Transverse Joint Details with Tight Bend Diameter U-Bars for Accelerated Bridge Construction

The main objective in Phase I was to test two joint details (i.e., U-bar and headed bar details) and select the best-performing joint detail for Phase II.

A deck thickness of 184 mm (7 $1/4\mathrm{in}.$) was selected for the transverse joint specimens to accommodate the required reinforcement clearances. To determine the controlling load case for a transverse joint, the transverse joint should be positioned over an interior support in a continuous span bridge system. The required steel area for the transverse joint was then determined. The reinforcement in the transverse joint was designed as if the joint was located over an interior pier of a continuous span bridge system with the deck compositely connected to the girder. The negative moment developed in these locations would create large tensile forces in the deck, which would require more longitudinal steel in these regions. The composite cross section and the negative moment used in the flexural calculations were taken from an example in Chap. 9.6 of the PCI bridge design manual (PCI 2003). The negative moment value that was used in the flexural calculations was $6,558\mathrm{kN}/\mathrm{m}$ ($4,837\mathrm{kip}/\mathrm{ft}$). The amount of longitudinal deck reinforcement was determined by a conventional flexural design using the composite section. The centroid of the reinforcement was assumed to be at midheight of the deck, and the required amount of reinforcement was determined for both 414 MPa (60 ksi) and 517 MPa (75 ksi) yield strengths. Table 1 contains the results of the designs.

The reinforcement spacing was conservatively selected to provide the same detailing across the joint in the specimens with the Grade 75 reinforcement as that used in specimens with Grade 60 reinforcement. Because of the use of the U-bar detail, the top and bottom layers of the primary joint reinforcement had to have the same spacing (by the nature of the U-bar, it consists of two layers of reinforcement). In summary, the reinforcement that crossed the joint consisted of top and bottom layers of No. 5 rebar spaced at 114 mm (4.5 in.) as shown in Table 1. The top layer of reinforcement parallel to the joints consisted of No. 4 rebar spaced at 305 mm (12 in.) and the bottom layer of reinforcement parallel to the joint was determined to be No. 5 rebar spaced at 152 mm (6 in.).

The joint overlap length, which is defined herein as the distance between the reinforcement bearing surfaces, was determined based on the expected development length of a U-bar. The ACI 318-08 (ACI 2008) equation for determining the development length of a standard hook in tension was used to calculate the approximate development length of a U-bar. This equation does not directly apply to the U-bars that were used, because the U-bars do not meet the dimensional requirements for a standard hook, namely the $3d_b$ bend diameter used in the U-bar fabrication violated the minimum $6d_b$ bend diameter specified in ACI 318-08 (ACI 2008). Eq. (1) shows the ACI development length equation for a standard hook in tension (ACI 2008)where $l_{dh}$ = development length from the tail of the hook; ${\psi }_e$ = epoxy coating factor; $\lambda$ = lightweight concrete factor; $f_y$ = reinforcement yield strength; $\sqrt{f_c^{\prime }}$ = square root of the concrete compressive strength; and $d_b$ = diameter of the bar.

Using Eq. (1), the development length was calculated for a No. 5 bar, assuming a concrete compressive strength of 41 MPa (6 ksi) and a steel yield strength of 517 MPa (75 ksi), because DWR and SS were the materials used for this joint detail. The development length of a standard hook bar in tension for this situation was calculated to be 216 mm (8.5 in.). In the testing program, an overlap length of 152 mm (6 in.) was used for the U-bar detail.

Based on Li et al. (2010a,b), one layer of headed bars with varying overlap lengths [64 mm (2.5 in.), 102 mm (4 in.), and 152 mm (6 in.)], were tested and compared, and the lap length for the headed bar detail was recommended to be 152 mm (6 in.). Although the head was larger in the study done by Li et al. (2010a), for the case of the No. 5 headed bars in this study, with the small head size, the overlap length was also taken to be 152 mm (6 in.).

Because the behavior of the specimens was to be compared, the overlap lengths for the U-bar and headed bar joint details were made the same [i.e., 152 mm (6 in.)]. Two transverse lacer bars were added to each joint detail to provide continuity and confinement of the joints (Gordon and May 2006).

The main objective in Phase II was to test the best-performing detail to investigate and compare variations in parameters, including concrete strength, bar spacing, and overlap length.

Based on the test results from Phase I, U-bar details with DWR were selected for further testing in Phase II. The testing parameters of the joint detail for Phases I and II are listed in Table 2. Each specimen is labeled as follows: “H” represents headed reinforcement, “S” represents SS reinforcement, “W” represents DWR, “T” represents tension test, and the number represents the specimen number tested in chronological order. Specimen HT-1 utilized a headed bar joint detail made of conventional deformed bar reinforcement. Specimens ST-1 and WT-1 both utilized the U-bar joint detail.

Specimens HT-1, ST-1, and WT-1, along with connection details designed for the transverse joint, are shown in Fig. 2. Bar spacing and overlap length are the same for these three specimens. As shown in Fig. 2, one side of each specimen was lightly reinforced with four No. 5 bars and the other side of the specimen was reinforced with six No. 5 bars. The joint zone is where the rebars end and stagger with respect to each other.

