Links to: |
E1008 Roads and Drainage Homepage |
Ron Ayers Homepage |
Faculty Homepage |
USQ Homepage |
The final wearing surface of a constructed road may be either gravel, a sprayed seal, asphalt or cement concrete. Except in the case of gravel surfaces, the purpose of the wearing course is to waterproof the pavement and to provide a durable, skid resistant, dust free surface with good riding qualities. The most common types of dust free surfaces in Australia are sprayed bituminous surfacings and asphalt.
The following are the most common sprayed surface treatments and their major characteristics:
Before a bituminous surface is applied to a pavement, the pavement should be:
The construction of a sprayed bituminous surfacing usually involves the following steps:
Step 1. Sweeping. The surface is swept to remove all loose dirt, stones and foreign material. Drawn rotary brooms are used to sweep the pavement with supplementation by hand sweeping when necessary. Brooming should extend 300mm clear of the area of pavement to be sealed
Step 2. Preparing Binder. Binder is delivered to the job site from the refinery by:
Step 3. Preparing Fluxed or Cutback Bitumen. Refinery cutbacks are available. However fluxing or cutting back is more commonly carried out in the field immediately prior to spraying. The desired amount of flux and/or cutter is pumped from drums into the tank of the sprayer and circulated.
Step 4. Incorporating Adhesion Agent. Adhesion agents of the amine type readily dissolve in hot bitumen but lose their effect if maintained at high temperature (160° – 180°) even for a few hours. Adhesion agents are therefore added to the hot bitumen immediately prior to sealing.
Step 5. Preparing Aggregate. The supply of aggregate has to be arranged well in advance of sealing work to permit sampling and testing, and for final design (ALD and binder application rate). An additional 5% should be obtained to cover wastage. If the aggregate is to be precoated all aggregate particles must be completely but thinly coated. Precoat is generally applied at the rate of 6 to 12 l/m3.
Step 6. Spraying Binder. Spraying should not commence until the pavement temperature is at least 15°C. Spraying should not proceed when rain is threatening, and the most satisfactory results are obtained when the work is done in warm, dry weather. To ensure that the final seal is correctly positioned on the pavement, the edge line should be marked out. The sprayer driver then follows this line. The sprayer is initially positioned at a sufficient distance before the start of spraying to enable the required speed in the appropriate gear to be reached before spraying commences. In order to obtain uniform distribution the sprayer is driven consistently at the appropriate speed while appropriate pump speed or pressure in the spray bar is maintained at the desired application rate. The sprayer is dipped on completion of the spray run and the quantity of bitumen sprayed is recorded.
Step 7. Applying the Aggregate. The spreading of aggregate needs to be balanced with the spraying of binder, so that sprayed binder is covered as quickly as possible. The most common types of spreaders in use are truck mounted fantail or cockerel types. The spreading truck travels backwards with the tipping tray in the raised position so that aggregate flows freely into the spreader. The application rate is governed by the speed of the truck and the gate opening. Successful use of truck mounted spreaders depends heavily on the skill of the truck driver and the operator. With a single coat seal the objective is to achieve a complete cover of aggregate one stone thick with all stones lying on their flattest sides after compaction. Sufficient loaded trucks to cover the whole spray run need to be in position to follow the sprayer immediately after spraying commences. Areas under spread with aggregate need to have additional aggregate spread before rolling. Drag brooming can be used to correct over-spreading and evenly distribute aggregate over adjacent areas. In multiple application work the objective in spreading the second, finer aggregate is to apply sufficient particles to lodge within the voids between the coarser aggregate particles from the first spreading.
Step 8. Rolling. Rolling assists in arranging particles in their correct position (i.e. minimum dimension vertical) and presses them into the binder to achieve maximum adhesion. Rolling should commence immediately after the aggregate is spread. The amount of rolling required varies with conditions but as a guide one hour is required for about each 1500 litres sprayed. Self-propelled pneumatic tyre rollers are generally used but steel wheeled rollers may be employed successfully in some instances. Vehicular traffic may also be used to aid the compaction process but speeds must be kept very low.
Step 9. Drag Brooming. To improve the distribution of aggregate particles and to correct spreading unevenness, drag brooming is generally necessary.
The Picture Gallery contains pictures of the bitumen sealing process on four pages:
Traffic control is essential on any bituminous surfacing work, in order to protect workers, the public, plant, and the work itself.
During spraying operations it is necessary that all traffic be stopped from using the road while the binder is sprayed and until the aggregate can be spread and adequately seated. The work should be organised so that any delay to traffic is limited to less than 15 minutes. On highly trafficked roads, even a delay of 15 minutes may create traffic chaos, and some form of bypass of the job site may be necessary. In advance of the work site, traffic signs should be placed to warn motorists of likely delays, the possible need to stop, the presence of workmen and plant, and the need to reduce speed. All signs and sign layouts should be in accordance with the appropriate Australian Standards.
