De-icing

De-icing is the process of removing snow, ice or frost from a surface. Anti-icing is understood to be the application of chemicals that not only de-ice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or prevent adhesion of ice to make mechanical removal easier.

De-icing at SLC
Spray de-icing at Salt Lake City airport, 2010

Approaches

De-icing can be accomplished by mechanical methods (scraping, pushing); through the application of heat; by use of dry or liquid chemicals designed to lower the freezing point of water (various salts or brines, alcohols, glycols); or by a combination of these different techniques.

Trains and rail switches

Icy train brake
Ice build up in train brakes jeopardizes braking efficiency.

Trains and rail switches in arctic regions have large problems with snow and ice build up. They need a constant heat source in cold days to assure functionality. On trains it is primarily the brakes, suspension and couplers that require heaters for de-icing. On rails it is primarily the switches that are sensitive to ice. These high-powered electrical heaters efficiently prevent ice formation and rapidly melt any ice that forms.

The heaters are preferably made of PTC material, e.g. PTC rubber, to avoid overheating and potentially destroying the heaters. These heaters are self-limiting and require no regulating electronics; they cannot overheat and require no overheat protection.[1]

Aircraft

A U.S. Army C-37B aircraft transporting Army Chief of Staff Gen. Raymond T. Odierno, gets de-iced before it departs Joint Base Elmendorf-Richardson, Alaska
A U.S. Gulfstream G550 gets de-iced before departing Alaska in January 2012

On the ground, when there are freezing conditions and precipitation, de-icing an aircraft is commonly practiced. Frozen contaminants interfere with the aerodynamic properties of the vehicle. Furthermore, dislodged ice can damage the engines.

De-icing fluids typically consist of a glycol-water solution containing a dye and agents to protect the metal surface. A range of glycols are employed. Thickeners are also used to help the deicing agent adhere to the airplane body.[2]:43 Ethylene glycol (EG) fluids are still in use for aircraft de-icing in some parts of the world because it has a lower operational use temperature (LOUT) than PG. However, PG is common because it is less toxic than ethylene glycol.[3]:2–29[4]

When applied, most of the de-icing fluid does not adhere to the aircraft surfaces, and falls to the ground.[2]:101 Airports typically use containment systems to capture the used liquid, so that it cannot seep into the ground and water courses. Even though PG is classified as non-toxic, it pollutes waterways since it consumes large amounts of oxygen as it decomposes, causing aquatic life to suffocate. (See Environmental impacts and mitigation.)

Infrared heating de-icing

Direct infrared heating has also been developed as an aircraft de-icing technique. This heat transfer mechanism is substantially faster than conventional heat transfer modes used by conventional de-icing (convection and conduction) due to the cooling effect of the air on the de-icing fluid spray.

One infrared de-icing system requires that the heating process take place inside a specially-constructed hangar. This system has had limited interest among airport operators, due to the space and related logistical requirements for the hangar. In the United States, this type of infrared de-icing system has been used, on a limited basis, at two large hub airports and one small commercial airport.[2]:80–81 [5]

Another infrared system uses mobile, truck-mounted heating units that do not require the use of hangars.[6] The manufacturer claims that the system can be used for both fixed wing aircraft and helicopters, although it has not cited any instances of its use on commercial aircraft.[7]

Airport pavement

De-icing operations for airport pavement (runways, taxiways, aprons, taxiway bridges) may involve several types of liquid and solid chemical products, including propylene glycol, ethylene glycol and other organic compounds. Chloride-based compounds (e.g. salt) are not used at airports, due to their corrosive effect on aircraft and other equipment.[2]:34–35

Urea mixtures have also been used for pavement de-icing, due to their low cost. However, urea is a significant pollutant in waterways and wildlife, as it degrades to ammonia after application, and it has been largely been phased out at U.S. airports. In 2012 the U.S. Environmental Protection Agency (EPA) prohibited use of urea-based deicers at most commercial airports.[8]

Roadways

In 2013, an estimated 14M tons of salt were used for deicing roads in North America.[9]

