|TTI Research Supervisor:
Nauman M. Sheikh, P.E.
Associate Research Engineer
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Kurt Brauner, P.E.
Bridge Engineer Manager
Louisiana Transportation Center
1201 Capitol Road, Suite 605G
Baton Rouge, LA 70802
The objective of this research was to determine the impact performance of a single slope barrier restrained by keying it 1-inch into asphalt pavement. The barrier was constructed with a 40-ft long segment unconnected to the adjacent barrier segment. The objective of this test was to determine if the 40-ft long keyed-in barrier segment could successfully contain and redirect an impacting vehicle. The barrier was evaluated by performing American Association of State Highway and Transportation Officials (AASHTO), Manual for Assessing Safety Hardware (MASH) Test 4-12 with a single-unit truck. MASH Test 4-12 involves a 10000S vehicle impacting the barrier at a target impact speed and angle of 56 mi/h and 15°, respectively. Based on the results of the test, the keyed-in single-slope barrier with the unconnected 40-ft segment performed acceptably for MASH Test 4-12.
For MASH TL- 4, in addition to the Test 4-12 with the single unit truck, MASH also requires longitudinal barriers to be evaluated with the pickup truck (Test 4-11) and the small car (Test 4-10). Both of these tests were not considered critical and were thus not performed. Under this project, the keyed-in single slope barrier with a 40-ft segment length was primarily being evaluated for its potential to contain and redirect the MASH single unit truck. There were concerns that under the impact from this vehicle, the barrier segment might topple or have enough lateral rotation or movement that results in the barrier being unable to contain and redirect the vehicle. The results of the crash test showed that the barrier was successful in containing and redirecting the vehicle in a very stable manner, and with minimal deflection or rotation. Testing with the small car and the pickup truck is not expected to impart greater lateral load into the barrier segment compared to the single unit truck. Thus, there are no concerns for these vehicles to topple or breach the barrier. Furthermore, in past testing, the single slope barrier has been successfully crash tested with the small car and the pickup truck vehicles under MASH Test 4-10 and 4-11 conditions, respectively. Based on these facts, it was determined that Tests 4-10 and 4-11 of MASH are not critical and therefore not needed.
The 40-ft barrier segment in this project was tested keyed into 1-inch of asphalt. Using greater thicknesses of asphalt or restraining the barrier by keying it into concrete are also considered acceptable alternatives. Increasing pavement thickness and/or using concrete material increase the lateral restraint capacity of the keyed-in barrier system. These changes are not expected to negatively affect the performance of the barrier. It should, however, be noted that for MASH TL-4, the barrier should have a minimum 36-inch height. For this reason, the key-in design or the extent of overlays should not reduce the effective height of the barrier below 36 inches.
The 1-inch asphalt layer that keyed in the test installation barrier was constructed on an existing concrete pavement. In a field installation, having the underlying concrete pavement is not a requirement. Any base material such as asphalt, road base, soil, etc. that is well compacted and stable enough to construct and compact a 1-inch think asphalt pad for keying in the barrier is expected to result in similar barrier performance.
It is sometimes desired to have drainage slots along the base of the barrier. Presence of drainage slots decreases the total length of the barrier that is keyed into asphalt, thus reducing the lateral restraint provided by the asphalt key-in. If drainage slots are desired, the length of the barrier segment should be increased so that at least 40 ft length is keyed in. As an example, if it is desired to have drainage slots that are 2 ft long, spaced 10 ft on centers, the length of the barrier segment should be increased to 50 ft so that at least 40 ft of the barrier is restrained by the asphalt.