Manual Steel Placement (Chairs or Transverse Bar Assembly)
See FHWA’s CRCP Design and Construction Guidelines for the references included in this page. A new, more comprehensive FHWA CRCP Design, Construction, Maintenance and Rehabilitation manual is currently under development and this page will be updated upon its release in the spring of 2016.
In this method, reinforcing bars are attached to support assemblies prior to construction. These can consist of a variety of chair and support combinations, often tied to the transverse bars in some fashion. The supports must be sturdy enough to hold the bars within tolerances during placing and consolidating the concrete. For example, a recent project experienced problems when the plastic chairs proved to have inadequate stability and required strength- ening and additional ties at steel intersections. In addition, supports should have a base configuration that provides adequate support for the weight of the steel and concrete as well as workers walking on the steel without collapsing, sinking into the base, or impeding the flow of concrete during placement and consolidation.
Using pins to anchor the reinforcing steel mat to prevent movement during paving is an item of contention. While some believe pinning the mat may cause horizontal cracking and/or a weakened bond between the concrete and steel, others believe the procedure is acceptable.
The arrangement and spacing of the steel supports should be such that the reinforcing bars are supported uniformly and in the specified position and do not move when concrete is placed. Bars should not permanently deflect or be displaced. Spacing of the supports is a function of the size and spacing of the reinforcing steel, the support provided by the pavement base layer, and the design of the chairs. As a general guideline, the spacing should not exceed 3 ft (0.9 m) transversely or 4 ft (1.2 m) longitudinally.
Usually, transverse bars are first placed on the individual chairs, or else a prefabricated transverse bar assembly is used. The longitudinal bars are then arranged in position (staggered for lapping, as discussed in Section 7.2.3). Next, the longitudinal bars are tied and secured to the transverse bars to maintain specified tolerances. Experience indicates that tying or clipping the longitudinal bars to the transverse bars at 4 to 6 ft (1.2 to 1.8 m) intervals along the longitudinal bars produces satisfactory results. States that are experienced with CRCP do not allow welding of longitudinal and transverse bars. Examples of steel placed on different types of supports are shown in the figures to the right.
A Transverse Bar Assembly (TBA) is sometimes used in place of a chair support system and separate transverse reinforcing bars. A TBA consists of a transverse reinforcing bar and triangular metal legs with metal u-shaped clips that are welded to the transverse bar. TBAs are custom manufactured to project specifications, such as paving width and horizontal and vertical bar location.
The number and spacing of the triangular metal legs is determined by the requirements of support and rigidity for the bar mat. The triangles of the legs are oriented in the longitudinal place to avoid overturning of the mat during slipform paving.
The metal u-shaped clips are welded along the transverse bar at the accurate spacing position required by the longitudinal reinforcing bars, creating a uniform separation between them. The clips are sized to hold the longitudinal bars in place and allow a bit of give in the paving direction. The longitudinal bar is snapped quickly into the clip. Sometimes the clips are omitted from every other transverse bar. Wire tying at rebar intersections is not required; tying is only required at the splice areas.
One key advantage of the TBA is that it saves labor and time, mainly in the tying required at rebar intersections. A six person crew using TBAs can typically lay one-lane mile of bar mat per 8 hour shift. While the TBA itself is more expense compared to the traditional method, the bar mat installation using TBAs is most cost-effective in areas where labor rates are high.