Today's world is faced with various emerging and challenging issues such as acceleration of world population which is increasingly facilitating the amounts and types of wastes generation during the production and consumption processes. There has been waste disposal crisis globally due to creation of non degradable or non decaying wastes, attributed to the growing consumer behavior. In response to this challenge, one of the solutions is the process of recycling the wastes into more useful products through research and innovation practices.
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This has brainstormed individuals, private organizations and highway agencies to develop and implement wastes application through analyzing the feasibility, performance, and environmental suitability of reuse of the wastes and more so in highway construction. Highway construction is an expansive project but use of waste products in this industry makes the undertaking a cost-effective process, thereby ensuring safe and economic use of waste materials (Horan, Gifford, 2001, 9).
Commonly used recycled materials
One of the waste materials widely used in highway construction is recycled plastics. Virgin polythene is used as an additive or component to asphalt concrete and asphalt cement additive. When sandwich bags and plastic trash are recycled, a recycled low density polyethylene resin is obtained that is used to manufacture both cement and concrete asphalts in form of pallets. Recycled plastic can also be used in the manufactured of Portland cement concrete through use of recycled high density polyethylene which replaces fine aggregate with light weight concrete. After application of these recycled plastics in the construction process, flexural toughness of the concrete increase, compressive strengths are maintained and impact resistance of the concrete improves generally. Guardrail, a product of recycled wood and plastics has been brought forth to replace block in construction of guardrail posts. This facility is cheaper compared to the use of conventional block materials. The end product is also of high quality because they increase resistance against damage and deterioration, thus achieving reduction in overall life cycle cost. Noise pollution has also emerged in the processes of production around the globe, causing health, brain and social problems.
To evade this, noise walls made from recycled rubber and plastics have been developed. The shell of the wall is made of thermosetting, a byproduct of glass and polyester. The fill section of the wall is composed of ground, rubber tires and recycled plastics (Tibbets, 2002, 12).
In the developed and industrialized nations, glass is one of the waste products released in high quantities. It is later recycled to cullet, a component in the glass manufacturing industry. In the construction field, glass can be used as an aggregate in the asphalt pavements or asphalt base courses which satisfactorily serve the normal conventional pavement functions. Glass cullet has been approved for characteristics such as permeability, gradation, compaction, durability, specific gravity, thermal conductivity, and their strength and workability as an aggregate in the construction processes. In addition, recycled glass is being used in the manufacture of glass beads which are used as traffic control devices (Tibbets, 2002, 14).
According to Carpenter (2001, 19), "the combustion process is common and widely used in production and in the day to day activities. This happens mostly during coal combustion which releases wastes like fly ash, bottom ash and combined ash. Among other residues, ash is produced, especially when municipal solid wastes are burnt or depending on the type of coal burnt". This ash is characterized by different physical and chemical aspects of toxicity and leaching ability. When coal containing sulfur is burnt, sulfur dioxide is released. This calls for use of scrubbers to reduce the amount of sulfur dioxide used. Bottom ash or slag is larger with heavier particles that are found at the bottom of hopper after the combustion process, and complies with environmental standards of toxicity. Bottom ash has sufficient engineering properties and uses. It has been used as aggregate for light weight concrete, sandblasting abrasive and as an abrasive in pavement deicing products and materials.
Combined ash is the product of mixing both fly and bottom ashes, which are placed in landfills. Depending on the type of plant and coal source, it physically includes gradation, specific gravity and loss on ignition. Combined ash is used as partial fine aggregate replacement in the asphalt concrete mixes. Fly ash contains higher percentage of heavy metals such as cadmium and lead, leachate and other acidic components. In highway construction, incinerator residue is used as a partial aggregate substitute in the asphalt concrete base course. The performance of the substitute is equal to the conventional asphalt pavements (Carpenter, 2001, 21).
Ash can also be used in stabilized and unstabilized bases and sub-bases as an aggregate, because the original components are aluminium oxide, iron oxide, calcium oxide and silicon oxide. In the process of highway construction, coal fly ash can be used as mineral admixture to Portland cement concrete, road base stabilization, roller compacted concrete and soil stabilization. Ponded fly ash has also performed well as a component in stabilized aggregate base course. This kind of ash is first sluiced into a disposal pond. Tests have shown that this experimental mix outperforms conventional mix and the rutting rate is significantly lower. Results from the various applications show that cement-treated ash produce higher density and compressive strengths compared to conventional soil cement. Leaching tests upon cement-treated ash have shown compatible and accommodative levels of cadmium, lead and copper that meet legal drinking water specifications of various agencies (Foster, 2008, 13).
Highway related applications show a wide use of crumb rubber modified asphalt, which has good performance and environmentally friendly. Recycled rubber tires in combination with recycled plastics are recently being employed in making of noise walls' lightweight fill, channel slope protection, sub grade insulation and as an additive to the Portland cement concrete pavement. Ground scrap tires are also used in manufacturing Portland cement concrete. This happens when scrap tires are processed by removing the loose steel and fibers, which are finally ground.
The ground rubber then acts as a substitute for fine aggregate. However, when high amount of rubber is used, the flexural and compressive strengths decrease. Gravel surfaced roads in most times experience severe deterioration in seasons of spring thawing because of frost penetration from beneath. To counteract this scenario, tire chips have acted as effective insulation layers against the frost. Its performance has been approved by use of resistivity gauges, groundwater monitoring wells, weather station and thermocouples. Recycled tires can also be used to make blocks for constructing retaining walls and landscaping. Through recent technology, scrap tires can be converted into a form for making stakes or poles. The tires are split and flattened, rolled in a spiral fashion and finally form almost solid log of reinforced rubber material (Foster, 2008, 15).
