Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.9
In addition to the formal testing procedures specified by ASTM and the special
procedures described in the research references, some practical auxiliary tests, precautions
in evaluating tests, and observations that may aid the user in practical
applications follow.
Compressive Strength, ƒ�c . The standard test (ASTM C39) is used to establish the
quality of concrete, as delivered, for conformance to specifications. Tests of companion
field-cured cylinders measure the effectiveness of the curing (Art. 9.14).
Core tests (ASTM C42) of the hardened concrete in place, if they give strengths
higher than the specified ƒ�c or an agreed-on percentage of ƒ� c (often 85%), can be
used for acceptance of material, placing, consolidation, and curing. If the cores
taken for these tests show unsatisfactory strength but companion cores given ac-
celerated additional curing show strengths above the specified , these tests estab-
lish acceptance of the material, placing, and consolidation, and indicate the remedy,
more curing, for the low in-place strengths.
For high-strength concretes, say above 5000 psi, care should be taken that the
capping material is also high strength. Better still, the ends of the cylinders should
be ground to plane.
Indirect testing for compressive strength includes surface-hardness tests (impact
hammer). Properly calibrated, these tests can be employed to evaluate field curing.
(See also Art. 9.14.)
Modulus of Elasticity E c. This property is used in all design, but it is seldom
determined by test, and almost never as a regular routine test. For important projects,
it is best to secure this information at least once, during the tests on the trial
batches at the various curing ages. An accurate value will be useful in prescribing
camber or avoiding unusual deflections. An exact value of E c is invaluable for longspan,
thin-shell construction, where deflections can be large and must be predicted
accurately for proper construction and timing removal of forms.
Tensile Strength. The standard splitting test is a measure of almost pure uniform
tension ƒ ct. The beam test (Fig. 9.4a) measures bending tension ƒ r on extreme
surfaces (Fig. 9.4b), calculated for an assumed perfectly elastic, triangular stress
distribution.
The split-cylinder test (Fig. 9.4c) is used for structural design. It is not sensitive
to minor flaws or the surface condition of the specimen. The most important application
of the splitting test is in establishment of design values for reinforcingsteel
development length, shear in concrete, and deflection of structural lightweight
aggregate concretes.
The values of ƒ ct (Fig. 9.4d) and ƒ r bear some relationship to each other, but are
not interchangeable. The beam test is very sensitive, especially to flaws on the
surface of maximum tension and to the effect of drying-shrinkage differentials, even
between the first and last of a group of specimens tested on the same day. The
value ƒ r is widely used in pavement design, where all testing is performed in the
same laboratory and results are then comparable.
Special Properties. Frequently, concrete may be used for some special purpose
for which special properties are more important than those commonly considered.
Sometimes, it may be of great importance to enhance one of the ordinary properties.
These special applications often become apparent as new developments using new
materials or as improvements using the basic materials. The partial list of special
properties is constantly expanding—abrasion and impact resistance (heavy-duty
floor surfacings), heat resistance (chimney stacks and jet engine dynamometer
ƒ� c