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Affects of "Bubble Size" on Air Entraining Testing
Have you ever wondered why you can test for air content on fresh concrete and get a substantially different air content determination when you check that same concrete in its hardened state? Never wondered? Well, most people haven't, but you can bet specification writers and concrete suppliers have. Frustration has been rising in recent years when air-entrained, exterior concrete compressive strengths are lower than required, yet the field tests on the fresh concrete meet the project requirements. In an ideal situation, the concrete arrives at the job site (on time), the quality control technician tests the slump and temperature (correctly), which all meet the project requirements. The samples are field cured properly and transported to the laboratory for final curing and testing. But to everyone's surprise, the 7-day compressive strength test comes in much lower than anticipated.
Why you ask? Some speculate that the equipment used for measuring the air entrainment in the fresh concrete (ASTM C 231 Standard Test Method for Air Content of Freshly Mixed Concrete by Pressure Method) has difficulty detecting some of the entrained air bubbles - the really little ones. This traditional method for testing air content of fresh concrete is most reliable when the size of the air bubbles are greater than 50 microns. Smaller than that and the trouble begins. Your field test results may read 6%, but when tested in the hardened state, you might measure 10% or more. With your air entrainment level at that elevated amount, your concrete compressive strength generally decreases. Fortunately, although the strength has decreased due to this higher air content, there appears to be little affect on the abrasion resistance, creep, permeability, and drying shrinkage. Ultimately, the higher air content may actually increase the freeze-thaw durability of this troublesome concrete.
There has been research conducted that has touched on the difficulty in testing small air entrainment bubbles and the effects of air entrainment loss through pumping methods, but no solutions have yet been provided. An interesting section in a research paper about this phenomenon can be found in Kenneth Hover and Roger Phares' paper entitled Impact of Concrete Placing Method on Air Content, Air-Void System Parameters, and Freeze-Thaw Durability, 1996. Hover and Phares state, "It has been suggested that the discrepancy between fresh and hardened air contents may be due to the inaccuracy of the pressure meter in measuring the total air content of concrete mixes with very small entrained air bubbles. It has been quantitatively proposed that the air meter becomes increasingly less accurate as the bubbles become smaller."
So what's the deal? Is this random? How long has this been going on and is this a compatibility issue? Unfortunately, there are no definitive answers to these questions. In my conversations with the lab technicians who examine concrete at the microscopic level, I have been told that this a generally recent issue. Concrete they have examined that is 5-year old or more (from today) generally contains about a 1 percent higher air content in the hardened state than was tested during placement. More recently placed concrete have had much larger discrepancies in the air content between fresh and hardened states.
Although the precise solution to dealing with this quality control issue has not been fleshed out widely in the public or private sectors, in the meantime, consider this option: test the unit weight of the concrete at the batch plant prior to delivery. A low unit weight, measured at the plant, would likely indicate a higher than intended air content. Catching this at the batch plant might be the difference between wasting thousands of dollars on rejected, hardened concrete and a profitable project.
Gregory J. Bauer, PE (MN, ND, IA), Associate/Project Engineer
Braun Intertec
11001 Hampshire Avenue S | Minneapolis, MN 55438
612.221.3618 mobile | 952.995.2020 fax
gbauer@braunintertec.com
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