Performance of Corrugated Pipe Manufactured with Recycled Polyethylene Content (2011)

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A-1 T A B L E O F C O N T E N T S A-2 A.1 Procedures A-2 A.1.1 Melt Blending by Extrusion A-2 A.1.2 Plaque Preparation (ASTM D4703) A-2 A.2 Index Test Methods A-2 A.2.1 Density (ASTM D1505) A-2 A.2.2 Melt Flow Index and Flow Rate (ASTM D1238) A-2 A.2.3 Percentage Volatiles (TRI Method) A-2 A.2.4 Percentage Black/Color + Ash (ASTM D4218) A-3 A.2.5 Percentage Ash (ASTM D5630) A-3 A.2.6 Percentage Polypropylene (TRI Method) A-5 A.2.7 Tensile Properties (ASTM D638) A-5 A.2.8 Flexural Modulus (ASTM D790) A-6 A.2.9 Oxidative Induction Temperature (OITemp) (ASTM D3350) A-6 A.2.10 Oxidative Induction Time (OIT) (ASTM D3895) A-6 A.2.11 Pipe Deflection Tests (ASTM D2412) A-6 A.3 Stress-Crack Tests A-6 A.3.1 Notched Stress-Crack Tests (NCLS and NCTL) (ASTM F2136, D5397) A-6 A.3.2 BAM Stress-Crack Test (TRI Method) A-8 A.3.3 BFF Test (TRI Method) A-9 A.3.4 Junction Test (FDOT FM 5-572) A-10 A.4 Service-Lifetime Estimation Tests A-10 A.4.1 Stepped Isothermal Method (SIM) for Long-Term Creep Modulus and Strain (ASTM D6992) A-11 A.4.2 Stepped Isothermal Method (SIM) for Creep Rupture (ASTM D6992) A-11 A.4.3 BFF Test for Long-Term Stress Crack Resistance (TRI Method) A P P E N D I X A Procedures and Test Methods

A.1 Procedures A.1.1 Melt Blending by Extrusion All blending and extruding was performed on a laboratory line consisting of a Welding Engineers Model HT8-222-2251 Twin Screw Extruder, a custom-built cooling bath equipped with a chiller and circulator and a Berlyn Corporation Model HV1 Pelletizer. The extruder has counter-rotating screws at an L:D ratio of 24:1. The extruder temperatures were set at 115°C (240°F) for the feed zone, and 177°C (350°F) for the mixing and metering zones. The die temperature was also 177°C (350°F). The die was a single 6.35-mm (0.25-in.) diameter rod and the die was fitted with two, 1-in. diameter melt filter screen holders. All samples for extrusion were dry blended then gravity fed. The batch size was typically 2.2 lb (1 kg). The screw speed was set at 220 rpm, which produced an output rate of about 6.8 kg/hr (15 lb/hr). The residence time in the extruder was less than 45 seconds. Samples of recycled regrind chips were melt blended twice and recycled reprocessed pellets were melt blended once. All the experimental blends were melt blended three times to ensure optimum blending. Virgin resins were also blended in the same manner to duplicate the heat history of the blends. It is worth mentioning that the processing conditions used during this study are different than what is actually going on in the recycled suppliers’ plants or in the pipe manufacturers’ plants. First of all, the twin screw mixes much better than a single screw, or even a high-length/diameter single screw. So, the blends studied herein were optimally blended. Secondly, the study on the effects of polypropylene (PP) might not translate well to the plants because the PP may not be as well blended as in the study. And thirdly, the twin screw processing is very gentle compared to a large single screw. So, even though there were no signs of oxidation in the study on the twin screw, oxi- dation may still be an important consideration in the plants. A.1.2 Plaque Preparation (ASTM D4703) Sample plaques were prepared according to ASTM D4703, Practice for Compression Molding Thermoplastic Materials into Test Specimens, Plaques, or Sheets on a Pasadena Hydraulic Industries Model P215H Platen Press. Two sizes of plaques were prepared. The plaque used for the BAM test specimens was 15.24 cm × 15.24 cm × 1.27 mm (6 in. × 6 in. × 0.05 in.). The plaque used for all other tests was 17.78 cm × 17.78 cm × 1.9 mm (7 in. × 7 in. × 0.075 in.). The smaller mold used 50g of pellets while the larger one used 75g. The open cavity mold was charged with pipe pieces about 5 cm × 5 cm in between two Mylar release sheets and placed in the press, which was preheated to 190°C. Once the mold reached 160°C, the set-point temperature was reduced to 177°C and the mold quickly pressed to 20.68 MPa (3000 psi). After the plastic was allowed to relax for at least 5 minutes, the mold temperature was recorded and the cooling process begun. The mold temperature was lowered by slowly open- ing a needle valve with cooling water. The temperature was recorded every 30 seconds and the cooling was done at a rate of 15°C ± 2°C. Once the temperature was below 50°C, the mold was removed. A.2 Index Test Methods A.2.1 Density (ASTM D1505) The density of the samples was determined by ASTM