The
Importance of Testing Concrete Masonry Units
General
A testing program for concrete masonry
units is a valuable tool for assuring consistent product quality and
uniformity and for monitoring changes in production. Another important
benefit of testing is that it is a visible demonstration of a company
wide commitment to quality.
The economic benefit of testing is the
ability to detect costly changes in raw material quality or production
processes and to quickly address inconsistencies. Examples of these
inconsistencies are miss-weighing cement or under-weighing aggregates,
which can lead to product rejection or excessive material costs if
allowed to continue.
A complete testing program consists of
daily tests at the plant and regularly scheduled tests at an independent
laboratory. Since in-plant testing is for internal use, producers may
customize sampling or testing procedures to suit their needs. In
contrast, laboratory testing is for external use and must be conducted
in accordance with sampling and testing procedures defined by ASTM C140
(Standard Methods of Sampling and Testing Concrete Masonry Units). An
ongoing program, using these two types of testing, is recommended so
that results can be compared and correlated.
In-Plant Testing
The goal of in-plant testing is to
discover indications of changes in the equipment, materials or in the
processes which are affecting concrete masonry unit quality, cost or
uniformity. Procedures described in ASTM C140 should be used as
guidelines for conducting in-plant testing.
The main benefit of in-plant testing is
the ability to obtain immediate test results. Variations in material
properties, process changes and batching malfunctions can be quickly
identified and communicated to employees so that remedies can be
implemented.
Pictured right: Tests
for compressive strength are conducted in the lab at the World Center
for Concrete Technology. (The door is open for photographic purposes
only.)
Equipment
The basic equipment for in-plant
testing consists of a compression testing machine, screens for
determining aggregate gradations and a gram scale for weighing
aggregate. Conducting absorption tests requires tanks for submersion and
a scale for weighing concrete masonry units. Plants with several
production lines may benefit from obtaining equipment similar to
commercial testing laboratories.
Sampling
Sampling techniques vary depending on
the characteristics being monitored. The concrete masonry units for
in-plant testing are frequently tested immediately after curing which is
much earlier than units from inventory or from job sites. Testing this
early does not reveal the ultimate unit strength, but it does provide
the opportunity to discover the effects of different curing conditions
and locations within the curing chamber. Testing in this case would
involve a set of three units from the same pallet that would be compared
to other pallet averages.
Testing
In some situations, indicator
compression tests may be conducted without using the time consuming
gypsum cement or sulfur caps. Gypsum board or wood veneer may be
substituted and used to cushion the unit and minimize the effects of
stress concentrations. This method yields lower indicated strengths, but
is quite consistent and permits quick testing for indicators or major
differences. Usually strengths appear to be 10% to 15% lower because of
the capping method and moisture content.
Laboratory Testing
The frequency of laboratory testing
depends upon the practices of the marketplace. If the concrete masonry
units on a job site are tested frequently by the owner or inspector, it
may not be necessary for the producer to conduct extensive tests.
However, if testing is required for product submittals prior to
construction, a full compliment of current test results for the products
being supplied is necessary.
Generally, testing of one set of
samples every month or two is sufficient. If aggregates or cements are
changed, a test should be performed to determine the effects. Other
tests such as durability and shrinkage tests should be performed
annually. It should be noted that ASTM standards do not contain a
minimum age before use, but that the concrete masonry units comply with
the standard when used.
Sampling
A set of samples for ASTM C140 consists
of six concrete masonry units selected at random from the project site
or from salable inventory. After selecting the samples, the units are
immediately marked using a consistent identification method. The marking
should include the sample identification number, date of sampling and
the sampling location.
Whenever concrete masonry units are
sent to a laboratory, parallel samples should be taken and tested
in-plant to compare the results.
Testing
At the lab three of the concrete
masonry units are checked for dimensional conformance, moisture content,
absorption and density. The remaining three units are capped and tested
to determine the compression strength.
The required capping materials are
either sulfur or gypsum cement. These capping methods insure complete,
uniform contact with the testing machine and that the faces are parallel
to each other. ASTM C140 requires that the units be tested within 72
hours to minimize the effects of unnatural curing. Units from absorption
tests must not be used for compression strength tests since the
unnatural moisture introduced and removed using heat cures the unit and
may affect the strength results.
A test to determine the drying
shrinkage of the unit may also be performed. This test needs to be
conducted annually. Results are usually very consistent unless there
were major changes in the type of aggregate or curing process.
The specifications for both
interlocking concrete pavers and segmental retaining walls contain
requirements for durability. The specialized equipment and duration of
the test (up to 3 months) almost requires that this test be performed in
an independent laboratory. Assuming favorable testing of the concrete
masonry units, the laboratory may certify the results. Durability
testing consists of subjecting product samples to alternating freeze and
thaw cycles in either fresh water or in a saline solution. Periodically
throughout testing, the units are weighed until the weight loss reaches
the specified level.
Aggregates may also be tested at a
commercial testing laboratory. These tests evaluate gradation,
soundness, organic materials and deleterious materials.
Pictured right:
Concrete masonry units are tested for absorption and density in the
absorption tank.
Records
All test results, including both
in-plant and laboratory tests should be recorded for future reference.
Batch proportions and age at testing should also be included.
Additionally, a complete history of test results is valuable in the
remote possibility of a dispute.
Summary
A consistent testing program is
essential to insuring that concrete masonry units are of uniform
quality. Testing provides insight into performance of raw materials,
plant processes and equipment. Testing also demonstrates concern for
product uniformity and quality.
| References
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NCMA TEK 18-2, National Concrete
Masonry Association (NCMA), Herndon, Virginia, 1994.
Annual Book of Standards – Volume
04.05, American Society for Testing and Materials (ASTM), Conshocken,
Pennsylvania, 2000. |
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