A complete guide to Rubble Masonry

Find out more about rubble masonry as a timeless construction technique. Advantages, disadvantages, consolidation techniques, and examples

Rubble masonry is a fascinating construction technique that has evolved over centuries. In this comprehensive guide, we will delve deep into rubble masonry, starting with its technical definition, and then examining how it is constructed, what materials are used, and in what contexts this technique is particularly suitable.

Technical Definition of Rubble Masonry

Rubble masonry is a construction system that owes its name to its unique structure. It consists of two wall curtains, often made of solid bricks or stone, placed parallel to each other at a certain distance with connective elements between inner and outer masonry wall layers. This arrangement creates a sort of “sack” between the two walls, a real empty space that is usually filled with residual material such as fragments of bricks and gravel, firmly bonded by lime or cement mortar. This combination of materials, if executed correctly, creates a cohesive and durable structure, ready to withstand the challenges of time.
Rubble masonry is is certainly nothing new in modern architecture and dates back to antiquity. The Egyptians and Greeks, for example, extensively used it in their defensive systems and building structures. This technique has demonstrated its reliability over the centuries and is still appreciated for its strength and durability.

Rubble Masonry

Types of Rubble Masonry

Rubble masonry can be constructed using various materials for both the curtains and the infill. The most common materials include bricks, stone, tuff, and cementitious conglomerate. The choice of material influences both the performance and the aesthetics of rubble masonry, so it is essential to carefully consider which one is most suitable for a specific project needs. The most common types of rubble masonry are:

bricks: They are known for their durability and can be arranged in various configurations to achieve different textures and patterns;
stone: natural stone gives a rustic and charming appearance to rubble masonry. It is often used in restoration projects and historic constructions;
tuff: tuff is a porous and lightweight material, especially suitable in seismic areas. Its porosity allows it to absorb and dissipate seismic energy effectively;
cementitious conglomerate: in contemporary rubble masonry, cementitious conglomerate, reinforced or not, is commonly used for infill. This choice offers greater strength and durability over time.

Representing the material layers of Rubble Masonry with BIM Software

In a restoration or recovery project of existing structures, as well as in a new construction project, it is necessary to produce detailed graphics, complete with information about the as-built state and the works to be carried out. It is necessary, therefore, to correctly represent all the masonry that makes up the structure of a building, and to have evidence of all the layers that compose it and their characteristics (type of material, mechanical resistance of the various layers, state of conservation, etc.).

Creating material layers in Edificius

In this context, creating a 3D model of the building, using BIM software for architectural design, can be very helpful. With BIM software, you can assign material layers to each building envelope and represent all the layers of rubble masonry with thicknesses, materials, hatches, and even the physical properties of individual materials (mass, thermal transmittance, etc.). Additionally, you can view walls and material layers in both 2D and 3D and automatically extract detailed and always up-to-date plans, sections, and elevations.

Use of Rubble Masonry

Rubble stone masonry as a technique of building structures has not changed significantly over the centuries. Even the Romans used stones or bricks of various sizes, depending on local availability and the type of construction. The stones or bricks were carefully selected to ensure good adhesion with the mortar. The mortar was a mixture of sand, lime, and water. The Romans were known for the quality of their lime, which provided durability to the structures. In some cases, such as hydraulic works, concrete was used, consisting of lime, pozzolan, and gravel. Typically, two external walls of stone or bricks were constructed, placed in parallel with a space between them to accommodate the infill. It required skill and expertise to arrange the bricks or stones in geometric patterns to achieve a uniform and stable arrangement. Inside the space between the two external walls, a pour of mortar, concrete, or other filling material was carried out. This material was poured carefully to avoid air bubbles and ensure a solid bond between the external walls. During the pouring process, the material was leveled and compacted to ensure a uniform surface and maximum density. After filling, the external walls were leveled to create a uniform surface. This phase required special attention to ensure that the final structure was solid and stable. After construction, the rubble masonry required a settling period during which the internal material hardened and became solid. This process could take several days or weeks, depending on the conditions and materials used.
Today, the process involves raising the two curtains, generally made of bricks, to a height of about 60 cm, inside which metal reinforcement is placed. Subsequently, cementitious conglomerate is poured. This process is repeated until the desired height is reached. During the pouring process, it is essential to stiffen and brace the brick curtains to counteract the forces generated by the weight of the conglomerate.

The evolution of Rubble Masonry: historical insights

Rubble masonry, also known as “emplecton” among ancient Greeks, boasts a history dating back millennia. Its first recorded appearance dates to around 2900-2600 BCE, with the majestic Sadd el-Khafara dam near Helwan, Egypt, standing at a towering 14 meters in height. This ancient dam was built using the rubble masonry technique, ensuring remarkable structural resilience. In this context, the cladding of planks, stones, or bricks served to protect the underlying embankment and discourage enemy attacks.

Ancient Greeks, renowned for their architectural excellence, extensively employed rubble masonry in constructing defensive walls for their city-states. This technique became a pivotal element in safeguarding ancient Greek polis.

The use of rubble masonry has spanned the centuries and remains relevant to this day. During the Middle Ages, medieval defensive walls were often built using this technique, demonstrating its durability and long-lasting strength.

Innovation in construction has led to a “modern” version of rubble masonry. In this contemporary variant, walls are constructed with a concrete pour devoid of residual elements and internally reinforced with steel. This approach imparts greater elasticity, remarkable static and seismic resistance, while maintaining the unity between the external form and the typical internal structure of load-bearing wall constructions. These modern rubble masonry walls allow for a more efficient distribution of structural loads, freeing up interior spaces from constraints and enabling greater flexibility in interior organization.

