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We're about to finally implement in Ontario your PCB as a non proprietary "Type M" temporary concrete barrier for work zones.

 

My first set of questions is with respect to rebar. In Canada we use metric rebar which is different from US metric or US customary. Our standard rebar is Grade 400 rebar which has a min yield strength of 400MPa and a min tensile strength of 600MPa. I understand an ASTM A615M Grade 420 bar has a minimum yield strength of 420MPa. Will this be a concern for the PCB?

 

The next metric problem I have is our sizes are different. Your #4, #5 and #6 bars have nominal diameters of 12.7mm, 15.875mm and 19.05mm respectively. Our standard 10M, 15M and 20M metric bars have nominal diameters of 11.3mm, 16.0mm and 19.5mm respectively. Originally we were going to specify 15M bars throughout, including the loop bars which is discussed below. For the #4 stirrups, we would like to now consider using the slightly smaller 10M bars unless you have a real concern.

 

For the loop bars, you have specified different steel " #6 smooth A706 Grade 420 steel. What is the rationale for using the larger diameter low alloy non-deformed steel bars for the loops ? Is it for welding or other reasons? Would you have a concern with our Grade 400 deformed 15M bars for the loops, or will we have to specify low alloy 20M smooth bars or Grade 400 20M smooth bars?

 

The second issue I would like to discuss is anchoring to concrete bridge decks with 90mm thick asphalt overlays, which is standard in Ontario. We have been reviewing the capacity of the specified Red Head anchors against other anchoring systems with significantly higher capacities to try and accommodate the 90mm standoff. I note Florida DOT allows a 1" to 2" asphalt overlay for their anchors into concrete.

Keywords
  • Temporary Barriers
Other Keywords none
Date August 13, 2008


Response
Response
Date August 13, 2008
Attachment Ontario PCB Stirrups002.pdf


Response
Response

I have responded to you questions below in red.

My first set of questions is with respect to rebar. In Canada we use metric rebar which is different from US metric or US customary. Our standard rebar is Grade 400 rebar which has a min yield strength of 400MPa and a min tensile strength of 600MPa. I understand an ASTM A615M Grade 420 bar has a minimum yield strength of 420MPa. Will this be a concern for the PCB?

 

The US rebar standard is Grade 60 which has a 60 ksi yield. This converts to a 414 MPA. The difference in strength is negligible, so I would not be concerned about the grade.

 

The next metric problem I have is our sizes are different. Your #4, #5 and #6 bars have nominal diameters of 12.7mm, 15.875mm and 19.05mm respectively. Our standard 10M, 15M and 20M metric bars have nominal diameters of 11.3mm, 16.0mm and 19.5mm respectively. Originally we were going to specify 15M bars throughout, including the loop bars which is discussed below. For the #4 stirrups, we would like to now consider using the slightly smaller 10M bars unless you have a real concern.

 

With regard to the bar size, it appears that your 15M and 20M bars have larger diameters than our No.5 and No. 6 bars. However, the US No. 4 bar has a diameter of 0.5" while your 10M bars have a 0.445" diameter. That is a 21% reduction in area. We would not recommend using bars with that much of a reduction in area. Thus, we would recommend that you substitute your 15M bars in locations where you are currently using the 10M bars. We believe this is necessary based on the amount of damage we have observed these barriers having during full-scale testing. We believe that the current barrier reinforcement is approaching its safe minimum capacity, so we have been holding the line and making alternative designs be equally as stronger or stronger than the tested barrier configuration.

 

We would recommend that you use the 15M bars for the stirrups as well, but that may not be practical. If you have to use the 10M bars for the stirrups, we would recommend that you install additional 10M stirrups 4.5" from the each stirrup adjacent to the tie-down anchor pockets as well as an additional stirrup between the two stirrups on the end of each rail. See the attached sketch.

 

For the loop bars, you have specified different steel " #6 smooth A706 Grade 420 steel. What is the rationale for using the larger diameter low alloy non-deformed steel bars for the loops ? Is it for welding or other reasons? Would you have a concern with our Grade 400 deformed 15M bars for the loops, or will we have to specify low alloy 20M smooth bars or Grade 400 20M smooth bars?

 

The loop bar steel is a different spec because we have found that the small bend diameter can cause reduced ductility and toughness in some grades of steel which compromises the impact strength of the loop. As such, we current specify that the loop steel must have a minimum yield strength of 60ksi, a minimum tensile strength of 80 ksi or 1.25 times the yield strength " whichever is higher, and a minimum % elongation of 14%. A706 and A709 steel both meet that spec. Others may as well. The bars can be deformed or smooth as long as the steel is within spec. Some of our states prefer smooth, so it is on the drawings that way. Again, we would recommend that you use the 20M bar because of the area difference between the No. 6 bar and the 15M bar (30% difference in area)

 

The second issue I would like to discuss is anchoring to concrete bridge decks with 90mm thick asphalt overlays, which is standard in Ontario. We have been reviewing the capacity of the specified Red Head anchors against other anchoring systems with significantly higher capacities to try and accommodate the 90mm standoff. I note Florida DOT allows a 1" to 2" asphalt overlay for their anchors into concrete.

 

We do NOT recommend installing any of our concrete tie-downs with asphalt cover. Florida asked us about this as well. The issue is that the asphalt cover creates large bending moments in the anchors which cause them to fail at much lower loads than the designed and tested system. The original projects were for anchorage on concrete surface bridge decks, and we have never had a chance to develop an anchorage that works with concrete overlays. That said, Florida continues to use it because they have no other option. We cannot recommend this type of installation, but it is up to you if you want or need to use it. You may want to account for additional barrier deflection for tie-downs used with the asphalt overlay. This issue has been brought up several times by other states as well. We will try to submit this as a problem statement for next year's Midwest States Regional Pooled Fund.

 

Is there a convenient time on Thursday that I could call either of you to discuss the above? Thanks,

 

I should also note the concrete strength of the barrier should be f'c = 5000 psi. The barrier was designed based on this strength, but some plans have been found using 4,000 psi concrete.

 

Date August 14, 2008


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