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A thin protective wearing surface that is applied to a pavement or base course is called a bitumen road surface treatment (BST), which is also known as a seal coat or chip seal. Other names for this type of treatment are chip seal and seal coat. All of the following benefits can be provided by BSTs:
A layer that prevents water from penetrating through to the pavement underneath.
Increased resistance to slipping.
A filler that can be applied to surfaces that already have cracks or ravels.
A surface that reduces glare in wet conditions and has been enhanced to reflect more light for use during nighttime driving.
Since the 1920s, BSTs have been utilized, primarily on low-volume dirt roads, according to historical records. BSTs are being utilized more frequently as a method of preventative maintenance for flexible pavements that have a sound structure.
A BST can provide preventative maintenance against the impacts of water and sunlight, two factors that can contribute to the deterioration of the pavement structure.
Both a fresh wearing course and a waterproof covering over the existing pavement are produced by BSTs throughout the construction process. The use of a BST makes it more difficult for water to enter the base material, hence avoiding damage caused by freeze and thaw cycles in areas where temperatures fall below freezing.
Because it incorporates cover aggregate into the pavement, they also contribute to an increase in the surface friction of the roadway. The consequences of raveling, which can make the pavement slick and make it harder to stop, are mitigated as a result of this. The pavement surface is given a BST, which gives it an excellent, gripping texture.
BSTs should be placed on a pavement surface that is either completely free of or only mildly affected by distress. The average service life is between two and four years, but it is possible to achieve service lives of up to five years.
Materials
Asphalt and a cover aggregate are the two primary components used in the production of BSTs.
Asphalt
Together with aggregate, asphalt (in the form of asphalt binder, cutback asphalt, or asphalt emulsion) (uniformly graded).
In most cases, the asphalt is found in the form of an emulsion. On the day that the building is to begin, the weather conditions must be carefully monitored; ideally, the conditions should be warm with low humidity.
BSTs should never be built on days that are forecast to have rain or when rain is already falling. If the asphalt binder has not yet cured, rain can dilute it, which will bring the binder to the top of the cover aggregate. Once the water has evaporated, tires can pick up the loose aggregate or track binder across the surface.
Asphalt should be reduced.
Although cutback asphalt has traditionally been a possibility for BSTs, the solvent that is utilized (often gasoline or kerosene) is both pricey and carries the risk of becoming harmful.
The asphalt is dissolved in a solvent to create a cutback, which enables the asphalt to be pumped and sprayed without first having to be heated to extremely high temperatures. As the solvent is exposed to the air, it will eventually evaporate, leaving the asphalt binder behind.
The cutback will be considered cured once the solvent has been fully evaporated. Gasoline is used for setting times that are cut back quickly, whereas kerosene is utilized for setting periods that are cut back more slowly.
In recent years, because of pollution and health concerns, there has been a reduction in the usage of cutback asphalt.
Asphalt Emulsion
Asphalt emulsions are the more popular choice for use these days. An oil that is suspended in water is what makes up an emulsion. In this particular instance, the oil component is asphalt cement.
In order to make the oil and water combine, a surfactant, which is also known as an emulsifying agent, must be added to the mixture. Because the asphalt is dispersed throughout the water, the laying temperature for a BST can be substantially lower than the temperature required for a hot mix asphalt.
One of the advantages offered by a surfactant is that it may break up asphalt into very small droplets that can then be suspended in water. This is accomplished by reducing the surface tension that exists between the asphalt and the water. Two, the electrical charge of the emulsion is determined by the type of surfactant used.
The electrical charge of aggregate is often in the negative direction. Because charges that are opposing to one another will attract one another, it is essential to select an emulsion that has the opposite charge. This will make it easier for the asphalt to adhere to the aggregate. Cationic emulsions, which have a positive charge, are the most commonly utilized type.
After the asphalt, the component that makes up an asphalt emulsion that contains the most volume is the water. About a third of the total volume of the emulsion is composed of water.
The use of the emulsifying agent will assist in the process of asphalt particles becoming suspended in the water. It is essential to keep in mind that the emulsion will transform from a brownish-black color into a black color if it breaks, which happens when the asphalt and the water separate from one another.
In order to prevent the emulsion from breaking, the aggregate must be applied and rolled. After the emulsion is broken, the water will evaporate, but the asphalt will still be there on the road.
Bitumen surface
Asphalt cement is the principal component of bitumen surface treatment (BST). This asphalt is quite similar to the asphalt that is utilized in the process of hot mix paving. To increase the early chip retention or BST durability, it is possible to utilize asphalt that has been treated with latex or polymer in some cases.
Gravel or crushed stone are examples of aggregates that are frequently utilized in construction projects. These have to be spotless, dust-free, rock-like, and consistent in their consistency.
