C5602 Highway Maintenance

Filling in the Cracks

Both asphalt pavements and concrete pavements can develop cracks, though not always for the same reasons. Previously, we looked at some common types of cracks in asphalt pavements, their causes, and how to deal with them. In this edition of the RoadReady newsletter, we’ll do the same for cracking in portland cement concrete (PCC) pavements.

Since PCC shrinks as it sets and cures, some amount of cracking in concrete pavements is inevitable. In fact, the different types of concrete pavements have their own ways of dealing with this. For example, jointed pavements (JPCP or JRCP) are cut with a saw during construction so that the concrete will crack at the slab joint by design instead of elsewhere. In reinforced pavements (CRCP  or JRCP), the reinforcing steel is used to hold the concrete tightly together, so the shrinkage cracking that may appear is less of a problem.

Other forms of cracking in PCC pavements are a serious concern, however. Let’s take a closer look at four common types of PCC distress cracking:

• Linear cracks
• Corner breaks
• Punchouts
• Durability (“D”) cracking

Linear Cracks

Linear cracking, sometimes also known as panel cracking, refers to cracks that divide a slab into two or more pieces. These cracks will not necessarily run in a perfectly straight line, but they do extend across the entire slab; that is, each crack runs either edge-to-edge or until it reaches another crack. Linear cracking includes transverse cracks (running across the slab) and longitudinal cracks (running in the direction of travel) as well as cracks in a diagonal direction. Slabs with multiple linear cracks are considered divided or shattered slabs.

The cracks can develop due to a variety of factors. These include overloading, thermal expansion and contraction, moisture stresses, slab curling, and loss of support underneath the slab. Often a combination of factors is involved, and traffic loads usually contribute to some degree.

Linear Crack Repair

In mild cases, such as a slab with a single, narrow crack, a repair may be as simple as filling the crack with a sealant. If a dowel bar retrofit is being considered for the pavement, cracks that are working cracks (meaning they behave like joints, with a similar range of movement), should also have dowel bars installed to help with load transfer.

More extensive cracking will require at least a patch to repair. Patches will need to be full-depth, since these cracks are not just surface distresses but extend through the entire thickness of the slab. If the cracking is severe enough, the slab is shattered and the appropriate repair would involve complete slab replacement.

Corner Breaks

A corner break is a crack that runs across one corner of the slab. Like a linear crack, a corner break typically goes through the entire slab thickness so that the corner is fully broken off from the slab. The difference between corner breaks and diagonal linear cracks is in how close the crack is to the intersection of joints at the corner. Typically, a corner break is defined as being less than half of the slab length away from the actual corner (on both sides).

Corner breaks can be caused by repeated traffic loading and may be an indication of poor load transfer across the joint. Other potential factors include a loss of support along with warping and curling stresses.

Corner Break Repair

If the slab is otherwise in good condition, a corner break can be repaired with a full-depth patch. The patch will fix the primary crack as well as any additional cracking within the broken-off corner piece. If the corner break is partly due to loss of support in the base or subgrade, this should be remedied as part of the repair. Otherwise, another break may appear and the patch is not likely to hold.

Punchouts

Punchouts are localized areas where the slab is cracked and broken into several pieces. Unlike linear cracks, punchout cracking is primarily confined to one part of the slab, so the cracks do not extend from edge to edge. Punchouts come in many different shapes and sizes. However, most punchouts form near a linear crack or pavement joint that helps define the punchout area.

Causes of punchouts include repeated heavy loads, inadequate slab thickness, and loss of foundation support. Punchouts may be the product of a localized construction deficiency, such as honeycombing in the concrete or inadequate consolidation while the pavement was being built. In continuously reinforced concrete pavements, punchouts are of particular concern and may indicate steel corrosion or an inadequate amount of steel for reinforcement.

Punchout Repair

A punchout can be repaired with a full-depth patch covering the punchout area. Take care to identify and resolve any underlying problems, such as loss of support or inadequate reinforcement. Fixing these will help prevent the issue from reoccurring.

