The most common modern asphalt paver is composed of the following components: engine, hydraulic drives and controls, drive tracks, hopper, feeding conveyor, distribution augers, and the screed. The screed is made up of leveling arms, moldboard, end plates, burners, vibrators, and slope sensors. When the paving operation starts, the truck full of asphalt backs up to the front of the paver and slowly dumps hot asphalt mix into the hopper. One of the duties of the operator is to raise (and lower) the hinged wing compartments of the hopper to allow gravity to feed the asphalt down to the conveyor, as the hopper begins to empty.
Another duty of the operator is the monitor and adjust the conveyor speed to harmonize with the speed of the paver, and the demands of the project, i.e. lift thickness, area being covered, etc. Also flow gates are sometimes utilized to control the amount of asphalt fed from the end of the conveyor into the augur distributer. Many modern day pavers come equipped with an automatic feed control system, which regulate the feed with little to no operator input. Once the asphalt is fed down the conveyor, it is pushed to the back of the paver and onto the augur to be distributed evenly along the screed. There are 2 arms of the augur typically centered by the gear box. Each augur arm rotates to promote flow outwards to be placed. The screed is a very important part of the paving process and works to level off the asphalt and to partially compact it to the desired shape. The screed consists of the screed plate which flattens and compresses the asphalt, the depth crank which manually controls the screed angle and mat thickness, the screed heater, the screed vibrator which increases compaction quality.
Stopping and starting is generally undesirable when paving, especially when a straight lane of vehicle traffic is being placed as there will often be a bump or divet placed in the joint where the machine temporarily stopped. Screed extensions can also be used with most modern pavers which increase the overall placement width of the paver. The screed is typically kept hot to keep material from sticking and to avoid from mat tearing. The screed operates with many forces all working together in harmony to place the asphalt appropriately. A common configuration for a pass on a paver is to build the crown on one end of the screed placing a cold joint there, but with many models it is also possible to ‘crown’ the screed in the center, allowing asphalt in a road to be placed in one pass. This is considered by many to be a desirable way to place asphalt in a road due to the elimination of a cold joint, but many factors can influence this: contractor’s paver specifications, traffic maintenance, width of road, etc. It is also typical for cold joints to be kept from occurring in expected wheel paths, to keep traffic from aggravating the joint.
In terms of grade, there are many modern day add-ons and technologies on pavers when it comes to how the operator will provide control, be it automatically or manual. Manual grade manipulation is when the pavers elevation and wheel path influence the screed arms tow point, and in turn determines smoothness of the pavement. The ‘toe point’ is elevation and the point where the screed arm is attached to the paver. Under manual grade manipulation, the toe-point essentially controls the asphalt grade by affecting the angle of the screed and in turn the thickness of the mat. 1″ movement in tow point elevation will produce approximately 0.125″ movement in the leading edge of the screed. As the tow point rises in elevation, the screed angle increases which produces a thicker mat. On the other hand, when the tow point lowers in elevation, the screed angle decreases which produces a thinner mat. Because the screed angle takes time to adjust to tow point elevation changes, it is common for low points to receive a thicker mat and high points a thinner one.
When the screed tow point exists near the middle of the paver, this significantly reduces the transmission of small elevation changes in the front and rear of the tractor to the screed. The vast majority of smaller paving firms (specializing in drives, parking lots, etc), do not use automatic grade control and rely on manual grade manipulation. When grade is set in this fashion manually, the placement of aggregate base prior to asphalt pavement is of the utmost importance.
Since tow point elevation is so cumbersome to control manually, most modern pavers can typically be operated with an automatic screed control. This controls the tow point elevation using a reference/control separate from tractor body. Examples of these outside controls or references are curbs, skis, lasers, subgrade, etc. Since these references help control pavement grade, many call them “grade reference systems” or “automatic grade control”. The following are some modern systems commonly used:
- Stringline: This consists of taught string placed to controlled elevations independent of existing contours under/around the paver (often from a survey/layout crew). Stringlines provide accurate grades to within tolerance, but they are fragile and considered cumbersome to many. Lasers are often used in lieu of stringlines because they are aren’t as fragile and are just as accurate, if not more so. Lasers are most commonly used when frequent grade changes do not exist.
- Mobile reference Beam (Ski Beam). This consists of a reference/control system that moves with the paver (long beam/tube attached to the paver) and touches the road. There are also ultrasonic devices which don’t touch the road and gain their elevation data from sonic pulses. The premise of this system is to gather existing pavement grade data variations over a distance longer than the paver, and averages them for use in screed grade control. This data is longitudinal only, transverse or cross slope is typically preset. Ski beam lengths for contact devices are typically 25 feet minimum, with lengths closer to 40-60 being most commonly used.
- Shoe Joint Reference. This is a small shoe or ski attached to the paver that slides on an existing surface (most commonly curb). There are also ultrasonic devices which don’t touch the curb and gain their elevation data from sonic pulses. This kind of grade control essentially duplicates the surface being referenced.
To summarize, The most important task an asphalt paving machine operator has is to deliver asphalt with a consistent head of material ahead of the screed. This boils down to accurate regulation of the asphalt feed rate by way of conveyor speed, augur speed, flow gate openings, and consistent paving speed. If the amount of asphalt within the screed fluctuates, the angle of the screed placement will be affected and can cause bumps and waves in the finish mat. Also the amount of asphalt in the hopper should be kept above a certain amount and should never be empty during paving operations. This might leave the large, cold aggregate left in the hopper to move through the system and be placed on the mat without being mixed with hot or fine aggregate and can lead to segregation, temperature differentials, and inconsistent/low mat densities. The second most important aspect of operating an asphalt paver is assuring the pavement surface elevation is consistent and smooth, no matter the approach.