Quality Control Testing of Lightweight Insulating Concrete

Quality Control Testing of Lightweight Insulating Concrete

October 1999
ALITY
ESTING
OF LIGHTWEIGHT
INSULATING CONCRETE
BY KARL A. SCHAACK, P.E., RRC
L ightweight insulating concrete fill , like many other components of the
roofing assembly, is “manufactured” and installed in the field . Maintaining
proper procedures during the batching and installation processes is essential
to achieving the desired performance characteristics of the lightweight insulating
concrete. Several testing procedures can be implemented during and/or after the
placement in order to evaluate the properties of the lightweight insulating concrete.
Wet density, fastener pull-out resistance, compressive strength , and dry density are
some of the common physical characteristics determined by testing. These test procedures
not only evaluate the quality of the lightweight concrete fill but also determine
the adequacy of the lightweight concrete for receiving the new roof assembly.
Photo i, A i o-quart steel pail is commonly used for the “pail test.”
Interface • 5
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Ions or 1/3 cubic foot. After completely filling
and obtaining the weight of the filled
10-quart container, multiply the weight by
three to provide the weight per cubic foot
or the wet density. Independent testing laboratories
have a specific apparatus for determining
the field density of the concrete.
This apparatus consists of a steel vessel of
known volume attached to a balance scale to
determine the weight (see Photograph 2) The
wet density is then calculated similar to the
“pail method” described previously.
Pboto 2: A slul vrml of known volume altachtd lo a balance scalt may be used lo dettrmint weight.
Wet density found to be greater than the
specified range usually means too much
cement is being used in the mix. With wet
density below the specified range, the probable
cause would be use of too much water.
The sample of the lightweight insulating
concrete used for testing purposes should be
considered representative and not be collected
at the beginning or at the end of the
placement operation. If batching and placement
methods are terminated and restarted
Wet Density
Determination of wet density is performed during the placement
of the lightweight insulating concrete. The wet density
should be determined at various times during the day as the
lightweight insulating concrete is being batched and placed.
The wet density should be measured at both the hopper and the
point of placement. Wet density can be determined simply by
placing the batched mixture in a steel container of known volume
and weighing the filled container. By knowing the weight
and volume of the filled container, one can calculate the wet
density of the mix. A steel pail with a volume of 10 quarts is
commonly used by deck applicators fo~ performing this procedure
(see Photograph 1 ). The 10-quart container equals 2-1 /2 gal-
Pbotos 3 and 4: A spri11g-typt (fish) sea Ir (left) tmd a cuslo111-fabricattd
sbttt 111tlal holding clamp (right) 111ay bt ustd for pull-out mislanct tests
011 bast sbttt Jaskllm.
6 • Interface
during the workday, it is imperative that the
wet density be verified when the operations are resumed.
For cellular concretes, the density of the foam should also be
verified at regular intervals. The density of the foam is also
checked by weighing a full container of known volume ( 10-quart
pail). The foam density, therefore, is the weight of the foam
divided by the volume of the container. During the batching ..
process, the foam is introduced into the concrete via a manually-WI’
operated, trigger-actuated hose apparatus. Since this process
depends on the experience of the operator, the rate of foam discharge
from the apparatus should be measured. Again, this
process simply involves discharging the foam into a container of
known volume and determining the time required to fill the container.
Different types of scales can be employed to weigh the pail
used to verify the densities of the foam and/or lightweight insulating
concrete. Either platform or spring tension scales can be
used. Platform scales are typically more accurate but more diffi-
October 1999
cult to keep clean in the field. The spring tension scale is more
portable and can also be used to perform pull-out resistance tests
for the base sheet fastener.
ield Density
The field density should be verified to determine when the
lightweight insulating concrete has “cured” to a point that it is
suitable to receive the new roof. A common rule of thumb used
by field personnel is, “If foot traffic does not leave impressions
(footprints) in the lightweight insulating concrete, then the concrete
is suitable to receive the new roof.” However, there are
more accurate and scientific test methods to evaluate the suitability
of the concrete.
Fastener pull-out resistance is a relatively “quick” test to
determine if the concrete has reached an adequate “age” in order
to install the new roof. The fastener proposed for use in the new
roof assembly should be tested in several random locations
throughout the subject area. A typical test frequency is approximately
four tests per 100 squares of monolithic pour. It is recommended
that additional tests be performed on individual roof
elevations or independent pours at the same elevation. The minimum
pull-out resistance that is commonly required by membrane
manufacturers for the split shank fastener is 40 pounds. When a
minimum pull-out resistance of 40 pounds can be achieved, both
deck and roof membrane manufacturers recommend that installation
of the new roof can begin. This time frame is typically two
to four days after placement, depending on the type of lightweight
insulating concrete, substrate, and climatic conditions.
