2016 ~ Pavement Engineering

Introduction

A pavement with base and sub-base that allows the water to infiltrate or pass through the surface is known as Permeable Pavement. It not only helps to reduce the surface run off but also effectively traps the pollutants and solid particles in the water avoiding ground water pollution.

Evaporation of water at or below the surface also produces surface cooling, as opposed to the traditional heating of paved areas. This is especially beneficial in cities which experience extremely high temperatures in summer – traditional “blacktop” temperatures can make some public spaces unusable in warmer weather.

An Illustration of Permeable Pavements

Generally permeable pavement structure consists of a pervious / permeable surface course which may be of Asphalt concrete or Portland cement concrete laid over a layer of fine aggregates, which acts as a filter layer for infiltrating storm or rain water. Underneath this filter layer is uniformly graded gravel course to store water, which ultimately penetrates in the natural ground. General design practice requires an un-compacted natural ground, which allows relatively easier discharge of water and to avoid any standing water under the pavement structure that may cause structure failure.

Permeable pavements are generally recommended for residential, commercial and urban areas but not feasible for highways, motorways and streets with heavy traffic loading as these pavements are not designed with structural capacities to support such loads.

Advantages

Permeable pavements offer many benefits, both aesthetic and practical. Some of the benefits are listed below:

Reduces storm water runoff, including reduction of temperature, total water volume, and flow rate.
Treats water runoff.
Increases groundwater infiltration and recharge.
Provides local flood control.
Improves the quality of local surface waterways.
Reduces soil erosion.
Reduces the need for traditional storm water infrastructure, which may reduce the overall project cost.
Increases traction when wet.
Reduces splash-up in trafficked areas.
Extends the life of paved area in cold climates due to less cracking and buckling from the freeze-thaw cycle.
Reduces the need for salt and sand use during the winter, due to little or no black ice.
Requires less snow-plowing.
Reduces groundwater pollution.
Creates green space (grass ground cover, shade from tree canopies, etc.
Offers evaporative cooling.
Porous pavements reduce the volume of storm water, increase the recharge, control the peak rate, and offer a high out flowing water quality.

Disadvantages

Some of the disadvantages associated with these pavements are listed below:

Highly contaminated runoff can be generated by some land uses where pollutant concentrations exceed those typically found in storm water.
Heavy traffic loads can cause severe distresses which leads to pavement failure.
In cold climates, salts used for removal of snow, contains chlorides that could migrate through the porous pavement into groundwater.
Frequent maintenance is required to prevent the blockage of pores.
Additional maintenance cost compared to conventional pavement.

1. Alligator Cracking

Alligator cracking is a load associated structural failure. The failure can be due to weakness in the surface, base or sub grade; a surface or base that is too thin; poor drainage or the combination of all three. It often starts in the wheel path as longitudinal cracking and ends up as alligator cracking after severe distress.

FIX: Because a structural failure is taking place the only possible solution to alligatoring is to perform a full-depth patch.

2. Block Cracking

Block cracks look like large interconnected rectangles (roughly). Block cracking is not load-associated, but generally caused by shrinkage of the asphalt pavement due to an inability of asphalt binder to expand and contract with temperature cycles. This can be because the mix was mixed and placed too dry; Fine aggregate mix with low penetration asphalt & absorptive aggregates; poor choice of asphalt binder in the mix design; or aging dried out asphalt.

FIX: Less severe cracks measuring 1/2 inch or less can be sealed to prevent moisture from entering into the sub grade. More severe cracks should be fixed by removing the cracked pavement layer and replacing it with an overlay.

3. Longitudinal (Linear) Cracking

Longitudinal cracking are cracks that are parallel to the pavements centerline or laydown direction. These can be a result of both pavement fatigue, reflective cracking, and/or poor joint construction. Joints are generally the least dense areas of a pavement.

4. Transverse Cracking

Transverse cracks are single cracks perpendicular to the pavement's center line or lay-down direction. Transverse cracks can be caused by reflective cracks from an underlying layer, daily temperature cycles, and poor construction due to improper operation of the paver.

