Worth one's salt


Severe weather has seen an increase in the amount of salt we put on the highways, but what if we could use less – and achieve more?

There is increasing pressure on winter maintenance managers to keep roads open, free of snow and ice, 24 hours a day. This is no mean feat, given that winter maintenance is not an exact science, and operatives are very much at the peril of Mother Nature.

These managers’ only defence is the increasing knowledge and technological advances in winter maintenance techniques. It is a learning curve and, with each passing year, new legislative challenges and policies must be met in order to make roads much safer places on which to travel. In order to meet this challenge in the winter period 2003/2004, Dudley Metropolitan Borough Council sought an alternative solution to traditional maintenance practice. A new salt-additive was introduced.

The council prides itself on its forward-thinking, proactive approach to winter maintenance. This, coupled with its environmental ethos, has led it to set an example in one area of an industry which has seen relatively little change in the last 50 years.

So, last season, the authority ordered 200 tonnes of Safecote in order to conduct a trial. Dudley’s winter-maintenance operations team covers 352 km of roads. It uses a fleet that consists of two Whale Blazers and seven Econ spreaders, although John Crowther, the council’s purchasing manager, comments that the team is expecting a fleet of new Econ’s machines ‘with all the bells and whistles’ next year. When using Safecote, the team adjusted the gate head on the gritters, lowering them to slow the amount of treated salt coming out.

‘The product was a lot more free-flowing,’ explains Crowther. ‘When we were spreading, the treated salt didn’t bounce as much and adhered to the road surface. Due to this, we were achieving the same target spread with less product. It meant we could plan longer routes. ‘Unfortunately, due to the age of the gritters, we were limited in the alterations to settings that we could make, although next year, we will have much more flexibility to get the optimal performance,’ he adds.

Safecote is a blend of raw materials, engineered by United Molasses, a subsidiary of Tate & Lyle. It goes through a number of mixing processes so that its manufacture meets a consistent specification. Dr John Higginbotham heads the product’s research and development.

He explains that Safecote is a formulation of soluble carbohydrates, organic acids and naturally-occurring mineral ions. ‘The resulting high concentration – more than 60% solids – and variety of small organic and inorganic molecules, significantly reduce the freezing point of water by their “colligative properties”,’ he says.

Dudley, like other councils in the area, subscribes to the Met Office for weather forecasting services. It also uses road temperature sensors as part of the process that determines when it needs to go out and salt. Precautionary salting is carried out at 10g/m2, which is generally recognised as the industry standard when severe weather conditions are not expected.

However, as Mark Dutton, Safecote’s sales director explains, the addition of the product does not mean less than 10g/m2 can be spread. ‘It is important we separate the issues,’ he says. ‘We must always achieve 10g/m2 when precautionary salting, the code of good practice states this is our requirement.’ Instead, he says, the use of the product means gritters need to be recalibrated, ‘otherwise users will achieve 13-14g/m2 when precautionary salting. This will enable them to achieve a much higher g/m2 in the target zone. They can then recalibrate to use less material and still achieve the desired g/m2.’ Like many UK depots, Dudley has no salt barns and all its material is stored outside.

‘We used Safecote-treated salt all winter and stored the material outside as we do with normal salt,’ Crowther says. ‘We didn’t see any adverse effects in the stockpile.’ However, after heavy rain, Crowther says there is a small amount of leaching from the stockpile.

‘No more than with salt, it was just more visible due to its brown colour,’ he says. ‘Operationally, we had no problems with outside storage.’ Safecote is applied to de-icing salt at 3% weight – 22.2 litres per tonne – using a high-pressure spray bar system to achieve a fine mist at the desired dosage rate. It encapsulates each grain and enhances the salt’s life by protecting it over a longer period of time, which allows the salt to reactivate in second and third phase incidents. As well as protecting the salt in storage, this encapsulation also mitigates the chloride in the salt attacking and corroding metal structures. Higginbotham points to corrosivity trials at CAPCIS (UMIST) and TRL.

He explains that the carbohydrate type used and the optimisation of the concentration is why there is a reduction of more than 80% in the metal corrosion traditionally attributed to chloride-based de-icers. ‘This inhibition has been achieved by the directed “complexation” of chloride ions to greatly inhibit their migration to metal cathodic surfaces. Furthermore, Safecote biologically consumes the dissolved oxygen in water present on metal surfaces or in concrete, preventing metal oxidation to rust or crumbling of concrete,’ he claims. ‘We’re also not washing down our gritters, following Safecote’s advice,’ Crowther says.

By not washing down, it leaves a brown residue, which acts as a corrosion barrier between the salt and the gritter body. After Dudley’s initial 200 tonne trial, it ordered only Safecote treated salt for the rest of the season for all of its seven routes. Its biggest test, however, came in January ‘when we had all the snow,’ Crowther comments. He says the difference was more than noticeable. ‘When using it in severe conditions, it worked very well. It reacted quickly and I was very happy with its performance.’ As a result, the council plans to restock with the product for the upcoming season.

But in terms of the research with Capcis, and the corrosion reduction which is claimed, he adds: ‘We will need to wait and see. There have been no adverse effects so far on any of our machinery or bridge decks and, I believe, we will see the benefits soon. ‘Again, TRL’s findings with regard to less damage being caused to the highways surface, we haven’t seen any adverse effects but it is too early to say if it has been less damaging. We will have to wait and see what happens over the next season or two to make comments.

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