Patch Concrete Foundation

You can pour then rest of the concrete on top without any problem.

This is the same thing that happens when you pour columns and then pour the slab. The concrete just sits on top of the previous layer.

Especially as it is foundations, this is even less of a problem since the waterproofing between the layers of concrete will not be an issue at all and so no roughening of surfaces is necessary. Just give the layer a good clean with the water jet beforehand.

A slab on top of a column does not need to develop bond because the stress between the two is pure compression.

The two sections of a footing must be tied together to make it act monolithically. Use dowels of sufficient number to develop the horizontal shear between the two sections. Do not rely on chemical adhesives. They are ineffective unless conditions are perfect.

Surely there are vertical bars between the lower bars that the OP mentioned ?

Not necessarily. If it is a square column footing, it could have horizontal bottom bars running each way with nothing but the column dowels vertically.

It might help if we had a sketch showing the size of the footing and the loads involved.

I would suggest that it is unclear as to what is actually occurting. More info would be welcome.

Umm, 20cm. Clean the surface good like a good construction joint, dowel it (if the project engineer is concened about it) and pour on top.

Where are you?

If there is a substantial amount of rebar sticking out for the new concrete to grab onto, you'd be OK. If not, (if it were me) I'd drill some over-sized holes in the concrete and insert some additional rebar, and cement them in place, and I'd use an acid wash over the surface of the existing concrete before I'd pour anymore concrete. -- But that's just me. You need to see what your local code requires.

I do not know enough about your foundation to know whether there is shear at the interface or not. If the bearing area on the soil is significantly larger than the column, there will be bending of the foundation and shear at this interface. The US code calls for a 1/4", say 6mm profile for dry joints. I would back chip the surface to the 6mm profile if there is any doubt.

We sometimes spray the surface of a day joint with a retarder, leave time for the rest to set up and then hose the unset concrete from the surface, leaving the aggregate profile. You should not need any extra reinforcing steel.

Check out how a "Silo" is poured. If the column or Footings, are critical in "required" strength, I would be very careful how I proceed. If it is a "racing Pigeon " cage, I think it could be alright., but do ensure that no rogering pigeons are allowed inside.

Mark N.

PS. This comment is from an Electrician, who knows stuff, and pigeons.

If this is a non-critical section of a foundation wall (meaning that it is not taking much lateral load from the backfilled soil), then you must roughen-up the old concrete surface where the new concrete will be bonding to the old. Also, make sure you thoroughly moisten (soak it long and good) the old concrete bonding surfaces with clean potable water prior to placing your new concrete. This will moisture in the old concrete will lessen the new concrete from drying out too fast where it comes in contact with the old concrete.

I'd play it safe and add the vertical and horizontal steel deformed rebars that must be embedded into holes drilled or notched cavities (pockets) made into the old concrete wall. Make sure you clean-out all drill holes and pockets of dirt and concrete dust so that the concrete can properly bond to the rebars.....use of a high pressure compressed air wand and nozzle is good for this procedure. Provide a minimum of 12-inches of rebar embedment for #4 rebar (1/2" or 12mm diameter). These bars will help transfer the loads placed on the wall, as well as provide Shrinkage and Temperature (S&T) steel that will lessen the effects of concrete shrinkage and help transfer concrete expansion and contraction forces due to temperature variations. They're a cheap insurance policy for all of you hard work and will help ensure a more durable wall. You'll also will need to provide at least 2-inches (50mm) of concrete cover (from outside bar face to the wall surface) over these bars to prevent corrosion of the steel.

A good idea to less the new concrete mass from drying out, and help it to properly cure, is to leave the wall forms in place for a solid 2 weeks. Otherwise, you're going to need to spray the new section of concrete (both sides of the wall) with a chemical called a "concrete curing compound", or provide a light continuous mist of water spray onto the concrete surface. Of course you may not want to spray water onto the foundation wall down in your basement as it may cause water damage. It may be acceptable to water spray the wall if you have no concrete floor slab in the basement and any water is capable of percolating into the underlying soil mass.

Good luck with your project!

The OP needs to share some more info with us. After rereading his posting I'm unclear if this "foundation" is the footing itself and/or the foundation wall. I was assuming it was the latter.....MY BAD!!!!!!