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| Figure 7.80: Corroded Dowel Bars | Figure 7.81: Stainless Steel Clad Dowel Bars |
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| Figure 7.82: Dowel Bars with Excessive Grease |
Figure 7.83: Properly Lubricated Dowel Bars |
| Major Topics on this Page | ||
| 9.1 | Dowel Bars | |
| 9.2 | Tie Bars | |
| 9.3 | Reinforcing Steel | |
Steel placement involves the positioning of dowel bars, tie bars and reinforcing steel during rigid pavement construction. This section discuss the placement of dowel bars, tie bars, and reinforcing steel during the construction process.
Dowel bars can be placed either before PCC placement by using dowel baskets, or after PCC placement by using an automatic dowel bar inserter. Their placement is crucial to proper joint load transfer. Skewed, shallow or excessively corroded dowels can fail causing faulting and/or cracking at the joint. This subsection covers:
Dowel bars must be protected from corrosion (see Figure 7.80); although joints are sealed to keep water penetration to a minimum, water will seep in over time and, combined with deicing salts, may corrode unprotected bars. Typically dowel bars are protected from corrosion by the application of epoxy coating or stainless steel cladding (see Figure 7.81). Additionally, dowels should be lightly coated with a lubricant such as grease or oil to prevent bonding with the PCC. The FHWA notes "...The dowel must be free to slide in the concrete so that the two pavement slabs move independently, thus preventing excessive pavement stresses. Only a thin coating should be used, as a thick coating may result in large voids in the concrete around the dowels" (FHWA, 1990a). Figure 7.82 shows dowel bars with excessive lubricant (notice how it is dripping off the dowel bars), while Figure 7.83 shows one with the correct amount of lubricant.
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| Figure 7.80: Corroded Dowel Bars | Figure 7.81: Stainless Steel Clad Dowel Bars |
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| Figure 7.82: Dowel Bars with Excessive Grease |
Figure 7.83: Properly Lubricated Dowel Bars |
Dowel baskets (see Figures 7.84 and 7.85) are simple truss structures used to hold dowel bars at the appropriate height before PCC placement. Typically, dowel baskets span an entire lane width and are fabricated from thick gauge wire. They are left in place after the PCC is placed but do not contribute to the pavement structure.
When using dowel baskets, the dowels must be properly aligned and the dowel basket firmly anchored to the base course. The FHWA recommends that the dowel baskets be secured with steel stakes with a minimum diameter of 8 mm (0.3 inches) embedded at least 100 mm (4 inches) in stabilized bases, 150 mm (6 inches) in treated permeable bases and 250 mm (10 inches) for untreated bases or subgrade. Further, a minimum of 8 stakes per basket is recommended.
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| Figure 7.84: Dowel Basket Placed on an HMA Base | Figure 7.85: Dowel Baskets Placed on a Lean Concrete Base in Advance of PCC Placement |
Dowel bar inserters are automated attachments to slipform pavers that allow the paver to insert transverse joint dowel bars as part of the slipform paving process. Dowel bar insertion usually occurs after the vibrator but before the tamper bar. Dowel bars are placed on the fresh PCC surface then pushed down to the correct elevation by a series forked rods. The rods are usually vibrated while they insert the dowel bar in order to (1) ease insertion and (2) help move the PCC back into the space created by the insertion.
Tie bars are typically placed after PCC placement either by hand or using a tie bar inserter attachment (slipform paving only). When one lane at a time is paved, tie bars are inserted at mid-slab depth and bent back until the adjacent lane is ready to be paved (see Figure 7.86). On slipform pavers, tie bars are inserted on slab edges that will become longitudinal joints (see Figure 7.87 and 7.88) and, if two lanes at once are being paved, pushed into a mid-slab area (similar to dowel bar insertion) that will later be cut as a longitudinal joint (see Figure 7.89 and 7.90). Like dowel bars, tie bars should be protected from corrosion.
 Figure 7.86 (Top): Bent Tie Bars Figure 7.87 (Right): Side Tie Bar Inserter |
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Figure 7.88: Tie Bar Insertion Equipment for Side Tie Bars |
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| Figure 7.89: Inserter for Mid-Slab Tie Bars (the area in which the tie bars are inserted will later be cut as a longitudinal joint) | Figure 7.90: Tie Bar Insertion Cartridge
Protruding Upward from the Center of a Slipform Paver |
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Proper reinforcing steel placement is crucial to CRCP performance. CRCP failures are usually associated with insufficient reinforcement bar lapping, unconsolidated PCC around the steel, improper position of the steel in the slab and extreme hot weather during construction. In general, CRCP seems to be less forgiving of construction errors than other types of rigid pavement (Burke, 1983). Reinforcing steel for CRCP can be placed by two general methods:
The most common method, the manual method (see Figure 7.91), involves hand-placing the reinforcing steel before the PCC is placed. Since the steel is located at mid-depth or higher in the finished slab, the reinforcing steel must be supported by small metal or plastic "chairs" in order to achieve this elevation before PCC placement. These chairs must be strong enough to hold the reinforcing steel in place during PCC placement, consolidation and finishing.
The typical placement process involves (1) placing the transverse bars (which function only as placement aids) on chairs (see Figure 7.92), (2) arranging the longitudinal bars on top, and then (3) tying the longitudinal bars to the transverse bars. Typically, they are tied or clipped to the transverse bars every 1.2 - 1.8 m (4 - 6 ft.) (Burke, 1983). Figure 7.93 shows reinforcing bars in their final position before PCC placement.
The chief advantage of the manual method is that it allows for easy checking of bar placement, height and lap distance. However, the manual method is slower and more labor intensive than mechanical methods.
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| Figure 7.92: Reinforcing Bar in Place (white items are support chairs) |
Figure 7.93: Reinforcing Bar in Place |
Reinforcing steel can also be placed mechanically. There are many variations of mechanical placement, however most of them involve picking up prepositioned, but not precisely spaced, reinforcing bars with a placer/spreader attachment and then holding the bars at a preset depth as the PCC is placed around them (see Figure 7.94). However, a number of states have found longitudinal steel placement deviations of ± 75 mm (±3 inches) in the vertical plane when tube feeders were used to position the steel (FHWA, 1990).
Figure 7.94: Mechanical Reinforcing Steel Placement
Additionally, CRCP pavements (and JRCP pavements when they were more routinely constructed) can be placed in two lifts. First, the bottom lift is placed, followed by reinforcing bar placement, and then finished with a second PCC lift. Although feasible, this method can be costly as it usually requires two passes of the PCC paver. However, some equipment companies offer pavers capable of placing two lifts simultaneously.
