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Publication Number: FHWA-HRT-06-115
Date: August 2006

Index, Structural Behavior of Ultra-High Performance Concrete Prestressed I-Girders

CHAPTER 5. UHPC GIRDER TEST RESULTS

This chapter presents detailed results from the four UHPC girder tests. The results from the flexure test on the 24.4-m (80-ft) AASHTO Type II girder are presented first, followed by the results from the three full-scale shear tests.

5.1 Static Flexural Testing

As discussed in chapter 4, Girder 80F was a 24.4-m (80-ft) AASHTO Type II prestressed girder. The girder was loaded in four-point bending by point loads located 0.91 m (3 ft) from the centerline and by roller supports located 12.0 m (39.25 ft) from the centerline. The girder contained twenty-six 12.7-mm, 1860-MPa (0.5-inch, 270-ksi), low-relaxation strands and no mild steel.

Figure 9 shows the applied load versus the vertical deflection response for this girder. The deflection is measured from the girder's bottom flange at the midspan centerline. The load-deflection response shows that the girder began to soften between 310 and 355 kN (70 and 80 kips) at a deflection of approximately 75 mm (3 inches). The girder exhibited significant additional capacity, reaching a peak load of 790 kN (178 kips) at a deflection of 470 mm (18.5 inches). A similar response can be observed in figure 10, which shows the applied load versus rotation at the girder support. Figure 11 provides a plot of the deflected shape of the girder at seven load steps throughout the test. Note that the data points closest to the supports were derived from the support rotation values.

Figure 9. Graph. Load versus midspan deflection response of Girder 80F. The behavior of the girder is basically linear elastic until between 300 and 350 kilonewtons (67 and 79 kips) of applied load with a deflection of approximately 75 millimeters (3.0 inches). The girder then began to soften before eventually failing after it reached a peak load of 790 kilonewtons (178 kips) at 470 millimeters (18.5 inches) of deflection. Unload/reload steps were completed periodically throughout the test to capture the residual stiffness of the girder. These steps also are shown in the graph, with the stiffness of the girder decreasing as the loading progresses.

1 mm = 0.039 inch
1 kN = 0.225 kip

Figure 9. Graph. Load versus midspan deflection response of Girder 80F.

Figure 10. Graph. Load-rotation response of Girder 80F. The graph shows the total applied load plotted against the rotation measured via the tilt meters located above the bearings. The shape of the curves for the response at each bearing is very similar to the shape of the response shown in figure 9. Softening begins soon after 0.5 degrees of rotation and failure occurs at approximately 3 degrees of rotation.

1 kN = 0.225 kip

Figure 10. Graph. Load-rotation response of Girder 80F.

Figure 11. Graph. Deflected shape of Girder 80F at selected load levels. This graph shows the deflected shape of the girder at 7 load levels throughout the test. The load levels are 0, 133, 267, 400, 534, 667, and 790 kilonewtons (0, 30, 60, 90, 120, 150, and 178 kips) of applied load. The results in this graph are based on the readings from the five potentiometers and two tilt meters located along the length of the girder. The graph shows how the initial response of the girder if elastic, and that as the distress in the girder becomes more pronounced, the curvature near midspan becomes much more intense.

1 m = 3.3 feet
1 mm = 0.039 inch

Figure 11. Graph. Deflected shape of Girder 80F at selected load levels.

The 16 strain gages located at the girder's centerline were used to create a strain profile over the depth of the girder throughout the test. Individual gages were used until their readings became unreliable due to cracking of the underlying concrete. The data from both sides of the girder correlated well. The neutral axis location and the curvature of the cross section could be computed from these results. Figure 12 shows the applied moment versus midspan curvature response. Similarly, figure 13 shows the neutral axis depth from the top of the girder versus the applied moment. Note that basic elastic section analysis indicates that the neutral axis at test initiation should have been located approximately 520 mm (20.5 inches) down from the top of the girder. This plot shows that the neutral axis began to rise at an applied moment of approximately 1,920 kN-m (17,000 kip-inches), which corresponds to a load of approximately 347 kN (78 kips). Also, note the stability of the neutral axis depth during the unloads/reloads.

The strain rosette on the south face of the girder near the west bearing was used to capture the data presented in figures 14 and 15. The data shown in these plots are not continuous because difficulties with the data acquisition system resulted in the loss of 10 data points in the middle of the test. Figure 14 provides the principal tensile and compressive strains at the rosette. Figure 15 shows the angle of the principal strains in clockwise degrees from horizontal on the south face. In both figures, note the linearity of the results. The tensile principal strain angle was approximately 40° from vertical, and the strain increased elastically up to more than 75 microstrain. No shear cracking was observed in the girder.

Figure 12. Graph. Moment-curvature response of Girder 80F. This graph shows the midspan applied moment plotted against the curvature of the girder at midspan as computed from the midspan strain profile. The basic shape of the curve is similar to the curves shown in figures 9 and 10. Nonlinear behavior becomes clearly apparent when the moment reaches 2,000 kilonewton meters at a curvature of approximately 0.000002 per millimeter. Failure of the girder occurs at a curvature of approximately 0.000015 per millimeter.

1 kN-m = 8.85 kip-inches
1 mm-1 = 25.4 inch-1

Figure 12. Graph. Moment-curvature response of Girder 80F.