Rubble Masonry consolidation

In the realm of construction, rubble masonry represents a fascinating yet complex technique. This construction method entails meticulous structural analysis to ensure its solidity and long-term reliability.

One of the main challenges in rubble masonry is its lack of monolithic behavior. The two external walls and the internal filling material do not always collaborate cohesively. The binding mortar, if present, often does not enable perfect adhesion between the filling material and the external stone or brick walls.

From a structural perspective, when working with significant wall thicknesses (beyond 100 cm), it’s essential to consider that, in the absence of precise data on the strength of the filling material, the load-bearing capacity of rubble masonry should primarily rely on the external walls.

During the construction of rubble masonry, separation planes often occur between the filling material and the external walls. These can result from an insufficient amount of binding mortar, sometimes entirely absent or pulverized over the years, compared to the inert part of the filling material. Additionally, phenomena like shrinkage or natural compaction of the material can contribute to such separation planes.

In particular, compaction of the filling material can lead to different static behaviors in the central part of the masonry. Due to its poor quality, this part can experience compressions that transmit expansion stresses to the external walls, jeopardizing the integrity of the entire masonry system.

To address these structural challenges, adequate consolidation of rubble masonry is essential. An effective method involves saturating voids through the injection of grouts completely free of free lime, with a super-fluid and dimensionally stable consistency, preferably pozzolanic or cementitious-based. Competently executed, this type of intervention can significantly enhance the compressive strength of the filling material, preparing it for subsequent actions such as reinforced plasters, structural reinforcements, and more. The injection’s goal is to restore the internal continuity of the material, correcting cracks and disconnections. This process strengthens the masonry wall and improves its behavior within the plane.

What makes material injection even more intriguing is that it represents a “passive” consolidation method. This means the intervention occurs within the structure itself, without requiring external alterations. The most suitable consolidating mixtures for this purpose are often fluid mortars based on natural hydraulic lime, injected up to refusal, into specific holes regularly spaced in the masonry. The process typically proceeds from bottom to top. These mixtures contribute to increasing the masonry’s strength without significantly altering its rigidity.

However, mortar injection does not exclude the possibility of surface reinforcement interventions. These interventions, often complementary to injections, may include the application of fiberglass nets (or other materials) to enhance the compactness and consistency of the masonry core before further reinforcements.

Another type of consolidation intervention can be repointing of mortar joints between stones. Following deep removal and cleaning of inconsistent residues, the mortar joint between the masonry stones is reinstated to restore its compactness.

Repointing can be achieved by applying a lime-based mortar formulated for maximum chemical and chromatic compatibility with historic masonry, allowing for an increase in its mechanical characteristics while preserving its exterior appearance.

Examples of contemporary architects

To demonstrate the applicability of rubble masonry in modern architecture, we can mention some prominent contemporary architects who have successfully employed it in their projects. Among these are Louis Kahn, Rafael Moneo, Julio La Fuente, and Massimo Carmassi. Each of them has contributed to revitalizing this ancient technique, proving that it can be used innovatively and remains relevant.

The Role of BIM in Rubble Masonry project and restoration

Building Information Modeling (BIM) methodology and architectural design software are invaluable resources for professionals involved in rubble masonry recovery, restoration, and design. They provide a comprehensive platform for design, analysis, material management, and collaboration, enabling more precise, efficient, and effective interventions. Here are some examples:

Construction Detail Modeling: One of BIM’s main features is the ability to create detailed and realistic 3D models of existing buildings, including rubble masonry. These models can capture every structural element, material layer, surfaces, and even decorative masonry details. This virtual representation offers the designer a complete view of the building before restoration work begins;
Structural Analysis: BIM software for structural calculation starts from the 3D model of the load-bearing structure to perform advanced structural analyses on existing rubble masonry. These analyses can detect weak points, areas requiring intervention, and provide data on structural stability. This helps the designer plan restoration operations more accurately;
Energy Analysis: Through BIM software for energy simulation, it’s possible to simulate the behavior of rubble masonry in different conditions. Thermal transmittance, interstitial condensation formation, and more can be evaluated. This helps predict how the masonry will respond to various situations and allows the designer to assess potential improvement measures;
Degradation and Crack Analysis: Using HBIM software, the 3D BIM model can be utilized for a detailed analysis of degradation phenomena, following the NOR.MA.L. terminology, and for identifying and classifying cracks using an orthophoto of the masonry;
Material Management: BIM allows efficient management of materials needed for restoration, including bricks, stones, mortar, and other specific materials used in rubble masonry. Accurate material management helps avoid waste and control costs;
Integrated Collaboration: BIM fosters integrated collaboration among various professionals involved in the restoration project, including architects, structural engineers, surveyors, art historians, and more. Everyone can work on the same model, share information, and make real-time changes. This enhances communication and coordination among team members;
Comprehensive Documentation: BIM can automatically generate complete documentation for the restoration project, including plans, sections, elevations, and detailed material lists. This documentation is essential for obtaining the necessary approvals and for the practical realization of the restoration;
Temporal Planning: BIM allows detailed planning of the restoration program, considering different phases and the time required for each operation. This is crucial for meeting deadlines and effectively managing labor;
Costs and Budgeting: BIM facilitates cost management by providing accurate cost estimates based on the 3D model and material data. This helps the designer plan a realistic budget and avoid surprises during project implementation.

Conclusion

In conclusion, rubble masonry is an optimal construction technique that blends history and modernity. The choice of materials, careful planning, and knowledge of existing masonry consolidation techniques are essential for the success of a rubble masonry project. This technique offers significant advantages, but it’s important to carefully assess both its positive and negative aspects before embarking on a project. With the right preparation, rubble masonry can create solid, resilient, and captivating buildings that stand the test of time.