Getting greener

With the potential to use local, recycled materials, concrete has much to offer in terms of sustainable construction. 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 minimal 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 efficiency. 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 loadbearing layers of roads. Water recycling and conservation is also a common feature of modern precast factories.

The precast industry’s biggest source of CO2 emissions is cement with a heavy carbon footprint. Cement holds up to 70 percent of the precast element’s total CO2 load.

“Cement manufacturing releases CO2 into the atmosphere when limestone is calcinated. 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 Aalto University, School of Engineering.

The precast industry works hard to reduce the CO2 emissions of cement by developing precast products and optimizing the cement content in concrete.

“The use of other cementitious materials, such as ground-granulated blast furnace slag from the steel industry and pulverized fuel ash from coal-fired power stations, is growing. Both of these additions have much lower embodied CO2 than cement,” Punkki says.

The material and energy efficiency of cement plants has been improved by utilizing waste materials from other industries as fuel for the cement kiln.

Pre-stressed hollow-core slabs stand as a good example of efficient raw material use. Hollow-core slabs provide savings of up to 45 percent in concrete compared with a plain cast-in-situ reinforced slab.

Precast production in controlled factory conditions has huge potential to improve the 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 products. It is also possible to use alternative binders in certain applications to decrease CO2 emissions,” Punkki says.

“Improving structural efficiency is also important. Longer spans and more advanced structures also save material and reduce the amount of cement. These savings can be most easily obtained by using pre-stressed concrete.”

Pre-stressed hollow-core slabs stand as a good example of efficient raw material use. Hollow-core slabs provide savings of up to 45 percent 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 30 percent because of the lower selfweight.

For an average apartment, this means savings of 14.4 tons of concrete and 275kg steel.

Good durability of concrete buildings has significant value in changing weather conditions. Worsening hurricanes and tornados challenge construction materials to withstand heavy rainfall and wind-blown debris.

According to a study by the Texas Technical University’s Wind Engineering Research Center, concrete wall systems were proven to withstand 100 percent of all known hurricane-force winds, and over 99 percent of tornado-force winds.

Good insulation and thermal properties are also beneficial to precast concrete. Dense precast concrete can act as a thermal sink and lightweight concrete can act as an insulator, and in some buildings you can see precast doing both. High thermal mass together with good insulation make versatile concrete as a very competitive material.

The hollow cores in precast floors can be used or pipes can be cast into slabs to form cooling systems that use up to 50 percent less energy than air conditioning.

The good insulation properties of precast concrete are particularly valuable in countries like India and the United Arab Emirates, which have humid weather and a high demand for construction.

In India, where severe energy deficiency is a big challenge for the rapidly developing country, there is growing interest in precast technology. The government of India intends to build more than one hundred smart, sustainable cities in forthcoming decades.

“India cannot afford to waste energy through ill-constructed buildings. This is why constructing energy-friendly buildings is vital,” comments Chander Dutta, the Managing Director of Elematic India.

“The good insulation properties of precast concrete are important, as well as smooth joints in the walls, which lead to less energy leakage when a house is cooled or heated.”