Figure 13. Graph. Midspan neutral axis depth from the top of Girder 80F. This graph shows the depth of the neutral axis plotted versus the applied midspan moment. The depth remains basically steady at approximately 525 millimeters (20.7 inches) until approximately 2,000 kilonewton-meters of moment is applied. By 2,500 kilonewton-meters the neutral axis is at 425 millimeters (16.7 inches). By 3,500 kilonewton-meters it is at 350 millimeters (13.8 inches). Failure of the girder occurs after the depth drops below 250 millimeters (9.8 inches) with a moment above 4,400 kilonewton-meters. The graph also clearly shows how the depth of the neutral axis is stable during each unload/reload stage of the test.

1 mm = 0.039 inch
1 kN-m = 8.85 kip-inches

Figure 13. Graph. Midspan neutral axis depth from the top of Girder 80F.

Figure 14. Graph. Principal strains in the web near the west support of Girder 80F. The rosette strain gages near the west end of Girder 80F allowed for the capture of the principal tensile and compressive strain in the web of the girder. This graph shows these strains as a function of the applied load on the girder. The responses are basically linear elastic until over 600 kilonewtons (134 kips) of load has been applied. A problem with the data acquisition system then led to the loss of a few data points. After the interruption of data capture, the responses again seem to be linear, however there is a slight offset to the responses. At girder failure, the principal tensile strain is approximately 100 microstrain.

1 kN = 0.225 kip

Figure 14. Graph. Principal strains in the web near the west support of Girder 80F.

Figure 15. Graph. Principal strain angles in the web near the west support of Girder 80F. This graph works closely with figure 14 to provide the principal tensile and compressive strain angles for the rosette at the west end of Girder 80F. The principal tensile angle was approximately 40 degrees from horizontal throughout the test.

1 kN = 0.225 kip

Figure 15. Graph. Principal strain angles in the web near the west support of Girder 80F.

Significant audible cracking-which started when the load reached 325 kN (73 kips) and continued throughout the remainder of the test-emanated from the girder. However, in general, these cracking sounds could not individually be correlated to cracks on the surface of the girder. In fact, the cracks were not visible without the aid of a highly volatile crack-revealing spray until the load had reached approximately 710 kN (160 kips). After this load was reached, many tightly spaced hairline flexure cracks were visible in the bottom flange and web near midspan.

The test was halted overnight just after a peak load of 623 kN (140 kips) was reached. The girder was locked in place with a deflection of 305 mm (12 inches). Before resuming the test, the cracks on the bottom flange were mapped using the volatile spray. Figure 16 shows photographic results of this mapping from six points along the length of the girder. The crack spacing near midspan was approximately 5 mm (0.2 inch). This spacing increased to 10 mm (0.4 inch) at 3 m (10 ft) from midspan, 25 mm (1.0 inch) at 4.9 m (16 ft), and 127 mm (5 inches) at 6.7 m (22 ft). No cracks were visible either to the unaided eye or through simple magnification devices.

As previously mentioned, Girder 80F exhibited a significant deflection capacity before failure. Figure 17 shows the girder carrying a load of 780 kN (175 kips) with a resulting deflection of 430 mm (17 inches). Soon after this photograph was captured, the girder reached its maximum applied load of 790 kN (178 kips), which corresponds to an applied moment of 4,370 kN-m (38,700 kip-inches). The maximum combined dead and live load moment was 4,800 kN-m (42,500 kip-inches). For comparison, recall from section 2.4.4 that a similar ultimate flexural capacity was attained experimentally by Russell and Burns during the testing of a 1.17-m (46-inch) deep decked I-girder.(15) That girder had a 41-MPa (6-ksi), 1.83-m (72-inch) wide top flange, and was prestressed with twenty-eight 12.7-mm (0.5-inch) strands.

After the maximum load was reached, the girder began to exhibit drastically decreased stiffness. The loading was stopped at this point. As the girder softened the load decreased, because the loading apparatus was hydraulically actuated. Approximately 1 minute before failure, a single gross crack was observed growing up from the bottom flange at the west load point. Unlike any other cracks in the girder, this crack was clearly visible to observers from a distance of 4.6 m (15 ft). Failure of the girder was dramatic, with the girder fracturing into two unconnected pieces. Failure occurred due to a combined tensile failure of the concrete matrix and the prestressing strands. At the failure location, the fibers pulled out and all the strands necked and ruptured. Figure 18 shows the north elevation after failure, and figure 19 shows the west failure surface.

Figure 16. Photo. Crack spacing on the bottom flange of Girder 80F at 305 millimeters (12.0 inches) midspan overall girder deflection. This figure shows six photographs of the cracking that was visible on the bottom flange of Girder 80F at one particular overall girder deflection. A volatile liquid was used to make the cracks visible. Crack spacing of 5 millimeters (0.2 inches) was observed at midspan and at 0.6 meters (2.0 feet) from midspan, spacing of 7.6 millimeters (0.3 inches) was observed at 1.8 meters (5.6 feet) from midspan, spacing of 10 millimeters (0.4 inches) was observed at 3 meters (9.8 feet) from midspan, spacing of 25 millimeters (1.0 inches) at 4.9 meters (16.1 feet) from midspan, and spacing of 127 millimeters (5.0 inches) at 6.7 meters (22.0 feet) from midspan.

1 m = 3.3 ft
1 mm = 0.039 inch

Figure 16. Photo. Crack spacing on the bottom flange of Girder 80F at 305 mm midspan overall girder deflection.

