Center for Environmental Excellence by AASHTO CENTER HOME  
skip navigation
CEE by AASHTO Home | Compendium Home | Online Compendium Help | Recent Updates | Inquiries | FAQs | State DOT Links
About Best Practices | Comment on Best Practices | Suggest A Best Practice | Volunteer to Vet Best Practices
Printer Friendly Version Print This Page    
 
« Back to Chapter 7 | Go to Chapter 9 »
Chapter 8 (Revised August 2013)
Winter Operations and Salt, Sand, and Chemical Management
8.5. Precision Application to Manage and Reduce Chemical Applications

Precision application equipment can represent a large cost in ever shrinking winter maintenance budgets, and purchase of this equipment often requires justification or benefit-cost analysis prior to purchase. Identified benefits of precision application equipment for deicing and anti-icing include (Kroeger and Sinhaa, 2004):

  • Reduction in accidents
  • Return on investment
  • Reduced chemical usage and improved environmental stewardship

Cost, benefit or effectiveness information related to material distribution systems have been conducted but were unable to produce a benefit-cost ratio (Colson, 1997; Nantung, 2001; Sharrock, 2002; Iowa DOT, 2000). Identified costs of material distribution systems included equipment costs, as well as the costs of maintenance and calibration. Identified benefits were cost savings, material savings, and improved material placement. The effectiveness of such systems was only identified by one document, which found such systems to be effective in producing bare pavement in a timelier manner. Veneziano et al. (2010) developed a web-based tool to assist winter maintenance manager is computing benefit-cost ratios and can be used for material distribution systems.

Some transportation agencies choose to leave a small amount of snow on the road before salt is applied in order to keep the salt from bouncing or being blown off the road surface by passing traffic or wind. This can increase the amount of salt required to "de-ice" or melt the snow packed on the road, and is not as efficient in retaining salt on the road as other methods (e.g., slower spreading speeds, pre-wetting, "zero-velocity" spreading, etc.)

Pre-wetting material, whether it be sand/grit or solid deicing product, with liquid (e.g., brine) before application allows the product to stick to road surface producing less bounce and scatter and allows the product to begin to worker sooner. MDOT conducted a bounce and scatter field test and found to optimize application of solid material it should be pre-wet and applied at vehicle speeds between 25 and 35 mph (Michigan DOT, 2012).

General overviews of technology available and disadvantages and advantages of their use are summarized below from the Transportation Association of Canada and a TRB Report on Snow Removal and Ice Control Technology. (Perchanok et al., 1993; TAC, 2003) Additionally, Clear Roads has funded the following projects - Comparison of Material Distribution Systems for Winter Maintenance Phase I and Development of a Totally Automated Spreading System currently underway, and reports are expected to be available to the public in early to mid-2013.

8.5.1 Spreaders, Spread Patterns, and Spreader Controls
< back to top >

The total amount of salt used for winter maintenance is significantly influenced by the characteristics of the spreader equipment.

  • Spreader controls should be capable of delivering several precise application rates.
  • The application rate should be consistent whether the spreader is full or nearly empty, regardless of material variations, or temperature changes.
  • When purchasing new equipment, transportation agencies should require test results from suppliers to confirm that the equipment will achieve precise application rates under all conditions.
  • Spreaders must operate in a severe environment of low temperatures, high moisture, poor visibility, and corrosion, often with limited maintenance. Controllers must be easy to load, and simple to operate.
  • Ideally, a spreader should be adaptable for other tasks, or the hopper should be easily removed so the trucks can be used for other operations during the summer.
  • Hoppers must be constructed so that all sand and salt can be easily removed from the body.
  • Spreaders should be fitted with screens to ensure that frozen clumps of material or other contaminating material that would jam the chain/conveyor mechanism are not loaded into the spreaders. Or the spreader has crushing system so that all applied material is the same size.
  • Cab shields should be fitted to assist in loading the spreaders to ensure that all loaded salt enters the box, and material is not spilled over the truck.
  • Spreaders should be manufactured from a material that will resist corrosion. Special chlorinated rubber primers and epoxy-based primers will increase coating life. Stainless and galvanized steel, and fiberglass bodies are available but can be relatively expensive. High strength, low alloy self-coating steel, used with good surface preparation and special primers has been proven to provide a cost effective body life of up to fifteen years. Manufacturers also supply spreader bodies constructed of fiberglass. These bodies are lighter and thus provide increased payload possibilities, but are also more expensive than steel.
  • Electrical wiring for controls and lighting, and hydraulic components must be enclosed in vapor proof, or sealed systems.
  • Neoprene spinners are frequently used to improve durability and spreading efficiency.

