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1、A bituminous surface treatment A bituminous surface treatment (BST), also known as a seal coat or chip seal, is a thin protective wearing surface that is applied to a pavement or base course. BSTs can provide all of the following: A waterproof layer to protect the underlying pavement. Increased skid
2、 resistance. A filler for existing cracks or raveled surfaces. An anti-glare surface during wet weather and an increased reflective surface for night driving. Historically, BSTs have been in use since the 1920s, primarily on low volume gravel roads. BSTs are increasingly in use as a preventative mai
3、ntenance procedure on flexible pavements of good structure. Purpose A BST offers preventive maintenance from the effects of sun and water, both of which may deteriorate the pavement structure. BSTs create a new wearing course, as well as a waterproof covering for the existing pavement. A BST makes i
4、t more difficult for water to enter the base material, and preventing freeze thaw damage for those locations with below freezing temperatures. BSTs also increase the surface friction of the pavement, due to the addition of the cover aggregate. This combats the effects of raveling, which can make the
5、 pavement slippery and stopping difficult. A BST gives good, gripping texture to the pavement surface. BSTs should be applied to a distress-free to moderately distressed pavement surface. A two to four year service life is common, while five year service lives are achievable. Materials BSTs are crea
6、ted using two main materials: asphalt and a cover aggregate. Asphalt Asphalt (as asphalt binder, cutback asphalt or asphalt emulsion) and aggregate (uniformly graded). The asphalt is typically an emulsion. Care must be taken with the weather on the day of construction ideally, a warm day with low hu
7、midity is preferred. BSTs should never be constructed on rainy days or when rain is predicted. Rain can dilute the asphalt binder if it has not yet cured, bringing the binder to the top of the cover aggregate; after the water evaporates, tires can pick up the loose aggregate or track binder across t
8、he surface. Cutback Asphalt While cutback asphalt is historically an option for BSTs, the solvent used (usually gasoline or kerosene) is expensive and potentially dangerous. A cutback is asphalt dissolved in a solvent, allowing the asphalt to be pumped and sprayed without heating it to high temperat
9、ures. The solvent evaporates into the surrounding air, leaving the asphalt binder. Once the solvent has completely evaporated, the cutback has cured. Fast setting time cutbacks use gasoline, while kerosene is used for longer curing times. Cutback asphalt usage has declined in recent years due to pol
10、lution and health concerns. Asphalt Emulsion Today, asphalt emulsions are more commonly used. An emulsion consists of an oil suspended in water. In this case, the asphalt cement is the oil component. A surfactant (also called an emulsifying agent) needs to be added in order to make the oil and water
11、 mix. Having the asphalt suspended in water allows the placing temperature for a BST to be significantly lower than for a hot mix asphalt. A surfactant offers two benefits one, it causes asphalt to form tiny droplets that will suspend in water by lowering the surface tension between the asphalt and
12、the water. Two, a surfactant determines the electrical charge of the emulsion. Aggregate will have an electrical charge, usually negative. Since opposite charges will attract, it is important to choose an emulsion with the opposite charge, which will enhance the binding of the asphalt to the aggrega
13、te. Cationic (having a positive charge) emulsions are typically used. The next largest (by volume) ingredient in an asphalt emulsion is the water. Water forms approximately one-third of the volume of the emulsion. Asphalt particles will, with the help of the emulsifying agent, suspend in the water.
14、It is important to note that if the emulsion breaks (when the asphalt and water separate from one another) the emulsion will change from brown to black. The aggregate must be applied and rolled before the emulsion breaks. When the emulsion breaks, the water evaporates, and the asphalt remains on the
15、 roadway. The asphalt cement is the primary ingredient in a BST. This asphalt is much like the asphalt used in hot mix paving. Sometimes, a latex or polymer modified asphalt may be used to improve early chip retention or BST durability. Aggregate Aggregates typically used include natural gravel or c
16、rushed stone. These must be clean and dust free, hard, and uniform. This is to provide a durable surface for traffic. It is common to limit the percent of material passing the No. 200 sieve (0.075 mm) to about 1 percent or less since excessive dust can be a serious adhesion issue for BSTs. Aggregate
17、 used in construction of a BST should be placed only one layer thick. The exceptions to this is when adding chokestone or a second BST layer (a two layer BST is often referred to as a “double shot” treatment). Placing too much aggregate will cause aggregate pickup, whereby the well placed stones are
18、 dislodged, and may cause automobile windshield damage. A small amount of excess aggregate, more than 5% but less than 10%, may be placed in turning and stopping zones. This will reduce tire scuffing in the freshly laid BST. Design Design of BSTs must take into consideration all of the points mentio
19、ned in the Asphalt and Aggregate sections of this article. Material must be of good quality and the correct properties. The design must also take into consideration the amount of binder and cover aggregate to apply. In order to prevent excessive chip loss, about 70 percent of the aggregate (and a mi
