Stresses Due to Restrained Shrinkage in Concrete Deck Overlays

The stresses, strains, and curvatures due to the shrinkage restraint of new concrete bridge deck overlays by the underlying older substrate are investigated. A time history analysis method is derived that, for each time increment, computes free-shrinkage and creep strains, enforces compatibility and equilibrium using a time-dependent stiffness matrix, and determines incremental mechanical and total strains. A new model for tensile creep strains has test-predicted ratios for experimental results reported by others that average 1.00, with a coefficient of variation of 11%. The resulting stresses and mechanical strains are nonlinear across the depth of the member, with large stress gradients in the top and bottom faces of the overlay at early ages of drying. Simplified analytical methods proposed by others are often not accurate: neglecting swelling of the substrate underestimates the mechanical strains, neglecting tensile creep markedly overestimates the mechanical strains, and assuming uniform free shrinkage through the overlay thickness initially overestimates the mechanical strains but subsequently underestimates them at older ages. The studies also found that application of a waterproofing membrane at the top of the overlay 3 days after the end of curing has very little effect on the maximum tensile stresses in the overlay. The age-adjusted equivalent modulus method accurately estimates the overlay tensile stress at early ages, but fails to predict the time of cracking.