ASTM C231 Air Content Test Explained

In this next series of blogs, I am going to examine the various ASTM Standard Test Methods used in testing concrete in a precast operation.  The first blog will be on ASTM C231, the Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.

ASTM C231 is the test method most commonly used by precast quality control personnel.  The NPCA Plant Certification program requires that this test be performed daily to verify the total air content present in the concrete sample.  Note that the air measurement attained in this sample may differ from the actual air content of the cured concrete it represents.

The scope of ASTM C231 specifically addresses concretes (or mortars) with dense aggregates.  It is not suitable to use this method when testing for air content of lightweight aggregate concrete or concrete made with air cooled blast furnace slag.  For these materials, ASTM C173 is to be used.  Also, ASTM C231 is not applicable for concretes often referred to as dry cast.  These mixes are not plastic, and they are often used for the manufacture of concrete pipe or masonry units.

While this ASTM standard shows two different meters, a Type A and a Type B, this blog will only refer to the Type B meter.  It is the experience of the author that this is the most widely used air meter by precast concrete manufacturers.  The Type B meter has an air pump, two petcocks, an air release valve, and a bleeder valve.  The bowl is typically ¼ cubic foot in volume, although if this bowl is used for computing density, the volume must be verified.  The bowl should have a diameter that is equal to the depth within a range of ±25%.

There is a strike-off bar and a strike-off plate for leveling the concrete sample.  The strike-off bar can be used when only using the sample for an air test using this standard.  If a density test (ASTM C138) will be performed with the same sample of concrete being prepared for the air test, then a strike-of plate must be used.

The air meter needs to be calibrated at least every three months.  ASTM C231 provides instructions for the calibration process in the appendix of the standard.  The calibration can be done by the plant personnel with knowledge of the procedure.  Documentation should be kept of the calibration, especially if the producer must provide evidence to a DOT or for Plant Certification.  The calibration date and initial pressure (if applicable) are to be present on the meter.

The test method is straight forward; the bowel is filled as noted in the standard.  Using Self Consolidating Concrete does not require three lifts or rodding as required in conventional mixes.  The bowel is filled to approximately 1/8” over the top of the bowl.  The concrete is struck off, and the sealing surface is cleaned.  Refer to the test standard for the exact procedures.  When filling with water through the petcock, the second petcock is to remain open.  Once water is seen exiting the second petcock, the meter is to be jarred gently to expel any trapped air.

With the aid bleeder valve closed, the meter air chamber is pumped to the initial pressure mark for the meter.  It is very important to leave the petcocks open at this point.  Why?  This allowed the technician to verify that the seals between the air chamber and the bowel are not leaking.  If there is an air leak, bubbles will form in the petcocks.  Once the initial pressure is reached, wait a few seconds to allow the compressed air to cool.  Gently tap on the gauge to assure an accurate reading.  Add or bleed air as necessary.  Release air into the bowel.  Tap the gauge to make sure there is an accurate reading.  This reading is the apparent air reading.  The sample air content is the computed by subtracting the aggregate correction factor from the apparent air reading.

What is an aggregate correction factor?  ASTM C231 is very clear that even when using dense aggregates, it is possible to have air within the pores of the aggregate.  Since this may affect the air content within the matrix of the concrete paste, this must be calculated and subtracted from the apparent air reading.  See ASTM C231, section 6, for the exact procedure for computing the aggregate correction factor.