8.5.1.1 Spread Patterns

Salt and sand application methods can be modified to meet differing requirements.

  • Salt use sometimes can be reduced by applying the salt in concentrated locations (e.g. windrowed on the crown), rather than being spread uniformly or broadcast across the entire road surface.
  • In most cases solid or pre-wetted salt should be applied in a continuous narrow windrow along the centerline of the road. The concentrated mass of material minimizes the tendency of the material to bounce or be blown off the road by passing traffic. Salt going into solution drains down the crossfall of the road, and can migrate under packed ice and snow; a uniform section of road is then bared off initially along the center of the road to provide two-wheel stability for traffic. Application in a windrow is achieved without using the spinner, by dropping the material from a chute. Windrowing on the centerline will not work if the crown of the road is not consistently on the centerline, or the road surface is badly deteriorated which could cause the salt brine to pond in some areas.
  • Centerline application is also not appropriate if the entire road surface is slippery and immediate de-icing is required. In these situations, higher salt application rates may be needed across all traffic lanes.
  • Application ahead of the drive wheels can provide improved traction under the drive wheels of the spreader vehicle. Application close to the driver's cab also enables the driver to monitor the application to ensure that material flow has not been impeded.
  • Hopper Spreaders - Conventional hopper spreaders provide good control of material application and dependable service. However, they are the least versatile for other operations during the off-season. New hopper designs, including rear-discharge, slide-in units with a longitudinal agitator bar and belt conveyor, or augers used to create a slurry are gaining popularity, particularly for pre-wetted applications.

Work conducted by Bolet and Fonnesbech (2010) testing spreader material application versus the amount of winter maintenance product on the road recommend focusing on spreading quality, the stability of spreaders adjustments, and the development of simple verification tests for adjustments made through out a season.

8.5.2 Material distribution systems (MDS)
< back to top >

Material distribution systems are the front line in the application of anti-icers and deicers to the roadway. It is of great interest to agencies to apply the right amount of materials in the right location at the right time, and advanced material placement systems can assist in meeting these goals (Veneziano et al., 2010). Ideal systems should lead to uniform distribution of applied materials on the pavement and minimal material loss due to bounce and scatter; as such, they would enable the maintenance agency to adopt reasonably lower application rates without reducing the level of service. This could translate to significant cost savings and environmental benefits.

Material Distribution Systems have been documented to various degrees. In a nationwide survey, CTC & Associates (2010) collected information from snowy states to learn more about the best field practices pertaining to using and adapting material spreaders and related equipment. Results have shown that more than half of the agencies use more than one type of material spreader, while two-thirds of the agencies use more than one type of delivery mechanism to get material from the spreader to the pavement. The listed delivery mechanisms include: zero velocity spreaders, dual spinners, spinners, modified spinners, homemade chutes and other types of mechanism. Challenges in working with the material spreaders include bearing failure and hydraulic motor/gearbox failure; clogging, clumping or refreezing of material in the chute; corrosion; difficulty making the transition from spinner to chute in installations lacking a remote-controlled gate; frozen spinners and augers; repeated adjustments or breakage of conveyor chains; and snow accumulating on the back of the truck, plugging the opening to the delivery system.

8.5.2.1 Tailgate Spreaders and Reverse Dumping of Dual Dump Spreaders

The primary limitation of tailgate spreaders is the inconvenience of raising the dump box and the possibility that the box will not be raised high enough to ensure that sufficient material is dumped in the hopper to provide consistent delivery. The rear discharge restricts the operator view of the operation and ability to ensure that the material is being discharged at the right location. The vertical clearance and the upward and rearward shift of the center of gravity when the box is raised can cause instability and is a safety concern in some areas.

Dual dump spreaders were developed to overcome problems identified for tailgate spreaders while still providing a multi-purpose spreader that could be used year round. They function as regular rear dumping bodies when not being used to apply winter maintenance materials.