D1505, Standard Test Method for Density of Plastics by the Density- Gradient Technique. The test is performed with the use of a glass column filled in a specified procedure with isopropanol and water that creates a gradient of densities from around 0.910 to 0.970 g/cm3. Standard glass density beads are placed in the column for calibration. Samples are cut into small pieces (1⁄16-in. diameter), degassed under vacuum in isopropanol, and placed in the column. Once settled, the position is recorded from a graduated scale on the glass column and the density calculated from the position of the standard beads. A.2.2 Melt Flow Index and Flow Rate (ASTM D1238) The melt index was determined in accordance to ASTM D1238, Standard Test Method for Flow Rates of Thermoplastics by Extrusion Plastometer. A sample of plastic was heated inside a standard-sized cylinder for a controlled amount of time. Then, a weight was applied, forcing the molten plastic through a standard die. The mass of plastic that is extruded through the die in a specific period of time was reported. Two conditions were used for the polyethylene samples in this study. The first were a temperature of 190°C and a load of 2.16 kg (Melt Index) and the second were a temperature of 190°C and a load of 21.6 kg (Flow Rate). The results are reported as grams per 10 minutes. The melt flow tests were performed on pellets after melt blending. A.2.3 Percentage Volatiles (TRI Method) Duplicate 2.000 g ± 0.100 g samples were placed in an alu- minum weighing boat and heated at 175°C for 1 hour in a forced air oven. Upon cooling, the final weight was obtained and the percentage volatiles were calculated. This test was performed on the as-received pellets, before melt blending. A.2.4 Percentage Black/Color  Ash (ASTM D4218) This was done by the muffle furnace method described in ASTM D4218, Test Method for Determination of Carbon Black A-2

Content in Polyethylene Compounds by the Muffle-Furnace Technique. About 1 g of sample was placed in an aluminum dish and heated in a small muffle furnace for 3 minutes. Upon cooling, the final weight was obtained and the percentage black/color + ash was calculated. A.2.5 Percentage Ash (ASTM D5630) This was generally done in accordance with Procedure B of ASTM D5630, Ash Content of Plastics. The sample mass was 1 g and the ashing temperature was 800°C. After heating for at least 10 minutes, the sample was cooled, reweighed, and the % Ash determined. A.2.6 Percentage Polypropylene (TRI Method) The percentage polypropylene was determined with a differential scanning calorimeter (DSC) and by a procedure developed for this project. A combination DSC curve with both high-density poly- ethylene (HDPE) and PP melting profiles is shown in Fig- ure A-1. Notice that there is an overlap between the two peaks near 136°C. The area under each of these two curves is the heat of fusion, which relates to the percentage crystallinity of the two polymers. The tail of the HDPE curve will vary, depend- ing upon the size and quality of the crystallites formed dur- ing cooling of the plastic from the melt. One would normally integrate the PP curve from 100°C to 175°C to determine the heat of fusion. However, in a sample containing both HDPE and PP, the end of the HDPE curve will cut off a portion of the PP curve. So, a method was devised to account for the miss- ing portion of the PP curve. The first step was to obtain a bag full of PP chips from a recycler who separates out PP caps. Then, 15 different colored chips were analyzed by DSC to determine the Heat of Fusion, the Melting Onset Temperature and the Melting Peak Tem- perature. The results are shown in Table A-1. Next, a master curve was prepared by determining the heat of fusion of the PP starting at different initial temperatures. This is demonstrated in Figure A-2. The four sets of onset temperatures and heats of fusion represent the area under the curve starting at temperatures of 130, 135, 140, and 145°C. This represents what would happen at different HDPE overlap temperatures in a blend of HDPE and PP. This was done for all 15 cap samples and a master curve was generated by plotting the average heat of fusion as a func- tion of overlap temperature. This is shown in Figure A-3. When a blend of PE and PP is analyzed, the temperature at the end of the HDPE peak is noted along with the area under the PP curve. This is shown in Figure A-4. The inset graph is the portion of the curve from 135°C to 180°C. Notice the overlap temperature is 139.1°C and the heat of fusion for the PP curve is 3.18 J/g. A-3 Figure A-1. DSC melting profiles for HDPE and PP.

