PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 279

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7.5.1 Compressive Strength Test

The compressive strength test is the test most commonly performed on
hardened concrete. Compressive strength is one of the main structural design
requirements to ensure that the structure will be able to carry the intended
load. As indicated earlier, compressive strength increases as the water-
cementitious materials ratio decreases. Since the water-cementitious mate-
rials ratio is directly related to the concrete quality, compressive strength
is also used as a measure of quality, such as durability and resistance to
weathering. Thus, in many cases, designers specify a high compressive
strength of the concrete to ensure high quality, even if this strength is not
needed for structural support. The compressive strength of normal-
weight concrete is between 20 MPa to 40 MPa (3000 psi to 6000 psi). In the
United States, the test is performed on cylindrical specimens and is stan-
dardized by ASTM C39. The specimen is prepared, either in the lab or in
the field, according to ASTM C192 or C31, respectively. Cores could also be
drilled from the structure following ASTM C42. The standard specimen size
is 0.15 m (6 in.) in diameter and 0.30 m (12 in.) high, although other sizes with
a height-diameter ratio of two can also be used. The diameter of the specimen
must be at least three times the nominal maximum size of the coarse aggregate
in the concrete.
In the lab, specimens are prepared in three equal layers and are rodded
25 times per layer. After the surface is finished, specimens are kept in the
mold for the first Specimens are then removed from the
mold and cured at either in saturated-lime water
or in a moist cabinet having a relative humidity of 95% or higher, until the
time of testing. Before testing, specimens are capped at the two bases to en-
sure parallel surfaces. High-strength gypsum plaster, sulfur mortar, or a
special capping compound can be used for capping and is applied with a
special alignment device (ASTM C617). Using a testing machine, speci-
mens are tested by applying axial compressive load with a specified rate
of loading until failure (Figure 7.30). The compressive strength of the spec-
imen is determined by dividing the maximum load carried by the speci-
men during the test by the average cross-sectional area. The number of
specimens and the number of test batches depend on established practice
and the nature of the test program. Usually three or more specimens are
tested for each test age and test condition. Test ages often used are 7 days
and 28 days.
Note that the test specimen must have a height-diameter ratio of two.
The main reason for this requirement is to eliminate the end effect due to the
friction between the loading heads and the specimen. Thus, we can guarantee
a zone of uniaxial compression within the specimen. If the height-diameter
ratio is less than two, a correction factor can be applied to the results as in-
dicated in ASTM C39.
The compressive strength of the specimen is affected by the specimen
size. Increasing the specimen size reduces the strength, because there is a
greater probability of weak elements where failure starts in large specimens

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