Cast-in-Place Concrete Connections for Precast Deck Systems (2011)

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433 Chapter 13 Transverse Joint Details for Accelerated Bridge Construction: Fatigue Evaluation 13.0 Introduction This chapter presents a summary of the investigation of the U-bar detail used for the transverse joint (i.e., tension tests). In this study, eight specimens connected by a U-bar detail utilizing a 6 in. lap length were fabricated and tested. The analytical parametric study conducted to provide the database of tension forces to be applied to specimen was summarized in Chapter 10. The forces applied to the tension specimens were intended to simulate the service live load demand anticipated in the transverse joint at a pier in a continuous decked bulb-T bridge or full-depth precast panel on girder bridge. Static and fatigue tests under tension loading were conducted. Test results were evaluated based on tensile capacity, cracking, displacement and steel strain. Based on these test results, the U-bar detail was deemed to be a viable connection system for the transverse joint. 13.1. Experimental Program 13.1.1. Specimen Dimension A total of four specimens were fabricated for the static and fatigue testing, with two different closure pour materials used in the transverse joint. Each specimen consisted of two panels (Part 1 and Part 2 in the figure) connected by one of the closure pour materials (overnight and 7-day cure) as shown in Figure 13.1.1. Each panel was 15 in. wide, 32 in. long and 7.25 in. deep. The female-to-female shear key was provided at the vertical edge of the end with the U-bar extended in the specimen length direction.

434 4'' 5.5'' 1.125'' 2.5'' 4'' 5.5'' 1.125'' 2.5'' Shear Key Detail 32'' 32''7 .2 5 '' 15 '' Panel 1 Panel 2 See "Shear Key Detail" Centerline of Joint Figure 13.1.1: Dimension of transverse joint (tension) specimen 13.1.2. Reinforcement Layout and Strain Gage Instrumentation Figure 13.1.2 displays the reinforcement layout used in the tension specimens. The reinforcement layout was the same as that used in WT-1 and WT-2, discussed in Chapter 9, except that the welded joints connecting the threaded rods to the U-Bars were kept outside of the concrete specimen. This facilitated repair of the welded connection if it were to fail before the specimen. There were four layers of reinforcement in each panel along the specimen depth direction with a 2 in. cover at the top and 1 in. cover at the bottom. The straight bars simulated the transverse reinforcement while the U-bars simulated the longitudinal reinforcement that comprised the transverse joint connection reinforcement in the bridge deck. The reinforcement details in the specimen were as follows: #5 straight bar spaced at 6 in. at the bottom along the specimen width direction; #4 straight bar spaced at 12 in. at the top along the specimen width direction. The #5 U-bars projected out of the panel to splice with the U-bars in the adjacent panel in the transverse joint. The spacing of the U-bars was 4.5 in. and the overlap length (the distance between bearing surfaces of adjacent U-bars) was 6 in. The interior diameter of bend of the U-bar was 3db.

435 2" 1" 178" 6' 6" 4"1' 6" 714" 412" 6' 4"6" 112" 1'-3"412" 4"6" 412" (TYP) (TYP) #4 Bars (3d) (TYP) Strain Gage Detail #5 Bars (TYP) (TYP) (TYP) #5 Bars #4 Headed Lacer Bars Figure 13.1.2: Reinforcement layout in transverse joint (tension) specimen The strain gage configuration was modified based on the test results described in Chapter 9. Figure 13.1.3 depicts the strain gage layout in the specimen for the tension test. Gages were placed at 2 and 6 in. away from the bend of the U-Bar. The strain gage diagrams have notations indicating the U-Bar identifier and the location on the bar. The U-Bars are represented by “UB” and the lacer bars are indicated by “LB.” The distance from the bend of the U-Bar to each gage is shown at the bottom of the diagram. All distances indicated on the diagrams are in inches and measured from center-to-center. A gage was placed at the midpoint of the lacer bar.

