Why Roads Fail Prematurely

December 24, 2018
Listed in Road Maintenance

In a perfect world, pavement would last forever. Unfortunately, this is not the case. Before deciding on the proper repair, we need to understand why there may be premature distresses. Before discussing why roads fail prematurely, we need to start with, “What is a road?”

What is a Road?

A road allows transportation from point A to B in all weather and traffic conditions. While a basic definition, this does not answer the question of why we build roads. Essentially, we build roads to reduce the stress on the native material (i.e., subgrade) under the pavement. To do this, we have to place good materials on the subgrade to spread out the load. Figure 2 shows how the load is spread out by the pavement. A thicker pavement will result in less stress on the subgrade. Figure 3 shows how pavement deflects under a wheel load. As the pavement flexes, there will be a combination of compression (pushing) and tension (pulling) stress in the pavement. This can eventually lead to cracking due to fatigue.

The amount of deflection and stress in the pavement is also related to the amount of moisture in the subgrade soils. If the subgrade soils are wet, there will be a great deal of deflection under the wheel loads. The deflection will be much less for the same soil when it is well drained. The excess moisture in spring thaw will result in higher stresses in the pavement.

Larger loads and thinner pavements result in more stress on the pavement. Pavements will fail sooner than expected if:

• There are heavier loads than expected
• There are more loads than expected
• The pavement is too thin for the traffic loads
• The materials used in the pavemnet are weaker than expected

Understanding Fatigue

Fatigue is the failure of a material due to repetition of many loads. The larger the load, the fewer the number of cycles needed to cause failure. In a pavement, the result is typically cracking or rutting. Roads with heavier trucks or weakened pavement during spring thaw are more susceptible to fatigue failure.

To understand this, take a paper clip and bend it back and forth until it fails. To simulate the summer, bend it to 45° each time. Count how many cycles it takes to fatigue the paper clip. To simulate spring thaw, bend to 90°. A very weak pavement with heavy loads might be like bending the paper clip to 180°.

Premature Failure

Pavements fail prematurely because of many factors. When boiled down to the basics, there are four primary reasons pavements fail prematurely:

• Failure in design
• Failure in construction
• Failure in materials
• Failure in maintenance

Generally when a road or street fails before we expect it to, one of these four factors is the primary cause. Multiple factors can occur, but usually one of the four is the most critical.

The think system
Think of a section of highway in your municipality that never seems to last as long as you expect. Pick one of the four factors above that you think is most likely to be the prime factor to premature failure. As you read the section below, see if your choice stays the same.

Design

Most roads are not specifically designed. They have evolved from paths and trails to the pavements we have today. This does not mean we need to go out and have a full-blown engineering design done for every road repair. In fact, most roads work just fine.

However, there are still many issues that need to be examined. Do we understand the conditions on the road? What is the traffic level? Has anything changed since the last major improvement? Is anything likely to change?

For low-volume roads, the most important design challenge is accounting for weather and drainage conditions. If the drainage is done correctly, and the road is built to certain minimums of thickness and quality, it should hold up just fine.

However, there are still many failures due to design.

Under-designed

A road that cannot handle the loads is under-designed. This could be due to a failure to account for conditions such as an increase in truck traffic. For instance, new roads to industrial and commercial areas should be designed. Before the municipality takes over a road, it must feel confident the road will last as long as possible. Get an engineer to help design the road if you are not sure. Ask for a professional engineering certification that the road will last the desired number of years.

Failure to account for conditions

Even if the road is built to a quality standard, there may be premature failure if any conditions remain unaccounted for in the design. The condition assessment problem that leads to the most premature failures is a lack of good drainage. This is not a construction or materials problem in many cases. When inspecting the road before work is done, the quality of the drainage MUST be assessed. Failure to do so will almost always result in premature failure.

Changes after construction

If you build it, they will come. As soon as you build a smooth section of pavement, vehicles that had detoured in the past may suddenly decide to use the new roadway. If you failed to anticipate this increased traffic, your road may fail too soon. This can be especially bad if there is extra truck traffic in the spring during the thaw.

Examples of Failure in Design

• Overlay too thin for traffic load (Too thick is also undersirable as it's a waste of money.)
• Failure to account for spring in the middle of the roadway
• Use of a chip seal over a badly cracked road
• Using asphalt cement stabilizer when fines content is too high

Construction

Just as design can lead to premature failure, poor quality construction can cause a roadway to fail early. Many construction failures do not appear as defects for several years, so it can be difficult to determine the reason for the failure. Whether the work is done in-house or by contract, it is important to get the job done right.

If you are doing the work yourself, are you ready? Has the crew been trained? What training do they need and where can you get the training? Municipalities have some of the best snowplow crews anywhere. Part of that expertise is experience. You do something enough and you get pretty good at it. Part of the expertise is training. Riding with that old-timer can be some of the best training you can get.

If you are contracting the work, are you ready? Do you need an inspector for the work? Is the inspector trained and ready to make sure the municipality gets what they pay for? What kind of contract are you using?

Construction may be the most difficult step because there are so many questions to be asked and answered. The problem with not asking the questions is that we usually do not get a second chance to do the work again.

Fortunately, experience is a great teacher, and for most operations some basic training and practice is enough to make sure the work is done right. Complicated and specialized work can still be problematic and failures due to construction can occur.

