Research article
Green roof substrate physical properties differ between standard laboratory tests due to differences in compaction

https://doi.org/10.1016/j.jenvman.2020.110206Get rights and content

Highlights

  • We evaluated variation in substrate properties among standard laboratory tests.

  • Differences in compaction accounted for differences in properties among tests.

  • Tests with low compaction produced results closer to simulated in situ conditions.

Abstract

Green roofs are expanding internationally due to the well documented benefits they provide for buildings and cities. This requires transferable knowledge of the technological aspects influencing green roof design, particularly substrate properties. However, this is made difficult due to differences in substrate testing methods referred to in green roof guidelines and standards. Therefore, we tested a green roof substrate using laboratory-based methods from European (FLL), North American (ASTM) and Australian (AS) green roof guidelines and standards to determine how these methods vary in characterising substrate physical properties (bulk density, water permeability and water holding capacity at field capacity (WHC)). Further, we compared the results from the laboratory-based methods with measures of bulk density and WHC in green roof platforms to determine whether standard methods accurately represent substrate properties in-situ. Results from the standard test methods varied due to differences in sample compaction. The standard test methods that employ Proctor hammer compaction (FLL and ASTM) had greater bulk density (at field capacity and dry) and lower water permeability than Australian standard methods that employ free-fall compaction. WHC did not differ among the standard methods. The Australian standard method better reflected bulk density at field capacity and WHC of the substrate under in-situ green roof conditions. For mineral based substrates, our results suggest that for the FLL and ASTM testing methods, a single Proctor hammer drop will produce a degree of sample compaction equivalent to the free-fall method (AS) and be more representative of bulk density in-situ. Subtle changes in testing procedures would allow for more direct comparison of substrate properties between standard methods and help enable the international transfer of knowledge for substrate design.

Introduction

The substrate or growing medium is often regarded as the most important component affecting green roof performance (Ampim et al., 2010; Beattie and Berghage, 2004). This can be attributed to the ‘ecosystem services’ provided by green roofs being directly associated with substrate physical properties (Vijayaraghavan, 2016), including stormwater retention (Stovin et al., 2013; Szota et al., 2017), delayed peak flow of stormwater runoff (Berndtsson, 2010; Stovin et al., 2013), improved building energy efficiency (Jaffal et al., 2012) and noise attenuation (Yang et al., 2012). Further, the long-term success of green roofs is largely determined by substrate depth and physical properties, as these influence water retention, plant survival and growth (Jennett and Zheng, 2018).

The challenge with engineered substrates (which may contain natural, synthetic and modified components) is finding the right balance between low bulk density, rapid drainage and optimal properties (i.e. high plant available water and high cation exchange capacity for nutrient storage) to support plant growth and canopy coverage (Ampim et al., 2010). For example, low bulk density and good air-filled porosity enables a substrate to be lightweight, free draining and facilitate plant respiration. However, these properties need to be balanced against sufficient water retention capacity (Rowe et al., 2006; Thuring et al., 2010), stability for plant anchorage (Dunnett and Kingsbury, 2004) and resistance against decomposition (Vijayaraghavan, 2016). The desirable physical characteristics of a green roof substrate include low bulk density, sufficient permeability and high WHC (Graceson et al., 2014; Hill et al., 2016). For stormwater mitigation, substrate permeability and WHC are important properties as they influence rainfall runoff and retention from green roofs (De-Ville et al., 2018a).

Testing substrate formulations to select those with optimal physical properties becomes critical to achieve desired outcomes (Vijayaraghavan and Raja, 2014). This is achieved by several established and recognised green roof standards and guidelines that provide testing methodologies and performance criteria for bulk density (for estimating structural loads), water permeability (estimating drainage) and WHC (as an indicator of rainfall retention) (ASTM International, 2015; FLL, 2008). Of these the German FLL (2008) guidelines for green roofs are the most widely used and are internationally recognised as a leading source of authority (De-Ville et al., 2017; Dvorak and Volder, 2010).

First published in 1982, the FLL guidelines are based on field observations, empirical research and substrate development within the green roof industry (Dvorak and Volder, 2010; Oberndorfer et al., 2007). The FLL guidelines include rigorous laboratory testing procedures for measuring substrate physical and chemical properties, and provide performance target ranges for different green roof configurations (Fassman and Simcock, 2012; FLL, 2008; Kazemi and Mohorko, 2017). As a result, the FLL guidelines have been a primary reference for the development of testing procedures, standards, norms and guidelines of other climatic regions throughout Europe, North America, Asia and Australia (Ampim et al., 2010; Dvorak, 2011; Kazemi and Mohorko, 2017). However, variation in climate and application of green roof technologies mean that the FLL substrate performance targets are not always relevant for designing substrates in other regions or for novel green roof applications (Dvorak, 2011). For example, compared with Germany many ecoregions in Mediterranean Europe, North America and Australia have considerably warmer and drier climates, where substrates with greater water holding capacities are often prioritised (e.g. Ampim et al., 2010; Farrell et al., 2013; Ntoulas et al., 2015; Rowe et al., 2006). This has led to the development of standards and guidelines in other regions.

