Geosynthetics June/July 2023 - 11

THE ASTM D1505 METHOD
GEOMEMBRANE WELDING
BELOW FREEZING
Q: I want to measure the density of a mineralfilled
composite polymer mixture by the
ASTM D1505 method. What liquid will be
used for density measurement (density >1)?
The products to be tested are made of either
polypropylene or polyethylene with calcium
carbonate and additives. We will be adding
50%-70% CaCo3 to the resin.
A: ASTM D1505, Standard Test Method for
Density of Plastics by the Density-Gradient
Technique, uses a graduated glass column
which is charged with two solutions, such
that one is greater than and one is less than
the density of the material to be tested.
The liquids are charged into the column by
slowly adding varying amounts of each using
a tremie tube. The liquid at the bottom of
the column is of greater density and, as the
column is filled, the density decreases until
the top of the column contains only the
lighter density liquid. The density gradient
will be uniform through the length of the
column. Standard glass spheres of known
but varying density are introduced into the
column along with the specimens using a
charging basket. This basket is slowly lowered
through the column without disturbing
the density gradient, distributing the glass
spheres and specimens according to their
relative densities. Using the glass spheres as
reference points, the density at the locations
where specimens are neutrally buoyant is
calculated by interpolation. ASTM D1505
is probably the most sensitive, but also
painstakingly difficult to run and expensive
test method. Values to four-to-five-decimalplace
accuracy are routinely obtained.
This method can be extended to highdensity
materials greater than one if suitably
dense liquids can be selected. I have heard
of people using surfactants and cornstarch
instead of alcohol for this purpose. However,
preparing the column is difficult (requiring a
constant temperature) and column stability is
also a challenge. Have you tried ASTM D792
specific gravity with a density measurement
kit for an analytical balance? Although not as
accurate as ASTM D1505, this test would lend
itself to much less uncertainty and is a lot
easier to set up and run.
Q: Can geomembrane welding be done below
freezing (32 degrees Fahrenheit [0 degree
Celsius])? If it can be done, is there a method
or preferred procedure to assure its success?
A: If the soil beneath the geomembrane
is frozen, the heat from the welders may
thaw moisture from the subgrade and allow
water to condense on the underside of the
geomembrane before the seam is welded.
This possibility may be eliminated using
suitable seaming boards or slip sheets
pulled in advance of the seaming activity.
For sheet temperatures below 40°F (4°C),
shielding, preheating, a slower seaming rate,
etc., may be required. These techniques are
summarized in GRI-GM9, Practice for Cold
Weather Seaming of Geomembranes. The
practice also outlines more frequent seam
testing and other precautions to ensure
quality in cold weather. In all cases, a sharp
and frozen subgrade must be avoided so that
stress concentration on the geomembrane
can be negated.
Please note that the above commentary and
information contained within the GRI-GM9
practice does not supersede the directions
of the project engineer and the formal plans
and specifications. In most cases, a formal
written exemption needs to be obtained
before such work can commence in cold
weather. This is much more complicated
than modulating the pressure, speed and
temperature of the wedge welder. In all
cases, a specific seaming window tailored
for the geomembrane type at hand and
contingent on changing environmental
conditions is needed. All this is checked with
increased frequency prior to startup and
destructive seam testing by on-site Certified
Quality Auditor (CQA) personnel. G
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Geosynthetics June/July 2023

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