Knowles Applies Principles of Air-Entrained Concrete to Dry-Mix Shotcrete

Knowles Industrial Services is proud to announce the successful application of air-entrained dry-mix shotcrete. While air-entrained concrete has been an industry standard since its conception more than 80 years ago, the process of introducing porosity to dry-mix shotcrete is a newer development that Knowles has been closely following. As customer demand for freeze-thaw resistant shotcrete has increased over recent years, we saw an opportunity to expand our client portfolio.

What is Air-Entrained Concrete?

Air-entrained concrete is concrete that has been chemically altered to contain small, controlled, and evenly-spaced air bubbles throughout the material’s structure. The process is achieved by mixing a surfactant into plastic concrete which in turn creates air bubbles while mixing. These air entraining admixtures are useful in harsh environments that have frequent freeze-thaw cycles, as the air bubbles help make concrete significantly more crack resistant. These air bubbles provide a controlled space for water to occupy and permit the expansion of freezing water without creating internal stresses that can eventually cause cracking within the concrete.

Air-Entrained Shotcrete in Action

One of our key hydro-electric/utility clients recently requested a concrete application that could withstand the extreme winter weather cycles in New England. The Owner and Design Engineer mandated a minimum of 4% air entrainment by volume to resist damage caused by freeze/thaw cycles. Due to difficult site access requirements for conventional cast-in-place concrete application, this project was a strong candidate for dry-mix shotcrete (pneumatically-applied mortar). Knowles had never before been tasked with air-entraining shotcrete. The contract award was contingent upon successfully proving the ability to air-entrain dry-mix shotcrete prior to commencing construction.

Knowles has been following this relatively new dry-mix air entrainment concept for some time as it affects our core business and is just now starting to take hold in the industry with meaningful data. Dry-mix shotcrete, by the nature of the process, typically contains up to 4% air. Dry-mix has been documented to provide a very durable material up to .012 inch air spacing. In an effort to bring this technology to the project, Knowles performed a series of test panels at our Portland, Maine office on June 19th, 2015, and had cores tested by the country's foremost authority on ASTM C457. We have obtained outstanding air void system results with the proposed air entrainment admixture used for these tests.

Read more about the testing process in our full report on air-entraining shotcrete