Building in Stone
Building in Stone
Content
Relative carbon impact of related building materials
Stone, a commodity not a luxury; building a resilient future in stone
"Stone extracted in the morning, fitted in the afternoon" was the mantra of post-war French construction sites. Widely available with new mechanical extraction methods, more efficient cutting saws, and powerful cranes, stone became the most economical material for rebuilding affordable housing in France.
In contrast, the art of building with structural stone in Britain was largely forgotten. With new regulations and codes favoring steel and concrete, stone has been removed from the modern construction toolbox. Yet stone has much to offer; it is strong, slow to age, easily maintained, non-combustible, and versatile. Importantly, stone has the ability to greatly reduce the carbon footprint of new buildings. Sometimes referred to as "natural concrete," limestone, granite, and sandstone, if used structurally in solid pieces, are easy to repurpose and require very little energy to reuse.
The biggest misconceptions surrounding stone include its perceived cost, potential challenges in specification, and the belief that it is expensive to shape and labor-intensive to install. To make it more affordable, we must reconsider the modern understanding of stone, question rigid aesthetic selections, challenge demanding and unnatural structural conditions, as well as costly finishes and processing methods. By embracing the diversity of stone colors and textures, educating engineers on its mechanical properties, informing architects about its raw potential, and engaging with quantity surveyors to clarify cost structures, stone can emerge as an optimal construction material.
Using prefabricated modules of stone, optimizing these modules from specific quarries, and designing buildings accordingly, as well as developing bricks to maximize the output of extraction sites, we can make stone more relevant than ever. The conditions are right to expand the use of this versatile resource. With its increasingly efficient extraction, low use of water, and easy and efficient repurposing—any stone building is a quarry to be—we can embark upon a New Stone Age.
We hope this modest book will lay the foundations for a Renaissance in stone building.
A short history of the misuse of stone in the 20th century
The 20th century marked the decline of construction in stone, the victim of rising energy costs, wars that decimated a skilled and knowledgeable workforce, and the fashion for new materials. To understand this rapid decline and witness the change from structural to veneer, take a walk in the heart of the City of London.
The journey begins at Bank station, the site of architect Sir John Soane’s 19th-century Bank of England building, considered his finest work. Soane’s building, mostly demolished in the 1920s, is our reference for load-bearing masonry. Constructed in Portland Stone brought to the site on barges, it utilized the full strength of limestone, demonstrating a complete understanding of the material by engineers, makers, and the architect.
27 Poultry Lane by Edwin Lutyens tells another story. Built in 1924 with a steel structure, the engineers’ material of choice in post-war Britain, steel was seen as easy to specify and control during its manufacture. The stone is now a self-supporting structure acting as a screen with a classical vocabulary. Across the street stands the iconic postmodern No. 1 Poultry by James Stirling, designed in 1985 and completed twelve years later, in 1997. Its pink and cream sandstone from Australia and the UK is only between 30mm and 50mm thick—a veneer hooked on stainless steel fasteners—a solid concrete shell embellished with a mineral make-up.
The lack of certification and codes ensures stone is left out of the materials conversation. During the late 20th century, architecture was not always concerned with the environmental impact or reuse of materials. Across the road sits a behemoth of a building, the Bloomberg Headquarters by Foster and Partners, completed in 2017, an exoskeleton of large Yorkshire sandstone pieces, patiently selected and fabricated off-site, then intricately connected to a structural concrete core. A tight embrace of two structural materials, leaving a legacy of wasted material and intensive fossil fuel use.
In the center of London, Lutyens, Stirling, and Foster wanted a heart made of stone but built only a skin. However, their desire to use stone proves that architects and clients still want stone buildings as a symbol of authenticity, reassurance, and legacy. In the story of structural stone, the material can either push the limits of new technological developments or fall victim to progress and fashion. As we move towards more responsible construction methods, being both frugal and conscientious, stone, the low environmental impact material of excellence, just might be the solution.
From stereotomy to the digital age, new technology is central to stone engineering
From the cutting devices employed by 17th-century water mills to the cable wire cutting saws, steam-powered planers, and industrial tip blades used to increase the speed of cutting granite and marble in the 1900s, stonemasons have never shied away from progress.
Stonemasonry was at the forefront of applied geometry with the use of stereotomy, the art of cutting volume through spatial geometry, producing some of the most daring vaults, bridges, and civil and military infrastructure. The stone design knowledge of the past compares to the most powerful parametric software used by stonemasons today.
Contemporary advanced scanning and imaging techniques allow quarry operators to map out their resources with unprecedented accuracy. This not only minimizes waste but also ensures the responsible and sustainable use of precious stone reserves. Modern tracking and triage software ensure that a stone plant can develop a range of the most efficient modules, from large ashlars for quick building to bricks.
