Report on

ENGINEERING COMPARISON BETWEEN FLEXIBLE AND RIGID PAVEMENT

Photo courtesy WSDOT

Submitted by .

Md.

Tahmidul Islam Farabi06 02 03 0314th Year 2nd SemesterDepartment of Civil Engineering Ahsanullah University of Science & Technology

Pavement

The road pavement is the actual surface on which the vehicles will travel. It's purpose is two fold, to provide friction for the vehicles and to transfer normal stresses to the underlying soils.

Typically, pavements are built for three main purposes:

1. Load support : Pavement material is generally stiffer than the material upon which it is placed, thus it assists the in situ material in resisting loads without excessive deformation or cracking.

2. Smoothness : Pavement material can be placed and maintained much smoother than in situ material.  This helps improve ride comfort and reduce vehicle operating costs.

3. Drainage : Pavement material and geometric design can effect quick and efficient drainage thus eliminating moisture problems such as mud and ponding (puddles).

Types of pavements

1. Flexible pavement2. Rigid pavement

The pavements can be classified based on the structural performance into two, flexible pavements and rigid pavements. In flexible pavements, wheel loads are transferred by grain-to-grain contact of the aggregate through the granular structure. The flexible pavement, having less flexural strength, acts like a flexible sheet (e.g. bituminous road). On the contrary, in rigid pavements, wheel loads are transferred to sub-grade soil by flexural strength of the pavement and the pavement acts like a rigid plate (e.g. cement concrete roads). In addition to these, composite pavements are also available. A thin layer of flexible pavement over rigid pavement is an ideal pavement with most desirable characteristics. However, such pavements are rarely used in new construction because of high cost and complex analysis required.

Flexible pavement

Are those pavements which reflect the deformation of subgrade and the subsequent layers to the surface. Flexible, usually asphalt, is laid with no reinforcement or with a specialized fabric reinforcement that permits limited flow or repositioning of the roadbed underground changes.

The design of flexible pavement is based on load distributing

characteristic of the component layers. The black top pavement including water & gravel bound macadam fall in this category.

Flexible pavement on the whole has low or negligible flexible strength flexible in their structural action). The flexible pavement layers transmit the vertical or compressive stresses to the lower layers by grain transfer through contact points of granular structure.The vertical compressive stress is maximum on the pavement surface directly under the wheel load and is equal to contact pressure under the wheels. Due to the ability to distribute the stress to large area in the shape of truncated cone the stresses get decreased in the lower layer.

As such the flexible pavement may be constructed in a number of layers and the top layer has to be strongest as the highest compressive stresses.

To be sustained by this layer, in addition to wear and tear, the lower layer have to take up only lesser magnitude of stress as there is no direct wearing action die to traffic loads, therefore inferior material with lower cast can be used in the lower layers.

Rigid pavement

The rigid characteristic of the pavement are associated with rigidity or flexural strength or slab action so the load is distributed over a wide area of subgrade soil. Rigid pavement is laid in slabs with steel reinforcement.

The rigid pavements are made of cement concrete either plan, reinforced or prestressed concrete.Critical condition of stress in the rigid pavement is the maximum flexural stress occurring in the slab due to wheel load and the temperature changes.

Rigid pavement is designed and analyzed by using the elastic theory.

Basic Comparison of Flexible and Rigid Pavement 1. Deformation in the sub grade

is transferred to the upper layers

2. Design is based on load distributing characteristics of the component layers

3. Have low flexural strength

4. Load is transferred by grain to grain contact

5. Have low completion cost but repairing cost is high

6. Have low life span

7. Surfacing cannot be laid directly on the sub grade but a sub base is needed

8. No thermal stresses are induced as the pavement have the ability to contract and expand freely

9. Thats why expansion joints are not needed

10. Strength of the road is highly dependent on the strength of the sub grade

1. Deformation in the subgrade is not transferred to subsequent layers

2. Design is based on flexural strength or slab action

3. Have high flexural strength

4. No such phenomenon of grain to grain load transfer exists

5. Have low repairing cost but completion cost is high

6. Life span is more as compare to flexible

7. Surfacing can be directly laid on the sub grade

8. Thermal stresses are more vulnerable to be induced as the ability to contract and expand is very less in concrete

9. Thats why expansion joints are needed

10. Strength of the road is less dependent on the strength of the sub grade

11. Rolling of the surfacing is needed

12. Road can be used for traffic within 24 hours

13. Force of friction is less Deformation in the sub grade is not transferred to the upper layers.

11. Rolling of the surfacing in not needed

12. Road cannot be used until 14 days of curing

13. Force of friction is high

Load distribution

Flexible pavement

Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading.  A flexible pavement structure is typically composed of several layers of material.  Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below.  Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (see Figure 1).  

