Modular plastic structural composites having a web section disposed along a horizontal axis and at least one flange section disposed along a horizontal axis parallel thereto and integrally molded to engage the top or bottom surface of the web section, wherein said composite is formed from a mixture of (A) high density polyolefin and (B) a thermoplastic-coated fiber material, polystyrene, or a combination thereof. Composites molded in the form of I-Beams and bridges constructed therefrom are also disclosed.