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Air-Content Test (ASTM C 231) An air-entraining admixture is added to the concrete mix so that enough air will be entrained to improve workability and durability of the mixture, but not enough to reduce strength substantially. Air-entraining cements may also be available for use in some military situations. The desired amount of air is generally from 4.0 to 7.5 percent of the total mix.The equipment for determining the percentage of entrained air is included in the boxed test kit. The basic tool is the pressure type of indicator, as illustrated in figure 13-21. The equipment furnished in these kits varies with the manufacturers. Each kit contains the complete equipment for conducting the test, including a detailed instruction pamphlet and the calibration procedure for the particular meter. Before the air content of a concrete mixture can be determined, the entrained-air indicator must be calibrated accurately, and the correction factor for the aggregate contained in the concrete must be determined. To perform the test, follow the procedures contained in NAVFAC MO-330 and in the instruction book furnished with the meter. The instruction book also describes the calculations for determining the entrained-air content. Unit Weight (ASTM C 138) The unit weight, or density, of concrete varies with the amount and density of the aggregate, the amount of entrapped or entrained air, and the water and cement contents. Conventional concrete used in structures, such as buildings and pavements, has a unit weight in the range of 140 to 150 pounds per cubic foot (pcf). For other types of concrete, the unit weight ranges from 15 pcf for lightweight insulating concrete to 400 pcf for heavyweight concrete. To determine the unit weight of freshly mixed concrete, you will need a cylindrical metal measure (container) of either 1/10-, 1/5-, or 1/2-cubic-foot capacity. If necessary, you should calibrate the measure before performing the test procedures. To calibrate the measure, you first determine the tare weight of the measure, and then fill the measure with water at room temperature. Then determine the temperature, density, and weight (in pounds) of the water. To determine the density of the water, use table 13-3 and interpolate, if necessary. Next, calculate the calibration factor of the measure by dividing the density of the water by the weight of the water required to fill the measure. The ASTM procedures for determining the unit weight are summarized as follows:
Table 13-3.Density of Water 1. Fill the measure with fresh concrete consolidated in three layers, as described for the air-content test. After each layer is rodded, tap the sides of the container 10 to 15 times with a rubber or rawhide mallet to remove any air pockets. 2. After filling and consolidating, strike off the top surface, taking care to leave the measure level full.3. Clean all excess concrete from the exterior of the measure. Then weigh it and determine the net weight of the concrete inside the measure by subtracting the tare weight of the measure from the gross weight of the measure and concrete. 4. Calculate the unit weight by multiplying the net weight of the concrete by the calibration factor for the measure. |
 
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