Pits are the most frequent type of failure found in bituminous wear surfaces. Pits can be caused by faulty drainage, frost action in the base, settlement of the base, or heavy traffic. A small crater may be surrounded by a large area that is becoming progressively smaller. Pits can be repaired with a hot or cold premix patch or a penetration patch. If there are many potholes in a large area, the paving system should be completely replaced or replaced, depending on the type of bituminous material used in the original pavement.
first stage (preparing cold mixture)
As for sustainability, the gradation of the aggregates and the amount of bitumen used are as important in patching mixes as they are in the construction of new pavements. If the ingredients are not in the right proportion, the patch may move or crumble and is likely to break in a short period of time. Because aggregates vary greatly in quality and grading, the amount of bitumen used in the mixture is variable. The finer the aggregate or the denser the gradation, the more bitumen is required to coat the increased surface area in a given volume. For most crushed stone, 2 quarts of bituminous material is typically used for 1 cubic foot of crushed stone. Fine sand will require more bitumen. For any gradation, the true bitumen content of the mixture is indicated with all particles light black. When the mixture is thrown into the pile, the particles should slowly roll or crawl over each other.
- application of bitumen
Asphalt kettle or bituminous distributor is commonly used for heating, transporting and applying bituminous material. An accurate thermometer is essential for controlling the temperature. The bitumen must be heated to mixing or spraying temperature. Safety precautions must be observed for heating bituminous materials. The bituminous material must be continuously agitated while heating, either with a pump attached to the equipment or by shaking. For surface patching, a sufficiently precise distribution of bituminous material can be achieved by hand spray by keeping the spray nozzle at a working distance and moving at a uniform rate. Bitumen is distributed from pouring vessels only to fill the cracks. If it is necessary to use pouring utensils for surface patching, the application should be swabbed for uniform coverage.
A pug mill or similar mixer gives best results for cold mixes. If a pug mill is not available, one of the more practical mixes with open-grade aggregates can be mixed in a mortar mixer. Another method is to spread the aggregate over an abandoned section of pavement, apply the bituminous material with a bituminous distributor and mix with a motor grader. The mixture can be used immediately or stored. For smaller quantities, open-graded mix can be mixed by hand with shovels.
Phase 2 (pit repair)
For best results, the patch should have the same density and release water as effectively as the surrounding area. Hand tamping of the area below the surface is usually required. Successive layers of patching material should be no more than 3 inches deep. Below are instructions for repairing a pothole or burn area.
The area around the pit should be marked with the edges of the field parallel to the direction of traffic (fig-1). The marked area should include all the surrounding vulnerable material.
- delete failure zone
The failed area should be removed (fig-2), including the base material which may be weakened. A rectangular hole should be cut along the vertical edges to hold the patching material against the pressure of traffic. All loose and defective material should be removed. When a patch is placed near the shoulder of a road or airfield, the patch should be applied on the shoulder as shown in Fig-2.
- filling the base
To replace the base, the bottom of the hole should be refilled in thin layers with new base material and each layer should be thoroughly tamped. The base material should have about an inch of crushed stone. If the wear surface is more than 2 inches thick, the pit should be filled to a level below the wear surface. If the wear surface is less than 2 inches thick, it should be refilled within 2 inches of the top of the hole.
- applying tack coat
A light tackle coat should be applied over the new base material and on the sides of the hole and around the edges. The tack coat provides a bond between the new base material and the patching mixture. The tackle coat should become sticky before the patching mix is applied.
- leveling and raking
For hand patching, the premixed material must be shoveled into place, not dumped or dropped. The patch is leveled by slight raking. Dumping or discarding the premix creates a compressed area that must be rotated or moved to obtain a uniform texture. Heavy raking is used only for feather-edged patches. The coarse material is pushed with the back of the rake towards the center of the patch. For small surface applications, the aggregate is dispersed with shovels as evenly as possible. A little sweeping or raking is required for even application. The bituminous patch must be compacted to obtain the required density and to seal the aggregate. For small repairs where roller access is impractical, or for larger repairs power rollers are inaccessible, the patch should be tamped by hand, leaving a slight crown for further compaction.
- to compress
The top of the patch should be lightly sprinkled with wet sand to compact the surface so that the roller and traffic do not lift the mixture as it hardens. Tamper or rollers must be used for compaction or a truck can be driven slowly over the patch.