Chapter 2: Materials

2.0 Storage and Handling

Key aspects of storing and handling any product include safety and quality.  It is essential to ensure safety in handling at all times and to maintain quality so that the material’s properties remain within specified requirements from manufacture to the intended end use.

This section covers storage and handling of:

• Conventional asphalt
• Modified asphalts, including asphalt rubber and MBs
• Asphalt emulsions
• Aggregates

2.1 Conventional and Modified Asphalts

When handled properly, asphalts may be reheated or maintained at elevated temperatures without adverse effects.  If asphalt is thermally abused in storage, handling or application, it may harden and compromise service properties.

2.1.1 Avoiding Problems During Storage

The main methods of avoiding potential storage problems are to ensure that equipment is properly designed and in good working condition, and that correct procedures are established and followed.  All tanks should be designed in accordance with a recognized standard (e.g., American Petroleum Institute 650).  General design considerations include tank shape, tank foundations, tank thickness, and tank access.  Best management practices require a secondary containment around all tanks.  Vertical tanks yield the highest asphalt-to-tank-volume ratio of all tank configurations.  Vertical tanks with a cone-shaped roof are preferred, although temporary storage in horizontal tanks is acceptable. 

The operational tank design considerations relate to:

• Minimizing the risk of overheating:  The tank requires accurate thermal sensors positioned in the region of the heaters and uniformly distributed throughout the tank.  The probes should be in thermal wells and removable for cleaning and calibration.  Heating may be accomplished via heat transfer (oil or steam), electric coils, or direct fired.  As asphalt is a good insulator, the heating rate must be controlled to prevent localized overheating, particularly when direct-fired systems are used.  The heat capacity may be estimated as 0.5 and heating rate should be limited to 25°C (77°F) per hour (17).
• Minimizing oxidation and loss of volatiles:  To minimize oxidation and loss of volatiles, contact with air must be minimized. This may be accomplished by designing pressure-tested, fully enclosed tanks. To avoid air entrainment, all circulation lines should re-enter the tank under the liquid level.  Filling a tank should start at the bottom; the asphalt should not be allowed to freefall, as this can result in entrapped air.  Venting is an essential safety precaution and cannot be eliminated to reduce oxidation.
• Maintaining asphalt homogeneity:  To maintain asphalt homogeneity and avoid temperature variation, asphalt should be mixed on an intermittent basis.  This may be done through circulation or by using side mixers under the liquid level.  Because vortex mixing entrains air, its use should be avoided.  When adding fresh asphalt to a tank, circulation is necessary to stabilize temperature and combine the existing material with the fresh material.
• Minimizing heat loss:  To conserve energy, all tanks should be insulated with fiberglass or rock wool insulation.  This insulation should be at least 50 mm (2 in) thick and sheathed in aluminum or galvanized steel at least 0.7mm (0.03 in) thick.  Additionally, lines should be insulated and heat traced with electric tape, steam or oil.

Safety hazards can arise from:

• High temperatures:  Because asphalt must be stored at such high temperatures, there are safety issues involving burns.  If the material comes in contact with water, it can rapidly expand, resulting in foaming and explosive boil-over.  Burns may be avoided by always using the correct safety apparel.  Additionally, all lines and surfaces must be thoroughly insulated.
  
  Due to the potentially hazardous side effects of water contacting high-temperature asphalt, steps need to be taken to avoid this interaction.  Water is slightly lighter than asphalt and will move to the top of tanks.  However, cold water may migrate to the bottom of a tank during transport.  Water entrapment in tanks can be avoided by using watertight cone-topped tanks, ensuring that tanks are watertight and hatches are sealed.  Water-finding gel should be used to check tanks before filling.  If water is present, the asphalt should be heated through the range from 92 to 125°C (198 to 257°F) at a rate of 10 to15°C (50 to 60°F) per hour.  Silicone antifoaming agent at 0.1% can also be added.  Pipes and any additives that are to be blended with the asphalt need to be checked for water.
• Flammable or explosive atmospheres:  Asphalts normally have flash points exceeding 250°C (482°F).  However, flammable atmospheres may form if contaminated by light products (e.g., products created from cleaning or flushing lines).  Ignition sources may include sparks, or static electricity.  With this in mind, proper grounding is important along with the use of shielded electric motors.
• Presence of toxic materials:  Fumes can be generated when asphalt is heated.  These fumes contain particulate asphalt, hydrocarbon vapor, and sulfide gases.  The latter are highly toxic and tend to build up in headspaces.  Proper venting is required to dispose of these fumes.

