Air entrainers are mostly surface-active substances (soaps from natural resins or synthetic nonionic and ionic tensides) with defoaming agents. Air entrainers are used to entrain microscopic air bubbles into the concrete. The purpose of entraining air into concrete is to achieve a proper air void system (the correct amount of air and the correct size and spacing of the air bubbles) in the hardened concrete. In the fresh concrete this requires a stable air system, which is able to withstand transport, placing and compaction. The effect of air entrainers on frost resistance is very significant. A mix that would normally fail after less than 60 freeze–thaw cycles may last for more than 1000, with no damage. The effect is far greater than increasing the strength, or reducing the permeability of a mix. The presence of air bubbles can help to relieve the internal pressures provided the bubbles are small and closely spaced through the paste phase.