Carpet industries around the world also produce carpet waste fibers. These fiber wastes have been incorporated in the manufacture of alphalt pavements and Portland cement concrete. Recycled fibers obtained from old carpets can also be used for concrete reinforcement, when the fibers are mixed with concrete in standard drum mixer. Flexural and compressive strengths comparison between concrete free of fibers and concrete with a percentage of virgin polypropylene fibers indicate good appreciable effects markedly handle the issue of compressive strengths. The toughness indices indicate that the more the amount of carpet used, the higher the concretes energy absorption abilities.
On this same field, waste nylon fibers can be used to reduce the level of plastic shrinkage cracking. This is performed when the fibers are packed in water soluble bags and latter added to fresh concrete in the time of mixing. There is no observed or suggested difference from the conventional concrete with reference to flexural and compressive strengths. Increased addition of nylon fibers into Portland cement concrete panels proportionately reduces the plastic shrinkage and heating. This is because when nylon fiber is used in construction and heating panels, the rate of radiation increases and thus helps in prevention of cracking.
Recycled fibers have also been proved to be increase efficiency when used in dense graded asphalt mix and polymer modified. Tests made on these include resilient modulus test, indirect tensile strain, lottman stripping tests and indirect tensile stress. According to these tests, there is no appreciable deviation from the performance of commercially available fibers. In the real application, addition of asphalt is primarily required to coat the surface area of those fibers compared to the thickness of the fiber films (Allen, 1998, 10).
Another waste known as roofing shingle waste can be used in the make of asphalt pavements. When they are added to dense graded mixes, they result to a reduction in optimum content of neat binder, develop and enhance the ability to densify under compaction, increasing the component of plastic strain in permanent deformation measurements. It also reduced cold tensile strengths, of which the impact was determined by the type of shingle waste and grade of asphalt cement. However addition of more roofing shingle wastes reduced the potentiality for thermal tracking.
The same material can be used in stone matrix asphalt mixes, which results to an admirable content of neat binder content. Shingles are ground uniformly to resemble coffee grounds and then added to drum mix plant to undergo a recycling process for the construction of asphalt pavement. So far, no pavement construction performance has been reported or detected since the recycling process commissioned. Old roofing material is also applied when making asphalt cold patch, which lasts three months more than the conventional cold patch material (Allen, 1998, 22).
Flue gas desulphurization waste is another common waste. In the highway construction network, it is used in stabilized road bases to act as an embankment and pavement material. It is trusted to make dry weight of great early strength and high sulfate resistant Portland cement. So far, there is no any evidence of physical deterioration associated with the use of the waste as waste embankment.
According to current researches, some materials have been categorized to compose the best set of wastes to use in highway construction. Such materials include rubber tires, recycled plastics, roofing shingle wastes, glass, carpet fibers, municipal solid waste combustion ash, blast furnace, steel slags, coal ash byproducts, fly ash, FGD waste and bottom ash (Horan, Gifford, 2001, 11).
In larger part of the world, there is an increasing concern on the public, federal government, state regulators and industries to explore, invent and develop the use of recycled materials in the highway environment. Although this is an historical process, it is still new in the developing countries. The use, management and regulation of recycled materials is usually jurisdictional, a role attached to state transport and environment agencies. Most of the by-product materials are generated in the transport sector, municipal sector, industrial sector and mining sector.
The wastes released from the transport sector include reclaimed asphalt and Portland cement concrete pavements, street sweepings, dredge materials and excess fill. Materials from industrial sector include blast furnace slag, nonferrous slags (phosphate, copper, and zinc), steel slag, coal combustion residues (fly ash, boiler slag, bottom slag, and flue gas desulphurization wastes), kiln dusts (lime-kiln and cement), baghouse dusts (smelters and asphalt plants), slags and foundry sands. Materials from municipal sector include biosolids, scrap tires, waste glass, wood waste, roofing shingle scrap, wastewater sludge ash, petroleum contaminated soils, municipal solid waste combustor residues, demolition and construction debris. Materials from the mining sector include quarry waste, mill tailings and phosphogypsum.
These materials are used to replace conventional ones in fabrication of highway appurtenances like guardrails, signs and bridges, used as substitutes for pavement structure, as supplementary and aggregates cementitious materials especially in asphalt and Portland cement concrete, stabilized base and subbase or granular, fill, landscaping materials and as substitute embankment (Tibbets, 2002, 16).
Mining industry generates high amounts of by-product materials, inaccessibility of mining operations with respect to the major metropolitan zones where demand for highway construction is high and there exist deficiency. This fact limits the potentiality of using large amounts of these resources widely.
In conclusion, the world faces and will continue to face environmental conservation problems due to cultural, economic, social and political diversity. The aspect of cooperative advantage is driving environmental conditions far from the safe side because countries are utilizing the available resources ignoring chances of possible harm to the rest of the world. The changing consumer behaviors and competition is calling for hi-tech production means, thus employment of production means which are harmful to the atmosphere. The problem has not been the emissions, but the disposal mechanisms. Existing landfills are filling up.
One solution to this mayhem is to develop and implement beneficial uses for the residues. Highway construction uses great amounts of diverse wastes. Emphasized use of residue byproducts in lieu of virgin materials relieves disposal burden and avail inexpensive and advantageous construction products. Most of the waste materials used in the highway construction have limited applications in other fields to an extend that only such constructions can utilize those particular wastes. Again, most of them are non degradable and non-decaying such that they are very harmful to the environment and accumulation of such can extend adverse impacts upon the lives of individuals (Carpenter, 2001, 27).
Therefore, use of recycled materials in highway construction should be imposed and implemented by all nations to contain the wastes disposal crisis threatening the world.
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