ABSTRACT:

BACKGROUND:

In the last several decades, the world has seen higher trends of urbanization than ever before. In 1800, 90% of the United States lived in rural environments (Department of Commerce, 1996). Yet 2007 marked the beginning of a new era – the urban area – in which the global urban population finally surpassed the global rural population (United Nation, 2018). The predominant reasons for urbanization are 1) rural-urban migration, 2) urban geographical expansion, and 3) transformation of rural settlements into urban settlements. Out of these, the most notable factor is rural-urban migration (Cohen, 2006, Young, 2013). In his analysis of regional migrations and inequalities, economist Alwyn Young noted that one of every four or five individuals who grow up in rural environments migrate to urban areas (2013). There are many economic benefits tied with rural-urban migration. Across 65 countries, the rural-urban gap accounts for 60% of mean income inequality (Young, 2013). Additionally, there is an improved state of social welfare in urban areas where, on average, education, water, electricity, and sanitation are more accessible (Cohen, 2006). The United Nations predicts this rampant urban trend will continue, where an estimated 23 of the population (over 6.5 billion people) will live in urban areas by 2050. By contrast, the rural population (currently 3.4 billion) will drop to 3.1 billion by 2050. It is becoming increasingly clear that, when we look towards the future, we are looking towards an urbanized world. 

There are many impacts, both positive and negative, when looking at higher rates of urban density, but for the purpose of this paper only three negative impacts are relevant: increased water run-off and flooding, alongside its corresponding water pollution, and the Urban Heat Island Effect. These impacts pervasively affect urban areas and as time goes on; these issues will continue to worsen. A promising solution to help mitigate both these is green infrastructure, but there remain many knowledge gaps in the field which require further research.

Impacts of Urbanization

Urbanization has led to an increase in water run-off and flooding. Run-off is a phenomenon whereby an environment experiences more precipitation than the terrain can handle, resulting in excess water flowing across the surface (Balasubramanian, 2017). Within urban environments, precipitation run-off can easily lead to flooding as the free water is unable to percolate into the ground. The construction of cities within the United States and the world has magnified the scale of urban flooding during the 20th century (Leopold, 1968). Research affirming this assertion has been ubiquitous and consistent. In 1962 South Korea, roughly 8% of the Seoul metropolitan region was impervious [made of cement or similar materials that are unable to absorb water]. By 2010, roughly half of the region’s surface was impervious, which is a natural consequence of urbanization as more undeveloped land is covered in infrastructure. In 1962, only 11% of rainfall was converted into surface run-off, whereas in 2010, the percentage increased to 49% (Lin et al., 2016). Researchers Yingkui Li and Cuizhen Wang analyzed St. Charles County, Missouri – which has undergone major urbanization within recent decades – and found that the average direct run-off has increased by 70% from 1982-2003 (2009). A comparable study was conducted on the Zhujiang Delta of Southern China, which found similar results: an increase of annual run-off depth of 8.10 mm (Weng, 2001). All three studies found that increased run-off was highly correlated to urban expansion (Lin et al., 2016, Li & Wang, 2009, Weng, 2001). As increases in run-off volume lead to higher flooding rates (Mazalena et al., 2021), urbanization-influenced water run-off can implicate serious risks of flooding. 

This risk is exacerbated due to the increasingly disastrous effects of climate change. When people often think of “climate change”, their next thought is often “global warming”. While climate change indeed triggers rising temperatures, it additionally describes large-scale shifts in weather patterns (United Nations). For example, precipitation in the Northern Hemisphere has increased by 0.1% - 0.5% each decade in the 20th century, which is highly correlated to the increased amount of atmospheric CO2-eq: annual CO2 emissions rose by an average of 2 billion tonnes during each decade of the 20th century (Dore, 2005; Ritchie & Roser, n.d.). In the last decade, flood, drought, and storm related deaths increased by 15 fold in climate vulnerable regions (IPCC, 2023). Studies show that the effects of climate change increased the magnitude of Hurricane Harvey by 20% - 40% and similarly affected a 2016 Louisiana flooding (Risser, 2017, van der Wiel, 2017). Unfortunately, the problem will only worsen. The Intergovernmental Panel on Climate Change (IPCC) predicts with high confidence that when global temperatures rise 1.5 ºC above pre-industrial levels (estimated to occur as early as the 2030s) rainfall and flooding events will increase in both magnitude and frequency (IPCC, 2023). While the creation of the National Flood Insurance Program was intended to help relieve the effects of urban flooding, it has had troubles self-financing and addressing vulnerabilities in urban environments (Ntelekos et al., 2009). With over 100 annual deaths and over $3 billion in damages within the United States (National Weather Service), it is necessary to find a solution that addresses water run-off and flooding as the problem will only worsen. 

There are two additional consequences of increased urban run-off: 1) the run-off eventually outflows into bodies of water (i.e. creeks, reservoirs, bays, etc.), and 2) the increased run-off becomes a major transport vector of pollutants. 1) is quite straightforward as water run-off will inevitably reach either a treatment plant or outflow into natural bodies of water. Over one third of all water run-off eventually flows into streams and rivers (U.S Geological Survey). Because a large portion of urban run-off reaches waterways, its cleanliness is of great importance. However, much of urban run-off is incredibly polluted and serves to contaminate waterways and affect ecosystems. Heavy metal contaminants are often introduced into urban environments via cars and roadways: batteries are sources of lead and zinc, lubrication oil and grease are sources of lead, brake linings and brake pads are sources of copper and lead, and tires are sources of lead and zinc (Wijeyawardana et al., 2022). Lawns and fertilizers are sources of phosphorus and nitrogen.

 Zgheib et al. analyzed the composition of stormwater run-off in Paris. Their findings showed high levels of copper (550 µg/kgdw), lead (283 µg/kgdw), and zinc (1865 µg/kgdw). These concentrations are higher than recognized guidelines by a scale of 3.10, 2.79, and 5.92, respectively (2012). Run-off can also gain contaminants in the form of larger solid particulates –suspended solids – defined as particles larger than 2 µm. Zhu et al. analyzed road-deposited sediment (road particle buildup) and found high levels of zinc, lead, copper, chromium and nickel (mean size 50 µm) with mean concentrations 1054, 539, 175, 142, and 131 mg/kg. Run-off has also been characterized to have high concentrations of phosphorus (concentrations of 200 µgP/L) and nitrogen (ranging from 280-10,000 µ/L) (Cowen & Lee, 1976, Jani et al., 2020). In a landmark study analyzing 14 urban sites with various tests to determine toxicity (using Daphnia Magna, Microtox, submitochondrial particles, etc.) Marsalek et al. found 60% of observed run-off was toxic (a third being severely toxic) (1999). 

When the polluted water outflows into streams and reservoirs, it contaminates the water and negatively impacts biodiversity. Li et al. found that the survival rate of Daphnia pulex decreased significantly (p<0.001) in a 48 h timespan when introduced to roadway run-off (2023). Of 24 juvenile coho salmon, 96% became immobilized within hours after being exposed to collected road run-off (Chow et al., 2019). The introduction of nutrients like phosphorus and nitrogen into water ecosystems can cause eutrophication, a phenomenon in which the increased nutrient levels result in algae blooms, degrading water quality. A review article by Akinnawa et al. notes several consequences of eutrophication: algae blooms generate green-colored clouds decreasing rates of photosynthesis in aquatic creatures, death of algae result in low levels of dissolved oxygen, some species of algae release toxins which create aquatic dead-zones. It is necessary to find solutions to treat contaminated urban water in order to preserve our water supplies and water ecosystems.

While water run-off, its contamination, and flooding are significant results of urbanization, there are additional consequences that require attention. One such consequence of urbanization is The Urban Heat Island (UHI). 

