Potholes and the Freeze/Thaw

A simple definition of a pothole:

“Any pavement defect involving the surface, or the surface and base, to the extent that it will cause significant noticeable impact on vehicle tires and vehicle handling.”

Asphalt is brittle in cold temperatures. When spring thaws saturate the soil that supports the asphalt, the pavement becomes vulnerable to cracking and breaking as heavy traffic passes over. The more severe the winter weather, the more likely it is there will be a pothole problem. The freeze-thaw cycle can be a very significant winter event associated with potholes.

The eastern United States experienced two severe winters, 1976-77 and 1977-78. The winter of 76-77 was bitterly cold and the winter of 77-78 was warmer and experienced record snowfalls. One of the most prominent and hazardous results of the winter of 77-78 over the winter of 76-77 was the large number of potholes in the roadways. This was associated with the large number of freeze – thaw cycles experienced during the wetter/warmer winter. A freeze – thaw cycle is defined as when the air temperature crosses 32° F and then returns to the original side.

Lake Mills’ winter of 2012-13 is regularly experiencing a freeze – thaw cycle. The conditions for potholes are prime. The advantage of all these freeze – thaw cycles is that it reduces the possibility of flooding this spring, but on the other hand, many of the roadways are going to need significant work.

Four causes of potholes:

1.    Roads that have insufficient thickness to support traffic during winter/spring thaw cycles without localized failures.

2.    Poor drainage, which will usually cause failure in combination with thin pavements, but can also affect thick pavements and new overlays.

3.    Failures at utility trenches and castings.

4.    Miscellaneous paving defects and cracks left unmaintained or unsealed from water intrusion.

When water and traffic are present at the same time, potholes form in two ways: fatigue failure and raveling failure.

Fatigue failures produce classic, bowl-shaped potholes when water weakens the soil under the pavement and traffic flexes the asphalt up and down until it breaks apart. This process is rare in thicker pavements (three to four inches) and common in thinner pavements of under three inches.

Raveling failures occur when water and traffic wear away adhesive asphalt films and stone aggregate from the surface of the road. Raveling is also common at the edge of the paved roadway or at cracks and joints where pavement has not been properly sealed. Potholes that maintenance crews have filled with cold mixes (liquid asphalt with solvents) on an emergency basis in cold weather often generate raveling failures.

Drainage
Poor drainage is a major contributor to pothole problems. Standing water and subsurface water weaken pavement support and contribute to heaves and cracking from frost penetration and freeze-thaw cycles. This year the large quantity of snow we received also prevented the gutters from being kept open. The maintenance of drainage features offers the greatest return for most municipalities in the spending of annual funds for maintenance of roads with regard to potholes.

There are several options for improving drainage. Preventive maintenance includes clearing debris and foliage from road storm drains, and culverts. Sometimes paved and lined ditches may be necessary for a permanent drainage solution. Curbs, storm drains, and street geometrics can help efficient movement of water away from the road. Street geometric or designs that provide for sloped pavements, adequate curb grades, adequately sized storm drains and sewer pipe sizes reduce the number of low spots and improve drainage.

In Lake Mills, our standard pavement specification is a minimum of six (6) inches of three (3) inch graded crushed limestone below the usual three (3) inches of ¾ inch crushed limestone base course prevents the capillary action of drawing groundwater toward the colder surface (similar to humidity condensing on a cold window).

Preventive Maintenance Programs and Pavement Inventories

A policy of only repairing potholes and blocked drains is not a maintenance program. Detecting early signs of roadway failure and taking prompt preventive maintenance action (crack filling and chip seal-coating) before potholes form can result in less expensive, more effective road maintenance. Using well-trained crews and limited funds to best advantage requires an expert inspector or engineering aide. Such experts are expensive, but their skill in identifying areas in need of attention will more than pay for them. Comprehensive inventorying of all roads in a local system by pavement type, thickness, and condition of roadway allows street departments to coordinate and prioritize maintenance efforts for maximum economy and effect.

Utility Cut Control

A study in New York City showed that utility locations accounted for more than 85 percent of pavement defects. This booklet recommended strictly enforced quality control requirements for pavement utility cuts and a first-class system for coordinating installation of underground utilities with roadway resurfacing programs. The latter will help prevent poor communication that can sometimes result in tearing up a road that one crew resurfaced in July for another crew to make utility improvements in August.

This report is probably high for Lake Mills, but we have our problems. Trenches and structures installed before or after the pavement will behave differently because the subgrade portion of the pavement system has been disturbed to a much greater depth than the surrounding pavement.

Intersections, Utility Castings, and Other Common Problem Areas

Intersections, manholes, and other utility castings are "major pothole generators." Numerous utility structures at intersections break up pavement continuity and weaken pavement strength. Starting, stopping, and turning traffic increases wear on intersection pavement. In addition, it is difficult to slope intersections for good drainage. The booklet suggested intersections could be improved by removing the top inch of pavement with a milling machine or heating the surface with infrared heaters. Crews can then rework (overlay) the surface to achieve a smooth finish that drains well.

