Foam Index

Foam Index test is a rapid method to determine the relative levels of Air Entraining Agent (AEA) needed during concrete mixing, with or without mineral additives like combustion fly ash, that control air void volumes within cured concrete.Bubbles do not cover entire surfaceBubbles cover entire surface Foam Index test is a rapid method to determine the relative levels of Air Entraining Agent (AEA) needed during concrete mixing, with or without mineral additives like combustion fly ash, that control air void volumes within cured concrete. AEA surfactants are added into concrete mixes to impart stable air microbubbles to give air void volumes between ~ 5%–10%. Besides increasing workability, adding the correct amount of AEA during concrete mixing improves its resistance to cracking during freeze and thaw cycles. Adding too much AEA during mixing decreases concrete strengths. Fly ash from pulverized coal combustion is a typical mineral admixture utilized in the production of concrete, where it partially replaces cement and improves concrete workability, pumping characteristics, hardness and resistance to alkali attack. However, cement and components of fly ash adsorb AEA’s to different extents, making it more difficult to routinely impart the correct amount of entrained air into concrete. The Foam Index Test was developed to measure capacities of AEA for cement-mineral admixtures during concrete mixing. The Foam Index Test is a laboratory titration procedure which determines the AEA absorption on fly ash or fly ash and cement mixtures. An AEA is titrated into a fly ash and cement suspension and the suspension is shaken. The added AEA leads to foam formation on top of the liquid surface, which initially behaves in an unstable manner. At the endpoint of the test, the AEA absorption sites are “saturated” and the AEA contributes to foam formation on the top of the mixture. The amount of AEA required to obtain stable foam depends on fly ash quality, fly ash-cement interactions, and characteristics of the AEA. Some fly ashes, typically those with high unburned carbon contents, tend to adsorb high amounts of AEA, i.e. more AEA is needed to obtain stable foam.