Figure 17. Photo. Girder 80F after approximately 430 millimeters (17 inches) of deflection. This photograph provides a glimpse down along the north face of Girder 80F after 430 millimeters of deflection has been applied to the girder.

Figure 17. Photo. Girder 80F after approximately 430 mm (17 inches) of deflection.

Figure 18. Photo. Girder 80F immediately after failure. This photo shows the failure location for this girder. The girder severed into two pieces at the west load point.

Figure 18. Photo. Girder 80F immediately after failure.

Figure 19. Photo. Failure surface of Girder 80F including (a) overall west failure surface These two photos show the full west failure surface and a closeup of the west failure surface. The closeup shows that the fibers pulled out of the corresponding failure surface and that the strands necked and ruptured.

Figure 19. Photo. Failure surface of Girder 80F including (b) closeup of west failure surface showing pulled-out fibers and necked strands. These two photos show the full west failure surface and a closeup of the west failure surface. The closeup shows that the fibers pulled out of the corresponding failure surface and that the strands necked and ruptured.

Figure 19. Photo. Failure surface of Girder 80F including (a) overall west failure surface and (b) closeup of west failure surface showing pulled-out fibers and necked strands.

5.2 Static Shear Testing

The results from three shear tests of full-scale AASHTO Type II prestressed UHPC girders are presented in this section.

5.2.1 Girder 28S

The first shear test was completed on Girder 28S. The test specimen had an overall span of 8.54 m (28 ft) and a shear span of 1.98 m (6.5 ft) resulting in a shear span-to-depth ratio of 2.17. The east bearing on this girder was placed 1.22 m (4 ft) from the end of the girder to minimize the effect that the debonding of the strands would have on the test results.

As shown in figure 3, Girder 28S was originally the west end of Girder 80F. Before the Girder 28S test, the girder was examined for damage that may have resulted from the Girder 80F test. Significant flexural cracking was observed toward the midspan of Girder 80F (i.e., the west end of Girder 28S). Additionally, a longitudinal hairline crack was observed along the base of the web that went from 0.3 m (1 ft) east of the east bearing to just west of the load point. This crack may have either occurred due to the failure of Girder 80F or during fabrication or shipping of the girder.

The response of Girder 28S to the applied loading is shown in figures 20 through 22. Figure 20 shows the applied load versus the vertical deflection response at the seven instrumentation lines. Figure 21 focuses on the data from the tilt meters at the east and west bearings. Figure 22 shows a plot of the deflected shape of the girder at six load levels. Recall that the point load was applied 6.6 m (21.5 ft) from the west support. Note how the location of maximum deflection shifts east as the test progresses. This shift is a direct function of the softening of the east end of the girder as shear cracking progresses.

The potentiometer readings nearest to the load point clearly indicate that the girder began to show softening behavior at a load of 1,110 kN (250 kips). The girder still had a significant reserve load capacity and reached a peak load of 2,220 kN (500 kips). At this peak load, the shear load carried by the east shear span was 1,710 kN (384 kips). For comparison, Tawfiq found that decked AASHTO Type II girders composed of 55 to 83 MPa (8 to 12 ksi) HPC and containing shear reinforcement carried approximately 1,200 kN (270 kips) of shear before failure.(11,12)

Half of the prestressing strands extending from the east end of the girder were instrumented to measure strand slip. Figure 23 shows the strand slip behavior throughout the test. Only the instrumented debonded strands are shown because all of the fully bonded instrumented strands showed less than 0.0508 mm (0.002 inch) of slip throughout the test.

Results from the four strain rosettes are presented in figures 24 through 27. The tensile principal strain and strain angle values are presented in the first two figures. The compressive principal strain and strain angle are presented in the second two figures. Note that the strain angles are measured in clockwise degrees from horizontal as viewed from the south face of the girder.

Figure 20. Graph. Load-deflection response of Girder 28S. This graph shows the applied load versus vertical deflection response of Girder 28S, including the responses from all seven potentiometers. The behavior of the girder is basically linear elastic until nearly 1,000 kilonewtons (224 kips) of load is applied. The behavior then softens and failure occurs after over 2,100 kilonewtons (472 kips) of load has been applied.

1 kN = 0.225 kip

Figure 20. Graph. Load-deflection response of Girder 28S.

Figure 21. Graph. Bearing rotation response of Girder 28S. This graph shows the rotation of the girder at the east and west bearing as measured by the tilt meters attached to the girder web. The east bearing rotation is basically linear elastic until the peak load is reached. The west bearing rotation is initially less than the east bearing until its rotation per load begins to increase after 1,200 kilonewtons (270 kips) of applied load. At the peak load, the rotation of the west bearing is approximately 0.45 degrees, which is slightly more than the 0.42 degrees that the east bearing reached at this load level.

1 kN = 0.225 kip

Figure 21. Graph. Bearing rotation response of Girder 28S.

Figure 22. Graph. Deflected shape of Girder 28S. This graph shows the deflected shape of the girder at six different load levels throughout the test. The load levels include 0 kilonewtons, 445 kilonewtons (100 kips), 890 kilonewtons (200 kips), 1,330 kilonewtons (300 kips), 1,780 kilonewtons (400 kips), and 2,180 kilonewtons (490 kips). At earlier load levels the overall response seems to be primarily elastic with the maximum deflection occurring near midspan. As the load increases toward failure, the location of maximum deflection moves toward the load point. The shear deflections that occur near the east bearing are also evident in the graph.

