7.4
Total Voids in Concrete
Total voids in concrete encompass gel water volume, capillary pores, and entrapped air. Gel water (retained within the cement hydration products) and…
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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Q1: What are the three main components that make up total voids in concrete?
Total voids in concrete consist of gel water, capillary pores, and entrapped air. Gel water is retained within cement hydration products through physical adsorption onto hydrate surfaces. Capillary pores are unoccupied spaces in the hydrated cement paste, while entrapped air becomes trapped during mixing and can reach up to 120 mils in size, substantially larger than capillary voids.
Q2: How much space is required for cement hydration products compared to unhydrated cement?
During complete hydration, approximately two cubic inches of space are needed for the hydration products of one cubic inch of unhydrated cement. This significant volume expansion occurs because the gel, which is the product of cement hydration, contains both solid substances and water physically retained or adsorbed onto the extensive surface area of the hydrates.
Q3: What is gel water and why is it important in concrete?
Gel water is the volume of water physically retained or adsorbed onto the extensive surface area of cement hydration products. This water is a key component of the gel formed during cement hydration and contributes to the overall porosity of the concrete structure. Understanding gel water volume is essential for assessing the hydration process and concrete performance.
Q4: How do capillary pores differ from entrapped air voids in concrete?
Capillary pores are small, irregularly shaped spaces within hydrated cement paste that range from nanometers to a few micrometers in diameter. Entrapped air voids, typically spherical, are substantially larger, reaching up to 120 mils. The distribution of capillary pores is considered a more precise indicator of hydrated cement paste characteristics than total porosity alone.
Q5: Why do entrapped air voids negatively impact concrete strength?
Entrapped air voids can reach up to 120 mils, making them substantially larger than capillary voids. These large voids reduce the effective load-bearing area and create stress concentration points within the concrete matrix. Understanding and controlling void volume and distribution is essential for optimizing concrete's strength and permeability properties.
Q6: What factors influence the size and distribution of capillary pores?
The size and distribution of capillary pores are largely influenced by the water-to-cement ratio and the degree of hydration. Lower water-to-cement ratios generally produce finer, more densely distributed capillary pores. The distribution of these capillary voids is believed to be a more precise indicator for assessing hydrated cement paste characteristics than total porosity.
Q7: How do voids affect both concrete strength and permeability?
Voids significantly impact concrete by affecting both strength and permeability. Larger entrapped air voids weaken the concrete structure, while capillary pores influence water penetration and durability. To optimize concrete's properties, controlling the volume and distribution of all void types is essential for achieving desired strength and resistance to environmental degradation.