Care should be taken if evaluation of the lightweight concrete
is determined by only performing pull-out resistance tests
on fasteners. The concerns are twofold:
1) When the tests are usually performed, the concrete has
not reached 28 day strength; and
2) Galvanized steel fasteners reportedly can gain additional
pull-out resistance as a bond develops between the calcium
hydroxide in the Portland cement and the zinc oxides
in the galvanized coating on the legs of the steel
fastener.
Consequently, the pull-out values can
increase in time due to these factors.
The pull-out resistance test for base
sheet fasteners can be performed using
rudimentary tools. These tools include a
spring-type (fish) scale (commonly purchased
in a hardware/sporting goods
store) and a custom fabricated sheet
metal holding clamp (see Photographs 3
and 4). The scale should have a range of 0
to I 00 pounds with one-pound increments.
Although considered to be somewhat less than
scientific, this method can provide
the user with the relative quality of
the lightweight fill.
A more sophisticated and reliable
piece of equipment is a hand-o perated
tensile tester that is specifically
October 1999
Photo 6: Both rigid
steel and ‘jlexible fabri c” lifter
feel are available for testing
base ply fa steners and the
stress plate.
engineered for pull-out strength testing of fasteners (see
Photographs ). Comten Industries of Petersburg, Florida manufactures
several types of these test stands. Two types of hand-operated
testers are available. The “Series 30 I” tester has an analog
read-out and the “Series 341” tester has a digital read-out. A
motorized digital fastener tester (“Series 381 “) is also available.’
These testers are commonly utilized for testing screw-type fasteners.
Both rigid steel and “flexible fabric” lifter feet are available
for specifically testing base ply fasteners and the stress plate
(see Photograph 6). The flexible fabric lifter foot was designed to
mimic the actual movement of the base sheet and membrane as
it pulls on the fastener during uplift. A national standard,
ANSI/SPRI FX-1, was developed by SPRI to provide guidelines
for performing pullout tests. ‘
Another easily performed test to verify the density of the
“cured” lightweight insulating concrete uses a hand-held penetrometer.
The penetrometer commonly utilized is one that is
designed for performing field and laboratory evaluations of initial
set of concrete mortars. This testing apparatus is comprised
of a hand-held cylindrical tool (7 inches long x 3/4-inch diameter)
with a circular probe measuring I/20th square inch of surface
area. This piece of equipment is manufactured by ELE
International/Soil Test Products Division of Lake Bluff, Illinois
and classified as a “Concrete Mortar Penetrometer.” The test
involves pushing the shaft of the penetrometer into the lightweight
insulating concrete (see
Photograph 7). The tool has
a direct read scale on a
range of 0 to 700
psi. A reading is
obtained from
forcing the
shaft into the
concrete at a
constant rate
to a known
depth, approxi-
Photo 7: A handheld
penetrometer is
used to verify the density
of the “cured” LWIC.
Interface • 7
mately 1/4 inch. The reading is divided by three to obtain the
relative compressive strength of the lightweight insulating concrete.
However, the test does not provide sufficient repeatable
data or the precision to use as a single source of evaluation for
lightweight insulating concrete. Using this test method together
with the base sheet fastener pull-out test and visual observations
can provide useful information to assist project personnel in
determining the “readiness” of the lightweight concrete fill.
Compressive Strength
Testing for the compressive strength of newly-installed lightweight
concrete should be performed in accordance with ASTM
C-495 “Standard Test Method for Compressive Strength of
Ligh~eight Insulating Concrete.” This method covers the
preparation and testing of molded cylinders for lightweight concretes
with oven dry weights not exceeding 50 pd. The test
specimens are molded from a sample of the lightweight concrete
mi xture obtained from the batching equipment prior to placement.
The mixture is placed in “molds,” stored, and specifically
cured. The mold can be either individual, cardboard cylinders, or
an EPS carton type of mold with multiple (four) cylinders per
carton (see Photograph 8). The size of the mold and resulting
cy linder varies from that commonly used for structural concrete.
The mold for these cylinders is three inches in diameter by six
inches long, compared to 6-inch diameter/ 12-inch length for
structural concrete.
The molding process consists of placing the “wet” mixture in
two to three approximate equal layers (lifts ). After each layer is
placed in the mold, the concrete should be “leveled” by either
tapping the sides of the mold or lifting and gently dropping the
mold until the top surface of the respective layer has subsided to
a plane. The ASTM standard has specific procedures for curing
which generally involve initial, moist curing followed by ovendri
ed curing. It is critical that the samples are dried prior to testing.
The procedure is a multi -step process as follows:
Step l : Moist cure (70 degrees Fahrenheit, +/- 10 degrees)
in the mold for the first seven days.
Step 2: Strip the mold and continue moist curing in the
8 • Interface
Photo 8: Test specimens are molded in EPS cartons.
appropriate environment (70 degrees Fahrenheit,
+! – 10 degrees) for the following 18 days.
Step 3: Oven dry ( 140 degrees Fahrenheit, +/- 5 degrees) ~
for the last three days of the 28-day cycle.
Step 4: The sample is then allowed to cool to the touch
in ambient air prior to testing.
