HMA Patching

Patches are a common method of treating an area of localized distress. Patches can be either full-depth where they extend from the pavement surface to the subgrade (Figure 1) or partial where they do not extend through the full depth of existing pavement (Figure 2).

Full-depth patches are necessary where the entire depth of pavement is distressed. Often times, the underlying base, subbase or subgrade material is the distresses root cause and will also need repair. Partial depth patches are used for pavement distresses like raveling, rutting, delamination and cracking where the depth of crack does not extend through the entire pavement depth.

Patching material can be just about any HMA or cold mix asphalt material as well as certain types of slurries. Typically some form of HMA is used for permanent patches, while cold mix is often used for temporary emergency repairs.

Pothole Patching

One form of patching, pothole patching, probably receives the greatest amount of public attention. Pothole patching procedures cover a wide range of methods and intentions from permanent full-depth patches to temporary partial depth patches. Two general patching procedures are described next.

Semi-Permanent Pothole Patch (Figures 3 and 4) (from FHWA, 1998¹)

1. Remove all water and debris from the pothole.
2. Square up the pothole sides so they are vertical and have in-tact pavement on all sides.
3. Place the patching material into the clean squared-up hole. The material should mound in the center and taper down to the edges so that it meets flush with the surrounding pavement edges.
4. Compact the patching material starting in the center and working out toward the edges. Compaction can be accomplished using a vibratory plate compactor or a single-drum vibratory roller. Check the compacted patching material for a slight crown. This is done so that subsequent traffic loading will compact it down to the surrounding pavement height.

Throw-and-roll (from FHWA, 1998¹)

1. Place the patching material into the pothole without any preparation or water/debris removal.
2. Compact the patching material using the patching truck tires (usually 4 to 8 passes).
3. Check the compacted patch for a slight crown. If a depression is present add more patching material and compact.

Although it may seem that the semi-permanent technique would produce a higher quality patch than the throw-and-roll technique, the FHWA's Long Term Pavement Performance (LTPP) Study found that the "throw-and-roll technique proved just as effective as the semi-permanent procedure for those materials for which the two procedures were compared directly" (FHWA, 1998¹). Since the semi-permanent technique is more labor and material intensive, the throw-and-roll technique will generally prove more cost effective if quality materials are used.

Utility Cut Patches

Installing, replacing or repairing underground utilities is one of the most common reasons for patching, especially in urban areas. Any utility cut, whether a trench for a pipe or cable or a small "keyhole" opening, must be backfilled and patched to match the surrounding surface. A patch always reduces the structural integrity of the pavement to some degree, even when it is installed well, but many patch problems come from poor construction practice, and a properly constructed utility patch can perform comparably to the pavement around it.

T-Section (Bridge) Patches

In a T-section or bridge patch, an additional strip of the pavement surface, typically 150 to 300 mm (6 to 12 inches) wide, is cut away on each side of the trench before the patch is placed. The surface patch then bridges the trench, and its edges rest on the original, already compacted base beside the trench; in cross section, the trench and patch together form a T. Offsetting the patch joints from the trench walls avoids a plane of weakness running straight down through several layers, and water that enters at the surface joint is less likely to travel all the way into the trench.

A T-section patch costs more than a patch over the trench alone, because more pavement is removed and replaced. Whether that cost is justified depends partly on the remaining service life of the pavement. When the pavement is close to a scheduled rehabilitation, the patch will only be needed for a short time and a less durable patch may be sufficient. Who is responsible for building and paying for the patch, the utility or the road owner, can also affect the choice.

Construction Quality

The type of patch matters less than the quality of construction:

• Compaction. Both the trench backfill and the patch must be compacted to the required density. Limited working room, and backfill that differs from the existing base, make this difficult, and without proper compaction the life of the patch is significantly reduced. A T-section patch is partly supported by the existing base, but it does not compensate for poorly compacted backfill. Compaction of the backfill can be checked with a device such as the Dynamic Cone Penetrometer.
• Joint. The joint between the patch and the existing pavement is a path for water. Where moisture susceptibility is a concern, the joint must be compacted to the desired density, and it should be sealed to keep out water and debris.

Calculator

Patch quantity

FHWA pothole and full-depth repair practice

Units

US customary SI (metric)

Distressed length, ft

Distressed width, ft

Margin into sound pavement, each side, ft

Patch depth, in.

Number of patches this size

Compacted density, pcf

CalculateExample values. Your agency’s repair details set the margin into sound pavement.

@misc{pavint-hma-patching,
  author = {{Pavement Interactive}},
  title = {HMA Patching},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/maintenance-and-rehabilitation/maintenance/hma-patching/}},
  note = {Accessed [date]}
}