Concrete Field Sampling
ACI Concrete Field Sampling 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 American Association of State Highway and Transportation Officials (AASHTO). Concrete for use in highway bridges is defined as Class A, B, C, P, or S. Class A concrete is generally prescribed unless another class of concrete is more appropriate. Class B concrete is for footings, pedestals, massive pier shafts, and gravity walls. Class C concrete is prescribed for thin sections. Class P concrete is used when compressive strengths in excess of 4,000 psi (28 MPa) are needed and Class S is used for concrete deposited underwater. The requirements for these five classes are prescriptive in that a minimum cementitious materials content and maximum water-cementitious materials ratio are specified in addition to the performance requirement of a minimum compressive strength. For many high performance concrete applications, performance based specifications are more applicable.See the AASHTO Construction Specifications for further details. 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. 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) 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. 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 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 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 Field Testing Field test must meet project specifications: Slump
Air Entrainment Unit Weight 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 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 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 Acceptable
Strengthening of the structure or removal / replacement is required!! $$$$$$$$$$$$$$$$$$$$$$$$$$$$$