Brine Spreading Trial

Publication type:Report
Published date: 25 October 2011

Authors: TRL - M Evans, R Jordan and T Rasalingham
Document Reference: 510390
Assessment Level: Definitive evidence
Topics: No related topics


Executive Summary

Background

As part of a project to investigate benefits of (i) pre-wetted salting with uniformly graded salt and (ii) liquid (brine) treatments, TRL prepared a business case which examined the economic viability of using brine treatments on the Highways Agency’s motorway and trunk road network (Jordan, 2011).

The analysis considered the savings that might be obtained when whole routes are treated with brine rather than pre-wetted salt, and estimated the potential savings from the following options:

  • Pre-wetted salting replaced with brine treatments using brine spreaders
  • Pre-wetted salting replaced with brine treatments or pre-wetted salting using combination spreaders

The business case concluded that there could be operational and financial benefits if the Agency had some spreaders capable of spreading brine over whole routes.

With brine spreaders, it was found that there may not always be direct cost benefits, but the overall cost benefit to society could be high when indirect costs (i.e. the cost of corrosion to vehicles and structures and environmental damage) were included in the analysis. However, the overall cost benefit was estimated to be much higher with combination spreaders capable of spreading brine in addition to dry, pre-wetted and treated salt.

For this reason it was concluded that combination spreaders were the most suitable for brine treatments over whole routes, particularly where there was a significant risk of heavy icing and snowfall.  The Agency is currently replacing its winter fleet and there may be an opportunity to purchase some combination spreaders to enable the brine treatment of whole routes in addition to pre-wet spreaders as part of the replacement procurement process.

To undertake the business case it was necessary to make a number of assumptions regarding, for example, the loss of brine and pre-wetted salt due to trafficking, the minimum coverage of de-icer across the carriageway, and the performance of spreaders.  These assumptions were based on previous research by TRL, a comprehensive review of relevant literature and discussions with winter service experts.  However, it was clear that the values assigned to some of the parameters in the analysis had an important influence on the outcome of the business case, it was therefore important that a further programme of research was undertaken to check, and if necessary modify, the values used in the analyses.

Brine treatments were suggested for slip road areas that are lightly trafficked, especially in low humidity and low temperature conditions.  They were also suggested for Managed Motorway hard shoulders as these must be free of ice before being opened to traffic but they are without the benefit of trafficking that encourages the dissolution of solid de-icers prior to opening.  The treatment of narrow spread widths (e.g. hard shoulders and slip roads) with brine using the current pre-wet spreader fleet would reduce the expenditure on spreaders for brine treatments.  Therefore, if the efficacy of such treatments could be proven, it would be possible to introduce them across the whole of the Agency’s network.

Recommendations for further work were contained in the business case (Jordan, 2011), and TRL was subsequently commissioned by the Agency to investigate these key areas.  The work undertaken is described in this report.  The work programme included:

  • Performance trials on a spreader with a brine spreading capability to determine the brine distribution profile immediately after spreading, including the minimum spread rate and maximum spreader speed that can provide a sufficiently uniform distribution
  • Road trials with a spreader with a brine spreading capability to investigate residual salt levels of brine after trafficking and further investigation of trials reported in the literature
  • Trials to determine the effect of traffic adjacent to the spreader when spreading brine
  • A survey of the saturators and the salt types being used by Service Providers to produce brine for pre-wetted salting, including an assessment of whether brine can be produced efficiently and cost effectively with brown salt from UK sources
  •  Identification of the routes most suitable for combination treatments
  •  Trials to investigate the feasibility of spreading brine using the pre-wet spreaders from the new winter fleet, to determine if they can be used to treat slip roads and the hard shoulders of Managed Motorways.   (Based on the results of this work, guidance for Service Providers has been developed for inclusion in the Network Management Manual)

A brief summary of the findings of the research is given below.  Based on the findings of the work, some of the assumptions in the original business case were modified.  The findings of the revised business case are presented in this report and are summarised at the end of this Executive Summary.

