Yes, concrete is a porous material that allows water to pass through its structure. Although it appears solid and impermeable, concrete is a composite material made from cement, aggregate, and water. This composition results in a complex internal network of tiny voids and channels. Porosity refers to the measure of these internal voids, and they significantly influence how the material interacts with moisture.
Understanding Concrete’s Porosity
The presence of these internal pathways is a natural and unavoidable result of the chemical reaction that forms concrete, known as hydration. When cement and water mix, only a portion of the water chemically bonds with the cement particles to form the hardened paste. The initial mix is designed to be highly workable, which requires more water than is necessary for this chemical process.
The excess water that does not react with the cement eventually evaporates as the concrete cures. As this water leaves the structure, it creates a network of interconnected spaces called capillary pores. The total volume of these pores is directly related to the initial water-to-cement ratio. A higher ratio, such as one above 0.60, leaves behind a greater number of these voids, which negatively impacts the material’s strength and durability.
Mechanisms of Water Intrusion
Water travels through the hardened cement matrix primarily through two physical mechanisms.
Capillary Absorption
The most common is capillary absorption, also known as capillary suction. In this process, the adhesive forces between water molecules and the pore walls are stronger than the force of gravity, allowing water to be pulled into and through the tiny, interconnected capillary channels. This capillary action is the dominant way moisture moves through concrete that is not fully saturated.
Permeation
The second mechanism is permeation, which occurs when water is forced through the material under hydrostatic pressure, such as from a high water table or prolonged standing water. Water can also bypass the internal pore structure by flowing through larger defects like micro-cracks and air voids.
Degradation Caused by Permeation
Water intrusion acts as the medium that carries aggressive agents deep into the concrete structure, leading to various forms of degradation.
Corrosion of Steel Rebar
One significant issue in reinforced concrete is the corrosion of the steel rebar. Water transports dissolved chlorides or carbon dioxide into the material, which eventually breaks down the passive protective layer on the steel. The resulting rust occupies a volume significantly larger than the original steel, creating internal expansive pressure that causes the concrete to crack and flake off, a process known as spalling.
Freeze-Thaw Damage
Damage from repeated freeze-thaw cycles is another major deterioration mechanism in cold climates. Water that penetrates the pores expands by approximately 9% when it freezes, generating immense internal stress. This cyclical expansion leads to the breakdown of the cement paste, causing internal cracking and surface scaling.
Chemical Attack
Chemical degradation, such as sulfate attack, is also facilitated by the movement of water. Sulfate ions present in groundwater or soil are carried into the concrete, where they react with the hydration products. This reaction forms expansive compounds, primarily ettringite and gypsum, which build up pressure within the pores. This accumulation leads to internal cracking, swelling, and a loss of strength.
Strategies for Water Resistance
Limiting water ingress is a primary goal in concrete durability engineering, and several methods are used to achieve this. One effective way to reduce porosity is by maintaining a low water-to-cement ratio during the mixing process. This often requires the use of specialized chemical admixtures, such as superplasticizers, to ensure the mix remains fluid enough to be placed and compacted correctly.
Proper curing is also an important preventative step. By keeping the concrete surface moist for an extended period, the hydration reaction can proceed further, creating a denser, less porous structure. For existing structures, exterior barriers like penetrating sealers are applied to the surface to repel water. These sealers, often based on silane or siloxane compounds, react with the material to create a water-repellent layer within the pores.