What is Concrete leaching?
Concrete leaching is a common issue visible through stains left on the concrete surface (Figure 1) or in extreme cases stalactite growth (Figure 2). Leaching occurs when water seeps into concrete through pores or small cracks and dissolves the calcium hydroxide (Portlandite) from the cement paste. When this dissolved Portlandite reaches the surface it reacts with environmental CO2 to create a calcium carbonate build up known as Efflorescence.
Concrete leaching of calcium ions increases the porosity and degrades the pore structure, resulting in a weakened matrix with lower compressive strength.
What causes Concrete leaching in concrete structures?
Concrete leaching is primarily caused by water penetrating through structural elements and dissolving the cement paste with many factors contributing to the total leaching potential.
Concrete porosity plays a pivotal role in controlling water ingress. Intelligent mix design limits porosity by utilizing low water to cement ratios, blended cements, suitable aggregate gradings, fine filler materials, and in some instances specifically designed densifying admixtures.
Structures most at risk of concrete leaching include those in constant contact with water especially under pressure, in coastal structures subjected to wave and tide action, or exposed to hot water in industrial applications.
Effective water proofing systems such as hydrophobic impregnation, impregnation and coatings are extremely effective at eliminating moisture ingress through the concrete surface the leaching it creates.
What are the signs of Concrete leaching in concrete structures?
Concrete paste dissolution.
How can I identify Concrete leaching in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
How can I prevent Concrete leaching in concrete structures?
Concrete leaching can be prevented by creating low porosity concretes, providing suitable surface protection for the specific environment they are placed in and designing suitable drainage to limit accumulation of water on the concrete surface.
Specifically designed admixtures systems are available to prevent concrete leaching. SIKA supply two such solutions that can be added during concrete mixing to increase density, reduce porosity and leverage hydrophobicity to stop water penetration.
Sika WT-240 P is a hydrophilic, permeability reducing admixture that reacts with the concrete’s pore solution to form crystalline deposits, blocking the pores and increasing the density of the concrete; limiting leaching potential.
Alternatively, Watertight concrete powder is a solid, hydrophobic, pore blocking admixture. It electromagnetically repels water away from pores providing up an 80% reduction in hydrostatic permeability.
Other defects such as early age cracks, expansive cracks, eroded concrete, spalling or honeycombing can accelerate leaching of concrete structures, therefore, ensuring an all-round durable concrete created with intelligent design details, good quality workmanship and a good surface finish is vital.
How can I repair the damage from Concrete leaching on concrete structures?
Surface protection systems such as concrete coatings or impregnations.