Add backwall approach slab dowels to existing 50+ year old bridge

[boz9999]

Guys

We are replacing a approach slab on a state road project in florida. The approach slab is for an existing bridge approx. 50 years old. The standards call to dowel and expoy #8 bars into the existing bridge's backwall which the approach slab will be poured onto. However my concern is that since the backwall is old we dont want to start spalling the concrete and make a bad situation worse. So my question is

1. There is an existing sleeper slab that was demo-ed to below the approach slab. Can we just dowel into that concrete to provide a resistance to the lateral movement of the slab or do the dowels serve another purpose besides just the lateral movement (possibly to ensure the approach slab doesnt jump up or down on the backwall with traffic loading.

 

[Qshake]

The dowels are to prevent separation of the approach slad and the abutment that will ultimately lead to the settlement of the slab and the big bump typically experienced before riding on a bridge. The dowels also act to transmit load into the backwall and into the bearing beam/pile. This, of course, assumes there is no haunch or corbel for the slab to sit on at the abutment wall. Details for this vary by state.

The concern I would have is not effects from drilling/epoxying the hole but the capacity of the backwall connection to the bearing beam. Typical backwalls are 12 inches wide and reinforced with #5 or #6 bars. So there isn't much capacity there.

You say this is a standard from the DOT, you may want to inquire with them what their philosophy is regarding this.



Regards,
Qshake

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[boz9999]

Thanks for the reply.

When we demo-ed the existing slab, there was no dowels connecting the approach slab to the backwall and therre did not appear to be any movement of the approach slab, however, there was concrete pavement behind the slab so there was no where it could move. But we did dowel into the wall and poured the slab, however the road had to be opened to traffic by 6:00 AM and the concrete only had 18 hours to hydrate and cure, so the early breaks indicated a compressive strength of 2500 PSI, far below the required 5500 PSI design strength, however the slab appears to be holding up well, no cracks yet.

Oh and on the bottom mat of steel the contractor thought the transverse #5 bars were at 12" centers, however the standards called for 9" centers so they used the correct spacing for the rest of the mat but we we allowed the contractor to just put a #5 bar in between every other space to prevent any more delays and a lower PSI strength. So one half of slab had 50% of spaces at 6" and the rest had still 12" but we had more bars than called for so the area of steel was in there for that half of the slab, is this acceptable to a structural engineer?


Also i have question about the rebar configuration in say bridge decks and approach slabs

Why does the longitudinal bars in the bottom and top mats in bridge decks go in between the transverse bars (in a typical view looking in the direction of traffic) but the approach slab has the long bars on the bottom and top with transvere in the middle?


Thanks