from neighboring operations can fall on it. Sharp
8 below. The tip must be shaped to an angle of
105 with a flat pinnacle of 0.5 mm. The conic tip
tools may be rested or dropped on it. Some equip-
ment and workers may traffic on it. Next, hot as-
must terminate in a cylindrical section of about 7
phalt concrete is laid and vigorously compacted.
mm in diameter by about 5 mm in length. The
Over 85% of the volume of a typical asphalt con-
cylindrical section that is inserted into the pen-
crete is composed of rock aggregates that have
etrometer must be 12.7 mm (0.50 in.) in diameter
varying degrees of angularity and sharpness. Vir-
by about 50 mm in length (approximately 2 in.).
tually all aggregates are harder than the membrane
The tip must be free of striations and preferably
material. In hot asphalt concrete, the aggregates
polished with sand paper with grit of 100 or finer.
are held in place by the cohesion of viscous bitu-
The membrane specimen is cut to a manage-
able size. The size of a standard letter paper (8.5
men binder. The potential for membrane puncture
11 in. [21.6 27.9 cm]) is suggested. If the mem-
is significant. Therefore, it is desirable that the
sheet membrane be able to resist puncture as much
brane uses a sheet of paper to protect its sticky
as possible. However, the properties of sheet mem-
side, this sheet must be removed and replaced by
brane materials vary among sources. Furthermore,
a sheet of aluminum foil. This ensures that the
the specifications reported by manufacturers are
puncture values are not affected by the properties
developed by differing test methods. It is nearly
of the temporary backing sheet. At this stage, the
impossible for a project engineer to compare sheet
specimen is laid on a 6-mm- (about 0.25-in.-) thick
membranes by the values reported by different
steel plate. This steel plate may be approximately
the size of half of a standard letter paper (6.5 8.5
manufacturers as the puncture resistance of their
in. [15.6 21.6 cm). An electrically insulating layer
products.
Several standard test methods for puncture re-
is required to separate the steel plate from the
sistance evaluation are in use. Perhaps the most
metal base of the apparatus. This insulating layer
frequently used puncture standard test method is
may be implemented by a letter-size card stock or
similar material (8.5 11 in. [21.6 27.9 cm]).
ASTM E-154. In this method, the membrane speci-
men is punctured while supported by a frame,
To detect when a puncture occurs, an ohmme-
much like in an acoustic drum. However, in a field
ter is utilized by placing one of its electrodes
bridge deck, the membrane is almost continually
pinched between the loose steel plate and the in-
supported by the deck. In view of the lack of con-
sulating layer, and the other held in contact with
sensus among manufacturers about a puncture test
the metal body of the penetrometer. This way, the
standard, coupled with the unreal drum frame
ohmmeter will initially indicate an open circuit
specimen support, the research team decided to
and it will suddenly change the display value
devise and propose a new test method.
(magnitude is irrelevant) when the circuit is
bridged by the contact of the puncture tip with
the specimen's conductive substrate. This event
New puncture test apparatus
The new test method is relatively easy and in-
instructs the operator to immediately release the
expensive to implement. The test apparatus is
puncture load. Figure 9 shows a schematic of the
commercially available from various sources such
puncture failure detection system. The load is ap-
as Soiltest, Inc., and Gilson Company, Inc. It is
plied at a rate of approximately 20 lb (9.1 kg) per
marketed under the name "Acme Laboratory Pen-
second. The Acme Penetrometer is equipped with
etrometer," Soiltest catalog number CT-426 and
an arm that holds the maximum load that was ap-
Gilson catalog number HM-570. The apparatus
plied during the test. This load is then recorded as
was originally designed to meet the requirements
the puncture load for one instance of the test. A
of ASTM C403 and AASHTO T 197, which mea-
minimum of 20 instances of puncture tests should
sure the rate of hardening of mortar sieved from
be conducted for each membrane sample. The test
concrete mixtures, i.e., mortar setting times. This
result is reported as the average puncture load for
apparatus (Fig. 7) is fairly common in material test-
the number of instances of the puncture test con-
ing laboratories. The apparatus is portable, does
ducted for the membrane being evaluated. In ad-
not require electricity to operate, and it weighs
dition, the coefficient of variation* for the test set
about 25 lb (11.4 kg). In addition to the commer-
should be reported.
cially available apparatus, a special, but simple,
metal tip must be made for this new test method.
*The coefficient of variation in a set of tests is calcu-
The tip can be made of common carbon steel, and
lated by dividing the standard deviation by the aver-
it must conform to the geometry shown in Figure
age and then multiplying this ratio by 100.
10
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