Testing technology, with more accurate instruments, can record and study in more depth how stone structures react to different conditions, from loadings in demanding structures to fire testing. Finite element analyses help predict potential weaknesses in natural materials, allowing for more accurate and safe use. This non-destructive testing, coupled with the scanning of blocks, ensures the optimization of material use from quarry to installation and maintenance. Ultrasound machines are currently being used to assess the structural integrity of stone blocks in certain projects. In the realm of stone extraction, technology has unlocked new possibilities.
Computer Numerical Control (CNC) technology has breathed new life into the age-old stone-cutting and carving craft. It offers unparalleled precision, allowing for the creation of intricate designs with a level of accuracy that was once unthinkable. From ornate facades to delicate sculptures, CNC has expanded the artistic horizons of stonemasons.
Automation and robotics have also entered the stone processing arena. These technologies streamline the cutting, polishing, and finishing processes, reducing labor-intensive tasks and enhancing efficiency. As a result, stone products can be manufactured more quickly and consistently.
The digital age has brought stone closer to architects and designers. Computer-aided design (CAD) software allows for complex 3D modeling and simulations, enabling professionals to visualize projects in detail before a single stone is cut. This fosters collaboration and creativity, pushing the boundaries of what can be achieved with stone.
Finally, emerging from the understanding that stone, with its high compressive strength, behaves like concrete, using cables and rebars for post and pre-tensioning creates endless possibilities, from prefabricated structural beams and columns to optimizing walls and arches.
The natural inertia of stone
Natural stone has numerous advantages when it comes to thermal efficiency. However, its efficiency will vary depending on its porosity and density. Generally, stones with a density similar to concrete will display the same thermal behavior, absorbing and releasing heat in response to internal conditions.
In hot climates, much of the unwanted heat gains will be absorbed by the thermal mass in exposed floors and walls, smoothing temperature fluctuations and reducing overheating. During the winter, stone will store heat during the day and redistribute it at night.
However, stone is not a great insulator. These sketches demonstrate two possible ways to deal with thermal mass in walls.
Resolving cold bridging in structural stone
Digging deeper; respectful extraction in well-managed quarries
The world of stone extraction has undergone a remarkable transformation, dispelling the lingering misconceptions that once plagued quarries and mines. Dimensional stone quarries—those dedicated to extracting solid, cubic stone—have emerged as the standard of sustainable practice, setting them apart from their massive clay or aggregate quarry counterparts. The stone industry has shifted its gaze towards innovative product utilization, now seeking to harness all of the stone’s potential, from stone bricks and load-bearing stone to coastal defenses and soil improvement applications. The result is the responsible and sustainable use of stone resources, leaving no waste behind.
Technological advancements continue to play a pivotal role in this evolution. Advanced scanning and imaging techniques have empowered quarry operators to craft meticulous resource maps, minimizing waste and ensuring the efficient management of stone reserves. Meanwhile, automation and robotics have revolutionized stone processing, streamlining tasks and enhancing product consistency.
The sustainability journey extends to quarry restoration. The extraction process is untainted by chemicals, ensuring zero contamination of the surrounding environment. Dimensional stone quarries can facilitate a rolling program of restoration. After the commercial life of a quarry concludes, restoration efforts breathe new life into the land. Examples range from Sites of Special Scientific Interest designation to the proposed Eden Project in Portland, showcasing imaginative restorative land use.
As we look to the future, companies in the stone industry are not just redefining quarrying and processing; they are pioneering a sustainable legacy, bringing together industry and nature—marking the present as a transformative era for the stone sector.
Maximising stone's potential
Keeping stone under pressure
If we had to label stone with one adjective, it would be strong. Stone is often pictured as the preferred material of the ancient builder. Due to its formation, with minerals compacted over millions of years, stone lends itself to being in compression. Jurassic limestone has 40 to 50 megapascals (MPa) of compressive strength; for granite, this increases to 200 MPa, enough to support any heavy construction. By comparison, basic concrete and reinforced concrete start at 50 MPa.
Stone is made of layers and should be positioned correctly when placed under pressure, ensuring that the load is applied perpendicular to its layers. As stone was formed by minerals deposited in the Earth’s mantle or on the floor of ancient oceans, unlike steel, it behaves poorly under tension. This is why stone buildings have always been designed with compression in mind.
A recurrent issue with the modern design of structural stone is the lack of experience and knowledge in the field. The limited design guides on dimensional stone in the Masonry Eurocodes complicate matters. It is important to remember that much of the design process for stone structures can be addressed by applying basic engineering principles. Any gaps in knowledge can be bridged by low-cost and reduced-scale testing. Destructive compressive and flexural testing provided by testing specialists or quarry owners will provide valuable information on the stone’s compressive and flexural capacities.
