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 (posts), and in horizontal beams (lintels). 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 (SSIPs), 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 (a steel rod) 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 (compared to steel or wood), so in isolation cannot be safely used in wide spans under tension.
"Post-tensioned stone increases the failure load of stone in bending, but also the stiffness of a structure by reducing joint cracking. This method of construction is widely used for concrete structures, but the advantages of using similar techniques with stone are only just being realised." 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 strength tested) 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:
"The term 'engineered timber' is already commonly used in construction, so why not a structural 'engineered stone'? ... The most exciting possibility for the stone industry... is the possible creation of a system of engineered stone for framed, or partly framed, structures. This would exploit the compressive strength of stone, which can be greater than that of concrete, combined with post-tensioning by stainless steel rods. Walls, columns, beams and slabs could all be made from small pieces of factory-sawn stone, cut and pre-drilled to a design of standard components."
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 (relative to concrete). 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 (except for box girder bridges), but these studies were not published.
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