Honeycombing of concrete.
Honeycombing is the term used to describe areas of the surface that are coarse and stony. It may be caused by insufficient fine material in the mix, perhaps due to incorrect aggregate grading or poor mixing. This can be corrected by increasing the sand and cement content of the mix and by proper mixing, placing and compaction. Alternatively, honeycombing may be caused by leakage of grout or mortar fraction from the concrete at construction or formwork joints. The obvious solution here is to ensure that joints are well sealed and leak-free.
Small, shallow areas of honeycombing are probably mainly cosmetic. However, deeper areas will lead to a local reduction in the protection to the reinforcement from the concrete cover and hence possibly durability problems in the future.