Effect of Concrete Overlay on Deck Rating

[LobstaEata]

I have been reviewing deck rating calculations for a bridge in New England (USA), when I ran across the rating engineer using all but 3/4" of a 1 1/2" polymer modified concrete bridge deck overlay in the compression zone calculations. I'm looking for someone that might have information regarding the reliability of PMC overlay bond with the reinforced concrete deck, which would logically be the controlling criteria for including a concrete overlay in the compression zone of the deck rating.

Use of an epoxy bonding compound betwen a bridge deck and concrete overlay has traditionally produced reasonably good results, but I have no idea how well the bond would be with a PMC overlay.

Does anyone here have experience with this subject or point me in the right direction to research it?

Thanks much,

Steve

 

[3Fan]

Our state DOT does not allow ANY overlay to be included in the helping of the rating of a structure. In essence we add in the overlay as dead load only. I'm not sure if the DOT has provided testing for this blanketed statement, but this is our guidance.

 

[LobstaEata]

@3Fan: Score one for the "do not include" side of the debate. Thanks.

This NE State, for which the rating is being produced, apparently has allowed A PMC overlay to be used on prior ratings. This practice is questionable in mind mind however, due to numerous delaminations and intermittent spalls of PMC overlays over the years leading to this state no loner using PMC overlays at all. I therefore am of the opinion that it would be unwise to count on the bond between the overlay and substrate to the extent that would justify use in strength calculations.

 

[bridgebuster]

I would consider the overlay as dead load unless it was mechanically bonded, ie. the original deck was removed to a point below the top mat of rebars.

 

[3Fan]

LobstraEata - it's your stamp going on the rating, I'd go on the side of being conservative.

bridgebuster - side note, how far down below the top mat of steel would you suggest going to be able to include a SDC (superplasticized dense concrete) overlay as part of the original deck?

DOT wants to remove 1" of the orignal 7.75" deck and then slap on a 2.5" SDC overlay. There is only 1" of clear cover to the top mat. Therefore the top mat would be exposed. I keep fighting them that this isn't a good idea. But if we removed say another 5/8" to get below the top mat (transverse bars anyhow), I might be a little happier to let that slide as I could see including the overlay as part of the original deck then.

Oh ya and the original deck was cast in 1948!

 

[bridgebuster]

3Fan - rule of thumb (pardon the pun) if you can get your index finger under the bars you're OK. However, I've generally seen 3/4" to 1 1/2". My preference (nothing scientific about it)is about 1", just because it's easy to measure.

Coarse aggregate for a typical mix would have 90-100% passing a 1/2" sieve and 15% passing a 1/4" sieve.

Hope this helps.

 

[bridgebuster]

Forgot - 100% passing a 1" sieve.

 

[TomDOT]

After re-checking my binder from the FHWA bridge maintenance class, they have three types of deck repairs: Type A is above the the top layer of rebar. Type B is at least 1" below the top layer of rebar and Type C is a full-depth. In the field, I have typically seen 1" clearance around the rebar for repairs to ensure aggregate lock.

 

[LobstaEata]

@3Fan: No, my stamp is not going on the calcs, thank goodness. On the other hand, I have been tasked with checking the calculations of the younger PE preparing them, so my initials will likely be added to the calc sheets. I'm probably going to have to challenge the State DOT's policy on this one. Always a thrill to do that!

 

[MichSt]

Do you have a current inspection report for the bridge? IS the overlay currently debonded/spalled, etc? If not, I would agree with the state to rate it with the overlay included in the capacity. When/if it debonds the rating would be updated at that point. Just have your assumptions stated clearly.

 

[LobstaEata]

@MichSt: Interesting approach. I tend to err on the conservative when I am unable to predict the potential for failure of a certain structural element that would lower the safe capacity load rating of the system. Of course, If I was protected from prosecution under the State's sovereign immunity, then perhaps I might feel differently. Thanks for your post. It's good to hear from the other side of this issue.