Disadvantages of this spreader are the high weight compared to a regular dump truck, and the need to raise the body while driving to move the material to the front of the truck. This reduces the truck's stability and care is required by the operator to ensure that sufficient material covers the cross conveyor at the front to maintain a precise application rate. The pivots have been a source of failure and replacement is expensive.

8.5.2.2 Multipurpose Spreaders

Multipurpose spreaders incorporate most benefits of the other spreaders. A recent design makes use of a U-shaped box to ensure that no material hangs up in the box and that all material can be easily removed from the box at the end of the shift. Material is either discharged in a windrow using a chute for concentrated action, or spun across the lane using spinners. The spreader provides precise application rates and all the advantages of distribution in front of the rear wheels. Cross conveyors are easily removable during the summer so that there is no tare weight penalty. The units are lightweight and provide year round use, and the body can be easily switched to carrying construction materials (simply by installing a pan or tray across the floor conveyor). As these units can carry substantial loads, care must be exercised to ensure that adequate truck components, axles, springs, and wheels are specified to carry the load. This is particularly important on combination units that are also equipped with snow plows.

8.5.2.3 Rear-Discharge Spreaders

Based on the premise that no salt particle should be placed dry onto the road surface, and that fine salt is the gradation of choice for prompt dissolving and melting, certain spreader design characteristics cater better to liquid and fine salt use in pre-wetted applications. The salt must be of a fine gradation in order for it to retain the brine moisture content and fine salt does not travel as easily on certain chain-type conveyor systems. These spreaders allow a "high-ratio" or "slurry" salt application rates up to 255 liters per ton of salt, or at a ratio of 30:70 liquid-to-solid by weight.

This requires a large capacity of liquid onboard and adequate pumping capability that may not be possible or practical on a conventional retro-fitted unit. They are either frame-mounted or slide-in, rear-discharge v-hoppers can stand on self-contained stilt legs in the maintenance yard, and remain tarped until needed. Pre-wetting liquid can be applied directly on the spinner, or in the hopper using an auger, or both. Areas that only have access to coarser salt may find that the liquid component must be reduced since saturation can be achieved with less liquid. To solve this problem installing a crusher on the hopper to reduce the particle size may help.

8.5.2.4 Electronic Spreader Controls

Currently, the vast majority of road agencies use spreader systems that are adjustable as to amount of material applied per lane mile. Spread rates can be manually reset by in-cab controls. The Minnesota DOT developed a spreader control that used on-vehicle friction sensors and vehicle location to automatically adjust a zero-velocity spreader (Erdogan, 2010). The controller that was developed was found to adequately apply granular materials up to speeds of 25 mph.

Modern spreaders use electronic groundspeed spreader controls to provide consistent, accurate application rates. The truck speed is monitored from the truck's speedometer drive, and the spreader output is adjusted to maintain a steady output at the set rate per kilometer. Both open loop and closed loop systems are available to monitor material flow and provide increased accuracy of the spread rate (closed loop systems provide confirmation of the actual application rate). Electronic controllers automatically increase the output rate if a second spinner is actuated (if so equipped) to treat truck climbing and turning lanes. With some electronic units, calibration settings can be applied electronically using infrared controls.

Manufacturers can now provide units that record information about the amount of salt used, the time it was used, and the associated application rate, for analysis and control by the transportation agency. Information that is captured can include: amount and type of material applied, gate position, run time, blast information, average speed, spread width/symmetry, etc. Units are also available that incorporate global positioning systems (GPS) for automated vehicle location (AVL) and to identify where the material was discharged (either generating a passive history or a live transmission). There is currently no industry standard format in place for this information reporting; it is difficult to compare and combine the information from the units supplied by the various manufacturers.

Hoppers configured to allow the snowplow to carry and spread both liquid and granular materials in different amounts are becoming popular, especially in areas sensitive to certain chemicals and materials. A more advanced version of such systems has been patented, which claims to enable "coordinated application of a plurality of materials to a surface simultaneously and in desired proportions and/or widths automatically and/or selectively" (Doherty, 2005).

8.5.2.5 Rearward Casting Spreaders (including Ground-Speed and Zero-Velocity Spreaders)

Casting material rearward has shown potential for salt use reduction by increasing the percentage of applied salt that is retained on the road, and in the required location on the road. This is a concept by which the salt is discharged rearward at exactly the same speed as the spreading vehicle is traveling forward. The two velocity components cancel each other causing the salt to drop on the road as if the spreading vehicle was standing still.