In order to take maximum advantage of this property, material layers are usually arranged in order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom. This section describes the typical flexible pavement structure consisting of:

Surface course : This is the top layer and the layer that comes in contact with traffic.  It may be composed of one or several different HMA sublayers.

Base course : This is the layer directly below the HMA layer and generally consists of aggregate (either stabilized or unstabilized).

Subbase course : This is the layer (or layers) under the base layer.  A subbase is not always needed.

Figure 1: Flexible & Rigid Pavement Load Distribution

Rigid pavement

Rigid pavements are so named because the pavement structure deflects very little under loading due to the high modulus of elasticity of their surface course.  A rigid pavement structure is typically composed of a PCC surface course built on top of either (1) the subgrade or (2) an underlying base course.  Because of its relative rigidity, the pavement structure distributes loads over a wide area with only one, or at most two, structural layers (see Figure 1).    

This section describes the typical rigid pavement structure consisting of:

Surface course : This is the top layer, which consists of the PCC slab.  

Base course : This is the layer directly below the PCC layer and generally consists of aggregate or stabilized subgrade.

Subbase course : This is the layer (or layers) under the base layer.  A subbase is not always needed and therefore may often be omitted.

Distress

Flexible Pavement Distress

Fatigue (Alligator) Cracking

Bad fatigue cracking Fatigue cracking from frost action

Fatigue cracking from edge failure

 

Description: Series of interconnected cracks caused by fatigue failure of the HMA surface (or stabilized base) under repeated traffic loading.  In thin pavements, cracking initiates at the bottom of the HMA layer where the tensile stress is the highest then propagates to the surface as one or more longitudinal cracks.  This is commonly referred to as "bottom-up" or "classical" fatigue cracking.  In thick pavements, the cracks most likely initiate from the top in areas of high localized tensile stresses resulting from tire-pavement interaction and asphalt binder aging (top-down cracking).  After repeated loading, the longitudinal cracks connect forming many-sided sharp-angled pieces that develop into a pattern resembling the back of an alligator or crocodile.

Problem:  Indicator of structural failure, cracks allow moisture infiltration, roughness, may further deteriorate to a pothole

Possible Causes: Inadequate structural support, which can be caused by a myriad of things.  A few of the more common ones are listed here:

Decrease in pavement load supporting characteristicso Loss of base, subbase or subgrade support (e.g., poor

drainage or spring thaw resulting in a less stiff base). 

o Stripping on the bottom of the HMA layer (the stripped portion contributes little to pavement strength so the effective HMA thickness decreases)

Increase in loading (e.g., more or heavier loads than anticipated in design)

Inadequate structural design

Poor construction (e.g., inadequate compaction)

Repair: A fatigue cracked pavement should be investigated to determine the root cause of failure.  Any investigation should involve digging a pit or coring the pavement to determine the pavement's structural makeup as well as determining whether or not subsurface moisture is a contributing

factor.  Once the characteristic alligator pattern is apparent, repair by crack sealing is generally ineffective.  Fatigue crack repair generally falls into one of two categories:

Small, localized fatigue cracking indicative of a loss of subgrade support.  Remove the cracked pavement area then dig out and replace the area of poor subgrade and improve the drainage of that area if necessary.  Patch over the repaired subgrade.

Large fatigue cracked areas indicative of general structural failure.  Place an HMA overlay over the entire pavement surface.  This overlay must be strong enough structurally to carry the anticipated loading because the underlying fatigue cracked pavement most likely contributes little or no strength (Roberts et. al., 1996).

Bleeding

BST bleeding in wheelpaths

BST bleeding in wheelpaths

HMA bleeding from over-asphalting

 

Description: A film of asphalt binder on the pavement surface.  It usually creates a shiny, glass-like reflecting surface (as in the third photo) that can become quite sticky. 