1 mm = 0.039 inch
1 m = 3.3 ft

Figure 22. Graph. Deflected shape of Girder 28S.

Figure 23. Graph. Strand slip in Girder 28S. This graph shows the applied load versus strand slip behavior for the six debonded and instrumented strands in the east end of the girder. None of the strands showed any slippage until the applied load was over 1,900 kilonewtons (427 kips). After this load, the debonded strands started to show small amounts of slip, with the greatest slip occurring in the uppermost of the strands in the bottom flange. All strands showed less than 0.75 millimeters (0.03 inches) of slip when the girder reached its peak load.

1 kN = 0.225 kip
1 mm = 0.039 inch

Figure 23. Graph. Strand slip in Girder 28S.

Figure 24. Graph. Principal tensile strain in the web of Girder 28S. This graph shows the principal tensile strains in the web of the girder as measured at rosettes 1, 2, 3, and 5 plotted against the load applied to the girder. Rosettes 1, 2, and 3 all show similar behavior through the 1,000 kilonewtons (224 kips) load level where they show approximately 100 microstrain. After this load level, the response becomes more erratic, most likely due to cracking in the web in the vicinity of and under the strain gages in the rosettes. Rosette 3 maintains a similar slope until 235 microstrain is reached at the peak load, but with rosettes 1 and 2 show decreased strain values throughout the postcracking portion of the girder’s response. Rosette 5 shows a reduced slope initially, only reaching 35 microstrain at the 1,000 kilonewtons (224 kips) load level. The slope of the load-strain response then increases until approximately 40 microstrain is reached at an applied load of 1,800 kilonewtons (404 kips). The strain then begins to increase until 90 microstrain is achieved by the time the peak load is reached.

1 kN = 0.225 kip

Figure 24. Graph. Principal tensile strain in the web of Girder 28S.

Figure 25. Graph. Principal tensile strain angle in the web of Girder 28S. This graph shows the angle of the principal tensile strains displayed in figure 24. These angles are measured in clockwise degrees from horizontal on the south face of the girder. Most importantly, rosettes 1, 2, and 3 display tensile strain angles of approximately 55 degrees from horizontal through shear cracking of the girder.

1 kN = 0.225 kip

Figure 25. Graph. Principal tensile strain angle in the web of Girder 28S.

Figure 26. Graph. Principal compressive strain in the web of Girder 28S. This graph shows the principal compressive strains in the web of the girder as measured at rosettes 1, 2, 3, and 5 plotted against the load applied to the girder. All of the rosettes show similar behavior through the 1,000 kilonewtons (224 kips) load level where they show approximately 175 microstrain. After this load level, the response becomes more erratic, most likely due to cracking in the web in the vicinity of and under the strain gages in the rosettes. The response of rosette 5 stiffens such that this rosette never reaches beyond 200 microstrain. Rosette 1 shows the greatest strain level reaching over 1,100 microstrain by the time the peak load is reached.

1 kN = 0.225 kip

Figure 26. Graph. Principal compressive strain in the web of Girder 28S.

Figure 27. Graph. Principal compressive strain angle in the web of Girder 28S. This graph shows the angle of the principal compressive strains displayed in figure 26. These angles are measured in clockwise degrees from horizontal on the south face of the girder. Most importantly, rosettes 1, 2, and 3 display compressive strain angles of approximately 35 degrees from horizontal through shear cracking of the girder.

1 kN = 0.225 kip

Figure 27. Graph. Principal compressive strain angle in the web of Girder 28S.

Cracking and other damage to the girder were observed both audibly and visually throughout the test. The first cracking was heard at a load of 1,245 kN (280 kips). The cracking continued throughout the remainder of the test and seemed to primarily emanate from the eastern half of the girder. Throughout the test, the preexisting hairline crack at the base of the web continued to grow larger and longer. At a load of 1,560 kN (350 kips), this crack was clearly visible from 4.6 m (15 ft). Figure 28 shows the size of this crack at a load of 2,000 kN (450 kips). This crack, along with its branch that rose toward the load point beginning around 2,000 kN (450 kips), was the only gross cracking observed before girder failure. However, many other smaller cracks were present. These cracks were initially visible only through the use of an indicating spray but later were visible by short-range unaided viewing.

Figure 28. Photo. Crack at south base of Girder 28S web at a load of 2,000 kilonewtons (450 kips). This photo shows the longitudinal crack that was observed to run along the base of the web in the shear span. A branch of the crack that is moving up toward the load point can also be seen. Overall, these cracks are large for U H P C since they can be easily observed with the unaided eye.

Figure 28. Photo. Crack at south base of Girder 28S web at a load of 2,000 kN (450 kips).

Figure 29 shows the south face of the east shear span near the east bearing just after girder failure. Note the longitudinal crack at the base of the web, the vertical crack descending from the top of the top flange, and the crushed area in the web above the bearing. Figure 30 illustrates the damage recorded after girder failure.

The failure of the girder was precipitated by a number of events. First, the longitudinal shear crack at the base of the web continued to grow longer and wider throughout the test. Near the completion of the test, the width of the crack was sufficient that only minimal tensile load transfer via fiber reinforcement across the crack would have been possible. Just after the peak load was reached, the base of the girder web just above the bearing began to crush. This slow process (lasting approximately 90 seconds) was finally halted by the rupturing of the two strands in the top flange. Note that the instrumented top flange strand did not show any slip throughout the test and that the rupture was 1.22 m (48 inches) from the end of the girder. The failure mode of this girder will be discussed in more depth in chapter 6.

