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piece contains a particle or an air bubble, the number could The OIT values were consistently above 25 min, except for
be skewed in either direction. pipe sample L1, which was 13 min. This was a proprietary blend
For this reason, it is believed that for a specification of resin from a single supplier, an example of a fully formulated resin
blends containing PCR-HDPE, the density is of very limited one might obtain from recycled-resin suppliers. In this case,
value. It would be better to simply specify mechanical prop- the OIT was below a suggested value of 25 min. The values for
erties that demonstrate what the density is. Two examples are the blends chosen for this study varied based on the virgin resin
shown in Table 10. content, because both the HDPE and LLDPE resins were well
These are the required mechanical property values to stabilized.
ensure that the base resin density was in the range specified In conclusion, the main findings concerning the short-term
by AASHTO M294. Unfortunately, the yield stress and flex- properties of the trial pipes were the following:
ural modulus values do not fit neatly into cell classes them-
selves. Cell class 5 for yield stress is 3,5004,000 psi, so one 1. A direct measurement of the density of pipe containing
could specify a cell class of 5 or higher. If the density were too recycled content is of limited value, even when the result
high, the stress-crack resistance would suffer from too much is corrected for percentage carbon black.
homopolymer. The cell class 5 for flexural modulus is broad 2. The percentages of carbon black were below 2.0% in nine
(110,000160,000) but could be used. A resin too low in flex- of 15 samples, which might suggest a lack of control by the
ural modulus would also be below the minimum yield stress corrugated pipe manufacturers.
of 3,500 psi. 3. The break strains were significantly lower than predicted
The MI values were consistent and very close to the theo- on blends without carbon black. This could be caused by
retical values, except in a few cases. This suggested that, for poor carbon blending, the quality of the carbon black, or
the most part, the pipe formulations were made correctly. even the quality of the carrier resin in the carbon black
The percentage carbon black results were a little surprising. concentrate.
Each manufacturer was asked to control the carbon content
to 2% to 3%. The results showed that only six samples had the
Stress-Crack Test Results
correct amount of carbon black, the others were all less than
2.0%. It should be noted that AASHTO M294 allows from Notched Stress-Crack Tests (NCLS and 15% NCTL).
2% to 5%, so the manufacturers were asked to do something There were two different stress-crack tests performed on the
they normally do not do. plaques made from the 15 pipe samples. The first was the
The percentage ash and percentage PP were both close to the NCLS test (ASTM F2136), in which each sample is loaded at
theoretical values with the percentage ash having a linear corre- a constant ligament stress of 600 psi. The 600 psi is a result of
lation coefficient (R2) of 0.75 and the percentage PP's was 0.91. taking 15% of 4,000 psi, which is about the yield stress of the
The yield stress values showed that all the samples were PPI-certified resins for corrugated pipe. So, the test basically
above 3,500 psi except samples B3 and B5. These two were assumes that most of the samples tested will have a yield stress
slightly lower and actually were very close to their calculated near 4,000 psi. The NCTL test (ASTM D5397, Appendix A,
values. Either of these could easily be adjusted higher with Section A.3) applies a load based on the actual yield stress of
added virgin pipe resin or recycled PE homopolymer. the material tested. Since the yield stresses measured on the
The percentage strain-at-break values were all much lower 15 pipes ranged from 3,251 to 4,145 psi, many of the samples
than expected. Apparently, the carbon black has a large effect were not very close to 4,000 psi. Therefore, the NCTL test was
on the break strain. This is one area that should be looked at also used at an applied stress equal to 15% of the measured
because, historically, there has been little attention paid to yield stress. This means that the applied stresses varied from
carbon black for corrugated pipe. It is a very important aspect 488 to 622 psi.
for solid-wall pipe and PE resins used for geomembranes. Both the NCLS and 15% NCTL results are seen in Figure 34.
Specifications for recycled materials could improve the qual- Notice that the 15% NCTL results are almost always higher
ity of the carbon blacks used simply by setting a high standard than the NCLS. This is due to the fact that most of the yield
for break strain. stresses are less than 4,000 psi, so the 600 psi is a higher load
Table 10. Mechanical properties related to base resin density.
