Generally for bituminous or asphalt pavement, the aggregates constitute 88% to 96% by weight or more than 75% by volume. The AASHTO standard specifications provide that: “The aggregate shall consist of hard, durable particles of fragments of stone or gravel and sand or other fine mineral particles free from vegetable matter and lumps or balls of clay and of such nature it can be compacted readily to form a firm, stable layers. It shall conform to the grading requirements shown in table 3 when tested by AASHTO T-11 and 27”.
The following materials are classified under Item 300 of the DPWH standard specifications. The coarse aggregate material retained on the 2.00 mm (No.10) sieve shall have a mass percent of wear by the Los Angeles Abrasion Test (AASHTO T-96) of not more than 45. When crushed aggregate is specified, not less than 50 mass percent of the particles retained on the 4.75 mm (No. 4) sieve shall not have at least one fractured face. The fraction passing the 0.75 mm (No. 200) sieve should not be greater than two thirds of the fraction passing the 0.425 mm (No. 40) sieve. The fraction passing 0.425 mm (No.40) sieve shall have a liquid limit of not greater than 35 and a plasticity index range of 4 to 9 when tested by AASHTO T-89 and T-90respectively.
The presence of organic impurities in the intended for concreting road pavement may cause slow or non-hardening of the concrete. Under AASHTO T-21 standard test, the aggregate is treated with a mixture of Sodium Hydrochloride Solution and when the treated aggregate turns dark, organic materials are said to be present in the aggregate. The strength of fine aggregate is measured by the compression tests of sand-cement mortar. Soundness of fine aggregate is measured by their resistance to deterioration under the action of solutions of Sodium or Magnesium Sulfate. The sodium sulfate test is five cycle. The maximum loss under AASHTO specifications is 10%.
For coarse aggregate the requirement consists of crushed stone, gravel, blast furnace slag, or approved inert materials of similar characteristics or combination thereof having hard, strong durable pieces free from adherent coatings.
The Department of Public Works and Highways standard specifications classify aggregate under Item 703, and specifically provides that: Aggregate shall consist of hard, durable particles of fragments of crushed stone, crushed slug or crushed or natural gravel. Coarse aggregate is the material retained on the 2.00 mm (No. 10) sieve and shall have a percentage of water or more than 50 for sub-base and not more than 45 for base and surface courses as determined by AASHTO designation test T-96. Fine aggregate is the material passing the No. 10 sieve (2.00 mm) consisting of natural, crushed sand and fine minerals particles. The Fraction Passing the 0.075 mm (No. 200) sieve should not be greater than 0.66 (2/3) of the fraction passing the 0.425 mm (No. 40) sieve.
The strength of the road pavement will be increased if dust additives which denses the graded mixture is added. It is called mineral filler which reduces the void contents in the mixture. This dust additive is not the ordinary dust that is being found in our floor or tables.
Dust additive is classified into: Finely powdered limestone Slag Hydrated lime Portland cement Trap rock dust Fly ash
The DPWH standard specifications relative to mineral filler states that: “Mineral filler shall consist of finely divided mineral matter such as rock dust, slag dust, hydrated lime, hydraulic cement, flyash or other suitable mineral matter. It shall be free from organic impurities and at the time of use shall be sufficiently dry to flow freely and shall be essentially free from agglomerations.” AASHTO M-17 provides that: Percentage passing by weight shall be as follows: No. 30 (0.66 mm) sieve 50 No. 50 (0.30 mm) sieve 95-100 No. 200 (0.75 mm) sieve 70-100 AASHTO further stipulates that for all materials other than hydrated lime or Portland cement, the Plastic Index (PI) value shall be 4 or less.
Bituminous material or asphalt is a viscous (gelatinous) liquid used as binder for aggregates in road construction. At normal temperature, asphalt is either lightly thicker than water or hard but brittle material that breaks under a hammer blow when cold. Bituminous material is in liquid form when mixed or combined with aggregates. This liquid form maybe produced either by heating the hard asphalt, by dissolving in solvent or by emulsifying in water. However, there are bituminous liquid materials available and ready to use. The action of the asphalt binder depends on its type and the aggregate it is combined with. The purpose of the asphalt binder is to resist the abrasive force brought about by heavy traffic.
If the pavement is of the open type, consisting entirely of coarse particles and asphalt, heavy binder is needed requiring more asphalt. On the other hand, if the aggregates on the pavement contain fine particles, cohesion will be developed by the surface tension in the thin asphalt film surrounding these fine [articles hence, less viscous asphalt is required.
Asphalt cement is used as binder for almost high types of bituminous pavement. Asphalt cement is a semi- solid hydrocarbons retained after fuel and lubricating oils are removed from petroleum. The softest grade used for pavement is the 200-300 penetration and the hardest is the 60-70 penetration. Penetration refers to the consistencies of asphalt cement as describes under AASHTO T-49. It is the distance that a standard needle penetrates a sample under known conditions of loading time and temperature. Recently, the procedure used in grading asphalt cement is viscosity test rather than the penetration test.
Cutback or Liquid Asphalt The liquid asphalt is a petroleum product consisting of asphalt cement with a liquid distillate (diesel, kerosene or gasoline). The less viscous asphalt contains up to 50% diluent and the more viscous contains diluent as little as 15%. The use of cutback is being frowned for two reasons: It is a usable fuel. It is an air pollutant. Cutback or liquid asphalt is classified into: Slow Curing (SC) road soils Medium Curing (MC) cutback asphalt Rapid Curing (RC) cutback asphalt
Emulsified Asphalt is a kind of mixture wherein the minute globules or asphalt disperses in water. Asphalt content ranges from 55% - 75% by weight. Emulsion could be applied or mixed at normal temperature, because when the water content evaporates the asphalt remains.
Oxidized Asphalt and Road Tar Oxidized asphalt is suitable only for roofing and similar applications. Highway uses of oxidized asphalt are limited to water proofing of structures and filling joints of concrete pavement. Road tar is a by-product of the distillation process of coal. Tars are produced from gashouse coal tar; cook ovens tars and water gas tar.Bitumen – Rubber Mixture for road pavement was experimented in Holland way back in the year 1929. It was adapted in the United States in 1947 and later in European countries.
Epoxy Resins as Binders Epoxy binders are produced in clear, dark, rigid and flexible forms for application to either concrete or asphalt pavement. Hardening is attained by mixing resin and catalyst hardener immediately before application. The result is thermosetting. Meaning, it will not soften under the influence of heat or the action of solvent such as water or petroleum products. But the prohibitive cost of the resin has restricted its use to bridge surfacing and to other special non-skid seal coating surface only.
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