Archives: Case Studies

In 2008, the A45 near Northampton was resurfaced using a new asphalt technology designed to prevent reflective cracking – and, a decade later, the road surface is still performing well.

The product used is a high-performance asphalt stress-absorbing membrane interlayer (SAMI), which was developed by Tarmac to tackle the issues associated with concrete roads overlaid with asphalt.

David Markham, head of Asphalt Technology at Tarmac, explains: “There are more than 1,500 miles of concrete roads on our network, many of which have been overlaid with asphalt to provide a quieter, smoother, more skid-resistant surface.

“While they are durable, over time these asphalt surfaces can suffer from reflective cracking, caused by the concrete below, which ultimately leads to the roads failing.”

Causes of reflective cracking

Reflective cracking can be caused by a number of factors, including thermal movement, which occurs as the concrete slabs expand and contract horizontally. This is concentrated at the slab joints and is transferred to the asphalt layer above, causing ‘bottom-up’ cracks to develop in the surface.

Traffic loading can also have a significant effect: as heavy vehicles move across the joint from one slab to the next, vertical movement is created which puts additional stress on the asphalt overlay.

On the A45 site at Billing, the existing asphalt thin surface layer had suffered extensive cracking and been heavily over-banded to keep it serviceable. In 2008 this stretch was resurfaced using Ultilayer SAMI laid 25mm thick directly on top of the jointed concrete, using conventional paving equipment, with 50mm of high performance polymer modified binder thin surfacing on top.

The solution provided enhanced flexibility to accommodate movements in the jointed concrete, protecting the asphalt surface course, which is showing no sign of reflective cracking 10 years after it was laid.

Tarmac has worked with the University of Nottingham to measure its asphalt stress-absorbing membrane interlayer’s (SAMI) resistance to fatigue damage by running long-term studies on both strategic and local roads. The results have demonstrated that when SAMI is used to repair concrete roads the material’s fatigue resistance is over 200 times better than conventional asphalt.

Dr Nick Thom, Assistant Professor in Civil Engineering at Nottingham Transport Engineering Centre (NTEC) and the University of Nottingham, said: “The material that Tarmac has developed has a remarkable resistance to fatigue. As part of an overlay system it presents a very effective barrier to reflective cracking, most especially thermally driven reflective cracking, leading in many cases to a considerable enhancement in road life.”

Recent developments in both mobile asphalt heaters and materials have enabled the mixing of small quantities of asphalt at the point of use for minor highways defects and utility reinstatements.

This history of small hot asphalt mixers dates back to 1999, when Leicestershire County Council first approached RSL Fabrications to manufacture a machine that could be used to reheat pavement breakout material for re-use as a base layer on a housing estate. The innovation was a success and led to the development of larger static machines for this purpose.

Latterly there has been increasing interest from Tier 1 contractors to develop smaller mobile machines. Amey, for example, recognised the value of this technology to overcome the practical and commercial issues relating to completing small volume one-time hot permanent repairs for minor highway defects and utility reinstatements.

Traditionally this involves collecting small volumes of asphalt from permanently located asphalt plants, often far from the site, leading to significant costs and material wastage.

The use of asphalt heaters provides the ability to manufacture the exact volume of fresh, highly workable asphalt required 24/7 and at the point of use, overcoming out-of-hours issues and reducing costs and wastage.

In late 2015, Amey introduced the first prototype machine into its utility business in the North West. The asphalt heater provided a 250kg batch cycle capacity and was fed with pre-packed asphalt. Having proved the concept, Amey then worked closely with the manufacturer to improve the design and capability of the machines. During 2016, additional 250kg machines were added to the fleet, along with larger 500kg and 1,000kg versions.

David Ogden, Business Director at Amey, said: “This is exciting new technology, which we have developed with RSL Fabrications, and which we are now using on many of our highways accounts.

“Because we are no longer relying on asphalt plants to be open to get our supply of material, we are also now able to work during antisocial hours, which is good news for our customers as it means less disruption during the working day.”

While Amey optimised mixer performance, it also realised the importance of optimising the asphalt mix behaviour and availability of suitable product for its expanding fleet of asphalt heaters. At this stage Tarmac became involved and, using its in-house R&D facilities and expertise, began to formulate a material that was equivalent to bulk delivered asphalt, compliant with the same specifications and overcame concerns with regard to any potential detrimental impact on the bitumen during onsite reheating of the bagged asphalt.

