Airentraining admixtures

Air-entraining admixtures, typically wood resins or synthetic surfactants, stabilise the tiny air bubbles which become incorporated into concrete or mortar as it is mixed. The bubbles, which are between 0.05 and 0.5 mm in diameter, do not escape during transportation or vibration. They improve the workability of the mix, reduce the risk of segregation and greatly enhance frost resistance. However, the incorporation of void space within concrete decreases its crushing strength by 6% for every 1% of air entrained, thus for a typical 3% addition of entrained air, a reduction of 18% in crushing strength is produced. This is partially offset by the increase in plasticity, which generally produces a higher-quality surface and allows a lower water content to be used. The increased cohesion of air-entrained concrete may trap air against moulded vertical formwork reducing the quality of the surface.

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