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Determination of Maximum Safe Gap in Permanent Concrete Barrier


Agencies encounter situations where there are longitudinal gaps located within permanent concrete barrier. For example, small gaps ranging from one to four inches may occur when accommodating an expansion/contraction joint between permanent concrete barrier and concrete bridge rail. Larger gaps of up to 4 feet may occur due to accommodation of drainage features, such as an existing storm sewer catch basin or a manhole access point. Relocating these types of features is typically discouraged as the process would add significant costs and delays to the project. However, gaps in permanent concrete barrier can create a potential safety hazard for errant vehicles due potential vehicle snag on the downstream end of the gap and corresponding risks for rapid vehicle deceleration, increased occupant risk, and vehicle instability. Gaps in these barriers may also pose a structural issue due to the discontinuity in the barrier section. Currently there is limited guidance on the maximum, unshielded gap that meets current MASH safety performance criteria, which limits agencies ability to effectively address this issue.

Based on these concerns, a need exists to determine the maximum allowable unshielded gap in a permanent concrete barrier system under MASH TL-4 safety performance criteria. This issue directly addresses goals of the AASHTO Strategic Highway Safety Plan and the FHWA Roadway Departure Strategic Plan, including the importance of reducing the incidence and severity of roadside crashes. The AASHTO Technical Committee on Roadside Safety (TCRS) has also prioritized the combined use of in-service performance evaluations (ISPEs), computer simulation, and full-scale crash testing in its strategic plan.


The objective of this research is to determine the maximum allowable unshielded gap for permanent concrete barriers under MASH TL-4 impact conditions. The research should provide guidance for the maximum allowable gap for a range of permanent concrete barrier geometries.


Roadside safety hardware is critical for reducing severe crashes on the nation’s highways. There is an urgent need to develop guidance for the agencies. Knowing the maximum allowable unshielded gap and development of crashworthy barrier hardware would provide agencies with increased safety through protection of vehicle snag posed by the interruption of the permanent concrete barrier system. This research will provide a measure of assurance in the placement of permanent concrete barrier and a higher level of safety for the traveling public.

State Transportation Agencies continue to strive to reduce the frequency and severity of run off the road crashes while balancing community and environmental needs, accommodation of utilities, and effects use of limited transportation budgets. Construction costs and delays can be reduced by not having to relocate drainage and utility structures located within alignment of the new concrete barrier system. State Transportation Agencies may utilize the results of this research for policy development for the placement of permanent concrete barrier. It is anticipated that the results of this research will be incorporated into the AASHTO Roadside Design Guide.

In the absence of this research, needless severe injury and fatal crashes will likely continue to occur due to the hazard posed by excessive gaps within permeant concrete barriers. Continuing without such guidance will result in the inconsistent and potentially unsafe practices by user agencies.

Related Research:

Limited research has been conducted regarding gaps and shielding of gaps in permanent concrete barrier. Most of the existing research has dealt with evaluation of expansion gaps in concrete bridge rails. The Midwest Roadside Safety Facility (MwRSF) tested a Nebraska open concrete bridge rail with a 4.5-in expansion gap under the AASHTO PL-2 criteria. The barrier was evaluated at the gap location with a 5,399 lb pickup truck at 61 mph and 20 degrees and a 18,000 lb SUT at 51.9 mph and 16.8 degrees. Both of these tests were successful, but snag was evident in both tests. Additionally, the pickup truck test was conducted at a lower angle than prescribed NCHRP Report 350 or MASH TL-3 and TL-4, and no small car testing was conducted. More recently, TTI tested the Texas T224 bridge rail to MASH TL-4. As part of that evaluation, they successfully tested the 10000S vehicle across a 2” wide expansion gap. The passenger vehicle tests were not conducted across the expansion gap.

Similarly, temporary barrier designs in free-standing and anchored configurations have had gaps as large as 4-in. that have been successfully traversed by vehicle in MASH TL-3 crash tests. For example, the Midwest States F-shape PCB was tested to MASH TL-3 when anchored to a bridge deck with a 4-in. barrier gap. This test did exhibit some snag across the PCB joint, but the vehicle was safely redirected. The snag may have even been exaggerated in this type of system as compared to a permanent concrete barrier as the upstream barrier translate laterally to some degree prior the vehicle traversing the gap, thus increasing the exposure of the barrier face at the downstream end of the gap.

Additional research has been conducted for shielding of larger barrier gaps. MwRSF developed a steel cover plate for spanning a large expansion joint in a TL-5 bridge rail developed for Manitoba Infrastructure and Transportation. In this effort, a steel cover plate was incorporated to span a 6 5/8-in. gap in a 49 ¼-in. tall, narrow single-slope concrete bridge rail, and it was crash tested according to MASH test designation 5-12. It may be possible to develop a similar reinforced steel cover plates for bridging other type of barrier gaps. The bridge rail in this research was also designed with increased barrier reinforcement adjacent to the gap to compensate for the lack of continuity in the barrier section.

While this research provides some insight into the issue of gaps in permanent concrete barrier, it does not provide guidance on the maximum allowable unshielded gap for commonly used permanent concrete barrier profiles in accordance with AASHTO MASH TL-4.


The research contractor will require the following major tasks:

· Literature review of previous research related to barrier gaps and gap spanning systems

· Surveying state DOTs for permanent concrete barrier geometries and permanent concrete barrier gap configurations and scenarios, including expansion joints, drainage structures, etc.

· Analyze existing barrier geometries to determine a potential maximum unshielded gap length for permanent concrete barriers and determine a critical gap length and barrier geometry for evaluation through full-scale crash testing

· Evaluate the critical gap configuration through full-scale crash testing to MASH TL-4

· Provide generalized guidance for allowable gaps lengths for permanent concrete barrier

· Provide a summary report to document all design, analysis, testing, conclusions, and recommendations


The results of this research will assist state and local standards engineers to update the standard drawings and design manuals to provide guidance on the installation of permanent concrete barrier. With proper review, the AASHTO TCRS could assist implementation through adopting the research results into the future update of AASHTO Roadside Design Guide and MASH. Implementation could also be supported through a discussion of the findings at a TRB AKD20 meeting and the conduct of the TRB webinar.


The desired outcome of this research will be guidance for incorporation in the AASHTO Roadside Design Guide. The final product will include guidance to agencies to avoid placing permanent concrete barriers with gaps exceeding the maximum allowable limit.

Sponsoring Committee:AKD20, Roadside Safety Design
Research Period:24 - 36 months
RNS Developer:Hung Tang, Bob Bielenberg, Rod Troutbeck
Source Info:AKD20 Summer Meeting 2020
Date Posted:01/04/2021
Date Modified:02/10/2021
Index Terms:Barriers (Roads), Joints (Engineering), Concrete structures, Highway safety,
Cosponsoring Committees: 
Safety and Human Factors

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