion. The study focused on ensuring VDOT s successful implementation of CFCC strand as a replacement material for traditional steel strand. This work involved the fabrication and driving of bridge piles for the Nimmo Parkway Bridge over the West Neck Creek in Virginia Beach. METHODS Overview VDOT placed CFCC in the piles of the two bents of the Nimmo Parkway Bridge over the West Neck Creek in Virginia Beach. The strands and spiral materials were CFCC. Initially, two test piles were cast at the producer s Plant and driven. During driving, the test piles were instrumented and the dynamic response was compared to that of the conventional piles with steel strands. One year later, the production piles were cast by the same producer, but at a different facility, known as Plant. The design information for the conventional steel strand piles and the CFCC piles are summarized in Table. TABLE Design Information for Conventional and Carbon Fiber Composite Cable (CFCC) Piles Property Conventional Steel Strand Piles CFCC Piles Pile size (location) in square (in all Bents except in square (in Bents and ) in and ) No. of strands Strand diameter 0. in 0. in Strand pattern Square Circle Spiral Galvanized W. (0.-in CFCC (0.-in diameter) diameter) Initial tension per strand kips kips Minimum ultimate strength 0 ksi (low relaxation) 0 ksi (low relaxation) Area of strand 0. in 0. in Initial prestress per strand /0. = ksi /0. = 0 ksi Initial prestress/fu.%.% Fu = ultimate strength

5 Ozyildirim and Sharp 0 CFCC Properties The manufacturer indicated that the CFCC would meet the properties listed in Table (). It is important to note that the total elongation does not meet the requirements of ASTM A, i.e., a minimum value of.%. TABLE Manufacturer's Specifications for Carbon Fiber Composite Cable Property Limit Value Guaranteed tensile capacity (Pu) a Greater than. kn/mm ( ksi) Tensile modulus a Greater than kn/mm (, ksi) Elongation at break Equal to.% Specific gravity Equal to. Relaxation b Less than.% Creep strain c Less than 0.0 x - Coefficient of linear expansion d Less than 0. x - / o C (0. x - / o F) Specific resistance Equal to 000 cm (, in) Creep failure load ratio e Greater than 0. Fatigue capacity stress range f Greater than 0 N/mm ( ksi) Bending stiffness Greater than. kn/cm (. ksi) Heat resistance Greater than 0 o C ( o F) a Calculated by effective cross section. b 0. * Pu,,000 hr (0 ± o C) c 0. * Pu, 000 hr (0 ± o C). d 0 o C to 00 o C e At million hours f x cycles at 0. * Pu Test Piles Concrete Properties The mixture proportions given in Table were used in the fabrication of both the CFCC and conventional test piles at Plant. A commercially available air-entraining admixture, a retarding admixture, and a high-range water-reducing admixture were also added. The specified -day compressive strength was,000 psi, and the release strength was,00 psi. The specified air content was % to %. Because of the addition of a high-range water-reducing admixture, the maximum slump of in was permitted provided there was no visible segregation. TABLE Mixture Proportions of Concretes for Piles (lb/yd ) Ingredient Plant Plant Type III portland cement Type F fly ash Coarse aggregate (No. ) Fine aggregate (natural sand) Water Maximum water cementitious material ratio Calcium nitrite (gal/yd ) Concrete properties of the piles at the fresh and hardened states were determined. Table lists the tests conducted at the fresh and hardened states and the specification that governed each test.

6 Ozyildirim and Sharp 0 0 TABLE Concrete Properties and Related Specifications Test Specification Fresh concrete Slump ASTM C Air content ASTM C Temperature ASTM C Unit weight (density) ASTM C Hardened concrete Compressive strength ASTM C Elastic modulus ASTM C Permeability ASTM C0 Casting of Piles The fabrication of the piles including the end preparation of strands, prestressing, concrete placement, steam curing, and detensioning was documented. Driving Operation The driving of a pile is a physical process, as the repetitive blows to the pile head drive the pile into the ground. Therefore, piles were visually inspected for cracked or damaged concrete prior to and after placement. The piles were instrumented in order to determine the dynamic response for comparison to that of the conventional piles with steel strands. RESULTS AND DISCUSSION Test Piles Two -in- square test piles with CFCC were cast at the same bed. The prestressing bed was 0 ft in long to accommodate the piles; one of the piles, P, was ft long, and the other pile, P, was ft long. The CFCC in spools, couplers, and material for the preparation of the ends of the CFCC were shipped from the manufacturer s facility in Japan. One interesting side note is that since CFCC is not a ferrous material, Buy America requirements do not apply. CFCC is currently produced in Japan, so additional shipping time is required. The producer has indicated interest in establishing a CFCC fabrication facility in the USA in early 0. Strand Handling The CFCC was unspooled, placed on the plywood surface, and cut to length. Cutting was done by a saw with a carbide blade. The CFCC was handled with care to avoid rubbing and cutting, but a damaged strand was detected (Figure ). The gouge could have been caused by rubbing or by a sharp or heavy object. The damaged length was discarded. There was no other visible damage in the remaining strands, however, it is important to remain attentive when handling the strands and watch for damaged areas. The strands were placed in the forms treated with release agent. Paper was placed along the bottom of the form to create a barrier between the CFCC and the release agent to avoid contamination of the strand. In addition, while the strands were placed through the metal head plates, plastic bushings were used to prevent any scraping or damaging of the CFCC during the placement and stretching of the CFCC.