The UHI effect is the phenomenon in which solar heat gets trapped within cities, increasing urban temperatures higher than their suburban counterparts. Urban architecture displaces vegetation and is primarily constructed of cement, asphalt, and roofing tiles (Stone et al., 2001, Mohajerani et al., 2017). The UHI effect occurs when these urban materials (which have a much higher threshold for heat capacity) absorb sunlight (a mix of  solar energy, infrared and ultraviolet light) during the day which is slowly re-emitted during the late afternoon and night (Stone et al., 2001, Mohajerani et al., 2017). Fresh concrete has an albedo (i.e. ratio of reflected light to incident light) of 5%. Its corollary holds true: fresh concrete absorbs 95% of solar radiation (Pomerantz et al., 2003). Concrete’s low albedo elucidates how the UHI effect occurs – nearly the entirety of the sun’s heat gets trapped within a city. Within New York, the UHI effect can increase temperatures up to 8.0 ºC (Chen et al., 2006). The UHI effect has tangible ramification for urban dwellers: the CDC reported over 9,500 heat related deaths in urban areas from 2004-2018, averaging over 650 deaths a year (2020). As climate change will affect global temperatures over the course of the 21st century (EPA predictions anticipate 1.5 ºC in 2030, and 2 ºC in 2043 ), the UHI effect will inevitably intensify in coming years (Synthesis Report). A study on the Delhi urban center found that annual heat wave duration will increase from 2.9 days more than its rural surroundings to 13.8 days in the future, assuming greenhouse gas emissions continue to rise during the 21st century (Sharma, 2018). 

Additionally, it should be noted that mass urbanization has had a negative effect towards carbon emissions (Chen et al., 2022). This is quite logical because, as urban centers grow, their power consumption increases proportionally – in Europe, urban areas contribute to 60% of the population but consume 80% of electricity – which increases greenhouse gas emissions (European Commission, 2021). These greenhouse gas emissions will contribute to climate change, exacerbating issues like water run-off and the UHI effect furthermore. 

Urbanization has increased water run-off (leading to flooding) and worsened the UHI effect, while simultaneously releasing greenhouse gasses. Through its emissions of greenhouse gasses, it contributes to climate change which serves to exacerbate the aforementioned issues. As climate change will continue to worsen over the next decades, it is incredibly important to begin looking for solutions. Prominent solutions that have recently gained traction are the introduction of ecologically-friendly and green infrastructure to urban environments.

Pervious Concrete to Aid Urban Systems

Pervious concrete (or porous concrete) is a viable solution to help manage and filter water run-off and mitigate the UHI. Pervious concrete has larger aggregate sizes (often 10-25mm) than normal concrete. Typically, the total aggregate volume in porous concrete is 50-65% of total volume compared to 60-75% in conventional concrete (Chandrappa & Biligiri et al., 2016). The larger aggregate composition creates void spaces within the concrete – whereas in normal concrete, fine aggregates would fill such void spaces. The void space matrix is interconnected which allows air and other substances to pass through. This ability to let substances pass through the concrete is defined as permeability. Pervious concrete is often implemented in parking areas, pedestrian walkways, and areas with light traffic (Obla, 2010). Due to its pervious nature, it is an excellent method to combat urban water run-off as surface water will get trapped in the concrete. Collins et al. found porous concrete in North Carolina to reduce water run-off by 99.3% and delay water stream by 0.77 h in rainfalls of depth 3.1-89.9 mm (2008). Another study found porous concrete in New Zealand to reduce run-off by 30-60% and delay outflow by 1h in rainfall of depth 150 mm (Fassman & Blackbourn, 2010). As porous concrete can be implemented anywhere throughout urban environments, it is an ideal method to alleviate urban run-off.

Porous concrete additionally alleviates the Urban Heat Island effect. While it may seem counterintuitive due to pervious concrete’s composition, it is not the concrete’s composition which reduces temperatures but its water content. The reason green roofs are so effective in reducing temperature – besides its green coloring which increases albedo (i.e. light absorbed) – is the evapotranspiration, the process of plants releasing heat through water (Sherba et al., 2011, Smith et al., 2011). Porous concrete reduces in a similar way, through evaporative cooling. Shimazaki et al. studied pervious concrete in the Okayama prefecture, Japan, and found the mean surface temperature of pervious concrete to be 13-14 ºC cooler than that of asphalt pavement (2021). Liu et al. found similar results in Shanghai, finding porous concrete to reduce temperatures by 15.3 ºC (2020). Wang et al. found a 3.9 and 6.6 ºC surface temperature drop compared to asphalt and concrete, respectively, during the summer and a 0.7 and 0.6 ºC temperature drop during the winter (2022). Park et al. elucidated the importance of water within the porous concrete matrix: they found the mean surface temperature of dry pervious concrete to be 4 ºC hotter than that of wet pervious concrete (2019). Hence, pervious concrete is more efficacious in regions where rainfall is common.

Pervious concrete additionally serves as a filtration mechanism, cleaning polluted run-off. While pervious concretes are uniformly effective in filtering run-off, specific blends’ efficacy may vary; the mechanism of removing chemicals and particulates is complex, due to the many factors of size, distribution, and tortuosity (defined as ratio between shortest possible pathway and distance of real pathway of a water molecule) of voids (Wijeydarna et al., 2022, Encyclopedia of Electrochemical Power Sources, 2009). While I will attempt to elucidate the potential of the general concept of pervious concrete, it is important to know that each blend of pervious concrete is unique and has specific traits. 

 Jianming et al. found pervious concrete to remove 72.5% and 95% of total nitrogen and phosphorus, respectively, from run-off that passed through the system (2008). In a similar study, Zhang et al. (using pervious concrete with 5-10 and 10-5 mm aggregates) found the porous concrete to remove 85-95% and 40-70%, respectively (2015). The concrete in Jian et al.’s study additionally removed 57-98% of total suspended solids and the concrete used in Zhang et al.’s study removed 95% of total suspended solids (2008, 2015). Pervious concrete has additionally been found to filter heavy metals. In a lead removal experiment by Muthu et al., pervious concrete removed 84% of lead with a single pervious concrete filter. Undoubtedly, when passing through a pervious concrete pathway spanning distance, the efficacy will increase (2018). Another study by Muthu et al. found pervious concrete to have similar effects when filtering water containing cadmium, zinc, and copper (2019). 

Despite its variety of positive traits, pervious concrete has uniformly been recognized as having low compressive strength. Until universally sustainable and replicable methods of increasing strength are found, pervious concrete cannot be widely implemented into high traffic urban infrastructure. As Zhang et al. observes (consistent with findings across the field), as the aggregate size of pervious concrete increases, void space increases, and its compressive strength decreases (2021). Pervious concrete’s reduction in strength compared to traditional concrete varies between 2.8 MPa and 28 MPa (Tahiri et al., 2022). While studies have found supplementary materials such as rice husk ash, a mixture of zinc sulfide nanoparticles and adsorbents, and geopolymers to increase strength, these methods either weren’t sustainable or decreased porosity (Vieiera et al., 2020, Kahrizi et al., 2023, Huang & Wang, 2022). However, over time, compressive strengths have been increasing, and porous concrete can be implemented in low traffic areas and walkways. Porous concrete is an incredibly promising material that will radically shift the ecological state of urban areas when widely implemented. However, while it’s ecologically friendly, it does not introduce more green spaces to urban populations which is becoming increasingly important.

