Retro-fitting Concrete Shoulders

Most of the rigid pavements constructed before the mid 1980’s did not include a tied concrete shoulder. Until 1986, the AASHTO Guide for the Design of Pavement Structures did not consider or give credit for the inclusion of a tied concrete shoulder. Rarely were tied concrete shoulders incorporated in the pavement design. Prior to their routine use, it was possible to infer what the outside lane performance could have been by examining the urban freeways where frequent entrance and exit ramps encouraged the heavy volumes of truck traffic to use an interior lane where the lane edges were supported by another PCC travel lane.

The performance of these interior travel lanes, with heavy volumes of truck traffic, have greatly exceeded expectations. The reasons for the tremendous success of truck travel lanes supported at the edge by concrete pavement can be attributed to a combination of structural support from the tied concrete pavement reducing stresses in the travel lane, and the tied concrete pavement helping to keep water from infiltrating into the subbase.

Guidelines for retrofitting concrete shoulders are summarized [FHWA] as follows:

  1. General.
    • Concrete shoulders should be tied to the mainline with properly spaced and sized tiebars. Tied concrete shoulders will reduce pavement stresses and edge deflections. Tied concrete shoulders will also result in a tighter, easier to seal longitudinal joint that, when properly maintained, will effectively reduce water infiltration into the pavement structure.
    • Retrofitting tied concrete shoulders or lane widening will reduce edge stresses and deflections. The age, condition and remaining service life of the existing pavement play a significant role in determining whether a retrofit is practical. It is recommended that a retrofit be added only when an engineering and economic analysis indicates it to be a cost-effective solution.
  2. Thickness/Width.
    • Shoulders should be structurally capable of withstanding wheel loadings from encroaching truck traffic. On urban freeways or expressways, the shoulders should be constructed to the same structural section as the mainline pavement to ensure adequate load capacity at the interface between the mainline and shoulder; to provide for ease and economy of construction; and to prevent a “bathtub” condition under the pavement. This will also allow the shoulder to be used as a temporary detour lane during rehabilitation or reconstruction.
    • Consideration should be made to pave the slab a standard lane width to facilitate detours. The retrofitted concrete shoulder should be at least 3-ft wide.
  3. Subbase. It is recommended that the same type of subbase be used under the shoulder as under the mainline, especially on high-volume facilities. Care must be taken in designing the subbase cross-slope under concrete shoulders to avoid pocketing of water under the lane/shoulder joint and at the shoulder edge. Problems are often encountered at this location due to changes in subbase type, resulting in nonuniform support or difference in drainage characteristics.
  4. Transverse Joint.
    • Mainline pavement joints should be extended across the shoulder. All transverse shoulder joints should be sawn to a depth of 1/3 the slab thickness.
    • Where plain jointed (CPCD) concrete shoulders are used adjacent to continuously reinforced mainline pavement, the shoulder joints should be sawn at 15-foot intervals. Plain concrete shoulders should not be constructed integrally with continuously reinforced concrete pavement. Transverse saw cuts in the integrally constructed shoulders will propagate cracks across the mainline.
    • Where plain jointed concrete shoulders are used adjacent to jointed reinforced mainline pavement with skewed joints, intermediate joints should not be sawed in the shoulder. Skewed intermediate shoulder joints tend to propagate two parallel transverse cracks across the mainline pavement.
    • Where plain jointed concrete shoulders are used adjacent to jointed reinforced mainline pavement with perpendicular joints, intermediate shoulder joints are optional. However, intermediate joints should not be sawed if the shoulder is constructed integrally with the mainline pavement. Intermediate transverse saw cut in integrally constructed shoulders will propagate cracks across the mainline.
  5. Longitudinal Joint. Combined lane and shoulder or lane widening widths of 15 feet for the right (outside) lane and 16 feet for the left (inside) lane have generally performed satisfactorily. For widths greater than these, a longitudinal joint should be sawed and sealed.
  6. Keyway. Keyways are not recommended for use, and should never be used for pavements less than 10 inches thick. If used for pavements 10 inches 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.
  7. Tiebars
    • Tiebars are needed between the mainline pavement and concrete shoulders to keep the longitudinal joint tight so as to provide the necessary load transfer. Tiebars are typically placed on 30-inch centers at mid-slab depth. If tiebars are to be bent and later straightened during construction, Grade 40 steel is recommended, as it better tolerates the bending. When using Grade 40 steel, 5/8-inch by 30-inch tiebars should be used. When using Grade 60 steel, 5/8-inch by 40-inch or 1/2-inch by 32-inch tiebars should be used. These lengths are necessary to develop the allowable working strength of the tiebar. Multi-piece tiebars may also be used.
    • Tiebars should not be placed within 15 inches of transverse joints. When using tiebars longer than 32 inches with skewed joints, tiebars should not be placed within 18 inches of the transverse joints.
    • The structural adequacy of tiebars can be reduced through corrosion. Corrosion resistant tiebars are recommended.
  8. Reinforcement. The majority of concrete shoulders are plain with short joint spacing. They have performed well when placed adjacent to either plain jointed (CPCD), reinforced jointed, or continuously reinforced concrete mainline pavements. In cases where jointed reinforced or continuously reinforced shoulders are placed integrally with the same type of mainline pavement, the steel in the shoulder is normally placed at the same percentage as required for the mainline pavement.
  9. Lane Widening. A 2- to 3-foot integral widening of the mainline slab will reduce edge strains and deflections. To be effective, the travel lane should be striped at 12 feet with the edge of the slab being moved into the shoulder and away from traffic load applications. The remaining portion of the shoulder may also be paved.

References

Original article content and pictures contributed by TxDOT.