Zero Velocity Spreaders can optimize the use of deicing material through the controlled distribution of the material. The material is dispensed at the same velocity of the forward motion of the equipment. This helps reduce bounce and scatter allowing more of the material to remain on the pavement, saving up to 40% in de-icing material and reducing salt runoff to the surrounding environment. The zero-velocity spreader applies material in such a way that the material lands at a velocity that is zero relative to the road surface. The spreaders, which mix and spread liquid and solid deicers, use technology that enables plow trucks to apply products at speeds as fast as 35 miles per hour, which increases efficiency and safety in terms of the speed differential between plows and traffic.

In 1994 and 1995, Iowa was the FHWA test site for the zero velocity spreader, at that time a new concept in roadway chemical spreaders. Minnesota DOT tested eight zero-velocity spreaders that same season and discovered savings of 30% or more. Even in 1995 when the spreader was priced at around $10,900 compared to $2,000 to $2,500 for a common spreader, tests indicated that material savings compensated for the increased cost (original reference no longer valid). Zero-velocity spreaders allowed roads to be treated at speeds of 40 to 50 miles per hour while keeping up to 90 percent of the granular salt material on the road surface in the wheel tracks where it is most effective (Sharrock, 2002). Cost savings over two years were over $70,000 (1998 dollars). Nantung evaluated the use of a zero-velocity deicer spreader and salt spreader to determine their effectiveness for the Indiana DOT. The primary benefit of the system was viewed to be the more accurate placement of material, producing significant potential for cost savings (Nantung, 2001). Maintenance downtime was identified as a potential cost of zero-velocity systems, as the hydraulic nature of the system would require lengthier maintenance time compared to traditional spreader systems. Work conducted by Nixon (2009) found that three tested zero-velocity systems provided superior performance in terms of material retention on the road than the tailgate spreader or a chute system.

8.5.3 Pre-Wetting Solid Materials to Minimize Bounce and Improve Performance
< back to top >

Pre-wetting is defined as the approach of adding liquid products to abrasives or solid salts to make them easier to manage, distribute, and stay on roadways. The pre-wetting of solids is performed either at the stockpile (pre-treatment) or at the spreader. The liquid chemical helps accelerate the break-up of the snow/ice pack, and keeps the material on the roadway longer by preventing loss due to rebound and traffic. A survey conducted for this report found that the majority of survey respondents (88%, n=28) indicated they have implemented pre-wetting as a tool for reducing product application while maintaining or improving LOS. Pre-wetting was also identified by the respondents as one of the ten most common practices that have been implemented or modified by their agencies.

Pre-wetting has shown to increase the performance of solid products or abrasives and their longevity on the roadway surface, thereby reducing the amount of materials required (O'Keefe and Shi, 2005). Pre-wetting accelerates the dissolution of solids and enhances its melting action (TAC, 2003). An evaluation of pre-wetting salt in Canada showed that fewer chemical applications were needed, resulting in up to 53% less material used (Warrington, 1998). In Nebraska, pre-wetting salt reduced salt usage by 35%-40% (Keating, 2001). Relative to dry salt, pre-wetted salt (with 10-mm or smaller particles) has been proven to be better retained on dry roads and its spreading leads to less wasted salt and quicker deicing effect (Burtwell, 2004). It was found that pre-wetted rock salt applied along the centerline in a windrow had 96% material retention compared with 70% of the dry rock salt (TAC, 2004). In a case study, the use of brine to pre-wet salt allowed for a 15% reduction in product usage, as the pre-wetted salt exhibited equivalent ice melting performance, better adherence to the road surface, and less loss and scatter (personal communication, P. Noehammer, City of Toronto, March 28, 2012). Looker et al. (2004) compared the performance of dry rock salt and six pre-wetted salt mixtures in the laboratory. The rate of pre-wetting was explored at 4, 8, and 12 gallons of liquid chemical per ton of rock salt respectively, and the melting of compacted snow improved with the rate of pre-wetting. Pre-wetting salt slightly decreased its performance at relatively warm temperatures (-1°C and -5°C, 30°F and 23°F, respectively) in some cases but "all of the rewetted mixtures were effective at -10°C (14°F), unlike the dry rock salt".

