Abstract: A modular wall construction system includes a box-like block form of expanded foam plastic material such as polystyrene having opposite, parallel, spaced apart sidewalls and endwalls extending between upper and lower surfaces and defining an internal cavity for receiving concrete slurry. A plurality of transverse bridge members maintain spacing between the sidewalls at spaced locations along the length thereof. Each bridge member includes a central web extending between opposed tongues which are slidably received in T-grooves formed in the sidewalls. A cylindrical boss member is integral with the central web and substantially equidistant between the opposed tongues and has an outer peripheral surface and a vertically extending bore for slidably receiving a vertical reinforcement bar.