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46 3.2 FL-DOT Minimum Failure Times Shifted to 20C Extended to 1000 yrs 3.1 y = -0.2694x + 4.5695 3.0 Log Stress (psi) y = -0.2695x + 4.4306 2.9 R2 = 1 2.8 304 yrs 1002 yrs 500 psi 2.7 2.6 0 1 2 3 4 5 6 7 8 Log Time (hrs) Figure 46. Master curve from FL-DOT minimum times. 80C/650 psi. From the slopes of those lines and shifting back strain is also evaluated. The methodology also incorporates a to 80C, the time that represents 1,000 years of service for both vertical arching factor to account for load relief when the pipe samples is 200 h. is less stiff than the surrounding backfill. Table Mechanical Properties of Thermo- Designing Pipe with plastics Pipe in AASHTO LRFD Bridge Design Specifications Recycled Content Section 12 sets forth the requirements for the minimum cell class, the initial and 50-year design tensile strength and the ini- An evaluation of the existing design methodology out- tial 50-year modulus of elasticity. Recently, McGrath et al.'s lined in Section 12 of the AASHTO LRFD Bridge Design Spec- NCHRP Report 631 from project NCHRP Project 04-26, ifications was performed to determine its applicability to "Thermoplastic Drainage Pipe, Design and Testing" recom- pipe manufactured with blends of virgin and recycled HDPE. mends 75-year design tensile strength and modulus of elastic- In particular, the material properties needed to meet M294 ity requirements as well as service long-term tension strain and required for the AASHTO LRFD design methodology were factored compression strain limits. Note that the estimated assessed. modulus of elasticity values proposed in NCHRP Report 631 The design methodology in Section 12 of the AASHTO LRFD for AASHTO LRFD Bridge Design Specifications Section 12 are Bridge Design Specifications (and McGrath et al.'s recent recom- based on observed field performance of pipe designed with mendations in NCHRP Report 631: Updated Test and Design 50-year modulus and relaxation tests on PE pipe in parallel Methods for Thermoplastic Drainage Pipe (13) evaluates local plate tests to estimate 75-year modulus. Values are reported buckling resistance to check the structural capacity of profile to be reasonably conservative. wall plastic pipe. The total factored compressive and tensile The material properties studied in this work included the strains in a pipe wall due to thrust and bending are evaluated minimum cell class (density, MI, flexural modulus, tensile yield to ensure that specified strain limits are not exceeded. Bending stress), NCLS, NCTL, thermal stability, and oxidation induc- tion time. Long-term properties (50 and 100 years) included creep modulus, creep strain, yield stress, stress-crack stress, service long-term tension strain and factored compressive Table 22. Minimum times for a 1000-year strain limit. Properties based on parallel plate tests on finished estimated lifetime. pipe products included pipe stiffness, flattening, environmen- tal stress-cracking resistance (ESCR), and brittleness. Test Conditions Minimum Failure Time 80C/4.48 MPa (650 psi) 360 h The laboratory results demonstrated that pipe can be made 80C/3.10 MPa (450 psi) 1,401 h with blends of virgin and recycled HDPE to meet AASHTO 70C/4.48 MPa (650 psi) 1,647 h M294. Three (A5, B1, and L5) of the six final candidate for-