At present it is common to treat an overlaid concrete pavement system as a multilayered Kirchhoff plate. A number of finite-element computer programs use this idealization, which assumes that the original slab and the overlay have the same deflection profile, that is, that the two act as an effective, homogeneous plate. Environmental loads, however, are among the most significant factors, making the assumption of equality of the deflection profiles of slab and overlay unacceptable. A formulation proposed by Totsky is implemented into an existing finite-element program to account for the effects of layer separation and compressibility. The effects of combined traffic loading and temperature differentials on the stresses and deflections in the pavement system when the overlay curls away from the existing slab are examined. The product is a finite-element code, abbreviated ILSL2, which represents an extension of the ILLI-SLAB program. Practical illustrative examples of the use of ILSL2 pertaining to typical pavements are presented. When appropriate these results are compared with existing analytical or numerical solutions, including those obtained by using an earlier version of ILLI-SLAB. It was found that both layer compressibility and separation effects need to be accommodated in a reliable mechanistic model for unbonded concrete overlays. If these effects are neglected, significant errors may result.