ASTM C231: Testing Air Content of Concrete With a Type B Pressure Meter
Significance and Use of ASTM C231
This test’s purpose is to get the air content of a sample of concrete using a pressure meter. In the meter you have a known volume of air at a known pressure in a sealed air pot with an unknown volume of air being contained inside the sample of wet concrete. You set it to the percentage of air (your initial pressure) where the pressure is equalized inside the chamber, fill it with concrete and water so that the only air left is the air inside the concrete, and then release the pressure inside of the air chamber to get your apparent air content. The actual air content is found by subtracting the aggregate correction factor, which you can calculate using batch information and a test that is expanded on below, from the apparent air content.
Air content is very important to the project engineer. The amount of air can increase or decrease the strength of the concrete, can determine whether the concrete is protected from freeze-thaw conditions or is prone to weathering, and can affect the finish and cosmetic look of the concrete. For every percent of entrained air (air that is added to the mix and does not occur naturally in the concrete), the strength of the concrete goes down about 200-300 psi. Check with whoever’s in charge at the site to see the air content they are hoping for and notify them if the results come out unusually high or low.
This method is only used for regular weight concrete that has relatively dense aggregate. Lightweight concrete is done with the volumetric method because the aggregate is too porous and the test results will be thrown off with so much entrapped air. Also, this test should not be performed on any concrete with aggregate larger than 2".
Air Content Test Equipment
- A clean, working air pot, complete with gauge and bowl (see calibration for more specifics)
- A tamping rod – the 18 inch, 5/8 diameter rod (the big one, not the small one). Must have a smooth hemispherical tip and be between 4 and 24 inches longer than the measuring bowl.
- A mallet – must weigh 1.25 ± 0.50 lbs for air pots smaller than 0.5 ft³.
- A strikeoff bar – must be at least 1/8 inches thick and ¾ inches wide by 12 inches long.
- A water dropper – to fill the remaining space in the chamber with water until water comes out of the valves.
- Scoop – needs to be large enough to get a representative sample of concrete and small enough not to spill concrete during placement in the measuring bowl.
- Rag – for cleaning. Note that concrete stuck to the inside of the chamber can be dissolved with vinegar if you come back from the field and there is some you couldn’t remove.
- Bucket of water – for cleaning.
- A calculator – to get your aggregate correction factor
ASTM C231 Procedure
- Wipe down the inside and rim of the measuring bowl and the cover with a wet rag to ensure that the pressure meter seals correctly when it is closed. Make sure you are doing the test on a level surface away from moving vehicles.
- Take your sampled concrete (sampled according to ASTM C172) and mix it up thoroughly. You will put in 3 layers of equal volume, and use your tamping rod to strike 25 blows through each layer of concrete. Space out your blows over the surface of each layer, and know that each blow should penetrate the previous layer by about an inch. The bottom layer will be penetrated to the bottom. On the third layer you will want the concrete to be slightly above the rim of the bowl. After each layer is done, strike it 10-15 times with a mallet to get rid of air bubbles that could overinflate your air content value. I like to do it 12 times, 3 on each side of the bowl.
- Strike off the excess concrete, placing your strike-off bar in the middle of the bowl and using a back and forth sawing motion to clear off the concrete. Move it towards you while doing the sawing motion, then away, until there is a smooth surface on top of the bowl at the level of the rim.
- Clean off the edges of the rim of the measuring bowl, then clamp down the cover of the air meter. Make sure you have an air tight seal or the results of your test will be invalid.
- Open the petcocks on your air meter by pushing the levers into an upright position. Use the water dropper to squeeze water into one petcock until water squirts out the other end. You are filling it so the only air present inside the meter is the air that’s inside the concrete.
- Every air meter has its own calibrated initial pressure value. Pump air into the pressure meter until you have reached this line. You should pump a little bit past it and use a combination of opening the bleeder valve (looks like a small knob, and you will hear air coming out when it is opened) and lightly tapping the side of the gauge until it is stabilized at the correct initial pressure. Remember that the initial pressure area is past the zero mark, so make sure you go past the zero when you are pumping it.
- Close both petcocks or you’re going to get a face full of concrete when you release the pressure.
- Use the lever at the top to release the pressure in the measuring bowl. Lightly tap the side of the gauge with your finger and the side of the measuring bowl with your mallet until the pressure stabilizes.
- Read the air content value on the gauge. This is the apparent air content. You will need to subtract the aggregate correction factor from this air content to find the true amount of air in the concrete.
- Carefully open each petcock (go slowly or you will end up covered in pressurized concrete) and release the remaining air in the chamber. Remove the cover and dump out the concrete in an area allowed by those in charge on the site. Do not reuse this concrete, because water has been added, irreversibly changing the mix design. Clean out the inside of your air meter thoroughly.
How to Perform an Air Content Test With a Type B Pressure Meter
Aggregate Correction Factor
For every air test you do, you will need to account for the amount of air that is stored in the voids of the aggregate. Some concrete suppliers may give out the aggregate correction factor for each mix design they offer. Others may not have that information on hand, but there is a test to find the aggregate correction factor. This test is typically performed in laboratory conditions but may occasionally be performed out in the field.
To do it, you will need to run an air test on a sample of fine and coarse aggregate in the same proportions and moisture condition as the concrete sample under test. First, calculate the mass of fine aggregate, Fs, and the mass of coarse aggregate, Cs.
S= volume of measuring bowl
B= volume of concrete produced per batch
Fb= total mass of fine aggregate in the batch
Cb= total mass of coarse aggregate in the batch
You will then need to fill your measuring bowl 1/3 full of water, and place your Fs and Cs sized samples in after mixing them together. Add the mixture of aggregates in small scoops so you don’t get extra air trapped in there. Stir after each scoop. If any foam is created during this process, remove it. You want the aggregate to be completely inundated, so if it is necessary add additional water until it is covered.
After each layer of aggregate you add, you will also need to rod the upper 1 inch of aggregate 8-12 times, and tap the sides of the bowl a few times. When you have all the aggregate added it will sit for a period of time about equivalent to the time between water going into the mixer and the time of performing the air test.
Before you put the cover on, you will screw a straight tube into the bottom of one of the petcocks and a j-shaped tube into the top of that petcock. Then, you will do your air content on that aggregate mixture, following the steps of the procedure above from step 4 onwards to the point where the air is released into the bowl, step 8.
Then, you will remove water from the air meter and put it into your calibration vessel, by releasing water out of the J tube. The water in the calibration vessel represents the percentage of air in the aggregate that was entrapped and has now been removed.
The final aggregate correction factor is:
Acf = gauge reading - % of air removed
Pressure meters need to be calibrated often. We calibrate ours every 3 months. Your lab technician will call you and tell you to bring it in, and the calibration procedure in Annex A-1 of the ASTM will be performed. It will be helpful for you to read this annex as it can help you troubleshoot problems with the pressure meter.
Common problems with air meters include:
- the bowl changing volume due to concrete buildup left over with improper cleaning
- the valves becoming clogged due to concrete buildup (you will have trouble putting water in)
- the needle becoming bent and being unable to move due to touching the glass or the gauge itself
- the needle coming loose and offsetting the gauge reading
- the gasket seal being popped (you will hear the sound of air whistling out of the gauge).
If any of these problems occur before calibration do not hesitate to bring it in and have your lab tech take a look. It is vital to get the correct air content on tests and unfortunately the pressure meter is one of the most sensitive pieces of equipment. PLEASE keep your pressure meter clean, and check it over before every day of testing to make sure it is working properly. Take pride in your work and keep your equipment clean and operating smoothly.