5. Edge Cracks

Edge Cracks travel along the inside edge of a pavement surface within one or two feet. The most common cause for this type of crack is poor drainage conditions and lack of support at the pavement edge. As a result underlying base materials settle and become weakened. Heavy vegetation along the pavement edge and heavy traffic can also be the instigator of edge cracking.

FIX: The first stepin correcting the problem is to remove any existing vegetation close to the edge of the pavement and fix any drainage problems. Crack seal/fill the cracks to prevent further deterioration or remove and reconstruct to full depth fixing any support issues.

6.Joint Reflection Cracks

These are cracks in a flexible pavement overlay of a rigid pavement (i.e., asphalt over concrete). They occur directly over the underlying rigid pavement joints. Joint reflection cracking does not include reflection cracks that occur away from an underlying joint or from any other type of base (e.g., cement or lime stabilized).

FIX: For less severe cracks (less than 1/2 inch) crack sealing will prevent the further entry of moisture into the subgrade. If the cracks are more severe the removal of the cracked pavement layer followed by an overlay may be required.

7. Slippage Cracks

Slippage cracks are crescent-shaped cracks or tears in the surface layer(s) of asphalt where the new material has slipped over the underlying course. This problem is caused by a lack of bonding between layers. This is often because a tack coat was not used to develop a bond between the asphalt layers or because a prime coat was not used to bond the asphalt to the underlying stone base course. The lack of bond can be also caused by dirt, oil, or other contaminants preventing adhesion between the layers.

FIX: All of the areas exhibiting the “stretch marks” will need to be removed and will require a partial or full depth patch.

8. Pot Holes

Small, bowl-shaped depressions in the pavement surface that penetrate all the way through the asphalt layer down to the base course. They generally have sharp edges and vertical sides near the top of the hole. Potholes are the result of moisture infiltration and usually the end result of untreated alligator cracking. As alligator cracking becomes severe, the interconnected cracks create small chunks of pavement, which can be dislodged as vehicles drive over them. The remaining hole after the pavement chunk is dislodged is called a pothole.

FIX: Full depth replacement patch

9. Depressions (bird baths)

Depressions are localized pavement surface areas with slightly lower elevations than the surrounding pavement. Depressions are very noticeable after a rain when they fill with water.

FIX: Depending on the severity of the depression the asphalt may have to be removed and replaced (severe). Less severe depressions can be fixed by applying a thin surface patch or infrared patch.

10. Rutting

Ruts in asphalt pavements are channelized depressions in the wheel-tracks. Rutting results from consolidation or lateral movement of any of the pavement layers or the subgrade under traffic. It is caused by insufficient pavement thickness; lack of compaction of the asphalt, stone base or soil; weak asphalt mixes; or moisture infiltration.

FIX: If rutting is minor or if it has stabilized, the depressions can be filled and overlaid. If the deformations are severe, the rutted area should be removed and replaced with suitable material.

11. Shoving

Shoving is the formation of ripples across a pavement. This characteristic shape is why this type of distress is sometimes called wash-boarding. Shoving occurs at locations having severe horizontal stresses, such as intersections. It is typically caused by: excess asphalt; too much fine aggregate; rounded aggregate; too soft an asphalt; or a weak granular base.

FIX: Partial or full depth patch

12. Upheaval
Upheaval is a localized upward movement in a pavement due to swelling of the subgrade. This can be due to expansive soils that swell due to moisture or frost heave (ice under the pavement).

FIX: Full depth patch

13. Raveling (very porous asphalt)

Raveling is the on-going separation of aggregate particles in a pavement from the surface downward or from the edges inward. Usually, the fine aggregate wears away first and then leaves little "pock marks" on the pavement surface. As the erosion continues, larger and larger particles are broken free and the pavement soon has the rough and jagged appearance typical of surface erosion.

There are many reasons why raveling can occur, but one common cause is placing asphalt too late in the season. This is because the mixture usually lacks warm weather traffic which reduces pavement surface voids, further densification, and kneading of the asphalt mat. For this reason raveling is more common in the more northern regions(snow belt).

FIX: Apply a thin hot-mix overlay. Other solutions could include: sand seal, chip seal, slurry seal or micro-surfacing.

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