huangyhg

repairing thin concrete roof panel
fellers,
i need to replace a 24"x20" section of a precast roof panel that was damaged above an elevator shaft. the panels span 8', are 24" wide with 3-1/2"x3-1/2" ribs at the edge of the panel and are 1.25" thick in the center. there is a small 4x4 mesh in the center of the 1.25" middle portion that i will replace and also dowel into the edge ribs. the concrete is a very small aggregate (1/4" max?). what should i specify for the small area of concrete needed to be filled in? what is the smallest diameter bars i can use to dowel into the 3-1/2" rib to lap with new mesh reinforcing?
thanks

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looks a specialty works, special concrete with high adhesive and tensile/shear stregth. you may use drilled connectors, so, small size.
the major worry is loss of section causing the beam fail whilst on works, so chec the status with the loss of section standing.
if i see it right than existing "small 4x4 mesh in the center" is intact and not damaged.
1. carefully remove the damaged concrete.
2. clean the mesh rebar
3. clean the exposed conc face with steel wire brsuh to expose aggregates.
4.apply bonding agent.
5. place a form work below.
5. fill with good epoxy samll aggregate concrete.
6. better order concrete from same precast plant.
the only things i would add to dgkhan is:
- try to square the corners of the patch area (sawcut 1/4" deep and remove concrete)
- place a sealer around the edge of the patch work
i wouldn't worry about a bonding agent(just wet the existing surface with water) and i wouldn't order concrete from the precast plant (logistics nightmare)
if you're really concerned, you could even extend the patch to over the top of the adjacent joists, but this is not necessary with the reinforcement still intact.
agree with dgkhan's recommendations,except that these are roof panels and using an epoxy, even filled with small aggregate, will be a problem due to the significantly different coefficient of thermal expansion.
use a polymer modified concrete repair mortar such as sikatop 123. if the repair has to be watertight, go from the bottom side after repairing the top and cover the entire area with a carbon fiber overlay to about 6 inches beyond the repair on all sides.
these are precast channel plank which are commonly available. i would suggest just replacing the panel rather than going to the expense of patching. the roofing has to be redone anyway.

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