20、nimum of 50 percent) must be embedded in the residual asphalt the asphalt left after the water or cutback has evaporated. With an asphalt emulsion, the binder must rise near to the top of the aggregate, otherwise the residual asphalt will be insufficient to properly embed the aggregate. The goal is
21、to have the binder at about 70% of the chip height after the binder has cured. One procedure which is commonly used is the McLeod Design Procedure. For more information on this, please refer to the Minnesota Seal Coat Handbook. Distressed Pavement Surface distress is “Any indication of poor or unfav
22、orable pavement performance or signs of impending failure; any unsatisfactory performance of a pavement short of failure” (Highway Research Board, 1970). Surface distress modes can be broadly classified into the following three groups: 1、Fracture. This could be in the form of cracking (in flexible a
23、nd rigid pavements) or spalling resulting from such things as excessive loading, fatigue, thermal changes, moisture damage, slippage or contraction. 2、Distortion. This is in the form of deformation (e.g., rutting, corrugation and shoving), which can result from such things as excessive loading, cree
24、p, densification, consolidation, swelling, or frost action. 3、Disintegration. This is in the form of stripping. raveling or spalling, which can result from such things as loss of bonding, chemical reactivity, traffic abrasion, aggregate degradation, poor consolidation/compaction or binder aging. Thu
25、s, surface distress will be somewhat related to roughness (the more cracks, distortion and disintegration the rougher the pavement will be) as well as structural integrity (surface distress can be a sign of impending or current structural problems). Pavement must be repaired prior to the application
26、 of a BST. Pavement can be tested in order to help determine what repairs should be made. Common tests include wheelpath rutting, roughness, and surface friction. If structural failure is suspected, a falling weight deflectometer test may be conducted. Distress Correction Once the pavement structura
27、l distress has been quantified, the distress must be corrected before constructing a BST. An extensive pavement distress discussion (with photos) can be found at: HMA Pavement Distress PCC Pavement Distress Base Repair If the distress is so severe that the subgrade has been affected, the structure m
28、ust be replaced through a full-depth dig-out, which may be done in a variety of ways. The designer must specify the area and depth of the repair for each section of the pavement. Patching Patching may be done (in HMA, in PCC) to repair wheel ruts, edge raveling, and delaminated or pothole areas. Com
29、mon construction practices in placing HMA for patching include use of a spreader box, grader/blade patching, or using a paver. Results from using any of these methods will be satisfactory if they are properly performed. To avoid chip loss, all paved repair areas must be fog sealed prior to applying
30、BST. Fog sealing is the light application of an asphalt emulsion. Fog seals can also be placed on a new chip seal after the final brooming to assist in aggregate retention and prevent the shedding and shelling of the new aggregate. The fog seal gives an additional layer of waterproofing to the new w
31、earing course. In this type of use, the fog seal should be applied 3 to 14 days after the chip seal was placed, with brooming to be completed immediately before the fog seal is placed. Crack Sealing Crack seals are used for repairing transverse and longitudinal cracks. This is typically the use of a
32、 rubberized tar material or mixture of sand and asphalt emulsion to fill the crack. Cracks and joints 1/4 or greater should be cleaned of any incompressible material including old sealant, and then sealed prior to applying a BST. Slurry Seal A slurry seal ought to be applied at 25% of the design lif
33、e for maximum benefit in preserving the pavement structure. Maintenance with a slurry seal must be instituted before significant pavement deterioration occurs . Aggregate size, emulsion type, and any additives determine classification of the slurry seal. If distress is noted, crack sealing ought to
34、be done before applying a slurry seal. When a slurry is placed over dry and raveling pavement, a tack coat should be done before the slurry seal. Construction A single layer BST is constructed in the following steps: Surface preparation. Surface defects, such as potholes, are repaired and the existi
35、ng surface is cleaned (e.g., by a street sweeper). Asphalt material application. Typically, an asphalt emulsion is applied from a spray truck to the surface of the existing pavement (see Figure 1). Aggregate application. A thin aggregate cover (only one stone thick) is spread over the asphalt materi
36、al before it has set (see Figure 2). The aggregate usually has a uniform gradation. Aggregate embedding. A roller (usually a pneumatic tire roller) is used to push the aggregate into the asphalt material and seat it firmly against the underlying pavement (see Figure 3). Generally, about 50 percent o
37、f each aggregate particle should be embedded in the asphalt material (see Figure 4) after final rolling. About 70 percent of each aggregate particle will be embedded after several weeks of traffic. It is common to place an aggregate “chokestone” on top of the uniformly graded larger aggregates after
38、 embedment. Chokestone is essentially a finer aggregate gradation (e.g., less than 12.5 mm (1/2 inch) used to make a more dense aggregate matrix at the level of embedment (see Figure 5). This more dense matrix helps prevent excessive aggregate loss due to traffic. Multiple layer surface treatments are done by repeating the above process for each layer. Figure 9 shows a BST in Washington State.