A mat assembly particularly suited for heavy equipment transit and support, such as roadway construction and the like. The mat assembly is made up of a number of structural members preferably having a "double I-beam" cross-sectional shape. Each I-beam has spaced apart flanges with edges preferably formed in tongue and groove profiles. When butted together, the tongue and grooves of adjacent I-beams mesh. Filler blocks are disposed in the cavities between the webs of the I-beams. Tension members extending through the webs and filler blocks tie multiple I-beams together to form the mat assembly. Preferably, both the I-beams and filler blocks are formed of a plastic material, via extrusion or other molding methods. The resulting mat is capable of bearing very high loads yet is decay resistant.