Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Civil and Environmental Engineering

First Advisor

Vijaya K. A. Gopu


A structural product made of recycled plastics in standard sections similar to dimensional lumber is referred to as plastic lumber. The recycled plastics used in the manufacture of these members include high density polyethylene, polyethylene terephthalate and polystyrene. Plastic lumber is a viscoelastic material possessing mechanical properties that are time and temperature dependent. This structural product is currently used primarily in non-load-bearing and low-end structural applications because of the non availability of engineering data and concerns about its creep behavior. Creep, which is the time dependent deformation under constant stress, is high for plastic lumber and is temperature sensitive. A good understanding of the creep behavior of plastic lumber is important in the design of structural members since this process is often controlled by serviceability rather than by its strength. This dissertation presents an in-depth study of the flexural creep at different stress and temperature levels for plastic lumber. Different methods for modeling the creep response of plastic lumber are presented. These methods include rheological models and mathematical functions based on statistical analysis. The goal of this study was to develop models for predicting the long term behavior of plastic lumber utilizing relatively short duration creep test results. These predictive models are useful in that they eliminate the need to conduct elaborate and expensive long-term creep tests at different temperature levels to establish long-term response of these members. This investigation shows that the long-term deformation of plastic lumber elements under constant load can be several times the initial elastic deformation. In addition to presenting models to predict the creep behavior of plastic lumber, procedures for determining the strength, based on specified failure criterion, corresponding to different durations of sustained loading are also presented.