De-icing of roads has traditionally been done with salt, spread by snowplows or dump trucks designed to spread it, often mixed with sand and gravel, on slick roads. Sodium chloride (rock salt) is normally used, as it is inexpensive and readily available in large quantities. However, since salt water still freezes at −18 °C (0 °F), it is of no help when the temperature falls below this point. It also has a strong tendency to cause corrosion, rusting the steel used in most vehicles and the rebar in concrete bridges. Depending on the concentration, it can be toxic to some plants and animals, and some urban areas have moved away from it as a result. More recent snowmelters use other salts, such as calcium chloride and magnesium chloride, which not only depress the freezing point of water to a much lower temperature, but also produce an exothermic reaction. They are somewhat safer for sidewalks, but excess should still be removed.

More recently, organic compounds have been developed that reduce the environmental issues connected with salts and have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are often generated as byproducts of agricultural operations such as sugar beet refining or the distillation process that produces ethanol.[10][11] Other organic compounds are wood ash and a deicing salt called calcium magnesium acetate made from roadside grass or even kitchen waste.[12] Additionally, mixing common rock salt with some of the organic compounds and magnesium chloride results in spreadable materials that are both effective to much colder temperatures (−34 °C or −29 °F) as well as at lower overall rates of spreading per unit area.[13]

Solar road systems have been used to maintain the surface of roads above the freezing point of water. An array of pipes embedded in the road surface is used to collect solar energy in summer, transfer the heat to thermal banks and return the heat to the road in winter to maintain the surface above 0 °C (32 °F).[14] This automated form of renewable energy collection, storage and delivery avoids the environmental issues of using chemical contaminants.

It was suggested in 2012 that superhydrophobic surfaces capable of repelling water can also be used to prevent ice accumulation leading to icephobicity. However, not every superhydrophobic surface is icephobic[15] and the method is still under development.[16]

Chemical de-icers

All chemical de-icers share a common working mechanism: they chemically prevent water molecules from binding above a certain temperature that depends on the concentration. This temperature is below 0 °C, the freezing point of pure water. Sometimes, there is an exothermic dissolution reaction that allows for an even stronger melting power. The following lists contains the most-commonly used de-icing chemicals and their typical chemical formula.

Inorganic salts
Organic compounds
Alcohols, diols and polyols

(these are antifreeze agents and scarcely used on roads)

Fluid types

Deicing Copenhagen Airport
An aircraft being de-iced at Copenhagen Airport with an orange-colored fluid

There are several types of aircraft de-icing fluid, falling into two basic categories:

  1. De-icing fluids: Heated glycol diluted with water for de-icing and snow/frost removal, also referred to as Newtonian fluids (owing to their viscous flow similar to water)
  2. Anti-icing fluids: unheated, undiluted propylene glycol based fluids that has been thickened (imagine half-set gelatin), also referred to as non-Newtonian fluids (owing to their characteristic viscous flow), applied to retard the future development of ice or to prevent falling snow or sleet from accumulating. Anti-icing fluids provide holdover protection against the formation of ice while the aircraft is stationary on the ground. However, when subjected to shearing force such as the air flow over the fluid surface, when an aircraft is accelerating for takeoff, the fluid's entire rheology changes and it becomes significantly thinner, running off to leave a clean and smooth aerodynamic surface to the wing.

In some cases both types of fluid are applied to aircraft, first the heated glycol/water mixture to remove contaminants, followed by the unheated thickened fluid to keep ice from reforming before the aircraft takes off. This is called "a two-step procedure".

Methanol de-ice fluid has been employed for years to de-ice small wing and tail surfaces of small to medium-sized general aviation aircraft and is usually applied with a small hand-held sprayer. Methanol can only remove frost and light ground ice prior to flight.

Mono-ethylene, di-ethylene and propylene glycol are non-flammable petroleum products and similar products are most commonly found in automotive cooling systems. Glycol has very good de-icing properties and the aviation grade is referred to as SAE/ISO/AEA Type I (AMS 1424 or ISO 11075). it is usually applied to contaminated surfaces diluted with water at 95 degrees Fahrenheit (35 °C) using a cherry picker on a truck containing 1,500 to 2,000 US gal (5,680 to 7,570 L; 1,250 to 1,670 imp gal) for on-ramp or departure runway entry point application. Colour-dyed fluid is preferred as it can be confirmed easily by visual observation that an aircraft has received a de-ice application. Runoff of Type I fluid appears to turn slush a pink tinge, hence the term "pink snow." Otherwise, all Type I fluids are orange.

