Numerous factors can contribute to a bond not being formed between the concrete and the reinforcing steel. Most are directly related to construction techniques and quality control on the job site.
Consolidation problems top the list. Concrete placed at a very low slump and not consolidated can "hang" larger aggregate on top of the bar. That can create a void below the bar as the plastic concrete settles. However, a crack typically forms directly above the bar where that has occurred.
Bond formation may also be inhibited when placing an elevated concrete slab while the reinforcing steel is subject to continuous vibration.
The temperature of the steel at the time of concrete placement can also be a factor. Steel placed outdoors in direct sunlight in hot weather can become so warm, it can have a drying effect where it contacts the concrete, preventing good hydration and bonding. On the other hand, steel placed outdoors in freezing weather, even with heated concrete, can retard the set of the adjacent concrete.
Although you don’t describe the condition of the steel, that too can yield some clues. Very light rust is not usually a problem with reinforcement; it creates a roughened surface that may promote bonding. The cement is alkaline enough to strip light rust off and provide an acceptable bond. But there should be no loose scale or flaking. When that is present, failure is likely to occur where the mill scale separates from the reinforcing bars, effectively acting as a de-bonding layer.
Regarding concrete composition, the delamination suggests there may have been excess bleed water and the slab was floated off before the bleed had finished. The excess may have come from water added at the site, often an effort to improve workability, for example. Excessive bleed water may also have collected under the reinforcement bars, thus preventing the bars from fully bonding to the concrete.
Aggregate Research Industries’ Web site (www.aggregateresearch.com) includes topic-specific forums where forum members can ask questions and respond, creating an interactive discussion group. The questions and answers in this problem clinic were based on ARI concrete construction forum postings.