Figure 29. Photo. Tension failure of top flange and crushing of web at conclusion of test. This photo shows the south face of the girder in the vicinity of the east bearing. This photo shows the longitudinal crack at the base of the web, the crushed concrete zone in the web just above and to the west of the bearing, and the crack in the top flange of the girder above the bearing.

Figure 29. Photo. Tension failure of top flange and crushing of web at conclusion of test.

Figure 30. Illustration. Crack pattern at failure in Girder 28S. This illustration shows the distress that was observed on the south face of the girder at the conclusion of the test. The illustration shows the area of crushed concrete in the web above the bearing, the shear cracking in the web, the major crack at the base of the web that eventually grew up through the web toward the load point, the flexural cracking in the bottom flange below the load point, and the negative moment flexural cracking that occurred over the bearing.

1 m = 3.3 ft
1 mm = 0.039 inch

Figure 30. Illustration. Crack pattern at failure in Girder 28S.

5.2.2 Girder 24S

The second shear test was completed on Girder 24S. This girder had an overall span of 7.32 m (24 ft) and a shear span of 2.29 m (7.5 ft), resulting in a shear span-to-depth ratio of 2.5. The east bearing on this girder was centered 1.22 m (4 ft) from the end of the girder to minimize the effect that the debonding of the strands would have on the test results.

As shown in figure 3, Girder 24S was originally the east end of Girder 80F. Similar to Girder 28S, this girder was examined for damage that may have resulted from the Girder 80F test. Significant flexural cracking was observed toward the west end of the girder; however, the majority of this cracking was outside of the test span. Additionally, a single crack was observed that started in the east overhang region at the bottom flange and ended in the web just west of the east bearing. This crack probably resulted from the motion of this end of the girder immediately following the failure of Girder 80F.

Figure 31 shows the applied load versus the vertical deflection response of the girder. Individual curves are shown for each potentiometer, including Pot 4, which was located under the load point. Figure 32 shows the applied load versus the girder rotation at the east and west supports. Figure 33 shows the deflected shape of the girder at nine points throughout the test. Recall that the load was applied 5 m (16.5 ft) from the west bearing.

Figure 31. Graph. Load-deflection response of Girder 24S.This graph shows the applied load versus vertical deflection responses for the seven potentiometers. All of the potentiometers exhibit basically linear elastic behavior until over 1,500 kilonewtons (337 kips) of load has been applied. The responses then slowly begin to soften until failure occurs at 3,250 kilonewtons (731 kips). Pot 5 exhibited the greatest deflection at failure, approximately 54 millimeters (2.1 inches).

1 mm = 0.039 inch
1 kN = 0.225 kip

Figure 31. Graph. Load-deflection response of Girder 24S.

Figure 32. Graph. Bearing rotation of Girder 24S. This graph shows the rotations of the girder above the east and west bearings as a function of the load applied. The responses from the two tilt meters are virtually identical throughout the entire test. In both cases, the response is basically linear elastic until over 1,500 kilonewtons (337 kips) of load has been applied and rotations of approximately 0.30 degrees have occurred. The response then begins to soften slightly until approximately 1.2 degrees of rotation have occurred by the time the peak load is reached.

1 kN = 0.225 kip

Figure 32. Graph. Bearing rotation of Girder 24S.

Figure 33. Graph. Deflected shape of Girder 24S. This graph shows the deflected shape of the girder at nine different load levels throughout the test. The load levels include 0 kilonewtons, 445 kilonewtons, 890 kilonewtons, 1,330 kilonewtons, 1,780 kilonewtons, 2,220 kilonewtons, 2,670 kilonewtons, 3,110 kilonewtons, and 3,250 kilonewtons (0, 100, 200, 300, 400, 500, 600, 700, and 730 kips respectively). At earlier load levels the overall response seems to be primarily elastic with the maximum deflection occurring near midspan. As the load increases toward failure, the location of maximum deflection moves toward the load point. The shear deflections that occur near the east bearing are also evident in the graph.

1 m = 3.3 ft
1 mm = 0.039 inch
1 kN = 0.225 kip

Figure 33. Graph. Deflected shape of Girder 24S.

The deflection measurements nearest to the load point indicate that the girder began to show softening behavior at a load between 1,330 and 1,780 kN (300 and 400 kips). The girder still had a significant reserve load capacity and reached a peak load of 3,250 kN (731 kips). At this peak load, the shear load carried by the east shear span was 2,230 kN (502 kips). Again, this capacity is significantly above the decked AASHTO Type II shear capacity determined by Tawfiq for HPC.(11,12)

Half of the prestressing strands extending from the east end of the girder were instrumented to measure strand slip. None of the strands showed any slippage until after the girder failed. For this reason, the strand slip results are not plotted here.

Results from the seven strain rosettes are presented in figures 34 through 37. The tensile principal strain and strain angle are presented in the first two figures. The compressive principal strain and strain angle values are presented in the second two figures. Note that the strain angle values are measured in clockwise degrees from horizontal as viewed from the south face of the girder. Also, in these figures tension and compression are not always strictly correct. For instance, at certain load levels, Rosette 4 in figure 34 exhibits a principal tensile strain that is actually compressive. A subsequent comparison with figure 37 shows that the compressive principal strain for Rosette 4 is compressive and is far larger. In this case, both principal strains were compressive; therefore, the less compressive strain is presented as tensile.

