RapidAir is an image analysis system for automatic determination of the air content in hardened concrete according to the linear traverse method described in ASTM C457, “Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete,” or in EN 480-11, “Admixtures for concrete, mortar and grout – Test methods – Part 11: Determination of air void characteristics in hardened concrete.”

The measured parameters of the air-void structure are total air content, spacing factor, and specific surface.


A core is taken from the structure, sliced, ground, and lapped in the laboratory. The resulting surface is plane, smooth, and with sharp edges along the perimeter of air voids. Before final specimen preparation, the lapping quality is checked under a stereomicroscope.

The lapped surface is colored black with a hard stamp pad containing black ink. After heating the specimen to 55 ºC, a white zinc paste is applied to the surface with a rubber spatula. The zinc paste melts on the surface and flows into the voids.

RapidAir_1 After cooling to room temperature, excess paste is removed from the surface with a straight, sharp steel blade. The quality of the black-white contrast is checked under a stereomicroscope. The voids should be filled totally with white paste and no white regions should be visible on the surface. Finally, voids in aggregates and obvious cracks are colored black under the stereomicroscope using a black marking pen. The photo on the left shows a properly prepared specimen.

The preparation of a well-lapped specimen surface for analysis takes about 30 minutes. The RapidAir measurement is done within 12 min for linear traverse analysis and within 30 min for modified point count. This should be compared with a time of 4 to 6 hours normally required for manual analysis using a light microscope in accordance with ASTM C457 or EN 480-11.


RapidAir_2 Following contrast enhancement, the prepared specimen is mounted on a moving X-Y-Z stage positioned below a video camera.

The RapidAir control unit automatically moves the stage, and the software determines the portion of the total traverse length that passes through the white air voids, as shown in the magnified view to the right. After the scan is completed, the air-void parameters are determined in accordance with ASTM C457 or EN 480-11.

The specimen scan is saved automatically in a report file documenting the air content, spacing factor, and specific surface. In addition, graphical presentation of the air-void distribution and the raw data are available.

RapidAir_3 Prepared specimen positioned on the moveable stage ready for image analysis.

RapidAir_4 Overall view of RapidAir system in operation.

Correlation with ASTM C457 and Precision

As reported in Pade, C., Jakobsen, U.H. and Elsen, J., “A New Automatic Analysis System for Analyzing the Air Void System in Hardened Concrete,” International Cement Microscopy Association Conference, San Diego, CA, USA, April 2002, very good agreement was found between the air-void system parameters measured by the RapidAir system and by the ASTM C457 standard method. The study involved thirteen European laboratories. The standard deviations of the air-void parameters determined by RapidAir were as follows:

  • Air content: 0.37 %
  • Specific surface: 1.57 mm-1
  • Spacing factor: 0.011 mm

RapidAir_5Comparison between RapidAir and ASTM C457 determinations of air content and specific surface

RapidAir Ordering Number

The RapidAir-3000 system shown to the right comes as a complete system, ready to plug in and operate, including PC with software, control unit, and manual.

A one day course is offered separately by a RapidAir specialist.



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