There's mortar concrete

In the 1970s, researchers began looking into the possibility of making a defect-free cement as a replacement material for organic plastics and aluminium. This was prompted byÊthe increasing emphasis on non-combustibility of raw materials along with the rising cost of hydrocarbons and the high energy cost of production.

The major attraction of cement has always been the energy savings gained in manufacture.

To produce 1m3 of organic polymer or aluminium requires 10,100 gigajoules of energy. In comparison cement requires just 1,000.

Clearly cement has a major advantage in energy saving because it hydrates with just the addition of water under normal air temperatures. There is no need to heat it.

The big disadvantage is its low tensile and bending strength, and low fracture toughness due to the air-voids trapped within it. Removal of the internal voids, air pockets and capillary pores - the defects - from the cement paste was made possible by introducing a small proportion of water-soluble polymers that reduced inter-particle friction and surface tension and made the cement particles pack much closer.

The increase in compressive strength of up to 300Mpa was phenomenal, but its application was limited to injection moulding and extruding and not concrete structures. Until now that is.

Researchers in Denmark have developed an ultra-high-strength cement mortar that can be mixed on site and poured in place just like conventional concrete. High strength is achieved in a similar way by controlling the particle-size distribution of the cement to minimise the void spaces between the cement grains. Very finely divided silica sand is then added to this, filling the voids left by the cement particles. There is no coarse aggregate.

The silica has the added advantage of reacting chemically with the cement paste to become an integral part of the matrix. A dispersion surfactant or superplasticiser is also necessary to achieve workability for placing as it is an earth dry mix with a 0.2 water/cement ratio.

This supercrete is a fibre reinforced cement with strengths ranging from 150Mpa to 300Mpa. It was developed by Aalborg Cement in Denmark in 1986 and called Compact Reinforced Composite (CRC). It produces high-bond strength that is very similar to superglue, due to the large content of micro silica and steel fibres that can be added to the mix.

CRC can glue together joints between two reinforced concrete sections, similar to welding steel sections, and has been called weld cement.

Reinforcing bars need only have a bond length of eight times the bar diameter for a full tension lap as opposed to 30 or 40 diameters for a conventional concrete. For practical reasons the minimum gaps between joints are usually 80mm or 100mm wide to enable pouring and compacting of the cement mortar.

CRC has been used extensively in precast concrete work and has also been modified for site applications. It can be supplied as a dry mortar with the binder (cement), sand and steel fibres all mixed in. Water is the only ingredient added under controlled conditions. The high strength of CRC with its mortar-like consistency allows for very close rebar spacing, making it possible to precast thin, lightweight structural elements such as balcony slabs and staircases. The steel fibres are necessary to maintain the ductility of the material, but are not sufficiently robust to support applied loads nor control deflection, therefore reinforcement is necessary

Typical proportions for making 1m3 of a 200Mpa mix are: 1,000kg of CRC binder, 1,290kg of sand and 180kg of steel fibre. A conventional pan mixer can be used and mixing time is between five and eight minutes after the water is added.

The bundles of steel fibres are added part of the way through the mixing. The material is sensitive to temperature change as the high superplasticiser dosage tends to retard the hardening and should not be used below 5¼C. At 20¼C, the initial set will start after seven to eight hours, with the compressive strength of 60Mpa achieved after 16 hours.

Because of its very low water content, in hot weather and drying winds the surface should be covered as quickly as possible to prevent evaporation and the structure enclosed by tenting with tarpaulins. Surface finishing is a problem as the mix is very sticky. A spike roller used for screeding is effective for levelling the surface.



How much does CRC cost?

In Denmark there are three precast companies offering CRC precast slabs, staircases, manhole covers, slender beams, columns and fa‡ade panels.

The high concentration of cement and inclusion of steel fibres makes CRC expensive and it can be as much as £500/m3.

However, the reduction in section depth of a CRC structure means a much smaller volume is required - typically it's about a third of the volume of conventional concrete. For balcony slabs and staircase applications the price of CRC structures is equivalent to that of steel or conventional concrete.

Where CRC has been engineered to its full potential and a balcony slab made to cantilever the span without need of a supporting column, contractors have found it to be the cheapest option.

While compressive strength and ductility is greatly enhanced, stiffness is only slightly higher than normal concrete, which means that deflection has to be carefully considered and controlled by reinforcement. Allowances have also to be made for drying shrinkage because of the high cement content and that is catered for by detailing additional reinforcement.

The properties and challenges of CRC has fired the imagination of architects and engineers in Denmark, leading to a number of innovative and pioneering structures like the pencil-thin staircase at the Roskilde Library Building at the University of Roskilde and at Aalborg Cement's headquarters in Aalborg. n