ASTM C173 - Air Content of Freshly Mixed Concrete by the Volumetric Method
Significance and Use of ASTM C173
ASTM C173 is a test method that covers the determination of the air content of a sample of freshly mixed concrete. It can be used for any type of aggregate, but since it is not affected by the air content that is present in porous aggregate, it is the preferred test method used on lightweight concrete or concrete with highly porous aggregate. Knowing the amount of air present in the concrete helps engineers to predict the concrete's strength, the weight of the slab, and how well it can survive a freeze-thaw cycle. Lightweight concrete is used in the upper floors of buildings to lighten the load on lower floors, so if you're building a skyscraper, be prepared to perform this test a lot!
Equipment Needed for ASTM C173
- Air meter (roller meter) - must be made of a material thick and rigid enough to handle fieldwork, and not be sensitive to extreme changes in temperature or pH. When you put the top and bottom sections together, no water should be able to leak through.
- Air meter bowl - the bowl must have a volume that is at least 0.075 ft3 and be constructed with a lip at the top so it can be clamped together with the air meter top. The diameter should also be 1 to 1.25 times the height of the bowl.
- Air meter top - The air meter top should have a volume that is at least 20% larger than the volume of the bowl, and should come with a plug or cap that fits into the end of the neck and creates a watertight seal even when the meter is upside down. The neck of the air meter top should be see-through, and marked with a scale that goes from 0% (at the top) to 9% or more of the volume of the bowl. These markings should be in increments that are not greater than 0.5%, and should be accurately placed on the neck to be within 0.1% of the volume of the bowl.
- Funnel - The funnel must be sized to where the top end is wide enough so that it doesn't fall into the neck, and long enough to where it extends to just above the bottom of the top section. The bottom end should be constructed in a way so that when water flows out it doesn't disturb the concrete.
- Tamping rod - The tamping rod must be 5/8 ± 1/16 of an inch in diameter and at least 12 inches long, with the rod being made of an abrasion-resistant material and the ends of the rod being rounded to a hemispherical tip.
- Strike-off bar - For your strike-off bar, you can either use a 1/8 by 3/4 by 12 inch steel bar, or a plastic bar (must be equally rated in abrasion resistance to steel) that is 1/4 by 3/4 by 12 inch. It must be flat and straight.
- Calibrated cup - This cup can be made of metal or plastic, and must either have a capacity of 1.00 ± 0.04% of the volume of the air meter bowl, or be marked in increments of that volume. Only use the calibrated cup to add water when the concrete's air content exceeds 9% or the calibrated range of the air meter.
- Measuring vessel for isopropyl alcohol - Must be able to hold at least 1 pint, and have graduations that are not larger than 4 oz.
- Syringe - must have a capacity of at least 2 oz.
- Pouring vessel for water - should be able to contain approximately 1 quart.
- Scoop - Should be made of metal and able to scoop a representative amount of concrete without spilling into the bowl of the air meter.
- Isopropyl alcohol - You will need 70% isopropyl alcohol. If you have alcohol of a higher concentration it will need to be diluted by volume to get 70%.
- Mallet - The mallet should have a rubber or rawhide head and should weigh 1.25 ± 0.5 lbs.
ASTM C173 Procedural Flowchart
ASTM C173 Procedure
Sampling the Concrete
Obtain your sample in accordance with ASTM C172. If the concrete contains particles with a diameter greater than 1 1/2 inch, wet sieve enough concrete over a 1 inch sieve to fill your measuring bowl with a little extra. Try to do this in a way to minimize the disturbance of the mortar. Do not wipe mortar off of coarse aggregate particles that are retained on the sieve.
Filling the Bowl
1. Dampen the inside of the bowl with a rag or sponge.
2. Put in the first layer of concrete, going to about half the volume of the bowl. Rod this layer 25 times, but do not forcibly strike the bottom of the bowl. Tap the sides of the bowl 10-15 times with the mallet to close up the holes made by the tamping rod and release any leftover air in the concrete.
3. Add the second layer of concrete, going a little over the top of the bowl's rim. Rod this layer 25 times, penetrating an inch into the first layer. You are allowed to add more concrete if this layer subsides under the rim of the bowl. Tap the sides of the bowl with the mallet 10-15 times. After tapping the second layer, 1/8 of an inch or less of concrete over the rim is considered acceptable; if you are over this you can remove concrete to shave it down.
4. Strike off the excess concrete with the strike-off bar until the surface of the concrete is level with the rim of the bowl. Wipe off the rim's top and sides to create a smooth surface that will allow for a watertight seal with the air meter top.
Adding Water and Alcohol
1. Dampen the inside of the air meter top, including the gasket.
2. Clamp the top to the bowl, making sure that it fits flush on all sides and there is no space for concrete to escape. Insert the funnel.