Several factors can affect the results of the testing of
molded cylinders.
• The accuracy of the testing machine is critical. The maximum
load required to break the sample of lightweight
insulating concrete should be not less than 10% of the
maximum load range of the testing equipment being used.
Testing equipment commonly used for testing compressive
strength of structural concrete has a load range of
10,000 pounds. Ten percent of this load range equals
1,000 pounds, which exceeds the anticipated maximum
compressive strength of lightweight insulating concrete
that ranges from 200 to 400 psi.
• Actual dimensions of the specimen should be obtained.
The improper measurement/documentation of the crosssectional
area of the cylinder can have an impact on the
test results. Even though the cylinder mold is commonly
three inches in diameter, the actual diameter of the hardened
concrete cylinder should be measured to the nearest
0.01-inch (0.3 mm). The recorded diameter should be
determined by an average of two diameters measured at ~
right angles to each other at mid height of the sample.
The difference of t/ IOth of an inch larger than the actual
will result in a larger bearing surface which can reflect a
lower compressive strength reading of approximately
6-1/2%. The actual recorded height of the sample should
also be measured to the nearest 0.01-inch (0. 3 mm) for
volumetric/gravimetric analysis.
• Preparation of the specimens can also have an effect on
the sample. The concrete should be placed in the mold in
two to three lifts. After each lift is placed in the mold, the
mold should be tapped or raised, and dropped approximately
1 inch to allow the lift/layer to “settle.” The concrete
should not be rodded, as is typically done during
the molding of cylinders for structural concrete. After the
cylinder is molded, it should be left undisturbed for 16
hours and kept in the mold a minimum of seven days.
Compressive strength testing can also be accomplished with
casting of cubes, which is outlined in ASTM C-109, “Standard
Test Method for Compressive Strength of Hydraulic Cement
Mortars.” The typical size of cubes is 2 inches x 2 inches. Some
industry personnel indicate that cubes provide a more realistic
sampling of the compressive strength compared to that obtained
from cylinders. Some factors that can have a negative impact on
the compressive strength test results of cubes include the following:
1) An excessive amount of mix water utilized during the
batching process could wash away cement particles. 2) Performing
the test before the specimen is adequately “dry” can
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cause moisture to act like a lubricant causing “s lippage” within
the matrix. This results in crushing of the specimen, likely reducing
the test values. 3) Inadequate mixing can result in poor dispersion
of the cement and less than desirable test results.
Testing the physical properties of existing lightweight concrete
(“cured” concrete) can be performed in accordance with
ASTM C-513, “Obtaining and Testing Specimens of Hardened
Lightweight Insulating Concrete for Compressive Strength.”
This method covers obtaining and preparing samples of in-place
lightweight concrete (minimum 14 days old). In general, the
procedure consists of obtaining a bulk sample of the existing
(cured) lightweight insulating concrete and “shaving/shaping” the
sample down to the desired size and number of cubes. The bulk
sample obtained should not include any cracks, spalls, or be
otherwise damaged. The size of the shaped cubes is 2 inches x 2
inches (minimum), or 4 inches x 4 inches (maximum). The size
of the cube is typically determined by the maximum thickness of
the lightweight insulating concrete. A total of four cubes is recommended
for appropriate testing. Three of the cubes are used
for testing compressive strength and one is used for determining
the dry density. Since the samples are manually produced, the
actual dimensions of the cube should be measured to determine
the true size and bearing surface. The specimens should be ovendried
( 140 degrees Fahrenheit, +/ – 5 degrees) for three days prior
to performing the respective tests.
Dry Density
To obtain the dry density of the lightweight insulating concrete,
the oven dry weight should be determined first. To determine
the oven dry weight, cylinders and/or cubes (similar to
those prepared for the compressive strength testing), should be
molded. The molded samples are prepared and cured using the
same procedures as those for the compressive strength specimens.
However, after the 28-day cure, the specimens are placed
in an oven at 230 + 18 degrees Fahrenheit ( 110 + 10 degrees
Celsius) and weighed at 24-hour intervals until the loss of weight
does not exceed 1 %. Upon determining the oven dry weight and
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A Tenacious Expanding Adhesive
obtaining dimensions of the specimen, the dry density can then
be calculated.
Summary
By performing each of these tests, project personnel (specifier,
contractor, owner, etc.) can obtain assurance that newlyplaced
lightweight insulating concrete will have the desired
characteristics originally intended. This comprehensive process
will also ensure that the lightweight insulating concrete will provide
a suitable substrate to receive the new roof assembly and
provide the long-term serviceability expected.
REFERENCES
1. Comten Industries Literature
2. Choiniere, Stan, and SPRl’s Fasteners Subcommittee,
“Standardizing Pullout Test Procedures,” Interface, April
1999
Also: The American Society of Testing and Materials
(ASTM) •
ABOUT THE AUTHOR
SUPERIOR U P L I F T P E R F 0 R M A N C E … T 0
October 1999 Interface • 9