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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
    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

       molding of cylinders for
 Proper
 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                               R=Pl
    –   Finish & cover                                    bd2
      Storing Field Samples

   Once field samples are taken:
    – Prevent:
        evaporation
        exposure to direct sunlight
        Exposure to radiant heat sources
    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!!

   $$$$$$$$$$$$$$$$$$$$$$$$$$$$$

				
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posted:8/7/2011
language:English
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