2.1.2 Recommended Storage and Handling Temperatures and Times

Asphalt and modified asphalt are stored and handled in similar ways.  An exception is asphalt rubber, which is used shortly after manufacture.  Allowable storage times are product specific and take into account the rate of property change, which occurs during storage.  For example, an AR grade of conventional asphalt can be stored for several months, a PBA modified grade can be stored for several days, and asphalt rubber stored for only several hours.

2.2 Asphalt Emulsions

Asphalt emulsions are a convenient way of handling asphalt.  Because water is the carrier of the asphalt and the system is chemically stabilized, emulsions may be subject to settlement or breaking prematurely.  For this reason storage and handling are important issues.  Over time, emulsions will become coarser, undergoing property changes.  To avoid these problems, timely use is required.  Simple rules for storage and handling of asphalt emulsions are discussed below.

2.2.1 Handling

Handling of emulsions is not difficult.  By following the rules below potential problems can be avoided.

• Pumping:  Pumps are a way of doing work on an emulsion.  Pumps usually compress or shear the material they pump.  This results in a compressed emulsion.  If compression is too severe or occurs too often, the emulsion will become coarser by the mechanism of flocculation and coalescence and may revert to straight asphalt.  Pumps should be selected carefully, using expert advice.  Diaphragm pumps are gentle, but require high maintenance and should only be used if essential.  Centrifugal pumps are acceptable as long as the peripheral speed is less than 300rpm.  Positive displacement pumps may be used, but usually 2-3 thousandths of an inch must be shimmed from the gears to provide adequate clearance.  Lastly, old and worn pumps may be used. 
• Temperature:  Asphalt materials shrink when they cool.  In an emulsion, shrinkage means that the asphalt droplets move closer together.  That has a number of important consequences: the material can flocculate and may coalesce,  or the emulsion may settle out faster than desirable.  If the material is pumped when cold, the droplets are more compressed due to temperature-related shrinkage.  As a result, a pump that was not too tight in January may be far too tight in July.  If the emulsion actually freezes, the droplets become frozen in contact and the emulsion will revert to bitumen upon thawing.  For most emulsions, this happens if the emulsion temperature falls below 4oC (40°F).  Materials expand when they’re heated.  However, when water is heated, its evaporation rate increases enormously.  If the water evaporates, the droplets get closer together and can result in an emulsion reverting to asphalt by the action of flocculation and coalescence.  If any part of the emulsion gets hotter than 95oC (203°F), localized boiling may occur.  If this happens, the droplets fuse back into asphalt.  To minimize the risk of fusing the emulsion:
  • Heat emulsions gently according to specifications.
  • Use agitation while heating.
  • Warm pumps before use.
  • On bulk tanks in cold areas, use electrical heating where possible.
  • Do not apply direct heat to emulsions with fire or a blowtorch.

2.2.2 Transport Handling

Emulsions are generally stable enough to transport.  However, a common problem occurs when air enters the emulsion.  Air can cause the emulsion to break in the bubbles of air and CRS emulsions are particularly prone to break in this way.  These larger particles can “seed” the emulsion, causing settlement.  Problems also arise when transport tanks are not clean.  Mixing cationic and anionic emulsions can lead to breaking of the emulsion.

2.2.3 Storage

Procedures for storing asphalt relate equally to the storage of an emulsion.  When an emulsion is stored, it has a finite lifetime determined by the formulation, handling, and storage of the emulsion.

Asphalt is slightly heavier than water; as a result, asphalt particles settle to the bottom of the storage container.  When the particles pack in this way, they can stick together (flocculate and coalesce).  If that continues, the emulsion will eventually turn back into bitumen.  This settlement may be controlled to some extent by formulation.  If the emulsion particles are fine enough to start with, they will settle more slowly, allowing for longer storage life.  Flocculation and coalescence can also occur as the result of electrical attraction between particles.  If an emulsion is electrically unstable, it will flocculate and coalesce.  This process may not take the emulsion entirely back to bitumen, but the large particles formed as a result of this process will settle faster.

