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Word is Densit's bond

Densit is sometimes known as 'compact reinforced composite' or 'CRC Jointcast'. And the high strength 'concrete glue' being trialed at Cardington on both the SINSLAB trials and the first European Concrete Building Project structure promises to give structural engineers a whole new range of options.

Densit was first developed in Denmark by Aalborg Portland more than a decade ago. In its early form it was even demonstrated on Tomorrow's World. Basically Densit is a blend of Portland cement, microsilica, superplasticiser, fine aggregate and fibres. Its very high content of microsilica and superplasticiser and the optional use of special calcined bauxite aggregates means compressive strengths of over 400MPa have been recorded.

As used in Cardington, Densit contains around 6% of steel fibres with a length of 12.5mm and a diameter of 0.4mm. With a 4mm quartz sand aggregate the material achieves a compressive strength of around 150MPa. But it is Densit's very high bond strength that is attracting most interest from structural engineers.

Tests in Denmark, France and Spain have shown that with deformed bars a bond length of only 10 bar diameters - or even less - will give full transfer of stress. This offers the intriguing possibility of actually 'welding' precast concrete units together into a single structural element without the complex and inefficient joint details normally employed.

One of the first Danish buildings to trial the system used 200mm deep precast slabs with straight 8mm deformed bars projecting only 80mm. The slabs were placed 100mm apart, transverse reinforcement fixed, and the gap filled with Densit poured and compacted much like any other insitu stitch.

Load tests on the system showed that the joint was stronger than the slabs themselves. Fire tests revealed that an unprotected Densit stitch had at least a 60 minute fire resistance, and that, despite its low porosity, Densit had no tendency to spall explosively.

On the first ECBP building at Cardington, Densit stitches together both the permanent concrete formwork to the eighth floor and the precast stair units. For extra safety, the stair joints feature projecting hook bars connected with a transverse bar, but the floor planks use simple straight bars as in the Danish prototype. Both longitudinal and transverse stitches were used for the SINSLABtrialsnearby.

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