A-4 Heat of Fusion (J/g) Onset Temperature (°C) Peak Temperature (°C) 100.0 4.9 156.9 3.4 167.2 2.2 y = -0.035x2 + 8.851x - 475.57 R2 = 0.9995 70 72 74 76 78 80 82 84 86 128 130 132 134 136 138 140 142 144 146 Initial Temp (C) He at o f F us io n Table A-1. Average melting properties for 15 different colored PP bottle closures. Figure A-2. DSC curve for PP showing integration from four starting temperatures. Figure A-3. PP heat of fusion as a function of overlap temperature (average of 15 PP caps).

From the equation in Figure A-3, the heat of fusion for PP from 139.1°C to 175°C is 78.4 J/g. The percentage PP is then 3.18/78.4 × 100 or 4.0%. A.2.7 Tensile Properties (ASTM D638) The tensile properties were evaluated according to ASTM D638, Standard Test Method for Tensile Properties of Plastics. Type IV dumbbell specimens were used with a cross-head speed of 50.8 mm/min (2 in./min). The gage length for break strain was 50.8 mm (2.0 in.). The displacement was measured by cross-head travel. The tensile yield stress and the strain- at-break were determined. Five replicates were tested. A.2.8 Flexural Modulus (ASTM D790) The flexural modulus was determined in accordance to ASTM D790, Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials. Pro- cedure B was used according to ASTM D3350. The specimen was a bar, 12.7-mm wide by 7.63-mm long (0.5 in. × 3 in.). The thickness varied within the sample, which was a compression molded plaque of a nominal thickness of 0.125 mil (3.175 mm). The test span was 2.0 in. and the cross-head rate of deflection was 0.5 in. per minute. A Load vs. Deflection plot was obtained for each of the five specimens. Then, the displacement corre- sponding to a 2% strain in the outside surface was calculated by: where: D = displacement at 2% strain (in), r = strain (in/in), L = support span (in.), and d = specimen thickness. The required displacement varies with the thickness of the specimen. Therefore, each of the five test replicates will likely have a different displacement representing 2% strain. Once the displacement is determined, the corresponding load is found and the flexural stress at 2% strain is calculated by: where: σ = stress at 2% strain (psi), P = load at 2% strain (lbf), L = support span (in.), b = specimen width (in.), and d = specimen thickness. σ = 3PL bd22 D rL d= 2 6 A-5 Figure A-4. DSC curve for a mixed-color recycled HDPE.

