In order to further utilize fiber reinforced polymer (FRP) composites for strengthening of existing concrete structures, the load and resistance factor design (LRFD) approach is proposed as a design framework. The statistical nature of LRFD provides a good match for the application of prefabricated FRP composites in a reliability-based design approach. In this work, probabilistic models to describe tensile properties of prefabricated FRP materials are developed and durability test results are used with a degradation prediction model to determine the design value of FRP material properties used in strengthening of concrete over time. Stochastic variation in the FRP is characterized based on tensile testing of several sets of prefabricated FRP composites obtained from three suppliers. A general procedure to determine the design characteristic values of the FRPs is proposed using a two-parameter Weibull distribution. This procedure is then incorporated with predictive degradation equations derived based on the experimental durability data to predict the time-dependent tensile properties of the prefabricated FRP strips and to assess the reliability of the materials over time. The proposed predictive model is used on example girders for illustration of differences between the guidelines design property values and those determined based on considerations of time and reliability. A reliability analysis is performed to compare the results. The philosophy proposed in this work provides a sound approach of considering time-dependent material degradation and reliability to determine FRP material properties values for design so that the values used are reliable and accurate over time.