Tarmac embraced the initiative and the R&D process moved swiftly from concept and laboratory trials to full plant manufacturing at its Mancetter asphalt plant in Warwickshire, in conjunction with the Amey mixers, in a matter of a few weeks. Having demonstrated that the asphalt met all the required criteria, both as a bulk asphalt and in 20kg bags, network trials began in early 2017.

“In spring 2017, we launched the pre-packed Ultipatch Sitemix range, which includes asphalt concrete mixes, SMA and HRA Surface Course,” said Brian Kent, National Technical Director at Tarmac.

“The asphalts are now in use on highway and utilities contracts, in conjunction with RSL and Roadmender mobile heaters and we have completed network trials with Roadtech’s low volume mixer as well.

“The introduction of this innovative approach to asphalt has opened up a new way of working to the industry and has demonstrated the value of supplier/contractor collaboration in terms of tackling an issue effectively and quickly.”

Hanson spreads the benefits of warm asphalt

Use of warm asphalts promises environmental benefits of energy saving, lower emissions and enhanced durability and to this list can now be added technical advantages, including extended compactibility in cold weather and quicker reopening of roads to traffic.

Hanson is one contractor which has taken these technical advantages on board and has used warm asphalt to reinstate one of Cumbria’s busiest roads; the A66 at Crackenthorpe, west of Appleby.

“Carriageway closures were restricted to night time during last winter,” said Hanson North Technical Services Manager, Aggregates and Asphalt, Jon Sharp. “We had to get in at 19.30, complete the planing out and replacement work, and get away for traffic to be running by 08.00 the next morning.

“Time was short considering the amount of work we had to do each shift. We needed materials that could be compacted well despite the cold temperatures and which would harden rapidly. Our client Kier (contractor to Highways England) agreed to the low temperature solutions we proposed.”

The challenge was to lay two layers of base course at 90mm each plus a binder course of 70mm, adding up to a total of 250mm, plus a Tuffgrip thin surfacing of 40mm, all between 22.00 (after planing) and the morning cut off point.

Hanson proposed lower temperature asphalt for the base and binder courses. Use of a warm asphalt containing Nynas Nypave PX50 allowed the asphalt to be mixed at a lower temperature (140^oC, providing a 30^oC reduction).

This rapid turnaround meant Hanson could lay the subsequent layers sooner and to the right level of compaction. “Nypave PX50 allows for lower temperature compaction and rapid hardening of the asphalt which, in turn, permits two to three layers to be laid at night, in the cold, in short order,” says Nynas Asphalt Support Engineer Jukka Laitinen.

The asphalt – supplied from both Hanson’s Shap and Keepershield asphalt plants – proved highly suited to the job, even though subsurface conditions were less favourable than expected; and the contractor found itself planing out to a greater depth and having to install not three, but four layers, of base and binder.

Due to the issues relating to the lower levels, the programme was revisited. More time was provided for laying, meaning that speed of completion for the base and binder layers was no longer critical. This meant Hanson could move to a conventional 40/60 binder in its asphalt. However, it stayed with Nynas Nytherm PMB 75 bitumen for the A66’s 40mm thick Tuffgrip surface layer. Specially modified, the binder allows production of high performance asphalt mixtures at lower temperatures, which display enhanced compactability in cold weather. Hanson uses the product in summer too, for its early opening potential, as well as its environmental credentials which make it an obvious option when sustainability is on the agenda.

State of the art new asphalt plant near Bristol:

Wainwright has opened a new £9 million asphalt plant in Avonmouth, Bristol, to extend its reach across the South West.

Construction of the 4.5 acre site was completed earlier this year and the plant will have the capacity to produce 240 tonnes of asphalt per hour when fully commissioned.

The asphalt plant is one of the most advanced in the country and features a range of environmental and technological benefits.

These include the use of natural gas in the burner, reducing the amount of CO2 produced, and an integrated recycling facility which allows the input of two different grades of RAP (recycled asphalt planings) into the asphalt mix.

The site also benefits from solar PV panels, reducing grid electricity consumption by up to 30 per cent during peak production and up to 100 per cent at low energy demand. It features LED lighting to reduce power consumption and rainwater harvesting is used for site and equipment cleaning.

In total it is estimated that the sustainability features at the plant will save more than 200,000 tonnes of CO2 emissions each year.

Peter Barkwill, chief executive at Wainwright, said: “The new facility is a bold move for our 125-year-old quarrying company and we believe now is the right time to invest and build on our reputation for quality, service and efficiency.

“The site is ideally located within one mile of the South West motorway network giving us access to a range of additional markets throughout the region. Our new plant is by far the largest single investment the company has made and we are particularly pleased with the sustainable aspects of its design, which dovetail with its location on the edge of Bristol − which was the first UK city to achieve European Green Capital status.”

