Mixing and handling can significantly alter the air content of fresh concrete. Thus, field tests are used to ensure that the concrete has the proper air content prior to placing. Air content can be measured with the pressure, volumetric, gravimetric, or Chace air indicator methods. The pressure method (ASTM C231) is widely used, since it takes less time that the volumetric method. The pressure method is based on Boyle's law, which relates Figure 7.13 Consolidating concrete with an internal vibrator. pressure to volume. A calibrated cylinder (Figure 7.14) is filled with fresh concrete. The vessel is capped and air pressure is applied. The applied pressure compresses the air in the voids of the concrete. The volume of air voids is determined by measuring the amount of volume reduced by the pressure applied. This method is not valid for concrete made with lightweight aggregates, since air in the aggregate voids is also compressed, confounding the measurement of the air content of the cement paste. Main air valve Pressure gauge Pump Petcock B Air bleeder valve Petcock A - Air chamber - Clamping device "Extension tubing for calibration checks Bowl FIGURE 7.14 Pressure method apparatus for determining air voids in fresh concrete-Type B Meter. (reprinted, with permission, from ASTM C231, Figure 2, copyright ASTM Intermational, 100 Barr Harbor Drive, West Conshohocken, PA 19428)
Mixing and handling can significantly alter the air content of fresh concrete. Thus, field tests are used to ensure that the concrete has the proper air content prior to placing. Air content can be measured with the pressure, volumetric, gravimetric, or Chace air indicator methods. The pressure method (ASTM C231) is widely used, since it takes less time that the volumetric method. The pressure method is based on Boyle's law, which relates Figure 7.13 Consolidating concrete with an internal vibrator. pressure to volume. A calibrated cylinder (Figure 7.14) is filled with fresh concrete. The vessel is capped and air pressure is applied. The applied pressure compresses the air in the voids of the concrete. The volume of air voids is determined by measuring the amount of volume reduced by the pressure applied. This method is not valid for concrete made with lightweight aggregates, since air in the aggregate voids is also compressed, confounding the measurement of the air content of the cement paste. Main air valve Pressure gauge Pump Petcock B Air bleeder valve Petcock A - Air chamber - Clamping device "Extension tubing for calibration checks Bowl FIGURE 7.14 Pressure method apparatus for determining air voids in fresh concrete-Type B Meter. (reprinted, with permission, from ASTM C231, Figure 2, copyright ASTM Intermational, 100 Barr Harbor Drive, West Conshohocken, PA 19428)
Chapter2: Loads On Structures
Section: Chapter Questions
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