AASHTO T 348:13(2018) Standard Test Method for Air-Void Characteristics of Freshly Mixed Concrete by Buoyancy Change.
The change in buoyancy of the dish, as measured by a change in weight and recorded as a function of time, can be related to the number of bubbles of different sizes by an empirical correlation. Specific surface, spacing factor, and air content as specified by ASTM C457/C457M may be calculated from this data with the use of an algorithm.
4.1. An adequate air-void system in hardened concrete protects the cement paste from damage during freezing and thawing cycles under moist conditions. This air-void system can be characterized by the volume of entrained air, spacing between air voids, specific surface, and void-size distribution.
4.2. This buoyancy change test method is capable of testing the air-void system of concrete in situ. reflecting the history of the concrete as it is in place, not as it is prepared in a sample for testing.
4.3. The primary function of the buoyancy change method is to provide air-void size and distribution information for concrete mixture designs. This test method could be used by the mix designer to evaluate various mix proportion options during prequalitication. The effect of admixture combinations and admixture dosages on the air-void system can be evaluated, it can also be used by the approving agency as a quick laboratory check on mixes offered to them for approval.
4.4. During production, the adequacy of the air-void system can be verified for acceptance and feedback can be provided for manufacturing control. This method also allows rapid assessment of the efTect of production changes in the mixture or equipment or variations in placement conditions such as temperature, slump. and pumping on the air-void system. Characterization of the air-void system of the concrete shortly after production provides an assessment of the durability of the cement paste. Results are usually obtained within 2 h, allowing adjustments in the subsequent production.
4.5. This method yields results that generally correlate well with the results of a linear traverse measurement on hardened concrete, as prescribed in ASTM C457/C457M for characteristics of the air-void system. Discrepancies between the results of this method and the results of ASTM C457/C457M may be due to coalescence of bubbles in the analysis liquid or due to errors in the ASTM C457/C457M test. The buoyancy change method does not give a total air content result that can be directly correlated with the results of T 152 and T l96M/T 196.
4.6. For further discussion of the significance of characteristics of the air-void system, see ASTM C457/C457M, Section 5. Significance and Use.AASHTO T 348 pdf download.