Total voids in concrete encompass gel water volume, capillary pores, and entrapped air. Gel water (retained within the cement hydration products) and physically entrapped or adsorbed water are significant for the hydration process. For complete hydration, it's estimated that the space needed for the products of a cubic centimeter of cement doubles. Capillary pores constitute the unoccupied space within the hydrated cement paste, with their size largely influenced by the water-to-cement ratio and hydration degree.
During mixing, concrete traps a small amount of air, forming voids that can be significantly larger than capillary voids, potentially weakening the concrete's strength. These air voids vary in size, with entrapped ones reaching up to 3 millimeters, much larger than the finer capillary voids, which range from nanometers to a few micrometers in diameter. The distribution of these capillary voids is believed to be a more precise indicator for assessing hydrated cement paste characteristics than total porosity.
Air voids, usually spherical, contrast with the irregularly shaped capillary voids. The impact of these voids on concrete is considerable, affecting strength and permeability. To optimize the concrete's properties, understanding and controlling the volume and distribution of these voids are essential.
Consider a mass of concrete in which the total volume of voids includes the volume occupied by gel water, capillary pores, and entrapped air, each contributing to the overall porosity of the concrete structure.
The gel, which is the product of cement hydration, is made up of solid substances and water that is either physically retained or adsorbed onto the extensive surface area of the hydrates. The volume of this particular water is the volume of gel water.
It is estimated that one cubic inch of unhydrated cement would need approximately two cubic inches of space for its hydration products upon complete hydration.
Capillary pores are the spaces within the hydrated cement paste that are not occupied by the hydration products, and these pores can either be empty or filled with water.
During the process of mixing concrete, some air often gets trapped within the cement paste.
These entrapped air voids can reach up to 120 mils, which are substantially larger than the capillary voids and can negatively impact the concrete's strength.
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