Problem:  Loss of skid resistance when wet

Possible Causes: Bleeding occurs when asphalt binder fills the aggregate voids during hot weather and then expands onto the pavement surface.  Since bleeding is not reversible during cold weather, asphalt binder will accumulate on the pavement surface over time.  This can be caused by one or a combination of the following:

Excessive asphalt binder in the HMA (either due to mix design or manufacturing)

Excessive application of asphalt binder during BST application (as in

the above figures)

Low HMA air void content (e.g., not enough room for the asphalt to expand into during hot weather)

Repair: The following repair measures may eliminate or reduce the asphalt binder film on the pavement's surface but may not correct the underlying problem that caused the bleeding:

Minor bleeding can often be corrected by applying coarse sand to blot up the excess asphalt binder.

Major bleeding can be corrected by cutting off excess asphalt with a motor grader or removing it with a heater planer.  If the resulting surface is excessively rough, resurfacing may be necessary (APAI, no date given).

Block Cracking

    

Description: Interconnected cracks that divide the pavement up into rectangular pieces.  Blocks range in size from approximately 0.1 m2 (1 ft2) to 9 m2 (100 ft2).  Larger blocks are generally classified as longitudinal and transverse cracking.  Block cracking normally occurs over a large portion of pavement area but sometimes will occur only in non-traffic areas.

Problem: Allows moisture infiltration, roughness

Possible Causes: HMA shrinkage and daily temperature cycling.  Typically caused by an inability of asphalt binder to expand and contract with temperature cycles because of:

Asphalt binder aging Poor choice of asphalt binder in the mix design

Repair: Strategies depend upon the severity and extent of the block cracking:

Low severity cracks (< 1/2 inch wide).  Crack seal to prevent (1) entry of moisture into the subgrade through the cracks and (2) further raveling of the crack edges.  HMA can provide years of satisfactory service after developing small cracks if they are kept sealed (Roberts et. al., 1996).

High severity cracks (> 1/2 inch wide and cracks with raveled edges).  Remove and replace the cracked pavement layer with an overlay.

Corrugation and Shoving

  

Description: A form of plastic movement typified by ripples (corrugation) or an abrupt wave (shoving) across the pavement surface.  The distortion is perpendicular to the traffic direction.  Usually occurs at points where traffic starts and stops (corrugation) or areas where HMA abuts a rigid object (shoving). 

Problem: Roughness

Possible Causes: Usually caused by traffic action (starting and stopping) combined with:

An unstable (i.e. low stiffness) HMA layer (caused by mix contamination, poor mix design, poor HMA manufacturing, or lack of aeration of liquid asphalt emulsions)

Excessive moisture in the subgrade

Repair: A heavily corrugated or shoved pavement should be investigated to determine the root cause of failure.  Repair strategies generally fall into one of two categories: 

Small, localized areas of corrugation or shoving.  Remove the distorted pavement and patch. 

Large corrugated or shoved areas indicative of general HMA failure.  Remove the damaged pavement and overlay.

Depression

Depression in left lane and shoulder

    

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

Problem: Roughness, depressions filled with substantial water can cause vehicle hydroplaning

Possible Causes: Frost heave or subgrade settlement resulting from inadequate compaction during construction.

Repair: By definition, depressions are small localized areas.  A pavement depression should be investigated to determine the root cause of failure (i.e., subgrade settlement or frost heave).  Depressions should be repaired by removing the affected pavement then digging out and replacing the area of poor subgrade.  Patch over the repaired subgrade.  

Joint Reflection Cracking

Joint reflection cracking on an arterial

Joint reflection cracking on an arterial

Joint reflection cracking close-up

 

Description: Cracks in a flexible overlay of a rigid pavement.  The cracks 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).

Problem: Allows moisture infiltration, roughness

Possible Causes: Movement of the PCC slab beneath the HMA surface because of thermal and moisture changes.  Generally not load initiated, however loading can hasten deterioration.

Repair: Strategies depend upon the severity and extent of the cracking: 

Low severity cracks (< 1/2 inch wide and infrequent cracks).  Crack seal to prevent (1) entry of moisture into the subgrade through the cracks and (2) further raveling of the crack edges.  In general, rigid pavement joints will eventually reflect through an HMA overlay without proper surface preparation.