Manholes and other utility castings require special attention to prevent severe breakout problems. For thin pavements, a five-foot-diameter transition area around a manhole that thickens the pavement to at least four inches, and preferably to the full casting depth of eight inches, may help prevent deterioration. Some success has been achieved by placing Portland cement concrete collars around castings and filling the top one or two inches with hot asphalt mix. The asphalt to concrete system does not work well in Wisconsin conditions though.

Repairing Potholes

The method that the City uses most often is "place and roll’ or "quick response" pothole patching. This method usually utilizes a two-man patch crew in a small dump truck assigned to repair potholes in specific areas in the city. The areas are assigned on the basis of traffic volume. Unlike quality patching, this type of repair is not intended to repair fatigued sections of pavement. Place and roll patching addresses an immediate need for filling potholes in the road with asphalt mix and making the road safe for the traveling public. City pothole patching crews can usually fix the majority of potholes before complaints are registered. We will attempt to fill the hole within 24 hours of notification at the Street Department and the weather allows. This type of response is needed to mitigate immediate risk to vehicles traveling on a city-paved road. Without this type of response, frequent vehicular damage from driving through potholes could occur.

Repair of potholes by place and roll patching is accomplished by:

1.    Cleaning the hole with a broom (if needed).

2.    Shoveling a pavement repair material into the pothole to the correct depth.

3.    Raking the material smooth unless a self leveling material is used.

4.    Compacting the asphalt mix by rolling the patch with the tires of the patch truck, using a vibratory plate compactor, or (with very small holes) using a hand compaction device.

Work zones for this type of work are categorized as "moving" work zones. Traffic lane closures are not needed.

This type of pothole patch repair can last from a low of one day up through several years. How long the patch will last depends upon the quality of the material used and the environment that the patching material is placed in. Water infiltration into any asphalt patch material will greatly decrease the life of the patch. The life of a patch repair will frequently be short when the material is placed in potholes on severely deteriorated roads in wet weather conditions.

"Quality patching" is used to repair larger damaged or fatigued areas of asphalt pavement. The damaged pavement section can include:  multiple potholes, "alligator" or "block" cracking, pavement raveling, pavement rutting, or pavement shoving. Quality patching is usually utilized to repair a road section larger than a single pothole.

A four-man crew or larger is usually used to perform quality patch repairs depending on equipment and traffic. Equipment frequently utilized to perform this function includes: backhoe, pavement cutter or pavement grinder, dump truck to haul excavated material and to haul materials used for the repair (base gravel and asphalt), a steel drum roller or plate compactor, and a utility truck and trailer to transport the roller and needed work zone traffic control devices.

Quality patch repairs are accomplished by:

1.    Diggers Hotline must be contacted three days before any work is started.

2.    Square out and remove the area around pavement distress with either a pavement cutter or pavement grinder.

3.    Remove old asphalt pavement and, in some instances, the base material.

4.    Asphalt mix will be placed into the excavated repair area to a depth that will be compacted level with the surrounding existing pavement surface.

5.    The asphalt mix will be raked smooth.

6.    The asphalt mix will then be compacted to an acceptable density with a plate compactor or small steel drum roller.

Because of the equipment needed for the repair, traffic control devices for work zone safety, in accordance with the Manual for Uniform Traffic Control Devices (MUTCD), must be in place before quality patch work is started. Traffic lane closures are almost always needed for quality patching. This, in turn, mandates traffic channelization or the establishment of detour routes. Flaggers may be needed in high traffic volume areas.

Pavement repair accomplished by this method will always last longer than a "place and roll" patch. When conducted on hot mix roads with a majority of the road asphalt surface in good shape, quality patch repairs are generally preferable to place and roll patching.

Any repair patch or street cut in a paved road surface is considered by definition to be pavement distress. A quality patch performed using proper construction methods and under the best conditions does not restore the asphalt pavement to its original structural integrity. At best, a quality patch will downgrade a high severity distress condition to a low severity pavement distress. On roads with marginal asphalt mat surfaces or with base problems, the process of quality patching can actually damage the asphalt surface outside of the patch area. When this happens, the quality patch will be good, but the asphalt surface surrounding the patch will disintegrate. Road damage can, in effect, spread from the repaired area.

Quality patch pavement repairs require utilization of a significantly larger crew than what is needed for "place and roll" pothole patching. Response time for this type of repair will be much slower than a "place and roll" patch. More manpower and equipment resources are needed to conduct the work. A quality patch repair of a distressed road section will take much longer to complete than a "place and roll" pothole patch. A repair that might take five minutes for a "place and roll" response repair might take an hour or longer to fix as a quality patch. The quality patch usually is requires hot mix asphalt which is not available in Wisconsin until mid to late May.