436 UB-3 UB-4 UB-2 4-2 LB1-m 3-2 2-2 LB-1 LB-2 UB-5 UB-4 UB-3 UB-2 UB-1 6" 2-1 4-1 3-1 2" 4" 2"4" 2" 4" (a) Strain gage configuration for U-bars 534 in LB1-m (b) Strain gage configuration for Lacer Bar Figure 13.1.3: Strain gage configuration for transverse joint (tension) specimen 13.1.3. Specimen Fabrication The specimens were fabricated at Ross Prestressed Concrete Inc. in Knoxville, TN. Figure 13.1.4-a shows the panel reinforcement before placement of the concrete. One end of the wood form, in the length direction, was slotted at a spacing of 6 in. to fix the U-bars in place, and foam wedges were used to form the configuration of the shear key at the vertical edge of the panel. Holes were drilled in the other end of the form, so that the threaded rods attached to the reinforcement could be extended out of the specimen. The design concrete compressive strength at 28 days was 7000 psi. Companion concrete cylinders were cast with the panels.

437 (a) Before pouring (b) After pouring and curing Figure 13.1.4: Panel fabrication 13.1.4. Joint Surface Preparation The surfaces of the shear keys were sandblasted to prepare the joints for the closure pours. The purpose of the surface preparation was to remove all contaminants that might interfere with adhesion and to develop a surface roughness to promote a mechanical bond between the grout and the panel. After the removal of the deteriorated concrete, proper preparation provided a dry, clean and sound surface, which offered a sufficient profile to achieve adequate adhesion. As discussed in Chapter 12, there were many available methods to prepare the surface including chemical cleaning, mechanical cleaning and blasting cleaning. Sandblasting, which uses compressed air to eject the high speed stream of sand onto the surface, was used because it is a very effective method to process the surface of precast members under industrial conditions. Black Beauty 2050 sand was chosen for sandblasting to prepare the surface in this study. The profile of the surface after sandblasting is shown in Figure 13.1.5.

438 Figure 13.1.5: Profile of joint surface after sandblasting 13.1.5. Closure-Pour Materials The longitudinal joint, which is filled with closure-pour (CP) materials connecting the top flange of the adjacent DBT girders, is considered to be the structural element of the bridge deck. It is important for the selected CP material to reach its design compressive strength in a relatively short time for the purpose of accelerated bridge construction. In this study, it was decided to use two closure-pour materials, SET® 45 HW for overnight cure and HPC Mix1 for 7 day cure, as described in Chapter 11. 13.1.6. Testing Plan and Setup A total of four specimens were made, where each specimen consisted of two concrete panels connected with an overlapping U-bar detail and one of the selected closure pour materials. When grouting, two panels were positioned to satisfy the 6 in. U-bar overlap length and the 4.5 in. spacing of the U-bars in the joint zone (Figure 13.1.6-a). The wood form was provided at the bottom and at both ends of the joint to prevent leakage. After grouting, the transverse joint (tension) specimen consisting of 2 panels connected by the joint was ready for testing (Figure 13.1.6-b).

439 (a) Before grouting (b) After grouting Figure 13.1.6: Transverse joint (tension) specimen before and after grouting Two of the four specimens (two for each of the two selected CP materials) were tested under static tension (ST), and correspondingly, two of the four specimens were tested under fatigue tension (FT). Table 13.1.1 presents the loading matrix for the four specimens. Figure 13.1.7 shows the test setup and the linear voltage displacement transducers (LMT) instrumentation for each test. The same test setup and LMT instrumentation were employed as used in the tension tests described in Chapter 9. The Crack Comparator was used to measure the crack width. Table 13.1.1: Transverse joint (tension) specimen loading matrix Overnight Cure 7 Day Cure Static Fatigue Static Fatigue SET® 45 HW SET® 45 HW HPC Mix 1 HPC Mix 1

440 Figure 13.1.7: Tension test setup The compressive strength of the concrete panel f’c and the compressive strength of grouted joint f’cj at the time of testing for each specimen are given in Table 13.1.2.

