Bitumen and tar binders

BITUMEN AND TAR MATERIALS

Materials of this group (asphalt materials) include a great number of road, roofing, waterproofing and other materials, which are produced from bitumen and tar cementing materials. They are used as emulsions, pastes, mastics, mortars and concretes, roll and piece products with such characteristic features as high waterproofing and chemical resistance and adhesiveness.

An overwhelming majority of bitumen and tar materials is applied in asphalt concrete manufacture. Asphalt concrete coatings are used on highways of any load rating. They are characterized by sufficient mechanical strength, required elastic and plastic deformations, and good adhesion with tires and easy in mending. Asphalt carpets are widely used in aerodrome construction. Asphalt materials are also applied for waterproofing, anticorrosive protection of pits, pipes and water storages.

Bitumen and tar varnishes and paints are used for waterproofing concrete, wooden and metal structures, hot- and cold- asphalt mastics and mortars are applied for plastering damp-course, and roll materials - for membrane waterproofing.

Considerable part of civil and industrial roofs of buildings are constructed with use of bitumen, roll and mastic roofing materials. Effective and endurable roofing materials include glassy bitumen felt, overlay bitumen felt, bitumen felt with colored grit and other. Asphalt concrete properties are improved fundamentally by adding polymer additions.

Bitumen and tar binders

Common specifications. Bitumen and tar along with synthetic polymers are organic binders and consist of high-molecular hydrocarbons and their non-metal derivative mixes.

Bitumen and tar are produced from different materials and in different ways, but they are united by common properties, which is caused because of similarity of structure and composition. The most important components as for bitumen, so for tar are oils and resins, which are flow medium with dispersed solid phases in it: asphaltenes for bitumen, free carbon for tar.

Asphaltenes are high-molecular compounds with molecular weight between 900–6000, which contains mainly compounds of carbon with oxygen, sulphur and nitrogen. Modifications of asphaltenes, which appear at more deep oxidization, are carbenes and carboides.

Resins have molecular weight from 500 to 2000, their consistency changes from viscous and sticky to solid amorphous mass.

Bitumen and tars are characterized by a micellar structure. The kernels of micelle are asphaltenes in bitumen - particles of colloid sizes (<20 micrometers), and in tars is a free carbon. The shell of micelle in both cases consists of oils and resins. Aggregate state of bitumen and tars depends on correlation in them of oils and solid components, and also from a temperature. When the oil content of micelles is excessive, there is no contact and system is in a liquid state. Reduction of viscidity and transferring into the liquid takes place also on heating, and results in destruction of micelle. At lower temperatures, the concentration of micelles is increased, come in a direct contact and consequently, the system passes into a viscid or solid state.

The main qualitative parameters of bitumen and tars are viscidity, deformability and heat capacity. These properties depend mainly on their composition. While increasing content of oils, the viscidity reduces, deformation ability grows and the softening temperature decreases. Resins stipulate the cementing properties of bitumen and tars, give them plasticity, and increase tack (adhesion ability). Asphaltenes in bitumen and the free carbon in tars improve the softening temperature and solidness.

Viscidity of tars and liquid bitumen is determined by viscosimeters which measure time of flow through an opening with diameter 5 or 10 mm at a constant temperature.

Viscidity of the semisolid and solid bitumen is relatively estimated from the depth of penetration of certain diameter needle at a temperature of 25°С into the tenth part of millimeter (degrees of penetration). With the increase of temperature viscidity of tars falls quicker than that of bitumen, due to insignificant tar micelles firmness.

Plastic flow capacity is determined for bitumen. It is insignificant for a solid tar product (pitch). Deformability of bitumen is characterized by extension (inklength) - ability to stretch in a filament. The greater the extension, the higher plasticity will be. Solid bitumen and tar materials, as well as other materials of amorphous structure, don’t have a specific temperature of melting, but are characterized by the softening temperature, which is determined on a standard tester «ring and ball». A softening temperature is one at which bitumen or solid residue after the distillation of tar (pitch), embedded in a ring, under the action of steel ball is pressed out at a certain depth.

It is also usual for the flash temperature, fragility, solubility and other properties of bitumen and tar binders to be determined if it is required.

In the final analysis, the basic properties of bitumen and tars are determined by their chemical composition with which they are interconnected. So, increasing penetration and stretching will be accompanied by diminishing softening temperature and vice versa.