Property Range Equivalent to Density
Cell Class 4 (>0.9470.955 g/cm3)
Tensile Yield Stress (psi) 3,5004,100
Flexural Modulus (psi) 130,000160,000
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70
60
NCLS
15% NCTL
50
Failure Time (hrs)
40
30
20
10
NT NT NT
0
A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 L1 L2 L3 L4 L5
Pipe Sample
Figure 34. Stress-crack resistance of 15 pipe samples (NT no test).
than 15% of the actual yield stress. Two lines are shown on water at 80°C. Samples cut directly from pipe are evaluated
the graph. The higher one is at 24 h, which is the required under the conditions in Table 11.
NCLS time for virgin blends used to make AASHTO M294 The results are then used with the RPM model to estimate
pipe. The lower line is 18 h, which has been suggested for pipe the service lifetime at 23°C and an applied tensile stress of
samples made into plaques (14). 500 psi. The specification allows one to terminate the tests if
Three of the samples tested failed to meet the 18 h require- no failures occur under the conditions shown in Table 11
ment. Also, the value for plaques will soon be a minimum of after 110, 430, and 500 h, respectively.
24 h. Eight of the 15 met 24 h and seven were lower than 24 h. The BFF test uses a larger test specimen, namely an ASTM
D638 Type I dumbbell. The mold in which the plaques are
BFF Test. A test has been developed that combines some made for this test has been modified to make the ends of the
features from the BAM test with the criteria set in the FL-DOT specimen thicker than the reduced section of the specimen. A
100-year durability assessment (3). The test is a stress-crack drawing of the specimen is shown in Figure 35 and a picture
test without a notch. Whenever there is a critically sized defect of the head in Figure 36.
present, a crack will initiate at the defect and ultimately cre- The main difference in the specimens is the size. The
ate a running crack through the thickness and width of the junction specimen is a Type IV and the Fathead is a Type I,
test specimen. This test is important for blends containing as defined by ASTM D638. The Type I has a surface area of
recycled materials because it is very sensitive to contaminants. 1.12 in.2 while the Type IV has a surface area of 0.32 in.2.
It is complimentary to the FL-DOT junction test because Depending on the sample thicknesses, the Type I has 2.5 to
it can be performed on the same equipment and is run under 3.5 times the volume of the Type IV. This is important when
the same conditions. The FL-DOT protocol calls for the eval- one is looking for flaws or defects, like small silicone rubber
uation of the junction between the corrugation and the liner particles. The new specimen is called "Fathead" because the
with a stress-crack test on a 0.25-in. wide test specimen in D.I. plaque mold was modified to make the heads more than two
Table 11. FL-DOT junction test conditions.
Temperature (°C) Applied Stress (psi)
80 650
80 450
70 650
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Figure 35. The fathead test specimen.
times the thickness of the reduced width section. The use of
this test specimen has essentially eliminated grip failures.
Figures 37 and 38 show pictures of both the FL-DOT Junc-
tion Specimen and the BFF specimen.
Tests have been performed on compression molded plaques
from each of the 15 trial pipes. Preliminary results are pre-
sented in Table 12.
Almost all of the samples had average values greater than
100 h. The 3 VR1 samples from each of the 3 plants have COVs
of 14%, 19%, and 19%. The three samples of the 50% VR1 +
20% MD + 30% MCR had a COV of 27%, 40%, and 28%. The
three samples that only contained natural recycled HDPE had
COVs of 14%, 4%, and 21%. These results suggest that the
highest variability is in the MCR recycled content.
The failure surfaces were examined to determine exactly
where the specimens broke. There were five categories used to
characterize the fracture face. They were the following:
1. Rubber Particle--a clearly present soft particle.
Figure 37. Junction specimen.
2. Not Obvious--no visible sign of a crack initiator.
3. Gel--a classic unmelt; harder and darker than a rubber
particle.
4. Imperfection--an ambiguous flaw or void. Examination of fracture faces from the tests reported in
5. Tiny particle--too small to tell if it's hard or soft. Table 12 produced the following breakdown:
· Rubber Particle: 48%.
· Not Obvious: 28%
· Gel: 5%.
· Imperfection: 13%.
· Tiny Particle: 6%.
Rubber particles are involved in at least 48% of the cracks
observed.
BFF Test Reproducibility. It became clear early on that
the BFF test results were influenced by residual surfactants
in the exposure bath, reflected in the increasing failure times
over time. However, it was also learned that once a bath has
been used with surfactants, they are difficult to remove, even
after multiple rinsings. The results of repeated BFF tests over
time are shown in Table 13. The time period represented was
about 18 months. It is clear from the table that baths that have
Figure 36. Side view of fathead specimen. never contained surfactants are preferred for the BFF test.