3. Add at least 1 pint of water, followed by the selected amount of isopropyl alcohol. Depending on the type of concrete, the amount of cement, and what admixtures are added to it, this amount can change. Many concrete mixtures made with less than 500 lb/yd3 of cement and air contents under 4% may require less than 0.5 pt of alcohol. Some high-cement mixes made with silica fume that have air contents of 6% or more may require more than 3 pints of alcohol. Generally, you can establish an amount of alcohol for a given concrete mix and use the same amount over the course of its use on a job. If you need to add over 4 pints of alcohol, you might need to add less water at the start, but always add some to keep the alcohol from contacting the surface of the concrete.
4. Keep adding water until you see water appearing in the neck. Remove the funnel at this point. Add water up until the meniscus (bottom of the lens at the surface of the water) is at the 0 point.
5. Attach and tighten the plug to create a watertight seal at the top of the neck. Dry this area and the bottom clamp so you will be able to tell if the meter is leaking.
Inversion and Rolling Procedure
1. Flip the sealed meter upside down and shake it from side to side for up to 5 seconds, and then flip it back upright. Do this for at least 45 seconds (or 9 inversion cycles), until the concrete has broken free from the bowl and you can hear the aggregate rolling around inside.
2. Put one hand on the neck and put your other hand on the clamped bowl and top. Tilt the meter 45 degrees from the ground so that the edge of the base is touching the ground. Using the hand on the flange, roll the meter on the ground 1/4 to 1/2 of a turn back and forth several times, keeping contact with the ground and rolling with force. Turn the meter 1/3 of a turn and roll it back and forth. Keep doing this for about 1 minute. You should be able to hear the aggregate sliding in the meter. If any liquid is leaking you will have to start the test over from the beginning.
3. Set the roller meter upright and loosen the top to let the air pressure inside stabilize. Let the meter sit while the air rises to the top and the liquid level stabilizes. The liquid level is considered stable when it doesn't change more than 0.25% air within a 2 minute period. If it takes over 6 minutes for the liquid level to stabilize, or if there is enough foam to take up more than 2 percent divisions of air, the test is invalid and you will have to start over from the beginning, adding more alcohol this time.
4. If the liquid level is stable and the foam is not excessive, read the meniscus to the nearest 0.25% of air, and record your initial meter reading. If the air content is greater than the 9% range of the meter, add enough calibrated cups of water to bring the liquid level within the meter's range. Read the bottom of the meniscus to the nearest 0.25%, and record the number of calibrated cups that were added.
Confirmation of the Initial Meter Reading
1. Retighten the plug at the top of the air meter, making sure it is sealed, and repeat the 1 minute rolling procedure. Loosen the top and allow the liquid level to stabilize.
2. When the liquid level is stable, make a direct reading to the bottom of the meniscus and estimate to 0.25% air. If this reading has not changed more than 0.25% from the initial meter reading, record it as the final meter reading. If the reading has changed from the initial meter reading by more than 0.25% air, record this reading as a new “initial reading” and repeat the 1-min rolling procedure. Read the indicated air content again. If this reading has not changed by more than 0.25% air from the “newest initial reading”, record it as the final meter reading. If the reading has changed again by more than 0.25%, discard the test and start a new test on a new sample of concrete using more alcohol.
3. Take the apparatus apart. Dump the bowl out and examine the contents to make sure that there are no pieces of undisturbed, tightly packed concrete left in the bowl. If there are still some chunks of concrete stuck to the bowl, the test is invalid and you will have to start over from the beginning.
Roller Meter Calibration
It is important to calibrate your roller meter on receiving it, on an annual basis after that, and at times when you suspect it is damaged, because this test is very particular and you don't want faulty equipment throwing off your results.
First, you will need to know the volume of your air meter bowl. Weigh the bowl empty and record its weight. Make sure it is as clean as possible and dry.
Next, fill the bowl with room temperature water. Tare a glass plate on your scale, and use this glass plate to strike off your water, making sure that the bowl is filled to the brim and striking off the water in a way that creates no bubbles in the bowl. If you splash water, dry the area with a towel. Weigh the full bowl with the glass plate on top.
Take the temperature of the water and find the density of that water using that temperature. Here is a helpful water density calculator.
Subtract the empty weight from the full weight of the water, and divide that number by the density of your water in lb/ft3 to get the volume of the bowl.
(Full weight - empty weight) / (density of water at _ temperature)
Now that you have the volume of the bowl, you can determine the accuracy of the graduations on the neck of the top section of the airmeter by filling the assembled measuring bowl and top section with water to the level of the mark for highest air content graduation (9% usually). Add water in increments of 1.0% of the volume of the bowl to check accuracy throughout the graduated range of air content. The error at any point throughout the graduated range should not be more than 0.1% of air.
You can also check the volume of the calibrated cup, making sure it contains 1% of the volume of the bowl, by using the same procedure that was used to calibrate the bowl's volume.
ASTM C173 Procedure Video by SI Certs
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