On November 16th ComBAR®, Schöck’s glass fibre reinforcement, passed another significant test of its strength and material capabilities. A barrier wall reinforced with ComBAR® bars sustained, without any significant damage, the impact of a 36 tonne tractor trailer hitting it at 80 km/h. To view the movies please visit www.youtube.com/watch?v=xQDXnISGXhw and www.youtube.com/watch?v=Ld0sOU0MaOY.
Increasingly, the barrier walls on Canadian bridges are being reinforced with glass fibre reinforced polymer (GFRP) rebars, as their installation eliminates the corrosion problems encountered on walls reinforced with carbon steel rebar. The other alternatives, installing either stainless steel rebars or epoxy coated bars, are either more expensive or far less durable than glass fibre reinforcement.
The wall tested in November is a PL-3 category wall. This is the highest category wall in terms of the load and the impact speed which are to be sustain. It is designed for installation on bridges along national highways. After a fatal accident in 2008, where a tractor trailer punched through a GFRP reinforced PL-3 barrier wall in Manitoba and ran off a bridge, the Ministry of Transportation Ontario (MTO) placed a moratorium on the installation of GFRP rebars in walls of this category. It decided not to build any more GFRP reinforced PL-3 walls until they had been tested in a full-scale crash test.
The test on the ComBAR® reinforced wall was performed at the Texas Transportation Institute (TTI) in College Station, Texas. The TTI is part of the Texas A&M University System and as such one of the most renown and experienced institutions on crash testing of barrier walls and highway accessories.
The wall was designed in close coordination with the MTO by a team of experts including Professor Khaled Sennah of Ryerson University in Toronto. The reinforcing scheme was based on a MTO standard drawing for PL-3 walls reinforced with glass fibre reinforcement, such as ComBAR®. The final rebar layout for the crash test was chosen by Schöck on the basis of a design performed by the renown Canadian engineering office McCormick Rankin Corp. (MRC) in Mississauga, Ontario. In their design MRC adopted the yield-line theory used for the design of steel reinforced concrete elements to the ComBAR® reinforced wall.
On behalf of Schöck, Prof. Sennah acted as an independent consultant to the MTO for the crash test. As such he attended the coordination meetings at the MTO, checked the design and reinforcement drawings of the wall and inspected the construction, formwork and reinforcement of the wall. During the test he and a technician from Ryerson University collected extensive data on the deflection and acceleration of as well as the strains within the wall. In January of 2011 Prof. Sennah will perform static tests on the wall at TTI to determine the static load equivalent to the impact load sustained in the crash test. This load is needed to allow for future optimizations of the design of the ComBAR® reinforcement in PL-3 barrier walls.
The wall itself was built by technicians at TTI on an existing foundation at the testing facility in College Station. Concrete was poured only a week before the test. On November 16th it had nearly achieved the specified concrete compressive strength of 25 MPa. The wall was cast onto an approximately one meter wide concrete slab which cantilevered off an existing foundation at the testing facility. The geometry and reinforcement of the deck (conventional reinforcing steel) were similar to those of the bridge deck of an actual bridge. The wall was almost 40m (130 ft) long. 40 mm deep control joints were cast into the wall at a maximum spacing of six meters.
Geometry of deck slab and wall
The wall was entirely reinforced using ComBAR® bars. Vertical bars at the inside and outside face were anchored in the deck slab. Vertical ComBAR® bars with a core diameter of 16mm were spaced at 300mm along the inside face of the wall. At the outside face 12mm bars were used at the same spacing. At the two ends of the wall the spacing of both the inside and outside face vertical bars was reduced to 150mm over a length of 2.7 meters. Single headed bars with 16mm diameter were placed at the inside face of the wall parallel to the sloped surface at the bottom of the wall. The head was anchored in the slab. Five horizontal 16mm bars were installed along the inside and the outside face of the wall. The concrete cover was 50mm at the sides. The first horizontal bar was located 80mm from the top of the wall.
Reinforcement of the PL-3 wall prior to casting of the deck
The test was performed according to the newly released American Manual for Assessing Safety Hardware (MASH) which replaced the NCHRC report 350. While the requirements for crash tests using tractor trailers with a total weight of 36 tonnes are identical in both documents the tolerances on the vehicle speed and the impact angle are more stringent in MASH document.
The trailer was loaded with sand bags to reach the required total unit weight of 36,000 kg. The load was tied down by sheets of plywood which were bolted into the deck of the trailer. To achieve the required center of gravity above the pavement the sand bags were placed onto a steel frame. The tractor trailer was remote controlled out of a vehicle driving next to it. Initially it was pushed by another truck until it had reached a speed sufficient to engage the clutch in fifth gear. It then propelled itself up to the specified impact speed using its own engine.
The truck hit the wall about ten meters from its upstream end near a control joint at an angle of 15 degrees. Immediately after hitting the wall the tractor was pushed upward and away from the wall. The front axle of the tractor was broken in the process. The trailer was tilted toward the wall at this point. The second hit on the wall came from the front end of the trailer. Shortly after it had hit the wall its side was torn open by the load which had shifted toward the wall. Finally, the rear end of the trailer bed hit the wall near the top causing a punching shear crack which reached down about half the height of the wall. The truck then careened along the wall and veered off at its end coming to rest about fifty meters from the end of the wall.
Neither bending cracks nor any other signs of bending failure could be found along the wall after the test. The horizontal construction joint between the deck slab and the wall likewise showed no signs of damage.
Wall and truck after crash test
Punching shear crack in the wall
The extensive film material, taken by high-speed cameras, and the test data, obtained by accelerometers installed in the tractor and along the trailer, will be analyzed within the next couple of weeks by the TTI which will then issue a final report of the test. This report will be submitted to the MTO. A revision of the standard MTO drawing for PL-3 barrier walls reinforced with newest generation grade I GFRP rebars is to be undertaken on the basis of the test results.
On the basis of the results of the static tests a further optimization of the ComBAR® reinforcement layout is planned by setting up a finite element model of the wall and the truck impact. The equivalent static load will be used in this model.