Placing the Coat
Before the paving operation starts, an asphalt distributor is used to spray asphalt on the unpaved surface. This film of asphalt serves as the prime and tact coats. The coats are then allowed to cure before the actual paving resume. The purpose of having these coats is to prevent any slippage between the surface and overlay during or after the compaction.
Placing the Asphalt Mix
To start the paving operation, the paver is positioned properly onto the road. The screed of the paver is lowered onto block of the same depth of the loose asphalt mat that is going to be laid on the road. After that, the block can be removed and paving can start. As soon as the haul truck arrives at the job site, the paving inspector must check that the asphalt delivered must be in a satisfactory condition. The paving inspector usually check for these criteria listed below:
- blue smoke - blue smoke indicate that the mix is too hot.
- stiff appearance
- mix slumped in truck.
- lean, dull appearance - this indicates that the mix has insufficient asphalt.
- rising steam - too much moisture.
If there is any of the signs above is observed, the mix will be sent back to the batch plant to be reprocessed. After all conditions are satisfied, the haul truck can load the mix into the receiving hopper of the paver.
When loading the mix into the receiving hopper, the haul truck is placed carefully in front of the paver. The rear wheels of the truck should be in contact with the truck roller of the paver to avoid any misalignment with the paver. The paver will push the truck forwards as it paves the road. If skewness happens, the whole process will be delayed because they have to reposition the truck in front of the paver.
Most paver used are self-propelled paver. Each of them consists of two main units:
- tractor unit. -it includes the receiving hopper, slot conveyor, flow control gates, spreading crew, power plant, transmission, operator control for use on either side, and operator's seat. This unit will move the whole system forward.
- screed unit. -it is attached to the tractor unit by long screed arms on both sides of the machine. It consists of screed plate, vibrators or tamper bars, thickness control, crown control, and screed heater.
As soon as the the first load of asphalt mix has been spread, the uniformity of the asphalt texture should be checked. Operators will adjust the the appropriate adjustment points to correct any nonuniformity. Any segregation of materials also should not be allowed. Operation should be stopped immediately if any segregation is detected. The operators should also be aware of is the crown control. Pavement with crown has to be redone all over again. In addition to that, operators should continuously loosen the mix that clings to the sides of the hopper and push it back into the active mix. If the asphalt mix grow cold, it cannot be properly compacted and thus, looses its strength.
The last process of paving is compaction. This process is highly influenced by major mix proportion; (1) asphalt content: aggregate size, shape texture and distribution gradation; (2) filler content, and; (3) mix temperature. Appropriate rollers and rolling methods should be used in accordance with these proportion. There are several roller combinations used for maximum results:
- steel-tired static and pneumatic-tired rollers,
- vibratory and steel-tired static rollers, or
- vibratory rollers used in vibrating and static modes.
These combinations are highly recommended by the asphalt institute.
Rollers should be moved in a slow but uniform speed to achieve the best result. These rollers should also be in good conditions. Any irregularities in the rollers' performances will result in poor compaction of the asphalt; thus, the pavement will not last long. The rollers should not reverse suddenly while compacting because this action can displace the mix. If displacement happens, the whole mat should be loosened with lutes or rakes and restored to the original grade before rolling can restart. A pattern that is economical and provides the maximum compaction result should be established.