Grantee Research Project Results
Final Report: Cool and Sustainable Sidewalks
EPA Grant Number: SU839840Title: Cool and Sustainable Sidewalks
Investigators: Rowangould, Gregory , Morelli, Claude , Taha, Mahmoud , Antonczak, Brittany , Genedy, Moneeb , Montano, Stephen , Raby, Patience
Institution: University of New Mexico
EPA Project Officer: Callan, Richard
Phase: I
Project Period: October 1, 2019 through September 30, 2020 (Extended to September 30, 2022)
Project Amount: $24,999
RFA: P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2019) RFA Text | Recipients Lists
Research Category: P3 Awards , P3 Challenge Area - Sustainable and Healthy Communities
Objective:
This P3 project challenged a group of undergraduate and graduate students to design a sidewalk that can be built using recycled and less energy- and carbon-intensive materials with the thinnest possible thickness. Sidewalks and other transportation infrastructure constructed with Portland cement concrete (PCC) contribute to excess urban heat in several ways. First, the cement used in PCC has a very large carbon footprint owing to the chemical process and thermal energy required to manufacture it. PCC is therefore a large source of climate warming greenhouse gas emissions (GHGs). Sidewalks and other paved surfaces made from PCC also store a large amount of the energy from sunlight during the day as heat. This heat is slowly released throughout the day and into the evening hours contributing to the urban heat island (UHI) affect (where urban areas are warmer than less developed surrounding areas), especially during evening hours. Warming caused by GHG emissions and the UHI affect both increase energy demand for space cooling in warm climates, causing a positive feedback loop that increases GHG emissions and further warming. By using less carbon intensive alternatives to Portland cement and reducing the thickness of sidewalk slabs we can limit GHG emissions and the UHI affect, providing both long- and short-term solutions to a warming climate. Reducing sidewalk thickness and incorporating recycled materials would also reduce raw material use, further increasing sustainability. By revisiting the materials and techniques that cities and towns use to build sidewalks, we believe it is possible to identify more durable, environmentally sustainable, and cost-effective approaches than are commonly used today to build cool and sustainable sidewalks.
Summary/Accomplishments (Outputs/Outcomes):
A team of Civil and Environmental Engineering undergraduate and graduate students from the University of New Mexico (UNM) were recruited to participate in the project. The students were tasked with identifying several alterative concrete mix designs using recycled and/or lower carbon materials and evaluating their structural and thermal properties. The students were also tasked with evaluating the possibility of 3D printing concrete slabs, which could allow slab designs with voids that would reduce heat capacity. Based on these experiments and an assessment of each design’s lifecycle costs and carbon footprint, the students aimed to identify a promising alterative to further develop and present at the EPA P3 Design Expo in the Summer of 2020.
Despite logistical challenges and delays caused by the pandemic, two teams of students over the course of 3 years were able to complete the design challenge and present their work at the 2021 Virtual P3 Design Expo since the 2020 Design Expo was canceled. The project was successful in engaging a large number of students in a hands-on sustainability focused engineering research and design experience.
The student work revealed new insights about the possibility of designing thinner sidewalks out of alterative materials and with 3D printing. First, while thinner slabs will get hotter during the day, they also dissipate heat more quickly to both the ground and air than thicker slabs. This allows them to cool quickly once the sun sets and could help mitigate the UHI affect during the evening hours when it is most pronounced. Second, it is possible to make printable concrete mixes and slabs using PCC and alternatives incorporating waste materials (coal fly ash, slag, recycled rubber and waste black carbon) that achieve similar compressive strength as conventional cast-in-place slabs. A 3D printed reference slab (PCC) and carbon black based mix had compressive strengths that were within 20% of similar cast in place mixes. A recycled rubber-based mix had a substantially lower compressive strength. While the students did not have time given the various disruptions and logical challenged caused by the COVID- pandemic to fabricate more complex 3D printed slabs with void spaces to reduce thermal mass, conductivity and material use, the present results indicate that this may be possible from a technical and structural point of view. Third, using recycled materials lowered the albedo of each slab (they are darker in color), causing them to absorb more incoming solar radiation, although differences in their thermal conductivity and heat capacity had a strong moderating effect on overall heat gain and storage. The thin recycled rubber mix produced slabs that achieved the lowest peak temperatures but heat also dissipated more slowly from these slabs resulting in similar levels of night time heating. Similarly, the carbon black mix produced slabs that achieved the highest peak temperatures but also dissipated heat rapidly to both the ground and air. These results indicate that the mix designs tested by the students are likely to have similar impacts on the evening UHI.
Conclusions:
Overall, the results indicate that the ability to achieve thinner sidewalk slab designs has a high potential to reduce the UHI affect and GHG emissions associated with sidewalk construction. Recycled waste products can also be used to replace Portland cement and aggregate, further reducing lifecycle GHG emissions and raw material use with a moderate impact on strength. Further structural testing and field experimentation are required to determine how thin a sidewalk slab can be made while meeting structural performance requirements. 3D printing appears feasible from a technical perspective and may offer the capability to create slabs with unique structural designs that could mitigate heat storage and material use and a provide a more cost-effective solution to other pre-cast concrete solutions. Whether 3D printed slabs can compete on cost with traditional cast in place sidewalks was difficult to evaluate given that 3D concrete printing is an emerging technology. Cost competitiveness will likely depend on the ability to achieve superior performance over cast-in-place construction and falling costs as this technology matures.
Journal Articles:
No journal articles submitted with this report: View all 1 publications for this projectSupplemental Keywords:
Pedestrians, Urban Heat Island, 3D Concrete PrintingRelevant Websites:
Cool and Sustainable Sidewalks Design Expo Poster and Video
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.