804 - Concrete Barriers and Glare Screens

804 Concrete Barriers and Glare Screens 2012 STANDARD SPECIFICATIONS FOR CONSTRUCTION - SECTION 804

GENERAL

Barriers and glare screens are generally constructed by slipform methods. The concrete median or bridge deck forms the base for the barriers. Occasionally the median barrier will be constructed on a separate foundation instead of a median shoulder. Review the Standard Plan R-49 through R-54 Series for barrier details. Bridge barriers are reinforced as detailed on the plans. Median barriers contain no reinforcement, but are doweled to the concrete median.

Before any placement of a barrier or glare screen is started, a careful review of placement temperatures and likely cure conditions should be conducted. The narrow cross section of the upper portion of a barrier makes it highly susceptible to freezing. Also, since these items on bridge or road jobs are some of the last items cast, late season work is likely.

MATERIALS

Slump of concrete is critical for proper slipforming. Concrete must be low slump, yet workable. Admixtures, such as water reducers or super-plasticizers, may also be used to aid workability.

Only occasional slump tests will be required, as slump is self-policing in slipform operations; however, air content should be checked frequently, as adequate air entrainment is difficult in low slump concrete.

CONSTRUCTION

Adjustable mules may be needed in transition and superelevated median areas.

Concrete Barrier

Before slipforming starts, a string line is erected for vertical and horizontal control. For bridge barriers the slipform machine should be “dry run” over the reinforcement to check for specified cover on the steel. Any reinforcement not having proper cover should be corrected before placement of concrete.

Immediately behind the slipform machine, the barrier should be inspected for honeycomb and other surface defects. Mortar obtained by screening coarse aggregate out of the mix should be used for repair work. Use of mortar batched on site from sand and masonry cement should not be permitted, as this often results in a "plaster coat" which spalls off after exposure to weather.

The Contractor should continuously straightedge the top surface of the barrier and correct any irregularities.

Joints require close inspection. During joint forming, the joint tool should slide freely in the vertical direction. Jab and slide joint tooling can create a hidden sawtooth plane-of-weakness that may lead to undesirable cracking. Work with the finishers early to establish smooth internal joint creation. Make sure the plane-of-weakness joints are cut at least 1 inch (25 mm) deep to prevent random cracking. Joints should be finished with an approved edger.

Joints in the barrier should line up with like joints in the concrete shoulder serving as a base.

Plan stationing should be stenciled on each side of the barrier. Finally, the barriers should be given a light broom finish and cured with white curing compound. To avoid running and streaking, the compound should be sprayed in two layers. Each coat shall not be less than 1 gallon per 300 square feet (1 L per 7 m²).

Glare Screens

Glare screens are either an integral part of the barrier or added as a separate pour after casting the barrier. Reinforcement and other details are shown on the Standard Plan R-76 Series. Finishing and curing are similar to that described for barriers.

Checklist for Inspection

Item	Description
Concrete	Grade S3 concrete is required for barriers. Grade P2 may be used for glare screens.
Dowels	Dowels between barrier and base are to be spaced 6 feet (2 m) center to center on each side. Check to see that the base concrete or grout has bonded to the dowels.
Air Content	4-1/2 percent air content permitted for slumps under 1.5 inches (37 mm). Check mortar used for surface patching for air content.
Line and Grade	Top surface of barrier is to be checked with 10 feet (3 m) straightedge behind slipform machine.
Finishing	Finish with a light broom to obtain uniform appearance.
Joints	Expansion and plane-of-weakness joints are to be located as shown on Standard Plan R-49 Series. Check depth of plane-of-weakness joint (4 inches (100 mm) required to control cracking). These joints may be formed or sawed.
Curing	Apply white membrane curing compound at rate of 1 gallon per 300 square feet (1 L per 7 m2) for each of two coats. If complete coverage is being obtained without streakage or running, rate is probably ok.
Stenciling	Barrier wall to be stenciled for station locations on both sides.

On some slipform operations, there is a tendency for the Contractor to operate the slipform machine too fast and exceed the ability of the vibrators to perform properly. This causes an unsightly and unacceptable tearing in the face of the barrier. The inspector should be aware of this and monitor the speed of the slipform machine.

Keep in mind that some quantities must be documented at the time of placement while others can be checked later. An example of the former is areas where high-strength concrete is used and is to be paid for.

MEASUREMENT AND PAYMENT

Concrete barrier and glare screen is generally measured in feet (meters) parallel to their centerline. Refer to the Standard Specifications for Construction for details on additional pay items that may be associated with this work.