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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