High severity cracks (> 1/2 inch wide and numerous cracks).  Remove and replace the cracked pavement layer with an overlay.

Longitudinal Cracking

 

 

Longitudinal cracking as the onset of fatigue cracking

Longitudinal cracking from poor joint construction

 

Description: Cracks parallel to the pavement's centerline or laydown direction.  Usually a type of fatigue cracking. 

Problem: Allows moisture infiltration, roughness, indicates possible onset of alligator cracking and structural failure.

Possible Causes:

Poor joint construction or location.  Joints are generally the least dense areas of a pavement.  Therefore, they should be constructed outside of the wheelpath so that they are only infrequently loaded.  Joints in the wheelpath like those shown in third through fifth figures above, will general fail prematurely. 

A reflective crack from an underlying layer (not including joint reflection cracking)

HMA fatigue (indicates the onset of future alligator cracking)

top-down cracking

Repair: Strategies depend upon the severity and extent of the cracking:

Low severity cracks (< 1/2 inch wide and infrequent cracks).  Crack seal to prevent (1) entry of moisture into the subgrade through the cracks and (2) further raveling of the crack edges.  HMA can provide years of satisfactory service after developing small cracks if they are kept sealed (Roberts et. al., 1996).

High severity cracks (> 1/2 inch wide and numerous cracks).  Remove and replace the cracked pavement layer with an overlay.

 

Patching

Failing patch Patch over localized distress

Utility cut patch

 

Description: An area of pavement that has been replaced with new material to repair the existing pavement.  A patch is considered a defect no matter how well it performs.

Problem: Roughness

Possible Causes:

Previous localized pavement deterioration that has been removed and patched

Utility cuts

Repair: Patches are themselves a repair action.  The only way they can be removed from a pavement's surface is by either a structural or non-structural overlay.

 

Polished Aggregate

SMAs at the NCAT test track

5 years of wear

  

Description: Areas of HMA pavement where the portion of aggregate extending above the asphalt binder is either very small or there are no rough or angular aggregate particles.

Problem: Decreased skid resistance

Possible Causes: Repeated traffic applications.  Generally, as a pavement ages the protruding rough, angular particles become polished.  This can occur quicker if the aggregate is susceptible to abrasion or subject to excessive studded tire wear.

Repair: Apply a skid-resistant slurry seal or BST or overlay. 

Potholes

Pothole from fatigue cracking

Developing pothole

  

Description: Small, bowl-shaped depressions in the pavement surface that penetrate all the way through the HMA layer down to the base course.  They generally have sharp edges and vertical sides near the top of the hole.  Potholes are most likely to occur on roads with thin HMA surfaces (25 to 50 mm (1 to 2 inches)) and seldom occur on roads with 100 mm (4 inch) or deeper HMA surfaces (Roberts et al., 1996).

Problem: Roughness (serious vehicular damage can result from driving across potholes at higher speeds), moisture infiltration

Possible Causes: Generally, potholes are the end result of 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.

Repair: In accordance with patching techniques. 

Raveling

Raveling due to low density

Raveling from snowplot operations

From segregation

 

Description: The progressive disintegration of an HMA layer from the surface downward as a result of the dislodgement of aggregate particles. 

Problem: Loose debris on the pavement, roughness, water collecting in the raveled locations resulting in vehicle hydroplaning, loss of skid resistance

Possible Causes: Several including:

Loss of bond between aggregate particles and the asphalt binder as a result of:

o A dust coating on the aggregate particles that forces the asphalt binder to bond with the dust rather than the aggregate

o Aggregate Segregation.  If fine particles are missing from the aggregate matrix, then the asphalt binder is only able to bind the remaining coarse particles at their relatively few contact points.

o Inadequate compaction during construction.  High density is required to develop sufficient cohesion within the HMA.  The third figure above shows a road suffering from raveling due to inadequate compaction caused by cold weather paving.

Mechanical dislodging by certain types of traffic (studded tires, snowplow blades or tracked vehicles).  The first and fourth figures above show raveling most likely caused by snow plows.

Repair: A raveled pavement should be investigated to determine the root cause of failure.  Repair strategies generally fall into one of two categories:  

Small, localized areas of raveling.  Remove the raveled pavement and patch. 

