The phenomenon of separation between the asphalt residue and water is referred to as breaking or setting. The rate of emulsion setting can be controlled by varying the type and amount of the emulsifying agent. Since most aggregates bear either positive surface charges (such as limestone) or negative surface charges (such as siliceous aggregates), they tend to be compatible with anionic or cationic emulsions, respectively. However, some emulsion manufacturers can produce emulsions that bond well to aggregate-specific types, regardless of the surface charges. Although emulsions and cutbacks can be used for the same applications, the use of emulsions is increasing because they do not include hazardous and costly solvents.

Uses of asphalt

The main use of asphalt is in pavement construction and maintenance. In addition, asphalt is used in sealing and waterproofing various structural components, such as roofs and underground foundations. The selection of the type and grade of asphalt depends on the type of construction and the climate of the area. Asphalt cements, also called asphalt binders, are used typically to make hot-mix asphalt concrete for the surface layer of asphalt pavements. Asphalt concrete is also used in patching and repairing both asphalt and portland cement concrete pavements. Liquid asphalts (emulsions and cutbacks) are used for pavement maintenance applications, such as fog seals, chip seals, slurry seals, and micro surfacing.

Liquid asphalts may also be used to seal the cracks in pavements. Liquid asphalts are mixed with aggregates to produce cold mixes, as well. Cold mixtures are normally used for patching (when hot-mix asphalt concrete is not available), base and subbase stabilization, and surfacing of low-volume roads.

The consistency of asphalt is greatly affected by temperature. Asphalt gets hard and brittle at low temperatures and soft at high temperatures.

Asphalt’s temperature susceptibility can be represented by the slope of the line; the steeper the slope the higher the temperature susceptibility of the asphalt. However, additives can be used to reduce this susceptibility. When asphalt is mixed with aggregates, the mixture will perform properly only if the asphalt viscosity is within an optimum range. If the viscosity of asphalt is higher than the optimum range, the mixture will be too brittle and susceptible to low-temperature cracking. On the other hand, if the viscosity is below the optimum range, the mixture will flow readily, resulting in permanent deformation (rutting). Due to temperature susceptibility, the grade of the asphalt cement should be selected according to the climate of the area. The viscosity of the asphalt should be mostly within the optimum range for the area’s annual temperature range; soft grade asphalts are used for cold climates and hard-grade asphalts for hot climates.

Vocabulary:

A cutback — уменьшение.

Viscous — вязкий.

The solvent evaporates — растворитель испаряется.

The volatility of the solvents — летучесть растворителей.

Emulsifying agent — эмульгирующий агент. A high affinity — высокое сродство. Patching — исправление, ремонт.

Temperature susceptibility — температурная восприимчивость.

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2. Decide whether the following statements are true or false according to the text.

1. Asphalt used in pavements is produced in three forms: asphalt cement, asphalt cutback, and asphalt emulsion.

2. Asphalt cement is a blend of hydrocarbons of equal molecular weights.

3. At room temperatures, asphalt cement is a liquid material that cannot be applied readily as a binder without being heated.

4. The consistency of asphalt can hardly be affected by temperature.

5. Asphalt’s temperature susceptibility can be represented by the slope of the line.

3. Answer the following questions and give examples.

1.What functions does asphalt fulfill?

2.What forms can asphalt obtain?

3.What viscosity rate does asphalt have? Is it good or bad for its uses?

4.What is better to use as road pavement: asphalt or concrete?

5.What purposes can liquid asphalt be used for?

TEXT 19

1. Answer the following question and read the text below to check your answer.

We know about asphalt and concrete as two separate materials. Can they be blended in one?

Asphalt concrete

Asphalt concrete, also known as hot-mix asphalt (HMA), consists of asphalt binder and aggregates mixed together at a high temperature and placed and compacted on the road while still hot. Asphalt (flexible) pavements cover approximately 93 % of the 2,2 million miles of paved roads in the United States. The performance of asphalt pavements is largely a function of the asphalt concrete surface material. The objective of the asphalt concrete mix design process is to provide the following properties:

–stability or resistance to permanent deformation under the action of traffic loads, especially at high temperatures;

–fatigue resistance to prevent fatigue cracking under repeated loadings;

–resistance to thermal cracking that might occur due to contraction at low temperatures;

–resistance to hardening or aging during production in the mixing plant and in service;

–resistance to moisture-induced damage that might result in stripping of asphalt from aggregate particles;

–skid resistance, by providing enough texture at the pavement surface;

–workability, to reduce the effort needed during mixing, placing and compaction.

Regardless of the set of criteria used to state the objectives of the mix design process, the design of asphalt concrete mixes requires compromises. For example, extremely high stability often is obtained at the expense of lower durability, and vice versa. Thus, in evaluating and adjusting a mix design for a particular use, the aggregate gradation and asphalt content must strike a favorable balance between the stability and durability requirements. Moreover, the produced mix must be practical and economical.