It is important to prevent settling by mixing an emulsion prior to the start of flocculation or coalescence.  Once an emulsion has coarsened, remixing will not separate the larger particles again.  If it has coarsened too much, pumping may cause the emulsion to break.  The only way to prevent problems is to start with a very fine emulsion and keep it properly maintained.

Tankage:  While vertical tanks are preferred for plant storage, mobile storage may be achieved with a road tanker.  A road tanker increases the surface area of the emulsion exposed to air and can promote skinning.  However, if properly handled, this will not become an issue for fieldwork.  Guidelines for tankage include:

• Contents of bulk tanks should be slowly circulated at regular intervals.
• Frequency of circulation depends on the weather and how long the emulsion has spent in storage.
• Most emulsions only require circulation once a week in summer and once every five days during the winter.
• Circulation should be performed in the middle of the day, not first thing in the morning when temperatures are colder.
• The time of circulation is based on the size of the tank:  a 5000 Lt (1320 gal) tank should be circulated for 15 minutes while a 10,000 Lt (2640 gal) tank requires 20 minutes.
• Pumps must be flushed after use, but never into the emulsion tank.
• Lines and pumps should be warmed before use.
• Emulsion should not be left in the lines.

Table 4 shows the storage and application temperatures for commonly used emulsions.

Cleaning Procedures:  When working with emulsions, cleanliness is very important.  A sloppy operation will cause problems. For example, when an emulsion comes in contact with air, it can begin to break.  When a cationic emulsion comes into contact with metal, it can begin to break.  Thus, if a pump is not properly cleaned after use or if lines are left containing emulsion, they will clog.  The higher the performance of the emulsion, the more critical cleaning is.  Thorough cleaning should be done before equipment is stored.  Cleaning guidelines include:

• Flush equipment including hoses thoroughly with water.
• Flush equipment and hoses with kerosene, not diesel fuel, distillate or other solvent.  These materials may dissolve asphalt but they are also incompatible with the emulsion and may cause the emulsion to break rather than flush it away.  NEVER FLUSH INTO THE EMULSION TANK.
• Flush a second type with water.
• If a pump or line is already clogged with bitumen, apply gentle heat at the blockage.  Do not apply heat to the lines, as this will break the emulsion.
• Soak pumps with kerosene for an hour or more.
• Flush again with water after blockage is removed.

Rust, dirt, grass or other foreign material should be kept out of the emulsion.  This is especially important when working with cationic emulsions as they can break by reacting with foreign materials.

The main transport requirements are to ensure that correct pumping is used and that pumps are warmed in cool climates.  Clean tanks or a switch-load process should be followed.  Switch loading is a process by which materials are transported in tanks that last carried a compatible material and therefore do not require the tank to be cleaned between material switching.  Table 5 provides acceptable switch loading combinations.  Always pump into clean tanks and always transport full containers.

Emulsions are chemical systems. To avoid contamination, they should never be mixed with other types of emulsions or other chemicals.

2.3 Storage And Handling of Aggregates

Aggregates must be handled and stored in a manner that avoids contamination, minimizes degradation, and avoids contamination (20).  Specific guidelines are as follows:

• Stockpile areas should be clean and stable to avoid contamination from the surrounding area.
• Stockpiles should be on free-draining grades to avoid moisture entrapment.
• Stockpiles should be separated for different aggregate sizes to prevent intermingling.
• Segregation or separation of a blended aggregate is the primary concern.  Segregation occurs mostly with coarse aggregates but even slurry-combined aggregate may segregate in the stockpile or on handling if it gets too dry.  Segregation may be avoided by avoiding stockpiling in a cone shape.  Acceptable stockpile shapes are either horizontal or radial.  Making each end dump-load a separate pile, each adjacent to the next, makes horizontal stockpiles.  Radial stockpiles are made with a radial stacker (20).
• Degradation of the aggregate creating fines can be avoided by handling the stockpile as little as possible.  In chip seal or slurry surfacing applications, re-screening may be considered.