Air Content of Freshly Mixed Concrete by the Pressure Method (C 231)

Significance Concrete is not a solid, but rather a solid with void spaces. The voids may contain gas such as air, or liquid such as water. All concrete contains air voids, and the amount can be increased by the addition of an air entraining agent to the mix. When such an agent is used, the size of the voids drastically decreases and the number of voids greatly increases, providing a much greater dispersal of voids. Air entrainment is necessary in concrete that will be saturated and exposed to cycles of freezing and thawing and to deicing chemicals. The microscopic entrained air voids provide a site for relief of internal pressure that develops as water freezes and thaws inside the concrete. Without the proper entrained-air content, normal concrete that is saturated and is exposed to cycles of freezing and thawing can fail prematurely by scaling, spalling, or cracking. Care must be taken, however, not to have too much entrained air. As the air content increases, there will be a corresponding reduction in the strength and other desirable properties of the concrete. Typically, this strength reduction will be on the order of 3 to 5 percent for each 1 percent of air content. A concrete mix design proportioned for 5 percent air, for example, will be approximately 15 to 25 percent lower in strength if the air content were to double. Scope This procedure covers determination of the air content in freshly mixed portland cement concrete containing dense aggregates in accordance with AASHTO T 152 (Type B meter). It is not for use with lightweight or highly porous aggregates. This procedure includes calibration of the " Type B " air meter gauge and two methods for calibrating the gauge are presented. Concrete containing aggregate that would be retained on the 2 inch (50 mm) sieve must be wet sieved. Sieve a sufficient amount of the sample over the 1½ in (37.5 mm) sieve in accordance with the FOP for WAQTC TM 2. Apparatus • Air Meter – Type B, as described in AASHTO T 152. • Balance or Scale – Accurate to 0.3 percent of the test load at any point within the range of use (for Method 1 calibration only). • Verified external or internal calibration vessel of known volume (usually 5 percent ± of the volume of the meter base). • Tamping Rod – 5/8 inch (16 mm) diameter and …