Plastic forms for poured concrete

In a hollow foamed plastic form for concrete having a pair of opposed parallel side walls with planar upper and lower faces, and a pair of opposed end members, the improvement which comprises:

• (i) a lip coplanar with the outer surface of, and extending outwardly from the upper and lower faces of, each of said side walls and having a first height;
• and
• (ii) a plurality of like, outwardly extending, elongated locking members defined in registered spaced relationship on the upper and lower faces of each of said side walls inside said lip, each locking member on one side wall being spaced from a corresponding locking member on the other side wall at a distance corresponding to the length of each locking member;
• each of said locking members consisting of a pair of first and second elongated ribs of uniform thickness and a uniform height which height is nominally twice the said first height of said lip, said first rib having a length which is greater than that of the second rib by the width of said ribs, each rib having, in top view, a trapezoidal shape defined by
• (a) first and second parallel vertical planar surfaces which surfaces are substantially parallel to the side walls of said form,
• (b) a third vertical planar surface perpendicular to said first and second planar surface, and
• (c) a fourth vertical planar surface intersecting said first and second planar surfaces at angles of 45.degree. and 135.degree., respectively,
• said first rib of each pair being disposed more proximate to said lip than said second rib of the same pair with the third planar surfaces of the two ribs being coplanar and the second planar surface of said first rib being coplanar with the first planar surface of said second rib of that pair.