ASTM C1738
Understanding ASTM C1738/C1738M
ASTM C1738/C1738M is a standard practice established for the high-shear mixing of hydraulic cement pastes. This standard is vital in laboratory research, particularly in assessing the rheological properties of hydraulic cement systems. It has been demonstrated to provide a paste with properties similar to those in concrete from which the aggregate has been removed. The high-shear mixing process outlined in this standard significantly differs from that in ASTM C305, as it imparts a higher shear to the cement paste, crucial for accurate rheological analysis.
Test Procedure for ASTM C1738/C1738M
Apparatus Setup: Utilize a mixer composed of an electric motor with a controlled speed, a mixing container, and a tachometer. The motor should maintain preset rotational speeds (420 rad/s and 1047 rad/s) under load. A cooling unit is required to control the paste temperature, along with a specific mixing container and blade assembly designed for high-shear mixing.
Temperature Control: Ensure the mixing room's temperature and humidity conform to ASTM C511 requirements. Adjust the cooling unit to maintain the cement paste temperature within the specified range.
Material Preparation: Follow the specified materials and proportions as per the method for which the paste is being prepared. Use a feeding system capable of adding all cementitious material into the mixing container within 60 seconds.
Mixing Process:
Place the mixing container on the mixer and connect it to the cooling unit.
Adjust the water temperature in the mixing container to the desired level.
Begin mixing at 420 rad/s to stabilize the water temperature.
Add the dry cementitious materials uniformly to the mixing container while continuing the mixing process.
After adding the dry materials, switch to the second preset speed (1047 rad/s) and mix for 30 seconds.
Allow the paste to rest for 150 seconds. During the initial 15 seconds, scrape down the sides of the container to incorporate any dry materials.
Resume mixing at 1047 rad/s for another 30 seconds following the rest period.
Final Steps:
Immediately after the final mixing, stop the mixer and measure the paste temperature without touching the container's bottom.
The paste is now ready for further use or testing as required.
Significance and Application
ASTM C1738/C1738M is instrumental in creating a standardized approach for preparing cement pastes for rheological testing. By providing a methodology that simulates the shear conditions similar to those found in concrete, this practice allows for more accurate and representative testing of cement pastes. The high-shear mixing process ensures thorough blending of the cement and water, resulting in a homogeneous paste that is essential for reliable testing outcomes. This standard is particularly useful in research and development of new cement formulations and in studying the effects of various admixtures on the rheological properties of hydraulic cement pastes.