Pre-wetting with deicing agents helps abrasives stick to the roadway, as does pre-wetting with hot water (Perchanok, 2008; Perchanok et al., 2010). Pre-wetted abrasives tend to refreeze quickly to the road surface and create a sandpaper-type surface, which can cut abrasive use by 50% in cold temperatures (Williams, 2003). Nixon (2001) examined the use of pre-wetted abrasives on five different road types, including: high-volume paved roads, low-volume paved roads, low-speed paved roads, unpaved roads, and urban intersections. The only instance where using chemical products to pre-wet abrasives was discouraged was for unpaved roads because it could cause the road to thaw and become unstable. For all other conditions, pre-wetting increased performance and longevity of the abrasive. Dahlen and Vaa (2001) found that "by using heated materials or adding warm water to the sand it is possible to maintain a friction level above the standard, even after the passage of 2,000 vehicles". A new sanding method based on a mixture of sand and hot water has been adopted at some airports in Norway. This method showed promising results as a long-lasting effect was observed along with the prevention of sand from being blown to the side by operating aircraft. However, an event occurred where the treated surface lost its frictional properties (Klein-Paste and Sinha, 2006a). The authors (2006b) also reported that the wetting the sand with hot water before applying it onto the runway surface resulted in a sandpaper-like appearance, of which the performance in practice, optimization, negative effects, and limitations are also discussed. During the winter of 2003-2004, field tests comparing salt pre-wet with hot water versus pre-wet with brine showed similar performance on thick ice and that pre-wetting with hot water provided higher friction levels on thin ice and was more rapid at deicing (Lysbakken and Stotterud, 2006). Vaa and Sivertsen (2008) observed Norway's winter maintenance operations and found that mixing hot water and sand was an effective alternative to salting when temperatures were low. While a specific temperature associated with this operation was not specified, subsequent text indicated salting was performed down to 12ºF.

8.5.4 Fixed Automated Spray Technology (FAST)
< back to top >

Fixed Automated Spray Technology, FAST, systems aim to deliver anti-icers to key locations in a controlled manner, using pumps, piping, valves and nozzles or discs (Zhang et al., 2009). As an anti-icing strategy, it reduces the chemical usage by applying the chemical "just in time" (Pinet et al., 2001). Ideally, the application should be fully automated, using pre-programmed logic and real-time input from a number of atmospheric and pavement sensors on-site. There are sensitive structures and critical segments of roadway network that need to be free of snow and ice in a timely manner, before maintenance vehicles can even travel to the site and treat them. There are also high-risk areas far from maintenance sheds or areas that experience a high traffic volume, where it is desirable to apply the anti-icing chemical just prior to the frosting or icing event. FAST systems are able to deploy alternative anti-icers to treat specific areas (e.g., bridge decks), which would be impractical for mobile operations to accomplish. The anticipated benefits from FAST systems are site-specific, as a function of winter weather severity, traffic density, accident history, distance from maintenance yard, among other factors (Ye et al., 2012b). In principle, FAST systems should be deployed at locations that are remote, feature high traffic density and significant congestion, or feature considerable safety risk during wintery weather.

Experience with FAST systems in North America and Europe has revealed a mixed picture. Since the mid-1980s, hundreds of automated anti-icing systems have been used throughout Europe as an established tool to battle snow and ice conditions on highways, bridges, and airports (Gladbach, 1993; Friar and Decker, 1999). In North America, FAST is a relatively new technology that has gained popularity since the late 1990s (SICOP, 2004). Several studies have indicated reductions in mobile operations costs and significant reductions in crash frequency, resulting in favorable benefit-cost ratios (Keranen, 2000; Johnson, 2001; Lipnick, 2001; Birst and Smadi, 2009). Yet, there have been a variety of problems related to activation frequency, system maintenance and training. On balance, North American transportation agencies consider FAST to be an evolving technology and future improvements in system design, hardware, software, and installation techniques may help improve the reliability and user acceptance of FAST systems (Ye et al., 2012b). A survey conducted in 2007 revealed that the transportation agencies in North America were not planning to expand their number of FAST installations.

