Reinforcement ripple is the name given to a surface irregularity that sometimes occurs on the surface of large areas of flat concrete slabs. It takes the form of shallow troughs over the line of the reinforcement after the concrete has been finished. In some cases this is just in line with the upper bars in the top mat but in the worst cases ripples are formed in both directions, giving a quilted effect. The probable cause is vibrations set up in the reinforcement, which results in additional compaction of the concrete around the bars in the finished areas, resulting in the depressions seen.
It is an aesthetic, not a structural or durability problem. There appears to be no way of preventing it when the method of finishing the concrete is by a skip- or bull-float or similar methods. The only known way of overcoming the problem of reinforcement ripple is to carry out further finishing operations on the slab such as power-floating or power-trowelling, both of which prolong the finishing operation and therefore tend to be more costly.
For further information see Concrete Advice No. 06, Reinforcement ripple.