441 Table 13.1.2: Compressive strength of concrete panel and grouted joint Panel (psi) Joint (psi) Specimen Start of Test End of Test Start of Test End of Test ST-O 8462 4649 ST-7 8886 9974 FT-O 8462 8216 3994 4982 FT-7 8462 9162 9470 9505 13.1.7. Fatigue Loading Determination As discussed in Section 10.2, the resulting extreme fiber stresses at the top of the girder in the transverse joint associated with the maximum moments were -1.056 ksi, -0.306 ksi, 0.169 ksi, and 0.066 ksi under the negative design load, negative fatigue load, positive design load, and positive fatigue load, respectively, assuming uncracked sections. The negative design stress (-1.056 ksi) under the design load (-3140 kip-ft) was greater than the modulus of rupture of concrete. Thus, the transverse joint was reanalyzed assuming cracked section properties. Based on the cracked section analysis, the stresses of the U-bar in girder DBT65 were determined to be 35.6 ksi and 10.3 ksi under the negative design load (-3140 kip-ft) and negative fatigue load (-910kip-ft), respectively. The static and fatigue loadings during the test were determined as follows: Static Tension: P=2×2×0.31×35.6=44.1 kips Fatigue Tension: P=2×2×0.31×10.3=12.8 kips Static Compression: P=7.25×15×0.169=18.4 kips Fatigue Compression: P=7.25×15×0.066=7.2 kips In the determination of the tension loads, on one side of the joint there were three sets of #5 U-bars, on the other side of the joint, there were two sets of #5 U-bars, as shown in Figure 13.1.2. The tension load was determined by multiplying the desired stress (i.e., 35.6 ksi or 10.3 ksi, for the static and fatigue tests, respectively) by total cross-sectional area of the fewest number of bars crossing the joint (i.e., two sets of U-bars, which was two sets of two bars multiplied by the cross-sectional area of a #5 bar of 0.31 in.2). The compression load was found by multiplying the concrete cross-sectional area by the desired compressive stress.

442 In the tests, the compression load was not applied to be conservative. During the fatigue test, the applied tension load was cycled between 12.8 kips (6.4 kips on each actuator) corresponding to a negative fatigue load of -910 kip-ft and 0 kips for a total of 2 million cycles at a frequency of 4Hz. Before the cycling and at the end of 0.5, 1.0, 1.5, and 2.0 million cycles, a static test was conducted. A static tension loading was applied in several increments up to 44.1 kips (22.1 kips on each actuator) in order to produce the negative design load of -3140kip-ft, and unloaded to zero. Finally, the specimen was loaded to failure. 13.1.8. Tensile Capacity The tensile capacity was calculated as the product of the lightly reinforced area of steel, As=1.24 in2, and the U-Bar nominal yield strength of 75 ksi, as discussed in Chapter 9. The service live load was also determined in Chapter 9. The test results are compared with the calculation in Table 13.1.3. The strengths for the fatigue tests, FT-O and FT-7, shown in the table are strengths at end of the tests. Table 13.1.3: Tensile capacity Specimen Measured Calculated Panel Strength (psi) Joint Strength (psi) Failure Load (kip) Service Live Load (kip) Ultimate Load (kip) 75 ksi (U-Bar Nominal Yield Strength) 60 ksi (U-Bar Nominal Yield Strength) WT-2 7719 88.70 44.14 93.00 74.40 ST-O 8462 4649 67.85 44.14 93.00 74.40 FT-O 8216 4982 65.21 44.14 93.00 74.40 ST-7 8886 9974 93.49 44.14 93.00 74.40 FT-7 9162 9505 101.15 44.14 93.00 74.40 WT-2, discussed in Chapter 9, had the same reinforcement layout as the tension specimens tested in this phase. The difference was that WT-2 was poured as a monolithic specimen, while ST-O, FT-O, ST-7 and FT-7 were fabricated by joining two panels with one of the CP materials as shown in Figure 13.1.6. All of the specimens exceeded the nominal service live load capacity. However, only ST-7 and FT-7 exceeded the

443 calculated tensile capacity. In Section 9.2.4, it was concluded that tensile capacities were reduced by reducing the concrete strength. Please note that the longitudinal reinforcement is not continuous in the U- bar detail. And one reason that ST-O and FT-O have lower capacities is due to the lower strength of the joint material. Attention needs be paid to the moisture loss during the first 3 hours after placement, which may have caused the lower strengths in the tests. 13.1.9. Load Deflection Relationships Figure 13.1.8 compares the load-deflection curves of the fatigue specimens after 2 million cycles with those of the specimens subjected to static loading without fatigue cycles. The vertical axis labeled “Load” represents the total applied tension load, and the horizontal axis “Deflection” represents the relative displacement between the two joint interfaces. Figure 13.1.8: Load-deflection curves of the tension specimens The slopes for ST-7, FT-O and FT-7 were large, because the specimens deflected out of phase at the beginning of the load, and the FT-7 specimen had this out-of-phase problem through the test. ST-O developed more deflection than FT-O. After reaching the peak load, ST-7 exhibited more ductility than FT-7. The fatigue cycles had an effect on the deflection of joints. 0 40 80 120 -0.1 0 0.1 0.2 0.3 0.4 Deflection (in) L oa d (k ip ) Service Live Load FT-O ST-O ST-7 FT-7