Cracking and other damage to the girder throughout the test were observed both audibly and visually. The first cracking was heard at a load of 1,650 kN (370 kips). The cracking continued throughout the remainder of the test and seemed to primarily emanate from the eastern half of the girder. Up until just before failure, no gross cracking was observed in the girder. However, many small cracks were present. These cracks were only visible through the use of an indicating spray and were primarily shear cracks in the girder web.

The failure of this girder was sudden and dramatic. Toward the conclusion of the test, two parallel shear cracks appeared in the girder web, each clearly visible from 4.6 m (15 ft) away. These cracks were approximately on the direct line from the bottom flange bearing plate to the top flange load plate. One of the cracks ran from the bottom to the top of the web while the other ran from the bottom to halfway up the web.

Figure 38 shows photographs taken from a digital video of the failure. These two parallel shear cracks released in a brittle fashion, and the girder failed. As previously mentioned, none of the strands slipped until after the failure. The two top flange strands did break in three locations during the failure; however, this secondary failure was related to the large release of energy coming from the concrete tensile (shear) failure. Figure 39 shows the girder after failure. Figure 40 illustrates the crack and failure patterns recorded after the girder had failed.

Figure 34. Graph. Principal tensile strain in the web of Girder 24S. This graph shows the principal tensile strain responses of rosettes X, 1, 2, 3, 4, 5, and 6. Rosette X shows virtually no principal tensile strain throughout the loading of the girder. Rosettes 1 through 4 exhibit basically linear elastic behavior until over 1,500 kilonewtons (337 kips)of load has been applied. Rosettes 5 and 6 show slightly nonlinear behaviors indicating some possible load redistribution on the west side of the load point. As was observed in Girder 28S, the responses for this girder are somewhat erratic after shear cracking has occurred.

1 kN = 0.225 kip

Figure 34. Graph. Principal tensile strain in the web of Girder 24S.

Figure 35. Graph. Principal tensile strain angle in the web of Girder 24S. This graph shows the principal tensile strain angle as measured in degrees from horizontal on the south face of the girder plotted versus the applied load. Most importantly, the angle for rosettes 1, 2, and 3 converge toward being approximately 55 degrees from horizontal when shear cracking occurs. s

1 kN = 0.225 kip

Figure 35. Graph. Principal tensile strain angle in the web of Girder 24S.

Figure 36. Graph. Principal compressive strain in the web of Girder 24S. This graph shows the principal compressive strain responses of rosettes X, 1, 2, 3, 4, 5, and 6. Rosette X shows virtually no principal compressive strain throughout the loading of the girder. Rosettes 1 through 3 exhibit basically linear elastic behavior until over 1,500 kilonewtons (337 kips) of load has been applied. Rosettes 4 through 6 show slightly nonlinear behaviors indicating some possible load redistribution on the west side of the load point. As was observed in Girder 28S, the responses for this girder are somewhat erratic after shear cracking has occurred.

1 kN = 0.225 kip

Figure 36. Graph. Principal compressive strain in the web of Girder 24S.

Figure 37. Graph. Principal compressive strain angle in the web of Girder 24S. This graph shows the principal compressive strain angle as measured in degrees from horizontal on the south face of the girder plotted versus the applied load. Most importantly, the angle for rosettes 1, 2, and 3 converge toward being approximately 35 degrees from horizontal when shear cracking occurs.

1 kN = 0.225 kip

Figure 37. Graph. Principal compressive strain angle in the web of Girder 24S.

Figure 38. Photo. Failure of Girder 24S (a) 1/15 second before failure, (b) 1/30 second before failure, (c) at failure, and (d) 1/30 second after failure. This set of photos shows the rapid progression that occurred in the failure of Girder 24S. Two large shear cracks are visible in the first two photos, captured fractions of a second before the girder failed. Figure (c) shows the girder at the initiation of failure with the shear cracks extending to connect the load and bearing points. The final photo, (d), shows the girder just after failure.

Figure 38. Photo. Failure of Girder 24S (a) 1/15 second before failure, (b) 1/30 second before failure, (c) at failure, and (d) 1/30 second after failure. This set of photos shows the rapid progression that occurred in the failure of Girder 24S. Two large shear cracks are visible in the first two photos, captured fractions of a second before the girder failed. Figure (c) shows the girder at the initiation of failure with the shear cracks extending to connect the load and bearing points. The final photo, (d), shows the girder just after failure.

Figure 38. Photo. Failure of Girder 24S (a) 1/15 second before failure, (b) 1/30 second before failure, (c) at failure, and (d) 1/30 second after failure. This set of photos shows the rapid progression that occurred in the failure of Girder 24S. Two large shear cracks are visible in the first two photos, captured fractions of a second before the girder failed. Figure (c) shows the girder at the initiation of failure with the shear cracks extending to connect the load and bearing points. The final photo, (d), shows the girder just after failure.

Figure 38. Photo. Failure of Girder 24S (a) 1/15 second before failure, (b) 1/30 second before failure, (c) at failure, and (d) 1/30 second after failure. This set of photos shows the rapid progression that occurred in the failure of Girder 24S. Two large shear cracks are visible in the first two photos, captured fractions of a second before the girder failed. Figure (c) shows the girder at the initiation of failure with the shear cracks extending to connect the load and bearing points. The final photo, (d), shows the girder just after failure.

