Given the current challenging economic climate, you might think that building sustainable or green structures would be viewed as unimportant because cost for a project would rise. But the idea of sustainability isn't going away; rather it's become the center point for the design process to begin. Sustainable structures can cost less to build, use less energy, cost less to maintain over time, use fewer building materials, be healthier to live in, and emit less greenhouse gas into the atmosphere. Sustainable structures can reduce short-term and long-term costs, and are good for the planet and the social environment.
Making concrete more sustainable
It's common knowledge that manufacturing 1 ton of portland cement produces about 1 ton of CO2. But recent technical developments in the way we produce cement reduces the amount of CO2 production by about 8%. When calcium carbonates, such as limestone, are heated in a kiln, CO2 is produced as part of the reaction, accounting for approximately half the total amount. The other half comes from the fuel needed for production. But more efficient plants require less fuel. Recycled or alternative fuels (recycled oils, old tires, and solvents) are being used more.
Concrete can be made more sustainable by better distributions of aggregate. Well-graded or reasonably graded combined aggregates in mixes can reduce the amount of cement required and the amount of water needed.
There are also advantages to replacing some portland cement in a mix with pozzolans—such as fly ash and slag cement—which would otherwise end up in a landfill. Replacements in the range of 15% to 40% can produce higher strength, lower permeability concrete, enhancing the long-term durability of structures.
Using less water in concrete improves strength and durability. Larry Novak, the director of engineered buildings for the Portland Cement Association (PCA), says the old adage “If you can drink it, you can pour with it,” has changed to include gray water that can be used as mixing water, as long as accepted methods for testing ensures its suitability for concrete (ASTM C1602, Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete).
The use of admixtures makes all of this possible, increasing concrete sustainability by improving its qualities. Without admixtures, such as polycarboxylate superplasticizers, it's more difficult to place and finish highly sustainable concretes.
Building more sustainable structures
David Sheperd, LEED AP, the director of sustainable development for the PCA, says the embodied concrete energy (the energy required to build a concrete structure) represents only about 3% of the total energy required to construct and operate a concrete house with more than a 100-year expected life. For commercial buildings, it's 5% to 15%. Though owners tend to think about construction costs more than long-range costs, the push for sustainable building causes them to think more long term.
Concrete is perhaps the most adaptable of all construction materials. It can be adapted to meet a large number of specific needs. Novak says that engineers can reduce the cost of concrete in a structure by specifying higher performance mixes to reduce the size of building elements.
Novak says that involving mechanical engineers early in the design process allows them to take advantage of concrete's inherent thermal mass capacity to store energy, reducing the heating or cooling demand during the hottest or coldest times of the day. Changes in concrete mixes, delivery, and forming systems (see March 2009 Concrete Construction, “Concrete Superstructures”) changes the way super-tall buildings are constructed—from structural steel to mostly structural concrete.
Making concrete structures more sustainable also includes construction systems. For instance, by using flat-plate forming systems to construct building floors, floor-to-floor heights are reduced. This results in less material required for construction, drop ceilings aren't required, and less material is needed for curtain walls and vertical risers.
Sustainability also relates to the life of a structure, which in turn depends on concrete durability. Replacing a building because of fire, extreme wind events, blast, or seismic events is costly and expends valuable resources. Considering robustness, engineers can design reinforced concrete structures to resist extreme events.
Sustainable concrete buildings
Tilt-up and precast panel systems can provide the external walls for buildings. With increasing frequency, they include a center sandwich of insulation, adding to the thermal efficiency of structures. Ed Sauter, the executive director of the Tilt-Up Concrete Association, says that workers cast the exterior face of a wall panel first, insert polystyrene foam insulation along with connection hardware to tie the two concrete wythes together, and then cast the interior structural face of the panel. “This saves money but the real issue now has to do with building structures that can be flexible over a long period of time,” he says. For example, a warehouse may someday be converted to an office building, old finishes may need updating, or walls may need to be moved to provide new design appearances. The ability to make these changes is where technology is headed.
How concrete supports a sustainable building system
The many advantages of concrete can be used to enhance other building systems. The new Residence Hall and Recreation Center building currently under construction at North Central College, Naperville, Ill., is an example of systems working together. The $20 million project will be completed in nine months using precast double-wythe panels and precast concrete sandwich panels. The construction includes 250 dorm rooms and a 180x322-foot field house. The college decided to build a sustainable structure to reduce energy costs by 16%. The anticipated payback period is seven years to recover increased construction costs.
Here's the value concrete brought to the project: structural double wall panels filled with bio-based foam insulation; panel faces are very smooth and “paint ready;” installing these panels completes the interior and exterior surface plus insulation in one step; self-consolidating concrete was used in the panels with 40% recycled ingredients: slag cement, slag aggregate, and fly ash (by volume); recycled rebar in all panels; double-wythe floor panels with pre-installed radiant floor heating tubes; and air-tight panel connections that ensure good thermal efficiency.
Why concrete is sustainable
Durability is the key to sustainability. If the useful life of a structure can be extended by using concrete, that's a huge gain for sustainability. When concrete's natural benefits become the building blocks for other building systems, there are gains in energy efficiency. This is the direction for the next era. Developments in concrete technology will continue to improve the industry's ability to respond.