Bitumen and tar binders are hydrophobic materials which do not moisten in water and are water-insoluble. Values of their real and average density range between 0.9-1.3 g/cm³ and are practically equal. Due to high water resistance and insignificant porosity, they are successfully used as basic components of waterproofing materials which have high water impermeability.

Bitumen and tars are chemically inert to water solutions of mineral salts and acids, which allow them to be widely applied for corrosive protection. However, concentrated alkalis and strong concentrated acids which have oxidizing ability would have aggressive influence on these materials (e.g, 10% solution of alkali causes the saponification of bitumen).

Bitumen and tars are soluble in organic solvents, especially the non-polar type (dichloroethane, benzene, chloroform, lacquer petroleum and others like that).

In the conditions of moist environment or foul with microorganisms, bitumen do not protect organic materials from rotting. Unlike bitumen, tars are fully biostable. That is why tar materials can be advantageously applied in conditions of concealed flashing. Furthermore, tars have higher adhesiveness than bitumen on account of the enhanced content of surfactants (phenols, carbazole and others).

Both bitumen and tars are susceptible to deterioration (with respect to worsening of quality parameters, decline of longevity) through the action of atmospheric factors. This is the essence of the senescence process which involves the successive oxidization of components, which slowly flows at ordinary temperatures. A time dependent decline of the oils and resins in bitumen and tar materials results in the reduction of their plasticity and increases their fragility.

Types of bitumen and tars. Bitumen are divided into natural and artificial –oil-firing.

Natural bitumen are products of natural oxidization of oil which flows in the earth's crust for ages. Per se they meet rarely, mainly impregnate the porous mountain rocks - sandstones, limestones, dolomites. These bitumen rocks are called asphalt. Bitumen are extracted by organic solvents from them, at a content of not less than 10–15 %. Natural bitumen contain a negligible quantity of harmful admixtures - paraffin and are characterized by enhanced longevity. Frequently, asphalt rocks are ground down and are used as fillers of mastics and asphalt concrete.

In construction practice mainly, artificial oil bitumen are appied. Oil bitumen are produced as the finished products of direct distillation or oil cracking.

They are divided into two basic types of oil bitumen according to the production method use as - residual and oxidized (Fig. 15.1). Residual bitumen appears as the leftover after distillation of oils from black oils and tar oils. The output of residual bitumen from distillation of aromatic oils can be as high as 15%. Residual bitumen contains a little amount of asphaltenes. That is why they have viscid consistency and belong to the soft types.

The oxidized bitumen is produced by oxidization of fluxing oils or residual bitumen at the temperature range between 180 - 280°С. Under the action of oxygen condensation reactions take place, which results in concentration of bitumen asphaltenes due to the decreasing of oils and resins part, at such temperatures. Increasing oxidation of part of the asphaltenes in the bitumen causes a marked change of physical-mechanical properties, such as - increase the softening and flash temperature, or diminishing the penetration and stretch.

Depending on the function, bitumen may be distinguished as - road, structural, roofing and special. The basic standard requirements for the road and structural oil bitumen are shown in Table 15.1.

Three classes of liquid bitumen exist and are applied for using asphalt materials along with viscid bitumen These include:

- Bitumens which thicken quickly;

- Bitumens that thicken with moderate speed;

- Bitumens that thicken slowly.

Liquid bitumens can be produced from viscid bitumen with solvents.

Tars are used as construction binders in considerably less measure, than bitumen. This due to a series of reasons, which include; a narrow interval of thermal stability, enhanced capacity for senescence under the action of atmospheric factors, deficit of tar products, which are used as valuable chemical raw material, and their toxicity.

The condensed products of destructive (without access of air) distillation of solid fuel and oil produce tars. Coal tars are the most widespread. The processing of anthracite coal produces coke, coke gas, benzene, ammonia and coal resin - "crude tar". Ammonia water oils and solid remains – pitch are manufactured by processing of raw tar. In construction, refined or composite tars are mainly applied. Refined tars are produced by rectification of raw tars from water and other volatile substances, whilst the composite tars are produced from composition of pitch and tar oils.

Table 15.1

Common properties of oil bitumens

Date: 2015-12-18; view: 819