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Asphalt Width under TCB Transition to Bridge

Question
State IA
Description Text For the roadside TCB transition to bridge, a 2-inch asphalt surface is shown extending 12 inches behind the back face of the TCB.  Is it possible to reduce this to 6 inches?  If so, would this require a greater depth of asphalt?
Keywords
  • Bridge Rails
  • Temporary Barriers
Other Keywords asphalt
Date April 25, 2012


Response
Response

The approach transition between free-standing F-shape PCB and a rigid barrier was tested at MwRSF using a asphalt pad that extended for approximately 36" behind the back side of the PCB segments.

We do believe that it is possible to reduce the amount of asphalt behind the barrier, but there are concerns. Reducing the amount of asphalt behind the barrier too much could lead to a disengagement or shear punch out of a significant section of the asphalt. This would increase pin and barrier deflections which could adversely affect the safety performance of the system. As such, we would recommend a minimum of 2 ft of aphalt behind the back side of the PCB segment in the region of the transition. This should provide sufficient resistance for the anchors used in the transition.

Increasing the depth of the asphalt could potentially decrease the amount of asphalt needed, but component testing of the anchor pins in different asphalt thicknesses was conducted during the project and the difference between 2" and 6" of asphalt on the anchor loads was minimal. Thus, we don't believe that thicker asphalt would be effective unless it was much thicker.

One other item to note with respect to paved surfaces and PCB's is that we generally recommend that paving extend underneath and behind free-standing barrier segments as well. As a general rule, we recommend that the paved surface extend a minimum of 4 ft behind the back side of free-standing PCB segments to allowed for controlled deflection of the barrier on an even, consistent surface. As the barrier deflects, the paving helps provide consistent barrier to ground friction and prevents the back of the barrier from sliding off the pavement or tripping on uneven ground while engaged with the impacting vehicle.

Thus, for a total installation we would recommend 2 ft of asphalt behind the transition section and 4 ft of asphalt behind the free-standing barriers.

Date April 25, 2012


Response
Response

Due to width constraints, we would be interested in a thickness recommendation for an asphalt pad that extends only 12 inches behind the TCB transition.

Date April 25, 2012


Response
Response

As noted previously, the original development of the asphalt pin tie-down evaluated various pin diameters in asphalt thicknesses varying from 2"-6". The testing was limited, but there was little to no variation in the peak lateral loads on the pins when using 2", 4" or 6" of asphalt. Thus, we don't expect
increased
load capacity due to the increased thickness of asphalt.
In addition, this component testing did not evaluate short widths of asphalt behind the pin.

The addition of more asphalt thickness may prevent the shear breakout of reduced asphalt width behind the PCB segments. Thus, other potential combinations of asphalt thickness and width could potentially restrain the tie-down pins. We have conservatively recommended 2' of asphalt width behind the PCB segments mentioned previously in order to be conservative. Other combinations may work and could be confirmed through future component testing or full-scale testing.

Another option would be to replace the asphalt in the area of the PCB approach transition with a reinforced concrete slab. The reinforced slab should constrain the pins as well or better than the asphalt and should elminate the potential for shear breakout of the pins. We believe that a 6" thick concrete slab with with a single mat of no. 4 rebar would work. The longitudinal and transverse bar spacing should be 12" on centers. The concrete slab would only need to extend 6" behind the back side of the PCB segments.


Date April 26, 2012


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