Air Content
The inclusion of air in concrete mix design serves as a crucial element to enhance its durability when exposed to water during freeze and thaw cycles. Concrete admixtures used to introduce air create tiny, uniform, and consistent air bubbles that range from 0.01mm to 1mm in diameter throughout the cement matrix of the mixture. These bubbles remain in the hardened concrete as air voids, creating a system that permits water expansion during freezing without harming the concrete.
Moreover, incorporating air in concrete mix design provides several advantages, such as decreasing the water demand needed for mixing and curing, improving workability, and reducing issues with bleeding and segregation.
Why test for Air Content in Concrete?
While concrete that is not exposed to freeze/thaw cycles may not necessarily need air-entraining admixtures, testing for air content is still a crucial step to evaluate the specific characteristics of a concrete batch. Typically, non-air-entrained concrete holds around 1% to 2% entrapped air, and other admixtures used may unintentionally introduce even more air.
Excessive air content exceeding 3% can cause difficulties in finishing certain concrete mixes, resulting in surface defects and delamination. Conducting air content tests on fresh non-air-entrained concrete can prevent potential issues before they arise or serve as a valuable tool for troubleshooting any problems.
Where and when should you test for air content in concrete?
Introducing air-entraining admixtures to concrete produces beneficial air bubbles; however, the process of mixing, transporting, pumping, and handling the mixture can eliminate these bubbles. Therefore, air content testing is not only essential but also requires testing at the appropriate time and location. Although sampling concrete directly from the delivery truck can confirm its properties as delivered to the job site, providing more comprehensive information to stakeholders is often preferred. Thus, it is commonly specified that the concrete should be sampled at the point of placement, such as at the discharge end of a concrete pump. However, this can pose challenges in accurately sampling and testing fresh concrete, necessitating careful planning by the testing agency and cooperation with other trades. It is not uncommon for air content testing to be incorporated into the mix design process.
Pressure Methods for Air Content in Concrete
Air content testing is a critical process in concrete mix design that ensures the durability of the finished product when exposed to freeze/thaw cycles. One of the most common methods for air content testing is the pressure method, which employs two types of meters - Type A and Type B. Both meters use different techniques to apply air pressure to a concrete sample and compress the air voids, indicating the apparent air content, which is later corrected to exclude air voids in aggregate particles. In this article, we'll discuss the pressure method for air content testing, including its pros and cons, and where it is best used.
Types of Meters and Testing Procedures
The pressure method cannot be used for mixes containing lightweight aggregates, blast furnace slag, or other aggregates with high porosity. Detailed test procedures for both types of meters are detailed in ASTM C231 and AASHTO T 152. Both meters use similar methods to consolidate fresh concrete into the measuring bowl, or pot, by depositing it in three equal layers, each rodded 25 times with a tamping rod. The concrete is then struck off with a strike-off bar or plate, ensuring the volume in the given bowl is the same each time.
Type A Concrete Air Meters
Type A meters are fitted with a sealed graduated cylinder in the cover. After the sample is consolidated, the cover is clamped into place and filled with water. A specified pressure is applied to the assembly using an external pump or squeeze bulb, compressing the air voids in the specimen and lowering the water level. After releasing the pressure, the water level in the cylinder rises, and the difference between the two levels is read from the cylinder graduations to determine the air content.
Pros and Cons
Type A meters provide fast and reliable results for both normal weight and lightweight concrete mixes. They also offer the convenience of Air Meter Calibrators to check the accuracy of readings in the field. However, testing with Type A meters requires more time and water than Type B meters, and the graduations can be more challenging to interpolate. Calibration can also be affected by changes in barometric pressure, and determining an aggregate correction factor is necessary.
Best For
Type A meters are best for concrete mixes using lightweight aggregates.
Type B Concrete Air Meters
Type B meters incorporate an air chamber in the cover that is pressurized with an integral pump to a predetermined initial pressure once the cover is placed on the pot. A small amount of water is added to the assembled unit to displace free air, and the petcocks are closed. The pressurized air is released into the lower chamber and compresses the concrete specimen, and the calibrated gauge displays the pressure difference directly as the total percent air content.
