Mechanical concrete: A road building revolution
Making a leap in waste beneficiation, the mechanical concrete system of road building transforms waste tyres into ‘geo-cylinders’, which function as lateral reinforcement elements and make road construction more affordable, faster and more sustainable.
South African construction company Tiro Sechaba secured the licensing rights to the Mechanical Concrete technique and has been granted patent protection in South Africa. Operations director Robert Jooste says in addition to patent protection, Tiro Sechaba has secured Council for Scientific and Industrial Research (CSIR) and Agrément Board certification after exhaustive year-long testing of a 600m extended pavement.
Tiro Sechaba has now started a pilot project on a 2.6km section of road at the Fairview golf estate in Gordon’s Bay.
“Our CEO, Khutso Manthata, and business development manager Thato Keetse came across Mechanical Concrete on a visit to the USA and immediately recognised the potential for developing countries. The inventor was invited to visit and was satisfied with Tiro Sechaba’s credentials, which led to a partnership and licensing agreement,” says Jooste.
Mechanical concrete was invented by US civil engineer Samuel Bonasso and patented in the US in 2008. It was initially pioneered for the US Military, but once its performance had been proven, the extended benefits were realised. More than 1000km of road have been built over 15 years with extensive compliance testing and certification carried out by relevant US state authorities. The system is certified in six US states. Mechanical Concrete can be used where heavy and ultra-heavy vehicular traffic is experienced or where conventional reinforced surfaces result in regular failures and high maintenance.
Although expert supervision in overseeing construction is necessary, the technique requires only lower level contracting skills, which means road construction, particularly in rural areas, can be undertaken by emerging contractors with labour drawn from local communities.
“Adapting the technology was relatively simple and we calculate the cost per kilometre of a road constructed from Mechanical Concrete compared with conventional construction will see a cost saving of up to 30% – the cost at Fairview would be around R3 million/km for conventional as opposed to R1.9 million/km for mechanical concrete,” says Jooste.
He attributes this to a number of factors, including the elimination of about 80% of the layer works. This translates into a vast saving of materials and a reduction of construction time per kilometre – typically four weeks compared to eight in conventional construction.
How it works
The Mechanical Concrete system of road construction also promises a sustainable solution to the massive problem of scrap vehicle tyres.
Conventional concrete combines cement, water, sand and gravel aggregates. The cement and water undergo a chemical reaction that causes the mixture to harden, or cure; when poured into a mould the chemical reaction binds and integrates the whole into a desired shape of great compressive strength.
With Mechanical Concrete, a geo-cylinder offers mechanical confinement rather than chemical bonding or binding aggregates together. It functions as a lateral reinforcing element and shape retaining device. Together, the cylinder and aggregates form a three-dimensional mass capable of supporting and transmitting applied loads. The geo-cylinder is a term for one of the most common, highly sophisticated, yet wasted resource on Earth –truck and automotive tyres. Removing the sidewalls of the tyre and retaining the structural integrity of the wire and fabric contained within the rubber makes it a perfect device for containing aggregates.
To preserve overall structural geometry during stone-filling operations, each cylinder is attached to its neighbour with a simple mechanical fastener and once the honeycomb-like structure is built and filled with stone, the mass and internal friction maintain its structural geometry.
“We made some improvements and simplified the way the tyres are connected to each other by riveting, which is faster, and we developed a simple guillotine arrangement for slicing off the tyre sidewalls, which handles eight tyres in one pass,” Jooste explains. This is currently done in a depot in Pretoria where about two million recycled tyres are stockpiled. Tyres – now geo cylinders – are transported to construction sites. “The redundant sidewalls are supplied to Eskom in Middleburg as low grade power station fuel, so nothing is wasted,” he adds.
According to Jooste another advantage is the ability to use low-grade cement rather than proof quality Portland cement as a binder, which further reduces the cost/km without detracting from the quality and longevity of the finished roadway.
Around 10 to 16 car or truck tyres create one cubic metre of Mechanical Concrete and 10 000 tyre-derived cylinders can create a 3.5m wide, kilometre-long lane of roadway.
