Compressive strength and creep of recycled HDPE used to manufacture polymeric piling

Fiber Reinforced Polymer (FRP) composites represent an alternative construction material that can alleviate deterioration of steel, concrete, and timber piling in marine, water front, and aggressive environments. However, unlike conventional construction materials which have well documented creep behavior, there is virtually no reliable data on the creep behavior of FRP, particularly in compressive loading. In this study, an accelerated test method to predict the compressive creep of recycled HDPE used to manufacture piling is proposed. The method is based on the equivalence of strain energy density (SED) between conventional constant-stress creep tests and stress-strain tests, conducted at different strain rates. Good agreement between the creep rates obtained from a conventional test and SED predictions were achieved when two stress-strain experiments with strain rates differing by two or more orders of magnitude are used. Test results indicate the tested HDPE loaded in compression will creep by less than 1% in 100 years when loaded at an ultimate stress of 6.9 MPa (1000 psi). SED was also used as a basis for predicting the onset of tertiary creep. Finally, onset of tertiary creep was utilized for rational selection of the ultimate strength of viscoelastic materials, whose strength is inversely proportional to duration of loading. The ultimate strength of the recycled HDPE tested in this research ranged from 15 to 28 MPa (2200 to 4000 psi). However, the ultimate strength corresponding to 100 years of service was predicted to be 11.7 MPa (1700 psi) using SED, which represents 40-60% of the ultimate strength obtained from conventional stress-strain tests.