Large raveled areas indicative of general HMA failure.  Remove the damaged pavement and overlay.

Rutting

Mix rutting Mix rutting Rutting from mix instability

 

Description: Surface depression in the wheelpath.  Pavement uplift (shearing) may occur along the sides of the rut.  Ruts are particularly evident after a rain when they are filled with water.  There are two basic types of rutting: mix rutting and subgrade rutting.  Mix rutting occurs when the subgrade does not rut yet the pavement surface exhibits

wheelpath depressions as a result of compaction/mix design problems.  Subgrade rutting occurs when the subgrade exhibits wheelpath depressions due to loading.  In this case, the pavement settles into the subgrade ruts causing surface depressions in the wheelpath.

Problem: Ruts filled with water can cause vehicle hydroplaning, can be hazardous because ruts tend to pull a vehicle towards the rut path as it is steered across the rut.

Possible Causes: Permanent deformation in any of a pavement's layers or subgrade usually caused by consolidation or lateral movement of the materials due to traffic loading.  Specific causes of rutting can be:

Insufficient compaction of HMA layers during construction.  If it is not compacted enough initially, HMA pavement may continue to densify under traffic loads.

Subgrade rutting (e.g., as a result of inadequate pavement structure)

Improper mix design or manufacture (e.g., excessively high asphalt content, excessive mineral filler, insufficient amount of angular aggregate particles)

Ruts caused by studded tire wear present the same problem as the ruts described here, but they are actually a result of mechanical dislodging due to wear and not pavement deformation.

Repair: A heavily rutted pavement should be investigated to determine the root cause of failure (e.g. insufficient compaction, subgrade rutting, poor mix design or studded tire wear).  Slight ruts (< 1/3 inch deep) can generally be left untreated.  Pavement with deeper ruts should be leveled and overlaid. 

Slippage Cracking

Slippage cracking at a bus stop

     

Description: Crescent or half-moon shaped cracks generally having two ends pointed into the direction of traffic.

Problem: Allows moisture infiltration, roughness

Possible Causes: Braking or turning wheels cause the pavement surface to slide and deform.  The resulting sliding and deformation is caused by a low-strength surface mix or poor bonding between the surface HMA layer and the next underlying layer in the pavement structure.

Repair: Removal and replacement of affected area. 

Stripping

Core hole showing stripping at the bottom

Stripping at bottom of hole

Fatigue failure from stripping

 

Description: The loss of bond between aggregates and asphalt binder that typically begins at the bottom of the HMA layer and progresses upward.  When stripping begins at the surface and progresses downward it is usually called raveling.  The third photo show the surface effects of underlying stripping.

Problem: Decreased structural support, rutting, shoving/corrugations, raveling, or cracking (alligator and longitudinal)

Possible Causes: Bottom-up stripping is very difficult to recognize because it manifests itself on the pavement surface as other forms of distress including rutting, shoving/corrugations, raveling, or cracking.  Typically, a core must be taken to positively identify stripping as a pavement distress.

Poor aggregate surface chemistry Water in the HMA causing moisture damage

Overlays over an existing open-graded surface course.  Based on WSDOT experience, these overlays will tend to strip.

Repair: A stripped pavement should be investigated to determine the root cause of failure (i.e., how did the moisture get in?). Generally, the stripped pavement needs to be removed and replaced after correction of any subsurface drainage issues. 

Transverse (Thermal) Cracking

Large patched thermal crack

Smaller patched thermal crack

Small thermal crack

 

Description: Cracks perpendicular to the pavement's centerline or laydown direction.  Usually a type of thermal cracking.

Problem: Allows moisture infiltration, roughness

Possible Causes: Several including:

Shrinkage of the HMA surface due to low temperatures or asphalt binder hardening

Reflective crack caused by cracks beneath the surface HMA layer

top-down cracking

Repair: Strategies depend upon the severity and extent of the cracking:

Low severity cracks (< 1/2 inch wide and infrequent cracks).  Crack seal to prevent (1) entry of moisture into the subgrade through the cracks and (2) further raveling of the crack edges.  HMA can provide years of satisfactory service after developing small cracks if they are kept sealed (Roberts et. al., 1996).

