Flexural Strength of High Performance Concrete

Paper by SPRINGENSCHMID BECKHAUS from ISCR 8th 1998 Lisbon Portugal

High strength concrete (HSC) having a flexural strength considerably higher than that of ordinary pavement concrete would be a suitable material - to replace only a single lane of concrete pavement deteriorated by heavy duty traffic (see Fig. 1) or - for constructing new pavement for high load with limited thickness, e.g. for tunnels or bridges. A very high flexural strength of 8 to 10 N/mm 2 combined with a good workability was reached by a concrete mix with a cement content of 450 kg/m 3 and a water/binder ratio of 0.30 and 0.36. Silica fume (sf) was added at 4% of the cement content and the air content was 5%. The compressive strength was between 70 and 80 N/mm 2 . The increase of the flexural strength in comparison to normal strength concrete was about 50%. To prevent increased autogenous shrinkage as well as enhanced cracking tendency, the w/(c+sf)-ratio should not be lower than 0.30 and the silica fume content should not be higher than about 4%. Ordinary Portland cement CEM 132.5 R can be used. A cement replacement by 20% fly ash slightly decreases the flexural strength at an age of 28 days as well as considerably improving the cracking resistance. In spite of a comparatively low water content, water can evaporate from the concrete surface. Therefore curing for at least 3 days is necessary. Also an unfavourable temperature gradient over the cross-section whilst hardening can lead to a thermal prestress, which can further reduce the flexural strength. The frost-deicing agent scaling resistance of HSC with air entrainment showed better results than HSC without air-entrainment. Especially the cut surfaces of the HSC without air entrainment showed significant detorioration. Therefore air-entrainment is necessary for a good frost-deicing agent scaling resistance.

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