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ACI Concrete Field Sampling. AASHTO Concrete Classifications n Concrete for use in highway bridges...
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Transcript of ACI Concrete Field Sampling. AASHTO Concrete Classifications n Concrete for use in highway bridges...
ACIACI
Concrete Field Sampling
AASHTO Concrete ClassificationsAASHTO Concrete Classifications Concrete for use in highway bridges is defined as Class A, B, C, P,
or S. Class A is prescribed unless another class of concrete is more
appropriate.– Class A concrete shall contain six (6) sacks of cement to the cubic yard– The water content shall not exceed six (6.0) U.S. gallons
per sack of cement (W/C < .54) Class B concrete is for footings, pedestals, massive pier shafts, and
gravity walls.– Class B concrete shall contain five (5) sacks of cement to the cubic yard. – The water content shall not exceed six and a half (6.5) gallons per sack
of cement (W/C < .58 Class C concrete is prescribed for thin sections Class P concrete is used when compressive strengths in excess of
4,000 psi Class S is used for concrete deposited underwater
Frequency of Field SamplingFrequency of Field Sampling
ACI defines requirements of evaluation and acceptance of concrete on the job.
3 criteria establish the required minimum sampling frequency:
– Once each day a given class is placed, nor less than…
– Once for each 150 yd3 of each class placed each day, nor less than….
– Once for each 5000 ft2 of slab or wall surface area placed each day
Minimum of 5 tests are required if less than 5 batches are required.
When less than 50 yd3 of concrete are to be placed:
– strength tests not required if satisfactory evidence of strength has been submitted and approved (previous quality assurance discussion)
Frequency of Field SamplingFrequency of Field Sampling
Field Sampling & TestingField Sampling & Testing
A standard cylinder mold is 6” in diameter and 12” high.
One test result is considered the average of two cylinders at 28-days.
Not more than one test (2 cylinders) shall be taken from a single batch.
Water may not be added to the batch after the sample is taken.
Sampling The ConcreteSampling The Concrete
Proper molding of cylinders for compressive strength:– Molded on rigid surface free of vibration– Placed in 3 lifts– Rodded 25 strokes per layer– To consolidate the layers, tap the outside of the
cylinder lightly with the rod until holes close– Trowel finish the top surface – Cover with a plastic lid once bleed water has
evaporated
Sampling The ConcreteSampling The Concrete
Flexural Beams: – Modulus of Rupture for
Pavement Design Purposes
– 6” x 6” x 20” beams– Cast in two layers– Rodded once for each 2 in2 of surface area– Tapped with rubber mallet 10 times each layer– Spade along edges– Finish & cover
R=Pl bd2
Storing Field SamplesStoring Field Samples
Once field samples are taken:– Prevent:
evaporation exposure to direct sunlight Exposure to radiant heat sources
Lab Curing & TestingLab Curing & Testing
Once the samples arrive at the lab:– Demold– Store in moist condition
(limetank or moist-cure room)
– Store at 73.4o +/- 3o
– Tested at 28-days
Investigation of Low-Strength Test ResultsInvestigation of Low-Strength Test Results
If lab-cured cylinders from the field have average strengths below specified by more than 500 psi:
– Verify sampling & testing procedures: Properly surfaced specimens? Initial curing over 80° (in field)? Frozen cylinders? Extra days in the field? Impact during transport? Delay in curing at the lab? Improper testing procedure?
– Lab is held accountable
Low Results: What Then?Low Results: What Then?
– 3 cores should be taken from the structure in question for each substandard lab test.
– core tests shall be considered adequate if the average is equal to 85% of specified….and...
– and if no single test is less than 75% of specified
When Results are NOT AcceptableWhen Results are NOT Acceptable
Strengthening of the structure or removal / replacement is required!!
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