7 Ozyildirim and Sharp FIGURE Gouge detected on carbon fiber composite cable prior to tensioning. Strand End Preparation To prestress concrete using CFCC, the CFCC was attached to the steel strand through the use of a coupler (this method is sometimes referred to as double chucking ) and the steel was pulled with the hydraulic jack. The end of the CFCC was prepared so that wedges do not damage it during stressing. First, a mesh sheet made of layers of metal and plastic were wrapped around the end portion of CFCC. The end preparation of these two test piles were performed by the manufacturer s technicians, who flew in from Japan. The couplers were within the pile forms, and the technicians worked in the tight form space. Second, a braided grip was put on the mesh sheet (Figure ), which is made of stainless steel. The prepared end was held by the four-part wedges in a chuck barrel. A tool was used to ensure equal spacing between the wedges, and the wedges were pushed into the chuck barrel evenly. A specially designed apparatus with a hydraulic jack was used to push the wedges into the barrel. A mark on the wedge indicated how far to push. 0 FIGURE End preparation of carbon fiber composite cable with mesh sheet and braided grip (left) so that -part wedges are evenly pushed into the barrel (right). After completing the end preparation of each CFCC strand, the chuck holding the CFCC was placed on one end of the coupler as shown in Figure. The steel strand was then placed in the other end of the coupler, and a traditional chuck was used to secure it. After all couplers were assembled, they were staggered and a preload of kips was applied with the jack.

8 Ozyildirim and Sharp 0 0 FIGURE Carbon fiber composite cable and steel couplers joined (left), and couplers staggered and tensioned (right). Prestressing After applying a preload of kips, the jack was used to increase the load in increments of kips until the maximum tension of kips was achieved. Normally for steel, continuous and rapid loading is done by completing prestressing within 0 sec; however, with CFCC, longer prestressing is desired, about. to min. At each preload and then at maximum load, the extension of the CFCC was measured. There was no noticeable slippage, and the expected elongation occurred. For safety reasons regarding the use of a new strand material at this facility, the CFCC was kept prestressed overnight. On the following day, spirals that were already in the bed (placed before the strands) were tied to the CFCC with plastic ties. The CFCC spiral was light and could be carried easily by one person. There were no sags in the CFCC tendons after placement of the CFCC spiral; sagging would be normal with the conventional steel reinforcement because of the heavy weight of the steel. For each pile, two lifting devices were placed. The lifting was accomplished by inserted threaded rods. The rods were placed in cardboard tubes to avoid contact with the CFCC and the spiral. Later, the bolts and the cardboard were removed and the hole was grouted. The pile forms were then blown clean and ready for concrete. The spiral and lifting device are shown in Figure. Figure also displays corrugated plastic at the head of the pile where the dowels are placed for connection to the pile bent. The use of the plastic was to prevent contact between the conventional steel reinforcement and the CFCC to prevent a galvanic cell between the two that could cause corrosion of the reinforcing steel since the CFCC is a more noble material. Although the CFCC wires are coated with a plastic material that should prevent such activity, the use of the plastic pipe avoided the possibility of contact due to damaged coating on the CFCC fibers. Concrete Properties Concrete was batched in a central plant and delivered in trucks with augers carrying yd each. The first (B) and fourth (B) of the five loads were sampled for fresh and hardened concrete tests. These specimens were steam cured in the bed overnight and then brought to the VCTIR laboratory, where they were kept in a moist room until testing. The fresh concrete properties are given in Table. Workable concretes with proper air contents were achieved.

