BACKGROUND
    Unchecked, roadside vegetation growth can impede motorist vision at intersections and degrade the appearance of a roadside guardrail. In an effort to reduce maintenance costs and the safety risk to workers associated with hand mowing around guardrail, and amid recent environmental concerns regarding the use of herbicides to control roadside vegetation growth, there is a growing trend toward encasing guardrail posts in pavement mow strips. This pavement layer prevents vegetation growth in the vicinity of a guardrail installation and, thereby, reduces or eliminates the need for hand mowing or herbicide use.
    However, if not properly designed, a mow strip can negatively influence the impact performance of a guardrail system. A strong-post guardrail system relies on the ability of the posts to rotate through the confining soil medium to dissipate the energy of an impacting vehicle. The increased stiffness induced by the confinement of the pavement mow strip can lead to premature failure or fracture of a post. In turn, this post failure can lead to snagging or pocketing of a vehicle in the guardrail system and, ultimately, rupture or override of the w-beam rail element and/or overturn of the impacting vehicle. The mow strips may also influence safety by making the repair of guardrail installations more difficult and time consuming after they have been struck.
    The Texas Department of Transportation (TxDOT) sponsored research at the Texas Transportation Institute (TTI) to develop a crashworthy means of encasing guardrail in an asphalt or concrete mowing strip. The research approach consisted of dynamic bogie vehicle testing of steel and wood posts in various mow strip configurations, computer simulation, and full-scale crash testing. The research resulted in a design for guardrail in mow strip that meets the guidelines of National Cooperative Highway Research Program (NCHRP) Report 350. (3)
    The recommended design is shown in Figure 1. It includes provision for a “leave-out” area in the mow strip around the guardrail post that allows at least 7 inches of post deflection at the ground line. The top 4-6 inches of this leave-out area is backfilled with a low-strength, 2-sack grout mix that has a 28-day compressive strength of approximately 120 psi. The grout resists vegetation growth and is weak enough that it crushes under loading from the guardrail post during a vehicular impact, thus allowing the post to rotate within the leave-out area.

Figure 1. Current recommendations for installation of guardrail posts in mow strips.

    Some user agencies that have implemented the recommended design have experienced problems with proper placement and inspection of the grout backfill. Some contractors lack the understanding of how to properly prepare the low-strength grout and/or do not want to take the time to specially mix the small quantity of grout. Consequently, it is possible that higher strength concrete may be placed in lieu of the low-strength grout. Because of the similar appearance of grout and concrete, user agencies have a difficulty with inspection of backfill material.
    The mow strip systems successfully tested under this project are considered to be representative of the most severe confinement conditions allowable. Any increase in post confinement beyond that provided by the grout backfill material used in the leave-out sections formed around the guard fence posts should undergo additional analysis and full-scale testing. However, the researchers noted that other leave-out backfill materials (e.g. foams) may be acceptable as alternatives to the 2-sack grout provided their compressive strength does not exceed that of the grout. It was further stated that the strength of an alternative leave-out backfill material can be demonstrated through laboratory and/or dynamic bogie vehicle testing. Alternative leave-out backfill materials should also have a demonstrated ability to resist vegetation growth.

OBJECTIVE
    The objective of this research is to identify more cost effective and constructable solutions for the placement of guardrail posts in mowing strips. This will be accomplished through laboratory and dynamic impact testing and evaluation of alternative backfill materials for use in the leave-out area around the perimeter of the guardrail post.

BENEFITS
    The research will identify more cost-effective solutions for construction of guardrail with mowing strips which will address some of the constructability and inspection issues identified with the use of the 2-sack grout currently recommended for use in the leave-out area around guardrail posts in mowing strips.

IMPLEMENTATION
    If acceptable, cost-effective backfill materials are identified, the new design(s) for guardrail posts in mow strips can be incorporated into a revised standard detail sheet for installation of guardrail in mow strips. Design details will be submitted to AASHTO-ARTBA-AGC Task Force 13 for incorporation into “A Guide to Standardized Highway Barrier Hardware.”

WORK PLAN
    The proposed work plan consists of four tasks as described below.
Task 1 –- Identify Candidate Backfill Materials for Post Leave-out Sections
    Under this task, alternative backfill materials for the leave-out sections will be identified and procured for further evaluation. Specifications and installation procedures for the materials will be obtained from the respective manufacturer. Emphasis will be placed on materials that can be easily installed and inspected. The materials must also be durable and capable of resisting vegetation growth. Two-part polyurethane foams, rubber mats, and other materials will be considered.

