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.
Longitudinal Control Joints
Longitudinal control joints, otherwise known as contraction, hinged or warping joints are necessary to relieve stresses caused by concrete shrinkage and temperature gradients and changes in monolithically placed slabs that are wider than 14 ft (4.3 m) to control longitudinal cracking. These joints are generally formed by sawing the hardened concrete, but early entry saws have proven to work as well.
The recommended sawing depth is one-third of the as-constructed slab thickness to ensure an adequate weakened plane. Longitudinal control joints must be spaced to avoid sawing directly over a longitudinal steel bar. Care must be taken to not cut the tiebars across control joints.(85) Saw cuts less than one-third the slab depth may not be adequate to form a crack at the planned location and can lead to random longitudinal cracking.
If random cracking should occur, the CRCP transverse reinforcement will aid in holding the cracks together. In some cases, cross-stitching can possibly be used to ensure that longitudinal cracks will remain tight. If any tiebars are damaged during saw cutting operations, cross-stitching should be done at those locations.(93)
Longitudinal Free Joints
Longitudinal free joints are used to reduce the amount of steel needed in the transverse direction, to facilitate construction, and to isolate structural elements from the CRCP. These joints are typically placed at the edge of median concrete traffic barriers, at the top of wingwalls, mechanically stabilized earth walls, or cast-in-place retaining walls to isolate the pavement movement from the movement of the structures. Since the amount of transverse steel needed is based on subgrade drag theory, a longitudinal free joint at the centerline of the pavement may reduce the amount of transverse steel needed by one-half. Longitudinal free joints should only be placed where load transfer and joint movement in the horizontal or vertical directions is not a critical consideration.
Transverse Construction Joints
Transverse construction joints are installed at the end of each day’s paving operation or other placement interruptions (whenever the placing of concrete is suspended for more than 30 minutes). In early CRCP construction, these joints often failed. However, proper design and construction proved these non-working transverse joints could perform as well as random transverse cracks.
Transverse construction joints are formed by means of a suitable split header board conforming to the cross section of the pavement. The header board should be secured vertically in place perpendicular to the surface of the pavement. Longitudinal reinforcing bars should extend through the split in the header board and be supported beyond the joint by chairs. On the leave side of the header, reinforcing steel should be covered with plates of wood so as to allow easy removal of the concrete that is carried over the header. Before resuming paving, the header board is removed.
Transverse construction joints are typically smooth faced butt joints that do not benefit from aggregate interlock. These joints should not be edged or sealed. An important factor in good CRCP performance is continuity of load transfer; thus, special reinforcing bar arrangements are needed to replace load transfer capacity lost because of the smooth joint face and to handle early stress concentrations. Several States require adding 72 in. (1.8 m) long tiebars at the construction joints, placed adjacent to every other longitudinal bar. These additional bars typically have the same diameters, grade, and depth of the regular longitudinal steel bars. This additional reinforcement prevents movement and reduces the possibility of cracks developing in the adjacent slabs.
Pavement areas adjacent to both sides of the joint should receive additional consolidation from hand vibrators inserted into the concrete along the entire length of the joint. These areas should extend at least 10 ft (3 m) from the joint. Ensure vibrators do not excessively contact steel, forms, or base.
A recent report by the Texas Transportation Institute provides additional details for CRCP transitions, including details for transverse construction joints.(123)
Blockouts are needed to allow for obstructions in the CRCP, such as drop-inlets, manholes, and foundations for luminaries. These types of obstructions in CRCP should be avoided if possible or otherwise limited to outer edges of shoulders. Typically, the perimeter of the blockout is an isolation joint where the width of the joint is 1.5 in. (40 mm). Whereas an isolation joint in normally constructed with preformed fiberboard material, the blockout joint should instead use a compressible material that does not absorb water. In addi- tion, approximately 3 in. (75 mm) outside the perimeter of the blockout should be two concentric reinforcing bars of the same size and grade as the longitudinal reinforcing steel in the CRCP.
Construction Techniques for Controlling Crack Spacing
Field studies in Texas and elsewhere have investigated the control of crack spacing by inducers at prearranged locations.(70) The Texas study was conducted on CRCP that was constructed in hot weather (90 to 100°F or 32 to 38°C).
Crack induction was achieved by the use of two different methods: sawcutting a shallow notch in the pavement surface and metallic crack inducers. Early-age sawcutting techniques (consisting of a portable, light saw) were used for surface notching. Notches were made roughly 4 hours after concrete placement and resulted in cracks that initiated at the surface. Metallic crack inducers were placed in both single and stacked layer configurations and were anchored to the double layer of longitudinal reinforcement to provide support against the flow of fresh concrete during paving operations. Crack inducers initiate cracks at the interior of the pavement thickness.
Based on the limited data from these research studies, it appears that surface crack initiation (using the early-age notching technique) is more effective than interior crack initiation (using inducers) in controlling the crack pattern. New studies also show that plastic tape inserts can be effective in initiating transverse cracks. However, more work needs to be done to prove the viability of this technique.