There are many causes such as improper finishing methods and curing methods. Simple repairs can be done fill the void and prevent further deterioration.
1. Chip out the edges of the spalled area and use a hose to clean out the hole.
2. Mix up a batch of concrete repair mortar. If the hole is deeper than an inch, add some small gravel to the mix such as pea gravel. Make sure the repair mortar contains a bonding agent or add some liquid bonding agent to the mix.
3. Place the mortar in the hole and smooth with a trowel. Don't overwork it.
4. When it begins to harden brush it lightly with a damp brush for better appearance.
5. For curing, brush some clear sealer over the surface before it gets too hard and begins turning white. If you don't have any sealer, keep it covered with a damp towell for a few days.
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