RGR benefits

RGR can provide environmental, social and economic benefits to urban dense areas: air quality improvement, stormwater control, heat island reduction [direct radiation absorption and evapotranspiration by plants (Sailor et al., Reference Sailor, Hutchinson and Bokovoy2008)], heating and cooling load reduction, in addition to aesthetic and acoustic improvements (Hilten, Reference Hilten2005; Berardi et al., Reference Berardi, GhaffarianHoseini and GhaffarianHoseini2014; Chow and Bakar, Reference Chow and Bakar2017; van der Meulen, Reference van der Meulen2019). Cooling and heating load savings for a two-story building with a green roof comprising 40% plant coverage are estimated at approximately 6–49% for the whole building, and 12–87% for the top floor (Santamouris et al., Reference Santamouris, Pavlou, Doukas, Mihalakakou, Synnefa, Hatzibiros and Patargias2007). The temperature balancing effect of a green roof for a small building is more apparent than that of a multi-storied building (DeNardo et al., Reference DeNardo, Jarrett, Manbeck, Beattie and Berghage2005; Yao et al., Reference Yao, Chini and Zeng2020).

When retrofitting with a green roof, the building is provided with additional heat storage by adding to the roof mass, allowing for additional direct solar radiation absorption by plants and by providing a reduced cooling load due to water evapotranspiration (Ascione et al., Reference Ascione, Bianco, de'Rossi, Turni and Vanoli2013). Temperature and humidity reducing effects by green roofs in a high temperature and humidity climate have been reported (Pandey et al., Reference Pandey, Hindoliya and Mod2013). For relatively colder cities like Toronto, Canada, and New York, USA, approximately 50% of roof cover with green plants can instantly reduce the area temperature by 0.8–1°C (Rosenzweig et al., Reference Rosenzweig, Solecki and Slosberg2006; MacIvor et al., Reference MacIvor, Margolis, Perotto and Drake2016). During snowfall in Michigan, USA, for example, the roof substrate is approximately 10°C warmer than ambient temperature with herbaceous plants; with sedum plants, however, roof temperature is similar to ambient air (Eksi et al., Reference Eksi, Rowe, Wichman and Andresen2017). In another study of 325 m² retrofitted extensive green roof in Michigan, USA, it was found that green roof can reduce around 13% of heat flux in winter and 167% in summer compared to traditional gravel roof (Getter et al., Reference Getter, Rowe, Andresen and Wichman2011). Deeper snow layer in green roof reduces the heat flux fluctuation from and to the buildings. The winter thermal benefit of green roof depends on plant type, roof construction and outdoor weather condition of the building (Lundholm et al., Reference Lundholm, Weddle and MacIvor2014).

Green roofs contribute to stormwater runoff control by delaying water runoff, mitigating the large hydraulic pressure into the sewerage system and reducing the risk of flooding (DeNardo et al., Reference DeNardo, Jarrett, Manbeck, Beattie and Berghage2005; Teemusk and Mander, Reference Teemusk and Mander2007; Wolf and Lundholm, Reference Wolf and Lundholm2008; Bianchini and Hewage, Reference Bianchini and Hewage2012). Green roofs can contribute to sustainable drainage system development by exposing large permeable areas in the city (Stovin, Reference Stovin2010). Unfortunately, regular roof material comprising reinforced cement concrete adds chemical runoff and suspended material to rainwater, both of which are problematic for water reservoir ecosystems (Berndtsson, Reference Berndtsson2010; Van Mechelen et al., Reference Van Mechelen, Dutoit and Hermy2015; Shafique et al., Reference Shafique, Kim and Rafiq2018). The fertilized soil/growing substrate of a green roof can release 0.7–2.0 mg l⁻¹ phosphorus, 3–11.4 mg l⁻¹ sulfate and 15.6–31.3 mg l⁻¹ chemical oxygen demand (COD) (Bliss et al., Reference Bliss, Neufeld and Ries2009). In contrast, green roofs have an extensive heavy metal removal capacity; on average, they can remove 97% Cu, 99% Pb, 92% Cd and 96% Zn (Moran et al., Reference Moran, Hunt and Jennings2003; Hashemi et al., Reference Hashemi, Mahmud and Ashraf2015; Shafique et al., Reference Shafique, Kim and Rafiq2018).

