View Q&A

We have a project on I-80 coming up where we will be installing the "head ejection" median barrier.  I am requesting your assistance in developing a few options for anchoring the barrier into existing pavement.

As shown in the attached PDF, the barrier will be installed on three slightly different median pavement configurations.  In all cases, the existing unreinforced PCC slab is 10 feet wide and 12 inches thick.  Note that the barrier may shift left or right within the slab, but should not get any closer than 1 foot from the edge of the slab.

It would be preferable to use the same (or very similar) anchoring details for all three configurations.  The final pavement elevation must match existing.

Please let me know if you have any questions.

Thanks!

Dr. Faller has asked me to help you with the anchoring of the TL-5 median barrier to an existing concrete slab.  From your comments below, I'm assuming that you are wanting to dowel/epoxy  into the existing median slab and not use the asphalt keyway of the original (as tested) design.  If so, this could be accomplished in a couple of different ways:  1) the stirrups could be modified to be open at the bottom, extended in length, and placed into the slab " this option would resemble the stirrups used in the end section configuration.  2) The stirrups could remain the same and #5 dowel bars would be placed at 18" intervals to match up with and anchor the stirrups.  Note " the #4 dowel bars shown in the original report and design drawings for the TL-5 median barrier were used to anchor the rebar cage during casting. These bars were not considered in the strength analysis of the barrier.

Let me know if either of these two options sounds like what you had envisioned... or if I'm completely off base.

You got it.  Either of those options (or some version of those options) would be much more agreeable to us instead of removing a bunch of concrete and pouring a new, separate footer.  And you're right " we are not interested in using the asphalt keyway on this project.

Do I need to pick between the two options, or were you planning on evaluating both of them?

The two options are directly related since they would both include utilizing epoxy to anchor #5 bars to the slab (same spacing/intervals too).  Thus, the embedment depth and location of the holes/anchors would be identical.  As such, I can sketch up both options and you can choose between the two based on cost and constructability.

For both options, the spacing would always remain consistent at 18 inches " both front and back sides of the barrier.  However, the embedment depth would be a function of the epoxy strength and concrete strength.  You would have to use the manufacturers technical manual / recommendations on embedment to obtain full capacity of the rebar.  Does Iowa have a preferred epoxy, or is this open?

Recently, MwRSF has been utilizing the HIT-RE 500 epoxy from Hilti (1,800 psi bond strength) for our anchorage designs.  This product coupled with a concrete f'c  of 4,000 psi would require only 6 inches of embedment to ensure full capacity of a #5 bar.

Also, are you planning on casting the barrier with the 1/18 face slope or with a vertical face (design option discussed in report).  I only ask because if the 1/18 slope is being utilized, the dowels will be bent to match the slope of the stirrups " anchoring at an angle is usually not desired.

I would be interested in seeing a sketch of both options.  This would be extremely helpful when used to explain the options to others.

I'm not sure that we have a preferred epoxy, but we do have a list of approved sources.  I found the following passage in our Materials IM 491.11.  Appendix C is the only place I found a listing for Hilti RE-500.  Can you take a look and see if these are the types of systems we should be employing in this situation, or if we should limit the systems listed in Appendix C, or if we should provide a separate list of approved sources for this particular project?

Appendix C contains polymer grouts for dowel bar installation. Either an encapsulated chemical

anchor system or a pressure-injectable system with mechanical proportioning and mixing shall be

required to blend the material to uniform consistency.

To obtain approval for products under Appendix C, the laboratory evaluation will consist of bonding

a No. 5 reinforcing bar in a 4-inch deep 3/4-inch diameter hole in a concrete specimen and

performing a pullout load test. The test specimen shall develop a 40-pound minimum pullout load in

one hour and a 24-hour pullout load at a minimum of 10,000 pounds. The specimen will be kept at

laboratory temperature. Two specimens are needed to obtain the average of each pullout load.

Products meeting the requirements for Appendix C will also be placed on Appendixes A and B.

Manufacturers whose products require special equipment such as an injection or mixing equipment

shall recommend which equipment can be used with their product.

We are planning on having the barrier slipformed, so we will probably be using the 1/18 slope on the barrier face.  Would this require an additional bend in the stirrups in order to avoid drilling/anchoring at an angle?

Please the attached PDFs for the two epoxy anchorage options previously discussed.  Please note that the embedment dimension of 6" is based on Hilte's HIT-RE 500 epoxy  (bond strength of 1,800 psi).  If another epoxy is desired, then the embedment depth may need to be altered to ensure ultimate tensile capacity can be obtained.  Also, this epoxy anchorage retrofit design assumes the concrete slab that you are anchoring too has sufficient size and strength as to prevent movement, rotation, and damage to the slab.  In your case of a 10 ft by 12 in deep slab, this should not be an issue.

Option 1 is divided into an Option 1a and 1b.  1a keeps the stirrup angled to follow the barrier before being bent to vertical 2 inches from the base of the barrier.  1b has the sides of the stirrup being bent to vertical near the top of the barrier, thus eliminating the need for the small bend near the base. 

For Option 2, I recommend doweling in using straight bars during the epoxy/anchoring stage, letting the epoxy set, and then bending the tops of the dowels inward to match and tie to the angled sides of the barrier stirrups.  Stirrups would remain identical to original design.

Option 1 will save on material cost as the amount of steel is reduced, but Option 2, may be easier to construct during installation

Does anything change if we will be using epoxy-coated rebar for the cage and the dowels?

In a recent study on epoxy anchors, MwRSF did dynamic testing on both black rebar and epoxy coated rebar dowels.  The testing has been completed, but the report is still being put together.  Bob is the one finishing the report, so he would know the conclusions of this study better than I.  Unfortunately, he is out of the office until next week.  From my recollection, I believe there is a 5-10 percent decrease in anchorage strength expected for epoxy coated bars.  If this is correct, then the embedment depth would need to be extended slightly (1/2" to 1") in order to ensure full capacity of the rebar can be developed. 

The Hilti design manual lists an embedment depth of 5¾" is required to obtain ultimate capacity of a #5 bar " I rounded this up to 6" to be a little conservative and to get a nice even number.  The Hilti manual does not mention anything for the effects of epoxy coated rebar.  Thus, if my above statements are correct (Bob help me out here) then the embedment depth should be increased to minimum of 6.5".

Don't take any of this as accurate until Bob confirms this... 

Scott is correct that our recent dynamic testing of epoxy coated rods with chemical adhesives showed approximately a 10% reduction when compared to black steel. Thus the increased embedment of 6.5" indicated by Scott is warranted and should provide the necessary strength.

The other point of concern I had was with the splice length required between the cage and the dowels being epoxied into the pavement - whether using epoxy-coated bars increases the required lap length.

The required lap splice length for an epoxy coated #5 bar according to the ACI Code is 28 in.   The original sketches I sent you showed 30" long dowels embedded 6 inches into the slab and extended 24 inches into the barrier (for uncoated rebar).  Using 6.5 inches of embedment and extending 28 inches into the barrier, epoxy coated dowels would need to be 35 inches in length.