The parameters of the air-void system of hardened concrete determined by the procedures described in this test method are related to the susceptibility of the cement paste portion of the concrete to damage by freezing and thawing. Hence, this test method can be used to develop data to estimate the likelihood of damage due to cyclic freezing and thawing or to explain why it has occurred. The test method can also be used as an adjunct to the development of products or procedures intended to enhance the resistance of concrete to cyclic freezing and thawing (1).

Values for parameters of the air-void system can be obtained by either of the procedures described in this test method.

No provision is made for distinguishing among entrapped air voids, entrained air voids, and water voids. Any such distinction is arbitrary, because the various types of voids intergrade in size, shape, and other characteristics. Reports that do make such a distinction typically define entrapped air voids as being larger than 1 mm in at least one dimension being irregular in shape, or both. The honey-combing that is a consequence of the failure to compact the concrete properly is one type of entrapped air void (9, 10).

Water voids are cavities that were filled with water at the time of setting of the concrete. They are significant only in mixtures that contained excessive mixing water or in which pronounced bleeding and settlement occurred. They are most common beneath horizontal reinforcing bars, pieces of coarse aggregate and as channelways along their sides. They occur also immediately below surfaces that were compacted by finishing operations before the completion of bleeding.

For air-entrained concrete designed in accordance with ACI 201.2R and ACI 211.1, the paste-air ratio (p/A) is usually in the range 4 to 10, the specific surface (α) is usually in the range 24 to 43 mm^{− 1} (600 to 1100 in.⁻¹), and the spacing factor () is usually in the range 0.1 to 0.2 mm (0.004 to 0.008 in.).

The air-void content determined in accordance with this test method usually agrees closely with the value determined on the fresh concrete in accordance with Test Methods C 138, C 173, or C 231 (11). However, significant differences may be observed if the sample of fresh concrete is consolidated to a different degree than the sample later examined microscopically. For concrete with a relatively high air content (usually over 7.5 %), the value determined microscopically may be higher by one or more percentage points than that determined by Test Method C 231.

Application of the paste-air ratio procedure is necessary when the concrete includes large nominal maximum size aggregate, such as 50 mm (2 in.) or more. Prepared sections of such concrete should include a maximum of the mortar fraction, so as to increase the number of counts on air voids or traverse across them. The ratio of the volume of aggregate to the volume of paste in the original mix must be accurately known or estimated to permit the calculation of the air-void systems parameters from the microscopically determined paste-air ratio.

Of the parameters determined with this test method, the spacing factor () is generally regarded as the most significant indicator of the durability of the cement paste matrix to freezing and thawing exposure of the concrete. The maximum value of the spacing factor for moderate exposure of the concrete is usually taken to be 0.20 mm (0.008 in.). Somewhat larger values may be adequate for mild exposure, and smaller ones may be required for severe exposure, especially if the concrete is in contact with deicing chemicals. Care should be exercised in using spacing factor values in specifications since the standard deviation of that property has been found to approach one-fifth of the average when determinations are made in different laboratories. Hence, substantial differences in spacing factor may be caused solely by sampling and between laboratory variation. The factors affecting the variability of the test method are discussed in the section on Precision and Bias.

The air content and the parameters of the air-void system in hardened concrete depend primarily on the kind and dosage of the air entraining agent used, the degree of consolidation of the concrete, and its water-cement ratio. The values of the specific surface (α) and the void frequency (n) decrease rapidly with an increase of the water-cement ratio or the paste content if other conditions are not altered. Satisfactory values of specific surface (α) and spacing factor () require that the void frequency be larger than about 315/m (8/in.). An increase in the water-cement ratio or the paste content must be accompanied by an increase in the air content, if the spacing factor
() is not to increase. The air content can be reduced substantially by extended vibration of the concrete, without a significant increase of the spacing factor (), provided the concrete was adequately air entrained originally. Extended vibration is not, however, recommended as a field practice because of the dangers of excessive bleeding and segregation.

The void frequency (n) is a critical parameter in determining the magnitude of the specific surface (α) and the spacing factor (). Consequently, utmost care must be taken in conducting either microscopical method to observe and record all air-void sections intersected by the line of traverse. Recognition of air-void sections of small size, for example, 10 μm (3.94 by 10⁻⁵ in.) is essential to securing a correct evaluation of these parameters. For this reason, care must be taken to prepare extremely smooth and plane sections, the magnification employed should be not less than 50×, and the index point in the cross hairs (or other reticle device) must be observed precisely in relation to the area and periphery of the air-void section.

Provided the value of the specific surface (α) or the void frequency (n) is sufficiently high, a suitable spacing factor () will be obtained even when the air content is low. However, in order to obtain an air-void system that has both the volume capacity and the geometric parameters necessary to protect saturated mature cement paste during exposure to freezing, it is important to obtain concrete with an acceptably high air content (A) and a low enough spacing factor () to provide protection (12).

For concrete exposed to freezing and thawing while critically saturated, a minimum-compressive strength must be developed prior to the freezing exposure, in addition to the securing of adequate air entrainment if the concrete is to be protected properly. Such compressive strength must be at least 28 MPa (4000 psi).

1.1 This test method describes procedures for microscopical determinations of the air content of hardened concrete and of the specific surface, void frequency, spacing factor, and paste-air ratio of the air-void system in hardened concrete (1). Two procedures are described:

1.1.1 Procedure A,

the linear-traverse method (2, 3).

1.1.2 Procedure B,

the modified point-count method (3, 4, 5, 6).

1.2 This test method is based on prescribed procedures that are applied to sawed and lapped sections of specimens of concrete from the field or laboratory.

1.3 It is intended to outline the principles of this test method and to establish standards for its adequate performance but not to describe in detail all the possible variations that might be used to accomplish the objectives of this test method.

1.4 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.

1.5 This standard does not purport to address all of the safety concerns associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements see 8.3 and 10.1.