RGR challenges

Green roof technology is one partial solution for controlling pollution, climate change and food production, yet the establishment of this technology has been met with several challenges: space and sunlight unavailability in the city, high mass on the roof structure, high cost of green roof manufacturing and research knowledge gaps spanning life-cycle assessment and environmental impact, maintenance and irrigation system requirements (Shafique et al., Reference Shafique, Kim and Rafiq2018; Tang et al., Reference Tang, Mohseni and Wang2018; Susca, Reference Susca2019). Retrofitting a roof with plants is difficult with sunlight in urban centers. Since plants require at least 3 h of sunlight, shadowing by other buildings in an urban center such as Hong Kong is of major concern (Wong and Lau, Reference Wong and Lau2013). Life-cycle analyses of green roofs and their environmental impact are equally problematic. Polymers (polyethylene and polypropylene) used in green roofs produce large amounts of NO₂, SO₂, O₃ and PM₁₀ during manufacturing (Table 2) (Yang et al., Reference Yang, Yu and Gong2008; Bianchini and Hewage, Reference Bianchini and Hewage2012). When examining extensive and intensive green roof manufacturing in Chicago, USA, the polymer requirement for extensive green roof is approximately 4.6 ton ha⁻¹ of low-density polyethylene and approximately 237.5 ton ha⁻¹ of polypropylene, whereas for intensive green roof, the low-density polyethylene requirement is approximately 4.6 ton ha⁻¹ area of roof and the polypropylene requirement is approximately 522.5 ton ha⁻¹ area of the green roof (Yang et al., Reference Yang, Yu and Gong2008; Bianchini and Hewage, Reference Bianchini and Hewage2012). For an extensive RGR system, low-density polythene normally serves as the root barrier layer, where only 1.5–2% polymer by total mass is needed. For an intensive RGR system, the root barrier layer uses a lower proportion of low-density polythene (0.5–1% total polymer), whereas the drainage (72.1% total polymer) and water retention (27.03% total polymer used) layers incorporate higher proportions of polypropylene (Yang et al., Reference Yang, Yu and Gong2008; Bianchini and Hewage, Reference Bianchini and Hewage2012). Extensive use of polymers makes the RGR system questionable when trying to minimize adverse environmental impact.

Table 2. Comparison of particles released to air per 1 kg polymer produced for green roof construction (Yang et al., Reference Yang, Yu and Gong2008; Bianchini and Hewage, Reference Bianchini and Hewage2012)

Challenges are further impacted by roof type, local weather, material availability and labor costs (Yao et al., Reference Yao, Chini and Zeng2020). For example, irrigation is problematic for intensive type roofing systems in cold weather, as increased energy consumption is required to melt ice and surface frost (Teemusk and Mander, Reference Teemusk and Mander2007; Berndtsson, Reference Berndtsson2010). White roof pricing, from $45 to $65 m⁻², is dependent upon the materials and green roofs cost $20–25 more for each square meter of roof construction (Yao et al., Reference Yao, Chini and Zeng2020). Most green roof studies (66%) are performed in Europe and the USA, which delays the dissemination of technology in the Arabic and Asian regions where it would be more cost-beneficial because of their warm/tropical climate (Blank et al., Reference Blank, Vasl, Levy, Grant, Kadas, Dafni and Blaustein2013). One 50-year cost–benefit analysis revealed that for a 4400 m² green roof in Florida, USA, initial costs were $103,320 more than that of a typical white roof. Acoustic and air purification, and overall environmental benefits cannot often be quantified, rendering cost–benefit analyses difficult (Yao et al., Reference Yao, Chini and Zeng2020).

Another challenge in developing RGR systems is finding an optimal plant-growing substrate for green roof installations that demonstrates high stability, availability, low-cost, ability to retain organic content, high water retention capacity, adequate permeability and porosity, high filtration and sorption capacity and light in mass. Typical plant growth substrates that are commercially available do not meet all structural and hydrological requirements, proving an insurmountable barrier to green roof construction to date (Vijayaraghavan, Reference Vijayaraghavan2016; Shafique et al., Reference Shafique, Kim and Rafiq2018).