Benefit of vegetation within urban environments

Urbanization no doubt displaces native fauna and flora. Compared to non-urban, natural areas, urban areas only contain 8% and 25% of native bird and plant species, respectively (Aronson et al., 2014). The implications of urbanization on biodiversity and ecological upkeep are incredibly grim. Cincotta et al. has found there to be 25 major ecological hotspots across the world. 16 of the 25 hotspots, they found, had higher population density than the urban average. Using a statistical model, they estimated that 19 of these 25 hotspots will see significant population growth in upcoming years, substantially higher than the worldwide average (Cincotta et al,. 2000). As ecological zones are growing in population, it is essential to find ways to maintain biodiversity within urban areas. In a review article evaluating 1883 published articles, Filezilla et al. have concluded that green infrastructure significantly improves urban biodiversity (2019). Besides improving biodiversity, green infrastructure has various psychological benefits as well. Yañez et al. estimated the Eixos Verdis Plan (a Barcelonian urban plan to implement street greening) will reduce reported cases of poor mental health by 14.03%, visits to therapists by 13.37%, and cases of antidepressant use by 13.37% (2023). The Green Building Council of Australia reported that hospitals with green interiors increased time of recovery by 15% and reduced stay times by 8% (GBCA, 2022). Methods to introduce vegetation into urban areas are an increasing necessity due to their ecological and physiological benefits. 

Green Roofs as Mechanisms to Mitigate Increased Run-Off and UHI Effect

Green roofs have become a promising solution to many of the outlined issues. They significantly help to combat water run-off and the UHI effect. They introduce green spaces to urban environments, increasing biodiversity. Although promising, many constraints and knowledge gaps exist. Shahmohamad et al. point out that most green roofs require high levels of maintenance meaning a sustainable method of irrigation is necessary. While rainwater harvesting – the utilization of rainwater cisterns or additional water retention layers – has potential, it adds excess weight that may exceed building load capacities and its effectiveness varies by region. Greywater utilization is another form of sustainable irrigation. Greywater is wastewater from sinks, showers, washing machines, and dish washers (excluding sewage). It is both sustainable and eco-friendly, but it is complex and expensive as it necessitates a pipe system separate from sewage water. Dew water harvesting has been shown to produce sufficient water for seedlings, but is only possible in high humid areas. Lastly, the review article mentions smart irrigation, i.e. the use of smart technology to calculate water requirements, preventing over-irrigation. While smart irrigation saves water, its practice requires energy for both the technology and pumping involved. While green roofs are incredibly promising, maintenance and upkeep costs prove large-scale introduction into urban areas difficult (Shahmohamad et al., 2022, Mohd et al., 2015). 

Green roofs can indirectly harm environments as well. While green roofs may vary, they generally consist of a vegetation layer, a growing medium layer, a water retention layer, a filter layer, and a drainage layer (Berndtsson, 2009). All of these layers, except the growing medium layer generally contain polymers. This is because weight limitations of green roofs necessitate light and durable materials. Polymers are low weight, have high tensile and corrosion resistance, and are able to be tailored. Because of this, polymers are incredibly common in green roofs: the drainage and filter layers are often composed of 40% polypropylene while the water retention layer is often composed of 100% polymeric fibers (Bianchini & Hewage, 2012). However, the production of polymers is harmful towards the environment. There are multiple methods of production, but any form of production requires temperatures of over 120 ºC to heat the polymer and an additional cooling process to solidify the polymer. The energy and chemicals needed to produce polymers  release toxic chemicals into the air (Bianchini & Hewage, 2012). Additionally, the growing medium of green roofs can be victim to leaching. While green roofs diminish water run-off, when run-off does occur, it carries nutrients from the green roof. Bliss et al. found the growing substrate of a green roof can release up to 2.0 mg/L of phosphorus and 11.4 mg/L of sulfate (2009). Green roof leaching can contaminate water, and contribute to eutrophication with increased levels of phosphorus. While green roofs are a promising new technology, there are still many concerns and knowledge gaps in the field. The shortcomings of green roofs highlight the strengths of pervious concrete and vegetative pervious concrete (VPC).

Vegetative Porous Concrete 

Vegetative porous concrete combines the beneficial effects of porous concrete and vegetation in an urban setting. Vegetation concrete is quite literally vegetation grown directly onto porous concrete – hydroponically, or with soil. The porous structure of pervious concrete allows plants to take root directly in the substrate and the interconnected void chambers allow for both air and water. A thin coating of soil on the surface can provide plants with room for sprouting and serves as an initial source of fertilizer after sprouting (Kim & Park, 2016). As porous concrete is beginning to be implemented in urban areas, its ability to promote plant growth is highly valuable. As urban architecture displaces vegetation, utilizing concrete that can nurture growth would change the landscape of urban architecture and urban greening. Vegetative porous concrete could be used as an alternative to green roofs and to create green walls. It could introduce vegetation to parking lots, alongside walkways, and generally be implemented wherever there exists large concrete expanses. 

While the mechanical properties of porous concrete are compatible with plant growth, the chemical properties are not so synchronous. Plants are widely established to exhibit ideal growth in acidic environments (Ganapathy et al., 2023). When concrete is created by mixing cement, water, and aggregate, the cement undergoes a series of hydration reactions, ultimately binding to the aggregates. The reactions produce calcium hydroxide crystals – Ca(OH)2 – which significantly increase the alkalinity (state of low acidity, high pH) of the concrete. The reactions can increase the pH levels to 12-13 which restricts plant growth (Tang et al., 2018). To lower alkalinity of vegetative pervious concrete, researchers Ganapathy et al. added fly ash – a byproduct of coal combustion – to the concrete mix. The fly ash lowered the concrete pH from a range of 12.8-12.9 to 11.5-11.9 and consequently increased vegetation (perennial ryegrass) root length by 7.91% (Ganapathy et al. 2023). Ground granulated blast furnace slag  – a byproduct of furnace blasted iron – has additionally been shown to lower alkalinity while simultaneously increasing compressive strength (Kim et al., 2015). Both fly ash and blast furnace slag are byproducts of other industries meaning they are cheap, sustainable, and eco-friendly materials. While methods have been found to lower alkalinity, current porous concrete mixtures still maintain a high pH. Vegetative concrete is a very new concept, and more research is needed for its optimization. As Tang et al. state in their article on the development of vegetation concrete, “the development of vegetation concrete composite is still in its infancy” (2018). 

There are many knowledge gaps in the field. As previously mentioned, porous concrete has water-treating capabilities and captures pollutants that pass through it. A study should be conducted on the long term effect of contaminated water on vegetative porous concrete. It is important to know how the pollutants trapped within the concrete will affect the vegetation on top. To fill this gap, an experimental study could comparatively analyze the stress of the vegetation on porous concrete that has interacted with contaminated water versus the vegetation stress of pervious concrete that has not interacted with contaminated water. The study should be conducted over a long span of time to fully understand the long-term consequences of contaminants in vegetative porous concrete.

Studies should additionally find methods to further lower pH and find supplementary materials that will further promote plant growth. One such supplementary material is biochar.

Biochar is a type of granular charcoal created through the incineration of waste vegetation and has a high carbon composition and that drastically increases soil fertility. A study by Chidumayo et al. showed biochar to enhance seed germination rate by 30%, shoot heights by 24% and biomass production of 13% across seven plant species (1994). Biochar also has a developed pore structure and large surface area, which enhances the water holding capacity, nutrient utilization efficiency, and microbial abundance of surrounding soil (Oliveira et al., 2017). Research has shown that biochar remains efficacious in alkaline environments (Rees et al., 2016). 

Biochar has a strong ability to aid plant growth because of its chemical and structural composition. In 2019, researchers Zhao et al. studied the use of biochar in vegetative porous concrete by examining its effect on vegetative porous concrete’s strength, total void space, permeability, and growth of vegetation. They created 4 porous concrete blends containing cement, fly ash, blast furnace slag, coarse aggregate, and varying levels of biochar: 0 (control), 5.0 (blend 1 [b1]), 10.0 (b2)  and 20 (b3) kg/m3. Analyzing the effects on compressive strength, Zhao et al. placed cylindrical blocks of each blend into concrete compressive machines and recorded the force in which a first crack appeared. To observe how biochar affected the void structure of the concrete, the researchers compared the concrete’s (in conical form) dry and wet mass to find the total void space. They also measured the amount of water that passed through the concrete within a given time to find the permeability. On a cubical block, they placed a layer of soil and planted the vegetation seeds, observing growth over time. To select the optimum species to test, they grew Bermuda grass ryegrass, and tall fescue on vegetative porous concrete with 5.6 kg/m3 of biochar. All three species are perennial plants (able to live year round) with long life cycles. They are widely used around the world due to their ability to adapt to different growing environments (Zhao et al., 2019). Perennial ryegrass performed best (highest germination rate, plant height, and root length) and was selected to be used for uniform growth testing.