As shown in Table 2, specimens WT-2, WT-3, and WT-4 were tested. Two different concrete strengths [69.0 MPa (10 ksi) and 48.3 MPa (7 ksi)], two different rebar spacings [114 mm (4.5 in.) and 152 mm (6 in.)], and two different overlap lengths [152 mm (6 in.) and 114 mm (4 in.)] were considered in the second phase tests. The typical detail of Test Specimens WT-2, WT-3, and WT-4 are provided in Fig. 2(b) with varied $f_c^{\prime }$, $l$, and $s$ listed in Table 2.

The calculated tension capacity was the force determined by multiplying the area of steel of the lightly reinforced side of the specimens by the appropriate rebar yield strength assuming continuous reinforcement (i.e., not staggered as in actual specimens). Table 4 shows the calculated and tested tensile capacities. Although the actual tested yield strength varies a little between ST and WT as shown in Fig. 5, 517 MPa (75 ksi) steel nominal yield strength was used to calculate the tensile capacity for both ST and WT in Table 4. For Phase I, all specimens produced similar tensile capacities. HT-1 produced the lowest tensile capacity, which was to be expected because HT-1 contained conventional rebar with the lowest nominal rebar yield strength. The second highest tensile capacity was produced by the U-bar detail using SS reinforcement (ST-1). The largest tensile capacity was produced by WT-1.

Both WT-1 and HT-1 exceeded the expected tensile capacity. The additional capacity of Specimens WT-1 and HT-1 may be explained by the fact that actual yield strength is higher than design yield strength or that the steel reached the strain-hardening stage. However, Specimen ST-1 did not quite meet the expected capacity. The low capacity could have been because of the fact that the welds broke during testing at a load of approximately 289.1 kN (65 kip). The specimen was rewelded and tested to failure, but the specimen may have been damaged during the first unsuccessful test. ST-1 may have experienced damage that could have affected its behavior and tensile capacity during the second successful test.

All three specimens produced similar crack patterns up to and beyond the service loading. The tensile service load was calculated by using the service-level negative moment found in the design example of Chapter 9.6 of the PCI bridge design manual (PCI 2003) and the neutral axis found for the cracked composite cross section. The area of steel in the specimens was compared with the total required area of steel to determine the tensile service loads for the specimens. The tensile service load determined was 196.2 kN (44.1 kip) and 245.1 kN (55.1 kip) for headed bar and U-bar specimens, respectively.

The first cracks to appear were transverse cracks evenly spaced along the length of the specimens. The joint zone usually experienced transverse cracking after several other transverse cracks had already formed in other locations. Delayed transverse cracking in the joint zone may have been caused by the larger area of reinforcement in the joint region compared with that of the body of the specimens. The transverse cracks initially were found only on the surface of the concrete, and as the loading progressed, the cracks propagated through the entire thickness of the specimens. Additional loading produced longitudinal cracks that appeared above the main longitudinal reinforcement in the specimens. These longitudinal cracks appeared above the longitudinal reinforcement located in the lightly reinforced half of the specimen, or the top half of the specimens in this particular setup. When the capacities of the specimens were approached, diagonal cracks appeared close to the sides of the specimens. These diagonal cracks would usually propagate toward a transverse crack in the joint zone and lead to the failure surface for the specimens. Fig. 6 shows the crack patterns at failure. The written numbers by the cracks represent the total force applied in kips when the crack was formed.

A crack width gauge was used to visually determine the widths of the cracks. The average width of all cracks within the joint zone at 245 kN (55 kip) was found to be 0.254 mm (0.010 in.) for ST-1 [a comparable crack width measurement was taken at 200 kN (45 kip) for Specimen ST-1, and the average crack width was found to be 0.152 mm (0.006 in.)]. The average crack width at 196 kN (44 kip) for Specimen HT-1 was determined to be 0.305 mm (0.012 in.]. The last crack width measurement for Specimen WT-1 was taken at 178 kN (40 kip) and the average crack width was found to be 0.200 mm (0.0079 in.). Comparing the crack widths of the two joint details, one can see that the headed bar detail of Specimen HT-1 generated the largest crack widths at its service-level loading compared with Specimens ST-1 at its service-level loading and WT-1 at approximately 178 kN (40 kip).

All joint details produced adequate capacities and ductility in tension tests. Among the three specimens tested, WT-1 produced the largest tensile capacity without compromising ductility. Smaller crack widths at service-level loading were also produced by WT-1. The development of small crack widths increases durability by decreasing reinforcement corrosion, thus leading to longer deck life.

In terms of constructability and reinforcement costs of the joint, the U-bar detail created a less-congested joint, which made it the easiest to construct. Although headed bars layout could give designers more spacing options for top and bottom steel reinforcement compared with U-bars, the bearing heads of the headed bar detail require more space to accommodate the larger diameter of the rebar heads. This extra space would reduce construction tolerances and could, therefore, cause problems in placement of precast deck components. The U-bars can also be easily tied together to form a rebar cage, which would allow for easy construction in the precast yard when compared with the two single layers of reinforcement in the headed bar detail. The material costs were competitive between the conventional rebar used in the headed bars and the DWR, which are much lower than SS.

