Construction World March 2018

ENVIRONMENT & SUSTAINABILITY

Getting greener with PRECAST

resource efficiency of materials, energy, and processes. Compared to cast-in-situ, precast uses less of everything – less cement, less water, and less steel. It produces less waste on-site and in the factory. This makes the carbon footprint of precast much smaller than in cast-in-situ construction. “Concrete can be effectively heat-treated at the precast factory, thereby reducing the amount of cement needed for concrete prod- ucts. It is also possible to use alternative binders in certain applications to decrease CO 2 emissions,” Punkki says. The highly optimised and economical use of material makes precast pre-stressed hol- low-core slabs one of the most sustainable products in construction. They feature tubular voids running the full length of the slab, making the slab much lighter than a massive solid concrete floor slab of equal thickness or strength. In the cross section of hollow-core slabs, concrete is used only where it is actually needed. Areas, where concrete acts only as ballast, are replaced with hollows. For instance, in 200 mm hollow-core slabs, 49,9% of the cross section consists of voids. In 400 mm hollow-core slabs, this percentage may be as high as 55,6. This brings savings of up to 45% in concrete compared with a plain cast-in- situ reinforced slab. At the same time, the amount of pre-stressing steel can be cut by

Optimising cement content The precast industry’s biggest source of CO 2 emissions is cement with a heavy carbon footprint. Cement holds up to 70% of the precast element’s total CO 2 load. "Cement manufacturing releases CO 2 into the atmosphere when limestone is calcinat- ed. Making cement in a kiln at a temperature of over 1 400 Celsius also requires a great deal of heat energy," explains Professor of Practice in Concrete Technology, Jouni Punkki from Finnish Aalto University, School of Engineering. The precast industry works hard to reduce the CO 2 emissions of cement by developing precast products and optimising the cement content in concrete. “The use of other cementitious materials, such as ground-granulated blast furnace slag from the steel industry and pulverised fuel ash from coal-fired power stations, is growing. Both of these additions have much lower embodied CO 2 than cement,” Punkki says. The material and energy efficiency of ce- ment plants has been improved by utilising waste materials from other industries as fuel for the cement kiln. Less of everything Precast production in controlled factory conditions has huge potential to improve the

W ith the potential to use local, recy- cled materials, concrete has much to offer in terms of sustainable construc- tion. Precast concrete products consist predominantly of natural aggregates and the local availability of aggregates makes for low-carbon footprint deliveries to the precast factory. Moreover, materials are subject to mini- mal processing or chemical treatment. This results in concrete having a relatively low embodied energy value, unlike some other highly processed materials. In relation to recycling, modern precast factories enable high manufacturing effi- ciency. New precast factories are built with closed-loop recycling systems where all wet waste is automatically conveyed back to a central recycling plant. Disused concrete buildings can be crushed and used in landfill or the loadbear- ing layers of roads. Water recycling and conservation is also a common feature of modern precast factories. An increasing demand for sustainable construction options opens up new possibilities for precast concrete to be more widely used in housing projects.

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CONSTRUCTION WORLD MARCH 2018