Environmental Benefits of Plastic Piling
Durability
In the United States, the deterioration of conventional concrete, steel, and timber piling systems results in costs of nearly $1 Billion annually for repair and replacement. A significant increase in marine borer activity has been observed in many harbors and coastal waterways since the enactment of the Water Pollution Control (Clean Water) Act of 1972 (Fig. 1), resulting in widespread damage to marine timber infrastructures.
Fig. 1. Increase in Marine Borers’ Attack in NY-NJ Harbor
Teredo, Bankia, and Limnoria are the three most common and destructive borers. Teredo and Bankia (ship-worms) enter the wood as a larva and follow the grain, tunneling deeper as the worm grows. Numerous tunnels in a timber pile make the wood’s interior as holed as Swiss cheese (Fig. 2). Limnoria nibbles at the outside edges of timber piles, causing timber piling to lose up to 1 in. in diameter annually. The most effective method of reducing marine borer attack is pressure treatment with creosote and arsenate. However, treatment poses health risks and does not stop borers from attacking the wood completely.
Fig. 2. Typical Marine Borers’ Attack in NY-NY Harbor (Teredo on LHS, Limnoria, RHS)
Steel and concrete piling perform somewhat better than timber in waterfront environments, but they are by no means immune to deterioration (Fig. 3). The major cause of deterioration of steel piles is corrosion, especially in industrial and marine environments. The rate of corrosion in regular soils is believed to be on the order of approximately 0.03 mm per year, and it increases to 1.2 mm per year in the splash zone. Coatings containing heavy metals can slow the corrosion of the steel piles, but these treatments are both harmful to the environment and not entirely effective. The most destructive agents for reinforced concrete piles are sodium and calcium chlorides. These salts penetrate through the concrete cracks to the reinforcing steel and form an electric cell that causes the reinforcement to corrode. This corrosion process is accompanied by expansion, which tends to induce high tensile stresses in the surrounding concrete, causing it to crack and spall. In addition to salts, freezing and thawing of concrete further degrade it.
Fig. 3. Deterioration of Concrete (LHS) and Steel (RHS) Piling in NY-NJ Harbor
Against this backdrop, recycled polymeric piling offers a number of environmental advantages over conventional timber piling for light loading applications in coastal and water front environments. RPP also offers a creative solution for water-front construction . For example, coastal communities recovering from hurricane disasters are now required to build above the advisory base flood elevation, which may result in structures being elevated by as much as 25 feet above ground level, requiring large amounts of exposed piling. Use of RPP in these situations may be advantageous because it is unlikely to be attacked by termites, which feed on exposed timber piling.
Use of Recycled Materials
Use of recycled plastics to manufacture piling offers a solution to the mountains of solid plastic waste which are growing all over the United States and consuming valuable landfill space. It is widely recognized that we use more polymers today than 50 years ago. In the United States, polymers in the municipal solid waste stream have increased from less than 1 % in 1960 to 12 % in 2008, according to the United States Environmental Protection Agency figures. Although the overall recovery of plastics for recycling is only 7 % of plastics generation; HDPE milk and water jugs are recovered at a rate of 26 % of HDPE generation. Therefore, use of recycled HDPE to produce structural members is important because it creates a sustainable solution for use of 14 Million Tons of rigid plastic containers that annually ends in landfills in the United States alone.
Recycled polymeric piling (RPP) typically consists of a thermoplastic extruded recycled HDPE matrix reinforced with FRP or steel rods. Additives are used to improve mechanical properties, durability, and ultraviolet (UV) protection of the HDPE matrix. Foaming of the resin is used to make the product lighter. The HDPE matrix often contains a small percentage of glass fibers to enhance its mechanical properties. One linear meter of 13 in.)diameter RPP typically uses 800 recycled milk jugs in its manufacture. RPP is intended to replace timber piling in marine structures for low to medium load ratings.
Elimination of Creosote & CCA from the Environment
Treatment of timber using Creosote and Copper Chromium Arsenate (CCA) may pose a threat to marine life, particularly when a large number of piles is involved. Workers who handle creosote and CCA treated timber are also exposed to hazardous materials during manufacturing and installations. Additionally, treated timber present a growing environmental disposal problem since creosote is listed as a toxin by The Environmental Protection Agency.
Preserving Old Growth Forests
Wood products are becoming increasingly more expensive and difficult to obtain, particularly as regulations to protect old growth forests and the habitat of the spotted owl were enacted.
Primary References
- Iskander, M., and Stachula, A. (1999). “FRP composite polymer piling, an alternative to timber piling for water front applications,” Geotechnical News, 17(4), 27–31.
- Iskander, M. (2012). “Sustainable piling made of recycled polymers, state of the art review,” Journal of ASTM International, Vol. 9, No. 2, doi: 10.1520/JAI103677, ASTM.