Driving resource efficiency

The construction industry is facing increasing pressure from policymakers to become more resource efficient due to it being both the UK’s largest industrial producer of waste and largest consumer of natural resources. According to the UK Green Building Council, the sector uses more than 400 million tonnes of material every year and both the EU’s Circular Economy Package and the London Circular Economy Route Map (a scheme led by the London Waste and Recycling Board) highlight that construction’s support is critical to making the shift to a circular economy – a concept which aims to maximise the lifecycle of resources as much as possible. 

Highways and infrastructure construction is no different when it comes to the demand for materials.  Thanks to initiatives such as Highways England’s £15.7 billion Road Investment Strategy, significant improvements are planned for England’s strategic roads over the next five years which will require the sector to supply huge amounts of finite resources. This renewal programme also gives the highways sector a unique chance to lead the wider construction industry in the push to adopt a more sustainable approach to materials.

“Much of the inherent value of Britain’s road network is in the billions of tonnes of asphalt that make up the surface of its highways,” said Tim Metcalf, aggregates and asphalt director at FM Conway. “Asphalt is 100 per cent recyclable and, with over 50 per cent of the strategic road network likely to be resurfaced by 2020-21, vast quantities of the material could be recovered for reuse.”

While it is not unusual to incorporate 50 per cent recycled content asphalt into the base and binder courses of roads, the recycled content of surface course mixes is traditionally limited to around 10 per cent.  This is mainly due to concerns that high recycled content asphalt cannot provide satisfactory skid resistance for strategic routes: doubts that are being addressed through a trial with Transport for London (TfL).

FM Conway is using an asphalt surface course containing 50 per cent recycled aggregate on the A1 in Mill Hill, which will be compared against primary aggregate asphalts on an ongoing basis to monitor its durability and skid resistance.

“We’re confident the results will show that high recycled content mixes can provide both an environmentally sound and safe way of maintaining the nation’s strategic roads,” added Tim. “Change won’t happen overnight, but we hope that trials such as this one will encourage others to reassess the role that recycled asphalt can play in road maintenance plans.”

Thin layer surfacing adds steel to bridge’s asphalt:

A high performance steel slag based asphalt has been used to replace the surface of St George’s Bridge in Doncaster. The 620 metre dual-carriageway bridge, built in 2001, is a crucial gateway into the town centre, carrying around 44,000 vehicles a day. A refurbishment project for Doncaster Council included replacing corroded movement joints, bridge drainage works and allied junction improvements as well as the carriageway resurfacing.

The decision was taken to move away from the previous porous asphalt surface as its gradual deterioration in performance contributed to the corrosion of the original movement joints. Instead, a dense binder layer was chosen topped with a special thin surfacing material that displays both enhanced durability and high resistance to skidding.

“Steelflow is a mixture of 6mm steel slag aggregate and a tough, but very flexible, polymer modified binder,” said the asphalt producer Steelphalt’s Commercial Manager Dean Raynor. “We receive and process steel slag from a number of different steelworks, turning the material into a durable aggregate equivalent to stone with a PSV of 63.”

Bitumen supplier Nynas worked with Steelphalt to design Steelflow, which uses the company’s Nypol S89 polymer modified binder to provide enhanced flexibility and resilience.

St George’s Bridge is the first time Steelflow has been used on a major elevated structure, a decision supported by transport research consultancy TRL.

TRL’s assessment included visual inspections of in service sites, skid resistance surveys using SCRIM and laboratory testing. The in service sites assessed were in Rotherham (laid 2011), Flouch and Dodworth (both laid 2013).

“Visual inspections found the materials to be in good or excellent condition,” says TRL Senior Researcher in technology development, Stuart McRobbie. “SCRIM surveys showed that the skid resistance demonstrated by 6mm Steelflow was higher, by a significant margin, than the Investigatory Level (as per the Design Manual for Roads & Bridges) for any of the site categories on which it was laid.”

The work to remove the porous asphalt and resurface the bridge deck with around 15,000m² of new material was carried out at night to minimise disruption. The new surfacing comprises 50mm of conventional binder course and 20mm of Steelflow to provide impermeability, durability, longevity, ride quality and resistance to skidding.

Jukka Laitinen, Nynas Asphalt Engineering Support Consultant, said: “Long bridges by their nature can move up and down under load, which can challenge conventional asphalts. Our binder is engineered to ensure that Steelflow displays both extra resilience and enhanced flexibility, making it an ideal choice for St George’s Bridge.”