Replacing steel in fibre reinforced concrete with stiff polyethylene and adjusting concrete mix reduces carbon footprint without compromising strength or consistency of wet mixture
If human history can be divided into ages named after materials, we could now be said to be in the steel-and-glass age. But the artefacts of our previous epoch, the concrete age, are still all around us, and concrete underpins the shining structures now being built. It also comprises much of our road and rail infrastructure, but concrete ages, and materials that were used in its formulation decades ago are now deteriorating. Unfortunately, concrete produces more carbon dioxide emissions in its production than almost any other material, so engineers are searching for a more sustainable way of maintaining existing concrete structures.
At the Swiss Federal Institute in Lausanne (EFPL), Iranian PhD student Amir Hajiesmaeili is trying to develop a new generation of ultra high performance fibre reinforced concrete (UHPFRC), and after three years of research has found a mixture which dispenses with the steel fibres commonly used in current UHPFRC, but has similar mechanical properties, is 10 per cent lighter and whose environmental impact is 60-70 per cent lower.
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Hajiesmaeili is doing his PhD as part of the Swiss National Science Foundation’s national research programme Energy Turnaround, which focuses on the scientific and technological aspects of energy strategy changes. His project involved using a comprehensive packing model developed at EPFL’s Structural Maintenance and Safety Laboratory to formulate and test different mixtures of reinforced concrete components to find one which fits the criteria needed for repairing and maintaining concrete structures. These criteria include the mechanical properties of the dry concrete and the consistency of the wet mixture.
“After three years of this trial-and-error, we finally found the right recipe – one that also meets stringent building standards,” says Hajiesmaeili. The crucial components of this new mixture replacing the steel reinforcing fibres with stiff polyethylene fibre which clings well to the cement mixture, while also replacing half of the cement powder with crushed limestone. “The trick was to find a material that’s very strong and produces the right consistency,” Hajiesmaeili explains.
The Structural Maintenance and Safety Laboratory is headed by Prof Eugen Brühwiler, who has a 25-year interest in eco-friendly concrete and monitoring the condition of structures such as road and rail bridges. “With this material, we can add value to age-old structures by ensuring they will last for a long, long time,” says Brühwiler, whose lab has already overseen the structural reinforcement of more than 100 bridges and buildings in Switzerland. “This solution is also much more financially and environmentally sound than razing and rebuilding existing structures like bridges and historical monuments.”