 If the average person is asked what Portland cement concrete is made of, he'll probably think of stone or gravel, sand, cement, and water. But the last ingredient that most people would think of is air. That's right, air. All concrete mixtures have some air, from voids and the aggregates that are trapped in the mix. But nowadays, air is intentionally added or entrained in most concrete to improve its quality. An air-entraining cement or admixture produces billions of microscopic air bubbles throughout the concrete. This makes the concrete more workable, more durable, because it resists the effects of freezing, thawing, and deisers, and more watertight, and it can be just as strong as non-air-entrained concrete. In Washington, the specifications call for an air content of 5% by volume and accept results of from 1.5% below that to 1.5% above. In other words, 3.5 to 6.5%. Air content tests are just as important as the other tests and checks made to determine if the concrete is acceptable. As for the frequency of testing for structures, one test for every 100 cubic yards. For paving, one test for every 2,500 square yards. But any time that you suspect low or high air content, run another test. This program covers the pressure meter, an instrument that uses pressure to determine air content. All cover equipment, operation, and calibration. So let's get started. Before any concrete is brought to the job site, be sure that all test equipment is on hand and in good condition. And make sure that the work site is ready for the concreting to begin. The pressure meter has a pressure-tight base to hold the concrete sample. Be sure that the base is clean, inside, outer rim, and upper surface of the flange. Remember that any buildup inside the base will alter the volume and void the test results. The cover must have a smooth, clean outer rim and flange surface. When clamped together, the base and cover must be pressure-tight. Above the cover is the air chamber, and attached to it is a pressure gauge, with a dial graduated in both pounds per square inch and percentages of air content. There's a built-in hand pump for building up pressure. The valves and petcocks are for adding and draining water, and for releasing pressure from the air meter. Also needed are the following, a metal scoop, a standard steel tamping rod, a standard strike-off bar, a mallet, some wiping cloths, a container for water, and a rubber syringe. For calibration, however, you'll need some more equipment. A short, straight tube, a longer curved tube, and a calibrating vessel. All right, now let's see how to operate the pressure meter. First, put the concrete from the sample in the base until it's one-third full. Rod the layer 25 times, following a spiral pattern toward the center. Then tap the outside of the base 15 times with the mallet to get out the open spaces and air bubbles. Repeat the procedure of filling, rotting 25 times, and tapping 15 times for the second and third layers. After tapping the base for the third layer, strike off the excess concrete using a sawing motion. Next, wipe the flange clean to ensure a tight seal between cover and base. And clamp on the cover with both petcocks open. Now inject water through one petcock until all air is expelled through the other one. You'll know that all the air is out when some water comes out of the second petcock. Leaving both petcocks open, pump air until the hand on the gauge dial reaches the initial pressure point. Then wait a few seconds and stabilize the gauge hand at initial pressure by either pumping more air in or bleeding some off. Now close both petcocks and press down on the thumb lever. This releases the air into the base. Keep holding the lever down and gently tap the gauge with your fingers until the dial hand stabilizes. Be careful not to tilt the gauge at any time. Read the percent of air content, as indicated by the gauge hand, and record this percentage on the delivery slip. Now all you have to do is release the pressure and clean the equipment. By opening both petcocks, removing the cover, discarding the sample, and thoroughly cleaning the base, cover, and petcock openings. Okay, that's how the pressure meter should be operated. But now let's look at the calibration. Calibration is necessary to verify the operating pressure and to check the pressure gauge accuracy. Here's what to do. First, fill the base completely full of water. Screw the short piece of straight tubing into the threaded petcock on the underside of the cover. Clamp on the cover. With both petcocks open, inject water into the petcock with the tube attached below it until water comes out the other petcock. Pump air to a little beyond the initial pressure line. Wait a few seconds, and then stabilize the gauge hand at the initial pressure line by pumping or bleeding off as needed. Now close both petcocks and immediately press down on the thumb lever. The air is exhausted into the base. Wait a bit for the hand to stabilize. The gauge should read zero percent. If it doesn't, run another test. If two or more tests show a consistent variation from zero percent, change the initial pressure line to compensate for it and use this new initial pressure line on subsequent tests. Then screw the curved tube into the outer end of the petcock. Fill the calibrating vessel with water from the base by pressing on the thumb lever and controlling the flow with the petcock lever. Usually this vessel holds five percent of the volume of the base. Release the air at the free petcock. Open the other petcock and let the water in the curved tube run back into the base. There is now five percent air in the base or whatever percentage is represented by the vessel. Next, with the petcocks open, repeat the steps of pumping to initial pressure line, stabilizing the dial hand, closing both petcocks and immediately holding down the thumb lever, and waiting for the dial hand to stabilize. Read the percent of air on the dial. It should be five percent or whatever percentage of volume is represented by the calibration cylinder. If two or more tests show incorrect readings, off by more than two-tenths percent, remove the gauge glass and reset the dial hand to five percent by turning the recalibrating screw. Keep in mind that when the gauge reads correctly at five percent, more water can be drawn off, as you just saw, to check results at ten percent, fifteen percent, and so on. And with that, we finish air content testing with the pressure meter. But just a few more points before I finish. No test is valid unless it's performed correctly. In the same way each time, using properly calibrated equipment in good condition. Use the equipment carefully without damaging it. Keep it clean. Air content should be recorded on the delivery slips. Be neat, complete, and accurate in your documentation. And finally, inspect your actions. What to do when air contents don't comply with specifications. First, be sure you know the requirements. Check the concrete design and the specifications if you are unsure. When a test result is out of specification, run another test. In fact, any time you see or suspect something unusual, run a check test. Inspecting and testing the quality of Portland Cement concrete is a big responsibility. So give it your best effort. This completes part three, the final program of the series on concrete sampling and testing.