HGR structure and components

HGR is comparable to RGR for ecological and environmental benefits. HGR is a relatively new technology, with system components and HGR construction that are quite different from those of RGR systems (Fig. 2).

Fig. 2. HGR system components (modified from Tanaka et al., Reference Tanaka, Kawashima, Hama and Nakamura2017).

Roof slab-attached (or with limited gap) HGR systems comprise sealing and holding materials that are similar to RGR systems, with the exception of growing medium. The main problem with floating planters is that the substrate requires changing, and sometimes, planters need replacing after each germination cycle. One of the hardest challenges is identifying a substrate that can be used directly, instead of floating planters, because most current hydroponic growing substrates are not reusable, requiring additional support to keep them stable and water accessible (Hitti, Reference Hitti2018).

HGR benefits

HGR systems with floating planters can reduce a rooftop's temperature by 5°C and heat amplitude by 55% even without vegetation (Huang et al., Reference Huang, Chen and Tsai2016). Temperature was reduced with plants by another 3–5°C, and heat amplitude was reduced by an additional 16% under Taiwanese weather conditions (Huang et al., Reference Huang, Chen and Tsai2016; Tanaka et al., Reference Tanaka, Kawashima, Hama and Nakamura2017). Cooling load savings without plants is due to extra styrofoam, a cement board, and ceramic tile layer directly on top of the roof slab. Apart from annual energy savings, an HGR can reduce fluctuations in roof temperature, and is similar to RGR because of the evaporative cooling effect and heat-storing capacity of the roof materials (Song et al., Reference Song, Kim, Bang, Son, Waldman and Lee2013). HGR is effective at controlling urban water runoff, and at treating and reusing gray water and rainwater. One HGR system showed significant efficiency in removing anionic surfactants, and reduced turbidity by 88 and 75%, respectively, and COD was reduced by 81% when 8 days retention time was used to hold the gray water in the HGR system (Xu et al., Reference Xu, Yang, Zhang, Jin, Huang, Bei, Zhao, Kong and Zheng2020a).

Overall, extensive RGR provides more energy-savings than HGR systems because extensive RGR growing media have more thermal insulating capacity than hydroponic solutions used in HGR systems (Huang et al., Reference Huang, Chen and Tsai2016). HGR provides some advantages over RGR, as water or hydroponic solutions are usually 1.5–2.5 times lighter than traditional RGR growing media (soil or other). If a lightweight and reusable growing substrate could be found, HGR systems would be more attractive than RGR in all measures.

HGR challenges

The main challenge in developing new HGR systems is finding a reusable substrate that can provide long-term benefits. This is exemplified by a Singapore hospital HGR system that produced 190 kg cherry tomatoes annually, but the growing substrate (rockwool) was costly, not easily disposable, and had to be replaced after every growing cycle. As this HGR system is not directly attached to the roof (10–12 cm gap), it does not provide energy-savings equivalent to those of intensive or extensive RGR (Greenroofs, 2020). Research is lacking in HGR systems; this is likely why fruit/vegetable production life cycle and cost–benefit analyses have not provided any clarity, which is another potential barrier in establishing HGR technology (Xu et al., Reference Xu, Yang, Zhang, Jin, Huang, Bei, Zhao, Kong and Zheng2020a).

Stormwater control ability

PC permeability offers plants' root systems easy access to water and nutrients inside the matrix, rendering it feasible for sustainable hydroponics and in urban agriculture (Hitti, Reference Hitti2018). Since the overall hydrological properties of PC depend mainly on permeability, and permeability highly depends on grain size, other physical properties such as mechanical strength, and thermal capacity, and vegetative properties depend on permeability and grain size. When determining PC's stormwater control ability, these properties, in addition to climatic conditions should be considered.