In 1992, the Dead Sea Works began marketing a de-icer based on salts and minerals from the Dead Sea.[17]

In-flight aircraft de-icing

Pneumatic systems

B-17 on bomb run
Boeing B-17 Flying Fortress. The black strips on the leading edges of the tail, stabilizers and wing are de-icing boots made of rubber.

In-flight ice buildups are most frequent on the leading edges of the wings, tail and engines (including the propellers or fan blades). Lower speed aircraft frequently use pneumatic de-icing boots on the leading edges of wings and tail for in-flight de-icing. The rubber coverings are periodically inflated, causing ice to crack and flake off. Once the system is activated by the pilot, the inflation/deflation cycle is automatically controlled. In the past, it was thought such systems can be defeated if they are inflated prematurely; if the pilot did not allow a fairly thick layer of ice to form before inflating the boots, the boots would merely create a gap between the leading edge and the formed ice. Recent research shows “bridging” does not occur with modern boots.[18]

Electric systems

Some aircraft may also use electrically heated resistive elements embedded in a rubber sheet cemented to the leading edges of wings and tail surfaces, propeller leading edges, and helicopter rotor blade leading edges. This de-icing system was developed by United States Rubber Company in 1943.[19] Such systems usually operate continuously. When ice is detected, they first function as de-icing systems, then as anti-icing systems for continued flight in icing conditions. Some aircraft use chemical de-icing systems which pump antifreeze such as alcohol or propylene glycol through small holes in the wing surfaces and at the roots of propeller blades, melting the ice, and making the surface inhospitable to ice formation. A fourth system, developed by NASA, detects ice on the surface by sensing a change in resonance frequency. Once an electronic control module has determined that ice has formed, a large current spike is pumped into the transducers to generate a sharp mechanical shock, cracking the ice layer and causing it to be peeled off by the slipstream.

Air bleed systems

Many modern civil fixed-wing transport aircraft use anti-ice systems on the leading edge of wings, engine inlets and air data probes using warm air. This is bled from engines and is ducted into a cavity beneath the surface to be anti-iced. The warm air heats the surface up to a few degrees above 0 °C (32 °F), preventing ice from forming. The system may operate autonomously, switching on and off as the aircraft enters and leaves icing conditions.

Environmental impacts and mitigation

De-icing salts such as sodium chloride or calcium chloride leach into natural waters, strongly affecting their salinity.[9]

Ethylene glycol and propylene glycol are known to exert high levels of biochemical oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose propylene glycol.[3]:2–23

Sufficient dissolved oxygen levels in surface waters are critical for the survival of fish, macroinvertebrates, and other aquatic organisms. If oxygen concentrations drop below a minimum level, organisms emigrate, if able and possible, to areas with higher oxygen levels or eventually die. This effect can drastically reduce the amount of usable aquatic habitat. Reductions in DO levels can reduce or eliminate bottom feeder populations, create conditions that favor a change in a community’s species profile, or alter critical food-web interactions.[3]:2–30

In one case, a significant snow in Atlanta in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the Atlanta airport.

Some airports recycle used de-icing fluid, separating water and solid contaminants, enabling reuse of the fluid in other applications. Other airports have an on-site wastewater treatment facility, and/or send collected fluid to a municipal sewage treatment plant or a commercial wastewater treatment facility.[2]:68–80 [20]

The toxicity of de-icing fluids is another environmental concern, and research is underway to find less toxic (i.e. non-glycol-based) alternatives.[21][22]

