ACI

1. ACI Concrete Field Sampling

2. AASHTO 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

3. Frequency 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.

4. Frequency of Field Sampling • 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)

5. Field 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.

6. Sampling 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

7. Sampling 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

8. Storing Field Samples • Once field samples are taken: • Prevent: • evaporation • exposure to direct sunlight • Exposure to radiant heat sources • Store between 60o and 80o • Transport to lab within 48 hours

9. Field Testing • Field test must meet project specifications: • Slump • Air Entrainment • Unit Weight

10. Lab 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

11. Investigation 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

12. 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

13. When Results are NOT Acceptable • Strengthening of the structure or removal / replacement is required!! • $$$$$$$$$$$$$$$$$$$$$$$$$$$$$