Durability Cracking

Durability cracking, simply known as “D” cracking, usually appears as a pattern of closely spaced cracks near a joint, corner, or linear crack. The cracks typically run approximately parallel to each other and the cracking will progress outward over time, eventually leading to disintegration of the slab if it is not corrected. The cracks may contain characteristic dark-colored deposits if the concrete becomes saturated and material collects there.

Durability or “D” cracking is caused by a specific problem with the large aggregate in the concrete mix. If the aggregate is susceptible to expansion during freeze-thaw cycles, it creates stresses in the PCC that eventually cause the pavement to fracture.

Durability Cracking Repair

Due to the nature of the problem, durability cracking is challenging to deal with. A patch can repair the affected area, but does not address the root of the problem and will not prevent cracks from spreading through the rest of the pavement. In the past, some agencies would treat durability cracking by placing an asphalt overlay on top of the pavement. This can temporarily address the surface distress and improve the pavement’s ride quality, but still fails to prevent the progression of the cracking.

Ultimately, pavements with durability cracking will need to be completely replaced as the deterioration accelerates. Obviously, this makes it an expensive and time-consuming problem. The real solution is to test aggregates before they are used in PCC construction to ensure they are not susceptible to freeze-thaw expansion.

Crack Prevention

As we’ve discussed, cracks can sometimes be sealed, patched, or repaired by replacing the affected slab. These measures are useful options, but in many cases the cracking could have been prevented. Good pavement design and following best practices during construction will avoid a lot of problems. It is a lot easier to prevent crack problems in the first place, rather than trying to cure them once they are in your pavements.  Just “say no to cracks!”

Resource from :  http://www.pavementinteractive.org

In this day and age, we’ve come to expect a lot from our roadways. We rely on roads to carry all kinds of things from place to place, from people and goods to emergency services. When it comes down to it, roads are the real heavy lifters of society. But how do they stand up to all this weight? We know roads deteriorate over time, but how do vehicle loads themselves impact pavement, and what factors affect the extent to which pavement is damaged? In this edition of the RoadReady Newsletter, we’re going to explore roadway loading, including load distribution in a pavement, which kinds of vehicles cause the most damage, and other issues associated with pavement design.

Loading Mechanics

When a vehicle drives over a pavement, it induces bending stresses in the pavement layers below. Induced stresses vary with the relative location of the load to a given point in the pavement. Imagine using your hands to bend a pencil. The wood on one side of the pencil will be pushed together, while the wood on the other side will be pulled apart. Likewise, directly underneath a vehicle load, top layers of the pavement are compressed while those beneath it experience tensile stress. Away from the location of impact, this is reversed. with tension in the upper layers and compression in those below it. Damage occurs through repetitive cycles of loading and unloading. Cracking is generally a result of tensile stresses, as the pavement is pulled apart leaving voids. In flexible pavements, the asphalt concrete and base layers provides the resistance to both compressive and tensile loads. In rigid pavements, tensile load is carried by both the concrete and any reinforcing steel present in the pavement.

Trucks and buses are the largest contributors of roadway damage

Vehicle Type

Personal automobiles make up the vast majority of mileage traveled on roadways. Despite this, these vehicles contribute minimally to pavement deterioration. Heavier loads tend to damage a pavement much more severely than smaller ones. As a general rule, the relative damage that a load causes to a pavement structure is calculated by taking the ratio of load magnitude to the fourth power. In other words, your local highway would much rather do a lot of low-weight reps than max-out on the bench press. As a result, the damage caused by a modestly loaded freight truck is equivalent to that caused by thousands of passenger cars. Because of this, trucks and buses represent the real heavy lifting for our roadways. In fact, loads from motorcycles, passenger cars, pickup trucks, and vans are seen as negligible and are not typically factored into pavement designs.

