Typically, when concrete is placed into a form in a precast plant, it is placed continuously or as close to continuously as possible, so that no cold joints form. (Also, not normally a back and forth motion). More importantly, anytime new concrete is placed on existing concrete, care needs to be taken to watch the flow of the concrete and assure the new concrete is mixing with the existing concrete. And if it is not mixing well together, the simple solution is to use a vibrator or a rod to mix the two layers of concrete.