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Please accept my compliments on a very interesting and detailed new concept concrete barrier design.

Manitoba Infrastructure and Transportation is investigating options to replace an existing F Shape (TL4) concrete median barrier with a TL5 system. The cross section has yet to be finalised but the Vertical-Faced, Concrete Median Barrier Incorporating Head Ejection Criteria design that MwRSF developed has been part of our assessments. Construction of the new barrier is tentatively scheduled for summer 2013.

To help us with the new system for our Department, I am seeking your assistance to quantify and/or qualify a number of conditions and criteria for the TL5 concrete barrier as it relates to your barrier.

  1) In the report on this barrier, it appears that an L4000 concrete mix (28 MPa) was used for both the foundation as well as the barrier itself. a. What is the complete mix design for the L4000 mix? (weight, volume, cement content, water content, aggregate composition, air content, slump, etc.)? b. What would be the recommended concrete mix design for a slip formed installation of this design?

  2) What would be the consequences if the bottom of the barrier were not imbedded 75 mm below the finished grade of the adjacent asphalt pavement? I.e., the base of the concrete barrier is at finished grade (compacted aggregate or concrete pad). What size (diameter and length) of, and how many, vertical connecting pins (dowels) would be required to suitably attach the barrier to a concrete pad?

  3) We have a number of overhead bridge sign structure supports that are located in a narrow median (approximately 900 mm wide). These supports cannot be removed and consist of approximately two - 220 mm diameter aluminium poles centred in the existing F Shape concrete median (see photo below). What would be the suggested means to provide some protection to vehicles and occupants from these installation using this barrier. E.g. increase the height of the barrier (to what height; at what slope should the top rise e.g.. 10:1, 20:1; would the cross section remain or would it change at the top)? An end anchor system would be required on either side of the structure. The opening could/would be treated in some form using standard guardrail components and hardware such as thrie beam.

4) Are there any special considerations needed for this barrier design to address bridge piers located in the median. These considerations would primarily related to a possible change in the cross section from a symmetrical cross section to an asymmetrical system (possible vertical back face)? A taller system or a system located further from the pier could be considered as possible design options to address vehicle roll. (See the photo below; the median shoulder is significantly wider at the piers.) We do not yet know what access is required to the bridge piers and/or footings.

  5) Our local contractors have been contacted regarding this proposed slip form barrier construction. They have indicated to us that a minimum cover over the reinforcing steel should be 100 mm (4 inches). What would be the consequences, or benefits, to this barrier if the stirrups and longitudinal reinforcing were modified to accommodate the suggested 100 mm of cover as opposed to this barrier’s design of 65 mm? I appreciate you may not be in a position to answer these questions without detailed analysis and possibly testing but your learned opinion would be appreciated in this regard. Please do not hesitate to contact me at your convenience if you need any clarification. Thank you in advance for your time and consideration of my questions.

To help us with the new system for our Department, I am seeking your assistance to quantify and/or qualify a number of conditions and criteria for the TL5 concrete barrier as it relates to your barrier. 

1)      In the report on this barrier, it appears that an L4000 concrete mix (28 MPa) was used for both the foundation as well as the barrier itself. 

a.       What is the complete mix design for the L4000 mix? (weight, volume, cement content, water content, aggregate composition, air content, slump, etc.)?

b.      What would be the recommended concrete mix design for a slip formed installation of this design? 

The barrier was designed with a minimum concrete compressive strength (f’c)  of 4,000 psi (28 Mpa).  Cylinder testing indicated that we received a concrete mix with an f’c over 5,000 psi (35 MPa).  However, this increase in concrete strength would only result in a 2% increase in barrier strength.  Therefore, a minimum f’c of 4,000 psi (28 MPa) is still recommended.

Since the barrier was only 200 ft long, we did not slipform the test installation. Slipforming would have increased the test costs greatly for such a short segment of barrier.  As such, the exact mixture we used would not be ideal for slipforming.  I would recommend discussing the mixture ratios with contractor / slipformers as they know much more about concrete mixtures than I do.  The only important thing about the mixture is that it results in a minimum f’c of 4,000 psi (28 MPa).