Herein I will present a replication of Zhao et al.’s study examining biochar’s effect on vegetative porous concrete. In adapting the experiment to the high school setting, I will implement several changes. First, I will not perform a species selection test. As time and resources are limited, I will instead move straight to the experimental growth tests, using perennial ryegrass (as it performed best in the precursory tests). Over the first half of this school year I will replicate Zhao et al.’s growth testing. I hypothesize that biochar’s introduction to vegetative porous concrete will positively affect plant growth, and porous concrete’s structure will aid plant growth and promote microbial activity while its chemical composition will optimize the chemical environment of the porous concrete blocks. Biochar is a fine grained material, and may ultimately fill the necessary interconnected voids within the porous concrete. Based on these two suppositions, I predict that blend 1 (5.0 kg/m3) will furthest promote plant growth. As biochar concentrations increase, the void spaces will decrease, taking away necessary space for plant rooting. 

Following the Zhao et al. replicate study I will propose the novel question: How does biochar affect the filtration abilities of porous concrete? I will run contaminated water (with particulates to be determined) through concrete cylinders (identical to the cubes in composition), testing the outsourced water. Biochar is well documented to immobilize pollutants within soil (due to its adsorbent capabilities) so it is reasonable to suppose biochar will enhance vegetative porous concrete’s filtration capabilities. I hypothesize that blend 1, 5 kg/m3 will perform the best, removing the most contaminants. The initial implementation of biochar will synergistically improve porous concrete’s filtration abilities, but as more biochar is added, void spaces will become filled. This data will contribute the research in optimizing porous concrete as an eco-friendly urban material.

• Urbanization is on the rise. Coupled with climate change, urban run-off and its consequential flooding and contamination will occur more frequently and to greater degrees. The Urban Heat Island effect will worsen: temperatures will rise higher and heat waves will last longer. While green infrastructure is a useful technology in combating these issues, more research is needed for optimization. The research I intend to accomplish will contribute to the burgeoning field of vegetative porous concrete and will outline methods to optimize this nascent technology.

METHODS & PROCEDURE:

Junior Research

Junior year research replicated Perini et al. 's 2022 investigation of growth rate, water retention, and heat reduction on 11 different moss species. Due to time and resource constraints, the replicate study was limited to three species: Barbula unguiculata, Syntrichia ruralis, and Homalothecium sericeum. These three species had contrasting growth rates (fastest, median and slowest respectively) in Perini et al.’s initial study so a similar growth rate distribution was expected. A Barbula unguiculata culture was then created following the steps outlined by Perini et al. (2022). A 1% agar solution was created and was blended with B unguiculata (2:1 volume/weight ratio) (2022). The culture was then applied to eco-friendly substrate Super Felt® and cultivated the moss in a Johnny's Seeds seedling light cart (Product # 7026) set to a 16:8 hour light-dark photoperiod with 60% relative humidity (Perini et al., 2022). Following week 5, photos were taken weekly and uploaded to ImageJ, an image processing software. The image particles were to be analyzed in order to quantify the greenness of the growing moss. However, although observable growth should have been observed by the week 8 benchmark, by week 11 no growth had occurred. 

Following the growth test experiment, it was decided to analyze the water retention and heat reduction abilities of the three species. In their study, Perini et al. tested water retention and heat reduction abilities of varying green roof panel designs. Their experiment was adapted to test 10 cm x 10cm squares of raw moss. With current resources it was not possible to replicate their roof panel designs which were made of intricate layers of different materials. Water retention was analyzed by pumping 190 mL/min of water (through a Kamoer peristaltic pump [Model #‎KCPA600-US]) into a moss system (one moss square placed atop a funnel) for 20 minutes. A balance (Flinn Scientific, Product # OB2139) was placed underneath the moss system to determine the outflow volume. Each trial was filmed and lasted 30 minutes (measuring outflow even after the inflow stopped at 20 minutes). The aggregated water discharge was measured at every 30 second interval. 

To test heat reduction, a 16 in by 8 in bare concrete block (Home Depot Product # 312064973) (negative) and a vegetated block (with a 10 cm by 10 cm moss square on top) were placed underneath an artificial heat source, Fluker®’s Sun Spot Heating Lamp (Product # 27003). The initial surface temperature was recorded by reading the temperature of four regions with an Testo® infrared thermometer (Product # 830-T1), and again recorded 15 minutes later after exposure to the heat lamp. 

Senior Study General Overview

As the replicate study conducted in 11th grade was unsuccessful (no observable moss growth), the senior year experiment began by replicating Zhao et al.’s study, investigating the effects of biochar on porous concrete (2019). Porous concrete has pores and open internal space (void space) that can promote plant growth. Plant roots are able to embed themselves within the pores. The concrete simultaneously improves root structural stability for the plant. Biochar, a granular charcoal, increases soil fertility and has been shown to increase plant germination rate and growth rate (Chidumayo et al., 1994). This study analyzes how the concentration of biochar affects the growth abilities of porous concrete. Perennial ryegrass was chosen to grow atop the concrete substrate. Perennial ryegrass has a fast growth rate, can survive high amounts of stress and grows relatively long roots (which is important when examining concrete-root interactions). It is hypothesized that the concrete blend with the smallest implementation of biochar will most promote plant growth. While biochar is thought to be a beneficial addition, as more biochar is added the porous concrete void spaces will become filled in, detrimentally affecting plant growth. 

Safety Protocol

This study involves the use of fine particulate matters. Thus PPE (labcoat, goggles, gloves, surgical mask, and close toed-shoes) was worn throughout, and safety practices were informed by the MSDS(s) of following materials:Fly ash, MSDS; granulated blast furnace slag, MSDS; superplasticizer, MSDS; Quikrete® cement, MSDS Eyewear, a lab coat, and gloves reduced chemical exposure to skin, eyes, and hands in accordance with MSDS instructions. Facial masks and working with fine powders in the LabAire Systems DynamicFLO Fume Hood mitigated harmful material inhalation. Harmful materials were only used during the Porous Concrete Creation section of the experiment. Following which, PPE requirements were lifted.

Porous Concrete Creation

The porous concrete was created using the material blends outlined by Zhao et al (2019). The negative control concrete had 0 g of biochar per kg3 of concrete while the experimental arms contained 5 kg, 10 kg, 15 kg, and 20 kg of biochar per m3 (Zhao et al., Year). Beside the basic components of concrete (ie. cement, aggregate, and water), fly ash and blast furnace slag were introduced into the blends as they have been shown to lower alkalinity within concrete (Ganapathy et al., 2022, Kim et al., 2015). Cement becomes alkaline when exposed to moisture, so it is important to include additives such as fly ash (Quikrete®). Superplasticizer was also added to increase tensile strength (Zhao et al., 2019). 

Cubes were created for the plant growth testing. It was found that using plastic tupperware as a mold was most conducive to a successful curing. 15 cm cubes were created in quadruplicate for each blend (Zhao et al., 2019). 

Plant Growth Test

The plant growth test was conducted in a Johnny's Seeds seedling light cart (Product # 7026) set to a 16:8 hour light-dark photoperiod (Zhao et al., 2019) with a light intensity of 5600 ± 200 lux. The light intensity was measured by a Urceri Light Meter® (Product # SMT912). While Zhao et al. did not include the light conditions of their experiment, a study observing ryegrass growth by Zhou et al. grew their crop under a light intensity of 10,000 lux (1213). The Johnny Seeds seedling light cart was unable to produce a light intensity of such degree; the maximum obtained was 5600 ± lux (2023).

