AXIAL AND FLEXURAL PERFORMANCE OF CONCRETE PILES PRESTRESSED WITH CFRP TENDONS

Transcription

1 AXIAL AND FLEXURAL PERFORMANCE OF CONCRETE PILES PRESTRESSED WITH CFRP TENDONS Steven Schiebel, Graduate Research Assistant/ MS Candidate Antonio Nanni, Ph.D., PE, V&M Jones Professor of Civil Engineering Department of Civil Engineering University of Missouri-Rolla ABSTRACT This paper presents the results of a study on the axial and flexural performance of fully bonded FRP prestressed members such as those used in foundations of highway bridges. Two piles were constructed for this project. One was tested in the laboratory to evaluate flexural performance. The other was driven in the field and then tested under flexural loading. The response of each of the piles was according to expectations. The testing and its results are described.

2 INTRODUCTION General Two pile prototypes using CFRP tendons are described in this paper. One prototype has been tested under four-point loading at the University of Missouri-Rolla (UMR). The second prototype has been transported to a bridge construction site where it was driven alongside a pile with steel reinforcement. The second pile was then tested under static loading conditions by jacking against the steel reinforced pile. Details of these procedures are given in the sections to follow. CONSTRUCTION OF CFRP PC PILES General In all four pile-prototypes were used in this project. Two prototypes were prestressed with CFRP tendons and two with conventional steel tendons. The two steel PC piles were manufactured at an external source and the two with CFRP tendons were manufactured at the UMR structures laboratory. In order to perform prestressing on CFRP tendons, an anchor system consisting of a resin-grouted metal sleeve and conventional wedge-type anchor was developed and its performance was evaluated through tensile testing and PC member fabrication (Yan, 1999). The tensile testing proved the anchor system was capable of carrying the tensile force up to tensile failure of the CFRP tendon. The PC member fabrication proved the feasibility of the application of this proposed anchor system. Concrete confinement under axial loading was provided by the AASHTO standard steel spiral except at the end of the prototype. At the ends a specially manufactured CFRP spiral was used for confinement. During construction of the piles it was observed that special care must be taken in selecting tendon sections that are free of defects. A flaw in the tendon cross-section can lead to failure of the tendon during stressing operations. CFRP Spiral Manufacturing The fabrication of the two sections of CFRP spiral served two purposes. The first was to evaluate the use of CFRP spirals as a confinement material for axially loaded members. The second purpose was to evaluate the manufacturing process and is beyond the scope of this paper. Two identical sections of spiral were fabricated, one for each of the two piles. Each spiral section has 21 turns (five turns at 25.4-mm pitch and 16 turns at 76.2-mm pitch) for a total length of mm. The spiral sections were installed at one end of the piles, which would later be the end in contact with the ram in the case of the driven pile. The rest of the pile prototype was confined with conventional steel spirals. The manufacturing technique

3 is described below with the aid of photographs. A strand of carbon fibers consisting of ten 48K fiber tows (20-mm 2 cross-sectional area) was held together and pulled through a resin bath for impregnation. Once the fibers were fully impregnated, the strand was pulled through a circular dye to remove extra resin. After that, the impregnated strand was wrapped on a mandrel of appropriate diameter (see Figure 1). Finally, the spirals were cured on the mandrel at room temperature for 24 hours. The final product is shown in Figure turns 8.65 in. (220 mm) Mandrel Figure 1 Details of CFRP Spirals Figure 2 Photo of Final Products Fabrication of CFRP PC Members in the Laboratory Plan of Fabrication in the Laboratory: The decision was made to fabricate the piles in the laboratory, where operation procedure (from resin injection to tendon stressing and concrete casting) and environmental conditions were easier to control. A shorter bed was set up in the laboratory and the anchor system was improved (see the following sections for details). In addition to the piles, fully prestressed and partially prestressed rectangular FRP- PC beams were cast. Prestressing System: The prestressing system was designed for a 490 kn load, as required by this project. It was made of standard ASTM A-36 steel by a local fabricator. It consisted of two sets of bulkhead and brace (see Figure 3). The bulkheads worked as reaction at live end and dead end; the braces were used to hold the anchors and transfer the jacking force to tendons. An 890 kn jack was utilized to achieve the required prestressing force. It had a valve that could be adjusted to maintain a stable load. An electronic load cell was installed at the live end of the prestressing system to measure the tension force applied by the jack. High-strength steel threadbars were used to transfer the force from the jack to the bulkhead and then to the brace and tendons. The high-strength steel threadbar had a continuous rolled-in pattern of threadlike deformations along its entire length. The deformations allowed anchorage and couplers to thread onto the threadbar at any point. The 32-mm threaded bar used in the prestressing system had a guaranteed 834 kn ultimate tensile capacity from the manufacturer.