Equivalent Single Axle Load (ESAL)

As discussed in the last section, different types of vehicles contribute differently to pavement damage. Because of this, pavement engineers use a metric that combines the contributions of these different sources into a single number. This number is then used as the basis for the structural design of the pavement. The equivalent single axle load, or ESAL, is such a metric. One ESAL is defined as a single axle with a downward force of 18,000 lbs. Other values of ESALs for different loads indicate the relative damage they would cause compared to this base load. For instance, an axle with an ESAL value of five is about five more damaging to a roadway structure. The ESAL value relative to one is also known as a load equivalency factor, or LEF. The calculation of load equivalency factor for a given load depends on the type of pavement (flexible or rigid) as well as the strength of the pavement structure. Designers will use traffic data and growth patterns to estimate the number of ESALs that a roadway will be subjected to in its lifetime. This number is then used to determine how strong the roadway structure needs to be.

The ESAL system was developed in the early 1960′s and is still widely used. However, the 2002 Guide for the Design of New and Rehabilitated Pavement Structures, developed by the National Cooperative Highway Research Program, recommends a new approach known as load spectra. Load spectra rely on the same data inputs as ESALs, but rather than aggregating the data into a single number, individual counts of weight and axle distribution are maintained. For instance, load estimation based on ESALs will produce a single number (typically in the millions) to be used for pavement design. In contrast, a load spectra approach separates vehicles by different types of loads, and totals frequencies for each category.

An example of data input for a load spectra approach

Axles and Tires

In addition to the total weight of a vehicle, the number of axles and tires has an important effect on how a load is distributed to a pavement, and therefore how the pavement is damaged. Generally, the more contact points that the load makes, the smaller each individual load, and the smaller the cumulative damage to the pavement. As a result, tables for ESALs are generally given for both single axles and tandem axles. Tandem axles are pairs of axles which are spaced close together. For the same axle load, these greatly reduce damage to a roadway. For instance, for an 18,000 pound load, a tandem axle contributes about one tenth the ESALs that a single axle does. In addition to single and tandem axles, axles can also be classfied as dual tire or single tire, corresponding to the number of tires on each side of the vehicle. The distinction between dual and single tire is not as critical as single and tandem axles, but use of dual tire axles translates to a 10-20% reduction of pavement stress for a given single axle load.

Tire characteristics also affect the severity of roadway damage for a given load. For instance, wider tires will distribute loads over a larger section of a pavement. Tire pressure also impacts load distribution. Highly inflated tires will concentrate loads on the pavement surface, while tires under low pressure will tend to spread out the loads more.

Diagram of a typical axle and tire arrangement

Load Distribution

In addition to determining the total amount of vehicle loads a road will experience over its lifetime, pavement engineers must also estimate how these loads will be distributed among the individual lanes of a roadway. If you work out your arms all day, they’re going to get tired before the rest of your body. The same goes for roads, and designers must take this into account. For multi-lane roads, a “design lane” is typically selected which carries the critical (maximum) traffic loads. These loads are used to design the entire roadway. Loads can vary between lanes in a number of different ways. For instance, opposite direction of travel will not neccesarily be expected to carry the same amount of traffic. In addition, for multi-lane highways, large trucks are more likely to travel in the slower lanes. As a result, the outside lanes of a highway are generally subject to higher loads than those closer to the median.

Carrying the Load

Accurate characterization of roadway loads is critical to intelligent roadway design. On one hand, underestimating loads will lead to premature roadway failure, and increased maintenance and rehabilitation costs. Overestimating loads, on the other hand, will lead to over-designed roadways, which means increased and unnecessary upfront costs. To get the most out of roadway spending and meet the needs of the traveling public, design and construction activities must be guided by intelligent load estimation.



 

                Bituminous Surface Treatments

Bituminous surface treatments (BST) refer to a range of techniques that can be used to create a stand-alone drivable surface on a low volume road, or rehabilitate an existing pavement. Usually, the term is used to describe a seal coat or chip seal, which is constructed by spraying a layer of emulsified asphalt, and placing a layer of aggregate on top. BSTs can be applied directly to a base course, or on an existing asphalt pavement structure, and represent a low cost alternative to typical asphalt paving. In this RoadReady newsletter, we will focus on chip seals as a rehabilitation option. 