 Each concrete block was covered with a 5 cm thick layer (255 g) of Ocean Forest® potting soil (moisturized with a 35:24 g/ml soil-water ratio).  An exterior tape boundary of height 5 cm was placed on the concrete to ensure no concrete would fall out. 16 evenly spaced divots of depth 1 cm were created in a 4 x 4 grid pattern across the top face of the cement cube. A depth of 1 cm was chosen as Javaid et al. showed it to be the optimal ryegrass seed planting depth (2022). 16 Ryegrass seeds were placed into each respective divot and 10 g of soil was distributed across each hole to fill the divots. The plants were watered daily (150 mL).

Growth Measurement

 On day 10, 15, 20 and 25, shoot lengths were measured. Each data collection day, four shoots were randomly chosen to be measured per concrete block. Root length was measured on day 25. Four random roots were chosen per concrete block (Zhao et al., 2022). On each time point the average shoot lengths across each blend will be comparatively analyzed using a two-tailed independent t-test. On day 25 the root length data will be comparatively analyzed across concrete blends.

Filtration Testing

Porous concrete cylinders (89 mm diameter, 150 mm height) were created following the same blends as the cubes. Cylinders were created in triplicate across each blend. PVC pipe was utilized to create the cylindrical mold. Paper inserts were placed into the cylindrical molds and once concrete cured, the paper allowed for an easy concrete extraction. 

An empty cylinder with a sieve on its bottom face was designed on Fusion360 with the same diameter of the porous concrete cylinders. A 1 ppt salt solution was created using table salt and deionized water.

In each experiment, the cylinder was suspended directly above a concrete block while an empty beaker sat beneath the concrete to catch the outflow. 500 mL of the 1 ppt solution was measured. Using an Apera Multiparameter tester (Product # 182378), the salinity and TDS of the solution was measured and recorded. Then, the solution was poured into the cylinder and it evenly flowed through the concrete into the beaker. The salinity and TDS of the outflow was measured and recorded. 

The percent change of salinity and TDS was calculated for each block. Each block was tested three times, creating 9 data points for each arm of the experiment. A two-tailed independent t-test was then used to compare the outflow and the inflow for each arm to determine which blends significantly reduced or (increased) TDS and salinity. Then, the outflow of each arm was comparatively analyzed with a two-tailed independent t-test.

RESULTS AND DISCUSSION:

Figure 1a) depicts the total aggregate water run-off (mL) on the y-axis vs. time (seconds) on the x-axis while Figures 1a) and 1b) depict each species’ average aggregate run-off (mL) at the initial and final time point, respectively. Across all species, no additional water run-off was measured after the 23 minute mark as the aggregate outflow began to plateau after 20 minutes and 30 seconds. Hence, each trial length was truncated to a duration of 23 minutes. It was hypothesized that all species would significantly retain water and Barbula unguiculata would perform best.

The water inflow rate was 190 mL/min for the first 20 minutes and was then reduced to 0. The total water entering the system was 3800 mL. Comparatively, most water escaped the Syntrichia ruralis system across all trials: 3655 ± 373.15 mL (144.16 mL retained). Homalothecium sericeum performed second best, with 3360.59 ± 415.05 mL of outflow (439.41 mL retained). Barbila unguiculata retained the most water, with a total outflow of 3264.924 ± 79.17 mL (535.08 mL retained).

The outflow data of each species was first compared to the inflow at each time increment (two-tailed independent t-test; for all t-tests, values below p=0.05 are considered significant). At the 30 second mark, all three species retained a significant amount of water (B. unguiculata: p=0.0000006, H. sericeum: p=0.000005, S. ruralis: p=0.000005). However, by the 23 minute mark, only B. unguiculata and H. sericeum retained significant amounts of total water (B: p=7E-13, H: p=0.005). S. ruralis did not retain a significant amount of water at the 23 minute mark (p=0.26). The outflow data of each species at each time point was then comparatively analyzed (two-tailed independent t-test). Across all 23 minutes, Barbula unguiculata performed significantly better than Syntrichia ruralis. The p-value at the 30 second mark was p=0.03 and reached p=0.007 by 23 minutes. Barbula unguiculata did not perform significantly better than Homalothecium sericeum (p=0.24 at 30 seconds, p=0.54 at 23 minutes) nor did Homalothecium sericeum perform significantly better than Syntrichia ruralis (p=0.21 at 30 seconds, p=0.12 at 23 minutes). 

Across all trials, the standard deviation ranged from 1.23 mL (S. ruralis, 30 seconds, 1.95% of average) to 426 mL (H. sericeum, 23 minutes, 13.50% of average). While all trials were under nearly identical conditions, there was one confounding variable that changed from trial to trial: each moss square was placed onto and packed into the funnel apparatus differently. Because placement in the funnel affects the moss-square density (which naturally affects the moss-square water retention rate), it is believed that this confounding variable caused the large standard deviations.

The results do not fully support the hypothesis. At time t=23, only B. unguiculata and H. sericeum significantly retained water. Syntrichia. ruralis’s lack of retention is contributed to its thin layers and relatively low height (1 cm). B. unguiclata retained the highest average amount of water. The species’ dense growth network allows it to absorb higher amounts of water per area. However, its superior retention was not totally significant. While it performed significantly better than S. ruralis, it did not perform significantly better than H. sericeum (which, it should be noted, did not perform significantly better than S. ruralis). Future studies should examine the water-retaining abilities of different moss species with large stem lengths and expansive growth patterns.. New research will aid in finding an optimal moss species to serve as a component of green roofs and green architecture. 

By day 25, the control group (no biochar) had the longest average shoot lengths (31.75 ± 1.83 cm). Blend 3 had the second longest shoots (31.63 ± 2.29 cm) while Blend 1 (27.88 ±  4.17 cm) and Blend 2 (22.63 ± 3.12 cm) had the third and fourth, respectively. Although the control group had the longest shoots on days 20 and 25 (26.85 ± 5.28 cm and 31.75 ± 1.83 cm, respectively), on days 10 and 15 Blend 3 was recorded to have the longest shoots (18.75 ± 8.72 and 21.03 ± 3.07, respectively). It is likely that prior to day 15 the ryegrass was only interacting with the soil (of depth 5 cm) – only after day 15 did the plant interact with the concrete. Any non-germinating seeds were not included in data collection because ryegrass seed germination (occurring 4 cm away from the concrete) does not reflect any ryegrass-concrete interaction. Besides this, all shoots were equally analyzed with random probability. 

First, two-tailed independent t-tests were conducted to compare the shoot lengths across each blend. No p-values were significant (below 0.05). This is because (due to extenuating circumstances) only 4 out of 64 ryegrass root lengths were measured across each blend. The sheer lack of data makes any statistical analysis difficult. 

The same t-test was then conducted to compare the shoot length of each blend across all days. This set of tests revealed p-values below 0.05 for only two comparisons: control vs. Blend 1 (p=0.02) and Blend 3 vs. Blend 1 (p=0.006). These p-values show that the control group and Blend 3 (the two highest-growing arms, p=0.96 when compared) grew significantly longer than Blend 1. This data refutes the hypothesis which predicted that blend 1 would most aid vegetative growth. While the data yielded little significance, the control and blend 3 promoted the two highest mean growths. A proposed mechanism is that the introduction of biochar in smaller increments (5 and 10 kg/m3) filled in existing void space and interfered with the porous concrete’s void matrix structure. These biochar additives were not significant enough to provide additional nutrients to the ryegrass. However, while the 20 kg/m3 of biochar corrupted the matrix structure, this amount was able to additionally provide significant nutrients for the ryegrass, aiding in its growth. However, this mechanism is only conjecture, further testing is needed to corroborate it. A proposed experiment would be to conduct the same study but instead have 20 experimental arms, each with a 1 kg/m3 increment of biochar. This study would provide a more detailed trend of how biochar and porous concrete interact and aid plant growth.

