How Does Presence of Moisture Cause Deterioration of Masonry Foundations?
In the past two weeks we’ve talked about topics related to decay, deterioration, and natural entropy here in our historic buildings and in the universe beyond. The week before last, we talked about trapped residual moisture in interstitial spaces between wall cavities or within wall assemblies. Moisture trapped in cementitious or gypsum type building assemblies can cause exponential damage over time, as it accelerates deterioration and the breakdown of the constituent binding materials within these structures. And then last week we talked about the natural process of entropy, also related to decay and physical matter’s tendency to naturally seek lower states of energy.
Links to these recent articles follow below:
- Trapped Residual Moisture In Masonry Walls – Part I
- Entropy In Historic Masonry Facades And Structures
This week we are looking closely at case studies and the principles of masonry foundation decay and deterioration in the presence of excess moisture. The outline for this week’s and next week’s articles follows below, this week we will cover sections I through III:
- The anatomy and make up of foundations
- The mechanics of deterioration
- Defensive systems to protect masonry
- Restoration and overhaul of historic masonry
- The connection between building deterioration and climate change.
The anatomy and make up of foundations
The foundations of historic brick rowhomes in neighborhoods like Capitol Hill, Washington, D.C., embody a utilitarian yet crucial element of these structures. Constructed during an era preceding the widespread use of bulk-produced concrete, these foundations are emblematic of a practical historic approach to construction, reflecting the economic and material constraints of their time. Against the backdrop of economic challenges of the past, now with renewed interest in urban living, many of these historic rowhomes had fallen into disrepair, underscoring the need for comprehensive restoration and maintenance efforts.
At the very base of these rowhomes, footings comprised of locally sourced stone or brick form the initial connection between the structure and the earth beneath. These corbelled masonry footings serve as the foundational base upon which the entire building rests, distributed to counteract potential differential settling and uneven load-bearing. The foundation walls, ascending from the footings, serve as the load-bearing structural backbone of these historic rowhomes. These walls are constructed using a technique known as thickened or corbelled brick and mortar.
In contrast to the foundation composition, piers and pilasters, erected with the same brick and mortar materials as the walls, are often found at front and rear porches. These vertical supports are positioned along the outer edge of the porches.
While the design did not conventionally feature weep holes or drain pipes within the foundation interior walls, other moisture management strategies were employed. These measures included the use of moisture-resistant materials like Del Carmen slate tile embedded into a thickened mortar joint right in the base of the wall above the foundation. Specific buildings around Capitol Hill had terra cotta drain tiles installed at the exterior side of footings, but are rarely found at the typical historic row buildings.
The foundation walls of these historic rowhomes are composed of brick and mortar units, set and bonded most commonly using common bond, The English bond, or the Flemish bond are generally found in above grade facades. In most cases, pressed bricks at front facades were set in a running bond. The bricks themselves, often locally sourced and locally kiln fired, were typical of the construction methods employed during the era. Mortar, a mixture of sand, lime, and water, acts as the binding agent, securing the bricks into a cohesive structure. The execution of various brick bond patterns, such as Flemish bond with alternating headers and stretchers, not only contributes to the structural stability but also imparts a sense of visual symmetry and craftsmanship to the rowhome facades.
The mechanics of deterioration
The brick rowhome facades and foubdations in historic neighborhoods like Capitol Hill, Washington DC, constructed during the late 1800s and early 1900s, are currently at a point in their respective lifecycle when they are overdue for overhaul and restoration. The mechanics behind the deterioration of these masonry structures is based on the deleterious effects of the external environment.
At the core of understanding masonry decay lies the interplay of two main components: brick and mortar. Mortar, a blend of sand, lime, and water, serves as the binding agent that holds bricks together. Over time, the porous nature of mortar allows for a chemical reaction called carbonation, where carbon dioxide from the air reacts with the lime to create calcium carbonate. While this process enhances mortar resilience, it also renders it more susceptible to moisture intrusion. Prolonged exposure to moisture, especially rainwater and groundwater, can trigger freeze-thaw cycles. Moisture absorbed by the mortar freezes during colder periods, causing expansion and subsequent contraction upon thawing. This cycle can lead to cracks and fissures within the masonry, compromising its integrity.
The bricks themselves, though largely surviving intact, to this day, are not impervious to the effects of moisture. Being porous, they absorb moisture readily, leading to challenges such as efflorescence—an accumulation of soluble salts that forms a powdery substance on the brick surface. Additionally, repeated cycles of moisture absorption and drying can result in spalling, where the face of the brick flakes due to water-related expansion and contraction.
