Concrete cracks the stress problem
By expanding while hardening, a new concrete has a cracking resistance more than three times higher than the regular high-performance option, say researchers from the Swiss Federal Laboratories for Materials Science and Technology (Empa).
‘We could obtain a durable high-performance concrete with stable residual expansion by special mix design including calcium sulfoaluminate expansive cement, shrinkage-reducing admixtures and superabsorbent polymers,’ says Mateusz Wyrzykowski, Group Leader of Concrete Technology at Empa.
He notes that the formulation can reduce costs and labour demands as it enables pre-stressed concrete elements without any need for external pre-stressing beds, which are expensive. ‘The self-pre-stressed concrete elements reinforced with ultra-high modulus carbon ﬁbre-reinforced polymers (CFRP)…are slender, durable and have an excellent strength-to-weight ratio.’
When the concrete expands during hardening, it causes the embedded reinforcement to elongate. Wyrzykowski explains, ‘This leads to tensile stress in the reinforcement, and, per equilibrium, compressive stress in the concrete (hence, pre-stress). The compressive pre-stress offsets the risk of tensile cracking in concrete.’ As a result of this expansion, the concrete puts the CFRP bars in its interior under tension and thus automatically pre-stresses itself.
The researchers have demonstrated the capacity to pre-stress in several directions at the same time, for example, for thin concrete slabs or ﬁligree curved concrete shells. ‘Our technology opens up completely new possibilities in lightweight construction,’ asserts Wyrzykowski. ‘Not only can we build more stable structures, we also use considerably less material.’
By using CFRP instead of conventional reinforcing steel, they believe they have achieved full corrosion resistance of the reinforcement and reduced the thickness of the concrete element by up to 100mm. ‘With our invention, the process becomes significantly simplified – hence, less expensive.’
The researchers claim that their invention opens new markets for mass production of filigree (lightweight) pre-stressed concrete elements, e.g. for façade elements, electricity poles, etc.