Another likely explanation is that there were too many confounding variables (investigated in the following paragraphs) which renders the data flawed. Considering that this experiment’s results are completely contrary to Zhao et al.’s findings (who found the 5 kg/m3 biochar blend to promote the most growth), it is not a big leap to assume the data is flawed (). Before looking too closely into the data trends, one should look at the standard deviation of each data point. On day 10, the standard deviation of Blend 3 is 8.72 (46% of the mean). On day 10, 15, and 20, there are standard deviations higher than the range of that day. Overall, the data is very variable and inconsistent. 

Figure #4 depicts the root length measurements. Similar to the shoot length measurements, the graph shows high instances of standard deviation. The standard deviation across the root length data of blend 3 is higher than average root length of blend 1. Two-tailed independent t-tests reveal that there is no significant difference between the data sets. Due to extenuating circumstances, only 4 root lengths were measured across each arm. 

A lead contributor to the experiment’s inaccuracy is the few number of data points collected. Due to resource and time constraints, only 4 shoots were able to be measured from each block. Due to extenuating circumstances, only one out of four blocks was measured for each arm on day 25. On day 25 especially, the 4 data points collected are not representative of the 64 plants across each arm. This is true for the root length data as well, the 4 data points are not representative of the 64 ryegrass seeds sown across each arm. Measuring more data points would cause the data to become more consistent. Additionally, watering the plants was unexpectedly variable. Due to the growing apparatus, half of the blocks were somewhat inaccessible to water. The blocks were on the bottom layer of a grow cart and it was difficult to reach the blocks in the back and evenly distribute water across those blocks. As plant growth necessitates consistent access to water, this confounding variable was incredibly important. Due to the inconsistent data and unclear t-test results, it is recommended that the experiment be re-done with more data being collected on each day.

The control concrete block (no biochar) removed significant amounts of NaCl and dissolved solids: 2.12  ± 2.10 % and 2.74 ± 3.41 %, respectively (two-tailed independent t-test, salinity: p=0.03, TDS: p=0.03). Although having positive net percent changes, Blend 1, Blend 2 and Blend 3 did not significantly increase or reduce the salinity or TDS content of the water (p values ranging from p=0.057 to p=0.58).T-tests comparing the control and the three experimental arms (of varying biochar concentration) all come back significant for both salinity and TDS. These sets of t-tests logically support each other. They elucidate that the porous concrete with no biochar effectively served as a filter while each arm with biochar effectively did nothing. 

These findings completely refute the hypothesis which predicted that the inclusion of biochar (which has strong adsorbent properties) in porous concrete would strengthen its filtration abilities. A proposed mechanism is that the inclusion of biochar corrupted the porous concrete matrix space that is so effective in removing particulates. As the biochar was integrated into the concrete, it simply filled in void space and lost the adsorbent qualities it has as a raw material. While a non-significant trend, the salinity graph shows that with each addition of biochar, the porous concrete became less effective. This corroborates the narrative that larger additions of biochar further fill in void space, further rendering the void matrix ineffective. 

It is important to note that the standard deviations are incredibly high, some standard deviations are even greater than the average itself. One reason for this has to do with the salinity & TDS meter (the same meter which measures both values). The meter would sometimes show two different values for the same sample if tested twice. Ideally each sample would be tested three times and each reading would be averaged. However, due to time constraints each value was only recorded once. This issue raises questions of accuracy in regards to the meter. Additionally, after the testing was finalized, the meter was placed once more in the calibration liquid (12.88 mS). Although the meter signaled that it remained calibrated, the conductivity reading read 12.41 mS – 43 mS off from the actual value. Another potential explanation for the high standard deviation is that there could be a time-based variable that is unaccounted for. The concrete could perchance lose efficacy as each trial is conducted. A proposed study would be to create 27 experimental replicates of each blend and examine how the concrete’s filtration ability is affected over time. 

Future testing should examine how the different blends filter out lead and other heavy metals. Due to their larger particle sizes, heavy metals will interact differently with the porous concrete void matrix. Hence, it is still unknown if varying concentrations of biochar will aid or hinder the filtration process.

As urban populations continue to rise, porous concrete will become an increasingly relevant green-infrastructure. My experimentation corroborated that porous concrete is able to host plant growth and filter water with dissolved substances. Although my research hasn’t displayed that biochar is a beneficial additive of porous concrete, more testing is needed to confirm this. The field is still nascent and future research optimizing porous concrete will further improve the concrete’s ecologically-friendly capabilities.

SUPPLEMENTAL INFO AND FIGURES:

Water Retention Data Standard Deviation

Comparative T-Test Water Retention Data 

Shoot Length Data

Lab Notebooks (11-12)

WORKS CITED:

Aronson, M. F. J., La Sorte, F. A., Nilon, C. H., Katti, M., Goddard, M. A., Lepczyk, C. A., Warren, P. S., Williams, N. S. G., Cilliers, S., Clarkson, B., Dobbs, C., Dolan, R., Hedblom, M., Klotz, S., Kooijmans, J. L., Kühn, I., MacGregor-Fors, I., McDonnell, M., Mörtberg, U., … Winter, M. (2014). A global analysis of the impacts of urbanization on bird and plant diversity reveals key anthropogenic drivers. In Proceedings of the Royal Society B: Biological Sciences (Vol. 281, Issue 1780, p. 20133330). The Royal Society. https://doi.org/10.1098/rspb.2013.3330

Balasubramanian, A. (n.d.). Surface Water Runoff . Research Gate. https://www.researchgate.net/publication/320331329_SURFACE_WATER_RUNOFF 

Bianchini, F., & Hewage, K. (2012). How “green” are the green roofs? Lifecycle analysis of green roof materials. In Building and Environment (Vol. 48, pp. 57–65). Elsevier BV. https://doi.org/10.1016/j.buildenv.2011.08.019

Bliss, D. J., Neufeld, R. D., & Ries, R. J. (2009). Storm Water Runoff Mitigation Using a Green Roof. In Environmental Engineering Science (Vol. 26, Issue 2, pp. 407–418). Mary Ann Liebert Inc. https://doi.org/10.1089/ees.2007.0186

Cincotta, R. P., Wisnewski, J., & Engelman, R. (2000). Human population in the biodiversity hotspots. In Nature (Vol. 404, Issue 6781, pp. 990–992). Springer Science and Business Media LLC. https://doi.org/10.1038/35010105

Cowen, W. F., & Lee, G. F. (1976). Phosphorus Availability in Particulate Materials Transported by Urban Runoff. Journal (Water Pollution Control Federation), 48(3), 580–591. http://www.jstor.org/stable/25038539

Chandrappa, A. K., & Biligiri, K. P. (2016). Pervious concrete as a sustainable pavement material – Research findings and future prospects: A state-of-the-art review. In Construction and Building Materials (Vol. 111, pp. 262–274). Elsevier BV. https://doi.org/10.1016/j.conbuildmat.2016.02.054

Chen, X.-L., Zhao, H.-M., Li, P.-X., & Yin, Z.-Y. (2006). Remote sensing image-based analysis of the relationship between urban heat island and land use/cover changes. In Remote Sensing of Environment (Vol. 104, Issue 2, pp. 133–146). Elsevier BV. https://doi.org/10.1016/j.rse.2005.11.016

Chen, F., Liu, A., Lu, X., Zhe, R., Tong, J., & Akram, R. (2022). Evaluation of the Effects of Urbanization on Carbon Emissions: The Transformative Role of Government Effectiveness. In Frontiers in Energy Research (Vol. 10). Frontiers Media SA. https://doi.org/10.3389/fenrg.2022.848800

Chidumayo, E. N. (1994). Effects of wood carbonization on soil and initial development of seedlings in miombo woodland, Zambia. In Forest Ecology and Management (Vol. 70, Issues 1–3, pp. 353–357). Elsevier BV. https://doi.org/10.1016/0378-1127(94)90101-5

