Today we are looking at the ruins of a historic rubblestone masonry structure located in a remote wooded area. The building was constructed primarily of irregular stone units bonded with mortar, characteristic of early American historic rubblestone masonry. Notable features include the use of larger, more uniform stones at the corners, indicative of quoining techniques. The structure is largely in ruin, mostly collapsed, at this point, with various signs of deterioration, including mortar loss, biological growth, and some stone displacement, providing an excellent opportunity to study long-term degradation processes in historic masonry constructions.
Rubblestone masonry is a historic building method that uses uncut or roughly dressed stones of varying sizes, typically bonded with mortar. This technique has been employed for millennia due to its efficiency in using locally available materials and its structural robustness. The irregular nature of the stones, when properly interlocked, creates a stable matrix capable of distributing loads effectively. Examples of rubblestone masonry’s durability can be observed in structures such as sections of the Great Wall of China, Roman aqueducts, and medieval European castles, some of which have stood for over a thousand years. The longevity of these structures can be attributed to factors including the interlocking nature of the stonework, the use of compatible mortars, and the inherent mass and stability of thick masonry walls.
Quoining is a masonry technique involving the use of larger, often more regularly shaped stones at the corners of buildings. This method serves both structural and aesthetic purposes. Structurally, quoins reinforce the corners of buildings, which are typically subjected to greater stress concentrations. The larger stones used in quoining provide increased surface area for load distribution and enhance the interlocking between perpendicular walls. This improved corner stability is particularly beneficial in taller structures or those subject to lateral forces such as wind loads. The alternating pattern of quoins also helps to tie adjoining walls together more securely, reducing the risk of separation over time. From an engineering perspective, quoins contribute to the overall structural integrity of the building by strengthening what would otherwise be a vulnerable point in the construction.
Masonry deterioration is a complex process involving multiple physical, chemical, and biological mechanisms. One of the primary agents of degradation is water infiltration. Moisture can penetrate the porous structure of both stone and mortar, leading to various forms of damage. In regions experiencing freeze-thaw cycles, water expansion during freezing can cause internal stresses, resulting in cracking or spalling of stone surfaces. In warmer climates, the repeated wetting and drying cycles can lead to salt crystallization (efflorescence), causing surface erosion.
Biological growth, including moss, algae, and higher plants, can exacerbate moisture retention and introduce mechanical stresses through root systems. Some organisms also produce acids that can chemically erode stone surfaces. Additionally, atmospheric pollutants can react with stone minerals, forming soluble salts that contribute to deterioration.
Structural factors also play a role in masonry degradation. Differential settlement of foundations can induce stresses in the masonry, leading to crack formation. Thermal expansion and contraction cycles can cause cumulative damage over time, particularly at interfaces between materials with different thermal properties.
The composition and properties of mortar play a crucial role in the performance and longevity of masonry structures. Historically, lime-based mortars were predominant. These mortars, produced by burning limestone to create quicklime, then slaking with water to form lime putty, possess several advantageous properties for use with natural stone.
Lime mortars are characterized by lower compressive strength and higher porosity compared to modern Portland cement mortars. This relative softness allows for slight movements within the masonry without inducing excessive stress on the stone units. The higher porosity facilitates moisture transmission, allowing the masonry to “breathe” and reducing the risk of trapped moisture, which can lead to freeze-thaw damage or salt crystallization.
In contrast, Portland cement mortars, widely adopted since the late 19th century, exhibit higher compressive strength and lower porosity. While these properties can be beneficial in certain modern construction applications, they can be detrimental when used in historic masonry restoration. The rigidity of cement mortars can cause stress concentrations in softer stone, potentially leading to cracking or spalling. The lower permeability can impede moisture movement, potentially trapping water within the masonry and accelerating deterioration processes.
We can Help
Our company focuses on historic restoration more than modern building upkeep, maintenance, and construction, but our company understands both types of construction very well and a full picture well-rounded approach is needed in any niche in the construction industry. Although we focus on historic restoration, repointing, tuckpointing and historic brick repair, our company also has technical knowledge and competencies in the areas of modern and contemporary construction as well as we become one of the leaders in that area of the market today. Understanding both historic and modern or contemporary construction is useful because both aspects help understand the challenges and potential solutions for challenges in building science and construction.
We can help with a variety of historic masonry restoration needs and upkeep, from modest tuckpointing and or repointing to complicated and extensive historic masonry restoration. Infinity Design Solutions is a historic restoration specialist contractor specializing in both historic masonry restoration such as tuckpointing our repointing, and brick repair. If you have questions about the architectural details or facade of your historic building in Washington DC, reach out and say hello and if we can help we’ll be glad to assist you. You can email us or call us on the telephone at the following link: contact us here.
