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45
Table 20. Comparison between BFF and junction tests at 80ºC/650 psi.
Sample BFF Time (h) Junction Time (h) (COV) Junction/BFF
(COV) [liner breaks]
B1 414 ± 97 (23%) 1,076 ± 561 (52%) [4] 2.6
B5 228 ± 141 (62%) 578 ± 224 (39%) [4] 2.6
L1 208 ± 73 (35%) 362 ± 201 (55%) [4] 1.7
L5 334 ± 116 (35%) 1,342 ± 300 (22%) [4] 4.0
strengths are 1,124 and 1,067 psi, while the 100 year stress- that the design factor for a 100-year life is 0.33 (307 × 0.33 =
crack strengths are 979 psi and 950 psi for samples B1 and A5, 100). This is more conservative than the design factor of 0.50
respectively. used for solid wall water pipe.
Now, since there is uncertainty regarding the use of recy-
cled PE, a very conservative approach would be to use a design
The BFF Test for QC
factor of 0.10. That would mean that the test results will have
to show that the pipe will last for 1,000 years. The master curve
There is no question that the successful use of recycled PE
for 1,000 years is also shown in Figure 30.
depends on the stress-crack resistance of the manufactured
The slope of the line is reasonable for the resins used in
pipe. And, because of the inherent variability in recycled
corrugated pipe. The average slope from PPI (16), Hsuan (17),
resins, each lot should be tested for stress-crack resistance.
and this study on 17 data sets was -0.26, with only four values
Therefore, a QC test is needed that will relate to the long-term
over -0.27.
stress-crack resistance of the pipe, be of a reasonable duration The minimum FL-DOT times to generate the 1,000-year
of time, be sensitive to the base resin stress-crack resistance, lifetime curve are shown in Table 22.
and be able to determine the effects of contaminants. This shows that if a junction specimen lasts 360 h without
The BFF test is a good candidate. This test is done on a rel- failure in a 80°C bath under 4.48 MPa (650 psi) of load, it will
atively thin plaque made from the pipe. The test specimen is be estimated to last 1,000 years.
a modified ASTM D638, Type I dumbbell, which has a sur- Since it has been demonstrated that the BFF test is at least
face area of 7.2 cm2 (1.1 in.2). The thickness is about 1.1 mm 1.7 times faster than the junction test at 80°C/4.48 MPa
(0.045 in.), which means that even a 0.12 mm (0.005 in.) par- (650 psi), it would be conservative to set the requirement for
ticle will be 11% of the thickness. This ensures that the effects the BFF test at 200 h. Therefore, the proposed minimum aver-
of particles will be determined. The test does not evaluate the age failure time in the single point BFF test for QC is 200 h,
actual end product, like the FL-DOT junction test does. How- or 8.3 days. This is not an ideal time for a QC/quality assur-
ever, because of its thickness, larger surface area, and consis- ance (QA) test, but even longer times are used for HDPE
tent stress, the BFF test is the more aggressive test. In fact, geomembranes. The geomembrane manufacturers perform
results showed that failure times at 80°C/650 psi averaged 2.5 the test on every railcar of resin and some QA specifications
times faster in the BFF test compared to the junction test when require the test for each batch of resin, or sometimes every
12-inch pipe was tested. The results from four recent, side- 100,000 sq. ft. of material. Additionally, these tests are typi-
by-side tests are shown in Table 20. cally run for 300 or 400 h. A durability test like this one is crit-
This comparison is complicated by the fact that nearly all ical for the successful use of recycled HDPE in corrugated
the junction test specimens failed in the liner and not at the drainage pipe for highway applications. And every lot of
junction. The average of these data sets show that the BFF test recycled-containing resin needs to be tested because of the
is at least 1.7 times faster than the junction test. variable nature of recycled resins.
The FL-DOT 100-year service-lifetime protocol calls for The BFF stress-cracking master curves in Figures 42 and 43
the junction test to be run under three sets of conditions and independently support the value of 200 h for a QC test at
sets minimum time requirements for each condition. These
are shown in Table 21.
These values were determined with the use of POP factors Table 21. FL-DOT junction test conditions and
and the 95% lower confidence interval based on a Student-t minimum failure times.
distribution. When one applies the Popelar shifts, the 20°C
Test Conditions Minimum Failure Time
master curve can be generated. This is shown in Figure 46. 80ºC/4.48 MPa (650 psi) 110 h
The master curve shows that a pipe sample under 3.45 MPa 80ºC/3.10 MPa (450 psi) 430 h
(500 psi) of load will not stress crack for 304 years. This shows 70ºC/4.48 MPa (650 psi) 500 h