Chow, M. I., Lundin, J. I., Mitchell, C. J., Davis, J. W., Young, G., Scholz, N. L., & McIntyre, J. K. (2019). An urban stormwater runoff mortality syndrome in juvenile coho salmon. In Aquatic Toxicology (Vol. 214, p. 105231). Elsevier BV. https://doi.org/10.1016/j.aquatox.2019.105231

Cohen, B. (2006). Urbanization in developing countries: Current trends, future projections, and key challenges for sustainability. In Technology in Society (Vol. 28, Issues 1–2, pp. 63–80). Elsevier BV. https://doi.org/10.1016/j.techsoc.2005.10.005

D’Aguanno, B., Hofmann, D. W. M., Kuleshova, L. N., & Pisani, L. (2009). FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Membranes: Modeling. In Encyclopedia of Electrochemical Power Sources (pp. 764–774). Elsevier. https://doi.org/10.1016/b978-044452745-5.00882-0

Design or build a Green Hospital - Green Building Council Australia (GBCA). Green Building Council of Australia. https://www.gbca.org.au/green-star/why-use-green-star/why-design-or-build-a-green-hospital/ 

Dore, M. H. I. (2005). Climate change and changes in global precipitation patterns: What do we know? In Environment International (Vol. 31, Issue 8, pp. 1167–1181). Elsevier BV. ​​https://doi.org/10.1016/j.envint.2005.03.004

Fassman, E. A., & Blackbourn, S. (2010). Urban Runoff Mitigation by a Permeable Pavement System over Impermeable Soils. In Journal of Hydrologic Engineering (Vol. 15, Issue 6, pp. 475–485). American Society of Civil Engineers (ASCE). https://doi.org/10.1061/(asce)he.1943-5584.0000238

Filazzola, A., Shrestha, N., & MacIvor, J. S. (2019). The contribution of constructed green infrastructure to urban biodiversity: A synthesis and meta‐analysis. In M. Stanley (Ed.), Journal of Applied Ecology (Vol. 56, Issue 9, pp. 2131–2143). Wiley. https://doi.org/10.1111/1365-2664.13475

Ganapathy, G. P., Alagu, A., Ramachandran, S., Panneerselvam, A. S., Vimal Arokiaraj, G. G., Panneerselvam, M., Panneerselvam, B., Sivakumar, V., & Bidorn, B. (2023). Effects of fly ash and silica fume on alkalinity, strength and planting characteristics of vegetation porous concrete. In Journal of Materials Research and Technology (Vol. 24, pp. 5347–5360). Elsevier BV. https://doi.org/10.1016/j.jmrt.2023.04.029

Hannah Ritchie, Max Roser and Pablo Rosado (2020) - "CO₂ and Greenhouse Gas Emissions". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/co2-and-greenhouse-gas-emissions' [Online Resource]

Huang, W., & Wang, H. (2022). Multi-aspect engineering properties and sustainability impacts of geopolymer pervious concrete. In Composites Part B: Engineering (Vol. 242, p. 110035). Elsevier BV. https://doi.org/10.1016/j.compositesb.2022.110035

Jani, J., Yang, Y.-Y., Lusk, M. G., & Toor, G. S. (2020). Composition of nitrogen in urban residential stormwater runoff: Concentrations, loads, and source characterization of nitrate and organic nitrogen. In J. Aherne (Ed.), PLOS ONE (Vol. 15, Issue 2, p. e0229715). Public Library of Science (PLoS). https://doi.org/10.1371/journal.pone.0229715

Jianming, G.; Xu, G.; Lu, X. Experimental study on eco-environmental effect of porous concrete. J. Southeast Univ. 2008, 38, 794–798.

Kahrizi, E., Sedighi, M., & Rajaee, T. (2023). The effect of adsorbent-containing nanoparticles on the efficiency of porous concrete. In Construction and Building Materials (Vol. 408, p. 133696). Elsevier BV. https://doi.org/10.1016/j.conbuildmat.2023.133696

Kim, H.-H., Kim, C.-S., Jeon, J.-H., & Park, C.-G. (2016). Effects on the Physical and Mechanical Properties of Porous Concrete for Plant Growth of Blast Furnace Slag, Natural Jute Fiber, and Styrene Butadiene Latex Using a Dry Mixing Manufacturing Process. In Materials (Vol. 9, Issue 2, p. 84). MDPI AG. https://doi.org/10.3390/ma9020084

Kyle, C. (2016). The Application of Porous Concrete. https://repository.arizona.edu/bitstream/handle/10150/623460/capstone%20thesis.pdf?sequence=1 

Li, J., Xu, J., & Jiang, X. (2023). Urban runoff mortality syndrome in zooplankton caused by tire wear particles. In Environmental Pollution (Vol. 329, p. 121721). Elsevier BV. https://doi.org/10.1016/j.envpol.2023.121721

Li, Y., & Wang, C. (2009). Impacts of Urbanization on Surface Runoff of the Dardenne Creek Watershed, St. Charles County, Missouri. In Physical Geography (Vol. 30, Issue 6, pp. 556–573). Informa UK Limited. https://doi.org/10.2747/0272-3646.30.6.556

Lin, W., Ryu, S., & Cho, Y.-H. (2016). Performance of Permeable Block Pavements in Accelerated Pavement Test and Rainfall Simulation. In Journal of Performance of Constructed Facilities (Vol. 30, Issue 1). American Society of Civil Engineers (ASCE). https://doi.org/10.1061/(asce)cf.1943-5509.0000686

Liu, Y., Li, T., & Yu, L. (2020). Urban heat island mitigation and hydrology performance of innovative permeable pavement: A pilot-scale study. In Journal of Cleaner Production (Vol. 244, p. 118938). Elsevier BV. https://doi.org/10.1016/j.jclepro.2019.118938

Leopold, L. B. (1968). Hydrology for urban land planning - A guidebook on the hydrologic effects of urban land use. In Circular. US Geological Survey. https://doi.org/10.3133/cir554

Marsalek, J., Rochfort, Q. (1999). An exploratory study of urban runoff toxicity. (1999). In Water Science and Technology (Vol. 39, Issue 12). IWA Publishing. https://doi.org/10.1016/s0273-1223(99)00315-7

Mazalena, R. D., Ahmad, & Yusup, Y. (2022). Analysis of run-off potential for flash flood management in Rahakolowu Watershed. In IOP Conference Series: Earth and Environmental Science (Vol. 986, Issue 1, p. 012046). IOP Publishing. https://doi.org/10.1088/1755-1315/986/1/012046 

Mohajerani, A., Bakaric, J., & Jeffrey-Bailey, T. (2017). The urban heat island effect, its causes, and mitigation, with reference to the thermal properties of asphalt concrete. In Journal of Environmental Management (Vol. 197, pp. 522–538). Elsevier BV. https://doi.org/10.1016/j.jenvman.2017.03.095

Muthu, M., Chandrasekharapuram Ramakrishnan, K., Santhanam, M., Rangarajan, M., & Kumar, M. (2019). Heavy Metal Removal and Leaching from Pervious Concrete Filter: Influence of Operating Water Head and Reduced Graphene Oxide Addition. In Journal of Environmental Engineering (Vol. 145, Issue 9). American Society of Civil Engineers (ASCE). https://doi.org/10.1061/(asce)ee.1943-7870.0001551

M. Pomerantz, H. Akbari, S.-C. Chang, R. Levinson and B. Pon Heat Island Group Energy Analysis Department Environmental Energy Technologies Division Lawrence Berkeley National Laboratory Berkeley, CA 94720 

Ntelekos, A. A., Oppenheimer, M., Smith, J. A., & Miller, A. J. (2010). Urbanization, climate change and flood policy in the United States. In Climatic Change (Vol. 103, Issues 3–4, pp. 597–616). Springer Science and Business Media LLC. https://doi.org/10.1007/s10584-009-9789-6

NWS Preliminary US Flood Fatality Statistics (2023) National Weather Service. United States. [Web Archive] https://www.weather.gov/arx/usflood.