See also

References

  1. ^ 2012 Autumn & Winter Season (Drivers' Briefing). London, UK: First Capital Connect. September 2012.
  2. ^ a b c d e Technical Development Document for the Final Effluent Limitations Guidelines and New Source Performance Standards for the Airport Deicing Category (Report). Washington, D.C.: U.S. Environmental Protection Agency (EPA). April 2012. EPA-821-R-12-005.
  3. ^ a b c Environmental Impact and Benefit Assessment for the Final Effluent Limitation Guidelines and Standards for the Airport Deicing Category (Report). EPA. April 2012. EPA-821-R-12-003.
  4. ^ Stefl., Barbara A.; George, Kathleen F. (2014), "Antifreezes and Deicing Fluids", Kirk-Othmer Encyclopedia of Chemical Technology, New York: John Wiley, doi:10.1002/0471238961.0114200919200506.a01.pub2, ISBN 9780471238966
  5. ^ Rosenlof, Kim (2013-10-02). "Infrared De-icing Speeds Process and Reduces Cost". Aviation International News Online. Midland Park, NJ.
  6. ^ APS Aviation, Inc. (December 1998). Deicing with a Mobile Infrared System (Report). Montreal, Quebec. Report prepared for Transport Canada.
  7. ^ "Ice Cat Aircraft Deicing System". Bonner Springs, KS: Trimac Industries. 2004. Archived from the original on 2016-06-20. Retrieved 2016-05-29.
  8. ^ "Airport Deicing Effluent Guidelines". EPA. 2016-04-21.
  9. ^ a b Miguel Cañedo-Argüelles, Ben J. Kefford, Christophe Piscart, Narcís Prata, Ralf B.Schäferd, Claus-Jürgen Schulze (2013). "Salinisation of Rivers: An Urgent Ecological Issue". Environmental Pollution. 173: 157-167. doi:10.1016/j.envpol.2012.10.011.CS1 maint: multiple names: authors list (link)
  10. ^ Amanda Rabinowitz (February 25, 2008). "Beets Part of New Recipe to Treat Icy Roads". National Public Radio.
  11. ^ Richard J. Brennan (January 21, 2012). "Beet juice melts ice from winter roads". Toronto Star.
  12. ^ The alternatives to salt for battling ice: cheese, beets and ash
  13. ^ "About Magic Salt". 2007. Archived from the original on 2009-06-05.
  14. ^ "Thermal Energy Storage in ThermalBanks for under runway heating". ICAX Ltd, London. Retrieved 2011-11-24.
  15. ^ Nosonovsky, M.; Hejazi, V. (2012). "Why superhydrophobic surfaces are not always icephobic". ACS Nano. 6 (10): 8488–8913. doi:10.1021/nn302138r.
  16. ^ Hejazi, V.; Sobolev, K.; Nosonovsky, M. I. (2013). "From superhydrophobicity to icephobicity: forces and interaction analysis". Scientific Reports. 3: 2194. Bibcode:2013NatSR...3E2194H. doi:10.1038/srep02194. PMC 3709168. PMID 23846773.
  17. ^ Dead Sea product melts snow. (Dead Sea Works markets snow melting compound)
  18. ^ aopa.org pdf file Archived February 2, 2007, at the Wayback Machine
  19. ^ "De-Icer for Airplane Propeller is Made of Electric Rubber" Popular Mechanics, December 1943
  20. ^ Tom Gibson (September 2002). "Let the Bugs Do the Work". Progressive Engineer. Retrieved 21 February 2011.
  21. ^ U.S. Federal Aviation Administration. Airport Cooperative Research Program (April 2010). "Alternative Aircraft and Pavement Deicers and Anti-icing Formulations with Improved Environmental Characteristics." Research Results Digest 9.
  22. ^ SAE International (2011). "Issues and Testing of Non-Glycol Aircraft Ground Deicing Fluids." Archived 2013-02-02 at the Wayback Machine doi:10.4271/2011-38-0058
Air Ontario

Air Ontario Inc. was a regional Canadian airline headquartered in Sarnia then London, Ontario. In 2002 Air Ontario became Air Canada Jazz.