Poor Workmanship

The best laid plans often go astray. If the work is not done properly then it may not last. A very common problem in culvert installation is the failure to compact the backfill in thin even lifts. It may be faster to put in thick lifts, but coming back to fix the problem after settlement occurs is not a good alternative. Training and pride in your work go a long way towards overcoming workmanship issues.

Using incorrect equipment

Everyone knows you should hit a nail with a hammer, yet how many of us have used a wrench instead? Using the wrong tool in pavement maintenance can lead to premature failure. A rubber-tired roller should be used on a chip seal. A steel drum roller can crush and break the aggregate. NOT using a piece of equipment can also cause problems. Leaving gravel unrolled because no roller is available is a bad solution.

Using equipment improperly

Even if you have the right piece of equipment, it is important to use it correctly. A distributor with misaligned fans will lead to streaking. Using a compressor to blow out cracks can put water into the cracks. Know what a piece of equipment is for and how to use it properly.

Failure to follow plans

Do you have plans? Engineering drawings are not required for pavement maintenance, but writing down the steps and having a plan is a valuable tool. Examples of items in a good plan include stakeout, detours, materials, construction steps, and plans in case of poor weather. Without a plan, how do you know whether anything was done incorrectly?

Lack of training

Lynne Irwin likes to say: “How do you know what you don’t know, if you don’t know that you don't know it?” A crew cannot be expected to do something if they do not know how. Provide training for everyone. It can be on-the-job, tailgate talks over breaks, hour-long training at an association meeting, or all-day training from the Cornell Local Roads Program or a vendor.

Wrong time of year or poor weather

We cannot control the weather, but we can account for it. A surface treatment placed in October is not likely to work as well as one placed in July. On the other hand, if it rained during construction in July, it is not likely to do very well either. Know the limitations for the repair. You may have to do the work anyway, but you will be better prepared to overcome the problems that may arise.

Examples of Failure in Construction

• Failure to compact cold patch with truck tire
• Failure to place the aggregate in a chip seal before the asphalt emulsion breaks
• Using an air compressor without an oil/water separator to clean cracks (can introduce water and cause a loss of bond)
• Paving over a base that is not properly prepared
• Doing any work with asphalt emulsions after late October (or on any cold day)

Materials

Using the wrong material in the right place or the right material in the wrong place can lead to premature failure. Sometimes the problems are obvious. Sometimes the problem does not appear to be related to the material choice. Backfilling an underdrain trench with large stone is actually a materials problem. The stones will retain silt particles brought in by the drained water and will lead to premature plugging of the pipe. Be sure to select the correct material for the job.

Wrong material

The wrong material will lead to premature failure. Using a dirty gravel base is a classic example. The use of the less expensive material can lead to much larger expenses in the future. Understand what the limitations of a particular product are before you use it. Ask the vendor, other highway departments, or the New York State Department of Transportation. Get a clear picture of the best material for the job.

Material does not meet specifications

Once you select the material, make sure it meets specifications. By some estimates, 1/4 of the wire in the main cables of the Brooklyn Bridge did not meet the specifications. When the contractor was caught, the extra expense of more cable was paid out of their contract. Some failures are due to a material not meeting specifications. One recommendation is to always sample the materials on site. It is not always possible to go back and get a sample after the construction is complete.

Material installed incorrectly

If material is put in incorrectly, there can be premature failure. Is this a construction issue or a material issue? It could be both. It should be neither. Sometimes the problem is failure to install the item using a newer technique. For instance, Superpave asphalt concrete needs to be rolled differently than older Marshall Mix design asphalt concretes.

Incompatibility with other materials

Aggregate charge incompatibility is cited almost every time a chip seal fails. In reality, it almost never occurs. Much more common is using a dusty stone that does not adhere to the asphalt emulsion. When this problem occurs, the consequences can be dramatic.

Examples of failures in materials

• Chip sealing over a good quality surface gravel (a good surface gravel has too many fines to be a base gravel)

• Using an asphalt emulsion to seal cracks

• Using a cheaper cold patch that may last only a few hours

• Using a dusty or wet aggregate in surface treatment operations

Maintenance

The most common maintenance problem is that not enough maintenance is done. This is a budgetary, planning, and communication issue that is sometimes very difficult to overcome. Once we decide to perform maintenance we need to remember that ALL maintenance techniques can be designed to fit the conditions and need to be constructed properly using the correct materials. Premature failure of pavement maintenance is usually a failure of design, construction, or material.

Design

The most common design issue is a lack of design. The first step in design is selecting the correct repair to fix the problem. In too many cases, the choice of repair is made for non-technical reasons. In addition, many maintenance repairs are made without any design. Even a chip seal can be designed to obtain the best result for the municipality. Knowing what needs to be done to get the right repair is one of the most critical steps in the design of pavement maintenance.

Construction

Once a technique has been chosen, it needs to be done correctly. The failure to construct the maintenance repair properly is a major cause of premature failure. A classic example is the lack of truck tire rolling of cold mix patch in the winter. Instead of lasting several months, it lasts less than a day.

Material

Using the correct material is critical. It may be less expensive to buy cheaper gravel, patch, or emulsion, but can you afford the cost of replacement if it fails prematurely?

 

Source: Cornell Local Roads Program by Cornell.edu