Tensioned stone
Tensioned stone is a high-performance composite construction material: stone held in compression with tension elements. The tension elements can be connected to the outside of the stone, but more typically tendons are threaded internally through a drilled duct.
Tensioned stone can consist of a single block of stone, though drill limitations and other considerations mean it is typically an assembly of multiple blocks with grout between pieces. Tensioned stone has been used in both vertical columns, and in horizontal beams. It has also been used in more unusual stonemasonry applications: arch stabilization, foot bridges, granite flag posts, cantilevered sculptures, a space frame, and staircases.
Tensioned stone has an affiliation with massive precut stone, which is a central technique of modern load-bearing stonemasonry. It is also aligned with mass timber and straw structural insulated panels, which are all reconfigurations of traditional materials for modern construction that involve some pre-fabrication.
Tensioned stone methods
Tensioning is achieved with steel tendons or rods that are either threaded through ducts within the stone elements or attached to the stone externally. For internal tensioning, holes are drilled into the stone elements to form a duct; the tensioning tendon is threaded into the duct.The most common form of tensioned stone is post-tensioned stone, which also has the longest history. A second method, developed in the early 2020s, is pre-tensioned stone.
As with pre-stressed concrete, the pre- and post-tensioned methods can be used in different contexts: pre-tensioned stone may be more appropriate for prefabrication, while post-tensioning may be more suitable for on-site assembly.
Post-tensioned stone
For post-tensioning, once the stone components are in place, the tendons are tensioned using hydraulic jacks, and the force is transferred to the stone through anchorages located at the ends of the tendons, usually in combination with a plate. The tensioning process imparts a compressive force to the stone, which improves its capacity to resist tensile stresses that could otherwise cause cracking or failure.Pre-tensioned stone
In pre-tensioned stone, the tendon is held in tension with jacks while the remaining cavity in the duct is filled with epoxy grout. After the epoxy has set, the ends of the rod are released from the jacks, placing the stone under compression. A structural difference between pre- and post-tensioned stone is that, in the former, the tension element is adhered to the stone along its length, so compression is applied to the stone along the length of the duct, while in post-tensioned stone the pressure is applied through the end plates.Rationale
Increased strength
Stone has great compressive strength, so is ideal in compressive structures like stone arches.However, it has relatively weak flexural strength, so in isolation cannot be safely used in wide spans under tension.
For concrete, this problem has been long solved: in addition to conventional tensile reinforcement, engineers developed prestressed concrete methods starting around 1888. Such tension-reinforced concrete applications combine compressive strength with pre-stressed tensile compression for combined strength much greater than either of the individual components, and have been in wide use for decades. One of the early concrete engineers Eugène Freyssinet improved concrete pre-stressing methods, and it is claimed that he also applied post-tensioned concrete methods to stone. As for concrete, post-tensioning maintains stone in compression, thereby increasing its strength.
Energy use and carbon emissions
Stone is 'natural precast concrete' so only needs to be cut and tensioned prior to use in construction. Compared to concrete and steel, post-tensioned stone production has dramatically lower energy costs, with concomitant lower carbon emissions.Applications
Post-tensioned stone has potential to replace steel-reinforced concrete in some contexts, as, according to structural engineer Steve Webb "a post-tensioned stone beam is as strong as steel". "Post-tensioning offers new potential for the revival of masonry as a structural material". Post-tensioned stone has the potential to be used in conjunction with massive precut stone in a range of designs.In 2020, post-tensioned stone was featured prominently in "The New Stone Age", an exhibition at The Building Centre.
Architect James Simpson writes:
Benefits
Structural benefits
Advantages of tensioned stone relative to reinforced concrete
- Strength. Compared to standard concrete, many types of stone have increased compressive and tensile strength; this property contributes to the increased strength of the prestressed stone assembly.
- Slenderness. Less bulky beams, due to stone's greater compressive strength compared to concrete.