The purpose of asphalt concrete mix design is to determine the design asphalt content using the available asphalt and aggregates. The design asphalt content varies for different material types, material properties, loading levels, and environmental conditions.

To produce good-quality asphalt concrete, it is necessary to accurately control the design asphalt content. If the appropriate design asphalt content is not used, the pavement will lack durability or stability, resulting in premature pavement failure. For example, if not enough asphalt binder is used,

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not all the aggregate particles will be coated with asphalt, which will result in a less stable and less durable material. Also, if too much binder is used, aggregate particles may have too much “lubrication” and may move relative to each other upon application of the load, resulting in a less stable material. Typical design asphalt contents range from 4 % to 7 % by weight of total mix. Before the Superpave mix design method was developed there were two common asphalt concrete design methods: Marshall (ASTM D1559), and Hveem (ASTM D1560).

The Marshall method was more commonly used than the Hveem method, due to its relative simplicity and ability to be used for field control. Both methods are empirical in nature; that is, they are based on previous observations. Both methods have been used satisfactorily for several decades and have produced long-lasting pavement sections.

However, due to their empirical nature they are not readily adaptable to new conditions, such as modified binders, large-sized aggregates, and heavier traffic loads.

The Superpave design system is performance based and is more rational than the Marshall and Hveem methods. Many highway agencies are implementing the Superpave system. The discussion below is limited to the Superpave and the Marshall methods.

Additives

Many types of additives (modifiers) are used to improve the properties of asphalt or to add special properties to the asphalt concrete mixtures. Laboratory tests are usually performed and field performance is observed in order to evaluate the effect of the additives and to justify their cost. The effects of using additives should be carefully evaluated; otherwise premature pavement failure might result. The recyclability of modified asphalt mixtures is still being evaluated. A relatively recent development in asphalt paving is warm mix, which uses modifiers, or a modified process, to produce asphalt concrete at lower temperatures than conventional hot mix. Warm mix is described in the following section.

Fillers

Several types of fillers, such as crushed fines, portland cement, lime, fly ash, and carbon black, can be added to asphalt concrete. Fillers are used to satisfy gradation requirements of materials passing the 0,075 mm (No 200) sieve; to increase stability; to improve bond between aggregates and asphalt; or to fill the voids and thus reduce the asphalt required.

Extenders

Extenders such as sulfur and lignin are used to reduce the asphalt requirements, thus reducing the cost.

Polymer Modified Asphalt

Rubber has been used in asphalt concrete mixture in the form of natural rubber, styrene–butadiene (SBR), styrene — butadiene — styrene (SBS) or recycled tire rubber. Rubber increases elasticity and stiffness of the mix and increases the bond between asphalt and aggregates. SBS is the most common modifier for producing PMA. Scrap rubber tires can be added to the asphalt cement (wet method) or added as crumb rubber to the aggregates (dry method).

2. Decide whether the following statements are true or false according to the text.

1. Asphalt concrete, also known as hot-mix asphalt (HMA), consists of asphalt binder and aggregates mixed together at a high temperature and placed and compacted on the road while still hot.

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2.Stability or resistance to permanent deformation under the action of traffic loads, especially at high temperatures is a feature that can’t be achieved by this new material

3.The purpose of asphalt concrete mix design is to determine the design asphalt content using the available asphalt and aggregates.

4.Marshall and Hveem methods are far more rational than the Superpave design system

5.Rubber has been used in asphalt concrete mixture in the form of natural rubber, styrene — butadiene (SBR), styrene — butadiene — styrene (SBS) or recycled tire rubber.

3.Answer the following questions and give examples.

1.What is the difference between tar and asphalt cement?

2.Discuss the main uses of asphalt.

3.What are the ingredients of asphalt cutbacks? What are the ingredients of asphalt emulsions?

4.Name three uses of asphalt emulsion.

5.Why is asphalt emulsion preferred over asphalt cutback?

6.What is the most common polymer used for modifying asphalt cement?

7.Briefly describe the interaction that occurs when asphalt and SBS are mixed.

8.List three advantages of warm mix asphalt technology.

TEXT 20

Steel Bridges Construction:

Myths & Realities

MYTH 1: Concrete lasts forever without maintenance.

REALITY: Concrete is affected by the same environmental deterioration factors as steel. Its performance is also affected by quality of materials and design.

MYTH 2: Concrete bridges outlast steel bridges.

REALITY: There is no credible statistical evidence to support the notion that concrete bridges outlast steel bridges.

MYTH 3: Weathering steel performs only under ideal climatic conditions

REALITY: Weathering steel performs successfully when designed and detailed according to the published FHWA and Industry guidelines for its use. There are many cases of weathering steel bridges not con-forming to the guidelines that are also performing well.