2)      What would be the consequences if the bottom of the barrier were not imbedded 75 mm below the finished grade of the adjacent asphalt pavement?  I.e., the base of the concrete barrier is at finished grade (compacted aggregate or concrete pad).  What size (diameter and length) of, and how many, vertical connecting pins (dowels) would be required to suitably attach the barrier to a concrete pad?  

We have had discussions like this previously with various State DOTs.   I will refer you to our company’s Q&A website in which this topic has been discussed and options were sketched.  Please visit   http://mwrsf-qa.unl.edu  and search for Question ID No. 629 from June of 2012.

3)      We have a number of overhead bridge sign structure supports that are located in a narrow median (approximately 900 mm wide).  These supports cannot be removed and consist of approximately two - 220 mm diameter aluminium poles centred in the existing F Shape concrete median (see photo below).  What would be the suggested means to provide some protection to vehicles and occupants from these installation using this barrier.  E.g. increase the height of the barrier (to what height; at what slope should the top rise e.g.. 10:1, 20:1; would the cross section remain or would it change at the top)?  An end anchor system would be required on either side of the structure.  The opening could/would be treated in some form using standard guardrail components and hardware such as thrie beam. 

I’m assuming that the concrete barriers are going to be replaced with the TL-5 barrier being discussed here.  Yes, end section would need to be present on both sides of the structure.  Increasing the barrier height may limit trailer-box roll into the barrier, thus minimizing the possibility of box impacts to the support poles.  See below for common practices on height increases.  However, the barrier would have to get wider to accomplish an increase in height.

                I see two options for shielding this support:

(1)    Transition the barriers to single faced sections (again see below).  The barrier would need to be brought in front of the poles and may terminate on the downstream side of the poles.  This would incorporate two independent barrier segments that are not connected.  Each free end would face downstream and be protected from end on hits by the opposite barrier.  We refer to this as a “fish scale” scheme as the protection is continuous upstream but open downstream of the hazard.  For this option, you may need a wider median as it requires a barrier on each side of the support poles, so a minimum of 9” + 20” + 20” = 49 inches (54” by the time you include space between the poles and the barriers).  This option would provide TL-5 protection.

(2)    You could use thrie beam elements or structural tubes on both sides of the median to bridge the gap between each barrier end.  Of course, the rail elements would need to be tapered (end shoe design for thrie beam) to prevent vehicle snag.  This design would not likely meet TL-5 and may not meet TL-4 depending on the rail strength and height location.  However, it would provide protection for passenger vehcles.

4)      Are there any special considerations needed for this barrier design to address bridge piers located in the median.  These considerations would primarily related to a possible change in the cross section from a symmetrical cross section to an asymmetrical system (possible vertical back face)?  A taller system or a system located further from the pier could be considered as possible design options to address vehicle roll.  (See the photo below; the median shoulder is significantly wider at the piers.)  We do not yet know what access is required to the bridge piers and/or footings. 

I have seen similar designs to what you are proposing here.  Single sided versions of this barrier have been developed by a few State DOTs (sorry I can’t find the drawings right now) for uses in pier protection, slope separation, and sign bridge protection (like #3 above with a wider median).  Vertical back faces were used in most of these situations.  To accomplish this while still keeping the strength, the flat top portion of the barrier was extended backward.  Thus, the barrier width remained at 20 in. (508 mm).

                Taller systems to reduce trailer-box roll over the barrier have also been configured for pier protection installations. This was accomplished by extending the top sloped portion of the barrier upward.  Holding this slope constant ensures the  head ejection envelope is not violated.  I do not have a grasp on the quantity of trailer-box extent behind the barrier as a function of barrier height, only the general idea that taller barriers reduce the roll and extent of the box.  I have heard of State DOTs extending up to as high as 54 in. (1,372 mm).

5)      Our local contractors have been contacted regarding this proposed slip form barrier construction.  They have indicated to us that a minimum cover over the reinforcing steel should be 100 mm (4 inches).  What would be the consequences, or benefits, to this barrier if the stirrups and longitudinal reinforcing were modified to accommodate the suggested 100 mm of cover as opposed to this barrier’s design of 65 mm?

If an increase in clear cover is desired, the internal steel would likely need to be increased (larger bars or more bars) to account for the loss in depth.  A new reinforcement design would need to be configured such that the barrier strength remained the same as the original, as tested, version.