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.
Optimizing cement content
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.
Less of everything with precast
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.
Demanding weather conditions challenge construction
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.”
‘Zero carbon’ homes challenge construction
The energy associated with construction typically accounts for just 10 to 20 percent of a building’s energy use over its lifetime. In the 50-year lifecycle of an office building, the precast concrete elements account for less than 3 percent of the total CO2 load.
The roles of different phases in the service life of a building are about to change remarkably, precast concrete professional Jouni Punkki says.
“The proportion of the operational carbon footprint will decrease dramatically as both energy-efficient buildings and low-emission energy gain ground. The target of zero-carbon homes is already becoming well established in Europe,” Punkki says.
Punkki sees the trend as a positive challenge for the construction industry.
“Currently, the emphasis is on operational energy consumed. Regulations and taxation are made to support energy efficient solutions. The same is likely to happen within the construction sector.”
In Punkki’s vision, real-estate taxation and permitted building volumes could be based on emissions.
“In the future, the beneficial sustainability characteristics of precast concrete, particularly local products, well organized material recycling, and improving resource efficiency, are favorable for the companies in the field,” Punkki believes.
In recent years, environmental certification of buildings has become increasingly popular in Europe. The Building Research Establishment Environmental Assessment (BREEAM), developed in the UK and used around the world, is known as world's most widely used environmental assessment method and rating system for buildings.
BREEAM sets the standard for best practice in sustainable building design, construction, and operation. It encourages designers to think about low-carbon, low-impact design, minimizing the energy demands created by a building while utilizing low carbon technologies.
The famous library of Birmingham, rated ‘Excellent’ by BREEAM, stands as a good example of the sustainable characteristics of precast concrete. The high rating was made possible by ensuring thermal mass integral to the building’s energy strategy and the concrete structure and design worked to deliver efficient operational energy use. Moreover, materially efficient post-tensioned concrete slabs, transfer walls, and arches were used. The durability and versatility of the concrete structure will ensure a long life-cycle for the building.
BREEAM is also starting to gain a foothold outside Europe. China’s first BREEAM ‘Outstanding design’ certification was awarded in 2014 to one of the country’s leading developers, Franshion, for the sustainable building exhibition center in Changsha, Hunan Province.
The Living Lattice is the flagship building of the new Meixi Lake Eco City, one of eight new exemplar eco cities in China. Named for its multi-level matrix of floorplates, courtyards and gardens, the building is designed to harmonize with the local climate, culture, and landscape, enabling it to benefit from passive natural lighting and ventilation.
In many cases, precast products incorporate materials such as blast furnace slag (GGBS) from the steel industry and fuel ash (PFA) from coal-fired power stations that might otherwise go to waste. As a rule of thumb:
Substituting 50% of cement with
GGBS = 40% less CO2
Substituting 30% of cement with
PFA = 20% less CO2
It is possible to specify products with over 70% replacement material. Materials such as microsilica, glass, limestone powder, and china clay waste can also replace Portland cement or primary aggregates.
Source: British Precast, The Little Green book of Concrete