Every source-control system puts a material directly in the path of runoff bound for a creek, which makes the material a design decision rather than a procurement line item. BGI makes no stormwater quality claim for the Tree Nurturing System (TNS™). The material question is separate from quality and concerns what the water meets on its way through. That material is precast pervious concrete: mineral aggregate bound in cement, with nothing else as standard. This page covers the whole materials story, from what the unit is made of to where it goes at end of life.
Both kerb units are precast products of PPC Australia, made to a patented permeable concrete formula. Structurally the material is conventional concrete with the fine fraction removed – coarse mineral aggregate held in a cement paste, with the void network that sand would normally fill left open. Those interconnected voids are the working part, passing water at up to 500 mm per minute when tested to ASTM C1701. The unit carries a Class D trafficable load rating at the tray.
The ingredient list is deliberately short, with mineral aggregate, cement and water as standard and nothing else in the water path. The age of the recipe works in its favour, because concrete's behaviour in water, under load and over decades is among the most thoroughly characterised of any construction material on earth. When a council asset engineer asks what this unit is made of, the answer is a material that their standards, testing regimes and demolition contractors have understood for a century.
All three are mineral aggregate and cement as standard, with the full water path set out step by step on How it works.
Because the kerb line is where a catchment's runoff converges, the unit is the final surface the water contacts before entering the system built to protect the receiving water. BGI's design position is that this convergence point should contribute nothing of its own to the water passing through it.
Mineral aggregate and cured cement paste deliver that, with constituents that are geologically stable minerals and a hydrated cement matrix. The unit carries no organic binder to age, polymer to weather or additive package to mobilise. Water moves through the void structure by gravity, contacts stone and cured paste, and continues to the manifold. BGI has commissioned no leachate testing on the water-path elements, which contain no constituent of concern to test for. Where an authority requires characterisation for a specific approval, BGI would work with PPC Australia to obtain it.
The one caveat, stated fully: freshly crushed concrete in bulk produces strongly alkaline leachate, with reported pH values above 12, falling toward neutral as the material carbonates (Engelsen et al., 2012). The alkalinity is a property of crushed recycled aggregate stockpiles and does not apply to a cured precast unit in service. It is also the reason we describe the water path as contributing no constituent of concern and make no claim that concrete is inert in the abstract.
The question rarely asked at tender is where a unit goes when it is lifted out at the end of its service life, decades from now. It is the harder half of the circularity question – and the one the TNS™ can answer with a national statistic in place of a promise.
Concrete is the most successfully recovered material stream in the country. Building and demolition materials achieved an 84% resource recovery rate in 2022–23, up from 76% in 2016–17 (DCCEEW, 2024). Recycled concrete aggregate has established, liquid markets as road base, aggregate and hardstand. A TNS™ unit lifted from a kerb line by a demolition contractor in 2065 enters that mature, high-volume, domestic stream the same day. The recovery pathway is the ordinary operation of the Australian construction industry, with no bespoke take-back scheme, specialist processor or dependency on a supplier still existing.
"Recyclable" and "recycled" are different claims: the figures above describe a national material stream, not a guarantee about any individual unit. What a specific unit meets at end of life depends on the demolition contractor and receiving facility, decades from now. A spent unit is renewed by replacement and is never lifted, cleaned and reinstalled.
The strong claim is the true one: the unit is made of the single most recovered construction material in Australia, and its end-of-life pathway already exists at national scale.
This page describes BGI's own system, its materials, design decisions and end-of-life pathway, and compares the TNS™ with no other product.
Cement production has a significant footprint and BGI does not pretend otherwise. This page concerns what enters the water and where the unit goes; no comparative life-cycle assessment has been run and none is presented.
Mineral composition of the porous elements is the standard specification, not a manufacturing limit.
BGI takes part in that research, set out under Research below.
We would rather publish a narrower claim we can hold than a broader one we would have to defend.
Units at the Bondi site, constructed in 2022, have kept their top edge and channel face intact, with the aggregate still fully bound in. The EnviroKerb® unit is precast by PPC Australia in a controlled environment as one permeable concrete mix through its full depth. It has no surface coat or topping that could separate from the body. The permeable concrete in the ETI is held inside a galvanised steel frame that protects its edges from wheels and sweepers.
A unit at end of service life is renewed by replacement and is never lifted, cleaned and put back. As concrete, the unit is recyclable through the same recovery stream as any other demolition concrete. BGI makes that claim for the material class, and makes no claim that individual units are reused.
Cement production does have a significant carbon footprint, which we accept without qualification. The materials case on this page concerns the water and the end of life, with no comparative life-cycle assessment run or presented.
BGI has commissioned no leachate testing on the unit, which is conventional mineral aggregate and cement with no constituent of concern. Where an authority requires characterisation for a specific approval, we would work with PPC Australia to obtain it.
Because the kerb line is where a catchment's runoff converges, the kerb unit is the last surface the water touches before the receiving system. Our design position, delivered through the material, is that the convergence point should contribute nothing of its own to the water passing through it.
What urban surfaces contribute to the water that leaves them – thermally, chemically, and as particles – is an active research field in which BGI holds a stake. Our existing research stream on reflected heat and pollutant mobilisation from hot urban surfaces falls inside this question. Our materials work extends it to how surfacing materials across a catchment behave over decades of Australian ultraviolet, heat, wetting and wear, and what reaches the kerb line as a result.
We are pursuing these as research questions as an industry partner of the ARC Training Centre for Climate-Resilient Water. The Centre is a national centre led by The University of Queensland with Griffith University and the University of Technology Sydney. The TNS™ itself has not been assessed by the ARC Training Centre.
The practical consequence for an asset owner is the same as it is for maintenance: we would rather hand you a measured answer in five years than a confident one now.
Where our own observations inform a question, we say so and we treat them as observations. They are not presented as findings until they have been measured.
Two sources carry the literature claims on this page – each verified against the publisher's own record. Everything else is product specification or a design position, stated as such.
Department of Climate Change, Energy, the Environment and Water. (2024). Resource recovery and waste material analysis 2024. Australian Government. https://www.dcceew.gov.au/environment/protection/waste/publications/national-waste-resource-recovery-reporting/resource-recovery-waste-material-analysis-2024
Engelsen, C. J., Wibetoe, G., van der Sloot, H. A., Lund, W., & Petkovic, G. (2012). Field site leaching from recycled concrete aggregates applied as sub-base material in road construction. Science of the Total Environment, 427–428, 86–97. https://doi.org/10.1016/j.scitotenv.2012.04.021
If your project or procurement policy asks something this page does not answer, ask us. Materials questions go to the people who made the design decisions described here.