Permeability

Highly permeable PC provides high infiltration capacity, a longer delay time and reduced peak flow rate for stormwater. With a longer delay time, PC provides more evaporation, resulting in a lower heat island effect (Ahammed, Reference Ahammed2017). PC pavement has proved highly cost-effective as an infiltration system with high runoff control up to 100 mm h⁻¹ of rainfall rate. These properties may be used to explore the applications of PC as a roadway, pedestrian pavement, an artificial riverbank, parking lot or other locations where water cycle control within an urban area is essential (Lin et al., Reference Lin, Ryu and Cho2016). Runoff of two permeable interlocking concrete pavements (PICP-1 and PICP-2), concrete grid pavers (CGPs) and PC with conventional asphalt pavement was compared to determine the hydrologic benefits of concrete permeability. After 1 year, it was determined that high concrete permeability processes the lowest peak flow and a large delay time in stormwater runoff. The water retention and controlling capacity sequence obtained was PC > PICP-1 > PICP-2 > CGPs, in comparison with regular asphalt pavement, demonstrating that all permeable concrete offered substantially better stormwater controlling capacity than conventional asphalt (Collins et al., Reference Collins, Hunt and Hathaway2008). A 2-year runoff comparison of inclined permeable pavement (200 m²) and conventional asphalt road (850 m²) showed that permeable pavement had exceptional runoff control with a reduced peak flow rate when compared to conventional road structures in Auckland, New Zealand; daily, 2-year, 5-year and 10-year storm flows demonstrated a flow rate variation for asphalt pavement that ranged from 0 to 48.6 mm h⁻¹, with a sharp peak following the fluctuation of rainfall. PC showed a more level runoff flow profile, with less variation (0–14.9 mm h⁻¹) regardless of the rainfall depth and fluctuation (Fassman and Blackbourn, Reference Fassman and Blackbourn2010). A summary of comparative PC and asphalt studies for runoff, infiltration and stormwater delay time is provided in Table 3.

Table 3. Comparison of PC and conventional asphalt pavement surface runoff, time delay and water retention studies

CGP, concrete grid pavers; CTB, cement-treated base; ESAL, equivalent single axle load; GB, granular base; GPB, granular porous base; GSB, granular subbase; NPTB, non-porous cement-treated base; PC, porous concrete; PCTB, porous cement-treated base; PICP, permeable interlocking concrete pavement; PPBP, permeable paving block pavement; SP-B, porous herringbone shaped block.

Pollution control and water purification capacity

Pollution control and stormwater purification are necessary, as the presence of higher nitrogen (N) and phosphorus (P) content causes eutrophication. As effective N and P removal is in high demand, the water purification capacity of PC, determined by different casting parameters [binders, grain size, binder–aggregate (B/A) ratio, etc.] and physical properties (void ratio, porosity, permeability), is promising. If PC is placed anywhere among the water channel such as a pedestrian walkway, riverbanks or anywhere before the final water destination (river or sea), it could reduce total N (T-N), total P (T-P) and total suspended solids (TSSs), as well as other heavy metals in the water as pre-treatment (Jianming et al., Reference Jianming, Xu and Lu2008; Winston et al., Reference Winston, Arend, Dorsey and Hunt2020). Due to the pre-treatment system in the water cycle, the flow path becomes slower than usual, which helps in preventing soil erosion and in reducing the stormwater stress on the surface. PC can reduce to a maximum of 98% of metal from water (Sansalone and Teng, Reference Sansalone and Teng2004). An optimum PC design was recommended, with 10–20 mm grain size and 25% porosity, which could remove 72.5% of T-N, 95% of T-P and 95% of TSSs (Jianming et al., Reference Jianming, Xu and Lu2008). Without any chemical treatment, PICP reportedly reduced T-N by 65%, T-P by 90% and TSSs by 99.5%, just by flowing water through the concrete (Winston et al., Reference Winston, Arend, Dorsey and Hunt2020). The addition of nanoparticles is helpful in water filtration as well (Winston et al., Reference Winston, Arend, Dorsey and Hunt2020). Other PC components, including porosity, grain size, B/A ratio, different binder and reinforcement further affect water purification capacity.

