RELATIONSHIP OF RECYCLED PLASTIC LUMBER MECHANICAL PROPERTIES AND DESIGN CONSIDERATIONS

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1 RELATIONSHIP OF RECYCLED PLASTIC LUMBER MECHANICAL PROPERTIES AND DESIGN CONSIDERATIONS Vijaya K. A. Gopu and Roger K. Seals Department of Civil and Environmental Engineering, Louisiana State University Baton Rouge, LA 70810, USA ABSTRACT The mechanical properties of recycled plastic lumber members are influenced by a variety of factors. These factors include the composition of the feed stock; member size ; service temperature ; service stress level ; duration of loading ; and the orientation of loading. An extensive experimental study was undertaken to investigate the influence of these various factors on the strength and stiffness properties of plastic lumber. The results of the experimental study are presented in this paper and the relationship between the mechanical properties and the design considerations is highlighted. INTRODUCTION Plastic lumber, as defined in the ASTM Testing Standards, is a manufactured product composed of more than 50 weight percent resin and in which the product is generally rectangular in cross-section and typically supplied in board and dimensional lumber sizes, may be filled or unfilled, and may be composed of single or multiple resins. The products are currently manufactured predominantly with recycled post consumer or industrial plastic waste. The current focus of the plastic lumber industry is to enhance the application of this material in a wide variety of outdoor structures, such as, decks, docks, seawalls, piers, wharves, boardwalks, etc. While these structures can be classified as secondary structures, they nevertheless necessitate the application of engineering design principles for code and client acceptance. In order to satisfy engineering design requirements, the material (actually the facility) satisfy appropriate strength and serviceability criteria under service loads. While the strength requirements can be easily met by plastic lumber, the serviceability (deflection) requirements are of concern. The flexural stiffness of plastic lumber varies between ksi depending on the nature of the resin mix. Higher stiffness values can be achieved by using fillers such as fiberglass. Since temperature and creep under sustained loads have a significant influence on the stiffness of this material, it is important to accurately account for these effects in design. ASTM Committee D has been pursuing the development of testing and product standards, specifications and application guidelines for plastic lumber products for several years. Standard test methods for density, compression, flexure, creep, shear, connections, and linear expansion have been developed. These test methods provide a uniform and consistent means to evaluate and report the mechanical properties of plastic lumber products. The non-homogeneity of the plastic lumber product arising from the manufacturing process (which produces a definite core-shell cross-section) yields different strength and

2 stiffness values at a standard laboratory temperature depending on the member size and the orientation of the member with respect to the applied load. SCOPE AND OBJECTIVES The objective of this paper is to report the results of a comprehensive mechanical properties testing program and highlight the implications of these findings on the design of structural members and systems using the plastic lumber products. The testing program involved both short term and long term (creep) tests and was conducted in according with the ASTM standard test methods. The results of the test program show the following : a) the range and variability of the mechanical properties between products produced by different manufacturers b) the influence of the member non-homogeneity on the mechanical properties evident from the change in values for different sizes and member orientations c) influence of temperature and creep on the strength and stiffness of the members It is pertinent to point out that ASTM testing standards for plastic lumber products require that they be tested using the entire cross section and not a coupon because of the concern for non-homogeneity. The results reported in this paper are for the entire element and not coupons cut from the total section. Thus, the reported properties are product properties not material properties. EVALUATION OF THE INFLUENCE OF MEMBER NON-HOMOGENEITY A series of flexure and compression tests were conducted on plastic lumber products of different sizes and obtained from different recycled plastic lumber manufacturers. These tests were conducted in accordance with the ASTM test standards. The flexural test specimens were tested in both flat-wise and edge-wise loading conditions. The tests were conducted in the laboratories of the Department of Civil and Environmental Engineering, Louisiana State University. The work was supported by the Louisiana Quality Support Fund Industrial Ties Program. The test results obtained from this series of tests are presented in Tables 1 and 2. The results of these tests clearly show that the mechanical properties are dependent on the manufacturer and a single value cannot be assigned to plastic lumber products. The influence of non-homogeneity is evident from the variation in the properties with member size and orientation of loading with respect to the member axes. The tests also show that the variation in the mechanical properties for these products is relatively low and the penalty associated with the use of a lower 5 th percentile values at a given confidence level is within manageable limits. The results presented enable the designer to establish the basic strength and stiffness properties but do not account for influence of temperature or creep.

