Potholes are the most frequent type of failure found in bituminous wearing surfaces. Potholes may be caused by defective drainage, frost action in the base, settlement of the base, or heavy traffic. A small pothole may be’ surrounded by a large area that is progressively failing. A pothole may be repaired with a hot or cold premix patch or a penetration patch. If a large area has many potholes, the paving system must be entirely reworked or replaced, depending on the type of bituminous material used in the original pavement.
Stage – 1 (Preparation of the Cold Mix)
For stability, the gradation of 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 accurately proportioned, the patch may shove or ravel and it is likely to break down in a short time. Because aggregates vary greatly in quality and grading, the quantity of bitumen used in the mix is variable. The finer the aggregate or the denser the gradation, the more bitumen is needed to coat the increased surface area in a given volume. For most crushed stone, 2 quarts of bituminous material usually are used for 1 cubic foot of crushed stone. More bitumen will be required for fine sand. For any gradation, correct bitumen content of the mix is indicated by a dull black color with all particles coated. When the mix is thrown into a pile, the particles should slowly roll over each other or creep.
Application of Bitumen
The asphalt kettle or bituminous distributor usually is used to heat, transport, and apply bituminous material. An accurate thermometer is necessary for controlling temperatures. The bitumen should be heated to the mixing or spraying temperature. Safety precautions for heating bituminous materials must be observed. The bituminous material should be constantly agitated while it is being heated, either with a pump attached to the equipment or by stirring. For surface patching, sufficiently accurate distribution of bituminous material can be made with a hand spray by keeping the spray nozzle at a distance from the work and moving it at a uniform rate. Bitumen is distributed from pouring pots only for crack filling. If it is necessary to use pouring pots for surface patching, the application should be broomed for uniform coverage.
A pug mill or similar type of mixer gives the best results for cold mixes. If the pug mill is unavailable, one of the more workable mixes with open-graded aggregates can be blended in a mortar mixer. Another method is to spread the aggregate on an abandoned section of pavement, apply the bituminous material with a bituminous distributor, and mix with a motor grader. The mix may be used immediately or stockpiled. For small quantities, the open-graded mix may be hand-mixed with shovels.
Stage – 2 (Repairing a Pothole)
For best results, patches should have the same density and should shed water as effectively as the surrounding area. Hand-tamping of the area below the surface is usually necessary. Successive layers of patching material should not be more than 3 inches deep. Instructions for repairing a pothole or burned area are given below.
The area surrounding the pothole should be marked off with the sides of the area parallel to the direction of traffic (fig-1). The area marked should include all surrounding weak material.
Removing of Failure area
The failed area should be removed (fig-2), including base material that may be weak. A rectangular hole with vertical edges should be cut to hold the patching material against the push of traffic. All loose and defective material should be removed. When a patch is placed adjacent to the shoulder of a road or airfield, the patch should be keyed to 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 tamped thoroughly. The base material should consist of approximately ¾ inch crushed stone. The pothole should be filled to the level of the bottom of the wearing surface if the wearing surface is more than 2 inches thick. If the wearing surface is less than 2 inches thick, it should be refilled to within 2 inches of the top of the hole.
Applying Tack Coat
A light tack coat should be applied on the new base material and on the sides and around the edges of the hole. The tack coat provides a bond between the new base material and the patching mix. The tack coat should become sticky before the patching mix is placed.
Leveling & Raking
For hand patching, premixed materials should be shoveled into place, not dumped or dropped. The patch is leveled by slight raking. Dumping or dropping the premix produces a compacted area that must be turned or moved to obtain a uniform texture. Heavy raking is used only for feather-edging patches. The coarse material is pushed to-ward the center of the patch with the back of the rake. For small surface applications, the aggregate is spread with shovels as evenly as possible. Slight sweeping or raking is necessary for uniform application. Bituminous patches must be compacted to obtain the required density and to seal the aggregate. For small repairs where use of the roller is impracticable, or for larger repairs not accessible to power rollers, the patch should be hand tamped, leaving a slight crown for further compaction.
To compact the surface the top of the patch should be sprinkled lightly with wet sand so that the roller and traffic will not pick up the mix while it is hardening. Tampers or rollers should be used for compaction or a truck may be driven slowly over the patch.