The terms form streaking and sand streaking often are used interchangeably because the underlying causes are very similar. And both conditions are aggravated by harsh concrete mixes containing too few fines, overly wet mixes, and excessive vibration.
Form streaking results from the loss of bleedwater, paste, or mortar through leaks in the forms, either through the joints between form elements or around form-tie openings. As a result of this leakage, areas of fine or coarse aggregate, usually with no specific orientation, are exposed on the concrete surface. Sand streaking results from excessive bleeding along the form face. As the bleedwater moves upward, it carries cement grains and very fine aggregate particles along with it. When the forms are removed, vertical streaks of these aggregate particles are visible.