Boeing 247

The Boeing Model 247 was an early United States airliner, considered the first such aircraft to fully incorporate advances such as all-metal (anodized aluminium) semimonocoque construction, a fully cantilevered wing and retractable landing gear. Other advanced features included control surface trim tabs, an autopilot and de-icing boots for the wings and tailplane."Ordered off the drawing board", the 247 first flew on February 8, 1933 and entered service later that year. Subsequent development in airliner design saw engines and airframes becoming larger and four-engined designs emerged, but no significant changes to this basic formula appeared until cabin pressurization and high altitude cruise were introduced in 1940, with the Boeing 307 Stratoliner.

Brine

Brine is a high-concentration solution of salt in water. In different contexts, brine may refer to salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of solutions used for brining foods) up to about 26% (a typical saturated solution, depending on temperature). Lower levels of concentration are called by different names: fresh water, brackish water, and saline water.

Brine naturally occurs on the Earth's surface (salt lakes), crust, and within brine pools on ocean bottom. High-concentration brine lakes typically emerge due to evaporation of ground saline water on high ambient temperatures. Brine is used for food processing and cooking (pickling and brining), for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, and may pose an environmental risk due to its corrosive and toxic effects, so it requires wastewater treatment for proper disposal or further utilization (fresh water recovery).

British Rail Class 930

The British Rail Class 930 was reserved for former Southern Region electrical multiple units and diesel-electric multiple units converted for departmental use. Originally the series was reserved for de-icing and, later, sandite units. In recent years, however, other types have also been given numbers in this series. Electric and diesel units are dealt with separately

British Rail Class 936

The British Rail Class 936 was reserved for former electrical multiple units not from the South-East, converted for departmental use. Units were converted for various tasks, including application of sandite, and de-icing duties.

Merseyrail Units

Scottish Units

British Salt

British Salt Limited is a United Kingdom-based chemical company that produces pure white salt. The company is owned by Tata Chemicals Europe after a buy out from private equity company LDC in April 2010. It is based in Middlewich, Cheshire, employs 125 people, and produces approximately 800,000 tonnes of pure white salt every year.LDC bought British Salt from its previous owners, US Salt Holdings LLC in 2007, investing £35m in the company. A management team has taken a minority stake. US Salt had bought British Salt from its previous owners, Staveley Industries plc in 2000 for £80m. In 2005, British Salt acquired New Cheshire Salt Works Limited, known as NCSW Limited. This acquisition was referred to the Competition Commission who approved the purchase. Since the purchase the NCSW site in Wincham has been closed and the 198-acre (0.80 km2) site sold to Chantry Developments.The salt is extracted from strata that lie approximately 180 metres below ground. Bore holes are drilled into the strata and water is forced down to dissolve the salt. The resulting brine solution is pumped along 5 km of pipes back to the surface and direct into the Middlewich factory for purification and water evaporation to produce the pure salt. It is estimated that there are salt reserves sufficient for 200 years. The main uses for the salt products include:

Water softeners

Chemical industry

Food processing

Animal feeds

Textiles and tanningDuring the severe weather experienced in the UK in February 2009, British Salt also started to supply low-grade salt for de-icing of roads, after local authorities announced they were running very low on salt used for gritting due to the unexpected weather.

Calcium chloride

Calcium chloride is an inorganic compound, a salt with the chemical formula CaCl2. It is a white coloured crystalline solid at room temperature, highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide.

Calcium chloride is commonly encountered as a hydrated solid with generic formula CaCl2(H2O)x, where x = 0, 1, 2, 4, and 6. These compounds are mainly used for de-icing and dust control. Because the anhydrous salt is hygroscopic, it is used as a desiccant.

Continental Airlines Flight 1713

Continental Airlines Flight 1713 was a commercial airline flight that crashed while taking off in a snowstorm from Stapleton International Airport in Denver, Colorado on November 15, 1987. The Douglas DC-9 was operated by Continental Airlines and was a scheduled flight to Boise, Idaho. Twenty-five passengers and three crew members died in the crash.

Deicing boot

A deicing boot is a type of ice protection system installed on aircraft surfaces to permit a mechanical deicing in flight. Such boots are generally installed on the leading edges of wings and control surfaces (e.g. horizontal and vertical stabilizer) as these areas are most likely to accumulate ice and any contamination could severely affect the aircraft's performance.

