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Meshing together

Glass fibre mesh plus highly polymer modified asphalt layers are pushing the boundaries of highway maintenance technology, NCE reports from the A1.

More from: Pass us the salt

Least disruption and best performance were key requirements of the Highways Agency when work began to repair a heavily used and well worn 4km section of the A1 in Cambridgeshire.

Cracking and movement of the concrete dual carriageway, plus reflective cracking through the concrete’s thin layer asphalt surfacing, had to be put right in the most effective way.

The principal problem was the site’s congested nature. The A1 at Eaton Socon is dual, two lane only with no hard shoulders. Installing contraflows is not a viable option.

This in effect ruled out full depth reconstruction, crack and seat of the concrete or very thick overlays, as any of these would have involved unacceptable lengthy possessions of the narrow route.

“At Eaton Socon in one stint we managed to put down 6,000m² of GlasGrid in only 50 minutes”

Howard Cooke, ARS

A solution that required only weekend working was sought and found. It was decided to leave most of the concrete in place but to cover the carriageways with a coated glass fibre grid reinforcement embedded within very high strength asphalt.

The asphalt specifi ed is bound with polymer modified bitumen (PMB), providing high durability and fl exibility, to allow − in conjunction with the glass fibre mesh − a structural overlay of only 110mm thickness. This minimalist form of reinstatement meant the contract could be carried out in just 16 weekends with comparatively little disruption to traffic.

“We’ve used glass fibre grid reinforcement before but not in such combination with PMBs,” says Highways Agency project sponsor Nick Cook. “We were anxious to keep full depth concrete repairs to a minimum and with this in mind, the chosen solution appears an effective way of restoring the carriageways.”

Complicating matters was the fact that the original concrete had been laid years before as part of concrete carriageway trials, with some bays containing reinforcement. “Certain sections were moving, others were not,” says Cook. The carriageways had received their asphalt thin surfacing in the early 1990s which, although cracked, “had lasted remarkably well. But the time had come for major repair”.

Chips being laid

Chips being laid

The Eaton Socon contract falls within the Agency’s East & South Eastern Asset Management Framework and the project was managed by one of the framework’s five principal contractors − Interserve Project Services. Outline design was commissioned from consultant URS, suitable proposals were sought from surfacing contractors and an early contractor involvement form of contract was used to refi ne the work.

Tarmac National Contracting obtained the surfacing contract with specialist subcontractor Asphalt Reinforcement Services (ARS) providing design guidance and installing its GlasGrid mesh. Before the mesh could go down, however, a considerable amount of preparatory work had to be carried out.

“We started on site early last September, with planing off old material and tackling the first of the FDRs,” says Interserve framework manager Steve Hall. FDR stands for full depth reconstruction, and relates to the areas of concrete slab too unstable to leave in place. Here the concrete was removed and replaced by asphalt. Hall calls himself a stickler for detail and has the site’s statistics at his fingertips. “There were 68 FDRs in all, amounting to 6,800m² in surface area, roughly 8% of the pavement reinstated. For the job overall, we carried out 87,000m² of surfacing.”

“There were 68 FDRs in all, amounting to 6,800m² in surface area, roughly 8% of the pavement reinstated.”

Steve Hall, Interserve

First of the asphalt to be applied was a 20mm thick polymer modified pad coat, or regulating layer, designed to iron out differences in level of the concrete slabs and provide a uniformly flat base for the GlasGrid.

Produced by Saint-Gobain Technical Fabrics in the USA, a GlasGrid has a minimum strength of 100kN and is specifically intended to guard against reflective cracking. It is self adhesive and although requiring skilled handling, can be laid at speed.

“Our proud claim to fame is that, at Eaton Socon, in one stint we managed to put down 6,000m² in only 50 minutes,” says ARS technical directorHoward Cooke. The material is laid by mechanical applicator. Altogether 38,000m² of GlasGrid was installed on the southbound carriageway, over two weekends, with 45,000m² going down on the northbound carriageway, over three weekends.