Once the stress is found, the modulus is determined by dividing by the strain (0.02). A.2.9 Oxidative Induction Temperature (OITemp) (ASTM D3350) The procedure for this test is found in ASTM D3350, Spec- ification for Polyethylene Plastics Pipe and Fittings Materials. An unweighed chip of material was placed in an open DSC sample pan and heated at 20°C/min in air until an exother- mic oxidation occurred. The onset temperature of the oxida- tion peak is reported. A.2.10 Oxidative Induction Time (OIT) (ASTM D3895) This test was done in accordance to ASTM D3895, Test Method for Oxidative-Induction Time of Polyolefins by Differ- ential Scanning Calorimetry. A sample weighing from 5–10 mg was placed in a DSC under nitrogen and heated to 200.00°C. Once the temperature was steady, the atmosphere was changed from nitrogen to oxygen. The test continued until an exo- thermic peak was obtained. The time from the introduction of oxygen to the onset of the exothermic peak is the oxidative induction time (OIT). A.2.11 Pipe Deflection Tests (ASTM D2412) The stiffness and flattening characteristics were determined by ASTM D2412 as modified by AASHTO M294. The testing machine used was a Lo-Tes Model LT-10, manufactured by Plowman Brothers (now Varicore). Duplicate pipe samples, about 12-in. long were deflected at a rate of 0.5 in./min, with a 5 lb preload and the load/deflection curve obtained. One pipe specimen was tested along the mold lines and the other was tested 90° offset from the mold lines. The test was continued up to a 40% linear deflection of the inside diameter occurred. The pipe stiffness was calculated by: The peak load and peak deflection were also recorded, which can be considered the buckling point of the pipe. A.3 Stress-Crack Tests A.3.1 Notched Stress-Crack Tests (NCLS and NCTL)(ASTM F2136, D5397) There are two notched stress-crack tests that have been used during this project. The NCLS test is ASTM F2136, Stan- dard test Method for Notched, Constant Ligament-Stress (NCLS) PS psi Load lb Deflection in. Length in.( ) = ( ) ( ) ( ) Test to Determine Slow-Crack-Growth Resistance of HDPE Resins or HDPE Corrugated Pipe. The NCTL test is ASTM D5397, Standard Test Method for Evaluation of Stress Crack Resistance of Polyolefin Geomembranes Using Notched Constant Tensile Load Test. The appendix of D5397 describes a single load test that is very similar to F2136. The main difference between the two test methods is the applied load. For D5397, the applied loads are based on percent- ages of the measured yield stress of the material to be tested. For F2136, the applied load is a constant 600 psi (4.14 MPa). This is the applied load that would be used in D5397 if the measured yield stress was 4,000 psi (27.59 MPa) and the load was 15% of the yield stress. The constant applied stress of 600 psi has been accepted for pipe resins in the density range of 0.948–0.955 g/cm3. For most of this study, the NCTL test was performed at an applied stress equal to 15% of the materials yield stress. This was done for all materials submitted for characterization. The rea- soning was that samples of recycled resins were obtained with yield stresses from 3,300 to 4,500 psi (22.76–31.03 MPa). Apply- ing a constant stress of 600 psi would load these materials from 13 to 18% of their yield stress. Since stress crack times are highly dependent on applied stress, it was believed that the NCTL test at an applied stress of 15% of yield would produce more accu- rate results. Eventually, however, resin blends for considera- tion for AASHTO will require NCLS test results. Besides how the load is determined, the tests are run basi- cally the same way. Small dumbbell shaped specimens are cut and a face notch equal to about 20% of the specimens thick- ness is cut in a controlled manner with a razor blade and a notching device. Five specimens are then hung on a lever-load frame and placed in the 10% Igepal CO-630 surfactant solu- tion. The loads are applied and the timers reset to zero hours. The variables that need to be controlled in this test include the plaque preparation, the notches, the loads and the expo- sure environment. Of these, the environment seems to be the most dynamic. It was determined that for these laboratories, the Igepal baths required changing about every 4–5 weeks. A.3.2 BAM Stress-Crack Test (TRI Method) This test was introduced by researchers in Germany at the Bundesanstalt für Materialforschung und-prüfung (BAM). It was developed for evaluating textured coatings for Geo- membranes (landfill liners). It was used successfully by the researcher on both textured Geomembranes and heat-bonded seams (1, 2). The test involves the use of the same load frame as the NCTL or NCLS tests, except that special clamps are needed to test the 15.24-cm × 1.27-cm (6-in. × 0.5-in.) test specimens. The test is normally run under a constant load of 580 psi (4MPa), at 80°C in 5% Igepal CA-720. However, different A-6

