A team of researchers from the University of Cambridge, working in partnership with industry, has helped to develop the first piece of 3D-printed concrete infrastructure to be used on a National Highways project.
The structure – a type of retaining wall known as a headwall – has been installed on the A30 in Cornwall, where it’s providing real-time information thanks to Cambridge-designed sensors embedded within its structure. The sensors provide up-to-date measurements including temperature, strain and pressure, which are used to create a ‘digital twin’ of the wall that could help spot and correct faults before they occur.
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Headwall structures are normally made in limited shapes from precast concrete, which requires formwork and extensive steel reinforcement. But by using 3D printing, the team – including specialists from Costain, Jacobs and Versarien – could design and construct a hollow, curved wall with no formwork and no steel reinforcement. The wall gets its strength not from steel, but from its geometry.
The wall – which took one hour to print – is roughly two metres high and three-and-a-half metres across. It was printed in Gloucestershire at the headquarters of advanced engineering company Versarien using a robot-arm-based concrete printer. Making the wall using 3D printing significantly reduced the cost, materials and carbon emissions.
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Over the past six years, Professor Abir Al-Tabbaa’s team in the Department of Engineering has been developing new sensor technologies and exploring the effectiveness of existing commercial sensors to get better-quality information out of infrastructure. Her team has also developed various ‘smart’ self-healing concretes. For this project, they supplied sensors to measure temperature during the printing process.
Temperature variations in different layers of the 3D-printed wall were continuously monitored to detect any potential hotspots, thermal gradients or anomalies. The temperature data will be correlated with the corresponding thermal imaging profile to understand the thermal behaviour of the 3D-printed wall.
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‘Since you need an extremely fast-setting cement for 3D printing, it also generates an enormous amount of heat,’ said Al-Tabbaa. ‘We embedded our sensors in the wall to measure temperature during construction, and now we’re getting data from them while the wall is on site.’
In addition to temperature, the sensors measure relative humidity, pressure, strain, electrical resistivity and electrochemical potential. The measurements provide valuable insights into the reliability, robustness, accuracy and longevity of the sensors.
A LiDAR system was also used to scan the wall as it was being printed to create a 3D point cloud and generate a digital twin of the wall. ‘Making the wall digital means it can speak for itself,’ said Al-Tabbaa. ‘And we can use our sensors to understand these 3D-printed structures better and accelerate their acceptance in industry.’
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The Cambridge team developed a type of sensor known as a PZT (piezoceramic lead-zirconate-titanate) sensor, which measures electromechanical impedance response and monitors changes in these measurements over time to detect any possible damage. These smart sensors can show how 3D-printed mortar hardens over time, while simultaneously monitoring the host structure’s health.
Eight PZT sensors were embedded within the wall layers at different positions during the 3D-printing process to capture the presence of loading and strain, both during the construction process and service life after field installation.
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The team, which included experts in smart materials, automation and robotics and data science, also developed a bespoke wireless data-acquisition system. This enabled the collection of the multifrequency electromechanical response data of the embedded sensors remotely from Cambridge.
‘This project will serve as a living laboratory, generating valuable data over its lifespan,’ said Al-Tabbaa. ‘The sensor data and digital twin will help infrastructure professionals better understand how 3D printing can be used and tailored to print larger and more complex cement-based materials for the strategic road network.’