Figure 38. Photo. Failure of Girder 24S (a) 1/15 second before failure, (b) 1/30 second before failure, (c) at failure, and (d) 1/30 second after failure.

Figure 39. Photo. Failed Girder 24S (a) south elevation and (b) bottom flange near bearing. The photo in figure (a) shows the south face of the girder after failure. This photo depicts a traditional shear failure of a concrete beam with a diagonal shear crack running from the load point to the bearing. Figure (b) shows a closeup view of the bottom of the south face of the girder near the bearing. Intact strands are visible in the photo, as is a thin slab of U H P C that broke away from the bottom flange of the girder.

Figure 39. Photo. Failed Girder 24S (a) south elevation and (b) bottom flange near bearing. The photo in figure (a) shows the south face of the girder after failure. This photo depicts a traditional shear failure of a concrete beam with a diagonal shear crack running from the load point to the bearing. Figure (b) shows a closeup view of the bottom of the south face of the girder near the bearing. Intact strands are visible in the photo, as is a thin slab of U H P C that broke away from the bottom flange of the girder.

Figure 39. Photo. Failed Girder 24S (a) south elevation and (b) bottom flange near bearing.

Figure 40. Illustration. Crack pattern at failure in Girder 24S. This illustration shows the damage that was present as viewed on the south elevation of the girder after the conclusion of the test. The locations of the dominant shear cracks that resulted in the girder failure are noted. The shear cracks that were present both east and west of the load point are also shown. In the immediate vicinity of the shear failure plane, the shear cracks were spaced as closely as 3 millimeters (0.1 inches).

1 mm = 0.039 inch

Figure 40. Illustration. Crack pattern at failure in Girder 24S.

5.2.3 Girder 14S

The third shear test was completed on Girder 14S. As shown in figure 3, this girder was one end of an untested 9.2-m (30-ft) girder. This specimen had an overall span of 4.27 m (14 ft) and a shear span of 1.83 m (6 ft), resulting in a shear span-to-depth ratio of 2.0. The east bearing on this girder was placed 152 mm (6 inches) from the end of the girder, similar to where it would be placed in practice. Note that half of the bottom flange strands are debonded to 0.91 m (36 inches) from the end of the girder.

Figure 41 shows the applied load versus the vertical deflection response of the girder. Similarly, figure 42 shows the applied load versus the girder rotation at the east and west supports. Figure 43 shows the deflected shape of the girder at nine points throughout the test. Recall that the load was applied 2.44 m (8 ft) from the west support.

The deflection measurements nearest to the load point indicate that the girder began to show softening behavior at a load of between 2,000 and 2,220 kN (450 and 500 kips). The girder still had a significant reserve load capacity and reached a peak load of 3,410 kN (766 kips). At this peak load, the shear load carried by the east shear span was 1,950 kN (438 kips).

Half of the prestressing strands extending from the east end of the girder were instrumented to measure strand slip. Figure 44 shows the strand slip results throughout the test. The results for 11 of the 13 instrumented strands are shown. The LVDTs attached to the remaining two strands were not providing reliable results at the conclusion of the test; therefore, the results from these instruments were disregarded.

Results from the six strain rosettes are presented in figures 45 through 48. The tensile principal strain and strain angle values are presented in the first two figures. The compressive principal strain and strain angle values are presented in the second two figures. Note that the strain angle values are measured in clockwise degrees from horizontal on the south face of the girder. Also, similar to the Girder 24S results, tension and compression are not always strictly correct with regard to the sign of the principal strains.

Cracking and other damage to the girder throughout the test were observed both audibly and visually. The first cracking was heard at a load of 1,600 kN (360 kips). The cracking continued throughout the remainder of the test and seemed to primarily emanate from the eastern half of the girder. At a load of approximately 3,100 kN (700 kips), a larger crack became visible that could clearly be seen from 4.6 m (15 ft) away. This crack continued to grow longer and wider as the applied load was increased. Figure 49(a) shows the crack at the peak load carried by the girder. Subsequent to this point in the test, the girder continued to soften while the displacement increased and the load decreased. The crack continued to grow toward the bearing and load points until a secondary crack formed. Figure 49(b) shows the girder after the formation of the secondary crack in the bottom flange 0.91 m (36 inches) from the end of the girder. This is precisely the location to which half of the bottom flange strands were debonded.

Figure 41. Graph. Load-deflection response for Girder 14S. This graph shows the applied load versus vertical deflection results from the six potentiometers attached to the bottom flange of this girder. The responses are basically linear elastic until approximately 2,500 kilonewtons (562 kips) of load has been applied. Later, each response shows some softening before the peak load of 3,410 kilonewtons (767 kips) is reached. After the peak load is reached, the responses then show a temporary decrease in load capacity then a steadying of response at around 2,500 kilonewtons (562 kips) until the first strand ruptures occur at a load point deflection of just over 25 millimeters (1.0 inch).

1 mm = 0.039 inch
1 kN = 0.225 kip

Figure 41. Graph. Load-deflection response for Girder 14S.

Figure 42. Graph. Bearing rotation for Girder 14S. This graph shows the rotation of the girder at middepth over the east and west bearings. The responses from the east and west bearings are very similar throughout the loading. The responses are basically linear elastic until approximately 2,500 kilonewtons (562 kips) of load has been applied. Later, each response shows some softening before the peak load of 3,410 kilonewtons (767 kips) is reached. The shape of these responses is very similar to the responses shown in figure 41.