temperatures were explored to determine appropriate condi- tions for evaluating recycled materials. A different surfactant is necessary because the cloud point of CO-630 is 55°C, so it is immisible at 80°C. The cloud point of CA-720 is 82°C. Because there is no notch in the test specimens, cracks will only grow if there are critical defects in the specimen. This makes the test useful to evaluate the effects of contamination, especially for recycled plastic. Test results on geomembrane seams showed clearly that the test was sensitive to both the inherent stress-crack resis- tance of the sheet and the defects where cracks could initiate (2). The results also showed that very good stress-crack resis- tance resins were much more forgiving, in terms of critical defect size. Therefore, by choosing a better resin, one can lessen the effects of defects that are present. BAM Test Procedure 1. Specimen preparation: Five samples, 0.6 in. × 6 in. (1.52 cm × 15.24 cm) were cut from the plaque with a die. The edges of these samples were shaved with a sledge microtome to remove any defor- mations caused by the cutting die and to reduce the sam- ple width to about 0.5 in. (1.27 cm). The shaving operation is shown in Figure A-5. The result is five test specimens, 0.5-in. wide by 6-in. long with perfectly smooth edges. 2. Test Apparatus: The test was run with the use of a BT Technology stress crack fixture that had been modified with special grips and weights. The top grips were simply 1-in.2 (2.54-cm2) smooth faced, stainless steel grips with a screw in each corner for tightening. These can be easily made from 1-in. bar stock, 1⁄4-in. thick. The top of the back plate was tapped to accept 1⁄16-in. all thread. The back plate of the bottom grip was 1.5- in. (3.81-cm) long and had a 3⁄16-in. (4.76-mm) hole at the bottom so it could be mounted to the frame. The grips are shown in Figure A-6. The ones pictured show serrations on the outside. The insides of the grips are smooth. The test requires more weight than can be added to a typical NCTL weight tube. The tubes used had 1⁄8-in. all- thread running through the length of the tube. The tube was reduced in length by 4 in., and in its place were added two or three, 1-in. × 1-in. × 2-in. brass blocks. A combina- tion of brass blocks and zinc coated steel shot was used to obtain the required loads. This is shown in Figure A-7. Exposure bath: A Blue M Model 1140A was used to per- form the tests. A solution of 5% Igepal CA-720 in de- ionized (D.I.) Water was the test solution. The solution was found to have a limited life of around 300 h, so it was made fresh at the beginning of each test. The bath had an external stirrer along with a magnetic “flapper” to help control bath temperature. 3. Procedure: The test specimens with the shaved edges were carefully mounted in the grips. The pair of screws closest to the ends of the specimens were tightened first, followed by the pair of screws closest to the center. The amount of pressure on the screws is critical. If the grips are too loose, the specimen slips out. If the grip is too tight, a stress crack is initiated at the edge of the grip. Once the specimens are mounted, the five station load frame is placed in the bath, the pre-weighed weight tubes hung immediately, and the timers reset to zero. This way, as the test specimens heated up, the load was applied gradually. After the specimens failed or 300 h was reached, the test was terminated. The failed specimens were examined to be sure they did not break at a grip, and the crack faces A-7 Figure A-5. Edge shaving of BAM test specimens.

were examined microscopically to determine if the source of the crack could be identified. In almost every case involving post-consumer recycled (PCR) bottles (natural or mixed colored), the crack started at a piece of silicone rubber. A.3.3 BFF Test (TRI Method) This test method was first used in these laboratories as a means for classifying the cracking resistance of post-consumer recycled HDPE for a commercial client. The BAM test (1) was much too severe, so a test was needed that could distinguish poor stress cracking materials. The first tests were performed with the testing apparatus used for the BAM test, under the same 580 psi of load, but the exposure environment was changed from a surfactant to D.I. water. This environment was inspired by the Florida Department of Transportation (FDOT) durability protocol, also performed in D. I. Water (2). The test worked very well at 580 psi, but it was changed to 650 psi to match the FDOT testing conditions. And eventu- ally, the test specimen was changed from the bar used in the BAM test to an ASTM D638 Type I Dumbbell. This was done to open up the possibilities for the test to be performed on unmodified stress-crack testing devices. The current version of this test takes some aspects of the Bam Test, with the exposure conditions of the FL-DOT test and uses a test specimen with a Fathead, so the test is referred to as the BFF Test. Test Specimen The specimen is basically an ASTM D638 Type I Dumbbell with a couple of modifications. First, a 7⁄32-in. hole is drilled into each head portion so that the specimens can be mounted with a screw in a conventional stress-crack testing device. However, because of the holes, the tabs on each side of the hole are narrower than the reduced section of the dumbbell. Therefore, the highest stressed areas are the tabs, so all the specimens would naturally fail at the tabs. To get around this, a 6–in. square, open-faced mold was modified to produce plaques that were about 45 mil in the center and over 90 mil on the edges. This way, the tabs are under less stress than the A-8 Figure A-6. Grips for the BAM test. Figure A-7. Weights for the BAM test.

