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Hi Dr. Faller,

I have looked through the AASHTO Design and Construction Manuals
and I can’t find an acceptable tolerance for the following situation.  We
have two parapets meeting at an intentional bond breaker and their depths don’t
align such that there is a smooth surface with the traffic.  What would be
an acceptable offset such that snag would not be a concern?

There were two studies that addressed raised or depressed asperities in the face of concrete barriers. One study was performed by CALTRANS and the other study was performed by TTI as part of a NCHRP study. I think the guidance from those two studies may be applicable for guiding your uneven vertical surfaces in concrete barriers. 

We have previously suggested a maximum lateral offset of 1” for concrete. For steel hardware, we have observed problems with a 3/8” offset and thus guided a value of around ¼” or less. With concrete, there may be fracture if short. However, in long solid sections, it could be more problematic. I might suggest the use of 45” bevels or flatter with a 1” maximum offset. It may be better to limit concrete offsets to ½” or less to lower risks.

http://mwrsf-qa.unl.edu/view.php?id=632

http://mwrsf-qa.unl.edu/view.php?id=371

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