Constructing Shoulders and Auxiliary Lanes
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.
Typical shoulder designs for CRCP traffic lanes include:
- Jointed plain concrete pavement placed after the mainline traffic lanes without dowels (when not expected to carry traffic). Tiebars are used to provide load transfer.
- HMA placed adjacent to an extended outside lane of the mainline CRCP slab. The mainline slab should extend at least 2 ft (0.6 m) into the shoulder area to provide an interior loading condition for traveled lanes.
- CRCP with the same cross-section as main lanes. Often times a State will widen the shoulder so that it may serve as a traffic lane when needed.
Key features to consider in the design and construction of shoulders and auxiliary lanes include the following:
- Amount of load transfer provided by the shoulder (or auxiliary lane) through the design life of the pavement.
- Ability to prevent the intrusion of moisture to susceptible layers under the loaded area of the pavement.
- Maintenance requirements.
- Ability to use shoulder for regular traffic (emergencies, increased capacity, and/or parking).
Concrete shoulders should be tied to the mainline by either extending the transverse steel from the mainline CRCP into the shoulder with a longitudinal contraction joint provided at the juncture of the traveled lane and the shoulder, or through a longitudinal construction joint with properly spaced, aligned, and sized tiebars. Tiebars are required between the mainline slab and concrete shoulder to provide designed load transfer and prevent water infiltration into the pavement and base structure.
Tiebar installation at the shoulder follows the same construction practice as provided in Section 7.4 at longitudinal mainline construction joints. Almost all states have abandoned the use of Grade 40 (Grade 300) tiebars and the practice of bending the bars, due to joint separation issues.
Some agencies are now using a multi-piece threaded tiebar. One half of the bar is tied to the reinforcement in the CRCP traffic lane. After concrete is placed, the other half is threaded into this bar.
Keyed longitudinal joints are not recommended and should never be used for pavements less than 10 in. (250 mm) thick. If used for pavements 10 in (250 mm) or greater in thickness, keyways should be placed at mid-slab depth to ensure maximum strength. Proper concrete consolidation, both above and below the keyway, is essential and the joint must be tied with reinforcing steel as previously recommended.
Where plain jointed concrete shoulders are used adjacent to CRCP, the tied concrete shoulder should be sawed transverse to the direction of traffic to a depth of one-third the pavement thickness at no more than 15 ft (5 m) intervals. If the shoulder will be used for mainline traffic, or substantial parked truck traffic, consideration should be given to use of dowels at these joints to prevent faulting and provide additional load transfer. Plain jointed concrete shoulders should not be constructed integrally with or before the mainline CRCP, as the transverse saw cuts in the integrally constructed shoulders would propagate as cracks across the mainline CRCP.
Corrugations that are impressed in plastic concrete have proven to be an effective contrast between shoulder and mainline pavements alerting drivers they are moving onto the shoulder. The width and depth of the corrugations are dependent on roadway average speed. In a 50 to 70 mph (80 to 110 kph) range, a width of 4 to 6 ft (1 to 2 m), and a spacing of 60 to 100 ft (18 to 30 m) are appropriate.(89) Care should be taken to make certain that the impressed corrugations meet the plan details throughout the hardening process and that the concrete is not weakened by late
The FHWA Technical Advisory T5040.29 recommends that shoulders be constructed of the same materials as the mainline pavement to facilitate construction, improve per- formance, and reduce maintenance costs.(96) The Advisory explains that tied concrete shoulders in lieu of flexible shoulders will minimize problems associated with infiltration of surface water into foundation through lane-shoulder longitudinal joints. However, if HMA shoulders are selected, the following guidelines should be considered:
- Include anti-stripping agents in the HMA mixture used on the shoulder.
- Include proper subdrainage such as edge drains beneath the lane/shoulder joints, or daylighted treated permeable bases, so as to drain water infiltrating the lane-shoulder joint and to keep the entire base structure free of moisture.
- Ensure that the HMA is compacted to adequate density, particularly at lane-shoulder interface.
The use of full-width CRC paved shoulders is desirable for many reasons. However, the additional cost of this design may not be warranted on all projects. Some agencies build widened outside lanes instead of using full-width paved shoulders. A monolithic widening of at least 1 ft (0.3 m) outside of regular painted edge strips is commonly used. Widened lanes are only effective when travel lanes are striped at 1 ft (0.3 m) to help guide vehicles within the traffic lane and not on the widened area. Widened lanes reduce the number of edge loadings; thereby reducing development of edge punchouts. Placement of rumble strips on the shoulder portion of a widened lane should also be considered.