How does air content affect concrete?
A. Intentionally entrained air voids improve the resistance of concrete to damage from cycles of freezing and thawing. Any air voids reduce the strength of concrete, with about a 5% reduction in strength for each 1% increase in the volume of air voids. Air voids, however, also improve the workability of concrete.
How do you control the air content in concrete?
Adding a pumping aid along with an AEA increases air c o n t e n t . Dust on the aggregate decreases air content. Crushed-stone concrete entrains less air than gravel concrete. Diluting AEA with hard water prior to batching decreases air content.
What determines air content concrete?
A Type B pressure meter is used to determine the air content of normal-weight concrete. The air content is read at the dial, which is calibrated for each apparatus. The aggregate correction factor (explained in ASTM C 231) must be subtracted from your reading to obtain the net air content.
What is the approximate air content (%) in air entrained concrete?
The amount of entrained air is usually between four and seven percent of the volume of the concrete, but may be varied as required by special conditions.
What happens if air content is too high concrete?
A.: Strength loss is the major effect of too much air. Each percent increase in air content can reduce compressive strength by 2% to 6% for moderate-strength concrete.
What causes high air content in concrete?
Air content will increase if the mixer is loaded to less than capacity and will decrease if the mixer is overloaded. However, in very small loads in a drum type mixer, air becomes more difficult to entrain.
How do you reduce air voids in concrete?
Lower the viscosity of cement by adding sand or fly ash. This allows large aggregate to move more freely and reduces the amount of air entrapment. Use aggregate that is more uniform in shape. Irregular shaped pieces of aggregate tend to make the concrete less fluid.
What is the difference between entrapped air and entrained air?
It is important to note that entrained air is not the same as entrapped air. Entrapped air is created during improper mixing, consolidating and placement of the concrete. Entrained air is intentionally created by adding a liquid admixture specifically designed for this purpose.
What causes high air in concrete?
Forming Air in Concrete Entrained air is produced during mechanical mixing of concrete that contains an air-entraining admixture. The shearing action of mixer blades continuously breaks up the air into a fine system of bubbles.
How do you remove air from concrete?
Techniques used during vibration can eliminate most surface voids. Vibrating both the outside and inside of your mold will draw most air and water bubbles away from the surface of the concrete. Hammering the mold can eliminate any residual voids.
How do you fix air bubbles in concrete?
Slurry is a mixture of cement paste, pigment, and water. It is used to fill in the small air bubbles in the surface of the concrete….
- Step 1 – Polish surface if desired.
- Step 2 – Mix Slurry.
- Step 3 – Apply Slurry.
- Step 4 -Wait for it to Cure.
- Step 5 – Wipe Up Slurry.
What is the difference between air entrained concrete and trapped air in concrete?