Crack and seating methods are being brought to Australia after a long history in the US and elsewhere. What are they and when are they useful?
In July a Badger vehicle began its work, making its way along part of the M1 Pacific Motorway. The much-used road already sees a good deal of punishment from travellers, with an estimated 70,000 vehicles going between the Central Coast, the Hunter and Sydney every day.
The Badger will undertake in situ demolition of concrete pavement between the Tuggerah and Doyalson interchanges, where the motorway is being widened to provide three lanes in each direction. The Badger rambles along at a human’s walking pace, pounding the concrete slabs with a dozen 550-800kg hammers. It will break pavement into pieces, producing “a well graded, rubblised material that is substantially debonded from any reinforcement,” explains the company that has brought the machine to Australia.
There is great potential to put the multi-headed breaker to work in “crack and seat”-type applications, where a slab is broken and made to sit better under the road. Following this, a roller makes sure the resulting mini-slabs are fitted together snugly, and the road is then re-covered with asphalt.
Cracking and seating “was not used extensively before the 1980s” notes a paper from late in that decade by the US’s National Cooperative Highway Research Program. Fractured slab methods (to which we can add rubblisation and seating) have become well established in the US and elsewhere in the 1980s and beyond, though Australia has been a slow adopter.
“It’s new technology to Australia, but it’s not new. That’s a key point; it’s been proven overseas,” Marty LaNauze, Business Manager at Concrete Pavement Recycling, tells inspire.
“It’s been used overseas extensively for a number of years, but we’re the first ones to put our money where our mouth is and bring a machine over.”
Making a road last longer
Methods of cracking, breaking, rubblising and then seating allow a portland cement road base to become semi-flexible and able to better resist reflection cracks. Reflection cracks can happen when the slab moves and the asphalt can’t properly match it. This can be in the case of thermal shifts, slabs moving excessively or excess loads passing over the top of them.
The Asphalt Institute explains the relationship between temperature shifts and these cracks: “Cooling causes the opening of joints and cracks and results in tensile stress developing in the asphalt overlay above these openings.
“When the stress exceeds the tensile strength of the asphalt overlay a reflection crack forms. The key to delaying or eliminating reflection cracking is to minimise or eliminate the stresses and deformations developed in the overlay.”
It’s also noted that reflection cracking can be prompted by horizontal strains (from thermal shifts) and vertical strain (from wheel loads).
The methods of combating cracks in overlay by creating them in the slab can be broken into three categories. These are cracking for plain (unreinforced) concrete. Then there is breaking for reinforced pavements, and which requires more fracturing energy. Then there is rubblising, which can be applied to either.
According to the article mentioned above, crack and seat requires slabs to be broken into pieces 1.5 to 3 feet (approximately 500cm to 1m) in length and break and seat 12 to 18 inches (300cm to 500cm). Different suggestions appear elsewhere, such as in the guide cited in the sidebar. Each method can use a pneumatic roller (for cracking and breaking) or a tandem vibrating roller (for rubblising) for seating.
A 1982 National Asphalt Pavement Association-sponsored review in the United States describes the tradeoff involved: “The smaller the cracked pieces, the more the potential for reflective cracking is reduced, but also the more the structural strength of the concrete pavement is reduced.”
“It’s really for where concrete pavements are getting toward the end of their design life and they are starting to fail and break down,” says LaNauze of where the method makes the most sense.
Bringing back a breaker
It wasn’t until 2014 that the appeal of these technologies found a serious Australian audience.
An Australian Asphalt Pavement Association International Knowledge Study Tour to the US exposed visitors to best practices in pavement engineering. Delegates Tony Wehl of RPQ and Nick Argyropoulos from NA Group saw the potential to bring the methods back home, and launched joint venture Concrete Pavement Recycling (CPR) the next year at the International Flexible Pavements conference.
CPR received an MHB Badger Breaker in June from Antigo. According to the company, there are 50 Badger Breakers in operation worldwide.
Of the Breaker and what it offers, LaNauze explains that “it’s not a silver bullet, it’s just another tool in the chest to be able to provide a good value for money solution.”
LaNauze says that it offers advantages such as speed over hammer guillotine machines – covering about 8000 rather than 500 to 1000 square metres per day – and allows breakout concrete to be kept in place if a designer wants that.
“Environmentally, there’s no material to truck out and recycle: it all stays in situ. There’s no trucks required to bring new base materials then. Everything that’s there gets recycled in place,” he says.
“The old concrete layer becomes the new sub-base,” adds Joseph Grainger, Vice President and General Manager at Antigo International.
Antigo has been involved in pavement rehabilitation since 1982, using a Wirtgen guillotine machine for crack and seat and removal/recycling. It formed Badger State Highway Equipment Inc in 1988 to manufacture its own equipment, and later introduced multi-headed machines.
“The MHB Badger Breaker was developed in 1995 as a multi-head breaker optimised for concrete pavement rubblising as the demand for that fractured slab technology began to increase,” Grainger explains.
“The MHB has been modified over time and is now widely used to rubblise, crack and seat, and break concrete pavements for removal.”
Overseas demand led to the forming of Antigo International in 2002, and it serves Europe out of a UK depot.
“They have also been sent to many countries outside of the North American and European markets including India, Turkey, Saudi Arabia and South Africa, amongst others,” Grainger says. The domestic US market currently sees significant demand for rubblisation work.
Grainger says the multi-head machines are very different from guillotine-style hammers, which are not as well suited to urban environments. Guillotine-style units are mostly required for airport work, and are “used for concrete typically 300mm thick or greater.”
“Airports are typically where thicker concrete shows up so this is a natural market for this type of machine,” he adds.
Airport environments are also better than urban areas at tolerating the considerable vibration and noise these generate. One 2015 paper published by the California Department of Transport warns that crack and seat work “can cause damage to surrounding structures in close proximity to the roadway, especially shallow culverts and buildings with eligibility for the National Register of Historic Places.”
The multi-head breakers are a much gentler affair, with smaller hammers producing lower vibration levels. Noisewise, LaNauze says the output is 98 decibels, comparable to a cold milling machine.
As for where crack and seat will prove useful, the many kilometres of ageing concrete roads, cost savings, and environmental benefits all point to massive application possibilities, he believes.
“It’s really for where concrete pavements are getting toward the end of their design life and they are starting to fail and break down,” LaNauze says.
“And there’s a lot of pavement in New South Wales, but also in Queensland, that was made mid-last century and in the 70s and 80s. When those pavements were put in they weren’t designed for the long haul. They weren’t built to take the types of vehicles and the weight of vehicles that are on them at the moment, but also the volume of traffic.”
“There is a lot of pavement that is getting toward the end of its design life and we needed a way to treat that pavement; we can’t just leave it and hope for the best or rip it all out and replace it with new stuff.
“We need to be looking at the shortages of materials coming up. There is a big drain on resources at the moment with all this new infrastructure, so we should utilise the resources we have already got and that the state road authorities have already paid for.”
This story first appeared in the July edition of inspire magazine. Read the original and more here.