444 13.1.10. Crack Width Development During the tests, the entire specimen was monitored for crack development, and crack widths were measured. The crack development within the joint zone is presented in Figure 13.1.9. “Crack 1”, “Crack 2”, “Crack 3”, and “Crack 4” are shown in Figure 13.1.10, marked as “①”, “②”, “③” and “④,” respectively. Cracks 1 and 2 were cracks along the upper and lower joint interfaces, respectively. Crack 3 was a longitudinal crack observed within the joint, and Crack 4 referred to the diagonal or transverse cracks within the joint. The crack development observed in ST-O and ST-7 was similar. From Figure 13.1.9, the width of “Crack 1” grew faster among all of the cracks due to the less reinforcement at the joint interface of the “Crack 1” location. There were two U-bars that crossed the joint interface at the “Crack 1” location and three that crossed the joint at the “Crack 2” location. Crack 2 had the smallest measured crack widths throughout the tests as would be expected. For all four specimens, the first crack developed along the joint interfaces with less U-bars, Crack 1 as shown in the figures. As the tension loading was increased, transverse cracks continued to appear in various locations within and outside the joint zone. For ST-O, the first transverse crack occurred at approximately 20 kips, which was about 30% of the tensile capacity. For FT-O, the first transverse crack occurred at nearly 10 kips, about 15% of the tensile capacity. For ST-7, the first transverse crack developed at 30 kips, 32% of tensile capacity, and for FT-7, the first transverse crack developed at 30 kips. Longitudinal cracks began forming inside the joint zone at the following loads: 40 kips for ST-O, 20 kips for FT-O before cycling, 40 kips for ST-7, and 44.1 kips for FT-7 after 0.5 million cycles. The longitudinal cracks formed above the longitudinal reinforcement in the specimen, which relates to the longitudinal reinforcement in the deck of precast bridge deck system crossing the transverse joint. Such cracks initiate splitting cracks associated with bond failure. Diagonal cracks appeared in the joint, as the specimens approached capacity.

445 (a) ST-O (b) ST-7 Figure 13.1.9: Load-crack width curves of static tension specimens 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.1 0.12 Crack Width (in) L oa d (k ip ) Crack 1 Crack 2 Measured Failure Load Service Live Load Crack 3 Crack 4 0 20 40 60 80 100 0 0.02 0.04 0.06 0.08 0.1 0.12 Crack Width (in) L oa d (k ip ) Crack 1 Crack 2 Measured Failure Load Service Live Load Crack 3Crack 4

446 (a) ST-O (b) ST-7 Figure 13.1.10: Cracks within the Joint Figure 13.1.11 shows the crack width-fatigue cycle curve (CW-N) for the fatigue tests representing the maximum crack width within the joint after various number of fatigue cycles under specified loadings. For FT-7, Crack 1 and 2 along the joint interfaces developed before the fatigue cycling, and one longitudinal crack developed in the static test after 0.5 million cycles. The longitudinal crack width was less than 0.008 in. all through the cycling. For FT-O, seven cracks developed during the static test before the fatigue cycling, including two cracks along the joint interfaces, two transverse cracks and three longitudinal cracks in the joint. No new crack occurred in the interim static tests. Crack 1 along the joint interface with two U-bars had the largest width all through the fatigue cycling, while all the other cracks had widths less than 0.008 in. From Figure 13.1.11, it can be seen that the width of the cracks within the joint was dependent upon the applied load. The crack widths increased with increased loads, as expected. Under the same loading, the crack widths were observed to increase with increasing numbers of fatigue cycles, particularly for the specimens with the overnight cure material. The joint with the 7-day cure material developed smaller crack widths than the overnight cure material, and the cracks in the joint with the 7-day cure material tended to stabilize under fatigue loads.

