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I will provide my general comments below under the individual questions and realizing that Roger has already done a great job in addressing each item.

Ron

From: Elle, Michael (DOT) [mailto:michael.elle@state.mn.us]
Sent: Friday, October 09, 2015 10:13 AM
To: Bligh, Roger <RBligh@tamu.edu>; Ronald K. Faller <rfaller1@unl.edu>
Subject: Concrete Median Barrier re-bar questions

Roger and Ron,

I am sending this to both of you, as this has become an urgent constructability question that I need some answers for before a meeting on Monday morning.

Two Questions:

1. Two our three primary concrete contractors are claiming that their slip forming operations would go much better if the rebar clearance from the face of the barrier was more than two inches.  I have seen other states with 2" or 2.5" minimum listed, but never a maximum.  What should the maximum clearance be?  Could we state 2" min and perhaps 3" or 4" maximum.

**Per my recollection, we have not experienced significant discussions on clear cover, except when developing the TL-5 barrier depicted in the attached report. This barrier was planned for implementation under slip-forming operations. For it and based on contractor feedback, we settled on a general clear cover of 2.5 in. I have not considered or recall seeing a maximum clear in standard plans. However, if both maximum and minimum clear covers are desired, the barrier capacity should be based on a maximum clear cover during testing or with design calculations. I am not overly found of clear covers of 4”. I would be more supportive of a 3” maximum clear cover as long as adequate structural capacity is provided.

2.  The current design allows for two vertical 8" bars on the base (every two feet) if the footing is poured separately. Could these be placed closer to the center to help in slip form operations? Better yet could they be placed one every foot, on center or perhaps 4" just off center alternating from side to side every foot?

**Without some additional analysis, it is somewhat difficult for me to make further guidance pertaining to number of shear bars, resistance to overturning, placement, etc. Shear bars provide some overturning resistance, but the short length reduces effectiveness. Asphalt keyways on front and back can provide similar resistance to shear. I personally like to see vertical stirrups in barriers. Although the current plans show a particular dowel size, length, and spacing, it is unclear as to the background of these selections. I could further investigate next week if needed. I seem to recall that this configuration may have evolved from the results obtained in the SwRI 1976 barrier study.

I am sorry for the short response and can provide more thoughts next week if needed.

From: Bligh, Roger [mailto:R-Bligh@tti.tamu.edu]
Sent: Friday, October 09, 2015 12:33 PM
To: Elle, Michael (DOT) <michael.elle@state.mn.us>; Ronald K. Faller <rfaller1@unl.edu>
Subject: RE: Concrete Median Barrier re-bar questions

Hi Mike,

A few comments for your consideration in regard to your questions.

1). We noted on your current drawings that your clear cover is currently specified to be 2” +/- ½”.   Increasing the clear cover will slightly reduce the moment capacity of the barrier because it reduces the “d” distance to your tension steel.  However, you may get some contribution from your compression steel that could counter some of this loss.   You also have a relatively wide barrier section that helps provide capacity.  For example, the 36 inch barrier is probably wider and stronger than it needs to be for TL-4 impacts.  With all this in mind, we would be comfortable with a 3 inch clear cover.  One note -- with increased clear cover (e.g., 3 inches) on the side and top of the barrier, there is an increased chance of having chunks of concrete lost at the top corners of the barrier during severe impacts. 

2). Having a continuous slip-formed barrier helps tremendously in developing required barrier capacity due to the inertial resistance offered by the large barrier mass.  There is also a good deal of adhesion strength that exists between the two pours (footing and barrier) that is not accounted for in the design process.  This being the case, either of your alternate anchorage options are likely o.k.   However, it should be noted that the short 4” embedment will not come close to developing the strength of the #8 bars dowel bars.  A better detail would be to use an “L” shaped bar with the leg of the “L” bar in the footer and with a taller vertical projection into the barrier.  The size of the anchor bars can be reduced to something like a #6 bar and they could be placed along the center of the barrier at a wider spacing.  I suspect that your contractors my simply “stab” the #8 dowel bars into the concrete footer after it has been placed.  The “L” bars could still possibly be stabbed into the footer and perhaps tied together (if needed) using a #3 bar along the vertical leg.  I noted that TxDOT is using #6 “L” bars on 8-ft centers when a single slope barrier is cast onto a concrete deck or pavement.  They use a few bars at 2-ft spacing near the ends of the barrier run.  I have attached a detail sheet for your reference. 

Please note that these are general comments.  Analyses can be performed to be more precise with recommendations, but this would require more time to complete.  Please let me know if you have any additional questions or if we can be of any additional service. 

Have a great weekend.

Best regards,

Roger