FAST is not an "off-the-shelf" system that can be purchased and installed right away at any given site. Customized design of the installation (e.g., spray logic) at each site after studying the site specifics and conditions is suggested (CERF, 2005). The FAST systems have been found to not spray when wind speed is greater than 15 mph and when pavement temperature drops below 12°F (Birst and Smadi, 2009). Note that there are two distinct hydraulic system design philosophies of FAST systems (Barrett and Pigman, 2001; Ye et al., 2012b). The first (referred to as Type I), more common in North America, utilizes a pump located in a pump house to deliver the fluid to the nozzles some distance away. The delivery pressure needs to be rather high to overcome the hydraulic head loss in the delivery lines. In these systems the flow is metered by the size of the nozzle orifice. The reliability of these systems becomes more problematic as the nozzles get farther away from the pump. In the second design philosophy (referred to as Type II), common in European systems, the pump at the pump house is used to fill a small pressurized vessel (tank) located in close proximity to each individual nozzle. When the signal to activate is given, a valve on the small pressure vessel is opened and the liquid is discharged through the spray head. This reduces the effect of the head loss, delivering a fixed amount for each activation.

8.5.5 Calibration
< back to top >

Whether materials are distributed by spreaders or FAST systems, the application rates that these pieces of equipment are set at must be accurate. In order to ensure an accurate amount of material is being distributed, equipment must be calibrated. Calibration ensures that materials are being applied at the appropriate rate/setting for a given material and storm scenario. Conversely, equipment that has not been calibrated may be over applying materials, resulting in wasted product, added financial cost, and environmental impacts.

Regardless of the equipment that must be calibrated, a calibration policy should be in place at an agency to ensure that the application rate settings for different materials are correct. When a material is applied by weight (e.g., pounds per lane mile) or fluid measure (e.g. gallons per lane mile), calibration should be by weigh or fluid quantity, respectively. Calibrations should be made when a piece of equipment is acquired or installed; calibration should also occur prior to and at points during the winter season, as well as whenever a new material is to be used. Calibration should also occur after repairs have been made to equipment or when material usage calculations indicate a significant discrepancy. Of course, the identification of material use discrepancies relies on accurate tracking of material use by operators through appropriate data records.

As stated, a thorough approach to calibration is necessary to account for the different anti-icing, deicing and sanding materials that may be distributed by equipment. Consequently, calibrations for both liquids and solids should be carried out on each piece of equipment for all materials that will be dispensed by that piece of equipment. Records of calibration should be kept digitally and as a paper file for redundancy. When a controller is replaced, recalibration for the vehicle must be performed. Concurrent with calibration, the spread pattern for a particular piece of equipment should also be checked. This will further ensure that materials are being placed properly, reducing waste and environmental impacts. Similarly, the different components of the equipment being calibrated should be checked to ensure they are operating as intended. Such components include ground speed controls, belts and chains, chutes, nozzles, gates, spinners and deflectors.

Frequent calibrations should be conducted by maintenance staff that has been trained at least yearly so that they understand the importance of proper calibration and conduct it diligently. Training should provide an overview of calibration, its steps, and other pertinent information. This training may be conducted at any organizational level, but is being used most commonly at the districts. Calibration training can be integrated with other training to maximize the use of operator's time while also providing information related to other aspects of winter maintenance, such as discussions of application rates and material quantities. Note that for agencies that employ contracted maintenance, the equipment of contractors must also be calibrated frequently (by their own personnel), with agency staff carrying out calibration checks when necessary (e.g., prior to start of winter season, at points during the winter, etc.) as specified by the maintenance contract.

In conjunction with calibration, spreader and sprayer equipment should be set up so that they are mechanically restricted from applying more than a maximum amount of material approved for a given set of routes. This will further ensure that excessive materials are not used during anti-icing and deicing operations. Application equipment should also be set up so that materials are only applied in the travel lane, avoiding scatter or bounce that can lead to material leaving the roadway and impacting the roadside environment.

Spreader calibration is a straightforward process that can be completed with a minimum of tools and equipment. It consists of calculating the pounds or gallons per mile of material that should be discharged at different controller settings and vehicle speeds (Salt Institute, 2007). Spreaders must be calibrated individually, as the same models used on two different vehicles can have varying application rates. Different calibrations must also be made for different types of materials for different spreader units. The goal of spreader calibration is to ensure that materials are being discharged at appropriate rates, minimizing wasted materials (and producing cost savings) and reducing environmental impacts. The equipment used for calibration can be quite basic and includes a scale for weighing, a canvas or bucket/collection device, chalk, crayon or other markers, and a watch with second hand (Salt Institute, 2007).