Performance trials on a brine spreader

Trials were carried out on the TRL Test Track to determine the brine distribution profile immediately after spreading with a brine spreader.  The brine spreading mechanism on the brine spreader used was the same as that on combination spreaders supplied by the same manufacturer.  The main aims of the trials were to determine:

  • The brine distribution when spreading a three lane motorway symmetrically from Lane 2 and a two lane road asymmetrically from Lane 1
  • Whether brine treatments can be carried out at a faster speed than 64km/h
  • Confirm brine can be spread effectively at a minimum spread rate of 10g/m² as assumed in the business case

The trials were carried out on both 14mm stone mastic asphalt (SMA) negatively-textured thin surfacing and hot rolled asphalt (HRA) surfacing.  The volumetric patch texture depth of the SMA was high at 2.5mm.   A proportion of the brine spread on the negatively-textured SMA could not be collected using TRL’s wet wash vacuuming method.  A correction factor was applied to the results from the performance trials to account for the under recovery and determine the brine distribution after spreading..

The trials showed that brine can be spread evenly to the lanes when two and three lane spreading with a brine spreader.  However, the distribution profile was not uniform across the target spread width and there were distinct dips in the profile at the edges of each lane.  The brine was targeted within the lanes effectively with very little wastage (<10 per cent) and it is anticipated that the distribution could be better optimised for brine spreading by adjustments to the nozzle alignment/position.

Comparison of the results from trials with the same spread rate and spread width has clearly demonstrated very similar distributions when spreading at 64km/h and 80km/h.  There was no significant difference in the uniformity of the brine spread across and along the carriageway at the two speeds.  The jets of brine spread to the outer lanes appeared to be disrupted more by the increased spreading speed, however this was not seen as detrimental to the final distribution.  The results therefore showed that spreading can be carried out at 80km/h as effectively as at 64km/h.

There was no evidence of any significant change in the uniformity or targeting of the lanes when spreading at a target spread rate of 10g/m² compared to the higher rates, demonstrating that brine can be just as effectively spread at this rate.

Road trials to investigate residual salt levels after trafficking

Based on previous work in Europe by Bolet (2008), a 15 per cent loss of brine after two hours of trafficking had been assumed in the original business case prepared by TRL.  In order to investigate this assumption on the type of road surfacing found on the Agency’s network, two road trials were carried out on the M3 in Hampshire to measure residual salt levels after two hours of trafficking after spreading.  Lane 1 of the trial site was surfaced with a negatively-textured thin surfacing with a volumetric patch texture depth estimated to be 2mm and Lane 2 surfaced with HRA.

Using the same brine spreader as used in the performance trials, brine was spread at 80km/h with the road open to traffic.  The target spread rate was 20g/m² with spreading from Lane 1 across both Lanes 1 and 2.

The brine collected in each lane in the road trials was compared with the brine distribution measured in the performance trial, which was assumed to be the initial distribution on the road.  The percentage loss in Lane 1 (negatively-textured thin surfacing) was estimated to be 53 and 59 per cent in Trial 1 and Trial 2 respectively.  For Lane 2 (HRA surfacing) the loss was estimated to be 36 and 44 per cent in Trial 1 and Trial 2.  For the lanes overall, the average loss was very consistent between the trials at 48 per cent in Trial 1 and 49 per cent in Trial 2.

The strip to strip variation in the brine collected was very low, the maximum minus the minimum amount collected from the strips being 26 cent of the average in Trial 1 and 12 per cent in Trial 2.  This is lower than the 30 per cent measured in road trials carried out by TRL with 6.3mm pre-wetted salt (Jordan et al, 2011A)

Averaged for both trials, the loss for the negatively-textured thin surfacing was 56 per cent and for the HRA surface 40 per cent.  The losses in these trials were therefore significantly greater than the 15 per cent loss for brine measured in the previous studies and which was assumed in the original business case for brine spreading.

Trials to determine the effect of traffic on brine distribution

Observations of driver behaviour during day-time liquid spreading (with potassium acetate that is considered to be representative of brine spreading) were taken when the weather was very poor with heavy snow/sleet.  Drivers appeared unconcerned with the spreader and overtook without hesitation.  Due to the poor visibility conditions it is likely the drivers were less aware of what was being spread than they would be on a clear day.  The effect of the traffic on the de-icer distribution would likely have been minimal in this scenario.