1 kN = 0.225 kip

Figure 42. Graph. Bearing rotation for Girder 14S.

Figure 43. Graph. Deflected shape for Girder 14S. This graph shows the deflected shape of this girder at nine load levels throughout the test. These load levels include 0 kilonewtons, 670 kilonewtons, 1,330 kilonewtons, 2,000 kilonewtons, 2,670 kilonewtons, 3,400 kilonewtons, 2,670 kilonewtons (0, 150, 300, 450, 600, 764, and 600 kips respectively), and after the peak load at 2,490 kilonewtons and 1,330 kilonewtons (599 and 299 kips respectively). Initial behavior is basically elastic, and later behaviors clearly show the shear deformations that occurred in the shear span.

1 m = 3.3 ft
1 mm = 0.039 inch
1 kN = 0.225 kip

Figure 43. Graph. Deflected shape for Girder 14S.

Figure 44. Graph. Strand slip in Girder 14S. This graph shows the strand slip response for the 13 L V D Ts that were attached to the prestressing strands extending from the east end of the girder. The fully bonded strands in the bottom row of strands and the top flange strand showed no slip throughout the test. The other fully bonded strands showed small amounts of slip after the peak load was reached. The debonded strands began to show slip very early in the loading of the girder. After approximately 2,200 kilonewtons (494 kips) of load, the rate of slip increased until each of them showed over 2 millimeters (0.08 inches) of slip by the time the peak load was reached. Slippage continued as the peak load was surpassed until 8 to 10 millimeters (0.3 to 0.4 inches) of slip had occurred by the time the first fully bonded strands ruptured.

1 kN = 0.225 kip

Figure 44. Graph. Strand slip in Girder 14S.

Figure 45. Graph. Principal tensile strain in the web of Girder 14S. This graph shows the principal tensile strain responses of rosettes 1, 2, 3, 4, 5, and 6. Rosettes 1 through 3 exhibit basically linear elastic behavior until over 1,300 kilonewtons (292 kips) of load has been applied. As was observed in Girders 28S and 24S, the responses for this girder are somewhat erratic after shear cracking has occurred.

1 kN = 0.225 kip

Figure 45. Graph. Principal tensile strain in the web of Girder 14S.

Figure 46. Graph. Principal tensile strain angle in the web of Girder 14S. This graph shows the principal tensile strain angle as measured in degrees from horizontal on the south face of the girder plotted versus the applied load. Most importantly, the angle for rosettes 1, 2, and 3 converge toward being approximately 55 degrees from horizontal when shear cracking occurs.

1 kN = 0.225 kip

Figure 46. Graph. Principal tensile strain angle in the web of Girder 14S.

Figure 47. Graph. Principal compressive strain in the web of Girder 14S. This graph shows the principal compressive strain responses of rosettes 1, 2, 3, 4, 5, and 6. Rosettes 1 through 3 exhibit basically linear elastic behavior until over 1,700 kilonewtons (382 kips) of load has been applied. As was observed in Girder 28S and 24S, the responses for this girder are somewhat erratic after shear cracking has occurred.

1 kN = 0.225 kip

Figure 47. Graph. Principal compressive strain in the web of Girder 14S.

Figure 48. Graph. Principal compressive strain angle in the web of Girder 14S. This graph shows the principal compressive strain angle as measured in degrees from horizontal on the south face of the girder plotted versus the applied load. Most importantly, the angle for rosettes 1, 2, and 3 converge toward being approximately 35 degrees from horizontal when shear cracking occurs.

1 kN = 0.225 kip

Figure 48. Graph. Principal compressive strain angle in the web of Girder 14S.

Figure 49. Photo. Girder 14S at (a) peak load and (b) postpeak load of 2,650 kilonewtons (595 kips). The photos in this figure show the south face shear span of this girder when the peak load was reached and after the load had dropped to 2,650 kilonewtons (595 kips). At peak load, a large shear crack was beginning to form in the base of the web. The second photo shows this crack having extended along the line between the bearing and the load point. A second crack has also formed running vertically up through the bottom flange to connect with the primary shear crack.

Figure 49. Photo. Girder 14S at (a) peak load and (b) postpeak load of 2,650 kilonewtons (595 kips). The photos in this figure show the south face shear span of this girder when the peak load was reached and after the load had dropped to 2,650 kilonewtons (595 kips). At peak load, a large shear crack was beginning to form in the base of the web. The second photo shows this crack having extended along the line between the bearing and the load point. A second crack has also formed running vertically up through the bottom flange to connect with the primary shear crack.

Figure 49. Photo. Girder 14S at (a) peak load and (b) postpeak load of 2,650 kN (595 kips).

Failure of the girder occurred when prestressing strands in the bottom flange began to rupture. The vertical crack in the bottom flange at the end of the debonding length continued to widen as, sequentially, the fully bonded strands broke. As more displacement was imparted into the girder, more strands ruptured until all 12 fully bonded bottom flange strands had ruptured. Figure 50 illustrates the crack and failure patterns that were recorded after girder failure.

Figure 50. Illustration. Crack pattern at failure in Girder 14S. This illustration shows damage that was visible on the south face of this girder after the conclusion of the test. Tightly spaced shear cracks are shown paralleling the primary shear crack. Shear cracks were also visible in the west shear span.

1 mm = 0.039 inch

Figure 50. Illustration. Crack pattern at failure in Girder 14S.

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