Q.: A ready mix truck arrives at the jobsite with 8 cubic yards of concrete. Travel time was 10 minutes. We discharge a yard, then take a slump and air test. The air content is 6% and slump is 2 1/2 inches. We add 10 gallons of water to increase the slump, then discharge 5 more yards in about 15 minutes. If we test again, where will the slump and air be? And right after the water was added, what would the change in air content have been if we had run another test? Does temperature have an effect on air content? How about relative humidity?
A.: Adding water at the jobsite increases both slump and air content. The rule of thumb for slump is that 1 gallon of water added to a cubic yard of concrete increases slump by 1 inch. Air entraining agents form air bubbles by a frothing action. The wetter the concrete, the more frothing action you get. So up to a point, the higher the slump, the higher the air content. If the concrete gets too sloppy, however, some air may be lost.
Adding 10 gallons of water to 7 yards of concrete probably increased the air content slightly. Air content decreases after prolonged mixing or agitation but agitating for only 15 minutes after adding water isn't likely to reduce the air content or slump significantly.
Relative humidity doesn't affect air content, but temperature does. For a given dosage of air-entraining agent, as the concrete temperature rises, the air content decreases.