reduced section so nearly all of the failures occur in the reduced section. A drawing of the fathead test specimen is shown in Figure A-8. Test Apparatus The tests were performed with the use of a BT Technology NCTL stress crack frame. A picture of the test set-up is shown in Figures A-9 and A-10. The test specimens are mounted in the frame, which is then placed in a constant temperature bath. Then, the weight tubes are attached to the back of the lever arm to apply the stress. Exposure Conditions Screening Test: Five specimens are placed in deionized water at 80°C and under an applied stress of 650 psi., and the times- to-failure recorded. A.3.4 Junction Test (FDOT FM 5-572) The junction test is a Florida DOT test method (FM 5-572) and involves a test specimen cut from the pipe itself. An ASTM Type IV dumbbell specimen is used and it is cut from the pipe in a way that leaves the junction between the corru- gation and the liner intact in the center of the test specimen. A cross section of the specimen is shown in Figure A-11. The specimen is loaded in tension and placed in D.I. Water under 650 psi at 80°C. The test was performed on six pipe A-9 Figure A-8. Fathead specimen. Figure A-9. BFF test set-up—front. Figure A-10. BFF test set-up—back. Figure A-11. Cross section of junction specimen.

samples and the results compared to the BFF test results under the same conditions. A.4 Service-Lifetime Estimation Tests A.4.1 Stepped Isothermal Method (SIM) For Long-Term Creep Modulus and Strain (ASTM D6992) The Stepped Isothermal Method (SIM) is a special form of Time-Temperature-Superpositioning (TTS) that has been used to extrapolate short-term creep results (∼24 h) into long-term estimates of creep behavior (50, 100 years). It was originally developed in these laboratories on polyester (PET) geogrids used for reinforcement applications. The application of SIM to PET has been verified and validated by several other labo- ratories comparing the SIM results to conventional creep tests performed at room temperature. It has also been used by others on other PET fibers, Kevlar, and Polyethylene Naphthanate (PEN). It has also been used by TRI to examine PP buried struc- tures and most recently on HDPE resins used for corrugated drainage pipe. Only preliminary validation tests have been performed on PP, but the results are favorable. The main difference between PET and HDPE is their respec- tive temperature dependencies at temperatures below 80°C. HDPE’s properties change at a higher rate with temperature than PET’s properties. In fact, the low-temperature depend- ency of PET strength was the main reason SIM was developed. The sample-to-sample variability could be as large as the dif- ference in creep rates at two different temperatures. A com- parison for the two materials is shown in Figure A-12 below. Time-temperature-superpositioning (TTS) has been used for decades and it is the basis for the validation procedures for polyethylene pipe materials in ASTM D2837 and Plastics Pipe Institute (PPI) Technical Report TR-3. TTS is used to project the long-term hydrostatic strength of pressure pipe. Basically, increasing the temperature of a process like creep, stress relaxation, or slow crack growth is equivalent to perform- ing the test at longer and longer times. The higher the tem- perature, the longer the accelerated time. In the case of traditional TTS, tests are performed at various elevated temperatures on different samples and the results shifted to a lower target temperature. Because of the sample- to-sample variability, the result of TTS can be uncertain and requires tests on many test specimens. SIM is a form of TTS in which behavior at multiple temperatures is observed on a single test specimen, which reduces the uncertainty of the behavior due to sample-to- sample variability. An example SIM test for HDPE was performed under the following conditions: Sample: Type I Dumbbell. Strain Measurement: Extensometer. Initial Temperature: 20°C. Temperature Steps: 7°C (20, 27, 34, 41, 48, 55, 62, 69, 76) Stress: 1000 psi. Dwell Time: 10,000 seconds (2.78 h). The raw, unshifted data are shown in Figure A-13. There are nine temperature steps shown on the plot, so the highest temperature was 76°C. Notice that the sample yielded catastrophically during the early part of the 76°C step. A-10 y = -0.0117x + 1.2639 y = -0.003x + 1.069 0 0.2 0.4 0.6 0.8 1 1.2 10 20 30 40 50 60 70 80 90 Temperature (C) R el at iv e Pr op er ty PE PET Figure A-12. Temperature dependence of PET and PE.

