Air Voids Test
The Significance of Air Content in Concrete Mix Design
Air content in concrete plays a pivotal role in ensuring its longevity and robustness, especially when it comes in contact with freeze/thaw cycles. Here's a comprehensive look at why air content is crucial in concrete mix design and the various methods used for testing.
Air Void Test Equipment (CREDIT: construction.net)
Why Entrain Air in Concrete?
The primary objective of entraining air in concrete is to bolster its resilience during freeze/thaw cycles when water is present. Air-entraining admixtures facilitate the creation of tiny, consistent, and stable air bubbles, ranging from 0.01mm to 1mm in diameter, within the concrete mix. Once the concrete hardens, these bubbles transform into air voids, forming a buffer system. This buffer allows the expansion of freezing water without causing any harm to the concrete structure.
Apart from freeze/thaw resistance, air entrainment offers several other advantages:
It reduces the water requirement for mixing and curing.
It enhances the workability of the mix.
It mitigates issues like bleeding and segregation in the mix.
The Need for Air Content Testing
Even if concrete isn't vulnerable to freeze/thaw cycles, assessing its air content remains vital. While non-air-entrained concrete usually contains 1-2% entrapped air, certain admixtures might unintentionally increase this percentage. Industry standards such as ACI 301-16 and ACI 302.1R-15 highlight that air content exceeding 3% may necessitate added finishing effort, leading to surface flaws and delamination. Hence, testing fresh concrete's air content can pre-empt potential problems or serve as a diagnostic tool.
Timing and Location for Air Content Testing
Concrete's journey from mixing to placement can affect its entrained air content. For precise results, not only is the testing crucial, but so is its timing and location. While some might advocate for testing the concrete as it arrives at the construction site, it's often more insightful to collect samples at the point of placement, like the discharge end of a concrete pump. However, this requires meticulous planning and collaboration between teams.
Methods of Air Content Testing
1. Pressure Methods
Type A and Type B Concrete Air Meters: These devices employ distinct techniques to exert air pressure on a concrete sample, compressing its air voids. The variance in volume post-compression helps deduce the apparent air content.
Type A Meters: After consolidating the sample, the cover is fitted, and water is added. A specific pressure is then applied, compressing the specimen's air voids. The air content is then determined based on the water level difference.
Type B Meters: Here, an air chamber in the cover is pressurized. The pressurized air then compresses the sample, and a calibrated gauge displays the air content directly.
Super Air Meter (SAM): A modified Type B meter, SAM not only determines air content but also characterizes the size and distribution of air bubbles.
2. Volumetric Methods
These tests begin with a consolidated concrete specimen of known volume. Post-agitation, the difference in fluid levels is recorded as air content. Instruments used include the Roller meters and the Chace Air Indicator Kit.
3. Gravimetric Method
Part of ASTM C138, this method determines concrete properties like unit weights, yield, and air content. The procedure involves consolidating fresh concrete into a unit weight measure, weighing it, and then performing various calculations.
Conclusion
Understanding and controlling the air content in concrete is imperative for producing a robust and long-lasting concrete structure. Utilizing appropriate testing methods ensures that concrete maintains its integrity and performance over time. Should you have further inquiries or need clarity on the topic, always refer to industry standards or consult a concrete specialist.