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Guidance and Policy Design and Construction Guide - Chapter 4

Design and Construction Guide

U.S. Department of Transportation

Federal Highway Administration

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Section 4: Specific Considerations for Constructing the Safety EdgeSM in Conjunction with New Concrete Pavements or Concrete Overlays

This section identifies issues that need to be monitored during the construction of PCC pavements. All of the PCC demonstration projects that were used for preparing this Guide included the use of slip-form paving to form the Safety EdgeSM. A benefit of forming the Safety EdgeSM during PCC slip-form paving is the sensitivity to edge slump is reduced.

4.1 Design Features and Shape

The Safety EdgeSM is designed to create a 30 degree finished angle relative to the pavement cross slope as shown in Figure 14 and should be constructed on a similar structural base as the adjoining monolithically placed lane/shoulder. During design, the structural base width needs to account for the Safety EdgeSM width.

Figure 14. Recommended Safety EdgeSM configuration for PCC pavements and overlays.
Simple diagram of roadside cutaway showing the safety edge extending out and down from the edge of the pavment at a rise to run ratio ranging from 1:12. to 1:2.0. The safety edge is approximately 5 inches high with the base of the roadway being an additional 2 inches minimum.
* Note, Recommended Rise to Run ratio range 1:1.2 to 1:2.0. The range of slope is equal to 26° to 40°.

4.2 Equipment – PCC Safety EdgeSM Devices

This part describes the modifications that must be made to a slip form concrete paver to form a Safety EdgeSM. It also notes the essential attributes of a concrete paver that are necessary to create an effective Safety EdgeSM. When a slip form paver is not being used, the Safety EdgeSM needs to be manufactured or created as part of the forms – one side of the forms will have an angle of 30 degrees rather than being vertical.

Currently, there are no commercially available Safety EdgeSM devices for PCC pavers or forms. Custom devices, however, can be fabricated by modifying the finishing pan of the paver (refer to Figure 15). The pan is essentially configured to form an inverted curb under the pan close to the end gate. The steel components of the assembly need to be robust in order to resist bending during paving operations from the force of the plastic PCC. The following provides a description of the modifications that were made to a Gomaco paver that was used on one of the PCC demonstration projects and identifies some of the issues or items that should be considered in fabricating the Safety EdgeSM profile.

  • A template was created for the Safety EdgeSM profile to ensure correct dimensions. The template was used to manufacture a stainless steel profile pan of the desired shape and dimensions. Portions of the existing profile pan were removed to fit in the new Safety EdgeSM profile. [Note: The contractor decided against welding the Safety EdgeSM profile to the bottom of the existing profile section or pan because of a concern that the welds might tear the finished PCC.]
  • A 2-inch finish tail on the stainless steel Safety EdgeSM profile was fabricated to help finish the portion of the edge where it goes from slope to vertical. When going through existing intersections, driveways, or where the grade is not exact on the outside edge, the 2-inch finish tail did drag in some locations. For the demonstration project, the contractor cut one inch off of this tail and it performed well, but cutting 0.5 inches from the tail would have also worked well.
  • Adjusting bolts were fitted to the Safety EdgeSM profile pan. One adjustment bolt was on the sloped section in case an adjustment had to be made if the edge profile was not finishing properly.
  • In setting up the paver, care was taken to position the vibrators in the specific locations for proper consolidation of the Safety EdgeSM-near the breakpoint.

Unlike the Safety EdgeSM attachments for AC layers or overlays, the modifications made to the concrete paver cannot be easily removed and replaced. The modifications can be removed, but the parts must be cut from the pan using a torch or grinder. Touchup of the pan is required to return the unit to service on standard PCC pavements.

4.3 Concrete Mixtures

Demonstration projects completed to date have used standard concrete mixes. No changes to the mix were needed to accommodate the SE.

4.4 Safety EdgeSM Construction

This part of the Guide discusses items of interest to PCC paving projects when a Safety EdgeSM is to be placed as part of the new construction or rehabilitation project.

PCC Placement

Normal paving procedures are usually sufficient for forming the Safety EdgeSM on mainline sections of the roadway using slip form paving operations and equipment. Material properties of the hardened concrete from the edge and the interior of the pavement have been demonstrated to be similar.

Figure 15. View of the Safety EdgeSM device from front of paver.
Photo of the Safety Edge device taken from the front underside of a paver.

Transitioning Between Different Edge Profiles

It is recommended to have a transition from a no Safety EdgeSM section to a Safety EdgeSM section to avoid a vertical edge perpendicular to traffic (i.e. similar to beginning section of bridge concrete rail/parapet where the bottom of the rail/parapet is modified to reduce the probability of vehicle tire snagging).

An important issue to be addressed is the intersection of cross roads where a vertical edge is required. Due to the fixed nature of the edge and shoe assembly, the contractor can consider different options depending on the number and extent of sections with different edge profiles. In either case, additional labor should be anticipated to tie into intersections or other areas requiring different edge profiles when paving using the Safety EdgeSM concept. The following provides an overview of the two options that were used on some of the demonstration projects.

  • If there are only a few intersecting roadways, driveways or limited areas where a vertical face is required along the edge of the pavement, the contractor can consider placing the safety edge using normal paving operations. The Safety EdgeSM is sawcut and removed to create a vertical edge. The sawing of the Safety EdgeSM can be completed at the same time the transverse joints are sawed. This option has worked fine, but does waste some PCC and requires additional sawing.
  • Another option is to box out the areas requiring different edge profiles and place forms. The PCC in these limited areas is placed by hand using normal construction practices in order to tie into pavement intersections, driveways, and other features.

Vibration

Generally, the standard spacing of the vibrators are sufficient to properly consolidate the fresh concrete at the Safety EdgeSM. Consolidation of the material within the Safety EdgeSM should be checked at the start of any project. If slip form pavers are not being used, hand vibrators need to be used to ensure that the PCC flows into the lower part of the Safety EdgeSM. As noted at the beginning of this section, all demonstration projects included the use of slip form pavers in construction the Safety EdgeSM.

Curing

Care should be taken to insure the specified curing procedures are properly followed for all surfaces at the edge of the pavement, including the Safety EdgeSM. The Safety EdgeSM, however, does not require any special curing requirements beyond that for typical PCC layers.

Sawcutting

Transverse control joint sawcutting can be stopped at the breakpoint of the Safety EdgeSM and not continued through the sloped surface of the edge. Experience shows the contraction cracks normally form at the end of the sawcut and extend through the Safety EdgeSM as planned (refer to Figure 16).

Figure 16. Sawcut and crack at the formed joint on one of the Safety EdgeSM demonstration projects.
Close up photo of a sawcut in the surface of a portland cement roadway with a crack extending from the edge of the cut down the safety edge surface to the shoulder.

4.5 Quality Measurement

The same properties used or identified by the owner agency to measure the quality of PCC pavement layers for acceptance should also be used on projects that include the Safety EdgeSM. The acceptance plan should not be modified for projects that include the Safety EdgeSM. More importantly, the air void content, strength, and/or smoothness requirements appropriate for a typical project should not be relaxed simply because the Safety EdgeSM is added to a PCC overlay or new construction project. Thus, the only difference between projects with and without the Safety EdgeSM is the slope of the edge itself.


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Page last modified on August 12, 2013.
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000