Grain size

Two different aggregate grain sizes, 5–10 and 10–20 mm, were tested for water filtration capacity with a 30, 40 and 50% B/A ratio (volumetric) in the total blend. Both grain sizes, with the same specific gravity (2.69) had identical water absorption capacity at 0.66% (Park and Tia, Reference Park and Tia2004). The small grain size concrete had high T-P and T-N removal capacity (Park and Tia, Reference Park and Tia2004). Compressive strength increased from 3 to 6 N mm⁻² after varying the B/A ratio from 30 to 50%. The DO, T-N and T-P in the standard sample were 7.90, 2.06 and 0.53 mg l⁻¹, respectively. For 5–10 mm grain size, DO consumption was in the range of 0.38–0.42 mg l⁻¹ depending on the binding agents, T-N elimination was approximately 3.0–3.25 mg l⁻¹, and T-P elimination was approximately 1.18–1.35 mg l⁻¹ (96% of reduction). The PC with 10–20 mm aggregates showed DO consumption in the range of 0.05–0.18 mg l⁻¹, T-N elimination of 1.0–1.5 mg l⁻¹ and T-P elimination of 0.4–1.17 mg l⁻¹. Therefore, the PC comprising 5–10 mm aggregates removed around 1.5–2.8 times more pollutants than the 10–20 mm PC (Park and Tia, Reference Park and Tia2004). Another study compared water purification of two different PCs having 5–10 and 10–15 mm grain sizes, with a varying void ratio of 20–30% (Zhang et al., Reference Zhang, Kanemaru and Nakazawa2015). The concrete behaved similarly with respect to T-N, T-P and DO reduction. However, smaller grain sizes resulted in a higher reduction of carbon. Most importantly, the alkalinity of the PC sample itself did not affect the pH of flowing water (Zhang et al., Reference Zhang, Kanemaru and Nakazawa2015). For the TiO₂-soaked recycled aggregates, the nitrous oxide (NO) absorption rate of the PC increased from 71 to 80% due to the change in aggregate size, from 15–20 to 5–10 mm (Xu et al., Reference Xu, Jin, Hu, Li, Chen, Shen, Wu and Fang2020b). Studies investigating the effects of concrete type, grain size and porosity in water purification are listed in Table 4. To summarize, 90–95% of pollution can be removed from water by using PC as regular or reinforcing something on it. In general, fly-ash, CASC and slags have better performance than Portland cement as a binder. The reinforcement, addition of nanoparticles or a photocatalyst also significantly improves the water purification capacity of PC.

Table 4. Comparison of water purification properties for different PC types

CASC, calcium sulfoaluminate cement; COD, chemical oxygen demand; PC, porous concrete; PICP, porous interlocking concrete pavement; T-N, total nitrogen; T-P, total phosphorus; TSS, total suspended solid.

Plant-growing capacity of PC

Research exploring the plant-growing capacity of PC has expanded. PC-grown vegetation can be limited due to its alkalinity (pH) and sodic characteristics, but this may be partly solved with improvements to PC mixtures and additives (Hitti, Reference Hitti2018). To be considered as a structurally sound and feasible growing substrate in a rooftop hydroponic system, PC should meet other requirements as well. Because all other commercial inorganic rooting substrates, including rockwool, have common issues surrounding disposal after one or two rounds of germination (Inden and Torres, Reference Inden and Torres2001; Jones, Reference Jones2016; Hitti et al., Reference Hitti, Chapelat, Wu and Lefsrud2021), the possibility of long-term use, reuse or multiple growing cycles with PC must be explored. Common growing substrates (rockwool, perlite, carbonized rice hull, cypress bark, coconut coir, etc.) have minimal strength and very short lifetime when exposed to outdoor weather conditions (Bougoul et al., Reference Bougoul, Ruy, De Groot and Boulard2005). The reusability and non-adverse disposal after its lifetime make PC an inevitable growing substrate in the field of hydroponics (Hitti, Reference Hitti2018). To investigate PC's plant-growing potential, different model plants, including grasses (turf grass) plants with hypocotyl extension (radish, Raphanus raphanistrum), and leafy greens (romaine lettuce, Lactuca sativa), leafy greens and fruit (tomato, Solanum lycopersicum) were propagated in PC, serving as a hydroponic substrate (Fig. 3) (Hitti, Reference Hitti2018). When compared to growth in rockwool with the same nutrient solution, germination was comparable. Fresh mass was lower for all plants grown in PC when compared to rockwool, yet dry mass for radishes grown in concrete was 125% more than the rockwool-grown control (Fig. 3) (Hitti, Reference Hitti2018, Hitti et al., Reference Hitti, Chapelat, Wu and Lefsrud2021). This previous work further underlines the importance of permeability, porosity and water retention capacity when improving PC admixtures. In the context of plant growth substrates, healthy plant growth and development, PC runoff should be considered, not only when determining potential harm to urban ecological systems, but also when optimizing nutrient concentrations in hydroponic solutions. They can be easily manipulated for favorable plant growth outcomes in PC, as is commonly done for other hydroponic growth substrates. Nevertheless, plant species screening is required to investigate which plant parameters (stem height and diameter), types (grasses, flowers, short season vegetables, etc.) and root systems (e.g., fibrous, tap roots, adventitious) are conducive to this hydroponic substrate, particularly when challenged with abiotic and biotic stress factors on a roof top setting.