Deicing fluid

Ground deicing of aircraft is commonly performed in both commercial and general aviation. The fluids used in this operation are called deicing or anti-icing fluids. The initials ADF (Aircraft Deicing Fluid), ADAF (Aircraft Deicer and Anti-icer Fluid) or AAF (Aircraft Anti-icing Fluid) are commonly used.

Ice protection system

Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft surfaces (particularly leading edges), such as wings, propellers, rotor blades, engine intakes, and environmental control intakes. If ice is allowed to build up to a significant thickness it can change the shape of airfoils and flight control surfaces, degrading the performance, control or handling characteristics of the aircraft. An ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it can grow to a dangerous thickness.

Levis De-Icer

The Levis De-Icer is a High voltage direct current (HVDC) system, aimed at de-icing multiple AC power lines in Quebec, Canada. It is the only HVDC system not used for power transmission.

In the winter of 1998, Québec's power lines were toppled by icing, sometimes up to 75 mm. To prevent such a damage, a de-icing system was developed.The Levis De-Icer can use a maximum power of 250 MW; its operation voltage is ±17.4 kV. It can be used on multiple 735 kV AC power lines.

When there is no icing, the Lévis De-Icer installed at Hydro-Québec's Lévis substation operates as static VAR compensator improving the stability of the AC lines.

London Underground sleet locomotives

Sleet locomotives were redundant London Underground cars converted to help with the removal of ice that built up on the conductor rails. The main batch of eighteen tube-gauge locomotives were built between 1938 and 1941 from motor cars originally built in 1903. They were refurbished in the 1960s using equipment removed from redundant T-stock vehicles, and were joined by a pair of surface-gauge locomotives in 1961. In addition to de-icing duties, some of them were also used for experiments in clearing leaves from the running rails. They had all ceased to operate by 1985. One of the tube-gauge locomotives subsequently went to the London Transport Museum, and the surface-gauge cars went to the Spa Valley Railway.

Road ecology

Road ecology is the study of the ecological effects (both positive and negative) of roads and highways (public roads). These effects may include local effects, such as on noise, water pollution, habitat destruction/disturbance and local air quality; and wider effects such as habitat fragmentation, ecosystem degradation, and climate change from vehicle emissions.

The design, construction and management of roads, parking and other related facilities as well as the design and regulation of vehicles can change their effect. Roads are known to cause significant damage to forests, prairies, streams and wetlands. Besides the direct habitat loss due to the road itself, and the roadkill of animal species, roads alter water-flow patterns, increase noise, water, and air pollution, create disturbance that alters the species composition of nearby vegetation thereby reducing habitat for local native animals, and act as barriers to animal movements. Roads are a form of linear infrastructure intrusion that has some effects similar to infrastructure such as railroads, power lines, and canals, particularly in tropical forests.Road ecology is practiced as a field of inquiry by a variety of ecologists, biologists, hydrologists, engineers, and other scientists. There are several global centers for the study of road ecology: 1) The Road Ecology Center at the University of California, Davis, which was the first of its kind in the world; 2) the Centro Brasileiro de Estudos em Ecologia de Estradas at the Federal University of Lavras, Brazil; 3) The Center for Transportation and the Environment, North Carolina State University; and 4) the Road Ecology Program at the Western Transportation Institute, Montana State University. There are also several important global conferences for road ecology research: 1) Infra-Eco Network Europe (IENE), which is international, but focused primarily on Europe; 2) International Conference on Ecology and Transportation (ICOET), which is also global in scope, but primarily focused on the US; 3) Australasian Network for Ecology & Transportation (ANET), which focuses on the Australasian (sub)continent; and 4) a potential Southern African road ecology conference, being considered by the Endangered Wildlife Trust.

Snow removal

Snow removal or snow clearing is the job of removing snow after a snowfall to make travel easier and safer. This is done by both individual households and by governments and institutions.

