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Precast Concrete Barrier

Question
State MT
Description Text

I will summarize the issues that
were brought up by the MDOT and what I was hoping you would be able to provide
a professional opinion on or facts relating to those issues.  Again,
the issues brought up are by the MDOT in comparing what our supplier had
done when pouring the barrier to the Standard Drawing/Specification for
the precast Truck Barrier.  I make the assumption that the MDOT
does not have crash testing for the truck barrier per the March 7, 2014 e-mail
from Matt Strizich.  How or where the MDOT came up with the dimensions,
rebar placement, slope angles, or any other element of the Truck Barrier is yet
un-identified. (I have included a picture we took of a form with the cage in it
for your reference.  Please note the barrier are poured upside down and
you are looking at the bottom of the barrier.  I will reference the bottom
of the barrier even though it is at the top of the picture.)



Issue 1: The opening in the face of
the barrier (blockout) is specified to be 2-1/4" high in the MDOT Standard
Drawing.  Due to differences in form fabrication, the height of the
opening varies in dimension from 2-1/4" to 4" high. 
The concern that was pointed out to me by the MDOT was that if the blockout is
4" high instead of 2-1/4", there is a reduced dimension of concrete
coverage for the lowest horizontal bar and bottoms of the hair pin bent
bars through that section of the barrier. [In your opinion, does this
compromise these barriers? ] This ties into Issue #2 in the fact that the
supplier was placing the bottom horizontal rebar several inches up on the
hairpin bars which actually results in excessive clearance from the form. (See
picture - the horizontal bar that is sitting just above the spacer wheel should
actually be near the ends of the hairpins or closer to the bottom of the
barrier.) 



Issue 2: Bottom horizontal straight
rebar is not properly spaced.  Again referencing the picture, the
horizontal bar closest to the bottom of the barrier should be tied to the
hairpin bars near the tape measure.   [In your opinion, is
their any tolerance in the spacing of the rebar and would the barrier be
compromised if the horizontal bars are placed as shown in the picture vs the
standard drawing?]



Issue 3: Optional End Loops are
being installed upside down.  The 90 degree bends are facing towards the
bottom of the barrier as you can see in the picture whereas the MDOT
Standard Drawing show the  bends facing towards the top of the barrier.
[Would this have any negative effect on the truck barrier?]



All of these issues have been
corrected to meet the MDOT's Standard Drawing moving forward, but we are trying
to get the MDOT to accept the barrier previously poured.  I will
send follow up pictures that we took of the forms with rebar in them
waiting to be poured.  In these you can see what was happening and
hopefully formulate a professional opinion as to the effects it may or may
not have.



 

Keywords
  • Temporary Barriers
Other Keywords none
Date March 14, 2014


Response
Response

To clarify, you are wanting to use the tall, truck PCB for passenger car applications under NCHRP TL-3? If the 14” SBP issue is not resolved, all other issues noted below are only secondary.

I will summarize the issues that were brought up by the MDOT and what I was hoping you would be able to provide a professional opinion on or facts relating to those issues.  Again, the issues brought up are by the MDOT in comparing what our supplier had done when pouring the barrier to the Standard Drawing/Specification for the precast Truck Barrier.  I make the assumption that the MDOT does not have crash testing for the truck barrier per the March 7, 2014 e-mail from Matt Strizich.  How or where the MDOT came up with the dimensions, rebar placement, slope angles, or any other element of the Truck Barrier is yet un-identified. (I have included a picture we took of a form with the cage in it for your reference.  Please note the barrier are poured upside down and you are looking at the bottom of the barrier.  I will reference the bottom of the barrier even though it is at the top of the picture.)

 

Issue 1: The opening in the face of the barrier (blockout) is specified to be 2-1/4" high in the MDOT Standard Drawing.  Due to differences in form fabrication, the height of the opening varies in dimension from 2-1/4" to 4" high.  The concern that was pointed out to me by the MDOT was that if the blockout is 4" high instead of 2-1/4", there is a reduced dimension of concrete coverage for the lowest horizontal bar and bottoms of the hair pin bent bars through that section of the barrier. [In your opinion, does this compromise these barriers? ] This ties into Issue #2 in the fact that the supplier was placing the bottom horizontal rebar several inches up on the hairpin bars which actually results in excessive clearance from the form. (See picture - the horizontal bar that is sitting just above the spacer wheel should actually be near the ends of the hairpins or closer to the bottom of the barrier.)

For the 12” wide PCB at top, a 4” tall scupper/slot may not significantly affect the large section’s capacity or performance in free-standing applications as compared to the 2.25” tall slot. However, the 14” height to slope break point may be the bigger issue.

Issue 2: Bottom horizontal straight rebar is not properly spaced.  Again referencing the picture, the horizontal bar closest to the bottom of the barrier should be tied to the hairpin bars near the tape measure.   [In your opinion, is their any tolerance in the spacing of the rebar and would the barrier be compromised if the horizontal bars are placed as shown in the picture vs the standard drawing?]

For the over-designed truck barrier used in passenger car applications, it may not be a big deal for minor changes in rebar position. However, the 14” height to slope break point may be the bigger issue. For 6” wide PCBs at top, changes in rebar position may have a more dramatic effect of barrier capacity due to a much reduced level of reserve capacity. Inward movement of longitudinal bars will reduce a barrier’s bending capacity above a vertical axis.

Issue 3: Optional End Loops are being installed upside down.  The 90 degree bends are facing towards the bottom of the barrier as you can see in the picture whereas the MDOT Standard Drawing show the  bends facing towards the top of the barrier. [Would this have any negative effect on the truck barrier?]

I do not believe that a 180 degree change in bend will greatly change anchorage capacity or barrier performance.

All of these issues have been corrected to meet the MDOT's Standard Drawing moving forward, but we are trying to get the MDOT to accept the barrier previously poured.  I will send follow up pictures that we took of the forms with rebar in them waiting to be poured.  In these you can see what was happening and hopefully formulate a professional opinion as to the effects it may or may not have.

Date March 14, 2014


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