3 Table 1. Compressive Strength and Stiffness Properties Manufacturer Member Size Average 3% Strain Average Secant 1% Strain 1 2x X X x X X x X X Table 2. Flexural Strength and Stiffness Properties Manufacturer Member Size & Orientation Average 3% Strain Average Secant 1% Strain 1 2x4 Flat X6 Flat X x4 Edge x6 Edge N/A N/A x4 Flat X6 Flat X x4 Edge x6 Edge N/A N/A x4 Flat X6 Flat X x4 Edge x6 Edge N/A N/A EVALUATION OF THE INFLUENCE OF TEMPERATURE AND CREEP ON THE MECHANCIAL PROPERTIES The flexural strength of plastic lumber is defined as the stress at 3% outer fiber strain in ASTM D Since the testing standard requires the specimens to be strained at the rate of 1% per minute, the strength values are determined in a 3 minute test duration. Normal duration of loading for the design of structural elements is generally taken as 10 years of cumulative duration. The creep characteristics of plastic lumber require a quantitativeevaluation of the influence of load duration and temperature on the strength and stiffness properties in order to develop a design methodology.

4 An extensive creep test program carried out on 2x4 and 2x6 plastic lumber members at different stress levels. The tests were conducted for a duration of 1000 hrs and an emperical creep model was developed based on the test results. This creep model is given as follows : ε = ( E E -7 t ) e (T/58.948) σ 1.3 (1) where ε = strain in the outer fibers at time, t t = elapsed time after initial loading at which strain is being evaluated in hours T = service temperature in F σ = calculated outer fiber stress (sustained) in psi The stress levels required to induce a desired strain level at two different specified service temperatures and eight different durations of the stress are presented in Tables 3 and 4. Table 3: Stress required to induce desired strain level (For T=100 F and failure strains of 1%, 2% & 3%) Duration of Load Stress required to induce desired failure strain 1% strain 2% strain 3% strain 1 min min min day days months year years Table 4: Stress required to induce desired strain level (For T=122 F and failure strains of 1%, 2% & 3%) Duration of Load Stress required to induce desired failure strain 1% strain 2% strain 3% strain 1 min min min day days months year years

5 CONCLUSIONS The results of this study clearly show that the mechanical properties of plastic lumber are dependent on the manufacturer, i.e. the resin mix used as feedstock, the member size and orientation of loading. The results of the creep study show the need to adjust the flexural strength and stiffness values to account for the effect of creep and temperature. The basic concepts for developing load duration and temperature correction factors are presented. The need for enhancing the stiffness properties, particularly the creep characteristics, of plastic lumber is evident from the study. Methods to improve the stiffness and creep properties of these members by addition of fillers such as fiberglass needs to be aggressively explored if this product is to be utilized in a wide variety of structural applications. REFERENCES ASTM Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastic Lumber American Society of Testing Materials D , Philadelphia: ASTM ASTM Standard Test Method for Compression Properties of Plastic Lumber and Shapes American Society of Testing Materials D , Philadelphia: ASTM ASTM Standard Test Methods for Compressive and Flexural Creep and Creep-Rupture of Plastic Lumber and Shapes American Society of Testing Materials D , Philadelphia: ASTM Levitan, Janet E., Evaluation of the Mechanical Properties of Recycled Plastic Lumber., M.S. Thesis, Louisiana State University, Gopu, V.K.A. and Seals, Roger K., Mechanical Properties of Recycled Plastic Lumber and Implications in Structural Design, Composites International, Session 7-C, Cincinnati, OH, Martinez, Jose Noe, Flexural Creep Analysis of Recycled Polymeric Structural Elements, Ph.D. Thesis, Louisiana State University, 1999.