Obla, K. (2010). Pervious Concrete: A Review. Indian Concrete Journal. https://www.researchgate.net/publication/267793052_Pervious_concrete_-_An_overview

Oliveira, F. R., Patel, A. K., Jaisi, D. P., Adhikari, S., Lu, H., & Khanal, S. K. (2017). Environmental application of biochar: Current status and perspectives. In Bioresource Technology (Vol. 246, pp. 110–122). Elsevier BV. https://doi.org/10.1016/j.biortech.2017.08.122

Populations of States and Counties of the United States: 1790 - 1990 (1996). U.S Department of Commerce Bureau of the Census. https://www2.census.gov/library/publications/decennial/1990/population-of-states-and-counties-us-1790-1990/population-of-states-and-counties-of-the-united-states-1790-1990.pdf

Rees, F., Sterckeman, T., & Morel, J. L. (2016). Root development of non-accumulating and hyperaccumulating plants in metal-contaminated soils amended with biochar. In Chemosphere (Vol. 142, pp. 48–55). Elsevier BV. https://doi.org/10.1016/j.chemosphere.2015.03.068

Risser, M. D., & Wehner, M. F. (2017). Attributable Human‐Induced Changes in the Likelihood and Magnitude of the Observed Extreme Precipitation during Hurricane Harvey. In Geophysical Research Letters (Vol. 44, Issue 24). American Geophysical Union (AGU). https://doi.org/10.1002/2017gl075888

Shahmohammad, M., Hosseinzadeh, M., Dvorak, B., Bordbar, F., Shahmohammadmirab, H., & Aghamohammadi, N. (2022). Sustainable green roofs: a comprehensive review of influential factors. In Environmental Science and Pollution Research (Vol. 29, Issue 52, pp. 78228–78254). Springer Science and Business Media LLC. https://doi.org/10.1007/s11356-022-23405-x

Sharma, R., Hooyberghs, H., Lauwaet, D., & De Ridder, K. (2018). Urban Heat Island and Future Climate Change—Implications for Delhi’s Heat. In Journal of Urban Health (Vol. 96, Issue 2, pp. 235–251). Springer Science and Business Media LLC. https://doi.org/10.1007/s11524-018-0322-y

Shimazaki, Y., Aoki, M., Karaki, K., & Yoshida, A. (2022). Improving outdoor human-thermal environment by optimizing the reflectance of water-retaining pavement through subjective field-based measurements. In Building and Environment (Vol. 210, p. 108695). Elsevier BV. https://doi.org/10.1016/j.buildenv.2021.108695

Smith, K. R., & Roebber, P. J. (2011). Green Roof Mitigation Potential for a Proxy Future Climate Scenario in Chicago, Illinois. In Journal of Applied Meteorology and Climatology (Vol. 50, Issue 3, pp. 507–522). American Meteorological Society. https://doi.org/10.1175/2010jamc2337.1 

Stone, B., Jr., & Rodgers, M. O. (2001). Urban Form and Thermal Efficiency:How the Design of Cities Influences the Urban Heat Island Effect. In Journal of the American Planning Association (Vol. 67, Issue 2, pp. 186–198). Informa UK Limited. https://doi.org/10.1080/01944360108976228

Synthesis Report of the IPCC Sixth Assessment Report (AR6) (2023). The Intergovernmental Panel on Climate Change. https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf

Tahiri, I., Dangla, P., Vandamme, M., & Vu, Q. H. (2022). Numerical investigation of salt-frost damage of pervious concrete at the scale of a few aggregates. In Cement and Concrete Research (Vol. 162, p. 106971). Elsevier BV. https://doi.org/10.1016/j.cemconres.2022.106971

Tang, W., Mohseni, E., & Wang, Z. (2018). Development of vegetation concrete technology for slope protection and greening. In Construction and Building Materials (Vol. 179, pp. 605–613). Elsevier BV. https://doi.org/10.1016/j.conbuildmat.2018.05.207

Van der Wiel, K., Kapnick, S. B., van Oldenborgh, G. J., Whan, K., Philip, S., Vecchi, G. A., Singh, R. K., Arrighi, J., & Cullen, H. (2017). Rapid attribution of the August 2016 flood-inducing extreme precipitation in south Louisiana to climate change. In Hydrology and Earth System Sciences (Vol. 21, Issue 2, pp. 897–921). Copernicus GmbH. https://doi.org/10.5194/hess-21-897-2017

Vieira, A. P., Toledo Filho, R. D., Tavares, L. M., & Cordeiro, G. C. (2020). Effect of particle size, porous structure and content of rice husk ash on the hydration process and compressive strength evolution of concrete. In Construction and Building Materials (Vol. 236, p. 117553). Elsevier BV. https://doi.org/10.1016/j.conbuildmat.2019.117553

Vidal Yañez, D., Pereira Barboza, E., Cirach, M., Daher, C., Nieuwenhuijsen, M., & Mueller, N. (2023). An urban green space intervention with benefits for mental health: A health impact assessment of the Barcelona “Eixos Verds” Plan. In Environment International (Vol. 174, p. 107880). Elsevier BV. https://doi.org/10.1016/j.envint.2023.107880

Wang, J., Meng, Q., Zou, Y., Qi, Q., Tan, K., Santamouris, M., & He, B.-J. (2022). Performance synergism of pervious pavement on stormwater management and urban heat island mitigation: A review of its benefits, key parameters, and co-benefits approach. In Water Research (Vol. 221, p. 118755). Elsevier BV. https://doi.org/10.1016/j.watres.2022.118755

WENG, Q. (2001). Modeling Urban Growth Effects on Surface Runoff with the Integration of Remote Sensing and GIS. In Environmental Management (Vol. 28, Issue 6, pp. 737–748). Springer Science and Business Media LLC. https://doi.org/10.1007/s002670010258 

Wijeyawardana, P., Nanayakkara, N., Gunasekara, C., Karunarathna, A., Law, D., & Pramanik, B. K. (2022). Improvement of heavy metal removal from urban runoff using modified pervious concrete. In Science of The Total Environment (Vol. 815, p. 152936). Elsevier BV. https://doi.org/10.1016/j.scitotenv.2022.152936

World Urbanization Prospects 2018. (2018). United Nations Department of Economic and Social Affairs. https://www.un.org/development/desa/pd/news/world-urbanization-prospects-2018

Young, A. (2013). Inequality, the Urban-Rural Gap, and Migration*. In The Quarterly Journal of Economics (Vol. 128, Issue 4, pp. 1727–1785). Oxford University Press (OUP). https://doi.org/10.1093/qje/qjt025

Zgheib, S., Moilleron, R., & Chebbo, G. (2012). Priority pollutants in urban stormwater: Part 1 – Case of separate storm sewers. In Water Research (Vol. 46, Issue 20, pp. 6683–6692). Elsevier BV. https://doi.org/10.1016/j.watres.2011.12.012

Zhang, R., Kanemaru, K., & Nakazawa, T. (2015). Purification of River Water Quality Using Precast Porous Concrete Products. In Journal of Advanced Concrete Technology (Vol. 13, Issue 3, pp. 163–168). Japan Concrete Institute. https://doi.org/10.3151/jact.13.163

Zhang, Y., Li, H., Abdelhady, A., Yang, J., & Wang, H. (2021). Effects of specimen shape and size on the permeability and mechanical properties of porous concrete. In Construction and Building Materials (Vol. 266, p. 121074). Elsevier BV. https://doi.org/10.1016/j.conbuildmat.2020.121074

Zhao, M., Jia, Y., Yuan, L., Qiu, J., & Xie, C. (2019). Experimental study on the vegetation characteristics of biochar-modified vegetation concrete. In Construction and Building Materials (Vol. 206, pp. 321–328). Elsevier BV. https://doi.org/10.1016/j.conbuildmat.2019.01.238