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ASTM C31 - Making and Curing Concrete Test Specimens in the Field

For more training, check out our online ACI course with two practice tests.

When strength is used as a basis for acceptance of concrete, specimens must be molded and cured according to ASTM C31. Cylinder molds can be metal or plastic, so long as they are non-absorbent, non-reactive to concrete, and maintain their shape and dimensions under all conditions of use. They must not leak when water is poured into them (empty).

  1. Tamping Rod must be 3/8" dia (+/- 1/16-inch) for molds less than 6" in diameter or width (beam molds).
  2. Vibrators must produce at least 9,000 vibrations per minute, and vibrating element must be no bigger than one-fourth the diameter of the cylinder mold or one-fourth the width of the beam mold.

Vibrators or rodding are entirely interchangeable, but vibrators must be used for slumps 1" or less.

Cylinders

  1. When rodding cylinders, fill in: 2 layers (25 rods) for 4" dia cylinders; 3 layers (25 rods) for 6" dia cylinders; 4 layers (50 rods) for 9" dia cylinders.
  2. Tap outside of molds 10 - 15 times with a mallet or an open hand for each lift.
  3. When vibrating cylinders, fill in: 2 layers for all cylinder diameter sizes, but insert vibrator once for 4" dia cylinders, twice for 6" dia cylinders, and four times for 9" dia cylinders for both layers.
  4. Insert rod or vibrator 1 inch into the underlying lift.
  5. Cylinder must be at least 3 times the size of the coarse aggregate (aggregate larger than 2" must be wet-sieved out of the mix).
  6. Cylinder molds must be twice the length of the diameter.

Beams

  1. When rodding beams, fill in: 2 layers (rod once for every 2 sq. in. of top surface area of beam) for beam widths 6 - 8 inches; 3 or more equal layers, layers not to exceed 6" depth (rod once for every 2 sq. in. of top surface area of beam).
  2. When vibrating beams, fill in: 1 layer for beam widths 6 - 8 inches, 2 or more layers for widths greater than 8 inches (inserting vibrator uniformly until the surface of the concrete has become relatively smooth and large air bubbles cease to break through the top surface).
  3. Tap outside of molds 10 - 15 times with a mallet for each lift.
  4. Beams must be at least 3 times the size of the coarse aggregate in the smallest cross section (ex. a beam 6 x 9 x 20, smallest cross section is 6 inches, which means aggregate cannot be larger than 2 inches for this size mold).
  5. Beam mold dimension--the length must be at least 2 inches greater than three times the depth (ex. if depth is 6", length must be 20" min.). Width must not exceed 1.5 times the depth (ex. depth is 6", width can be 6" to 9" max.). Unless specified by the project, depths and widths must be at least 6 inches.

When planning to mold and cure concrete specimens, it is required that tests for slump, temperature, and air content be performed (unless entrained air is not of concern to the project).

Standard Curing

During initial curing, the cylinders must be stored in a temperature range from 60 - 80 degrees F in an environment that prevents moisture loss for up to 48 hours. If the concrete design strength is 6,000 psi or greater, the initial curing temperature must range from 68 - 78 degrees F. Specimens should be shielded from direct sunlight or radiating heating devices, if used. A minimum-maximum thermometer shall record the temperature of the initial curing period, and then recorded upon retrieval of cylinders.

For final curing, cylinders or beams must be placed in their curing storage no later than 30 minutes after removing from the molds. Cylinders shall have free water maintained on their surfaces at all times and remain in a constant temperature of 73.5 degrees F, +/- 3.5 degrees F. Cylinders and beams can be placed in moist rooms or water storage tanks, but beams must be moved into water saturated with calcium hydroxide at the same temperature at least 20 hours prior to testing. Prevent the drying of the surfaces of the beams between the removal of the water storage tank and testing.

Field Curing

Store cylinders or beams in or on the structure representing the area from which the sample was taken. Protect the surfaces of both in the same manner the structure is protected, as best as possible. For beams, at the end of the initial 48 hours (+/- 4 hours) after molding, remove the molds and place them at the structure location they represent. If representing pavements of a slab on grade, bank the sides and ends with earth or sand that must be kept damp, leaving the top surface exposed. As with standard curing, remove the beams from the field and store in water saturated with calcium hydroxide for 24 hours (+/- 4 hours) prior to testing.

Transporting

Protect cylinders with suitable cushioning material to prevent damage from jarring. Also prevent moisture loss by wrapping specimens in wet burlap or plastic, if not contained in plastic already. Transportation time of cylinders shall not exceed 4 hours.

Reporting

Report the following for each sample brought into the laboratory:

  1. ID number (such as project number written on specimens)
  2. Location of concrete represented by the samples
  3. Date and time
  4. Name of individual molding the specimens
  5. Slump, air content, and concrete temperature
  6. Curing method