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A mixed blessing

Concrete Admixtures

A dramatic improvement in precast concrete production efficiency is promised. Dave Parker reports.

Concrete may be one of the most energy efficient construction materials, says MAC managing director Mario Corradi, but traditional precast concrete production still needs significant energy input.

'Accelerated curing and mechanical vibration not only use energy, they also make the precast factory a less pleasant and safe place to work, ' he adds.

'In assessing the energy input into the production process, we must also include manpower, material usage and equipment efficiency. We believe advanced admixture technology can transform precast production and make it much more sustainable.'

MAC is part of the Degussa empire, one of the world's largest chemical producers.

Originally a joint venture between US admixtures giant MBT and a Treviso-based company, MAC works closely with the UK arm still best known as Feb MBT. But with Italy generally acknowledged to be the world leader in precast concrete technology, it was inevitable that MAC's Treviso lab should take the lead in developing the new Zero Energy Concrete (ZEC) philosophy.

Eliminating the need for mechanical vibration depended on the development of a concrete that could both self-place and compact, filling even complex, heavily reinforced sections from just one delivery location.

Polycarboxylate superplasticiser technology promised to meet this need, although in the end, it was the further addition of specialised viscosity improvers that made the goal achievable (see box). A much more significant challenge was accelerating the rate of strength development to achieve the same effect as steam curing or other accelerative techniques.

Precasters use steam curing and high cement contents for one main reason - to enable them to strip their products out of the moulds as soon as possible. On a 24 hour production cycle, the 16 or 18 hour compres2. Even with insulated shuttering, hitting targets like these without some form of heat input was a formidable challenge.

Glenium ACE with its reengineered molecule solved this problem. Field trials revealed other benefits of the ZEC approach, says MAC technology director Rabinder Khurana.

'The extra fine powder content needed for a good selfcompacting mix produces a much superior concrete finish.

'Most precasters will opt for limestone powder, which also means a lighter coloured concrete. On the downside, they may have to install an extra silo.'

Successful trials in Europe were followed by detailed comparisons in the UK of ZEC versus a similar self-compacting mix using a standard Glenium superplasticiser cured at 60°C.

'This was in a factory that normally produces prestressed beams using heated formwork, ' Khurana explains.

'Target 18 hour strength is 40N/mm 2. Our trials showed that while the heated self-compacting mix achieved 71N/mm 2at 18 hours, the unheated ZEC mix hit 75N/mm 2in the same time.'

The client for these trials is planning two new factories based on ZEC technology. And several plants in Europe are now in full production.

Water levels Ever since modern concrete construction began more than a century ago, engineers have been aware of the need to minimise the quantity of water added to the mix. Most of the important concrete parameters: strength, durability, abrasion and chemical resistance are inversely proportional to the water/cement ratio. The problem has always been the conflict between the desire for a low water/cement ratio and the need to produce a fresh concrete that can be easily placed and compacted.

Cement will react chemically with almost a quarter of its weight of water - over years.

Any more than this minimum quantity will degrade the quality of the hardened concrete, but until the 1930s, concrete producers had no option but to add extra water to achieve enough workability.

Then the first chemical water reducing admixtures (WRAs) - or plasticisers - were introduced. It was to take many years before they achieved general acceptance by specifiers.

Mostly based on lignosulphonates - a byproduct of the wood pulp industry - these admixtures offered either a significantly increased workability at the same water cement ratio or a reduction of up to 10% in mixing water at the same workability with a consequent increase in compressive strength.

There were limits to the increase in workability that could be achieved, however.

Adding more lignosulphonate achieved only excessive retardation and air entrainment. Stable high workability concretes had to wait on the introduction of a new chemical technology in 1964.

Derived initially from sulphonated melamine formaldehyde condensates and later from sulphonated napthalene formaldehyde condensates, superplasticisers - or high range water reducing admixtures - introduced a new principle into concrete technology. Their negatively charged molecules swiftly clustered on the surface of the individual cement particles, producing a mutual electrostatic repulsion that dispersed the particles much more evenly through the mix.

A consequent reduction in internal friction meant that even less water was needed to achieve adequate workability - or that a much more fluid mix would remain stable during placing.

But there were still drawbacks, apart from the high price. The electrostatic effect had a limited life span, often less than 60 minutes.

Rheological stability depended on careful attention to the fine aggregate fraction, its quantity and consistency.

Shuttering had to be redesigned to resist the much increased hydrostatic pressures from the fluid superplasticised concretes they had to contain. And some vibration was still needed to ensure full compaction, especially in heavily reinforced sections.

True self-compacting concrete only became feasible with the 1989 introduction of polycarboxylate technology, which added steric, or molecular, repulsion to the electrostatic repulsion of earlier superplasticisers. These new admixtures could produce highly fluid concrete which retained its workability far longer, plus greater rheological stability at high workabilities.

They could also be used to reduce water/cement ratios below 0.3, yielding very high strengths.

But they were even more sensitive to aggregate proportioning and moisture content variations than the first generation superplasticisers - and they shared a common problem.

Cement hydration was initially restricted by the coating of admixture molecules on the cement particles. Not until these lost their charge and were stripped off could the water make unhindered contact with the cement. The ultimate plasticiser was still to be developed. Most major admixture suppliers have been busy improving polycarboxylates ever since.

MAC modifiers Despite pioneering polycarboxylates in the 1980s in the well-known Glenium range, Feb MBT and its associated companies under the Degussa umbrella was not ready to rest on its laurels.

MAC head of research and development Dr Ivana Torresan says one of the key priorities was to reduce the early retardation associated with superplasticisers.

'For insitu concrete the reduction in water/cement ratio soon compensates for the early retardation, but to gain maximum efficiency for the precast concrete industry we had to make sure cement hydration could start as soon as possible.

'The answer was Glenium ACE, which has a completely new shape of polycarboxylate molecule.' Instead of coating the cement grains and forming a barrier, she says, the ACE molecules attach themselves at one end only. The result is that a significant proportion of the cement surface is open to react with water. Hydration can start no later than in standard mixes, giving the high early strengths that precasters crave.

This was still not enough for the MAC team. Precast units are often complex and heavily reinforced. Completely eliminating vibration, a key part of the Zero Energy Concrete concept required a genuinely self-compacting concrete with better rheological properties than even the most advanced polycarboxylates. A different approach was needed, as MAC admixture R&D head Dr Roberta Magarotto explains.

'A deeper study of the rheology of very fluid mixes showed there are two key values which control their behaviour. The first is the 'threshold value', which is a measure of the force needed to initiate flow. The second is the resistance to an increase in the speed of flow, which is a function of viscosity. If the threshold value is too high, the concrete will have no selfcompacting properties, but if the viscosity is too low, segregation is more than likely.'

MAC's answer is Glenium Stream which creates a network of molecular links to stabilise the plastic concrete structure without impairing its flow characteristics. This, says Torresan, makes selfcompacting concrete more attractive for precasters and insitu contractors.

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