When the designer considers it appropriate, 90-day
compressive or flexural strength may be specified in
lieu of 28-day, but not both.
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Specified compressive strength (f'c) shall be as follows:
COMPRESSIVE STRENGTH
STRUCTURE OR PORTION OF STRUCTURE
[35 MPa 5000 psi at 28 days
[_____]]
[27.5 MPa 4000 psi at 28 days
[_____]]
[20 MPa 3000 psi at 28 days
[_____]]
[[_____] MPa psi at [_____] days
[_____]]
Concrete slabs on-grade shall have a 28-day flexural strength of [4.5]
[_____] MPa [650] [_____] psi. Concrete made with high-early strength
cement shall have a 7-day strength equal to the specified 28-day strength
for concrete made with Type I or II portland cement. Compressive strength
shall be determined in accordance with ASTM C 39/C 39M. Flexural strength
shall be determined in accordance with ASTM C 78.
a.
Evaluation of Concrete Compressive Strength. Compressive strength
specimens (152 by 305 mm 6 by 12 inch cylinders) shall be
fabricated by the Contractor and laboratory cured in accordance
with ASTM C 31/C 31M and tested in accordance with ASTM C 39/C 39M.
The strength of the concrete will be considered satisfactory so
long as the average of all sets of three consecutive test results
equals or exceeds the specified compressive strength f'c and no
individual test result falls below the specified strength f'c by
more than 3.5 MPa 500 psi. A "test" is defined as the average of
two companion cylinders, or if only one cylinder is tested, the
results of the single cylinder test. Additional analysis or
testing, including taking cores and/or load tests may be required
at the Contractor's expense when the strength of the concrete in
the structure is considered potentially deficient.
b.
Investigation of Low-Strength Compressive Test Results. When any
strength test of standard-cured test cylinders falls below the
specified strength requirement by more than 3.5 MPa 500 psi or if
tests of field-cured cylinders indicate deficiencies in protection
and curing, steps shall be taken to assure that the load-carrying
capacity of the structure is not jeopardized. When the strength
of concrete in place is considered potentially deficient, cores
shall be obtained and tested in accordance with ASTM C 42/C 42M.
At least three representative cores shall be taken from each
member or area of concrete in place that is considered potentially
deficient. The location of cores will be determined by the
Contracting Officer to least impair the strength of the structure.
Concrete in the area represented by the core testing will be
considered adequate if the average strength of the cores is equal
to at least 85 percent of the specified strength requirement and
if no single core is less than 75 percent of the specified
strength requirement. Non-destructive tests (tests other than
test cylinders or cores) shall not be used as a basis for
acceptance or rejection. The Contractor shall perform the coring
and repair the holes. Cores will be tested by the Government.
c.
Load Tests.
If the core tests are inconclusive or impractical to
SECTION 03300A
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