While many researchers refer to test methods from green roof standards and guidelines (Kazemi and Mohorko, 2017), the choice of methodology differs according to the type of study and research location. For example, research groups that investigated substrates in controlled environments (i.e. laboratory or greenhouse) typically use or refer to methods in the FLL guidelines (Berretta et al., 2014; De-Ville et al., 2018b; Eksi et al., 2015; Emilsson and Rolf, 2005; Fassman and Simcock, 2012; Young et al., 2014). Other researchers often use methods from standards or guidelines developed in their own country; for example, UK researchers tend to use the British Standards (BSI) (Graceson et al., 2013, 2014; Molineux et al., 2009), Australian researchers use Australian Standards (Farrell et al., 2012; Szota et al., 2017; Xue and Farrell, 2020) and North American researchers have used ASTM standards (Hill et al., 2016). Furthermore, some researchers characterise substrates with modified testing methods (Young et al., 2014) or use methods from other scientific disciplines (Bouzouidja et al., 2016; De-Ville et al., 2017; Hill et al., 2016; Olszewski et al., 2010). This means there are difficulties in comparing results between studies due to differences in apparatus design, procedures, terminology, units of measure, reference values and performance criteria. The scale of testing environment also varies, with measurements taken in laboratories (Graceson et al., 2014; Mickovski et al., 2013; Olszewski et al., 2010), controlled greenhouses (Cao et al., 2014; Farrell et al., 2013; Young et al., 2014) and elevated test beds or experimental platforms (Bates et al., 2015; Berretta et al., 2014; Bouzouidja et al., 2016; Fassman and Simcock, 2012; Matlock and Rowe, 2016; Schroll et al., 2011). We also found several examples where researchers do not report which standards, guidelines or specific procedures were used. All of these factors make direct comparisons of substrate analysis reported in the literature difficult, particularly to compare substrate performance across different regions, climates, and green roof types (Fassman and Simcock, 2012; FLL, 2008; Kazemi and Mohorko, 2017). Further, while many studies have characterised substrate physical properties, few have compared methods used in green roof standards or guidelines (exceptions include Fassman and Simcock, 2012).

When evaluating differences in substrate testing methodologies it is also important to consider whether these tests relate with green roof performance in-situ (Fassman and Simcock, 2012; Szota et al., 2017). For example, laboratory-based assessment of water holding capacity (WHC) has been shown to overestimate in-situ field capacity and therefore, potential stormwater retention (Fassman and Simcock, 2012; Szota et al., 2017). More realistic representations of substrate physical properties and reliable testing methods are needed to better predict hydrological performance of green roof configurations and substrate performance (Berretta et al., 2014; Fassman and Simcock, 2012; Li and Babcock, 2013; Szota et al., 2017).

Therefore, we tested a green roof substrate using laboratory-based methods from European, North American and Australian green roof guidelines and standards to determine how these methods vary in characterising substrate physical properties (bulk density, water permeability and WHC). Further, we compared results from laboratory-based methods with measures of bulk density and WHC in green roof platforms to determine whether standard methods accurately represent substrate properties in-situ.

Section snippets

Substrate used for testing

A single substrate “Burnley mix” was used for all measurements in this study. The substrate was developed as part of a larger research program supporting green roof design for Australian climatic conditions (Farrell et al., 2013). This substrate is widely used on Australian green roofs due to its availability of components and consistency in composition and properties. The substrate was designed to comply with the FLL (2008) guideline recommendations for extensive green roofs; lightweight, free

Comparison of standard methods (with and without modifications)

Bulk density (at field capacity and dry) was significantly greater for FLL and ASTM methods compared to AS3743 and modified AS3743 (24 h immersion) (P < 0.001; Fig. 2). The modified AS 3743 (Proctor hammer) and AS 3743 (Proctor hammer + 24 h immersion) treatments produced no significant difference in bulk density (at field capacity and dry) compared to the ASTM and FLL methods (Fig. 2).

There was no significant difference in WHC at field capacity among the three standard methods with values

Discussion

This study showed variability in the characterisation of substrate physical properties among the standard test methods. Differences in bulk density and water permeability were due to differences in sample compaction among standard methods. However, there were no differences in WHC among the standard methods. Further, we compared results from the laboratory-based methods with measures of bulk density and WHC in green roof platforms to determine whether standard methods accurately represent

Conclusions

The main aim of this study was to determine how laboratory-based methods from European, North American and Australian green roof guidelines and standards vary in characterising substrate physical properties (bulk density, water permeability and WHC). Our study showed that standard laboratory-based test methods characterise the physical properties of a green roof substrate differently due to differences in compaction. Compaction with a Proctor hammer according to the FLL and ASTM methods

Authors contribution

Richard Conn: Conceptualization, Investigation, Methodology, Data curation, Visualization, Writing- Original draft preparation, Writing- Reviewing and Editing. Joerg Werdin: Conceptualization, Investigation, Methodology, Writing- Reviewing and Editing. John Rayner: Supervision, Writing- Reviewing and Editing. Claire Farrell: Supervision, Conceptualization, Visualization, Writing- Reviewing and Editing, Funding aquisition.

Acknowledgements

We thank Nick Osborne for technical assistance with setting up the testing apparatus. We also thank Dr Chris Szota for help with data visualisation. This research was funded by Australian Research Council Linkage Grant LP130100731, supported by Melbourne Water and the Inner Melbourne Action Plan.

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