It should be mentioned here that the transition from one temperature to the next is an important variable in SIM test- ing. The time it takes for the specimen to equilibrate at the new temperature should be just a few minutes. Other things that occur during the transition time are thermal expansion or con- traction of the specimen as well as re-equilibration of the grips and extensometer. TRI excludes the data from the transition region, but keeps the time scale in place. The transitions then show up as blank spots in any plot with time as the abscissa. The next step in the analysis is to determine what we refer to as the virtual starting time for each step above the first one, or t’. This accounts for the effects of the creep that occurred at the lower temperature. This step is necessary because the specimen “remembers” what had occurred at the previous creep step. This also allows one to rescale the individual creep curves and get them all on a common time scale. The virtual starting time is found by plotting creep modu- lus vs. log time for the end of one step and the beginning of the next step. Then, one can adjust the virtual starting time until the slopes of the two steps align. A vertical shift is also added at this time to aid in the alignment process. This is illustrated in Figures A-14a to A-14d for the end of the 41°C step and the beginning of the 48°C step. Once this is done for each step, master curves can be pre- sented as either creep modulus or strain. Master curves for this data set are shown in Figures A-15 and A-16. From these two curves, one can obtain both the 50-year creep modulus and 50-year creep strain. In this case, they are 10,000 psi and 8.9% respectively. These represent the behavior of the material when placed under a 1,000 psi load for 50 years. A.4.2 Stepped Isothermal Method (SIM) for Creep Rupture (ASTM D6992) Additionally, if one runs separate experiments at several loads, a master curve of stress vs. time can be obtained. Fig- ure A-17 shows the results of SIM tests at 2,000, 1,500, and 1,000 psi. Subsequent tests revealed that better results were obtained at applied stresses of 1,500, 1,250, and 1,000 psi. The failure times at 2,000 psi were thought to be too quick for good extrapolations. The results from these test show that the 50-year strength of this material will be about 1,074 psi. A.4.3 BFF Test for Long-Term Stress Crack Resistance (TRI Method) Three sets of test conditions are required to perform a ser- vice lifetime prediction with the use of the BFF test. The con- ditions selected are the same ones used in the FDOT 100-year service-lifetime protocol. They are the following: 80°C at 650 psi applied load in water, 80°C at 450 psi applied load in water, and 70°C at 650 psi applied load in water. Service Lifetime Prediction The service lifetime is found by application of the rate process method (RPM), described in ASTM D2837. The basic equation is: where t = failure time (hrs) T = temperature (°K) S = stress (psi), and A, B and C are constants. Under the three sets of test conditions, the values of the constants are found by solving three equations with three unknowns. The three equations are the following: 1. 80°C/650 psi log t1 = A + B/353 + C log 650/353 2. 80°C/450 psi log t2 = A + B/353 + C log 450/353 3. 70°C/650 psi log t3 = A + B/343 + C log 650/343 Constant A can be found by subtracting equation 3 from equation 1: Constant C can be found by subtracting equation 2 from equation 1: 353 343 650 450log t log t C log C log C 1 2− = − = 353 2 813 2 653log t t1 2−( ) −log . . 353 343 353 343 10 353 log t log t A A A A lo 1 3− = − = = g t log t1 3( )− 343 10 log ( )t A B T C log S T= + + 1 A-11 0 5 10 15 20 25 0 20000 40000 60000 80000 100000 TIME (sec) ST R AI N (% ) Figure A-13. Raw SIM data.