Observations during night-time spreading were when the liquid being spread was more visible and the drivers showed more awareness of the spreading.  Drivers appeared to be more hesitant on approaching the spreader, in particular when not following another vehicle.  Drivers were often observed to hang back before over-taking, although not in the immediate area that the de-icer was being applied to the road surface.  Once passing through the spray, vehicles tended to pass quickly and typically passed through within three seconds.  When the spreader was spreading Lane 1 and Lane 2 of a four lane section, drivers would typically move in to Lane 3 to avoid the area being sprayed.

Survey of saturators and salt types used by Service Providers

A survey of saturators used on the Agency’s network was carried out to determine the types of saturator and brine salt being used by Service Providers to produce brine for pre-wetted salting and obtain information on their operational experiences.  The survey found that the current brine production capacity for pre-wetted salting is mostly sufficient for combination treatments.  However, production reliability has yet to be fully tested as the majority of the saturators have been operational for a maximum of two winter seasons. Also, the use of brine has been limited because many treatments last winter were with dry salt rather than pre-wetted salt.

The increased use of saturators for combination treatments will increase the likelihood of failures and problems occurring and hence likely result in additional maintenance costs.  However, the cost of cleaning the saturator and removing insolubles is likely to be proportional to the amount of brine produced.

Although brown rock salt is low in cost and generally easily available, its cost effectiveness as a brine salt will be affected by the cost of extra maintenance required to remove impurities from the saturator.  Most saturators are designed for use with purer white salt and the manufacturers recommended that only this type of salt be used for brine production.  The experience of Service Providers using saturators that can use brown salt is insufficient to recommend the use of brown salt and discussions with salt suppliers have indicated that the production rate would be reduced by the need to remove impurities on a regular basis, in particular if removal has to occur during a period of use  No data have been found that would suggest that the use of brown salt is cost effective.  Therefore, white salt is recommended for brine production.

The potential for brine from industrial processes to be supplied directly to Service Providers was investigated.  It was thought that continuity of supply may be a problem and much storage may be required at depots to ensure sufficient brine was available at times of high demands.

Revised business case

In revising the business case it has been assumed that a combination spreader would be available to make brine treatments or for pre-wetted salting.  Brine spreaders have not been considered because the previous work by TRL shows they are not economically viable on the Agency’s network under current circumstances.

Four revised factors have a significant effect on the business case:

  • The brine loss due to trafficking
  • The cost and type of salt that can be used for brine production
  • The minimum spread rate for pre-wetted salting
  • The number of saturators used for brine production

The factor that has the greatest effect on the revised calculations is the brine loss due to trafficking.  In the revised business case, the loss of brine due to trafficking has been increased from 15 to up to 40 per cent for brine treatments on typical surfacing.  Losses of 30 and 50 per cent have been used, respectively, for denser and more open textured surfacing than typical surfacing.

The saturator survey found that brine production with indigenous UK brown salt is not likely to be economically viable largely due to the additional maintenance costs.  Therefore, it has been assumed that white salt would be used for brine production.  However, two severe winters in recent years have increased the demand for imported white salt and fuel costs have also risen, resulting in an increase in the cost of white salt for brine production.  When the higher brine loss due to trafficking and the higher cost of salt are taken into account, it is estimated that there would be very few weather conditions where the direct cost of brine treatments would be less than that of pre-wetted salt.  The number of weather conditions where brine treatments would be the best option would be slightly higher if the indirect costs due to vehicle and structural corrosion and environmental damage are taken into account. However, the calculations only considered the number of weather conditions where a direct cost saving to Service Providers would be obtained for brine treatments.

The original business case also considered the use of white salt for the dry salt component of pre-wetted salt.  For these calculations, it was assumed that white dry salt would be used only if it enabled the minimum spread rate at which pre-wetted salt could be spread uniformly to be lower if the dry salt component was white salt rather than UK brown salt, i.e. where the use of white salt would reduce the amount of salt required for pre-wetted salting.  However, a number of Service Providers have obtained supplies of imported white salt during the recent salt shortages and this might continue if UK supplies cannot meet current demands.  In addition, a minimum spread rate of 8g/m2 has been adopted on the Agency’s network for pre-wetted salting with brown salt, thereby reducing the potential de-icer cost savings when brine treatments are made, since the business case assumed a minimum of 10g/m2.  Both of these factors have been taken into account in the revised calculations.