4 The entire system worked in such a way that the tendons were placed in the form longitudinally between two bulkheads fixed on the structural floor of the laboratory and stressed by the jack against the bulkheads (see Figures 3 & 4). Figure 3 Prestressing System Figure 4 Pile Fabrication Concluding Remarks Although there are still some problems left to be resolved, fabrication of PC member was accomplished in the laboratory. The resin anchor system proved to be an adequate method of tendon anchorage. Its installation requires skill and precision. The prestressing system used adaptable and serviceable in a laboratory setting. The quality of the tendon used is very important in prestressing applications. A flawed tendon can easily lead to failure of the entire system during prestressing operations. Future research should focus on the simplification of the construction procedure to make it applicable to field production. PILE TESTING AND PERFORMANCE EVALUATION Theoretical Moment-Curvature Prediction In order to predict the moment-curvature relationship of the pile prototypes an analytical model was formulated and a MathCAD (Mathsoft, Inc., Cambridge, MA) worksheet was devised based on this model. The usual assumptions were made when working with concrete members (i.e. plane sections remain plane, behavior of concrete is linear elastic up until cracking, and the contribution of the concrete to the tensile force after cracking is neglected). The stress-strain relationship of concrete was assumed to be parabolic, and the equivalent uniform stress block theory was used in computing the compressive force. The material characteristics of the tendons were based on experimental values obtained by Yan (1999).

5 The analytical model is based on strain compatibility and the equilibrium of internal forces. Cross-section geometry and material properties are entered into the worksheet and theoretical moment-curvature response is computed. Laboratory Flexural Testing Test Setup: The pile prototype tested in the laboratory was tested under a four-point loading arrangement (see Figure 7). The beam supports were 5.5-m apart, and the load was applied through a spreader beam to two points located 0.9-m apart. Deflection measurements were taken at mid-span, quarter-span, and at the support. Strains were measured in each of the eight tendons in the cross-section as well as on the compression face of the prototype. To obtain an accurate moment-curvature relationship for the beam two LVDTs were used. Each was setup to measure horizontal deflections over a 0.30-m gauge length, one on the compression side of the prototype and the other on the tension side. Knowing the distance between the two LVDTs and the gauge length over which they are recording allows for the development of the moment curvature relationship of the prototype. The loading of the prototype progressed in increments of 4.5-kN. The load was held at each new level for a few seconds and then decreased to a base load of 4.5-kN. This cyclic loading allowed for the observation of permanent deflection in the cross-section. The beam displayed small deflection up until cracking. Once the cross-section had cracked and the prototype stiffness was reduced a much greater degree of deformability was observed. The prototype recovered deflection well when unloaded. This elastic behavior continued until failure of the beam. An audible warning to failure was heard when the tendons started to lose their bond with the cross-section. No-slip of the tendons at the prototype ends was observed however. Failure of the prototype was obvious and sudden. A load pop was heard accompanied by a sharp decrease in the load at the moment of the first tendon failure. Test Results: Failure in the prototype resulted from rupture of the FRP tendons. The moment-curvature diagram displays the characteristic bi-linear response with two ascending branches and a decrease in stiffness after cracking. Cracking of the prototype occurred at approximately kn-m and the ultimate moment was approximately kn-m. The calculated moment due to the self-weight of the beam is included in the moment-curvature diagram for accuracy as well as comparison purposes. In the field test the pile will be tested in a vertical configuration thereby eliminating any effects due to the weight of the beam. The moment-curvature diagram is given in Figure 5. The loaddeflection diagram shows a corresponding behavior and is seen in Figure 6. Field Testing-Axial and Flexural Pile Driving: One of the objectives of this project was to construct full size members and test their performance. With the pile prototypes this included driving the piles to ensure proper performance. Though no real problems were expected from this procedure, it was

