Classic Portland cement has many virtues, not least the widespread availability of cheap raw materials for its manufacture. But it also has some drawbacks. From the structural designer's point of view, major problems can be caused by the volume changes that occur when the cement reacts with water and later, as free water evaporates from the concrete surface. Although in real concrete structures this shrinkage rarely exceeds 0.1% it can cause serious cracking if the designer fails to take adequate precautions.
One alternative is to use a different type of cement. But although so-called expansive cements were developed more than 50 years ago in France, Russia and the USA and have been extensively researched since, they have failed to make a major impact. Little more than 1% of the world's concrete uses expansive cement and most of this is in China. Blue Circle Special Cements technical sales manager Mel Turner explains.
'The key element in expansive cement is calcium sulphoaluminate, which is made in a conventional cement kiln at normal temperatures. The problem is the raw materials - limestone and bauxite.
'Bauxite is very expensive here and relatively cheap in China, so the Chinese produce millions of tons of CSA cement a year. Here, bulk expansive cement costs 40% more than standard OPC and a typical C35 mix would be about 50% more expensive.'
Blue Circle's expansive cement, dubbed Rockfast CSCC, typically consists of around 6% ground calcium sulphoaluminate clinker, 88% OPC and 6% calcium sulphate in the form of milled synthetic anhydrite. When mixed with water the CSA reacts rapidly with the calcium sulphate and the lime in the OPC to form expansive ettringite and incidentally accelerate the initial set.
An added benefit is that the early removal of free lime, which normally inhibits the formation of the main strength-giving calcium silicate compounds, promotes rapid strengthening of the OPC component. Virtually all the CSA reacts in the first few days.
Mixes with a total cementitious content of 350kg/m 3ormore and a water/cement ratio of 0.45 expand by around 0.1% in the first few days after pouring.
This expansion is restrained, usually by steel reinforcement.
Over the longer term the Portland cement component will shrink as normal. The objective, says Turner, is to leave the final concrete with a slight net expansion, which will prestress the steel, induce some compressive stress and reduce the risk of cracking even further. Technically, the product is known as shrinkage-compensated concrete.
Given the innate conservatism of the construction industry, shrinkage compensated concrete has always struggled to make an impact even in countries where it has been available for decades. Nevertheless, Blue Circle believes there is a market for the concept in the UK, especially in the industrial ground floor sector. Even though long strip construction, polyethylene slip membranes and steel and plastic fibre reinforcement have largely replaced the old technique of casting large floor slabs in a chequerboard pattern of small, steel mesh-reinforced squares to minimise the risk of shrinkage cracking, the problem of joints remains.
Joints are a floor contractor's main bugbear. Blue Circle reckons that by eliminating joints from a floor the use of shrinkage compensated concrete will actually show significant overall cost and time savings on a typical industrial floor project. Turner says this was proven by what happened at Longannet power station in Scotland last July.
(NCE 27 July/3 August. ) 'It was a 50m x 30m warehouse floor slab, 200mm thick, with mesh reinforcement to restrain the expansion. We reckon that the use of SCC saved between 5% and 10% on the cost of the long strip alternative.'
Hanson/Pioneer supplied the P35 concrete ready mixed.
Turner says the fineness of the CSA and the speed of its reaction means there is increased water demand and a need for waterreducing admixtures if the water/cement ratio is to be kept low. 'It needed a little bit longer mixing time and was slightly thixotropic in the early stages, ' he reports.
'Target workability in the mixer was 200mm slump, which had dropped to 125mm as it was placed. Otherwise handling, placing and screeding were normal.'
At least seven days' wet curing is essential, Blue Circle says.
This makes sure there is enough free water in the slab to complete ettringite formation. Main contractor Stuarts Industrial Flooring adopted a belt and braces policy, applying a curing spray before covering the surface with polythene sheeting.
Test results showed that maximum expansion was 0.103% after two days, falling to 0.099% after seven days. By 28 days net expansion totalled 0.048%.
To date the largest joint-free slab in the world is believed to be one in Indianapolis, which measured 4,500m 2. 'The only limiting factor seems to be geometry, ' says Turner. 'A length to width ratio of 3:1 seems to be the practical maximum. And provided that concrete temperature at placing doesn't exceed 30degreesC there's no problem with thermal cracking, even on slabs up to 400mm thick.'
SCC reacts to conventional concrete admixtures in exactly the same way as OPC mixes, Blue Circle claims. Its marketing strategy is to offer a full mix design service to specifiers, carrying out trials with local aggregates to check expansion and strength developments - and to get the workability at the point of discharge from the truckmixer just right. And Blue Circle points out that other forms of concrete structures could benefit from joint-free technology - not least bridge decks and water retaining structures.