Several studies have demonstrated that structural members made of post-consumer recycled polymers can be used in construction, but with some precautions. First, the compressive, flexure, and tensile strength depend on temperature and decrease during the summer. Second, addition of wood fiber generally improves the mechanical properties of the members. Third, high variability in the properties of materials from the same manufacturer, and even greater variability across different manufacturers. Finally, the properties varied by manufacturer, composition of the waste stream, and production season. All these studies support the use of recycled polymers to manufacture piling.
Typical Dimentions of RPP (Reinforced Polymeric Piling)
- Diameter = 10 to 16 inches (25 – 40 cm)
- Maximum length to accommodate shipping = 23 m
- Extruded HDPE matrix
- Fiberglass or Steel structural Reinforcement (6-16 bars)
- Additives to enhance durability (typically carbon)
- Foaming agent to reduce density at the center
Typical Properties of RPP Materials
The nominal properties of RPP component materials (FRP and HDPE) are shown in Table 1, along with those of conventional pile materials.
Table 1- Typical mechanical properties of piling materials
Note: The properties of HDPE and FRP are highly dependent on the temperature, material composition, foaming, and manufacturing procedures. The strengths reported in this table must be confirmed through actual testing prior to the final design of any structure.
Polymers exhibit a significant viscoelastic-viscoplastic behavior, which might have important consequences for creep behavior and soil structure interaction. For example, the ultimate compressive strength of recycled HDPE increased from 15 to 28 MPa (2200 psi to 4000 psi) when the rate of loading increased from 0.003 %/min to 3 %/min (Fig. 1). FRP (E-glass or fiberglass) reinforcement is a composite material consisting of two components: glass fibers and resin. The glass fibers are placed longitudinally along the length of the bars, forming a bundle that is glued together using the resin. A spiral support (stirrup) is also wound around the longitudinal bundle and glued to it, which makes it difficult to test coupons of E-glass in compression. The stress-strain of FRP is presented in Fig. 2. The ultimate compressive strength of coupons of FRP ranged between 140 and 275 MPa (20 and 40 ksi).
Fig. 1- Compressive stress versus strain behavior of recycled HDPE at indicated strain rates (in %/min)
Fig. 2- Compressive stress versus strain behavior of E-glass at indicated strain rates
FRP exhibited some viscoelasticity, although the ultimate compressive strength was also influenced by the unwrapping of the stirrup supports. The use of foaming agents in the HDPE matrix of RPP complicates the stress-strain response. When tested at the same strain rate, the physical and engineering properties of most polymeric piling typically exhibit a high coefficient of variation, particularly when recycled plastics are used. Nevertheless, for RPP this scatter can be largely attributed to the spatial distribution of the compressive strength and density within the specimens. The compressive strength and density of RPP increased exponentially with distance from the center of the pile (Fig. 3). The compressive strength was also found to be linearly proportional to the density (Fig. 4). Therefore, strength is inversely proportional to the degree of product foaming, which is related to density.
Fig. 3- Density distribution of foamed polymeric piling
Fig. 4- Linear relationship between density and compressive strength measured at 10% strain for foamed polymeric piling specimens.
All these studies support the use of recycled polymers to manufacture piling.
Primary References
- Iskander, M. and A. Bozorg-Haddad (2011). “Spatial distribution of the compressive stress-strain of recycled polymeric piling,” Journal of Testing and Evaluation, Vol. 39, No. 4, pp. 706-717, doi: 10.1520/JTE103198, ASTM.
- Iskander, M., and S. Hanna (2003). “Engineering performance of FRP composite piling,” Proceedings. Transportation Research Board Meeting, CD, Paper No: 03-2959, National Academy Press.
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