Asphalt reinforcement services

Among research tools used at Eaton Socon was computer software provided by Asphalt Reinforcement Services (ARS).

Anti-Reflective Cracking Design Software (ARCDESO) can be used to predict the growth of reflective cracking in asphalt; and through this, establish what is likely to be the best maintenance option.

ARS technical director Howard Cooke says that more conventional ways of analysing the right solution at Eaton Socon could result in an overlay of the existing concrete of at least 200mm and more. “ARCDESO pointed the way to an effective solution of nearly half that,” he says. “The new pavement at Eaton Socon is relatively low environmental impact, with a reduced carbon footprint.”

 

Strict rules apply to joints and overlaps. Once in place, the GlasGrid is sealed with a bond coat, again using polymer modified bitumen, and then chipped − that is, receives a layer of stone chips − to prevent asphalt supply vehicles sticking to the bond coat.

Then came the project’s tricky bit − the development, production and laying of a special 70mm deep asphalt binder course to add pavement strength, durability and flexibility.

“It is not by any means your usual form of asphalt layer,” states Tarmac’s technical manager Tim Smith. “We have in fact produced a polymer modified EME2 category asphalt, which sounds like a contradiction in terms, but isn’t.”

Creating the correct formula

EME2 is normally bound with conventional straight run bitumen which results in a stiff, high strength material. While looking to obtain an EME2’s level of strength, Tarmac was anxious to achieve good resistance to the propagation of cracks as well and that meant a degree of flexibility. “So we talked to our bitumen supply chain and came up with our new product.”

The specialist bitumen company that helped Tarmac with its product development for the innovative binder layer was Nynas. Nynas asphalt engineering support manager Jukka Laitinen explains: “PMBs can enhance the performance of an asphalt so that this displays better load spreading properties, firmer resistance to wheel tracking and crucially − in the case of Eaton Socon − a greater degree of flexibility. It is better able to accommodate movement from below, for instance.

“Polymer modified bitumens carry additional cost and traditionally in the UK have tended to be used in premium surfacing layers. There is a growing trend, however, because of the value they add in terms of stability benefits, to use PMBs in the lower levels of asphalt pavements. Again, Eaton Socon is a good case in point.”

“PMBs can enhance the performance of an asphalt so that this displays better load spreading properties, firmer resistance to wheel tracking and crucially a greater degree of flexibility.”

Jukka Laitinen, Nynas

Tarmac opted to use Nynas’ Endura Z4 polymer modified bitumen to produce what Smith believes is an outstanding binder layer for the A1. As a bespoke material, can it be deemed a one off, though? “No. If we have clients with similar needs we will use this particular asphalt again. It makes a very high performing layer for your pavement.”

The binder layer was subsequently topped with a 20mm surface course of Masterflex, Tarmac’s proprietary thin surfacing, again polymer modified. This time the bitumen supplier was Shell.

Surfacing work ended just before Christmas, with “mopping up” continuing into the New Year because of the snow. Weekend working meant working from Friday evening until Sunday lunchtime only, which makes the achievement of Interserve and its subcontractors all the more commendable. “We had other things to do apart from the surfacing,” says Hall. “These included raising 7,500 linear metres of perimeter crash barrier and 3,900m of double sided, central reserve barrier.”

The Agency’s interest in its newest stretch of rebuilt A1 will not diminish in the months ahead. Two control sections of asphalt have been laid within the length of the reinstated carriageway, one northbound, the other south. While made up of the same asphalt layers, these sections have no GlasGrid reinforcement. The object is to compare, over time, the performance of the reinforced carriageways with unreinforced.

It is also to see if reflective cracking matches that predicted by ARS’s design software. In general, the intention in installing GlasGrid is to achieve a greater than 90% crack free surface in the life of the new surfacing.

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