After consideration of capacity, service-level crack widths, constructability, and cost, the U-bar detail constructed of DWR was chosen for Phase II testing.

The Phase I experiments showed that the U-bar detail could develop adequate capacity with an overlap length of 152 mm (6 in.), a rebar spacing of 114 mm (4.5 in.), and two transverse lacer bars.

Table 4 includes the results of Phase II tests. Specimen WT-1, tested in Phase I, is included in the following discussion for comparison. Specimens WT-1 and WT-4 exceeded the theoretical tensile capacity. The tensile capacity of WT-2, which had a decrease in $f_c^{\prime }$ from 69  MPa (10 ksi) to 48 MPa (7 ksi), was 4.6% less than the expected capacity, and the tensile capacity of WT-3, which had a decrease in joint overlap length from 152 mm (6 in.) to 102 mm (4 in.), was 18.7% less than the expected capacity. Because the amount of steel was not varied among the test specimens, the tensile capacity must be related to the interaction between the concrete and steel as well as the steel arrangement. This interaction is discussed in the following section describing the strut-and-tie model (STM).

Fig. 7 shows the load versus deflection curves of the four W specimens. All the specimens had similar slopes after cracking of the concrete. As the loading increased beyond theoretical tensile capacity, the concrete began to form large cracks and crumble at the joint zone. In the case of WT-3 where the joint overlap length was reduced from 152 mm (6 in.) to 102 mm (4 in.), the specimen was observed to fail at a load that was almost 20% less than the expected failure load.

For all specimens, as noted earlier for the Phase I tests, the first visible surface cracks developed in the transverse direction and were located outside the joint zone. As the tension loading increased, transverse cracks continued to appear in various locations outside the joint zone. For WT-1, the first cracks to appear were transverse cracks evenly spaced along the length of the specimens. The transverse cracks initially were found only on the surface of the concrete, and as the loading progressed the cracks propagated through the entire thickness of the specimens. For WT-2, the first transverse crack to develop at the joint occurred at approximately 125 kN (28 kip), which was at about 30% of the nominal tensile capacity. For WT-3, the first transverse crack occurred at nearly 151 kN (34 kip), at about 45% of the nominal tensile capacity. For WT-4, the first transverse crack to develop near the joint occurred at 240 kN (54 kip) at about half of the nominal tensile capacity. All initial transverse cracks occurred on the side of the specimen that had a 51 mm (2 in.) cover measured from the reinforcement surface to the surface of concrete. As testing progressed, the cracks began to form throughout the thickness of the specimen.

For WT-1, additional loading produced longitudinal cracks that appeared above the main longitudinal reinforcement in the specimen. As in the Phase I tests, these longitudinal cracks appeared above the longitudinal reinforcement located in the lightly reinforced half of the specimen, or the top half of the specimens in this particular setup. Longitudinal cracks began forming inside the joint zone at the following loads: 267 kN (60 kip) for WT-2, 196 kN (44 kip) for WT-3, and 311 kN (70 kip) for WT-4. The longitudinal cracks formed above the longitudinal reinforcement in the specimen, which relates to the longitudinal reinforcement that crosses the transverse joint in the deck of the precast bridge deck system.

Diagonal cracks appeared in the joint as the specimens approached capacity. These diagonal cracks propagated toward the first transverse cracks that developed in the joint. The concrete could be easily removed from the specimen where the diagonal and transverse cracks met. The crack patterns at tensile failure for WT-1, WT-2, WT-3, and WT-4 can be seen in Fig. 8.

The lacer bars provided confinement of concrete within the joint and served as restraints for the U-bars. The lacer bars allowed ductile failure in all four specimens. An example of the deformation of the lacer bars can be seen in photograph in Fig. 9, in which the lacer bar is located such that it provides bearing for the U-bars. These bearing forces cause the lacer bar to bend. This interaction between the lacer bar and U-bars helps explain the ductile failure mode observed in the tests.

The crack widths for WT-2 and WT-4 at service level were 0.508 mm (0.020 in.) and 0.203 mm (0.008 in.), respectively. For WT-3, the crack width at service-level loading had exceeded the limits of the comparator of 1.27 mm (0.050 in.). Specimen WT-3, with the joint overlap length of 102 mm (4 in.), created the largest crack width at service-level loading.

The data provided by the strain gauges was not as consistent in proving increasing strain in the reinforcement. Also, the strains were smaller at each gauge located further from the bearing surface in the joint. According to the strain data, only a couple of U-bars in WT-2 and WT-4 yielded at failure, which was observed at the strain gauge closest to the bend. The data were consistent in showing that the reinforcement developed large strains where diagonal cracks occurred. In Fig. 10, one typical total applied force versus strain curve is shown for Gauge 4–2 for these specimens.