The Salt Institute's "Snowfighters Handbook" presents an overview of the steps and calculations employed in granular spreader calibration. The steps in the process include:

  1. Warm truck's hydraulic oil to normal operating temperature with spreader system running.
  2. Put partial load of salt on truck.
  3. Mark shaft end of auger or conveyor.
  4. Dump salt on auger or conveyor.
  5. Rev the truck engine to operating RPM (at least 2000 RPM).
  6. Count number of shaft revolutions per minute at each spreader control setting, and record.
  7. Collect salt for one revolution and weigh, deducting weight of container. (For greater accuracy, collect salt for several revolutions and divide by this number of turns to get the weight for one revolution.) (Salt Institute, 2007)

Similar procedures should also be used when calibrating equipment that applies liquid materials.

The Clear Roads pooled fund developed a calibration guide as part of a larger effort examining ground speed controller units (Blackburn, et al., 2009). This spreader calibration guide was developed for both ground speed controlled and manually controlled spreaders used to apply granular and liquid materials. The guidelines discuss various aspects of calibration and outline different procedures to use in performing such activities. Guidance is also provided regarding when calibration/recalibration should be performed, including:

  • When the spreader/controller unit is first put into service.
  • Annually, before snow and ice control operations begin.
  • After major maintenance of the spreader truck is performed and after truck hydraulic fluid and filters are replaced.
  • After the controller unit is repaired or when the speed (truck or belt/auger) sensors are replaced.
  • After new snow and ice control material is delivered to the maintenance garage location (Blackburn et al., 2009).

In general, the spreader calibration process can take between 10 minutes and 1 hour, depending on the number of staff involved, the type of controller (open or closed loop), the number of materials calibration is being done for, and even the age of the vehicle (new equipment requires added time to calibrate from scratch). Past experience from agencies has indicated that 1 to 3 staff is used to complete calibrations, with 2 staff members generally being most widely used.

8.5.6 Operational Support Equipment
< back to top >

Accurate records should be maintained of the locations of de-icing agents and abrasives application and the quantities of de-icing agents and abrasives used. Various types of equipment support the winter maintenance program either by helping manage the operations by generating useful data or by supporting the service delivery itself.

Equipment is available to assist with meeting the following necessary functions for environmental stewardship and effective minimization of materials application:

8.5.6.1 Material Usage Monitoring

Loader Mounted Electronic Weighing Equipment

Loading extra material onto a spreader can lead to overloading or the temptation to over apply the salt. In the past, operators tended to load a little extra salt as there was no exact method of determining the amount of material loaded, and they did not want to run out without completing the route. Overloaded trucks also contribute to contamination in the area of the salt storage facilities. Salt heaped above the side boards is thrown off the trucks as they negotiate curves to exit the yards.

  • With electronic scale control systems operators can more precisely load the right amount of salt. This device is a relatively inexpensive, durable, and accurate weighing device consisting of a transducer load cell mounted to the loader bucket arm. These devices can measure a predetermined load size for the scheduled route (length of route X application rate + a limited contingency amount for bridge decks, intersections, etc.). Models are available that will record with the loader in motion so that the loader operation is not impeded.
  • The units will record the amount loaded for future analysis. Though the equipment can be overridden, it provides the operators with a mechanism to accurately measure and control the amount of material loaded on the spreaders.

Truck Scales

Weighing the trucks as they enter and leave the maintenance yard is one way of determining the material loaded and the resulting spread rate for the serviced route. This function can be automated with a weigh-in-motion pad that tracks the equipment movement and can serve to reconcile the data from the spreader controller and other documentation.

Liquid Meters

Pump meters will likely be used to measure delivered brine, but not likely be on each pre-wet unit.

  • A meter should be in place at the brine supply facility, whether the source is hauled brine or manufactured brine, in order to track loading times and quantities.
  • A cross reference should be incorporated in the electronic log to identify the truck loaded for future reference.