                                                               When to Use a Chip Seal
 
Chip Seals are appropriate for a number of situations. They can be used as an inexpensive paving option for a low volume road when applied to a base course. On an existing road, they can be used to seal cracks, provide a new wearing course, and provide protection from sunlight and moisture. BSTs also provide a high degree of friction and skid resistance, creating a safe driving surface. Chip seals should only be used to rehabilitate minor distresses on a roadway, such as worn surfaces, raveling, and small cracks. They will likely perform inadequately when cracks become too wide or the pavement has undergone significant structural deformation.
Rural roads with low traffic volumes are the best suited for chip seals. In urban settings, additional stresses from turning and acceleration can cause problems before the chip seal has gained sufficient strength. In addition, chip seals tend to generate more roadway noise than traditional asphalt pavements, making them an unpopular choice in populated areas. There is no absolute traffic limit that prohibits the use of chip seals, however virtually all specifications in the U.S. and Canada limit their use to roads with daily traffic less than 20,000 vehicles. Many states also use either 5,000 or 2,000 as a maximum, indicating that there is some uncertainty over the correct suitable range.

                                                                                    Materials
 
Selecting the right materials is an important part of constructing a successful chip seal. This includes the emulsified asphalt as well as the aggregate. Emulsified asphalt simply refers to a mix of water and asphalt binder. Because the success of a chip seal depends on the aggregate becoming embedded in the asphalt layer, high viscosity asphalt should always be used. In addition, the setting time must be selected carefully based on weather and traffic control, which will be discussed later.
Chip seals are unique from other types of asphalt pavement in that they specify exclusively uniformly graded aggregate, or aggregate within a very small size range. While typical asphalt pavements use densely graded aggregates to reduce air voids, similarly sized aggregates in chip seals allow embedment into the asphalt and interlock between particles. Aggregate size is an important design consideration for chip seals. Large aggregates can be used with higher amounts of emulsified asphalt, and therefore provide a higher degree of crack sealing. However, these increased material quantities are more expensive, and larger aggregate produces more roadway noise. Apart from size, aggregate particles should be angular, not overly slender, and relatively free of clinging fine particles.

                                                       Construction Considerations

Before constructing a chip seal, any major deficiencies must be corrected in the existing structure.  For example, if alligator cracks are present, subgrade repair may be required. If potholes are present, these must be patched . Next, the surface should be swept to remove particles that could interfere with bonding of the asphalt and the existing pavement. After the asphalt and aggregate have been placed, chip seals are compacted similar to hot mix asphalt pavements. Different kinds of rollers have different advantages and disadvantages for chip seals. Pneumatic rubber tire rollers can provide more uniform compaction on an uneven surface, but tend to pick up individual pieces of aggregate on the tires. Steel wheel rollers on the other hand risk fracturing the aggregate. Pneumatic rubber tire rollers are generally selected over steel wheel rollers for this reason. Following compaction, it is important to keep traffic off of the new surface until moisture content has reached an acceptable level. How long this takes depends on whether the emulsion is slow, medium, or fast setting. Allowing traffic to use the road prematurely can result in the loss of particles from the pavement surface.  
Bituminous surface treatments represent a low-cost option for the rehabilitation of low-volume roads. Though not ideal for every project, they have become an attractive alternative in a world where thin maintenance and rehabilitation budgets are stretched over large roadway networks.

Seal Treatment Types

Crack Seals

A crack seal involves filling a crack in the pavement surface with an adhesive sealant, usually asphalt binder or an emulsion of asphalt mixed with water. Crack sealing is normally focused on cracks that are not load-related. Prompt treatment of individual cracks can help prevent pavement deterioration from accelerating as cracks grow, develop secondary cracks, and begin to spall around the edges. Sealing the crack will keep water and incompressible materials (dirt, sand, debris, etc.) from infiltrating the pavement structure.

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A proper crack seal involves more than just filling the crack. The crack should be routed to clean out any existing debris and create a reservoir for the sealant. A backer rod may be inserted to keep the sealant material from draining to the bottom of the crack. Crack sealing, as opposed to crack filling, is particularly recommended for working cracks (cracks that experience significant horizontal movement), reflective cracking, and in general for most transverse cracks.

