entrained-air indicator must be calibrated accurately,
and the correction factor for the aggregate contained in
the concrete must be determined.
To perform the test, follow the procedures contained
in NAVFAC MO-330 and in the instruction book
furnished with the meter. The instruction book also
describes the calculations for determining the
entrained-air content.
Unit Weight (ASTM C 138)
The unit weight, or density, of concrete varies with
the amount and density of the aggregate, the amount of
entrapped or entrained air, and the water and cement
contents. Conventional concrete used in structures, such
as buildings and pavements, has a unit weight in the
range of 140 to 150 pounds per cubic foot (pcf). For
other types of concrete, the unit weight ranges from 15
pcf for lightweight insulating concrete to 400 pcf for
heavyweight concrete.
To determine the unit weight of freshly mixed
concrete, you will need a cylindrical metal measure
(container) of either 1/10-, 1/5-, or 1/2-cubic-foot
capacity. If necessary, you should calibrate the measure
before performing the test procedures. To calibrate the
measure, you first determine the tare weight of the
measure, and then fill the measure with water at room
temperature. Then determine the temperature, density,
and weight (in pounds) of the water. To determine the
density of the water, use table 13-3 and interpolate, if
necessary. Next, calculate the calibration factor of the
measure by dividing the density of the water by the
weight of the water required to fill the measure.
The ASTM procedures for determining the unit
weight are summarized as follows:
Table 13-3.—Density of Water
1. Fill the measure with fresh concrete
consolidated in three layers, as described for the
air-content test. After each layer is rodded, tap the sides
of the container 10 to 15 times with a rubber or rawhide
mallet to remove any air pockets.
2. After filling and consolidating, strike off the top
surface, taking care to leave the measure level full.
3. Clean all excess concrete from the exterior of the
measure. Then weigh it and determine the net weight of
the concrete inside the measure by subtracting the tare
weight of the measure from the gross weight of the
measure and concrete.
4. Calculate the unit weight by multiplying the net
weight of the concrete by the calibration factor for the
measure.
Compressive Strength Test (ASTM C 39)
The compressive strength of hardened concrete is
determined from compression tests on standard
cylindrical specimens. As you know, compressive
strength tests are used during concrete mix design to
evaluate the performance of the materials and to
establish mixture proportions that will give the required
strength. The tests are used also to control the quality of
the concrete in the field.
“Compressive strength” is defined as the average of
the strengths of all cylinders of the same age made from
a sample taken from a single batch of concrete. At least
two cylinders, or preferably three, are required to
constitute a test. So, if tests are to be made at 7 and 28
days, you will need four or six specimens. The standard
specimen is 6 inches in diameter by 12 inches long and
is capped with a suitable material to provide a
smooth-bearing surface on each end of the specimen.
You learned the procedures for preparing and capping
compressive strength specimens in the EA3 TRAMAN.
If necessary, you should review those procedures. The
following paragraphs discuss only the procedures used
to perform compression tests on the prepared
specimens.
The equipment you will use to perform the
compression test is a compression-testing machine,
having a capacity of 250,000 pounds. An example of
that machine, shown with a test cylinder in place, is
illustrated in figure 13-22.
The procedures for conducting the compression test
are as follows:
1. Prepare the testing machine by cleaning the
bearing plates and, if needed, cleaning and lubricating
13-30