Sodium chloride

Sodium chloride , commonly known as salt (though sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/mol respectively, 100 g of NaCl contains 39.34 g Na and 60.66 g Cl. Sodium chloride is the salt most responsible for the salinity of seawater and of the extracellular fluid of many multicellular organisms. In its edible form of table salt, it is commonly used as a condiment and food preservative. Large quantities of sodium chloride are used in many industrial processes, and it is a major source of sodium and chlorine compounds used as feedstocks for further chemical syntheses. A second major application of sodium chloride is de-icing of roadways in sub-freezing weather.

Total dissolved solids

Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular (colloidal sol) suspended form. Generally, the operational definition is that the solids must be small enough to survive filtration through a filter with 2-micrometer (nominal size, or smaller) pores. Total dissolved solids are normally discussed only for freshwater systems, as salinity includes some of the ions constituting the definition of TDS. The principal application of TDS is in the study of water quality for streams, rivers, and lakes. Although TDS is not generally considered a primary pollutant (e.g. it is not deemed to be associated with health effects), it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants.

Primary sources for TDS in receiving waters are agricultural runoff and residential (urban) runoff, clay-rich mountain waters, leaching of soil contamination, and point source water pollution discharge from industrial or sewage treatment plants. The most common chemical constituents are calcium, phosphates, nitrates, sodium, potassium, and chloride, which are found in nutrient runoff, general stormwater runoff and runoff from snowy climates where road de-icing salts are applied. The chemicals may be cations, anions, molecules or agglomerations on the order of one thousand or fewer molecules, so long as a soluble micro-granule is formed. More exotic and harmful elements of TDS are pesticides arising from surface runoff. Certain naturally occurring total dissolved solids arise from the weathering and dissolution of rocks and soils. The United States has established a secondary water quality standard of 500 mg/l to provide for palatability of drinking water.

Total dissolved solids are differentiated from total suspended solids (TSS), in that the latter cannot pass through a sieve of 2 micrometers and yet are indefinitely suspended in solution. The term settleable solids refers to material of any size that will not remain suspended or dissolved in a holding tank not subject to motion, and excludes both TDS and TSS. Settleable solids may include larger particulate matter or insoluble molecules.

Ulmus americana 'American Liberty'

The American Elm cultivar Ulmus americana 'American Liberty' is in fact a group of six genetically distinct cultivars under a single name, although they are superficially similar. The Liberty elm is reportedly suitable for street planting, being tolerant of de-icing salts and air pollution. However, examples included in 10-year trials at Atherton, California to evaluate replacements for Californian elms lost to disease did not perform well. The late Professor Eugene Smalley summarized 'American Liberty' as "not as resistant as the Asian hybrids, but it still has the look of a classic American Elm" [2]

Water–cement ratio

The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix difficult to work with and form. Workability can be resolved with the use of plasticizers or super-plasticizers.

Often, the ratio refers to the ratio of water to cementitious materials, w/cm. Cementitious materials include cement and supplementary cementitious materials such as fly ash, ground granulated blast-furnace slag, silica fume, rice husk ash and natural pozzolans. Supplementary cementitious materials are added to strengthen concrete.

The notion of water–cement ratio was first developed by Duff A. Abrams and published in 1918. Refer to concrete slump test.

The 1997 Uniform Building Code specifies a maximum of 0.5 ratio when concrete is exposed to freezing and thawing in a moist condition or to de-icing chemicals, and a maximum of 0.45 ratio for concrete in a severe or very severe sulfate condition.

Concrete hardens as a result of the chemical reaction between cement and water (known as hydration, this produces heat and is called the heat of hydration). For every pound (or kilogram or any unit of weight) of cement, about 0.35 pounds (or 0.35 kg or corresponding unit) of water is needed to fully complete hydration reactions.However, a mix with a ratio of 0.35 may not mix thoroughly, and may not flow well enough to be placed. More water is therefore used than is technically necessary to react with cement. Water–cement ratios of 0.45 to 0.60 are more typically used. For higher-strength concrete, lower ratios are used, along with a plasticizer to increase flowability.

Too much water will result in segregation of the sand and aggregate components from the cement paste. Also, water that is not consumed by the hydration reaction may leave concrete as it hardens, resulting in microscopic pores (bleeding) that will reduce final strength of concrete. A mix with too much water will experience more shrinkage as excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners), which again will reduce the final strength.

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