Post-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 uses tendons threaded internally through a duct formed from aligned drilled holes.
Post-tensioned stone ("PT stone") could consist of a single piece, but drill limitations and other considerations mean it is typically an assembly of multiple components with mortar between pieces. PT stone has been used in both vertical columns (posts), and in horizontal beams (lintels). It has also been used in more unusual stonemasonry engineering applications: arch stabilization, flexible foot bridges, and cantilevered sculptures.
A closely related method is pre-tensioned stone. A duct is drilled into the stone is used to host a steel rod held in tension with jacks while the duct is filled with epoxy grout. After the epoxy has set, the ends of the rod can be released from the jacks, placing the stone under compression. Similar to pre-stressed concrete, the pre- and post-tensioned methods can be used in different contexts.
Post-tensioned stone also has a close affiliation with massive precut stone, which is the central technique of modern load-bearing stonemasonry.
Rationale
"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.".
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.
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 compresssion 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.
Post-tensioning is achieved steel tendons either threaded through ducts within the stone elements or along their surface. 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. 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.
Energy use and carbon emissions
Stone is 'natural precast concrete' so only needs to be cut (and strength tested) and post-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.
Potential for 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."