A system and method for forming concrete structures, using foam panels as forms, supported by vertical tracks with horizontal bridge pieces inserted through the forms. The bridge pieces are in the form of triangular webs with coaxial rings for supporting vertical rebars centrally between opposing forms. The coaxial rings on opposing bridge pieces key together to securely hold the vertical rebars. At the corners of the structure, corner retaining extrusions are used having fins for retaning the foam panels, and having various included angles such as 60°, 75°, 90°, 120°, 135°, 150° or 165°. Specific types of corner tie pieces for square, obtuse or acute corners are used to connect together the corner retaining extrusions at the inner and outer corners of the structure.