An apparatus and method for on site pouring of pre-stressed concrete structures. A form, having elongated sides spanned by first and second bulkheads at each end, is assembled on a building site. The form defines the size and configuration of a structure, and has an upper edge determining the upper surface of the structure. A transportable cable pre-stressing frame is provided, having a first end and a second end with respective outrigger assemblies and at least one pre-stressed cable extending therebetween. Adjustable means are provided for positioning and maintaining the frame over and generally in longitudinal alignment with the form, the pre-stressed cable lying below the form's upper edge. Concrete is poured into the form, covering the cable. After curing is complete, the portions of the cable extending outside the concrete structure are cut, releasing the tension on the cable and transferring it to the concrete.