Life-Cycle Assessment (LCA)
A more objective and complete measurement of the sustainability of a product is a life-cycle assessment (LCA). LCA, often called a "cradle-to-grave" study, is the study of the environmental impact of a process or product from raw material acquisition, energy inputs and emission outputs during production and use, and end-of-life management (recycling/disposal).
In 2008, the International Zinc Association (IZA) hired internationally renowned LCA experts Five Winds International and PE International to conduct a life-cycle inventory (LCI) and life-cycle assessment (LCA) for hot-dip galvanized steel. Utilizing data from worldwide sources regarding energy consumption and air/fluid/solid emissions measured during zinc production and during the actual galvanizing process, combined with analogous survey data collected from the steel industry, an LCA for hot-dip galvanized steel was compiled. The graphic below shows an overview of the impact of hot-dip galvanized steel from production to end-of-life.
Hot-dip galvanizing is unique in that all material and energy inputs and emission outputs are isolated to the production phase, as there is no maintenance required for 70 years or more in most environments. The following pages will examine each of the phases (production, use, and end-of-life) in more detail.
Determining the environmental impact of a product starts with raw material acquisition and the amount of energy input and emission output during production. For hot-dip galvanizing (HDG), the production phase requires three life-cycle inventories (LCI) as the finished galvanized product cannot be created without first mining and producing the steel and zinc. Therefore, the production phase for HDG includes the life-cycle inventories of steel, zinc, and the galvanizing process.
Long before galvanized steel is erected as long-span steel bridge or used to construct elements of airport terminal, production begins on the steel and zinc metals. To begin, more than 95% of structural steel manufactured in North America is fully recycled from previously used steel materials, making the initial environmental impact of using a steel frame minimal. By specifying a hot-dip galvanized (HDG) zinc coating for corrosion protection, you are choosing to utilize yet another abundant, recyclable, natural metal to further the cause of sustainability as 30% of all zinc produced comes from recycled sources.
Combining the three LCIs of steel, zinc, and the galvanizing process provides the environmental impact of producing 1 kg of hot-dip galvanized steel, up to the point it leaves the galvanizer's facility. The production phase is the only phase where energy is used and waste is created because of galvanizing's long-term, maintenance-free corrosion protection, so the initial environmental cost is the final cost.
|Production Phase |
|1 kg of HDG Steel||25.9 MJ||1.80 kg||0.00615 kg||0.000824 kg|
As mentioned, galvanized steel is unique in that all of the energy inputs and emission outputs are isolated to the production phase. Hot-dip galvanized steel's maintenance-free performance means no wasted energy or emission created on upkeep.
Therefore, the use phase is where hot-dip galvanized (HDG) steel gains an environment impact advantage over competitors. After the steel pieces have been fabricated, galvanized, sent to the site, and erected or put into place, there is nothing to do but wait. For 70+ years, galvanized steel will often remain maintenance free; no raw material or energy expended, no carbon footprint extending beyond the production phase.
Conversely, a painted structure requires regular, routine maintenance; i.e., the entire painting process and all of its environmentally harmful outputs must be repeated every 12-20 years. This means every decade or two, more paint chemicals and potentially volatile organic compounds (VOC's) will be expelled into the atmosphere and blasted paint particles made a permanent part of the waste stream.
There are also indirect costs associated with this continued maintenance, including exhaust from transport vehicles and particulate emissions caused by surface-preparation blasting, not to mention the disadvantage to the community caused by maintenance shut-downs and delayed power production. These additional indirect environmental costs during the use phase lead to environmental impact sometimes even higher than the initial production costs.
The table illustrates how HDG steel incurs no additional environmental cost during use and identifies as of yet unpublished paint costs P1, P2, P3, and P4. Though paint manufactures have yet to publish quantifiable data about their environmental cost during use we know that there are values there because it must be maintained. It is possible to get an idea of how the coating impacts an LCA based on published case studies.
|Use Phase||PED||GWP |
|1 kg of HDG Steel||0 MJ||0 kg||0 kg||0 kg|
|Painted Steel||P1 MJ||P2 kg||P3 kg||P4 kg|
End -of-Life Phase
The true beauty and sustainability of incorporating hot-dip galvanized steel (HDG) into transportation infrastructure is there really is no "end-of-life," only a return to production; cradle-to-cradle, rather than cradle-to-grave. At the end of its useful life, galvanized steel elements can be recycled into structural steel for construction of new infrastructure or other applications and the zinc captured for reuse in new galvanized coatings.
|End-of-Life Phase||Primary Energy Demanda|
|1 kg of HDG Steel||-8.61 MJ|
|aSteel is the primary component and is 100% recyclable, however, the zinc in the galvanized coating is also 100% recyclable. Paint on the other hand, becomes a permanent part of the waste stream.|
As indicated in the table above, LCA credits HDG steel with 8.61 MJ for every kilogram recycled. Utilizing hot-dip galvanized steel for corrosion protection reinforces the environmentally friendly nature of mass transit, such as light rail and bus systems, as the zinc used in the coating is also 100% recyclable. In contrast, after years of environmentally damaging maintenance, a painted coating becomes a permanent part of the waste stream or is burned off as emissions when it is put to rest.
The steel in both a hot-dip galvanized piece of steel or painted steel is recycled either way, but the coating is what makes the difference. Steel and zinc melt at difference temperatures. The zinc is collected in dust form and sold for re-purposing to zinc recycling companies.
Due to the LCA credit of 8.61 MJ/kg, resulting in a complete life-cycle primary energy use of 17.3 MJ, the complete life-cycle primary energy use for HDG steel is actually less than the primary energy used in the production phase. This, combined with the recyclability of the steel and zinc, means hot-dip galvanized transportation infrastructure expends no extra energy or materials in the end-of-life phase and will be ready to continue on to a new phase of production.
When looking at the table below, it shows almost the exact image of the production phase with the exception of the recycling credit. With hot-dip galvanized steel, emissions, energy output, and material are isolated to the production phase. Because there are often no emissions or energy requirement for hot-dip galvanized steel during the use and end-of-life phases, the initial environmental cost is the final environmental cost. Not only that, but both the steel and the zinc used are completely recyclable, making the transportation elements less harmful to the environment from cradle-to-cradle. Though steel, the primary component is recyclable regardless of which corrosion protection coating is chosen, the environmental impact of that coating is significant as case studies have proven.
|Complete LCA |
|1 kg of HDG Steel||17.3 MJ||1.801 kg||0.00615 kg||0.000824 kg|
|*Complete Life-Cycle energy use reflects production, use, and end-of-life credit|
While it is true that utilizing these recyclable, natural elements will support the already green efforts of mass transit, a level of sustainability is achievable in all transportation-related pursuits. By choosing the most environmentally-friendly corrosion protection system, you can take a step towards greener transportation solutions.