Concrete is inherently porous, a characteristic resulting from its fundamental composition as a composite of cement, aggregate (like sand and gravel), and water. The porosity is not a defect but a natural result of the chemical reaction between water and cement, known as hydration. The resulting solidified paste, which binds the aggregates, is permeated by tiny voids and channels that determine the material’s strength and durability.
The Internal Structure of Concrete
Hardened concrete contains several types of microscopic voids. The most significant are the capillary pores, which form when excess mixing water does not fully react with the cement. This unreacted water eventually evaporates, leaving behind a network of interconnected channels throughout the cement paste. The size and volume of these capillary pores influence how easily liquids and gases can pass through the concrete matrix.
Air bubbles are another type of void, categorized as either entrapped or entrained. Entrapped air voids are larger, irregularly shaped pockets trapped accidentally during the mixing and pouring. Conversely, entrained air voids are intentionally introduced, numerous, microscopic, and spherical. These physical spaces collectively define the material’s porosity.
The water-to-cement (W/C) ratio is the most influential factor controlling porosity. A higher W/C ratio means more water is available than needed for hydration, leaving behind a greater volume of capillary pores upon drying. Engineers strive to use the lowest practical W/C ratio, as reducing free water correlates to a denser, less porous, and stronger final product.
Practical Effects of Water Penetration
The presence of an internal pore network means water can be absorbed into the concrete, leading to several practical issues that affect longevity and appearance. One of the most damaging consequences in colder climates is freeze-thaw damage. Water absorbed into the pores expands by approximately nine percent when it turns to ice. This volumetric expansion creates immense internal hydraulic pressure within the concrete matrix, often exceeding the material’s tensile strength.
Repeated freeze-thaw cycles cause micro-cracking to propagate, leading to visible surface degradation. Damage manifests as scaling, where the top layer flakes off, or spalling, where larger chunks detach. This cyclical stress weakens the structure, accelerating deterioration.
Water movement through the pores also causes efflorescence, a common aesthetic problem. This occurs when water dissolves soluble salts, such as calcium hydroxide, present within the concrete. As moisture migrates to the surface and evaporates, the dissolved salts crystallize into a white, powdery deposit. This visual indicator confirms water is actively traveling through the porous structure.
The absorption of moisture creates an environment that supports biological growth and staining. Concrete’s porous nature allows moisture to linger beneath the surface, providing the damp conditions necessary for mildew, moss, and mold to take root. These contaminants cause permanent discoloration and staining.
Methods for Decreasing Concrete Porosity
Managing porosity involves strategies implemented during initial mixing and after the concrete has hardened. Proper curing immediately following placement is a primary method for maximizing cement hydration and reducing capillary porosity. Keeping the concrete moist and at a stable temperature ensures the maximum amount of cement reacts with the water. Effective curing results in a denser cement paste with fewer capillary channels, significantly lowering permeability.
Chemical admixtures are another strategy. Water-reducing admixtures, or superplasticizers, work by surrounding cement particles and creating an electrostatic repulsion. This action disperses the particles, allowing the mixture to flow with less total water content. By achieving the same workability with a lower W/C ratio, these chemicals reduce the volume of pores left behind by excess water.
Air-entraining agents are specialized admixtures that introduce billions of microscopic air bubbles (10 to 500 micrometers in diameter). These tiny, uniform voids act as relief valves for expanding water during freezing temperatures. The microscopic chambers absorb the pressure, preventing the formation of damaging internal cracks and protecting the concrete against freeze-thaw cycles.
After the concrete has set, specialized sealers are applied to block water absorption. Penetrating sealers (e.g., silanes and siloxanes) are absorbed deeply into the pores, chemically reacting to form a hydrophobic barrier. These sealers stop liquid water from entering while allowing water vapor to escape, maintaining breathability. Topical sealers form a physical film on the surface, effective against spills and abrasion. However, this surface barrier may trap moisture below, potentially leading to issues like whitening or delamination.