Recently, many bridges have failed to perform under service loads, scour, floods or earthquakes. Meanwhile, evidence suggests that a large number of bridges may be deemed structurally deficient without justification by present inspection and rating procedures. Many RC (Reinforced Concrete) slab bridges, which are still useable, are now being replaced before full utilization of their inherent capacity because of a lack of understanding and knowledge of the effects of deterioration and aging on these bridges. To establish procedures that allow for the full utilization of RC slab bridge capacity, a 38-year old sample was loaded to failure. The bridge, which was decommissioned due to its age and deteriorated state, endured the equivalent loading of 22 rating trucks before failure. The research was designed to study and determine the abilities of different NDE (Nondestructive Evaluation) techniques to detect damage and to examine processes for improving the condition evaluation and rating of concrete slab bridges. Accurate condition evaluation and rating of bridges is key for improving the present rating and decommissioning practice. Through this research a better understanding of the complex behavior of slab bridges at all their limit states has been achieved. Improvements in NDE and NLFEA (Nonlinear Finite Element Analysis) have been accomplished.