Romans, I

What did the Romans of the turn of the common era know better than twentieth century structural engineers? Concrete of course. A couple of broadcasts on BBC Radio Four, and a Scientific American article of a couple of years ago prompted me to compare and contrast Roman concrete with the now infamous RAAC [Reinforced Aerated Autoclaved Concrete]. The latter of course is now falling apart without notice in 1960s and '70s built structures all over the UK and beyond, just fifty years into their lifespan. By contrast, Roman concrete dating back 2000 years is still standing and faring very well, thank you: think the Pantheon in Rome: still featuring the largest unreinforced concrete dome in the world.

Most people would think, 'well concrete is just concrete, right?', and yes the basic principles are the same, but the mix of materials and how they are prepared are various, and variously effective under various conditions, so to speak. Concrete is essentially a mix of aggregate and some form of cement, usually lime or Portland cement wetted with water to form the concrete matrix and allowed to harden and cure. Various means of reinforcing the material can be employed, but the usual is steel bar of some sort, set into the concrete, either passively or under tension [pre-stressed] whilst the material hardens and cures: used for beams, lintels, etc.

RAAC is an engineered concrete, the base material of which was invented in the 1920s, known as Aerated Autoclaved Concrete, a lightweight form of concrete [one quarter the weight by volume of ordinary concrete] produced by promoting a hydrogen-producing reaction in the mix, forming bubbles, which, when the hydrogen itself escapes is replaced by air. The material is then moulded whilst it solidifies, cut to size and cured in an autoclave, to form durable, lightweight building materials such as the familiar Aircrete building blocks, which can easily be cut with a handsaw are lightweight, making handling much easier than with cinder blocks [I used to carve Easter Island Moai heads out of them when I was a teenager, using an old wood chisel and mallet].

RAAC is the steel-reinforced version of the material, and came to prominence during the postwar rebuilding boom in the UK, when lots of buildings were needed quickly for housing, schools and hospitals. The stuff was fine as long as the buildings themselves were regularly and adequately maintained, something that could not be guaranteed in the economically fluctuating, increasingly business-and-profit-led decades that followed, where public and social building infrastructure was increasingly sidelined as too expensive to properly monitor and maintain. As leaks in roofing material, windows and walls gradually, inexorably and inevitably develop, the aerated concrete becomes a sponge for the water ingress, leading ultimately to the degradation of the concrete itself, and the corrosion of the embedded steel reinforcement, which loses strength, and expands into its surrounding material, imposing destructive structural forces on it, leading to unpredictable failure.

Roman concrete, on the other hand has survived the fall of Rome itself, the early Christian era, Viking and Norman expansion, the Dark Ages, The Black Death, the Renaissance, countless wars, monarchs, empires, the Industrial Revolution, and into the modern era; still effective to the present day. They certainly built for longevity, make no mistake. But what is it in the nature of Roman concrete that makes it so resilient and gives it such longevity? The Scientific American article quotes from a paper published in Science Advances in 2023; I quote from the abstract to the paper. Roman concrete contains '... relict lime clasts, a ubiquitous and conspicuous mineral component associated with ancient Roman mortars. [Together, these analyses provide new insights into mortar preparation methodologies and provide evidence that] the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime, to create an environment where high surface area aggregate-scale lime clasts are retained within the mortar matrix.' The paper goes on to say that these lime clasts are the key factor in giving Roman concrete its self-healing properties: water ingress serves to produce chemical reactions within the concrete matrix which cause the previously unactivated lime to fill and heal any cracks caused by that ingress with 'new' concrete. We really do need to take leaf out the Roman playbook and start rethinking how we use concrete as a structural material, methinks...