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A state DOT recently came to us inquiring about mowstrips around guardrail posts. In addition, they forwarded the attached photos regarding their existing standard detail. As you can see from photos, the post has failed at point of asphalt overlay. Their current spec calls for 2" hot-mix over 6" GABC [Graded Aggregate Base Course]. They are also a state that is currently considering MGS guardrail, and wanted to know if this current spec would be OK to use with the MGS.(?)

We believe their current spec is too stiff for post to move upon impact, barring any cut-out behind the post. We also believe that specifying 18" subbase (2A modified) with a 2" overlay would perhaps better perform in allowing the post to push backward crushing 2" asphalt surface before post fails (as shown in photos).

Question: In regards to mow strips, what would you specify with MGS?

MwRSF Research

Several years ago, MwRSF developed a W-beam guardrail system for placement in soil/rock foundations. As part of this research effort, several design configurations were prepared to account for rock formations found below the soil surface and in the region where guardrail posts were to be embedded. These configurations included holes and/or slotted holes cut into the rock to allow for post embedment in combination with a special fill material (ASTM C33 coarse aggregate, size no. 57). Hole sizes varied as a function of rock depth. Under vehicular impacts, the steel posts were allowed to rotate in the soil/rock foundation system and allowed the metric-height, W-beam barrier to perform in an acceptable manner. This research was published in MwRSF Report No. TRP-03-119-03.

As part of this MwRSF effort and due to similarities to posts placed in subsurface rock, design guidance was also provided for the placement of steel or wood posts in pavement surfaces. For this scenario, MwRSF researchers recommended that leave-out sizes of 8"x18" and 10"x21" be provided for W6x9 steel posts and 6"x8" wood posts, respectively. In these configurations, the posts were positioned with the front face near the front side of the leave-out to allow room for adequate post rotation on toward the back of the hole. In addition, it was suggested that a backfill material with confined compression properties similar to ASTM C33 coarse aggregate, size no. 57, would possibly be acceptable for this application, although further testing should be conducted. These recommendations were also included in MwRSF Report No. TRP-03-119-03.

TTI Research

During this same general time period, TTI researchers were conducting research to investigate the dynamic response of guardrail posts placed in various mow strip configurations. From this research program, it was determined that wood and steel posts performed in an acceptable manner when:

(1) placed in mow strips with depths 8" or less (component testing program) and 5" (crash testing program),

(2) utilizing square leave-outs measuring 18"x18" or larger,

(3) installed with leave-out material having compression strength equal to or less than 120 psi (two-sack grout mix), and

(4) utilizing low-strength leave-out material with depths of 4" or less.

From the TTI program, W6x9 steel posts, 7" diameter round wood posts, and 6"x8" rectangular wood posts were all deemed acceptable for use in the above noted mow-strip and leave-out combination system. This research is published in TTI Report No. 0-4162-2.

FHWA Acceptance

The results from these two research programs were later combined into one FHWA acceptance letter, no. B-64B.

General Thoughts

To date, no dynamic post-soil testing has been performed with guardrail posts embedded in soil with an asphalt layer placed on the soil and measuring 2" thick or less. As such, it is difficult to know whether wood posts could rotate sufficiently without premature fracture or whether steel post rotation would be excessively inhibited. Over the years, MwRSF has had discussions with various state DOTs with the hope of conducting a bogie testing program with posts embedded in thin asphalt mow strips or mow-strip/leave-out combinations with 2" thick asphalt fill material. Unfortunately, this simple research program has never received funding. As a result, MwRSF has continued to utilize the prior leave-out guidance for installing metric-height and 31" tall W-beam guardrail systems in mow strips in the absence of any component or full-scale crash testing with posts placed in 2" thick asphalt layers.

On a side note, MwRSF is conducting a research study on a similar topic for the State of Wisconsin. In this ongoing effort, dynamic component testing is being performed on wood posts installed on slope terrain with a 2" asphalt layer placed over the soil surface to reduce erosion near bridge ends and within the approach guardrail transition. For this effort, component testing will occur with posts placed on 2:1 and 4:1 terrain. Unfortunately, no testing is budgeted for evaluating wood posts installed on level terrain with a 2" asphalt surface surrounding the posts.

Finally, we believe that the MGS could be installed in concrete or asphalt mow strips if configured with S3x5.7 weak steel posts spaced on 3 ft " 1½" centers. This opinion is based upon the prior successful crash testing program under MASH on the MGS bridge rail. For this system, S3x5.7 steel posts were inserted into steel tubular brackets which attached to the side of the RC bridge deck. In this configuration, acceptable barrier performance was obtained with both 1100C and 2270P vehicles at TL-3 of MASH. This testing would indicate that similar acceptable performance would be achieved with posts placed in rigid mow strips. Of course, additional considerations would be required in the terminal and transition regions which typically utilize stronger post sections as compared to S3x5.7 posts. However, it may be possible to utilize a half post-spacing in terminal regions as well.

In 2015, two research reports were published decribing the design of an MGS with weak posts placed in sockets in a mow strip. One report addressed component tesitng and anticipated damage to the mow strip resulting from a full-scale vehicle impact: https://mwrsf.unl.edu/reportResult.php?reportId=304, and another study evaluated the full-scale crash testing and design of the MGS installed in socketed foundations in conjunction with a mow strip: https://mwrsf.unl.edu/reportResult.php?reportId=315.

The results of the MGS with weak posts installed in conjunction with sockets and a mow strip were determined to be successful and the system was recommended for FHWA eligibilty.