The efficiency of Terracrete permeable pavers

As a sustainable drainage system, permeable interlocking concrete pavers (PICPs) provide a highly viable alternative for a host of stormwater and erosion control applications.

Grassed Terracrete pavers at Glendirk Estate, Constantia, Cape Town Ryan Moore conducting a drench test as part of his UCT postgraduate study Preparing a testing station for the three types of Terracrete block infill typically employed, namely soil, sand or gravel

Acommon practice in Europe for decades, PICP adoption in South Africa was initially sluggish, but has gained significant traction in recent years thanks to the pioneering efforts of early adopters. The latter include Terraforce, a leading precast concrete retaining block wall manufacturer based in the Western Cape with an expanding global licensee network.

Back in 1994, Terraforce obtained the rights to a German PICP product range. Tested by the Institute of Hydraulic Engineering at Karlsruhe University, this system was capable of infiltrating 400 ℓ/sec/ha and was launched locally in cooperation with Dave King of Inca Concrete Cape. Sadly, the venture failed in South Africa, as the system was designed to be installed mechanically, whereas hand packing was still the norm here.

Taking cognisance of this, Terraforce’s research and development (R&D) team set out to find a more practical, homegrown solution, which led to the launch of the Terracrete eco-paver in 1999. Featuring large internal openings, this product is suitable for installing hard-wearing, permeable eco-surfaces on roadways, parking areas, or for mixeduse installations such as stormwater detention ponds, as well as for general erosion control measures.

A market in transition

Terracrete is now widely accepted as a versatile, costeffective solution to be used in combination with conventional paving systems or as a standalone solution. However, when initially introduced, it still received a cool reception from the market, despite being purpose-designed for local labour and installation practices.

“Even the development of plastic moulds that allowed for production on a small scale with minimal capital outlay did not help much. Finally, during the years 2002 to 2004, specifiers started to accept the concept and sales began to pick up, and today their popularity is widespread thanks to an increasing emphasis on environmental stewardship,” explains Holger Rust, founder and head, Terraforce.

Today, Terracrete is available locally in the Western, Eastern and Northern Cape, KwaZulu-Natal and Gauteng, as well as internationally in Swaziland, Namibia, the

UAE, India and Australia.

Drench test with newly laid Terracrete blocks on the bank of the Nahoon River in East London

Early PICP research

Ongoing R&D studies have further bolstered the market growth for PICPs. These include alternative methods for creating permeable and efficient drainage/ infiltration in existing paved areas or in new areas paved with asphalt or interlocking clay/ concrete pavers.

In 2007, Rust worked together with Dr Soenke Borgwardt – landscape architect, consulting engineer and leading expert on the subject in Germany – to highlight findings about the best approach using different fill materials.

Borgwardt’s key findings about PICP are as follows: • A high rate of infiltration demands the maximum open surface area possible without compromising stability. • Infiltration rates are higher with coarse infill – 2-10 mm (ASTM No. 8) or 2-5 mm (ASTM No. 9) – and regular maintenance by vacuum washing the gravel is very feasible in a way that would not be possible with finer infill such as sand. The latter is also prone to clogging.

cert ai n test sites l i t y at

Double ring infiltrometer test to check the baseline p e rme a b i

• This method is very suitable for upgrading drainage of existing areas or new ones where the presence of heavy vehicles or high traffic volumes could compromise conventional permeable paving. • It represents an important management tool for affordable and sustainable stormwater control. “The incorporation of drainage lines along edges, around perimeters and at predetermined centres across parking/ storage areas or roadways makes it possible to upgrade existing ones or enhances drainage/infiltration of new installations. This method can also be applied when new service or irrigation lines must cut across existing parking areas while trees are being established at the same time,” Rust expands.

Terracrete PICP systems are ideally suited to this purpose due to their approximately 40% open spacing design. This compares to around 15% for conventional concrete paving blocks incorporating widened joints.

“In addition – and something that is often neglected – wider spacings allow tree roots to breathe, and for the underlying soil to absorb water and nutrients that get washed off the paved areas,” Rust continues.

Testing for permeability

Studies conducted between 2020 and 2022 by two graduate students from the University of Cape Town’s Department of Civil Engineering on the permeability of Terracrete pavers support Terraforce’s earlier research. Ryan Moore completed his research in 2020 and Banele Noqayi completed his in late 2021, both under the tutelage of Professor Neil Armitage.

Moor concluded: “Terracrete systems maintain a steady long-term infiltration rate with minimal maintenance required and these pavements can be aesthetically pleasing by increasing the green land on a property.” The key consideration to achieving this is to ensure that the in situ soil conditions are suitable or adaptable for PICP installations. For example, a clay soil exhibits poor permeability characteristics and therefore needs extra drainage measures.

Noqayi’s conclusions echoed Moore’s, stating that “the infiltration of Terracrete eco pavers is largely dependent on the underlying soil conditions,” reinforcing the need to design and install subsoil drainage layers where soils prove less permeable. He also found that vegetation increases the infiltration rate through the root system of the plants. This was especially the case where the pavers are not filled entirely to the top to leave space for plant growth. However, Rust adds here that this strategy may

Terracrete blocks used adjacent to traditional pavers to help with drainage of more impermeable surfaces

not always be acceptable, in which case loose gravel or stone infill is more appropriate, as stated in Moore’s research results.

“As with any engineered system, effective long-term performance is interdependent on professional design, proper choice of materials and aggregates, as well as accurate installation, supervision of the building process and maintenance,” adds Rust.

“Working together with our local and international licensees, we are committed to further developing the product and its applications to perfect the process, closely liaising with industry experts as we expand the frontiers of PICP locally and globally,” Rust concludes.

Adding water to the three Terracrete infill types to calculate the different infiltration rates they yield

THE BENEFITS OF TERRACRETE PERMEABLE PAVERS

• Promotes reduction of stormwater load into natural discharge channels and into sewage conveyance systems • Can reduce excessive flooding in urban areas • Encourages bidirectional flow of water, either into subterranean aquifers or to allow high water tables to release moisture into the atmosphere • Promotes the establishment of beneficial soil-borne microorganisms that can neutralise harmful contaminants • Can significantly reduce the effect of heat generating islands in urban areas • Can be employed to reverse the sealed nature of large expanses of asphalt to create infiltration lines that can also host significant tree cover