Low-intervention permeable paving overlays can transform worn-out paving and also deliver multifunctional sustainable drainage (SuDS), trees and numerous other benefits.
Interpave
To understand how, it’s worth reminding ourselves of the unique characteristics and attributes of modular concrete paving generally – as the trade body Interpave explains.
Concrete Block Paving technology is based on high-strength, interlocking units installed with granular material filled joints and laying course. It enables small displacements between blocks, while retaining interlock, to create a particularly durable, very long-life surface over structural layers below. Decades of use internationally have demonstrated its suitability for the most taxing applications. A wide choice of shapes, styles, colours and finishes – including natural aggregates – add a richness, diversity, visual interest and a human scale to the urban environment.
Re-usable Block Paving
Concrete Block Paving is slip-resistant, durable, strong and sustainable. Modular concrete paving delivers fast, low-cost installation and replacement, using weather-independent, ‘dry’ construction. There are no curing, hot-work or noxious fume problems and only small plant and equipment is needed, with noise and disturbance minimised. With an extremely long lifespan, blocks can be taken up and re-used without processing for repairs, changing layouts, or new schemes – saving carbon and meeting ‘circular’ economy’ criteria.
Concrete Block Permeable Paving (CBPP) enhances this technology as a well-established, multi-functional SuDS technique. It simply combines self-drained, safe and attractive surfaces for a wide range of applications with attenuation, storage, pollution treatment and conveyance of rainwater runoff. The difference with CBPP is angular aggregate – not sand – used to fill enlarged joints and as a laying course. Then, pavement layers of voided material below accommodate water, whilst still providing structural performance. CBPP can also accept additional runoff from adjacent impermeable paving and roofs.
The Key to Successful SuDS
By its very nature, CBPP requires no additional land-take for water storage or management, and no gulleys with related pipework. Its unique capabilities include source control and delivering a gradual flow of clean water, for example to open SuDS features, for amenity or biodiversity, to drainage systems or into the ground. More than 25-years usage has proven its adaptability on projects ranging from footpaths to container terminals.
Both construction and whole-of-life costs of CBPP have been shown to be lower than for conventional paving and drainage, and it requires only limited, straightforward maintenance without clogging problems. It’s important to remember that CBPP is unlike – and not to be confused with – permeable materials, which behave very differently.
Innovative Overlays
Concrete Block Permeable Paving Overlay is a deceptively simple, innovative approach to retrofitted CBPP, delivering SuDS as part of low-intervention repair, maintenance or regeneration. The CBPP upper layer is applied as an overlay onto the original structural road or paving base. In many cases, it can simply run kerb-to-kerb to form a shared surface level with the footway and flush kerb top. This approach also enables low-cost improvement works – perhaps as part of creating traffic calming, Low Traffic Neighbourhoods or Homezones.
The same blocks and grit bedding layer and jointing material as for permeable pavements generally are used. Water is attenuated, treated and conveyed within the laying course, enabling filtration of silt and retention/treatment of pollutants without clogging. A gradual flow of clean water can then be released near the surface, meeting SuDS requirements.
This approach provides numerous benefits including:
• Interception losses, managing runoff during regular rainfall events
• Attractive, popular surfaces with no puddles or potholes, for user safety
• Maximising re-use of existing road-base and its embodied carbon
• Optimising the original drainage regime, but below the surface
• Low-intervention, low carbon, no-heat retrofit installation
• Long-life with minimal maintenance and low whole-of-life cost
• No open gulleys – wildlife-safe
• Integral water filtration, with debris/litter remaining on the surface
• Straightforward access to below-ground services
• Sustainable re-use of blocks for reinstatement or changed layouts.
Refurbishment and Green Infrastructure
CBPP and trees, and other planting, have been proven to work together in synergy. CBPP – whether full-construction or in overlay form – can collect rainfall away from the canopy and convey it to the tree. It can then simply discharge horizontally into a raingarden, perhaps with overflow into an existing gulley. The raingarden stores water during heavy rain for SuDS, retains soil moisture during dry weather and provides additional water quality, as well as irrigation.
Alternatively, CBPP can be used over standard tree pits, proprietary tree planters, ‘Stockholm System’ or other structural soil installations, enabling irrigation and simple gas (oxygen/carbon dioxide) exchange essential to trees – without additional reservoir units or pipes requiring maintenance. CBPP also avoids tree root disruption common with other paved surfaces.
This approach is exemplified in Bridget Joyce Square, London, where a typical, adopted asphalt street and adjacent parking areas were transformed for community use, with CBPP overlay shared surfaces discharging to tree-planted raingarden basins. The basins provide water storage for SuDS to reduce overloading existing drains as well as for irrigation. Interpave revisited the project around 5 years after completion and noted that the permeable paving was performing well and experienced no problems during recent extreme summer storms.
Tests were also carried out demonstrating that infiltration rates of the permeable paving were more than double those recommended by ‘The SuDS Manual’ (CIRIA, 2015). The trees and other green infrastructure were healthy, substantial and particularly well-established. A case study exploring Bridget Joyce Square is available on Interpave’s website and other innovative project case studies will be published shortly.
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