9 Ozyildirim and Sharp FIGURE Spiral tied to carbon fiber composite cable with plastic ties (left) and lifting device placed (right). TABLE Fresh Concrete Properties Test Test Piles Production Piles B B B B Slump (in).... Air content (%).0... Density (lb/ft ).... Concrete temperature ( F) The hardened concrete properties are summarized in Table. The -day strength and permeability values are the average of two specimens, and the -day values are the average of three specimens. The compressive strengths exceeded the specified minimum -day strength of,000 psi at days. The elastic modulus values were high. TABLE Hardened Concrete Properties Test Age Test Piles Production Piles (days) B B B B Compressive strength (psi) Elastic modulus ( psi) Splitting tensile strength (psi) Permeability (C) steam cure Permeability (C) accelerated cure at 0 F = No data The permeability specimens for conventional cast-in-place concretes tested at days require accelerated curing. They are cured the first week at room temperature and for the next weeks at 0 F; they are tested at days. Accelerated curing enables the determination of long-term permeability at an early age of days. Pozzolans or slag cements in concrete show their effectiveness after the hydration reactions, which take time. Steam-cured specimens are not subjected to the weeks of 0 F curing because the high steam temperatures provide

10 Ozyildirim and Sharp accelerated curing similar to the curing for weeks at 0 F. The permeability values for the specimens were C for B and C for B at days when the specimens were kept moist at room temperature after the initial steam curing. These are high permeability values; for prestressed elements, VDOT specifies a maximum value of 00 C. The temperature of the beam reached F; however, the specimens were near the couplers that were exposed to lower temperatures (varying between F and F) to prevent slippage. Two of the specimens from each batch were also tested after curing at 0 F for days in accordance with the accelerated curing. The permeability values were 0 C for B and C for B, which are very low values indicating high resistance to the penetration of liquids. In specimens subjected to relatively low steam cure temperatures overnight, the high early temperatures for permeability reduction may not be attained. Small specimens do not generate as much heat as the large beams. Large beams would exhibit higher temperatures than the small cylinders and may exhibit reduced permeability at early ages. Placement The concrete was discharged into forms starting from one end of the bed to the other. Concrete was consolidated using the internal vibrators with rubber heads to prevent damage to the CFCC. During concrete placement, shifting of the spirals occurred and the spacing was altered. More plastic ties and discharging of some concrete to the bottom to hold the spirals in place were planned for future work, which resulted in better control of the spiral spacing. Upon completion of placement, the bed was covered and the specimens were placed at one end over a rack under the cover. A thermocouple placed a couple of inches inside the surface monitored the pile temperature during the steam curing. Another thermocouple was placed in the enclosure at each end of the bed. The plant used temperature-matched curing to determine the compressive strength for detensioning. Temperature-matched cure (TMC) molds have heating elements and were kept in the laboratory. The wireless unit near the forms sent the temperature signals to control the temperature of the TMC molds in the laboratory. The control unit matches the temperature of the member to that of the TMC mold. In the early afternoon, right after concrete placement, the temperature recording started showing that the enclosure temperature was about F and increased to 0 F in hours. The temperature of the concrete in the pile based on the thermocouple was about F when the 0 F enclosure temperature was reached. Then the enclosure temperature varied between F and F; the concrete temperature in the pile increased up to a maximum temperature of F at about A.M. on the morning of November, about hours after the addition of water to the first batch; the steam was terminated. The thermocouples measuring the enclosure temperature were where the couplers were; at this location, it is critical that the temperature does not exceed F so that slipping of the strand in the coupler is prevented. A maximum temperature of 0 F is specified for the concrete containing the strand, which is much higher than the F reached. After the termination of the steam curing, specimens representing the live and dead ends for release strength were tested at about A.M. The average of two TMC cylinders for the south end (dead end) obtained from the first load was,00 psi and for the north end (live end) obtained from the fourth load was,00 psi. Thus, the specified release strength of,00 psi was achieved and the piles were ready for detensioning and removal from the bed.

11 Ozyildirim and Sharp As shown in Figure, the steel strands at both ends of the bed were cut using a torch, which was done in a manner similar to that in conventional detensioning operations using a sequence. Then, the CFCC between the two piles was cut using a grinder with an abrasive blade. The piles were then lifted and stored next to the forms. There were no unusual large visible defects on the piles, and only bug holes were evident on the formed side surfaces. 0 FIGURE Torch was used to cut the steel strands at the ends (left), and the demolded beam show some tiny bug holes but no large voids (right). Driving Operation Two CFCC-reinforced piles were driven as test piles along with numerous other traditional steel reinforced piles during the construction of the Nimmo Parkway Bridge, which is shown in Figure. The CFCC-reinforced piles were driven using an open-end diesel hammer that had a ram weight of, lb and a stroke that was. to. ft. The initial test pile drive started at the - ft mark on the pile, and the driving operation proceeded until one of the piles reached the.0-ft mark and the other pile reached. ft. After one week, piles were driven again; one of the piles to the.0-ft. mark, and the other pile was driven to the.-ft mark. The dynamic analysis indicated that there were no differences in the driving response between these piles and the conventional piles with steel strands. The production piles were driven with no problems. In August 0 the bridge was opened to traffic.