High severity cracks (> 1/2 inch wide and numerous cracks).  Remove and replace the cracked pavement layer with an overlay.

Water Bleeding and Pumping

Water bleeding

Water bleeding up close CTB base pumping

 

through HMA cracks

Description: Water bleeding (left two photos) occurs when water seeps out of joints or cracks or through an excessively porous HMA layer.  Pumping (right-most photo) occurs when water and fine material is ejected from underlying layers through cracks in the HMA layer under moving loads.

Problem: Decreased skid resistance, an indication of high pavement porosity (water bleeding), decreased structural support (pumping)

Possible Causes: Several including:

Porous pavement as a result of inadequate compaction during construction or poor mix design

High water table

Poor drainage

Repair: Water bleeding or pumping should be investigated to determine the root cause.  If the problem is a high water table or poor drainage, subgrade drainage should be improved.  If the problem is a porous mix (in the case of water bleeding) a fog seal or slurry seal may be applied to limit water infiltration.

Rigid Pavement Distress

Blowup (Buckling)

Severe blowup Severe blowup

   

Description: A localized upward slab movement and shattering at a joint or crack.  Usually occurs in spring or summer and is the result of insufficient room for slab expansion during hot weather. 

Problem: Roughness, moisture infiltration, in extreme cases (as in the second photo) can pose a safety hazard

Possible Causes: During cold periods (e.g., winter) PCC slabs contract leaving wider joint openings.  If these openings become filled with incompressible material (such as rocks or soil), subsequent PCC slab

expansion during hot periods (e.g., spring, summer) may cause high compressive stresses.  If these stresses are great enough, the slabs may buckle and shatter to relieve the stresses.  Blowup can be accelerated by:

Joint spalling (reduces slab contact area and provides incompressible material to fill the joint/crack)

D cracking (weakens the slab near the joint/crack area)

Freeze-thaw damage (weakens the slab near the joint/crack area)

Repair: Full-depth patch. 

Corner Break

Corner break on a residential street

Corner break on a highway

   

Description: A crack that intersects the PCC slab joints near the corner.  "Near the corner" is typically defined as within about 2 m (6 ft) or so.  A corner break extends through the entire slab and is caused by high corner stresses.

Problem: Roughness, moisture infiltration, severe corner breaks will fault, spall and disintegrate

Possible Causes: Severe corner stresses caused by load repetitions combined with a loss of support, poor load transfer across the joint, curling stresses and warping stresses.

Repair: Full-depth patch. 

Durability Cracking ("D" Cracking)

     

"D" cracking at panel corners

Description: Series of closely spaced, crescent-shaped cracks near a joint, corner or crack.  It is caused by freeze-thaw expansion of the large aggregate within the PCC slab.  Durability cracking is a general PCC distress and is not unique to pavement PCC.

Problem: Some roughness, leads to spalling and eventual slab disintegration

Possible Causes: Freeze-thaw susceptible aggregate.

Repair: "D" cracking is indicative of a general aggregate freeze-thaw problem. Although a full-depth patch or partial-depth patch can repair the affected area, it does not address the root problem and will not, or course, prevent "D" cracking elsewhere.

Faulting

Faulting from ground level

Faulting in the truck lane

Up close near a bus stop

 

Description: A difference in elevation across a joint or crack usually associated with undoweled JPCP.  Usually the approach slab is higher than the leave slab due to pumping, the most common faulting mechanism.  Faulting is noticeable when the average faulting in the pavement section reaches about 2.5 mm (0.1 inch). When the average faulting reaches 4 mm (0.15 in), diamond grinding or other rehabilitation measures should be considered (Rao et al., 1999).

Problem: Roughness

Possible Causes: Most commonly, faulting is a result of slab pumping.  Faulting can also be caused by slab settlement, curling and warping.

Repair: Faulting heights of less than 3 mm (0.125 inch) need not be repaired.  Faulting in an undoweled JPCP between 3 mm (0.125 inch) and 12.5 mm (0.5 inch) is a candidate for a dowel bar retrofit.  Faulting in excess of 12.5 mm (0.5 inches) generally warrants total reconstruction. 

Joint Load Transfer System Deterioration

Exposed failure with rusted dowel bars

Patched failure

   

Description: Transverse crack or corner break developed as a result of joint dowels.