Groundwater, a significant external factor, impacts foundation deterioration. As rainwater seeps into the ground, it raises the groundwater table, exerting hydrostatic pressure against foundation walls. This pressure forces moisture into the foundation, accelerating deterioration. In almost all cases of foundation water entry, there are items that can be modified on the exterior of the building to reduce water flow around the foundation which in effect has a reduction in The amount of water flow down into the lower area of the excavated basement. Capillary action, where moisture travels through porous materials, draws moisture inwards into and through the foundation walls. Capillary action, coupled with groundwater presence, can channel moisture upwards, weakening the mortar and inducing erosion. This process manifests as efflorescence and spalling.
In summary, the mechanics behind the deterioration of historic brick rowhomes in neighborhoods like Capitol Hill stem from the natural interaction between moisture, hydration, and external elements like groundwater. These structures, dating from the late 1800s and early 1900s, were build with the material’s and technology available at that time. Understanding the dynamics of masony carbonation, freeze-thaw cycles, moisture absorption, and hydrostatic pressure helps historic restoration specialists, like IDS,to apply informed restoration strategies.
Defensive systems to protect masonry
To protect DC historic masonry structures and foundations, like the brick rowhomes of Capitol Hill, from the natural forces of moisture and deterioration, a few different types of defensive systems and restoration techniques can be helpful. These measures, ranging from brick tuckpointing or repointing and waterproofing systems to advanced drainage solutions, help in preserving the historic buildings of these neighborhoods.
Brick pointing, a nuanced and complicated trade involves the careful removal of deteriorated or damaged mortar joints and the replacement with fresh mortar. This process not only restores the structural integrity of the outer masonry facade but also enhances its visual appearance. Repointing safeguards against water infiltration by creating a renewed barrier that deters otherwise unabated moisture from seeping into the brickwork. The proper choice of mortas, and consistency is crucial to ensure compatibility with historic masonry, allowing for effective integration.
In the battle against moisture intrusion, modern advancements have introduced external synthetic polymer liquid-applied coatings as a more durable waterproofing alternative or option. These coatings act as a protective shield, creating a low permeability barrier against rainwater and groundwater. By forming a continuous membrane on the exterior surface of the masonry, dampproofing or waterproofing coatings prevent water from permeating the brickwork. The result is enhanced longevity for both the masonry foundation.
To mitigate hydrostatic pressure and water leakage associated problems at the foundation related to groundwater, dimple board and perforated foundation drain assemblies create passive pathways to relieve groundwater pressure. Dimple boards, with a synthetic surface, create an air gap between the foundation wall and the surrounding soil, st the exterior side of a building. Although they are called boards, they are not boards, they are somewhat rigid sheets of synthetic impermeable materials. The gap allows water to drain and run freely next to the foundation, reducing hydrostatic pressure against the masonry. Paired with perforated foundation drain pipes, excess groundwater can be effectively channeled away from the foundation, mitigating the amount of exposure to building material hydration and moisture infiltration.
Internal and / or external sump pump systems, along with passive gravity drain systems, can work together where the building and respective site layout do not allow for a fully passive system. Internal sump pumps are installed within basements or crawlspaces to collect and expel groundwater that accumulates below the foundation. These systems help prevent basement flooding and help maintain a controlled moisture level. On the other hand, passive gravity drain systems rely on the principle of natural water flow. Strategically placed drainage conduits and channels redirect water away from the foundation, using the existing site layout and associated gravity to guide moisture away from the building.
Interestingly, one of the largest contibutors to degradation and deterioration of building materials, exposure to ultraviolet rays in sunlight, doesn’t significantly affect brick above grade in facade constructions, and in a below ground foundation there’s almost no exposure to sunlight. Kiln-fired clay bricks are resistant to damage by ultraviolet (UV) rays from the sun due to their inherent composition and structure. Clay bricks are primarily composed of natural clay minerals and other additives, which are formed into a solid structure through the firing process in a kiln.
The firing process involves subjecting the clay bricks to high temperatures, typically exceeding 1000 degrees Celsius (1832 degrees Fahrenheit). This intense heat causes chemical and physical changes within the clay minerals, leading to the formation of a crystalline structure and the expulsion of moisture. As a result, the clay bricks become more stable and durable.
This crystalline structure developed during firing contributes to the bricks’ UV resistance. Ultraviolet rays from the sun consist of high-energy photons that can cause damage to materials by breaking chemical bonds and promoting degradation. The tightly bonded crystalline structure of kiln-fired clay bricks provides a level of protection against these high-energy photons.
Furthermore, the firing process eliminates any residual organic materials and volatile compounds present in the clay, reducing the susceptibility of the bricks to deterioration caused by UV exposure. The absence of organic matter prevents the development of cracks, fading, and other forms of degradation commonly associated with prolonged sun exposure.
In essence, the combination of the firing process, crystalline structure, and absence of organic components renders kiln-fired clay bricks highly resistant to damage by ultraviolet rays from the sun. This durability makes them a reliable and long-lasting building material for various applications, including historic masonry and contemporary construction.
To properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, contact us or fill out the webform below and drop us a line. We will be in touch if we can help.
