It’s the most used material in the world. You’ve stepped on it, driven on it, likely poured it, and quite frankly entrust your life to millions of tons of it every day. The essential ingredients of concrete are rocks, cement, and water. But there’s one more that’s just as important – any guesses? Bubbles! Water that naturally permeates outdoor concrete structures expands when it freezes. With nowhere to go, the expanding water generates stress in the cement matrix, and repeated freeze-thaw cycles will cause concrete to crack and crumble over a period of years. To avoid this stress, engineers carefully entrain millions of tiny bubbles (8% by volume) when they mix concrete, leaving voids in the cured material that provide space for expanding water to flow into.
On-Site Measurement in Concrete Needed
Entraining bubbles in concrete is not easy, and many factors related to the mix processing, and transport can destroy them. The US Department of Transportation needs a reliable way to verify the entrained air in concrete on the jobsite, before it is poured into a road or bridge. Existing measurement methods for fresh concrete are time consuming, prone to error, and measure only the total void fraction, giving no information about the bubble size (a critical parameter for freeze-thaw resistance).
Creare has developed an Instant Air Meter that provides an in-situ measurement of both total void fraction and mean bubble size. This handheld wand can be inserted into any fresh concrete sample and returns an accurate result at the touch of a button. It effectively gauges total void fraction by measuring the compressibility of the bubble-entrained mix, and the bubble size by measuring its time response. (Small bubbles respond quickly, larger ones more slowly.) By making jobsite entrained-air measurement more timely and accurate, Creare’s innovative sensor will reduce construction costs and increase the durability of our nation’s roads and bridges.
Marc Ramsey received his Ph.D. in Mechanical Engineering from Vanderbilt University, and he specializes in optics, acoustics, and compressible flows. His work at Creare often focuses on advanced sensors and diagnostics. His current projects include a volume sensor for a fluid bladder on NASA’s new space suit, an optical strain gauge for Army parachute canopies, and an ablation diagnostic for in-flight hypersonic projectiles tested by the Air Force.
This story was featured in the Fall 2018 edition of Creare’s People & Technology newsletter.