​Stone is a strong, durable, and abundant natural material that has been used to build beautiful buildings for millennia. 

If you think of the buildings that have impressed you the most, chances are that several stone structures, such as the Egyptian pyramids or the great cathedrals of Europe, will be on your list.

The use of stone as a building material has diminished over time, largely due to the apparent usefulness of concrete, which is essentially a reconstituted form of stone. Concrete is versatile; it can be formed into any shape and reinforced with steel bars to provide tensile capacity, bending strength, and reliable ductility.

Traditionally, stone is cut into blocks of a few hundred millimetres on each side, and the joints between these blocks, usually made from cementitious or lime-based mortars, cannot carry tension. To make a traditional masonry structure stable against lateral loads such as wind or seismic forces, a significant amount of stone is required. According to Professor Jacques Heyman in his authoritative book, ‘The Stone Skeleton’ up to 98% of the stone is only there for its mass, which keeps the joints closed using gravity.

However, we can also clamp the joints shut by prestressing or post-tensioning our stone structures using steel bars or strands. Additionally, we can automatically cut and mill stone blocks into any shape that can be modelled in a computer. Structural stone is often stronger than a concrete matrix, and in our experience, post-tensioned stone structures (designed using existing masonry codes of practice such as EC6) are roughly the same size as their reinforced concrete counterparts, making them materially equivalent in size, weight, and strength.

Concrete, like stone, is fundamentally extracted from the ground. However, unlike stone, the cement in concrete is processed using intense energy and generating massive carbon emissions. Natural stone, on the other hand, is simply cut into blocks using saws that can be powered by renewable electricity. This results in less processing, less material handling, and similar transport requirements for post-tensioned structural stone when compared with reinforced (or post-tensioned) concrete, but with far less embodied carbon.

Like precast concrete, stone panels can be accurately manufactured off-site and then simply assembled on-site, as we have been doing at Sagrada Familia in Barcelona. This reduces waste, increases speed, and transforms construction from a noisy, dirty community nuisance into an interesting theatrical performance. Using this approach, we can create entire building structures using stone columns, stone beams, stone slab panels, and load bearing (and lateral load resisting) stone façade panels - reducing embodied carbon by about 75% in the process.

This could result in beautiful sustainable buildings that will last for millennia, just like their predecessors.

 

Tristram Carfrae, Arup Fellow