Rising Damp - capillary suction in brick wall
Structural dampness is considered one of the most common and major causes of degradation in buildings. Rising damp from soils manifests itself commonly in the walls and floors of buildings through capillary action along the porous structure of construction materials. This issue has been a concern since ancient times and leads to multiple anomalies, resulting in a considerable decrease in building habitability conditions. Several laboratory tests were performed on a specimen of stone masonry and hydraulic lime and sand mortar, representative of this type of old building walls, to assess the progression of rising damp and its behavior under certain implemented measures.
Water, in various natural states and through different mechanisms, is a very active and prominent factor in the degradation of buildings. According to Freitas et al., the mechanisms that determine the transport of moisture in construction elements are complex. From a physical point of view, there are three fundamental mechanisms fixing humidity:
- Hygroscopicity
- Condensation
- Capillarity
These three mechanisms can explain, in most cases, the variation of moisture in building materials with a porous structure. According to Freitas, humidity in buildings generally affects the walls and, in particular, impacts parameters such as durability, leakage, deterioration of appearance, and thermal performance of materials and construction components.
Rising Damp
According to Young, rising damp is caused by capillary suction of the fine pores or voids present in all masonry materials. The capillaries draw water from the soils beneath a building against gravity, leading to damp zones at the base of walls.
Salt attack is the decay of masonry materials such as stone, brick, and mortar by soluble salts forming crystals within the pores of the masonry. As the salt crystals grow, the masonry is disrupted and decays by fretting and loss of surface layers. The salt commonly comes from the soils beneath and is carried up into walls by rising damp. When the dampness evaporates from the walls, the salts are left behind, slowly accumulating to the point where they cause damage. Repeated wetting and drying with seasonal changes leads to the cyclic precipitation of salts and progressive decay of the masonry.
During dry periods, when water evaporates from the wall, the salt is left behind (as salts cannot evaporate), and the salt solution in the wall becomes more concentrated. As more salts are brought into the wall, the solutions further concentrate as moisture evaporates. When the solution reaches saturation or super-saturation (depending on the type of salt), crystals begin to form. When the rate of evaporation from the wall surface is low (such as in humid climates or in cellars and basements with little air movement), the evaporative front may be at or near the surface, causing salt crystals to grow as long, thin needles extruding from the wall face. This phenomenon, known as efflorescence, appears as a relatively harmless white powder on the surface of new walls.
However, when the evaporation rate is much greater, the evaporative front will be inside the wall, and salts will crystallize within the pores of the masonry (sub-florescence). The force exerted by rapidly crystallizing salts is very high and sufficient to disrupt even the strongest masonry material. Crystal growth leads to either grain-by-grain loosening, producing fretting and crumbling of the surface (particularly in soft mortars), or delamination of a complete layer.
Cyclic wetting and drying is an important driver of salt attack decay. When salts first disrupt masonry, they enlarge the pores slightly. After a cycle of wetting and drying, salts fill the enlarged pores, and new crystal growth further disrupts the masonry and enlarges the pores more. Each cycle may produce tiny changes, but cumulatively, they result in the progressive decay of the masonry material. The amount of salt required to cause damage varies and depends on the type of salts, the nature and condition of the masonry, including its pore structure (pore size and distribution), and the cohesive strength of the material.
Rising damp is caused by capillary action (or suction) drawing water from the ground through the network of pores in a permeable masonry material. Capillary suction becomes stronger as the pore size gets smaller. If the pore size is fine enough, damp may rise many meters in a wall until the upward suction is balanced by the downward pull of gravity. In practice, the height to which water will rise in a wall is limited by the rate of evaporation of water from the wall surfaces. The evaporation rate for external surfaces is related to the nature of the masonry materials, surface coatings, climate, season, and siting.