A-12 22500 23000 23500 24000 24500 25000 25500 26000 26500 27000 27500 5.55 5.6 5.65 5.7 5.75 5.8 5.85 5.9 5.95 6 6.05 LOG TIME (sec) CR EE P M O DU LU S .xls, g6.4 22500 23000 23500 24000 24500 25000 25500 26000 26500 27000 27500 5.55 5.6 5.65 5.7 5.75 5.8 5.85 5.9 5.95 6 6.05 6.1 LOG TIME (sec) CR EE P M O DU LU S .xls, g6.4 22500 23000 23500 24000 24500 25000 25500 26000 26500 27000 27500 5.6 5.7 5.8 5.9 6 6.1 6.2 6.3 LOG TIME (sec) CR EE P M O DU LU S .xls, g6.4 22500 23000 23500 24000 24500 25000 25500 26000 26500 27000 27500 5.9 6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 LOG TIME (sec) (a) t’ = 0 sec (b) t’ = 10,000 sec (c) t’ = 20,000 sec (d) t’ = 28,100 sec CR EE P M O D UL US .xls, g6.4 Figure A-14. Curve alignments at different virtual starting times. 0 20000 40000 60000 80000 100000 120000 140000 160000 180000 -3 -2 -1 0 1 2 3 4 5 6 7 LOG TIME (hr) CR EE P M O DU LU S (ps i) REFERENCE TEMPERATURE - 23C 10,000 Hour 50 Year Figure A-15. Creep modulus master curve under 1,000 psi of stress.

And, constant B can be found by substituting the equations derived above into Equation 1. Example Problem: Given the following failure times, calcu- late the three constants. Determine A A = (353 log t1 − 343 log t3)/10 A = −20.50 80 650 110 33 2 041 30 1° = ± = = = C psi hrs t t COV 1log . % log .80 450 430 172 2 6332° = ± = =C psi hrs t t COV 2 = ° = ± = = 35 70 650 500 175 2 6993 % log .C psi hrs t t3 COV = 40% log t A B C log log t A B 1 1 = + + = + 353 650 353 353 353 + = − − C log B log t A C log1 650 353 353 650 Determine C C = 353 (log t1 − log t2)/2.813 − 2.653 C = −1306.10 Determine B B = 353 log t1 − 353 A − C log 650 B = 720.47 + 7247.09 + 3674.06 B = 11,628.95 Now,withtheuseof these constants one can calculate the fail- uretime,t, foranyother set of temperature and stress. For exam- pletheservice lifetime at 23°C and 500 psi would be found from: log log log . , t A B C t = + +( ) = − + 296 650 296 20 50 11 628 95 296 1306 10 296 20 50 . . log . −( ) = − + log 500 t 39 29 11 98 688 7 568 640 886 . . log , , − = = = t t h Years A-13 0 5 10 15 20 25 30 35 40 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 LOG TIME (hr) ST R AI N (% ) REFERENCE TEMPERATURE - 23C 10,000 Hour 50 Year 2.9 3.0 3.1 3.2 3.3 3.4 0 1 2 3 4 5 6 7 LOG TIME (hr) LO G Y IE LD S TR ES S (ps i) sim rupture regression Regression Line 23C Reference Temperature (time is dependent variable) 2512 1995 1585 1259 1000 10,000 hour = 1307 psi R UP TU RE S TR ES S (ps i) 794 50 year = 1074 psi 100 year = 1014 psi Figure A-16. Creep strain master curve under 1,000 psi of stress. Figure A-17. Long-term yield stress by SIM.

This value represents the average lifetime based on the test results. Long-Term Strength Once the constants are known, one can also calculate the 50 year strength at 23°C from equation 1. 50 years = 438,000 h log . . 438,000 A B C log S= + +( ) = − 296 296 5 641 20 50 + + −( )11 628 95 296 1306 10 296, . . log S The rate process method predicted a service lifetime at 23°C and 500 psi of stress to be 176 years and also deter- mined the 50-year strength would be 661 psi. This strength represents the stress under which stress cracks will not form for 50 years. These results can be presented graphically as in Figure A-18. 1669 74 6076 88 11 628 95 1306 10. . , . .= − + − log S 2.98 S, S psi= =log 950 A-14 y = -0.2702x + 3.3647 R2 = 1 y = -0.2266x + 4.2577 2.2 2.4 2.6 2.8 3 3.2 3.4 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 Log Failure Time (hrs) Lo g St re ss (p si) Average Predicted Lifetime Under 500 psi = 864 Average 50 Year Strength = 955 psi Figure A-18. BFF lifetime prediction.