MYTH 4: Optimization by weight is the best approach to economical design.

REALTY: Although this may be true in some cases, savings in material may sometimes be more than offset by increases in fabrication cost; in certain instances, adding weight may provide the least cost solution.

MYTH 5: Steel is not recommended for short spans.

REALITY: Due to changing designs and prices for both steel and concrete members, the relative economics of span and cost of each material has also changed. In many cases, the most economical steel span may be close to or the same as for the concrete design. In some cases where 250-foot to 350-foot segmental prestressed concrete spans are used, the most economical steel spans may be shorter than for concrete.

MYTH 6: Steel bridges require traditional fabricated steel bearings rather than bearing pads usually specified in prestressed concrete designs.

REALITY: There are simpler steel reinforced, or fiber reinforced elastomeric pads and preformed fabric pads that are both more economical and often mechanically superior to the traditional fabricated steel bearings.

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MYTH 7: Painting or repainting of steel bridges is an insurmountable problem.

REALITY: There are cost-effective ways of repainting existing bridges. For new construction, there are modern high-performance coatings which comply with EPA standards and which can provide a minimum service life of 25 years prior to first paint maintenance.

MYTH 8: Bridges at the end of their calculated fatigue life or those experiencing localized fatigue problems have to be replaced.

REALITY: Fatigue life applies only to details. Localized fatigue problems can generally be fixed quickly and easily with no reduction in live load capacity or life of the bridge.

During the life of a steel bridge structure, certain details may exhibit fatigue cracking. These localized fatigue cracks do not mean that the entire structure has exceeded its service life. Many fatigue cracks can often be easily repaired by drilling holes at the tip of the cracks to stop crack propagation, if the driving force is removed or in other cases, bolting splice plates over the crack. After this retrofitting is per-formed, there is no reduction in live-load capacity or remaining service life of the bridge. Much of this retrofitting and repair can be completed without interrupting traffic.

MYTH 9: Modular prefabricated short-span steel bridges are only temporary structures. REALITY: Modular prefabricated short-span bridges, as compared with so-called panel bridges,

are typically permanent structures.

MYTH 10: Modular prefabricated short-span steel bridges are limited to a one-size-fits-all scheme.

REALITY: Modular prefabricated short-span steel bridges are custom engineered to meet individual specific requirements.

MYTH 11: Timber decks on modular prefabricated short-span steel bridges do not hold up and the timber treatments leach harmful chemicals into the environment.

REALITY: Treated timber continues to be used as a decking material for permanent bridge installations. Properly treated and detailed the treated timber will last the design life of the bridge. Timber treatment continues to advance with preservative choices and manufacturing techniques to minimize impact on the environment.

MYTH 12: There are limited options with modular prefabricated short-span steel bridges. REALITY: There are numerous options available.

MYTH 13: Steel is not competitive for simple-span bridges less than 140 feet in length. REALITY: Prefabricated modular steel bridges compete favorably with other materials when

considering the greater use of shop labor vs. field labor, the speed at which they can be installed and the significant reduction in time required to close a given roadway to the public.

MYTH 14: Corrugated steel pipe or corrugated steel plate bridges do not last, as they tend to rust out.

REALITY: With proper attention to design details and appropriate coating one can expect a service life up to 100 years.

MYTH 15: Reinforced concrete pipe lasts forever.

REALITY: Concrete pipe is susceptible to deterioration from aggressive soils and road salts as well as lack of soil stability.

MYTH 16: Corrugated steel is flexible and not appropriate under high fills.

REALITY: Corrugated steel material for bridges has the ability to perform under fill heights exceeding 100 feet.

MYTH 17: Corrugated steel pipe and corrugated structural plate bridges cannot compete with comparable reinforced concrete structures.

REALITY: Corrugated steel bridges compete very favorably with reinforced concrete bridges.

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MYTH 18: If corrugated steel pipe or corrugated steel plate is used as a bridge one is obliged to disturb the natural waterway.

REALITY: There are options available to avoid disturbing the waterway.

MYTH 19: Although able to provide long-term protection under adverse environmental conditions, galvanizing on plate-girder and rolled-beam bridges is prohibitively expensive.

REALITY: Due to the relatively stable price of zinc metal over the past 20 years, the initial cost of hot-dip galvanized plate girder and rolled beam steel for bridges is very competitive with painted steel and even less expensive in many cases.

MYTH 20: Bridge joints are essential.

REALITY: Experience has shown that joint less bridge decks can be designed to provide a durable and cost-effective structure.

MYTH 21: Steel bridges require traditional fabricated steel bearings rather than bearing pads usually specified in prestressed concrete designs.

REALITY: There are simpler steel reinforced, or fiber reinforced elastomeric pads and preformed fabric pads that are both more economical and often mechanically superior to the traditional fabricated steel bearings.

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