Pre-Wet or All Wet?
Robert Holden, HESS
When discussing 'Pre-Wet', what is actually being reviewed? Generally it is accepted that a purely or primarily liquid material is most likely to be a 'De-icing' or 'Anti-icing' process. This procedure would be similar to what most commercial flyers are familiar with during winter travels. Pre-wet most accurately describes a ratio blending of solids (salt or grit) and liquids (brine or de-icing fluid) to create an emulsion or suspension. This seems to suggest a simple equation to be implemented operationally by road maintenance staff. However, this is merely one step in a series that need to be evaluated and incorporated into any action plan.
A ratio blend, by definition requires some technical or scientific formulation based upon the characteristics of each constituent. This is an area where more expert advice is required and welcomed! Information of varying complexity is available through agencies like the National Research Council, MoE, Transport Canada, industry associations and others. The most directly relevant facts to be aware of are the percentage of solids in suspension, the quality and the consistency of each solution. Natural brine, saline solutions and manufactured products all react to temperature and mixing differently. Coarse salts, as commonly used in Canada, dissolve slower than fine ground salt. Maintenance of the thawing emulsion during storage, transport and application will also vary the consistency of results.
Essentially the concept revolves around the density and volume of the thawing material. If you were to compare a cube of salt and an equal mass of granulated material, the surface area occupied by each is noticeably different. The surface contact area of the granular greatly surpasses that of the cube, permitting coverage of a larger area, thus resulting in a larger reactive spot on the road surface. Unfortunately, we most commonly use a coarse crystallized salt product which is notably less effective without additional agitation. In regions where high traffic volumes do not exist this type of product does not spread and the result is a slushy, pock marked ice/snow roadway. Also, the larger crystals tend to bounce and disperse towards the fringe of the driving surface. This adds to peripheral contamination and loss of effective thawing.
Now, by applying a salt-based emulsion or suspension to the 'Dry' material being distributed the effectiveness is greatly increased and the waste is minimized. The bounce effect is eliminated and the large crystals become mushy and better suited to traffic agitation and distribution. However, in low traffic areas the results are minimal. Also, the larger crystallines will stick on contact and the solution will leech away further reducing the effective area. The result in both of these situations is that to meet safety and performance criteria a greater volume of material is required. This may be achieved through higher dosage or multiple applications, or both. Either way it defeats the principal of pre-wetting and adds significantly to the cost.
How can the effective performance be maintained or improved whilst escalating costs are controlled is the next step requiring our attention. There are two avenues that must be pursued to solve these issues. One is Product Selection, and the other is Equipment Selection.
Product selection will have the single most immediate impact on the effectiveness of any thawing application. Coarse grit has been shown to be the best traction improvement material that can be used. Fine or ground salt has proven to be the best dry thawing agent commonly available. Natural brine, with a salt content approaching 25% of volume, seems to be the best 'wetting' medium. Choosing the ideal material and ratio will result in less material required for the same area to be covered. Blending and applying these materials is dependent upon traffic conditions, road classifications, meteorological factors and equipment. A remote, single carriageway, gravel surfaced, steep slope roadway requires a much different approach than a multi-lane metropolitan thoroughfare. Material consumption, application rate and safety factors vary immensely for each scenario. Any Winter Maintenance Plan has to account for these to be physically and fiscally effective.
A multi-level plan requires management, to ensure compliance and adjustment which brings us to the issue of equipment selection. Simplicity and longevity are the two primary criteria of all equipment managers. Both are relative terms and are impacted by cumulative advancements in technology. In the field of winter maintenance we have now reached a sea change in technology available to us. We have discussed the evolution of material science to address efficiency of the thawing medium, now we must examine the equipment evolution.
Traditionally a simple mechanical system was employed to discharge material from a storage container to the road surface. From a tailgate chute to adjustable hydraulic spinners, these methods have changed little in over fifty years. The materials revolution, environmental and fiscal constraints and the evolution of electronics now can be combined to bring spreader technology into the 21st Century.
Simplicity is still best, and the conveyor and spinner are essential to any commercial spreader. These have now been augmented by the use of computer modeling, electro-mechanical sensors and liquid application systems, all of which combine to create a machine that is flexible, manageable and simple to operate. By the careful selection of the thawing and traction materials today's spreaders can be pre-programmed and remotely monitored to blend, mix and apply any combination of products in almost any operational scenario. They not only do this more efficiently through sensing and comparative electronics; they do it more economically. The manager becomes better able to control a changing situation and the operator less distracted and better able to navigate the equipment. Record keeping is improved and critical decisions are made quickly and reliably, balancing safety and budgetary parameters. So much so, that a given vehicle can be dedicated to a route with a minimum of operator requirements. By placing the ability to manage the functional aspects of Winter Maintenance in the same hands as the operational management requirements, a simpler, efficient, economical and flexible system is established.
We are now at a crossroads where operational requirements and specifications, material selection, equipment selection and managerial procedures MUST be collectively considered. With this done a long range plan can be initiated that will carry the industry through the next decade and beyond.
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