Bug holes, air holes or voids, whatever one may call them, are in fact a big problem in the precast concrete industry. There are many articles written on how to make them disappear. It has not been totally achieved as of yet. Certain mix designs (consolidated concrete) that have been incorporated into the larger computer run batching plants are having more success than smaller plants with smaller batching systems.
Bee holes are leftover entrapped air and water or both. Precast concrete products that have a cylindrical shape are less likely to retain the air and water that create bug holes than a mold that is flat on the side and more than one to two inches high.
When these molds are being filled with concrete and vibrated, the air and water in the concrete need to escape with the vibration. As these particles of air and water move upwards to the top of the mold, eventually some of them collide and attach to each other. The scientific philosophy, (only one object can occupy a space at any one given time) applies here.
At this point, they are less likely to escape from the mold and cling to the side of the mold. When the parts are demolded, the air and water pocket has evaporated therefore leaving a void in the product.