12 Ozyildirim and Sharp FIGURE Driving of test piles (left) for the Nimmo Parkway Bridge, and instrumenting pile before driving (right). 0 0 Production Piles The test piles indicated that CFCC requires special end preparation and handling, but once cast it behaves as with conventional piles. In addition, the test piles provided the lengths needed for the remaining production piles. One year after the test piles the contractor ordered the production piles. They were cast at Plant with the mixture proportions given in Table. These values were similar to those used in the test piles. A commercially available air-entraining admixture, a water-reducing and retarding admixture, and a high-range water-reducing admixture were also added. The strand handling, end preparation, prestressing, and concrete placement were similar to those for the test piles. Four piles were cast in the bed. The preparation of the piles including strand placement, end preparation, placement into the couplers, prestressing, casting of concrete, curing, and detensioning required more than a -hour cycle. Each set of piles in a bed were completed within days from beginning of placement of reinforcement to removal from forms. Conventional piles were prepared and detensioned in a -hour cycle. Studies are ongoing at VCTIR to expedite the end preparation in order to achieve daily production with the CFCC piles so that from placement of reinforcement to the removal from forms can be accomplished within hours. Production piles were also steam cured. The temperature in the beam reached F. The specimens were kept at the end of the bed with the couplers. At this location, temperatures were lower than at the remainder of the bed to ensure that slippage within the couplers did not occur. The ends were prepared by plant personnel. The manufacturer s technicians did not come to prepare the ends, but the manufacturer s engineer was there to supervise the operation. A local crew fabricated the piles with no problems. Concrete Properties Two batches of concrete denoted B and B were tested; the fresh concrete properties are given in Table, and the hardened concrete properties in Table. Workable concretes and specified air contents were obtained. The -day compressive strengths had exceeded the specified minimum -day strength of,000 psi. The permeability values were high. However, the values were similar to the ones for the test batches without the accelerated cure. If accelerated curing had been used, very low values would have been expected.

13 Ozyildirim and Sharp CONCLUSIONS CFCC-reinforced piles can be fabricated using a local crew. CFCC should be handled with care to prevent damage to the strand. The couplers should be protected from high temperature to prevent slipping. Additional fabrication time is required because of the extra time needed to prepare the ends, place them into the chuck, and then into the coupler. Improvements in the end preparations are expected to provide daily cycles so that from the beginning with the placement of the reinforcement to the removal from the forms can be accomplished within hours. To achieve the long-term permeability of the concrete, accelerated curing is needed if the samples are not exposed to high temperatures during steam curing at the plant. During driving, CFCC-reinforced piles responded in a manner similar to that of conventional steel-reinforced piles. ACKNOWLEDGMENTS The authors thank the Virginia Department of Transportation and the Federal Highway Administration for their support of this research, particularly VDOT s Structure and Bridge Division; VDOT s Materials Division; and Ethan Bradshaw, William Ordel, Jonathon Tanks, Mike Burton, Lew Lloyd, and Gail Moruza from the Virginia Center for Transportation Innovation and Research. REFERENCES. Sharp, S. R., and A. K. Moruza. Field Comparison of the Installation and Cost of Placement of Epoxy-Coated and MMFX Steel Deck Reinforcement: Establishing a Baseline for Future Deck Monitoring. Publication VTRC 0-R. Virginia Transportation Research Council, Charlottesville, 00.. Grace, N. F., F. C. Navarre, R. B. Nacey, W. Bonus, and L. Collavino. Design-Construction of Bridge Street Bridge: First CFRP Bridge in the United States. PCI Journal, September- October 00, pp. 0-. Grace, N. F., T. Enomoto, G. Abdel-Sayed, K. Yagi, and L. Collavino. Experimental Study and Analysis of a Full-Scale CFRP/CFCC Double-Tee Bridge Beam. PCI Journal, July- August 00, pp. 0-. Tsuchida, S., T. Maruyama, and M. Kodera. Flexural Behavior of CFRP-PC Pile Containing CFRP Rods. Presented at the th Japan Society of Civil Engineers Annual Meeting, September.. Grace, N. F., and S. B. Singh. Design Approach for Carbon Fiber-Reinforced Polymer Prestressed Concrete Bridge Beams. ACI Structural Journal, May-June 00, pp. -.. Enomoto, T., N. F. Grace, and T. Harada. Life Extension of Prestressed Concrete Bridges Using CFCC Tendons and Reinforcements, 0. Enomoto,Grace,%0Harada_LIFE%0EXTENSION%0OF%0PRESTRESSED%0C ONCRETE%0BRIDGES.pdf. Accessed July, 0.. Grace, N. F., E. A. Jensen, C. D. Eamon, and S. Xiuwei. Life-Cycle Cost Analysis of Carbon Fiber-Reinforced Polymer Reinforced Concrete Bridges. ACI Structural Journal, September-October 0, pp. -0. CFCC: Carbon Fiber Composite Guide. Publication No. -H-SA. Tokyo Rope Mfg. Co., Ltd., Tokyo, Japan, Accessed July, 0.