Determining the Bond-Dependent Coefficient of Glass Fiber- Reinforced Polymer (GFRP) Bars

Transcription

1 Final Report Determining the Bond-Dependent Coefficient of Glass Fiber- Reinforced Polymer (GFRP) Bars By George Morcous, Ph. D., P.E. Eliya Henin, M.Sc., Ph.D. Candidate University of Nebraska-Lincoln, Omaha, NE Sponsored by Hughes Brothers, Inc. Date August, 5,

2 Introduction In recent years, research has been carried out on glass fiber reinforced polymer (GFRP) bars as an alternative to steel reinforcement in concrete structures. GFRP bars have several advantages over reinforcing steel bars, such as corrosion resistance, high strength, lightweight, and high thermal resistance. This report focuses on the experimental evaluation of the bond-dependent coefficient (k b ) of GFRP rods according to the ACI 440K-subcommittee procedures proposed by Benmokrane, B in This coefficient is used in calculating crack width to account for the degree of bond between the GFRP bar and the surrounding concrete. For GFRP bars having bond behavior similar to steel bars, K b is assumed equal to unity. Specimen Design and Fabrication To evaluate the bond-dependent coefficient (K b ) of GFRP bars, six beam specimens were fabricated and tested at the University of Nebraska-Lincoln (UNL) Structural Laboratory in the Omaha Campus. Two beams were made for each of the following bar sizes: #4, #6, and #8. Each beam was a 10 ft long, 8 in. wide and 12 in. deep. The beams were reinforced in the bottom with two GFRP bars in the longitudinal direction and in the top with 2#4 steel bars, in additions to #3 6 in. as shown in Figure 1. Figure 1: Elevation and section of the test specimen The fabrication process includes forming the beam sides and installing the reinforcement cages as shown in Figure 2. Strain gauges were attached to the GFRP bars before placing 6 ksi, 4 in. slump ready mix concrete as shown in Figure 3. Six concrete cylinders were taken for quality assurance and the beams were covered with wet burlap for curing. 2

3 Figure 2: Beam fabrication Figure 3: Strain gauges attached to GFRP bars Figure 4: Concrete slump test 3

4 Material Properties Table 1 shows the design of the mix used for beam fabrication, while Figure 5 shows the compressive strength versus age up to the time of testing (28 days). Table 2 presents the mechanical properties of #4, #6, and #8 used in this investigation as provided by Hughes Brothers, Inc. Table 1: Mix Design Material Design Quantity lb/yard 3 Sand 47B 1820 Lime Stone L3/ Cement Type I Fly Ash Class C HRWR 8 oz/cwt Figure 5: Concrete strength gain with time Table 2: Mechanical Properties of GFRP bars Sample # Area (in) Failure Load Tensile Strength Ultimate Moudulus of Elasticity (lb) (psi) Strain (psi) # , , ,016,267 # , , ,173,432 # , , ,194,395 4

5 Test Setup and Procedures Figure 6 shows the test setup of the specimen. Two-point load was applied using hydraulic jack, load cell, and two roller supports. The beam was simply supported using two roller supports placed 9 ft center to center. Specimen deflection was recorded using one potentiometer located at midspan. The following steps were followed to test each specimen. 1- Setup the beam as shown in Figure 6. 3'-6" 2' 3'-6" 6" 3" 10' 9' Figure 6: Test specimen and setup 5

6 2- Load the beam until the first flexural crack appears. At that stage, loading is held constant to measure the initial crack width with a microscope as shown in Figure 7. Figure 7: Measuring the initial width of the first crack with a microscope 3- Place extensometer at the level of the reinforcing bars to monitor the width of the first crack until the end of testing as shown in Figure 8. Figure 8: Placing the first extensometer to monitor the first crack width 6

7 4- Resume loading until the second flexural crack appears, and then hold the loading constant and measure the initial crack width with a microscope as shown in Figure 9. Figure 9: Measuring the initial width of the second crack with a microscope 5- Place another extensometer at the level of the reinforcing bars to monitor the second crack width until the end of testing as shown in Figure 10. Figure 10: Placing the second extensometer to monitor the second crack width 7