This is done so that the surface can withstand the weight of traffic. Due to the fact that excessive dust can pose a significant challenge for BSTs in terms of adhesion, it is usual practice to set the maximum percentage of material that can pass through a No. 200 sieve (0.075 mm) at roughly 1 percent or less.
When constructing a BST, aggregate should never be more than one layer deep in any given location. The only exceptions to this rule are when chockstone is added or when a second layer of BST is applied (a BST treatment with two layers is frequently referred to as a "double shot" therapy).
The placement of an excessive amount of aggregate will result in the aggregate pickup, which will result in the well-placed stones becoming dislodged and may result in damage to automobile windshields.
In the areas designated for turning and halting, you are allowed to insert a trace amount of surplus aggregate—more than 5% but less than 10%. Because of this, tire scuffing in the newly laid BST will be reduced.
Aggregate Shape
The shape of aggregate can either be described as being flat or as being cubical. Additionally, it may have a round or angular shape. These characteristics will cause the seal coat to behave in a variety of different ways.
In the event that an aggregate is flat, the BST will suffer from excessive chip loss in the portion of the roadbed that is not the wheel path, or it may leak into the wheel path.
This is because the pressure exerted by car tires causes the flat chips to settle into the asphalt on the side of their shape that is the most horizontal. When the tires go over the BST, the thickness of the BST decreases there. For high-traffic streets, flakiness indexes of 20% or below in the aggregate used should be the minimum requirement.
Flakiness is not a problem in low-volume applications since a significant number of tire passes are required in a single location for this phenomenon to occur. However, cubical aggregate is preferable because of its greater stability for the vast majority of applications.
Aggregate that is round has a greater chance of rolling and becoming displaced by traffic. Aggregates with an angular shape interlock with one another.
In regions that receive regular snow plowing, extra caution is required to ensure that a BST containing a round aggregate will embed correctly. This is because the snow plow may shave off the higher bits of stone in these regions.
On roads with high traffic, a double chip seal is likely to be the superior choice. A BST is said to be stacked when it is placed atop another BST. It is important that the aggregate on the layer below be roughly twice as large as the one on the one above it.
It will be less likely for the tiny stones on top to cause damage to the windshield, and the surface will normally be smoother than a single coat of sealant would be.
Aggregate Size
The gradation of the aggregate, as well as its size, is critical to the effectiveness of a BST. The distribution of large stones in relation to smaller stones within the aggregate is referred to as the "gradation." One-size aggregate or graded aggregate are the two kinds of aggregate that can be used for a BST.
One-size aggregate is a type of aggregate mix that consists of stones that are more or less the same size. If all of the aggregates are roughly the same size, there will be sufficient void areas for the asphalt to fill and bond the stones to the structure of the pavement.
Other advantages of using aggregate with a uniform particle size include increased tire-to-aggregate contact area, improved drainage between individual stones, and an easier time determining whether or not there is an adequate supply of aggregate. All of these advantages are made possible by the uniform particle size.
The term "graded aggregate" merely refers to the fact that the aggregate varies in size in some way. There is a wide variety of gradations, including thick graded and gap graded, for example.
Because graded aggregate typically has fewer air gaps than other types of aggregate, using it might present a number of challenges, one of which is that the binder might not be able to fill the spaces between the chips. There is a potential for issues ranging from hemorrhage to loss of aggregate.
It is strictly forbidden to utilize any aggregate that contains dust in a BST. Dust will hinder the aggregate from attaching to the asphalt binder, which will lead to difficulties with excessive chip loss.
Dust will also contribute to the overall poor quality of the asphalt. You have the option of either using a high-float emulsion, which contains wetting agents that help with bonding in the dusty aggregate, or washing the aggregate with clean drinkable water and then allowing it to air dry. Both of these methods are viable options for dealing with this issue.
Design When designing BSTs, you need to take into account all of the factors that are discussed in the sections of this article that deal with asphalt and aggregate. The material in question needs to have appropriate qualities and characteristics.
In addition to this, the design needs to take into account how much binder and cover aggregate are going to be used. A minimum of fifty percent of the aggregate, and preferably seventy percent, must be embedded in the residual asphalt, which is the asphalt that is left over after the water or cutback has evaporated. This is necessary in order to prevent an excessive loss of chips.
If you are using an asphalt emulsion, the binder needs to rise close to the top of the aggregate; otherwise, there won't be enough asphalt left over to adequately embed the aggregate. After the binder has been allowed to cure, the aim is to have the binder at around 70 percent of the chip height.
The McLeod Design Procedure is an example of a procedure that is frequently put into practice. Please consult the Minnesota Seal Coat Handbook for any further details pertaining to this topic.
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