Comparative carbon footprint of product estimate study

A comparative study of the carbon footprint of product (CFP) associated with production and application of reinforcing bars (rebar) was completed. This study compares Proxxima™ systems-based glass fiber reinforced polymer (Proxxima GFRP) rebar to conventional steel rebar to estimate the global warming potential (GWP). The system boundary is cradle-to-end-of-life. This study quantifies the GWP associated with the rebar used in making reinforcing structures for different applications. Concrete, energy used for rebar installation, and lifetime of the construction were considered to be constant between the cases and were excluded from the study. The study was conducted to be in accordance with the ISO 14067:2018 [1] CFP requirements and guidelines. Additionally, External Critical Review which followed ISO 14071:2024 [2] was completed by Sphera Solutions, Inc. to support comparative environmental footprint communications as defined in ISO 14026:2018 [3].

This study allows ExxonMobil and Proxxima™ systems part manufacturers, their customers, and stakeholders to understand the global warming potential (GWP) of Proxxima™ resin system through the rebar production process to Proxxima systems-based glass fiber reinforced polymer rebar, through installation into different construction applications, and to end-of-life versus the steel rebar alternative. It serves as an important illustrative example of the GWP of a Proxxima™ polymer-based composite part relative to steel. 

Construction applications included in this comparative study are industrial pad, warehouse pad, and continuously reinforced concrete pavement (CRCP). Industrial pads and warehouse pads are used as foundations for installations of industrial equipment at manufacturing sites and warehouse storage equipment, respectively. The functional unit for this study is the rebar system required for 1 square meter of concrete pad laid and 1 mile of lane for CRCP constructed. Proxxima™ GFRP rebar was also compared to steel rebar at the rebar manufacturing gate on the basis of kg CO2e per 100 ft rebar. Sensitivity analyses were conducted to evaluate the CFP of different rebar sizing and spacing within the industrial concrete pad and different thickness of concrete for the CRCP application.

In the Proxxima GFRP rebar system, raw material inputs (boundary 1), bar manufacturing (boundary 2), product transportation to installation site, waste transportation, and disposal (boundary 3) were included in the system. At end-of-life, Proxxima GFRP rebar is assumed to be crushed with the concrete system and disposed of with the rest of the aggregate. It should be noted that Proxxima resin system is still in its launch phase within this application and no Proxxima GFRP rebar installations have reached end-of-life. More work is to be developed within this area to determine the best end-of-life scenario for Proxxima systems-based composites.

The steel rebar system starts with the manufacturing of steel rebar, including raw material inputs (boundary 2). The rest of the system includes rebar transportation to installation site, waste transportation, and disposal (boundary 3). For steel rebar, the steel is mechanically separated from the concrete matrix, collected, sorted, and then cleaned before it is utilized by a steel manufacturer. That which is not removed from the concrete matrix is typically sent to landfill.

• Equivalent areas of steel and Proxxima GFRP rebar provide the same function [4]. Both products were installed with the same concrete mixture and thickness. This study assumes direct replacement of same gauge rebar between cases for each application, except for the CRCP designs. For CRCP, as there is no installed application, the Proxxima GFRP rebar design case was based on recent published studies on GFRP rebar [5].
• Concrete and energy used for rebar installation were considered to be constant between the cases and were excluded from the study Same lifetime and maintenance is assumed for Proxxima GFRP and steel rebar systems. An older published study assumes that 1/3 to 1/2 less maintenance is required for GFRP rebar when compared to steel [6], thus this is considered a conservative assumption.
• EPA data [7] on construction waste and debris disposition and USGS data on mineral commodities [8] were applied to all studied cases as end-of-life disposition for recycle rate and landfill. Cut-off method was applied for end-of-life with an allocation sensitivity done at end-of-life using the substitution approach.

The cradle-to-rebar manufacturing gate (boundary 2) CFP estimate comparison of rebar produced from Proxxima GFRP vs. steel shows that the Proxxima GFRP rebar has a lower estimated global warming potential (GWP) than steel rebar, across the steel production scenarios assessed.

In the production of steel rebar, one of the highest CFP contributors is the production technology of steel sourced. Three of the most common production paths for steel are blast furnace-basic oxygen furnace (BF-BOF), direct reduced iron-electric arc furnace (DRI-EAF), and scrap-based electric arc furnace (EAF). As shown in Figure 1, Proxxima GFRP rebar’s cradle-to-gate CFP estimate is about 70% lower than steel rebar (based on a global mix of production routes) regardless of rebar size. Proxxima GFRP rebar has an estimated cradle-to-gate CFP estimate that is about 40% lower than steel rebar using steel derived from scrap-EAF production. Proxxima GFRP rebar has an estimated cradle-to-gate CFP estimate that is about 80% lower than steel rebar using steel derived from BF-BOF production.

It should be noted that the majority of operating global steel production [9] and roughly 40% of additional steelmaking capacity that has been announced or is in construction [10] is through the BF-BOF process. One potential pathway to lessen the future GWP impact of steel rebar, is to instead consider an alternative material, such as a Proxxima GFRP rebar, that has a potentially lower GWP.

Cradle-to-end-of-life (boundary 3) CFP estimates were also conducted for rebar installed into specific construction applications. These comparative estimates show a similar result to the cradle-to-gate estimate of rebar above due to cradle-to-gate rebar production being by far the biggest contributor to the CFP build-up. Across all base case comparisons Proxxima GFRP rebar has a CFP estimate 62%-63% lower than steel rebar shown in Table 1.

As shown in the CFP estimate for rebar in the industrial pad application (Figure 3), installed Proxxima GFRP rebar has a comparable CFP to installed steel rebar even if utilizing larger rebar size and tighter spacing (compare Proxxima GFRP #4 at 200mm to steel #3 at 300)