No single system has yet proven to be suitable for all types of bridge deck overlays. But progress is being made in developing thin polymer concrete overlays that satisfy many requirements: low permeability to water and deicing chemicals; adequate skid resistance; high abrasion resistance; sufficient flexibility; and good bond to existing concrete. Polymer concrete bridge deck overlay systems that have been used include mixtures containing epoxy, coal tar modified epoxy, polyester, methyl methacrylate and polyurethanes.
LESSONS FROM PAST EXPERIENCE
Experience has shown that past failures of bridge deck overlays could be attributed to one or more of the following factors: inadequate surface preparation; concrete deterioration too far advanced; dissimilar physical properties of the overlay material and underlying concrete; chemical incompatibility of polymers and concrete; improper proportioning, mixing or application procedures; and the use of less technologically advanced materials.
REQUIRED PROPERTIES FOR OVERLAY COMPONENTS
Testing and evaluation programs have led to the conclusion that our bridge deck overlay binders need to have the following properties: modulus of elasticity between 90,000 and 150,000 psi; tensile elongation of at least 30 percent; minimum tensile strength of 2500 psi; and compressive strength between 5000 and 8000 psi.
Two methods have been used to apply thin polymer concrete overlays: the broom-and-seed method and the slurry method. In the broom-and-seed method liquid epoxy is applied to the prepared surface by roller, broom, squeegee or spray. Aggregate is then spread over the wet epoxy. In the slurry method, aggregate and liquid binder are mixed to form a very rich mortar or slurry which is then spread uniformly over the deck at the required thickness. Immediately after the slurry has been struck off, additional aggregate is broadcast into the resin-rich overlay to improve skid resistance.