Potholes are a major problem in all Midwestern cities. The freeze thaw cycles which usually occur in late winter can cause extreme stresses that damage the pavement. Snow and ice melts during the day and settles into cracks and holes in the pavement, then refreezes during cold night time temperatures. The ice exerts tremendous force that is great enough to fracture concrete and asphalt, causing potholes. Most potholes form in late winter and early spring when rain or snowmelt freezes overnight however, city crews repair potholes year around.
Pavement and Pavement Repair
Omaha's streets are an important asset and are the core City's transportation network. They are an important element of every neighborhood. New and rehabilitated streets enhance the appearance of a neighborhood. Older streets bordered by trees and containing landscaped medians are part of the character of many established neighborhoods.
Most streets in Omaha are constructed of concrete and many have been overlaid with asphalt but retain the concrete base. Omaha also has many brick streets which were built in the early 1900's and are quite serviceable. Brick streets also lend a unique character to neighborhoods that homeowners favor.
Some streets in residential neighborhoods were never paved and are a combination of dirt, gravel and asphalt grindings. Omaha's Public Works Department concentrates its maintenance programs on repairing concrete, asphalt, and brick surfaced streets.
Concrete appears to be a durable material that lasts forever; in fact, in some applications it lasts hundreds of years. However, pavement is exposed to the weathering actions of rain, snow, salt, hot summer and cold winter temperatures and traffic loads that damage and eventually destroys the structure of the concrete. Furthermore, many utilities are buried under the pavement and over time repairs are needed and the pavement is cut to conduct these repairs. These utility cuts can settle leading to faster deterioration of the pavement causing cracks and potholes.
A concrete street can last 50 years or more, but the actual life is more or less depending on the severity of conditions. Omaha owns many miles of streets 50 years and older, and we have specific maintenance programs designed to keep these older streets sound and functional.
Unfortunately, there is not enough funding to keep all streets in good condition. In particular, hundreds of blocks of paving have reached the end of their useful life and it is no longer cost effective to continue to invest money in maintenance and replacement is the most cost effective option.
Concrete pavement is an array of connected panels. In sound pavement, as shown in the photograph above, these panels are unbroken and the joints between them are small, saw cut grooves. This produces a nice smooth ride for drivers and also presents a clean, pleasing appearance.
As the pavement ages these joints open up and eventually the panels move independently. At that stage, the pavement begins losing its integrity and is changing from an excellent or very good riding surface to a poor surface with noticeable joints, potholes and cracks.
Good sound pavement
The picture at the right shows pavement that was built in the late 50's. It has experienced 60 years of freeze thaw cycles. The joints are with about an inch wide and have filled with sand and dirt. The transverse joints have become offset along the centerline as uneven movement occurred on either side of the street.
There are utilities under the pavement to include storm sewer, sanitary sewer, gas mains and water lines under the majority of streets in residential neighborhoods. Other buried lines include cables for phone, cable TV, and electricity. Occasionally these utilities must be dug up for maintenance or replacement. Almost any residential street older than ten years has pavement cuts to access the buried utilities. The older the street is, the more repairs and cuts there are and the more deficient the pavement becomes.
Utilities are located in a trench under the pavement. The soil in the trench is to be well compacted. Poor compaction may cause the soil in the trench to settle and the pavement loses support. Panels can crack under these conditions. Cracked panels remain serviceable for many years. It is not necessary or cost effective to replace every panel, especially if the cracks do not open and the panel does not offset across the crack.
Every crack behaves like a joint and joint ages and loses its ability to keep adjacent panels closely aligned. As these joints become "loose" the street rides rougher. Furthermore, individual panels lose support on their edges from adjacent concrete and can develop 45° cracks at corners.
Street creep is a noticeable symptom of a street undergoing many cycles of thermal expansion and contraction. The photo on the bottom right shows a curb on the inside of a 90 degree bend. The pavement has lengthened as the joints have widened. The curb on the inside of the curve has remained fixed. This caused the pavement to move away from the curb. Eventually about a two inch gap developed and was filled with asphalt. If left unfilled, this joint would have become eroded by the water running along the gutter and the pavement and curb may be undermined and eventually collapse causing a cave-in.
The life of pavement is extended by various types of maintenance.
Crack seal is a good, cost effective maintenance method that helps prevent damage from water and the effects of the freeze and thaw actions. Flexible, water tight material is placed in joints and pavement cracks to keep water, sand and dirt from filling the cracks. Joints in the concrete are designed to expand and contract to prevent damage during freeze-thaw action. As the panels cool they contract and a gap between the panels opens up. If this is sealed with a water tight material, it simply closes up again as the panels expand in hot weather. If dirt and sand works into the open crack, when the panels expand, the pavement pushes out and the street becomes a little wider and a little longer. Although the amount of thermal contraction is very small, over the length of a block the pavement can change by several inches as it goes from below freezing in the winter to well over 100 degrees in the summer. This expansion causes joints to open and close. This movement also produces internal stress within the concrete that is intense enough to crack the concrete slabs.
Crack sealed pavement
Omaha Public Works has an extensive crack seal program. Crews work throughout the summer and fall to seal joints and cracks in pavement.
Crews prepare the pavement for crack seal by cleaning out joints with compressed air.
The crack seal material comes as a brick of solid rubbery material. The bricks are carried in the truck and placed into the kettle where they are heated to about 400° F. A special gas fired kettle towed behind the crack seal truck keeps the material at the proper temperature and pumps it thru a wand for precise placement in the joints.
One worker places the proper amount of material in the joint while a second worker uses a squeegee to force the material into the opening.
Old Utility Cut
Although concrete appears to be a durable material that lasts forever, it has a finite life. At 50 years old, concrete streets have lost half of their structural integrity. Omaha Public Works has several maintenance programs that extend the life of pavement. Maintenance improves the ride of streets and also prevents water from standing in holes and working through the pavement. Maintenance work includes the repair and replacement of sections of pavement that have lost their integrity. Streets are patched with asphalt and whole panels are replaced when warranted. In some cases entire sections of street are resurfaced with a two to three inch asphalt overlay.
Asphalt repairs of concrete streets can be done quickly. One of the advantages of using asphalt to repair concrete streets is that the street is open to traffic within a few minutes after the repair is made. Concrete repair work requires a street to be closed for a period between two and seven days.
The first step in an asphalt repair is to remove dirt and loose concrete from the hole. This is done using an air wand with compressed air that is blown into the cracks and holes in the pavement as seen in the below picture.
Using compressed air to clean repair area (left)
The clean area is coated with an asphaltic cement coating called tack. This coating helps the asphalt adhere to the pavement. Applying tack (right)
Asphalt is placed in the hole, leveled with a rake and compacted with a heavy steel roller. The repaired area can be driven over as soon as the crew has left the area.
Raking fresh asphalt (left)
Compacting asphalt with steel roller (right)