- Durability offered by the stone's resistance to weather conditions. This reduces maintenance costs.
- Aesthetics. Instead of cladding concrete in stone, the load-bearing stone has the appearance of stone.
- Reduced embedded carbon. Post-tensioned stone causes the emission of <1/3rd of carbon dioxide greenhouse gas relative to concrete.
- Lower cost. A study of a 30-storey office block found that using PT stone floor panels was cheaper than concrete floors. Subsequent studies by Groupwork found that stone would be cheaper than concrete on most architectural projects, but these studies were not published.
Advantages of tensioned stone relative to untensioned stone
- Reduced material usage. By enhancing the structural performance of stone, post-tensioning allows for the use of thinner stone slabs or walls, reducing the overall material requirements and associated costs.
- Expanded design options. Tensioning provides additional design options, allowing architects, engineers, and sculptors to create more innovative and complex structures that would be difficult to achieve with traditional stone construction methods.
- Seismic resilience. Compared to conventional stone, PT stone structures can have improved seismic performance, as the compressive forces could help to maintain the integrity of the structure during ground motion.
Operational advantages
- Pre-assembly of the pieces can be done at ground level, reducing cost and improving safety.
- Easier review of the assembly, including testing strength and waterproofing of the mortar
- Easier to schedule tasks, as the prefabricated post-tensioned stone assemblies can be stored before using
Challenges facing adoption
- Load-bearing stone's cultural affiliation with traditional architecture
- Confusion with the use of cosmetic stone in high-end luxury construction and the perception of high cost
- Limited knowledge amongst most structural engineers, beyond elite firms like Arup Group, Buro Happold, and Webb Yates. Structural engineering training does not include stone. This is changing with increased attention from the Institution of Structural Engineers.
- The absence of international standards for engineering with stone
- Lack of stonemasons with knowledge of tensioned stone methods. Lack of concrete workers with prestressing knowledge who also work with stone.
- Absence of a large-scale industry for pre-fabricated PT stone
- By analogy with precast concrete, a lack of manufactured modular PT stone components
- The high cost of some early high-profile post-tensioned stone projects, notably Portcullis House
- Shortage of dimension-stone quarries that do routine strength testing and can give strength certificates for their stone components
- The need to develop better CAD-CAM software and hardware for automated CNC stone cutting of precise shapes. In the early 2020s, the dimension-stone industry in most countries was structured almost entirely for tiles and cladding.
- Existing workflows with concrete create a barrier to entry to a new material, which does not have the economies of scale of a new material like stone
- Unclear how to determine the design strength of stone, versus a homogenized material like concrete
History
Tensioned-stone footbridges with spans up to 40 m have been built in Japan, Switzerland, Germany, and Spain, and are sold commercially in spans of up to 20 m by Kusser Granitwerke.
Early uses of post-tensioned masonry
Tensioning using metal clamps across masonry joints is thought to have been used in stone construction since .Tensioning of masonry was used in the early 1800s: "In 1825 a posttensioning method for tunnelling under the River Thames was utilized in England. The project involved the construction of vertical tube caissons of 15m diameter and 21 m height. The 0.75m thick brick walls were reinforced and posttensioned with 25mm diameter wrought iron rods."
Seismic post-tensioning of brick and stone masonry buildings with steel bolts dates from at latest the 19th century, including following the Charleston earthquake of 1883, and on buildings in seismic regions of Italy. Seismic post-tensioning of masonry is done with considerably lower tension than pre-stressed concrete or modern tensioned stone.
In the mid-20th century, the Sydney Opera House shells were constructed from pre-cast concrete masonry beams that were assembled into a pointed-arch vault using post-tensioning. By 1982, post-tensioned masonry was sufficiently widespread to fill a book published by the Institution of Civil Engineers, though this was brick and precast concrete masonry. In 1985 and 1986, structural engineer Remo Pedreschi and others published studies of post-tensioned brick.