It is important that records be kept, in a standard form, of every aspect of bituminous surfacing work. This should include design, cost estimate, job instructions, actual work performed and cost statements. These records are necessary to:
Asphalt is manufactured by drying and heating aggregates and mixing them with bitumen at about 130° to 160°. Asphalt can be produced in two types of plants:
The aggregates are taken from storage in controlled amounts, passed through a rotary drier where they are dried and heated, screened to separate them into individually sized fractions, proportioned into the required amounts, mixed with hot bitumen, and then discharged into trucks. The mix is prepared one batch at a time, similar to the production of most cement concrete. Batch plants are flexible in that they may operate intermittently and they can readily change from one mix to another.
Asphalt is produced in a continuous process in these plants. They differ from batch plants in that the aggregates are more accurately metered initially so that the drying, heating and mixing stages are performed as one operation. The manufacturing process is a continuous one and is therefore more suited to the continuous production of a single mix.
The Picture Gallery contains photographs of Asphalt Production.
Asphalt is generally laid using an asphalt paving machine. These machines are capable of high outputs as well as being suitable for small jobs. Asphalt may also be laid by grader or hand raking but this is usually restricted to situations where a paving machine is not available. The finish obtained with these methods is normally inferior to a machine finish.
The laying of asphalt generally involves the following steps:
Step 1. Preparing the Surface. Where asphalt is to be laid on a granular pavement the surface should be swept to remove loose material such as dust and foreign matter. It is common practice to prime the surface some time in advance of laying the asphalt to improve the bond between layers. Where asphalt is to be laid over an existing bituminous surface, the surface should be swept and a tack coat of bitumen emulsion applied. The tack coat should be light but uniform in application.
Step 2. Transport and Delivery of the Mix. The method of transport needs to minimise heat loss, segregation and contamination. Delivery is best at a uniform rate suited to the spreading and compaction equipment being used. Waiting time and delays on site should be minimised so as to minimise cooling of the mix. Transport is usually performed with normal tip trucks, although they should have clean, smooth, metal tipping bodies to facilitate discharge of the mix. A minimum capacity of 6 tonnes is advisable to enable heat to be conserved. When carting in cooler or wet weather, the mix should be covered to reduce heat loss and to protect the mix from rain. If haulage occurs over a long distance the truck body may need to be insulated to minimise heat loss. The internal surface of the truck body should be coated with a release agent to enable the mix to be readily discharged into the paver.
Step 3. Placing the Mix. For best results self-propelled pavers need to be operated by skilled and experienced operators. At the commencement of each run the paver screed plate needs to be supported on blocks equal to the height of the uncompacted layer to be paved. The thickness controls on the screed are then adjusted to this depth. Before paving commences the screed plate must be heated to the paving temperature. During spreading the mat should be constantly checked to ensure faults in texture and/or surface shape are not occurring. Correct adjustment of the spreader to achieve the desired compacted thickness will only be achieved by trial and error as each mix is individual in its compaction characteristics. Laying of asphalt should be organised to occur when the weather is warm to hot and when no rain is likely. Laying temperature varies from one mix to another but is generally unsatisfactory if below 110°C.
Step 4. Compacting the Mix. The critical factor in achieving adequate compaction is the temperature of the mix. Adequate compaction will only be achieved while the bitumen is hot enough to act as a lubricant. The optimum time to roll an asphalt mix is when its resistance to compaction is low but when it is also capable of supporting the roller without the mix shoving. The best rolling temperature is determined by the interparticle friction of the aggregates, the gradation of the mix and the viscosity of the bitumen.
Rolling consists of three consecutive phases:
Breakdown rolling compacts the mix beyond the compaction imparted by the paver and obtains most of the required density. Intermediate rolling densifies and seals the surface. Finish rolling removes roller marks and other blemishes left from previous rolling. Rollers available for these operations include static steel wheeled, vibrating steel wheeled and pneumatic-tyred. Roller wheels are kept moist during the compaction process to avoid the hot bitumen sticking to the wheels. Rollers should move at a uniform speed, with the drive wheels of the roller nearest to the paver. Rollers must be kept in good condition so that the change in the direction of rolling is made smoothly.
Vibratory rollers can achieve a high degree of compaction with fewer passes than static rollers, and hence roller capacity is much higher. An important advantage is that vibratory rollers can apply the required compaction effort while the mix is still hot. One further advantage of vibratory compaction is that efficient compaction can be achieved at relatively low mix temperatures. The ability to compact at lower temperatures is particularly relevant when laying is carried out in cold or windy weather. The effectiveness of a vibratory roller is influenced by several factors, the two most important ones being the frequency of vibration and the amplitude.
The Picture Gallery contains photographs of Asphalt Paving.
Page last modified 7 July 1999.