It may all have happened a long time ago in a galaxy far, far away, but there’s a secret weapon in every Star Wars film that’s found almost everywhere humans live on Earth. It explains how the scrappy Rebel Alliance were able to defeat the mighty Galactic Empire, with all its high-tech weaponry, vast space armada and the Death Star.
Even with the help of Jedis, The Force, ewoks and a wookiee, Luke Skywalker and the rebels skirted annihilation on many occasions. We believe that their ultimate victory owed less to their mystic powers and allies, and instead to something more down-to-earth.
Read more: How to build a real lightsaber
In several key moments throughout the series, the survival of the rebels depends on their ability to escape the Empire’s clutches, by retreating to, and defending, fortified bases. These are spread out on remote planets throughout the galaxy, in often hostile environments, from snowbound Hoth to desiccated Crait.
It’s time to finally give the Rebel Alliance’s engineering corps the credit it deserves. The fate of the galaxy was decided not by light sabres – but by concrete.
The Snows of Hoth
At the start of The Empire Strikes Back, things are looking grim. The Empire have cornered the remnants of the Rebel Alliance on the ice planet, Hoth. Fortunately, General Leia and co are able to defend Echo Base long enough for their allies to evacuate. The base is built into subterranean caves, but concrete is still needed for the roofs and floors of the spacecraft hangars, and the defensive laser cannon emplacements.
But how could they make all that concrete when temperatures on Hoth are consistently below 0°C? Concrete doesn’t set by drying out, but by a hydration reaction, which consumes water. If the water in the concrete mix is frozen solid, it can’t take part in the hydration reaction, and so the concrete can’t develop its strength.
So how did the Rebel engineers get around this? A cheat solution for small structures is simply to make the concrete in a place it’s not freezing, and then bring it in. This is known as “pre-cast” concrete. Engineers are currently using this to upgrade the Rothera research station in Antarctica – the most similar place to Hoth there is on Earth.
But shipping all that concrete through space isn’t practical. Engineers on Earth have a better solution – they use special rapid-hardening cement that contains calcium aluminates. This reduces how long the “danger zone” lasts, during which the cement must be protected from freezing. This quick-hardening ability allowed the French Army to build gun emplacements quickly in the first world war, and could have helped fortify rebel defences on Hoth.
The salt lakes of Crait
In 2017’s The Last Jedi, the rebels were almost wiped out again by the pursuing Imperial fleet. They take shelter on the barren planet of Crait, covered by flat, dried up salt lakes. The rebels regroup in an old base built into a mountain that’s protected by colossal, metal blast doors, supported by a reinforced concrete structure. Outside lie the vast salt flats and the attacking Imperial forces. Compared to the laser cannons of the First Order, salt would seem the least of the rebels’ worries. But in fact, salts can be one of the most destructive substances for reinforced concrete.
Reinforced concrete contains steel rods to make it stronger and resist bending. But if CO₂ in the atmosphere and chloride in the salts diffuse into the concrete, the steel can corrode, blister and weaken the concrete by expanding. This is sometimes known as “concrete cancer” and has been a contributing factor in several engineering disasters on Earth, including the collapse of the Morandi bridge in Genoa in 2018. In that particular case it is believed that salts carried in the sea air accelerated the deterioration of the structure.
Were the Rebels foolish to build reinforced concrete bases where salt is everywhere? Actually, this type of corrosion needs a certain level of humidity in the air. Crait has very little water, or none at all. In such a dry environment, corrosion wouldn’t happen. So thankfully for General Leia and co, the mountain base was in a good enough condition to resist the bombardment – just long enough for Luke Skywalker to intervene. But how did they make all that concrete in the first place with so little water?
Given that concrete needs water to set, a planet without water would prove tricky. The Middle East on Earth is largely desert, but there’s still a lot of concrete production, much of it made with desalinated seawater, which is expensive. On planets where there isn’t even seawater, water is too dense and precious to use for anything but keeping humans alive. There’s no easy fix for this, but it’s possible the rebels took a similar approach to scientists designing constructions on our Moon – using alternative binding materials instead of cement, such as sulphur, which could be mined from lunar rocks.
Back on Earth
While the battle for the fate of the galaxy concludes in the cinema, the battle for the future of humanity continues here on Earth. Cement is the largest manufactured product on Earth by mass, and generates about 8% of anthropogenic greenhouse gas emissions, which is more than all of aviation. Concrete is the second most-used substance on Earth – only water is used more. The world as we know it would not exist without concrete.
But the concrete saga is by no means over. At the UK’s National Centre for Infrastructure Materials, our research is helping to tackle two of the biggest challenges – reducing the environmental footprint of producing concrete and making the sure that the lifetime of concrete infrastructure is as long and robust as possible. May The Force be with us …
This article was written in collaboration with Dr Sam Adu-Amankwah, Prof Susan Bernal Lopez, Prof Leon Black, Dr Juan Pablo Gevaudan and Dr Toby Lord (School of Civil Engineering, University of Leeds). Alastair Marsh receives funding from the Engineering and Physical Sciences Research Council (EP/R001642/1), and UK Collaboratorium for Research on Infrastructure and Cities (EP/P017169/1).