Pros and Cons
Type B meters provide fast and reliable results and are easy to perform, requiring minimal training. They are also widely used and have proven performance. Unlike Type A meters, changes in barometric pressure do not affect test results. However, they cannot be used for concrete mixes containing lightweight or porous aggregates, and determining an aggregate correction factor is necessary.
Best For
Type B meters are best for most concrete mixes using normal-density, low-porosity aggregates.
Conclusion
Air content testing is crucial to ensure the durability of concrete mixes, especially when exposed to freeze/thaw cycles. The pressure method is a reliable and widely used technique that employs Type A and Type B meters. Type A meters are best for lightweight aggregates, while Type B meters are best for normal-density, low-porosity aggregates. The pros and cons of each meter should be considered to determine the best testing method for a given mix. With proper testing and corrections, the finished concrete product will be stronger, more durable, and last longer.
Volumetric Method for Air Content Testing in Concrete
The quality of concrete is largely determined by its air content, which affects its strength, durability, and workability. Accurate measurement of air content is crucial in ensuring the performance of concrete structures. Two commonly used methods for determining air content in concrete are volumetric and pressure methods. In this article, we will focus on volumetric methods.
Volumetric air content tests begin by consolidating a known volume of fresh concrete into a container. A cover with integral graduated markings is sealed on the top, and the entire assembly is filled with water or a mixture of water and isopropyl alcohol to the zero point. The assembly is then agitated vigorously, and the air voids are washed out of the specimen. The difference in fluid levels before and after agitation is recorded as the air content.
Roller meters are the most commonly used apparatus for volumetric air content testing. They are usually constructed of aluminum (Roll-A-Meter), plastic (Volumetair Air Meter), or brass with a measuring bowl of at least 0.075ft³ (2.0L). The top section of the roller meter clamps to the bowl and includes a transparent graduated scale. After consolidation of concrete into the bowl, the top section is clamped in place, and the assembly is filled with water and measured amounts of isopropyl alcohol, as per ASTM C173 and AASHTO T 196. Alcohol is added to some concrete mixes to disperse foam that can interfere with accurate fluid level readings after agitation.
Pros
One of the most significant advantages of the volumetric method is that it can be used for all types of concrete mixes, regardless of aggregate type. Calibration of the apparatus is easier and requires less frequent calibration checks. There is no need for an aggregate correction factor, which simplifies the process.
Cons
The test is time-consuming and requires repeated cycles of agitation and resting until readings stabilize. The agitation method is physically demanding, making it difficult for some operators. Graduations on the scale are less precise than those in Type B gauges, and carrying enough alcohol for repeated testing can be inconvenient.
Best for
Volumetric methods are best suited for concrete mixes that use lightweight, porous, or slag aggregates. The less expensive plastic meters are best suited for occasional use.
Conclusion
Volumetric methods are an effective and widely used method for determining air content in concrete. They provide accurate results for all types of concrete mixes and are especially useful when testing lightweight or porous aggregates. While the test method can be time-consuming and physically demanding, its benefits outweigh the disadvantages, making it an essential tool for concrete testing.
Air Content by Gravimetric Method
The gravimetric method is a widely used procedure for determining the air content of concrete. This test is detailed in ASTM C138 and AASHTO T 121, which also determine the unit weights and yield of a concrete batch. To perform this test, unit weight measures, strike-off plates, tamping rods, and field weighing scales are required.
The test starts with freshly mixed concrete consolidated into a unit weight measure. The concrete is then carefully struck off and weighed. From there, a series of calculations are performed using the specimen weight and actual batch weights of the mix to determine the unit weight, density, yield, air content, cement content, and other properties of the mix.
One of the major benefits of the gravimetric method is that it provides information and data for a wide range of concrete properties using just one procedure. However, the calculations involved in this test can be time-consuming, and the accurate result requires complete knowledge of exact batch quantities. Additionally, slight variations in test procedures, mix proportions, or specific gravities can cause relatively larger errors.
The gravimetric method is best suited for testing trial mix designs and providing data for comparison to other tests. Overall, the gravimetric method is a reliable and widely used method for determining the air content of concrete, as well as other important properties of the mix.