447 (a) FT-O (b) FT-7 Figure 13.1.11: CW-N curve 13.1.11. Strain Development Figure 13.1.12 shows the strain-fatigue cycle curves (MS-N) for the fatigue tests representing the reinforcement strain in the joint after various number of fatigue cycles under service live load. The strain gage number and the loading are shown in the figure. The strain gage layout is shown in Figure 13.1.3. “P = 44.1 kips” means the specimen was subjected to a tension load of 44.1 kips in total when the strain measurement was taken. From Figure 13.1.12, it can be seen that the variation of the reinforcement strain after different fatigue cycles was not significant. The strains were larger in the two U-bar reinforcement (see gage numbers 2-# and 4-#) compared with the strains in the three U-bar reinforcement (see gage number 3-#). The strain was observed to increase with increase in distance from the bend of the U-bars. The lacer bar experienced low axial strains; it served to provide mechanical anchorage to the U-bars through dowel action. 0.000 0.010 0.020 0.030 0.040 0.050 0.0 0.5 1.0 1.5 2.0 Number of Fatigue Cycles (million) C ra ck W id th ( in ) P=44.1 kips P=30 kips P=20 kips P=10 kips 0.000 0.010 0.020 0.030 0.040 0.050 0.0 0.5 1.0 1.5 2.0 Number of Fatigue Cycles (million) C ra ck W id th ( in ) P=44.1 kips P=30 kips P=20 kips P=10 kips

448 (a) FT-O (b) FT-7 Figure 13.1.12: MS-N curves of fatigue tension specimens Figure 13.1.13 shows the load-strain curves representing the strain values in the joint zone (Strain gage 3-1 shown in Figure 13.1.3) for each specimen, which also show that the variation in the reinforcement strain observed after fatigue cycles was not significant in comparing ST-O to FT-O and ST-7 to FT-7. P=44.1 kips -500 0 500 1000 1500 0.0 0.5 1.0 1.5 2.0 Number of Fatigue Cycles (million) M ic ro st ra in 4-2 4-1 3-2 3-1 LB1-m P=44.1 kips 0 500 1000 1500 0.0 0.5 1.0 1.5 2.0 Number of Fatigue Cycles (million) M ic ro st ra in 2-2 4-1 3-1

449 Figure 13.1.13: Load-strain curve 13.1.12. Failure of Specimen The crack patterns at tensile failure for ST-O, FT-O, ST-7 and FT-7 are shown in Figure 13.1.14. The diagonal cracks propagated toward the first transverse cracks that developed along the joint interface. The concrete could be easily removed from the specimen where the diagonal and transverse cracks met. The lacer bars were intended to serve as restraints for the U-Bars to facilitate ductile failure in the specimens. An example of the deformation of the lacer bars can be seen in Figure 13.1.15. 0 20 40 60 80 100 120 0 1000 2000 3000 Microstrain L oa d (k ip ) ST-O FT-O ST-7 Service Live Load FT-7

450 (a) ST-O (1 in. cover side) (b) ST-O (2 in. cover side) (c) FT-O (1 in. cover side) (d) FT-O (2 in. cover side)

451 (e) ST-7 (1 in. cover side) f) ST-7 (2 in. cover side) (g) FT-7 (1 in. cover side) (h) FT-7 (2 in. cover side) Figure 13.1.14: Cracking in transverse joint (tension) specimens

452 Figure 13.1.15: Deformation of lacer bar 13.2. Conclusions Based on the parametric study and the experimental program, the following conclusions can be made: 1. The fatigue loading had no significant influence on the tensile capacity and reinforcement strains. 2. The fatigue loading was observed to have an effect on the deflection development, particularly for the joints with the 7-day cure material. 3. The fatigue loading had some effect on the measured crack widths in the specimens with the overnight cure material. Under the same loading, the crack widths were observed to increase after the fatigue cycles. 4. Undesirable wider crack widths will be developed at service load levels in transverse joints designed with higher grades of steel (e.g., 75 ksi compared to 60ksi) because smaller amounts of reinforcement can provide the required nominal strength. Under service loads, larger stresses would be expected in the smaller bars, which lead to wider cracks at service. It is recommended that 60 ksi nominal yield strength be used in the design of transverse joints, or that stresses in the reinforcement are limited at service. 5. Based on these tests and with the aforementioned caveats, the U-bar detail may be considered a viable connection system for transverse joints in continuous decked bulb-T and full-depth precast deck panel on girder bridges.