Typical road construction techniques rely on excavation of the proposed route to a predetermined depth, compaction of a sub-base, which may involve adding material, and then filling the excavation with layers of graded stone or aggregate, which is compacted with heavy rollers between each subsequent layer. This is known as ‘layer works’ – the size of the stone and the thickness of the layers are determined by the engineer in accordance with the required design specification. A top-wearing course of bitumen or tarmac is usually applied, with a specification determined by the road’s performance requirement, which is dictated by the anticipated volume and type of traffic. Cementitious or chemical binders are added to the layer works and the aggregate mass is contained within the edges of the route excavation.
With Mechanical Concrete, excavation and compacted sub-base are similar to conventional road construction, although the excavation depth is less because the honeycomb effect of the interconnected geo-cylinders filled with stone provides the road with stability and integrity by reducing the possible movement of the aggregate while allowing superior drainage. Binders are still added to the aggregates but conventional layer works are eliminated, save for a layer or ‘topping’ that can be a bitumen-wearing course or compacted fines in the case of a rural road.
Jooste says the tyre-derived geo-cylinders affordably create an almost indestructible base and eliminate most road maintenance problems, including potholes and ruts; road edge and ditch wall collapse; shoulder erosion; soft sub-grades; ditch and channel erosion and scour; slips and slides.
Value of scrap tyres
Scrap tyres are a global problem – in the UK alone 40 million tyres a year are scrapped, with only 10% being recycled.
“South Africa produces and imports nearly 240 000 tons of vehicle tyres annually, of which 170 000 tons become waste. Until recently only 4% of these were recycled. The Recycling and Economic Development Initiative of SA (REDISA) initiative has increased this to 19% and created 2300 jobs and 190 small businesses in collection and storage initiatives,” says REDISA director Stacy Davidson.
REDISA’s mandate is to create a tyre recycling industry and job creation opportunities, and encourage initiatives such as Mechanical Concrete. In accordance with the Department of Environmental Affair’s requirements, REDISA had to divert 50% of waste tyres as of the end of November 2015. REDISA’s Plan has met expectations and at the end of September 2015, 91 324 tons of waste tyres were diverted from landfill, overachieving the 2015 target by 5%.
Other uses for scrap tyres include playground equipment; matting products; and ground rubber crumbs melted into bitumen to create a rubber bitumen binder used in the manufacturing of asphalt, and chip and spray surfaces. These make but a tiny dent in the millions of scrap tyres stockpiled around South Africa, which in REDISA’s estimations amounts to between 60 to 100 million tyre carcasses.
“The widespread acceptance of Mechanical Concrete technology has the potential of being a major exit channel for scrap tyres, which can be put to good use in the development of the country’s infrastructure,” Davidson says.
Countries including the US and the United Arab Emirates offer to export their scrap tyres at a landed cost in South Africa of 50c/carcass. This adds to Mechanical Concrete’s viability as the essential geo-cylinder is available at minimal cost. REDISA supplies scrap tyres to Tiro Sechaba for free.
Early adopters curve
Many innovative products and processes come up against opposition from established players in any given industry, and Mechanical Concrete is no exception.
Manthata and Jooste say they have received enthusiastic responses from the professionals in consulting businesses and from potential clients who see their road maintenance rands stretching further for longer.
Established contractors, however, are less enthusiastic and express concerns over Tiro Sechaba’s exclusive rights to the technology, which requires them to become licensed for the process and pay fees. “We are engaging with the CIDB (Construction Industry Development Board) to assist with contracts between Mechanical Concrete and licensed operators,” says Jooste.
“This technology has the potential of opening the door to less skilled contractors, which is seen as a threat to established contractors’ profit margins due to greater competition. Less sophisticated equipment is required for Mechanical Concrete construction, which also lowers entry barriers, adding to conventional contractors’ concerns,” he adds.
Highlighting the potential application for this solution going forward, a 2013 South African Road Federation estimate stated South Africa had a combined surfaced and gravel roads backlog of some 203 393km. These roads fall into the condition category of poor or very poor, and have maintenance costs of some R197 billion. Hypothetically, South Africa’s road backlog could absorb about two billion scrap tyres by switching to mechanical concrete. Even attempting to demolish South Africa’s current tyre mountain of 100 million used tyres could result in more than 10 000km of roads being improved, with the added benefits of a substantially longer life and lower maintenance requirements.
By Robin Hayes
For the full article, see earthworks magazine Issue 30, February-March 2016.