Task 2 –- Perform Laboratory Tests of Selected Candidate Materials
    Selected candidate materials identified under Task 1 will be subjected to laboratory testing to quantify their compressive strength. The tests will be conducted using a Universal Testing Machine (UTM) or similar equipment. A computer-based data acquisition system will be used to record applied compressive load versus time. The results of the static laboratory tests will be used as an initial screening to determine if the compressive strength of a candidate material is within a reasonable range to warrant further investigation through dynamic testing. For materials such as foams, several tests will be conducted to quantify appropriate mix ratios of the component parts required to achieve the desired strength. Some candidate materials such as rubber matting will not lend themselves to compressive testing and, consequently, their evaluation under this task may be more subjective.

Task 3 –- Perform Dynamic Bogie Vehicle Impact Test
    Those candidate backfill materials passing the initial screening conducted under Task 2 will be further evaluated under this task through dynamic bogie vehicle impact testing. A 25-ft long x 3.5-ft wide x 5-in. thick concrete mow strip will be constructed on top of 6 in. of compacted road base materials for use in the test program. A riprap concrete with a minimum 28-day compressive strength of 2000 psi will be used for the mow strip. The concrete will be minimally reinforced with welded-wire mesh to control shrinkage cracking.
    The length of the mow strip is designed to accommodate four guardrail posts at a spacing of 6 ft-3 inch, which is the standard spacing for posts in a strong-post guardrail system. The posts will be embedded to a standard depth of 3 ft-8 inch in 12-inch diameter augured holes. The holes will be backfilled with NCHRP Report 350 standard soil. The posts will be positioned approximately 3 inches from the front edge of an 18-inch x 18-inch leave-out section. The new candidate backfill materials will be used in the top portion of the leave-out section.
    The posts will be impacted head-on by a bogie vehicle at an impact speed of approximately 22 mph. The TTI bogie vehicle weighs approximately 1800 lb and is configured with a sliding, reusable nose assembly. The crushable nose configuration consists of ten stages of expendable aluminum honeycomb material of differing densities as shown in Figure 2. After a test, the honeycomb material is replaced and the bogie is reused. These tests will approximate actual impact conditions and will provide an objective basis for determining which candidate backfill materials can be used without comprising the impact performance of the guardrail system encased in mow strip.
    The bogie vehicle will be instrumented with a pair of uniaxial accelerometers to obtain acceleration versus time data for each post impact. The acceleration-time history will be post-processed to derive a force-deflection history from which energy absorption can be quantified for each post. The force-deflection response, energy absorption characteristics, and the failure mode of the posts in a concrete mow strip with the candidate backfill materials in the leave-out sections will be compared with the results of similar tests of posts surrounded by grout backfill that were obtained in the previous TxDOT research study. (2) An acceptable alternate backfill material will be one that does not generate more resistance or confinement than the approved 2-sack grout mixture.

Figure 2. Bogie impact vehicle with reusable sliding nose assembly.

Task 4 –- Prepare Deliverables
    In this task, a final report will be developed that documents the testing and evaluation of alternative backfill materials for use in the leave-out sections around guardrail posts installed in pavement mow strips. Recommended design guidance for guardrail posts installed in pavement mowing strips will be included in the report. The report will be prepared in a format suitable for submittal to the Federal Highway Administration (FHWA).

 
FINAL REPORT:
ALTERNATIVE DESIGN OF GUARDRAIL POSTS IN ASPHALT OR CONCRETE MOWING PADS, May 2009

Mowstrip Drawings
Mowstrip Summary

BOGIE TEST VIDEOS:
Test B1 (Post with 2-sack grout):
Real-Time     High-Speed Perpendicular     High-Speed Ground Level at Post

Test B2 (Post in two part urethane foam):
Real-Time     High-Speed Perpendicular     High-Speed Ground Level at Postt

Test B3 (Post in full-strength unreinforced concrete wedge):
Real-Time     High-Speed Perpendicular     High-Speed Ground Level at Post

Test B4 (Post with TopHat™ recycled rubber mat):
Real-Time     High-Speed Perpendicular

Test B5 (Post with flat mat):
Real-Time     High-Speed Perpendicular     High-Speed Ground Level at Post

 
QUARTERLY PROGRESS REPORTS:
December 2008 Progress Report
March 2008 Progress Report

____________________
(1.) Baxter, J. R., “W-Beam Guardrail Installations in Rock and in Mowing Strips,” Memorandum HAS-10/B64-B, Federal Highway Administration, U.S. Department of Transportation, March 10, 2004.
(2.) Bligh, R. P., Seckinger, N. R., Abu-Odeh, A. Y., Roschke, P. N., Menges, W. L., and Haug, R. R., “Dynamic Response of Guardrail Systems Encased in Pavement Mow Strips,” Research Report 0-4162-2, Texas Transportation Institute, College Station, TX, January 2004.
(3.) Ross, H. E., Jr., Sicking, D. L., Zimmer, R. A., and Michie, J. D., “Recommended Procedures for the Safety Performance Evaluation of Highway Features,” National Cooperative Highway Research Program Report 350, Transportation Research Board, National Research Council, Washington, D.C., 1993.