Fog Seals

A fog seal is a light treatment in which an asphalt emulsion is applied to the pavement surface. The emulsion can renew an older pavement surface that may be experiencing some weathering and raveling. Fog seal treatment can help seal voids and small cracks in the pavement surface, but will be less effective if pavement distress is more extensive.

Slurry Seals

A slurry seal is a mixture of asphalt and well-graded fine aggregate. The asphalt is normally a slow-setting emulsion, and modifiers or mineral filler may be included in addition to the aggregate. Slurry seals typically work best with pavements that are in good condition but may be showing signs of aging. For example, a local municipality could use slurry seals as part of a preventive maintenance program to improve the condition of pavements that are still performing well after several years of service.

Treating a pavement with a slurry seal helps improve skid resistance and restores a uniform surface texture. It also seals the pavement surface to fill cracks and prevent the intrusion of moisture or air into the pavement.

Sand Seals

Like a slurry seal, a sand seal uses an asphalt emulsion along with sand or fine aggregate. However, the materials are not applied as a mixture, but one at a time. The emulsion is sprayed onto the pavement surface first, after which the sand is applied. The sand can be spread immediately for maximum stick, or wait until after the emulsion breaks and be rolled with a pneumatic tire roller. Sand seals primarily work to seal the pavement surface and improve skid resistance.

Scrub Seals

The materials for a scrub seal are similar to a sand seal, consisting of an asphalt emulsion and finely graded aggregate. However, as an additional step after the emulsion is applied, a broom is dragged over the pavement to push the asphalt into cracks and voids in the surface. The aggregate is then applied, followed by a second pass with the broom mechanism to force the aggregate into the emulsion. This scrubbing action may allow the seal to fill cracks that might otherwise need to be sealed individually.

Chip Seals

A chip seal uses a two-step process in which the pavement is sprayed with a coat of asphalt and then covered with coarse aggregate. The seal coat is normally an asphalt emulsion and should have a high viscosity to helpembed the aggregate in the asphalt. The aggregate should be uniformly graded and must be rolled after placement to ensure proper embedding. A chip seal is useful for improving surface friction and to seal a pavement with mild cracking that is not load-related. On low-volume roads, a chip seal may be used as a wearing course.

Chip seals can be applied in multiple layers if desired. While a typical chip seal involves a single coat of binder and aggregate, a double chip seal can be used if a harder, longer-lasting treatment is needed. In a double chip seal, after the first layer of aggregate has been rolled, another seal coat is applied. This is followed by a second layer of aggregate, which should be graded to about half the size of the aggregate in the first layer.

Sandwich Seals

A sandwich seal consists of an application of asphalt binder in between two layers of aggregate, hence the name. Sandwich seals are similar to a double chip seal, but without the first layer of binder. Instead, the coarse aggregate is placed directly on the existing pavement surface. This type of treatment might be considered for a pavement that exhibits bleeding or flushing.

Cape Seals

A cape seal is effectively a combination of two separate treatments, a chip seal and a slurry seal. The chip seal is placed first, followed within a few days or weeks by the slurry seal. This enhances the binding of the chips with the asphalt and prevents loose aggregate from being dislodged. Cape seals provide a new wearing surface and can address more significant deterioration than either of the treatments individually. They can be an effective treatment for pavements where some distress is present, but a more expensive overlay is not required.

Additional Tips on Seals

The right kind of seal treatment to choose will vary based on environmental and traffic conditions, the age and condition of the pavement, and other factors such as its intended service life. In some cases, combining multiple treatments may be helpful to get the benefits from each type. One possibility is to combine crack sealing with another type of seal as a surface treatment. Seal treatments can also be re-applied every few years for maintenance as long as the pavement remains structurally sound.

Another technique used with certain types of seals is to add rubber to the asphalt mix. Ground-up pieces of tire rubber can be added to the asphalt binder as a modifier. For example, rubber can add elasticity to the sealant used for a crack seal. Rubberized asphalt could also be used in a chip seal to try and reduce reflection cracking.

Another Preservation Tool

Seal treatments are a critical tool for asphalt pavement preservation. With proper design and taking into account the condition of the existing pavement, seals can be useful for any roadway, including those with higher traffic volumes. All it takes is selecting the right tool.