FLEXURAL STRENGTH OF REINFORCED CONCRETE
WHAT?
Flexural strength
is one measure of the tensile strength of concrete. It is a measure of
an unreinforced concrete beam or slab to resist failure in bending. It is
measured by loading 6 x 6-inch (150 x 150-mm)
concrete beams with a span length at least three times the depth. The flexural strength is
expressed as Modulus of Rupture (MR) in psi (MPa) and is determined by standard
test methods ASTM C 78 (third-point loading)
or ASTM C 293 (center-point loading). Flexural MR is about 10 to 20 percent of compressive
strength depending on the type, size and volume of coarse aggregate used.
However, the best correlation for specific materials is obtained by laboratory
tests for given materials and mix design. The MR deter- mined by third-point loading
is lower than the MR determined
by center-point loading,
sometimes by as much as 15%.
WHY?
Designers of
pavements use a theory based on flexural strength. Therefore, laboratory mix
design based on flexural strength tests may be required,
or a cementitious material
content may be selected from past experience to obtain the needed design
MR. Some also use MR for field
control and acceptance of pavements. Very few use flexural
testing for structural concrete. Agencies not using flexural
strength for field
control generally find
the use of compressive strength convenient and reliable
to judge the quality of the concrete as delivered.
HOW?
Beam specimens
must be properly made in the field. Pavement concrete mixtures are stiff (1/2 to 2 1/2-inch slump).
Consolidate by vibration in accordance with ASTM C 31 and tap sides to release
air pockets. For higher slump, after rodding, tap the molds to release air
pockets and spade along the sides to consolidate. Never allow the beam surfaces
to dry at any time. Immerse in saturated limewater for at least 20 hours before
testing. Specifications
and investigation of apparent low strengths should
take into account
the higher variability of flexural strength results.
Standard deviation for concrete flexural strengths
up to 800 psi (5.5 MPa) for projects with good control range from
about 40 to 80 psi (0.3 to 0.6 MPa). Standard deviation values over 100 psi
(0.7 MPa) may indicate testing
problems. There is a high
likelihood that testing problems, or moisture differences within
a beam caused from premature drying, will cause
low strength. Where a
correlation between flexural and compressive strength has been established in
the laboratory, core strengths by ASTM C 42 can be used for compressive
strength to check against the desired value using the ACI 318 criteria of 85
percent of specified strength for the average of three cores. It is impractical to saw beams
from a slab for flexural
testing. Sawing beams will greatly reduce measured flexural strength and should not be done. In
some instances, splitting tensile strength of cores by ASTM C 496 is used, but experience is limited on how to apply the
data. Another procedure for in-place
strength investigation uses compressive strength
of cores calibrated by comparison with acceptable placements in proximity to the
concrete in question.
WHAT?
Flexural tests are extremely sensitive to specimen preparation, handling, and curing procedure. Beams are very heavy and can be damaged when
handled and trans- ported from the jobsite to the lab. Allowing a beam to dry
will yield lower
strengths. Beams must be cured
in a standard manner, and tested while wet. Meeting all these
requirements on a jobsite is extremely difficult often resulting in unreliable and generally low MR values. A short period of drying can
produce a sharp drop in flexural strength. Many state highway
agencies have used flexural strength but are now changing
to compressive strength or maturity concepts for job control
and quality assurance
of concrete paving. Cylinder compressive
strengths are also used for concrete
structures. NRMCA and the American
Concrete Pavement Association (ACPA) have a policy
that compressive strength
testing is the preferred method of concrete acceptance and that certified
technicians should con- duct the testing.
ACI Committees 325 and 330 on concrete pavement construction and design
and the Port- land Cement Association (PCA) point to the use of compressive
strength tests as more convenient and reliable.
The concrete industry
and inspection and testing agencies are much more familiar with traditional
cylinder compression tests for control and acceptance of concrete. Flexure
can be used for design purposes, but the corresponding compressive strength should
be used to order and accept the concrete. Any
time trial batches are made, both flexural and compressive tests should be made
so that a correlation can be developed for field control.
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