6 nonetheless interesting to verify that the prototype maintained its flexural capacity after it was driven. Moment (kn-m Experimental Theoretical 0 5E-06 1E-05 2E-05 2E-05 3E-05 3E-05 4E-05 4E-05 Curvature (rad/mm) Figure 5 Moment-Curvature Relationship Load (kn) Deflection (m) Figure 6 Load-Deflection Diagram To prepare for the driving process the piles needed to have a tapered end cast onto them. Standard concrete piles in use have these driving tip that help to prevent twisting or displacement when an obstacle is encountered during the driving. The driving tips were anchored to the piles through four threaded anchor bolts that were doweled into the pile prototype. A nut was placed on the ends of the anchor bolts to further ensure a good transfer of forces from the tip to the pile prototype. The next step was to cast a pyramid shaped tip on the ends of the piles. A 55-MPa mix was used to accomplish this task. Two piles were transported to a construction site and driven in the field. One pile was prestressed with FRP tendon and the other with steel. The steel pile was driven to be used as a reaction for the static load test described later. The piles were driven with a Kobe K-13 single-acting diesel hammer with a weight of 12.8-kN. Extra care was taken in the

7 placement of the piles before driving in order to protect the instrumentation that was in place at the time. The piles were driven into a rocky-clay fill material on the approach to a new bridge. Neither pile was driven to bedrock. Once the pile was driven the pile head was inspected for damage. Aside from some minor chipping at the square edges no damage was observed. Test Setup: The test setup for the driven pile was more complicated. A jack was suspended from above to allow loading at a point 0.5-m. from the top of the piles (see Figure 8). The piles were restrained against horizontal displacement near ground elevation through the use of high strength steel rods and small spreader beams. Load readings were recorded at the point of load application and at the restraint. Deflections were measured at the point of loading, at the restraint and midway between the two. Rotation was measured at the base. Strains were measured at several locations. Compression strains were measured using 50-mm strain gauges on the compression face of the prototype. Tensile strains were measured through extensometers, or clip-gauges, on the tensile side of the prototype. Strains were also monitored in the tendons on the tensile side of the prototype. Test Results: The prototype tested in the field failed by rupture of the FRP tendons as well. Initially, the first crack was observed at the restrained cross-section. The crack propagated directly under the remaining extensometer. Once this had occurred the strain values calculated from the extensometer reading were inaccurate and the only usable data was of crack opening. The load-deflection behavior can be seen in Figure 9. Comparing the moment-curvature relationship of the two tested piles to the analytical moment-curvature shows a close match. The maximum moment of the laboratory tested pile prototype was about 12.3% lower than the theoretical maximum moment. The pile tested in the field showed even closer results, with a maximum moment only 7.5% below the theoretical. Figure 7 Lab Test Setup Figure 8 Field Test Setup

8 CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER STUDY The piles displayed good performance during the driving process and under flexural loading. Though this project shows that it is possible to manufacture PC members using CFRP tendons there is still much work to be done before this becomes a feasible application. A more standard approach needs to be developed for anchorage work as well as better control over tendon manufacturing. The presence of defects in the tendons can be detrimental to the prestressing procedures itself. This must be addressed. Through long term monitoring of this project a greater knowledge of the material in terms of durability and performance over the long term will be gained. Issues relating to creep-rupture and fatigue need to be better understood. Load (kn) Deflection (m) Figure 9 Load-Deflection Diagram ACKNOWLEDGEMENTS This project would not have been possible without the support of the Federal Highway Administration under Contract DTFH61-96-C Many thanks also go to the University Transportation Center (UTC) for its support, and Arnold Yan, whose previous work paved the way for this project. LIST OF REFERENCES Yan, Xiang, (1999). Constructability and Performance of Concrete Members Prestressed with CFRP Tendons. MSc Thesis Department of Civil Engineering, The University of Missouri-Rolla.