Concrete spalling occurs mainly due to 2 reasons:
1. Carbonation. Iron/steel rods corrode due to carbonation (penetration of gases such as sulphur dioxide, carbon dioxide etc) and increase in volume leading to cracks in concrete and ultimately spalling.
2. Corrosion of rods due to moisture penetration into concrete. Porous concrete/lack of water tightness.
. Concrete aggregates such as sand and jelly.
. Hack the surface to obtain a sound concrete surface.
. Remove all debris.
. With a rust removing agent clean the iron/steel rebars.
. Prepare a mix/bond slurry by mixing Latex Bonding Agent : Water : Cement in the ratio 1:4:8 by weight.
. Apply over the surface to be repaired using a hard broom or brush.
. Prepare a concrete mix with the Latex Bonding Agent as an admixture in the concentration of 5-10% by weight.
. While the bond slurry is still in the wet condition, apply the concrete mix using a trowel.
. Plaster over the concrete to get an even surface.
Voila... repaired concrete!