To find a stone’s design strength, the characteristic strength used must be equal to the stone’s lowest expected value (LEV), often up to 30% lower than the stone’s mean strength. The design strength is obtained by dividing the characteristic strength by a high material safety factor; in many instances, a figure of 2.3 is suitable. In a general scenario, the design strength of the stone ends up being approximately four times lower than its mean strength, leading to a safe final design with a very low utilization factor. Engineering expertise leverages the intrinsic characteristics of stone to design structures that blend aesthetics, durability, and efficiency.
New structural stone typologies
With new codes and regulations alongside advanced prefabrication technology, it’s time to rethink stone building typologies.
New configurations in structural stone, as well as hybrid use with CLT and concrete, showcase the huge potential of stone.
The low-carbon stone brick; rethinking the future of construction
The low-carbon stone brick has emerged as a game-changer in the realm of sustainable construction, offering many advantages over traditional clay bricks. Its production process, characterized by cutting rather than firing, ensures remarkable environmental credentials, with carbon intensity reductions ranging from 55% to a staggering 86%. This innovative approach not only minimizes the ecological impact but also introduces a cost-effective alternative to traditional brick manufacturing.
An outstanding feature of the low-carbon stone brick is its versatility in design and construction. Unlike traditional fired clay bricks, this stone variant allows for the creation of specialized bricks at a significantly lower cost. These bricks are sourced from historical quarries, providing an economically viable means of utilizing stone for cladding at a fraction of the cost.
Low-carbon stone bricks are easily incorporated into construction projects without the need for extensive redesigns and are compatible with conventional bricklaying techniques. Beyond its cost-effectiveness, the low-carbon stone brick boasts exceptional strength, typically surpassing traditional clay bricks with a higher compressive capacity comparable to class B engineer bricks. Its tactile quality, warmth, and inherent protective properties add to its appeal, as does the varied grain, veins, and occasional fossils visible on each brick’s surface.
As the construction industry addresses the urgent need for more sustainable practices, the low-carbon stone brick proves that creating low-carbon buildings can be achieved at a reasonable cost. With the possibility of local quarries supplying stone bricks, the construction sector now has the opportunity to source materials locally, further reducing its carbon footprint and ensuring a more sustainable future for construction.
Spolia; repurposing masonry
Spolia (from the Latin: 'spoils'; singular: spolium) is the name given to stone taken from an old structure and repurposed for new construction or decorative purposes. It results from an ancient and widespread practice whereby stone that has been quarried, cut, and used in a built structure is carried away to be used elsewhere. This is an early example of what is now referred to as the circular economy.
More than half of the total material used to construct a building is attributed to the main structure and envelope. Reusing components from existing, soon-to-be-demolished, or already deconstructed structures significantly reduces the need for manufacturing new components, in turn reducing the carbon cost of the building.
As we start to see the financial cost of carving new stone usurped by the carbon cost associated with its creation, the dismantling and repurposing of stone façades is becoming more common. The technology available to support the process of repurposing masonry has leapt forward, allowing us to provide a greater level of technical certainty.
The development of Light Detection and Ranging (LiDAR) and Ground-Penetrating Radar (GPR) survey accuracy allows for the overall volume of stone within a building to be determined. The existing building acts as a stone quarry, removing the expectation of building with new stone for all repurposing projects. However, an early understanding of the original construction is necessary for such a scheme to be successful.
The comparable commercial benefit of repurposed stone rather than new stone is bolstered when considering the substantial reduction in carbon emissions, the improved thermal performance of the external fabric, and the elimination of inherent steel frame corrosion risk defects. An optimization strategy undertaken with GPR scanning makes it possible to calculate the external stone thickness of masonry façades, enabling all stones to be thinned down to a consistent depth. Reducing wall thickness provides space for thermal improvements such as cavity construction and insulation or an increase in net lettable area.
When reusing components from a building that have been exposed to weathering, they must be evaluated for suitability. In repurposing projects, the first survey confirms the performance of the stone, and a suite of tests will provide an assessment of the long-term durability and performance in line with current expected test standards.
A coordinated strategy for deconstruction needs to be planned jointly by the demolition contractor and the masonry specialist. This ensures that the timing of each step aligns and that the strategy for removing masonry can be synchronized with the demolition requirements, including the lifting and logistical aspects.
It is widely understood that products should be used for as long as they remain functional and reused or repurposed to the greatest extent possible when they reach the end of their service life. Masonry is designed to be durable and long-lasting and can be removed when a building is renovated or demolished, allowing for repurposing. Spolia is a key component in recognizing the importance and value of reclaiming materials to challenge waste and create a robust and climate-conscious circular economy.