The other factor that has had an effect on the business case is the number of saturators used to produce brine.  It should not be necessary to purchase additional saturators because those already in use for pre-wetted salting should have sufficient capacity for the number of brine treatments estimated.  However, the amount of brine required each year for combination treatments (i.e. brine treatments and pre-wetted salting) would be higher than that required for just pre-wetted salting and an additional annual maintenance cost has been allowed for the saturators, due to increased usage.

The main conclusions from the revised calculations (based on the use of a combination spreader) are as follows:

  1. Based on the revised business case, the use of combination spreaders is not recommended where brown salt is used for the dry component of pre-wetted salt.
  2. If brown dry salt and white brine salt are used, it is estimated that there will be no direct cost savings if the brine loss due to trafficking is 30 per cent or more (i.e. on all types of surfacing on the Agency’s network).
  3. If white dry salt and white brine salt are used, there should be direct cost savings in areas where the weather conditions are not too severe, although this is more likely in such areas if white salt costs more than £46/tonne (c.f. £25/tonne for brown salt)
  4.  If brown dry salt and white brine salt are used, there will be an overall (direct and indirect) cost saving if the brine loss due to trafficking is 40 per cent in areas where the weather conditions are not too severe.  In areas with the most severe weather conditions considered in the calculations, there will be an overall cost saving if the brine loss due to trafficking is 30 per cent or less (i.e. on dense surfacing)
  5.  If white dry salt and white brine salt are used there will be an overall cost saving if the brine loss due to trafficking is 40 per cent or less (i.e. on typical or dense surfacing).  There should be an overall cost saving if the brine loss due to trafficking is 50 per cent (i.e. on open textured surfacing) in areas where the weather conditions are not too severe.

Assessment of routes most suitable for brine treatments

The combination spreaders available from the suppliers of the new winter fleet can spread brine to one, two or three lanes, but it is not currently possible to spread three lanes and a hard shoulder.

An analysis was carried out of winter service routes on the Agency’s network in Areas 1 and 2; two areas where the climate is likely to be suitable for combination treatments.  At least 22 routes have been identified in these areas which have no sections with more than three lanes which would be suitable for brine treatments with combination spreaders from suppliers of the new winter fleet.

The feasibility of brine spreading with pre-wet spreaders

Trials were carried out on the TRL Test Track to investigate the feasibility of spreading brine with pre-wet spreaders from the Agency’s current winter fleet.  The main aims were to determine:

  • The feasibility of spreading the hard shoulder of a Managed Motorway for the following scenarios:
  1. Spreading from the hard shoulder and
  2. Spreading from Lane 1 into the hard shoulder
  • The feasibility of spreading a two lane slip road asymmetrically from Lane 1, e.g. top-up treatments in low humidity and low temperature conditions

When driving and spreading from the hard shoulder, the trials showed that it is possible to achieve the target rate within the lane the spreader is driving in.  To achieve the target spread rate, the wastage outside the lane is relatively high and the total amount of brine discharged will therefore be required to be greater than the target amount for one lane.  The uniformity of the brine distribution along the carriageway is comparable with trials with a brine spreader.  Overall, the results of the trials would indicate there is potential to drive and spread brine in the hard shoulder using pre-wet spreaders.

Based on the results from these trials, spreading brine from Lane 1 to the hard shoulder would not seem to be a feasible method when using pre-wet spreaders with the current settings for pre-wetted salting.  It may be that different settings or modification of the spreaders, e.g. the design of the spinner, would result in a better distribution.  A more effective option would likely be the addition of nozzles to the pre-wet spreaders which could spray brine to the hard shoulder while driving in Lane 1.

The distribution profile across two lanes was clearly less flat compared with the distribution for the brine spreader.  As a result, to achieve the target spread rate across the whole width of the carriageway would require significant overspreading in the centre of the lanes.  If brine spreading was to be carried out as the only treatment on a section of carriageway, more work would be required to try and optimise the two lane distribution.  However, with the distribution as measured at present, there would be potential for use as a top up treatment to areas previously treated with dry or pre-wetted salt.

Based on the results, guidance for Service Providers has been developed for inclusion in the Network Management Manual for brine treatments with pre-wet spreaders.  Without accurate information on residual salt levels, surface temperatures and surface conditions to optimise the spread rate, it is recommended that treatments are made at the maximum brine spread rate that can be achieved with pre-wet spreaders, namely 40g/m2 for one-lane spreading and 20g/m2 for two-lane spreading.

Project Outputs: