MBT system offers host of benefits
01 October 2002
MBT's revolutionary admixture technology is helping precast concrete manufacturers improve their production process, boost output and save costs.
A new revolutionary polymer technology developed by Master Builders Technologies (MBT) offers precast concrete producers increased productivity and reduced costs.
MBT says its new Zero Energy system combines state-of-the-art Glenium polycarboxylate-based chemical admixtures, advanced Rheodynamic Concrete and its technical expertise to enable precast concrete producers achieve a variety of savings including higher productivity, reduced power consumption and more efficient use of manpower and resources.
"Concrete may be one of the most energy-efficient construction materials but traditional precast concrete production still needs significant energy input," says David Bowerman of MBT Middle East.
"Accelerated curing and mechanical vibration not only use energy, they also make the precast factory a less pleasant and safe place to work.
"And 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 as well."
MBT Middle East is part of Degussa which resulted from the merger of MBT-SKW Trostberg and Degussa-Huls. Degussa is one of the world's largest specialty chemical companies.
Ever since modern concrete construction began over a century ago, engineers have been aware of the need to minimise the quantity of water added to the mix.
Nearly all 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 fresh concrete that can be easily placed and compacted.
Cement will react chemically with something like a quarter of its weight of water - over a period of years, according to MBT sources. Any water in excess of 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, they add.
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 by-product 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 per cent in mixing water at the same workability with a consequent increase in compressive strength, MBT says.
There were limits to the increase in workability that could be achieved, however - adding more and more lignosulphonate achieved only excessive retardation and air entrainment, the company says.
Stable high-workability concretes had to wait for the introduction of a new chemical technology in 1964.
Derived firstly 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, the company explains.
A consequent reduction in internal friction meant 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 lifespan, often less than 15 minutes (in the Middle East region), it says.
Rheological stability depended on careful attention to the fine aggregate fraction, both its quantity and its consistency, MBT adds.
The company says that 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 the earlier superplasticisers, according to MBT.
These new admixtures could produce highly fluid concrete which retained its workability much longer, plus greater rheological stability at high workabilities, it points out.
According to MBT, 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.
The coating of admixture molecules on the cement particles initially restricted cement hydration, not until these lost their charge and were stripped off could the water make unhindered contact with the cement.
But the ultimate plasticiser was still to be developed, says MBT.
The company says it has been busy improving polycarboxylates ever since. Despite pioneering polycarboxylates in the 1980s and developing them eventually into the well-known Glenium range, MBT and its associated companies under the Degussa umbrella were far from ready to rest on its laurels.
"One of the key research priorities was to reduce the early retardation associated with all types of superplasticisers," says Dr Ivana Torresan of MBT.
"For insitu concrete use, 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 solution was Glenium ACE, which has a completely new shape of polycarboxylate molecule."
Instead of coating the cement grains and forming a barrier, the ACE molecules attach themselves at one end only, MBT explains. The result is that a significant proportion of the cement surface is open to react with water. Hydration can start no later than its standard mixes, resulting in the high early strengths that precasters crave, the company points out.
But this was still not enough for the MBT team as precast units are often complex and heavily reinforced, the company says.
Completely eliminating vibration - a key part of the Zero Energy Concrete concept - required a true self-compacting concrete with better rheological properties than even the most advanced polycarboxylates could manage. A different approach was needed.
"A deeper study of the rheology of very fluid mixes showed there are two key values, which control their behaviour," says a company spokesman. "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 self-compacting properties, but if the viscosity is too low then segregation is more than likely."
MBT's solution was to add viscosity modifiers based on long chain molecules derived from cellulose.
The spokesman explains: "Glenium Stream creates a network of molecular links that stabilise the plastic concrete structure without impairing its flow characteristics. This makes self-compacting concrete much less sensitive and more user-friendly - and a more attractive option for precasters and insitu contractors alike. Eliminating the need for mechanical vibration depended on the development of a concrete that could 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.
"Accelerating the rate of strength development to achieve the same effect as steam curing or other accelerative techniques was a much more significant challenge. Precasters use steam curing and high cement contents for one main reason - to enable them to strip products out of their moulds as soon as possible. On a 24-hour production cycle, the eight-hour compressive strength is usually the key parameter. Target strengths can be as high as 35 N/sq mm. Even with insulated shuttering, hitting targets like these without some form of heat input was a formidable challenge.
"Trials conducted by Emirates Precast Construction, Concrete Technology and MBT Middle East, demonstrated detailed comparison where you can get the same strength with Glenium ACE versus a similar self-compacting mix, as you can with traditional superplasticisers using steam curing at 70 deg C (See Figure 1).
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Figure 1: Early strength development. |
"It also removes the risk sometimes associated with steam curing, which can thermally stress concrete, inducing microcracks in the cement paste and leading to higher permeability, lower long-term strength, and reduced durability. Companies in the process of planning new operations or expansion on existing structures do not require investing in expensive steam curing equipment."
MBT says the Zero Energy System is designed to provide significant benefits in virtually all aspects of precast concrete production:
