ASTM C1874
Introduction to ASTM C1874
The ASTM C1874-20 standard is an essential guide for measuring the rheological properties of cementitious materials using a coaxial rotational rheometer. This method is integral for understanding the workability and flow properties of materials used in the construction industry, particularly those containing particles up to 1 mm in diameter.
Overview of ASTM C1874-20
The standard was developed to provide a systematic approach to measuring two crucial rheological properties of cementitious materials: plastic viscosity and yield stress. These properties are vital for evaluating the workability of materials like concrete and mortar in construction. The method uses a rotational rheometer with various spindle geometries to determine these properties based on the Bingham model. This model is essential in understanding the flow behavior of cementitious materials under different conditions.
Significance and Use
The rheological properties measured by this standard provide critical insights into the flowability of cement paste or mortar. These properties are influenced by various factors, including material composition and environmental conditions. Therefore, the test is crucial for assessing how specific materials or combinations of materials will behave during application. It's also important to note that these rheological properties can be sensitive to the sample's preparation and testing procedure, necessitating careful adherence to the standard to ensure accuracy.
Apparatus and Procedure
The apparatus used in this standard includes a rotational rheometer and various spindles, as detailed in Guide C1749. The container for the rheometer should be made of a non-reactive material like stainless steel and have specific dimensions and features to prevent slippage. Temperature control is also crucial, as the testing temperature can significantly impact the results.
The testing procedure involves selecting the appropriate rotational speed range and preparing the materials as per the guidelines in Section 10 of the standard. The rheometer is conditioned and standardized using a standard material to establish two constants: the ratio of shear rate over rotational speed (Kγ˙) and shear stress over torque (Kτ). The sample is then introduced into the rheometer, and the rotational speed is adjusted as per the procedure, recording the torque at each stage.
Calculations and Data Analysis
The raw data from the rheometer (rotational speeds and torque readings) are converted to shear rate and shear stress using the calculated constants. This data is then used to determine the plastic viscosity and yield stress of the material. The results are plotted as torque as a function of speed or shear stress as a function of shear rate, providing a visual representation of the material's rheological properties.
Conclusion
ASTM C1874-20 is a comprehensive and technically rigorous standard that plays a crucial role in the construction industry. By providing a standardized method for measuring the rheological properties of cementitious materials, it helps in determining the workability and flowability of these materials, ensuring their appropriate application in various construction scenarios. This standard, therefore, not only aids in quality control but also in the research and development of new materials and formulations in the field of construction.
ASTM C1874 Rheometer Test Procedure for Cementitious Pastes or Mortars
1. Preparation of Materials:
Begin by selecting appropriate materials and mix proportions based on the testing program's purpose. If evaluating field mixtures, use the concrete mixture design without coarse aggregate, ensuring that fine aggregates do not contain particles larger than 1 mm.
2. Mixing:
Mix the paste or mortar according to ASTM Practices C1738/C1738M or C305, respectively.
3. Apparatus Setup:
Set up the rheometer by placing the container in it following the manufacturer's instructions.
Attach the spindle as instructed, ensuring sufficient clearance between the spindle and the bottom of the container to avoid interference.
4. Apparatus Conditioning:
Turn on the rheometer temperature controller and set it to the desired temperature at least 30 minutes before testing or standardization. This ensures temperature stabilization within the container.
5. Standardization:
The standardization process involves establishing two constants, ratio of shear rate over rotational speed and ratio of shear stress over torque, using a standard material.
Introduce the standard fluid material into the measuring container and position the spindle with appropriate clearance.
Maintain the system (container, material, and spindle) at the specified temperature. Record this temperature before the first reading.
For torque measurement, start rotation at the lowest speed in the pre-selected range. Maintain this speed for 30 seconds and record the torque.
Gradually increase the rotational speed in at least ten steps, shifting to the next speed after torque stabilizes for 5 seconds, and record the torque for each speed.
Repeat the process while decreasing the speed, using the descending portion of the curve for calculations.
6. Calculations for Standardization:
Calculate the slope (S) and intercept (I) of the descending angular speed versus torque, using the linear part of the curve.
Calculate the standardization constants from the measured data and the values provided in the standard material certificate.
7. Testing Cementitious Pastes or Mortars:
Select a rotational speed range, typically from 0.1 s⁻¹ to 50 s⁻¹.
Prepare the materials as per Section 10 and maintain the measuring tool at the testing temperature (±2 °C).
Record the time from initial water contact with the cementitious material to the start of rheological testing.
Introduce the mixture into the measuring container, ensuring proper spindle clearance.
Start rotation at the lowest speed in the selected range, maintaining it for 20 seconds before recording the torque.
Gradually increase and then decrease the rotational speed in the pre-selected range, recording the torque at each speed after 20 seconds of continuous rotation or once the shear stress stabilizes.
8. Data Analysis:
Calculate shear rate, shear stress, plastic viscosity, and yield stress for the descending curve.
Plot the rheological measurements, either as torque as a function of speed or shear stress as a function of shear rate.
This procedure ensures a standardized approach to testing the rheological properties of cementitious pastes or mortars, providing crucial data on material consistency and workability.