Fig. 3. Potentiality of replacing rockwool with PC as a growing substrate (Hitti, Reference Hitti2018).

Non-cement alkali-activated PC containing 0.2% jute fiber further supports vegetation, despite alkaline (pH 9–10) growing conditions and water purification capacity of vegetated (with grass) PC reduced suspended solids, COD, T-N and T-P by 48.9, 49.1, 81.2 and 43.5%, respectively (Oh et al., Reference Oh, Cha, Park, Lee, Park and Park2014). Mixing of biochar at levels up to 5 kg m⁻³ enhanced ryegrass germination rate, root height and plant height by 12.0, 21.1 and 22.2%, respectively, after 25 days. However, by increasing biochar content from 5 to 20%, plant-growing ability decreased because of decreasing permeability, porosity and plant compatibility (Zhao et al., Reference Zhao, Jia, Yuan, Qiu and Xie2019); if used as a growing substrate, 24.9% porosity and 15.6 mm s⁻¹ permeability was recommended. To summarize, pH inside the PC must be within a suitable range for plant growth, and the pore matrix must be suitable for plant root growth. PC must have enough strength and porosity to hold the structure together during root expansion.

Other advantages of integrating PC into HGR

Apart from water purification, runoff control and plant-growing ability, PC could provide some additional benefits over other growing substrates, as it offers enough mechanical strength to be a part of the static building structure and offers more thermal resistance when covered with vegetation (Ouldboukhitine et al., Reference Ouldboukhitine, Belarbi and Djedjig2012). PC containing dolomite as an aggregate has a compressive strength of 16 and 19 MPa, and flexural strength of 2.9 and 3 MPa, after 7 and 28 days curing, respectively. This strength can be improved, up to 46.2 MPa, by using a superplasticizer in the concrete paste (Lian and Zhuge, Reference Lian and Zhuge2010). For durability, PC containing 25 and 30% water can tolerate 12 freeze–thaw cycles (Gao et al., Reference Gao, Zhou, Xu, Liu, Shen, Peng and Du2020). Therefore, PC's potential as an outdoor growing substrate is expected for 12 consecutive years, even in cold climates. Use of recycled aggregate when casting new PC may be considered, leading to improved management of waste materials and to reduce carbon footprint of new concrete manufacturing. However, recycled aggregate has a lower mechanical strength than new aggregate. Strength could be improved up to 79% by reinforcing PC with polymer (Bhutta et al., Reference Bhutta, Hasanah, Farhayu, Hussin, bin Md Tahir and Mirza2013). PC allows more conduction and convection heat transfer with higher heat-storing capacity in its body than typical growing substrates (Sun et al., Reference Sun, Gao, Geng, Li, Zhang and Liu2017). Without force convection, the heat transfer rate inside the PC decreases sharply after 50% porosity, at higher porosity the heat transfer is led by convection rather than conduction inside the porous body. The heat transfer rate of PC increases with decreasing porosity of its structure (Bessenouci et al., Reference Bessenouci, Triki, Khelladi, Draoui and Abene2011). Therefore, PC is expected to offer a more stable temperature profile when incorporated into an HRG design, resulting in an extensive hydroponic green roof (EHGR) system with different structural components.

