By Claude Goguen, P.E., LEED AP
There are many definitions of sustainability, but one of the simplest is: “The ability to meet the needs of the present without compromising the ability of future generations to meet their own needs.”
As the emergence of green buildings, green developments and LEED certified buildings continues to grow, building with sustainable products is quickly becoming the new normal. Precast concrete is a sustainable product for many reasons, but for the purposes of this article, we will concentrate on how steel reinforcement plays a part in sustainability.
Factors that define a building material’s sustainability include its durability, transportation cost, efficiency, recyclability and distance from resources.
Steel reinforcement plays an important role in precast concrete for a number of reasons: Steel handles tensile stresses, bridges cracks and provides ductility. Precast concrete is very strong in compression; however, when subjected to tensile stresses, it needs reinforcement that can absorb these loads. Durability of precastast concrete structures is essential to sustainable construction and the proper use of reinforcement has a direct impact on durability.
It is important to protect the reinforcement against corrosion. Enhancing the corrosion-resistance properties of the reinforcing materials in the structure can substantially improve the durability of the precast concrete structure. Precast concrete structures that use conventional reinforcing steel bars have proven their ability to provide long lasting, nearly maintenance-free service lives. In applications where the structure is exposed to chlorides, moisture and oxygen, additional measures may be used such as increasing concrete cover over the reinforcement, using corrosive inhibitors or using epoxy-coated, galvanized or stainless steel reinforcement.
Enhancing the ability to reduce transportation costs
When using precast concrete components on a project, it is important to optimize the size to reduce structural mass and the number of pieces for the project. Transportation energy, emissions and cost savings will result from the need to transport less material and thus reduce the overall transportation carbon footprint. Reinforcement, both conventional and prestressing, allows for longer spans and thinner structural members due to its enhanced ability to resist tensile stresses in the field.
Steel reinforcing bars in current practice are typically Grade 40, 60 or 75, which means they have a yield strength of 40,000, 60,000 or 75,000 psi. High-strength reinforcing steel products are also becoming more readily available. Grade 80 bars with a yield stress of 80,000 psi were adopted into the ASTM in 2009. ACI 318, “Building Code Requirements for Structural Concrete,” has adopted the limited use of Grade 100 steel for use as confinement reinforcement, such as ties or spirals, in compression members. Higher-strength reinforcing products can, in some instances, provide a proportional replacement for lower-strength materials, which can result in thinner and lighter pieces.
Smaller and more efficient buildings
Some prestressed slab systems can reduce the floor-to-floor and building height compared with a steel structure of similar bay sizes. This, in turn, reduces the material needed for vertical elements such as concrete, mechanical and electrical systems, elevators and curtain wall systems. The reduction comes not only from thinner slabs but also from shorter floor-to-floor heights once the building systems are incorporated. The amount of interior space to heat and cool is also lowered, which results in a lower carbon footprint for energy use. The energy required to vertically transport liquids, gases and cooled air, and to move occupants between floors, is also reduced.
Conventional reinforcing used in precast concrete generally has a very high level of recycled content, and this can be of particular interest to those trying to achieve a LEED level on their project.
The following statement has been prepared and approved by the Concrete Reinforcing Steel Institute’s (CRSI) Technical Committees regarding the percentage of recycled materials content for steel reinforcing bars:
“The vast majority of conventionally available reinforcing steel (ASTM A6I5 and A706) has recycled material content typically greater than 97 percent. Specialty reinforcing steel products, such as ASTM A1035 low-carbon, chromium steel and ASTM A955 stainless steel, have a recycled content typically greater than 75 percent.”
Wire and welded wire reinforcement is typically produced with 92 to 97% recycled material content. Reinforcing steel (bars, strand and wire) is also 100% recyclable at the end of the useful service life of the structure. This recycling loop keeps material from the waste stream.
When supplying products for a LEED project, simply request a statement from your steel supplier with information on the recycled content of the product. This will help contribute to points under MR4 (Materials and Resource) – Recycled Content section of the LEED guide.
Currently, under LEED section MR5 – Regional Materials, you can obtain points toward LEED credits if the materials or products have been extracted, harvested and manufactured within 500 miles of the project site. This may get a bit confusing with reinforcing as the extraction of the scrap used could be from anywhere in the world. A position statement from CRSI clarifies this point as follows:
• Point of Manufacture (final assembly): The fabricator takes the reinforcing steel and modifies it, therefore, considered to be the point of manufacture.
• Point of Harvest: The steel billet producer is considered the harvester or point of harvest.
Simply consult with your steel supplier as to the locations of these two points to verify whether or not they fall within the 500 mile radius of the project site, and therefore contribute to this credit.
Proper selection and placement of reinforcement is essential to quality precast concrete products, whether it’s for a bridge beam or a septic tank. Properly designed, concrete and steel, acting as one, can efficiently resist live and dead loads for more than 100 years. Steel that would otherwise be bound for landfills is successfully recycled in reinforcement and provides a long life in service.
The durability and environmental stewardship that result from using recycled steel reinforcing define sustainability. Engineering and architectural students, the designers and decision makers of the future, are now being exposed to sustainable construction as a primary subject matter. Understanding and maximizing the sustainable attributes of raw materials, like steel, will result in green precast concrete building designs and a better future for all of us.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.