Automated Vehicle Location (AVL)

  • Tracking equipment movements along with the services provided is possible via proven GPS receivers/ transmitters and software.
  • This electronic record can be actively followed real time or can be passively recorded for later analysis.
  • AVL can support a route optimization exercise, to rationalize the number of trucks required and thus the expected salt to be used on the roads serviced.
  • This equipment can provide operational support to greatly enhance the monitoring of salt usage, to demonstrate prudent usage and to correlate with the achievement of the required level of service.

8.5.6.2 Material Loading and Handling

Sand, deicers, and anti-icers should be stored and handled in a manner to minimize any contamination of surface or ground water. Care should be taken to prevent runoff from tanks or treated stockpiles. Covered storage for dry products is preferred.

Avoiding Contaminants to Materials

  • Liquid and solid products, and sanding materials should be free of contaminants known to cause water quality problems. Some of these include: arsenic, barium, cadmium, chromium, fluoride, lead, mercury, nitrate, selenium, other heavy metals, hydrocarbons.

Bulk Salt Handling by Loaders

  • Extensive environmental contamination has been identified in the area of salt storage yards. Much of this contamination results from poor salt handling practices.
  • Conveyors are available which are designed to allow salt trailers to dump directly into the conveyor for movement into the storage facility.
  • Loaders used to fill spreader vehicles are often fitted with buckets that are too large for the spreader hopper bodies, resulting in spillage. Though they have a slower production rate, smaller buckets are available for most loaders. Side dumping bucket attachments can also be used to provide quick precise loading.

Bulk Material Conveyors

  • Whatever equipment is used for moving salt, it should provide a way of tracking the flow so the quantities can be reconciled.
    • Pre-loaded drop-hopper loaders meter salt into spreader trucks.
    • Overhead silos can be pre-filled with salt to similarly meter salt into spreader trucks.
    • Pneumatic handling equipment can handle fine material that is used for either direct application onto the road or for blending with sand.

Sand/Salt Blend Mixers

  • Ideally, blended winter sand stockpile are put up in favorable, dry conditions. Relatively dry sand stored indoors should not require more than 1-2 percent salt by weight; more moisture in the sand may require more blended salt (up to 5 percent), but the purpose still is to keep the sand free-flowing, and not to support melt action.
  • Traditionally, blending took place on the apron to the storage shed, with several buckets of sand spread level, followed by one bucket of salt trickled on the surface; the resulting blend was loaded in the dome, and the process was repeated. This method is highly inefficient and inaccurate, and produced sporadic result on the pavement surface. Equipment to support high-production stacking and uniform, light blends now involves a form of dual-auger pugmill or a twin conveyor feed. In either case, two supply lines are metered to an accurate ratio and the final conveyor stacks the completed mixture.

Brine Production Equipment

  • The concentration should be checked with a hygrometer to measure the specific gravity of the solution. The percent of saturation is determined by reference to specific gravity charts for the specific solution temperature.
  • Water supply flow rates are a critical factor. Production sites may require cisterns to ensure adequate water supply where well production rates are poor.
  • Manufactured salt brine can be pumped directly into tanks mounted on the spreaders or transferred to holding tanks at the maintenance yards.
  • Stored brine will normally stay in solution as long as there is not evaporation or a drop in temperature below eutectic.
  • Corrosion inhibition requirements can complicate the brine manufacturing process.
  • Additives such as rust inhibitors may complicate long-term storage, in which case agitation or recirculation could be considered.

Brine Delivery Equipment

  • Sampling containers and a refractometer or hygrometer should be available for sampling and testing the concentration.
< back to top >
 
Continue to Section 8.6 »
 
Table of Contents
 
Chapter 8
Winter Operations and Salt, Sand, and Chemical Management
8.0 Introduction
8.1 Selecting Snow and Ice Control Materials to Mitigate Environmental Impacts
8.2 Reducing Sand Usage and Managing Traction Materials
8.3 Strategic Planning for Reduced Salt Usage
8.4 Stewardship Practices for Reducing Salt, Sand and Chemical Usage
8.5 Precision Application to Manage and Reduce Chemical Applications
8.6 Monitoring and Recordkeeping
8.7 Winter Operations Facilities Management
8.8 Training for Salt Management and Winter Maintenance Operations
  References
  Appendix A - Acronyms
   
Lists: Examples | Tables | Figures
Website Problems Report content errors and/or website problems
PDF Document Download Adobe Acrobat Reader