8 6- Resume loading until the width of any of the two cracks exceeds 1.0 mm or until beam failure. 7- Calculate the bond-dependent coefficient (K b ) using the following ACI R-06 equation: F w 2 f s 2 2 Kb dc E f 2 Where: w : Maximum crack width (mm). Use 0.7 mm. E f : Modulus of elasticity of FRP bar (MPa). F f : Stress in FRP reinforcement in tension (MPa). K b : Bond-dependent coefficient. β : Ratio of distance from neutral axis to extreme tension fiber to distance from neutral axis to center of tensile reinforcement. d c : Thickness of concrete cover measured from extreme tension fiber to center of bar (mm) s : Longitudinal FRP bar spacing (mm) Test Results # 8 GFRP bars Figure 11 shows the load-deflection relationship for the two beams with #8 GFRP bars. Figure 12 shows the load-strain relationship for the second beam only because the strain gauges of the first beam did not work. Figure 11: Load-deflection relationship for the two beams with #8 GFRP bars 8

9 Figure 12: Load-strain relationship for the second beam with #8 GFRP bars Table 3 illustrates the stresses and strains for the two beams of # 8 GFRP bars. Theses stresses were calculated at crack width 0.7 mm. Table 4 shows K b values for the two beams using #8 GFRP bars. It should be noted that the stress in the bars of the first beam was calculated according elastic crack theory, because the strain gauge did not work in the first beam. Test # Bar # Table 3: Stresses and strains in #8 GFRP bars in the two beams at crack width 0.7 mm Initial Crack Width (in) Initial Extensometer Reading (in) Load at Initial Cracking (kip) Final Extensometer Reading (in) Final Crack Width (mm) GFRP Strain E f (ksi) GFRP Stress (ksi) Load at Final Cracking (kip) Table 4: K b calculation according to ACI 440.1R-06 equation for #8 GFRP bars Test # Bar # w (mm) E f (Mpa) F f (Mpa) β s (mm) d c (mm) K b Average K b , ,

10 # 6 FRP bars Figure 13 shows the load-deflection relationship for #6 GFRP beams, while Figure 14 shows the load-strain relationship for the same beams. Figure 13: Load-deflection relationship for the two beams with #6 GFRP bars Figure 14: Load-strain relationship for the two beams with #6 GFRP bars 10

11 Table 5 illustrates the stresses and strains for the two beams with # 6 GFRP bars. Theses stresses were calculated at crack width 0.7 mm. Table 6 shows K b values for these two beams. Test # Bar # Table 5: Stresses and strains in #6 GFRP bars for the two beams at crack width 0.7 mm Initial Crack Width (in) Initial Extensometer Reading (in) Load at Initial Cracking (kip) Final Extensometer Reading (in) Final Crack Width (mm) GFRP Strain E f (ksi) GFRP Stress (ksi) Load at Final Cracking (kip) Table 6: K b calculation according to ACI 440.1R-06 equation for #6 GFRP bars Test # Bar # w (mm) E f (Mpa) F f (Mpa) β s (mm) d c (mm) K b Average K b , , # 4 FRP bars Figure 15 shows the load-deflection relationship for #4 GFRP beams, while Figure 16 shows the load strain relationship for the same beams. Figure 15: Load-deflection relationship for the two beams with #4 GFRP bars 11

12 Figure 16: Load-strain relationship for the two beams with #4 GFRP bars Table 7 illustrates the stresses and strains for the two beams with # 4 GFRP. Theses stresses were calculated at crack width 0.7 mm. Table 8 shows K b values for these two beams. Table 7: Stresses and strains in #4 GFRP bars for the two beams at crack width 0.7mm Test # Bar # Initial Crack Width (in) Initial Extensometer Reading (in) Load at Initial Cracking (kip) Final Extensometer Reading (in) Final Crack Width (mm) GFRP Strain E f (ksi) GFRP Stress (ksi) Load at Final Cracking (kip) Table 8: K b calculation according to ACI 440.1R-06 equation for #4 FRP bars Test # Bar # w (mm) E f (Mpa) F f (Mpa) β s (mm) d c (mm) K b Average K b , ,

13 Summary Table 9 summarizes the bond-dependent coefficient (k b ) of the tested GFRP bars, which were found to be 0.91, 0.86, and 0.70 for bar sizes #8, #6, and #4 respectively. Table 9: K b calculation for #4, #6, and #8 GFRP bars Test # Bar # w (mm) E f (Mpa) F f (Mpa) β s (mm) d c (mm) K b Average K b , , , , , , References Benmokrane, B., Test Method for Determining the Bond-Dependent Coefficient of Fibre- Reinforced Polymer (FRP) Rods (Second Draft) Submitted to ACI 440-K subcommittee, March,