Future challenges and gaps when considering PC applications

The main challenges for future PC applications are unclear costs and benefits, no optimization between mechanical, thermal and hydrological properties, insufficient application-specific solutions, actual maintenance and property restoration, and less load and speed-bearing capacity with more brittleness (Kuruppu et al., Reference Kuruppu, Rahman and Rahman2019). During the course of this review, additional knowledge gaps were identified in stormwater controlling capacities: (1) analyses of highly porous PC (porosity >20%) as a means of stormwater control have not yet been performed; (2) specific solutions for clogging and PC maintenance have not been considered; (3) stormwater control with vegetated PC has not been examined and (4) PC's infiltration rate, strength and water holding capacity in practical applications when covered by plants have not been determined. An evaluation of PC's stormwater purification capacity has highlighted several additional research gaps. PC used for water purification in previous studies has a low porosity range (10–20%). However, highly porous (>25%) concrete has the potential for water purification if similar compressive strength can be maintained. To our knowledge, no studies published to date have tested the water purification capacity of vegetative PC. Published research uses a grain size higher than 5 mm for bulk water purification and to maintain more strength at low porosity. Because smaller grain size improves the removal of some specific impurities from water, the water purification capacity of PC with smaller grain size (<5 mm) with similar permeability and porosity merits further investigation.

In this work, PC has been considered for stormwater control and water purification when covered with vegetation in a new EHGR system. Since vegetation present on concrete changes its strength, porosity and hydrological properties, the analysis of PC in a similar hydrological application and as a plant-growing substrate should be explored (Tang et al., Reference Tang, Mohseni and Wang2018). For better water filtration, the PC could be cast with a small grain size (2–3.2 mm) but with higher porosity (25–35%) and adequate compressive strength (3–5 MPa).

Prospects of PC as a green roof material

The prospect of combining PC with HGR technology, resulting in a novel EHGR system (Fig. 4), is expected to open up a new and sustainable area of building structure and masonry work. Vegetation on PC may further increase water holding capacity, filtering capacity and delay time significantly when it would be used in the EHGR system (Hitti et al., Reference Hitti, Chapelat, Wu and Lefsrud2021). Therefore, successful PC-integrated EHGR design and implementation could improve water management and reduce the heat island effect. In both the water and temperature management standpoints for urban settings, the PC-integrated EHGR system will be explored as a possible impactful and realistic tool that could replace typical RGR systems. Plant species screening will be required to determine maximum number of growth cycles permitted and as mentioned above, and which vegetation types may be well adapted to rooftop environments and hydroponic growth, with root systems that are favorable for this new application. With this, partial solutions to greening dense urban areas, increasing biological diversity in cities and the possibility of locally produced food crops could be explored.

Fig. 4. Schematic and 3-D view of proposed EHGR system with PC growing medium.

The proposed EHGR system setup would be similar to RGR to extract full heating and cooling load benefits by completely sealing the roof. A PC layer can be placed directly on top of the drainage layer, reducing the use of environmentally harmful polymers by approximately 95 ton m⁻² compared to extensive green roofs. The polymer reducing amount may be up to 142.5 tons in building 1 ha of green roof area of an EHGR system compared to intensive green roofs (Yang et al., Reference Yang, Yu and Gong2008; Bianchini and Hewage, Reference Bianchini and Hewage2012). A separate filter is not required for this new EHGR system, yet typical (used in RGR systems) membrane insulation can be installed on top of the roof slab. In the installed EHGR, the growing substrate (PC) will be submerged into hydroponic solution, and the drainage layer will be used in flowing and exchanging solutions from and to the system. If this EHGR system is installed on any roof, that roof will provide all the benefits of a regular extensive green roof in terms of hydrology and energy savings, with even fewer materials and less weight on the roof. In addition, the EHG–PC system provides all the benefits of an HGR in terms of easy vegetation and crop production potential. Therefore, the proposed EHGR systems will be a complete green roof technology that maximizes green roof benefits while using less materials and design complicacy.

Summary and future research directions

Hydrological properties, such as water retention capacity, pollution control and water purification capacity make PC attractive as a reusable hydroponic plant-growing substrate for green roofs. PC strength can be adjusted by varying grain size, binders, porosity and permeability during casting, expanding its applicability as a vegetation-covered structural material rather than adding more load to a building. PC as a plant-growing substrate represents a new direction to explore in green building technology. Primarily, grain size, B/A ratio and casting procedure could be further optimized. Lower pH and lightweight binders and/or aggregate types that convey high compressive strength are under investigation. PC leachate will be analyzed to determine runoff, and nutrient uptake and heavy metal accumulation analyses for PC-grown plants is being prioritized, to expand the list of plants that may be grown in this novel hydroponic substrate. There is room to improve PC durability, as well as easy and affordable PC recycling methods.