Permeable formwork serves as a filter that allows excess water and trapped air to escape from concrete surface. During compaction by vibrators, the fluid movement through permeable formwork drives out air and water, leaving behind a denser and stronger concrete. The movement of water results in a decrease in water in fresh concrete and fines cement particles from interior concrete shall be carried towards the formwork. Hence, it lowers the water cement ratio at concrete surface and enhances a higher strength near concrete surface.
In traditional formwork, during concrete compaction process the vibration tends to force water to the surface of concrete mass owing to the rearrangement of solid particles in concrete. As such, the concrete in concrete/formwork interface possess more water than the interior concrete. Consequently, the higher water cement ratio at concrete surface would lower the surface strength. Permeable formwork functions and solves this problem by allowing excess water to pass through.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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