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WHAT are Concrete Test Cylinders
Most commonly, the compressive strength of concrete is measured
to ensure that concrete delivered to a project meets the requirements
of the job specification and for quality control. For testing the compressive
strength of concrete, cylindrical test specimens of size 4 ´
8-inch (100 ´ 200-mm) or 6 ´ 12-inch (150 ´ 300-mm) are cast and
stored in the field until the concrete hardens in accordance with the
requirements of ASTM C 31, Standard Practice for Making and
Curing Concrete Test Specimens in the Field.
Most specifications require that technicians certified by the ACI
Field Testing Certification Grade I, or an equivalent program make
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test specimens in the field. When making cylinders for acceptance
of concrete, the field technician must test other properties of the
fresh concrete to include temperature, slump, density (unit weight)
and air content. This information should accompany the set of cylinders
made for a particular pour or pour location. A strength test
result is always the average of at least two specimens tested at the
same age. A set of 2 to 6 cylinders may be made from the same
sample of concrete at a
WHY Make Concrete Test Cylinders?
According to ASTM C 31, the results of standard-cured cylinders
are used for
- Acceptance testing for specified strengths,
- Verifying mixture proportions for strength,
- Quality control by the concrete producer
It is of prime importance that the specimens are made and cured
following standard procedures. Any deviation from standard procedures
will result in a lower measured strength. Low strength test
results due to procedures not in accordance with the standards cause
undue concern, cost and delay to the project.
The strength results of field-cured cylinders are used for
- Determining the time at which a structure is permitted to be put
into service,
- Evaluating the adequacy of curing and protecting concrete in
the structure, and
- Scheduling removal of forms or shoring
Curing requirements for field-cured cylinders are different from standard
curing and the two should not be confused. Refer to ASTM C
31 for details on curing field-cured specimens.
HOW to Make & Cure Cylinders
Equipment needed at the job site:
- Molds for casting cylinder specimens. Plastic molds are most
common.
- Tamping rod with hemispherical tip - 5/8-inch (15-mm) diameter
for 6 ´ 12-inch cylinders or 3/8-inch (10-mm) diameter for 4´ 8-inch cylinders, or a vibrator
- Rubber or rawhide mallet, 1.25 ± 0.50 lb. (0.6 ± 0.2 kg)
- Shovel, hand-held wooden float, and scoop,
- Wheelbarrow or other appropriate sample container,
- Water tank or curing box with provisions to maintain required
curing environment during initial curing period.
- Safety equipment as appropriate to handle freshly mixed concrete.
Sampling concrete from a ready mixed concrete truck:
It is very important to obtain a sample of concrete that is representative
of the concrete in the truck mixer. Sampling from concrete
delivery units should be conducted in accordance with ASTM
C 172 Standard Practice for Sampling Freshly Mixed Concrete.
Concrete should be sampled from the middle of the load. The first
or last discharge from the load will not provide a representative
sample. The concrete must be sampled by diverting the chute into
a wheelbarrow so that the entire discharge stream is collected. At
least two portions during discharge are necessary to obtain a composite
sample. The time elapsed between the first and final portion
of the composite sample must not exceed 15 minutes. The
minimum required size of the concrete sample is 1 cu. ft. (28 L).
Prior to Casting Cylinders:
Cover the sample with a plastic sheet to protect the concrete from
evaporation, sunlight, and contamination. Move the sample to
the location where the fresh concrete tests are to be conducted.
The testing location should be close to where the cylinders will
be stored undisturbed for the initial curing period. After the concrete
is transported to the location for casting the cylinders, remix
the concrete in the wheelbarrow. Begin the slump, density
(unit weight), and air content tests within 5 minutes and start
molding cylinders within 15 minutes after the sample was obtained.
Casting The Test Cylinders:
- Label the outside of the mold with the appropriate identification
mark. Do not label the lids or tops.
- Place the cylinder molds on a level surface
- Determine the method of consolidation
- For concrete with slump less than 1-inch (25-mm), concrete
should be consolidated by vibration
- For concrete with slump 1-inch (25-mm) or higher, either
rodding or vibrating is permitted.
- Determine the number of layers of concrete to be placed in the
mold
- For concrete to be consolidated with the tamping rod, place
concrete in 3 equal layers for 6 ´ 12-inch cylinders; and in
2 equal layers for 4 ´ 8-inch cylinders
- For concrete to be consolidated by vibration, fill the mold
in two equal layers.
- Place the concrete in the mold by distributing it around the inside
of the mold with the scoop. Consolidate the layer by rodding
25 times evenly distributed around the layer. When using a vibrator,
insert it long enough so the surface is smooth and large
air pockets ceases to break through to the top. Two insertions of
the vibrator are required for a 6 ´ 12-inch and one insertion is
required for a 4 ´ 8-inch cylinders. Avoid over vibration.
- Tap the sides of the mold 10-15 times with the mallet after each
layer in order to close any insertion holes formed either by the
rod or the vibrator
- Strike off the top with a wooden float to produce a flat, even and
level surface and cover with a plastic lid or a plastic bag.
Storing and transporting test cylinders:
- Move cylinder molds with fresh concrete very carefully by supporting
the bottom
- Place the cylinders on a flat surface and in a controlled environment
where the temperature is maintained in the range of 60 to
80°F (16 to 27°C). When the specified strength of the concrete is
greater than 6000 psi (40 MPa), the temperature range for initial
curing should be maintained in the range of 68 to 78°F (20 to
26°C). Immersing cylinders, completely covered in water is an
acceptable and preferred procedure that ensures more reliable
strength results. Temperature in storage, such as in curing boxes,
should be controlled using heating and cooling devices as necessary.
The maximum and minimum temperature during initial
curing should be recorded and reported.
- Protect cylinders from direct sunlight or radiant heat and from
freezing temperatures in winter.
- Cylinders must be transported back to the laboratory within 48
hours of casting. Some concrete mixtures may take longer to set
and these specimens may be transported at a later time. In any
event cylinders should not be moved or transported until at least
8 hours after final set.
Store cylinders to prevent damage and maintain moisture during transportation.
Travel time from the jobsite to the laboratory should not
exceed 4 hours.
CAUTION
Fresh concrete can cause severe chemical burns to skin and eyes. Keep
fresh concrete off your skin. When working with concrete use rubber workboots,
gloves, protective eyeglasses and clothing. Do not let concrete or
other cement-based products soak into clothing or rub against your skin.
Wash your skin promptly after contact with fresh concrete with clean water.
If fresh concrete gets into your eyes, flush immediately and repeatedly
with water. Consult a doctor immediately. Keep children away from
all freshly mixed plastic concrete.
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