Problem: Indicator of a failed load transfer system, roughness

Possible Causes: Load transfer dowel bars can fail for two principal reasons:

Corrosion.  If inadequately protected, dowel bars can corrode over time.  The corrosion products occupy volume, which creates tensile stresses around the dowel bars, and a severely corroded dowel bar is weaker and may fail after repeated loading.

Misalignment.  Dowel bars inserted crooked or too close to the slab edge may create localized stresses high enough to break the slab.  Misalignment can occur during original construction or during dowel bar retrofits.

Repair: Removal and replacement of the affected joint load transfer system followed by a full-depth patch for affected area.

Linear (Panel) Cracking

Large panel crack Panel cracks on a residential street

Panel cracking in the truck lane

 

Description: Linear cracks not associated with corner breaks or blowups that extend across the entire slab.  Typically, these cracks divide an individual slab into two to four pieces.

Problem: Roughness, allows moisture infiltration leading to erosion of base/subbase support, cracks will eventually spall and disintegrate if not sealed

Possible Causes: Usually a combination of traffic loading, thermal gradient curling, moisture stresses and loss of support. 

Repair: Slabs with a single, narrow linear crack may be repaired by crack sealing.  More than one linear crack generally warrants a full-depth patch.

Patching

Patch on a residential street

Large utility patch Patch with poor edges

 

Description: An area of pavement that has been replaced with new material to repair the existing pavement.  A patch is considered a defect no matter how well it performs.

Problem: Roughness

Possible Causes:

Previous localized pavement deterioration that has been removed

and patched Utility cuts

Repair: Patches are themselves a repair action.  The only way they can be removed is through an overlay or slab replacement.

Polished Aggregate

Aggregate after almost 40 years of wear

A closer look

   

Description: Areas of PCC pavement where the portion of aggregate on the surface contains few rough or angular aggregate particles.

Problem: Decreased skid resistance

Possible Causes: Repeated traffic applications.  Generally, as a pavement ages the protruding rough, angular particles become polished.  This can occur quicker if the aggregate is susceptible to abrasionsubject to excessive studded tire wear.

Repair: Diamond grinding or overlay. 

Popouts

Large popouts Popout close-up

   

Description: Small pieces of PCC that break loose from the surface leaving small divots or pock marks.  Popouts range from 25 - 100 mm (1 - 4 inches) in diameter and from 25 - 50 mm (1 - 2 inches) deep.

Problem: Roughness, usually an indicator of poor material

Possible Causes: Popouts usually occur as a result of poor aggregate durability.  Poor durability can be a result of a number of items such as:

Poor aggregate freeze-thaw resistance Expansive aggregates

Alkali-aggregate reactions

Repair: Isolated low severity popouts may not warrant repair.  Larger popouts or a group of popouts can generally be repaired with a partial-depth patch.

Pumping

Pumping in action Pumping evidence during HVS test

Pumping damage

 

Broken slabs with pumping evidence

Pumping evidence during HVS test

   

Description: Movement of material underneath the slab or ejection of material from underneath the slab as a result of water pressure.  Water accumulated underneath a PCC slab will pressurize when the slab deflects under load.  This pressurized water can do one of the following:

Move about under the slab. Move from underneath one slab to underneath an adjacent slab. 

This type of movement leads to faulting.

Move out from underneath the slab to the pavement surface.  This results in a slow removal of base, subbase and/or subgrade material from underneath the slab resulting in decreased structural support.

Problem: Decreased structural support of the slab, which can lead to linear cracking, corner breaks and faulting.

Possible Causes: Water accumulation underneath the slab.  This can be caused by such things as: a high water table, poor drainage, and panel

cracks or poor joint seals that allow water to infiltrate the underlying material.

Repair: First, the pumping area should be repaired with a full depth patch to remove any deteriorated slab areas.  Second, consideration should be given to using dowel bars to increase load transfer across any significant transverse joints created by the repair.  Third, consideration should be given to stabilizing any slabs adjacent to the pumping area as significant amounts of their underlying base, subbase or subgrade may have been removed by the pumping.  Finally, the source of water or cause of poor drainage should be addressed.

Punchout

Severe punchout

     

Description: Localized slab portion broken into several pieces.  Typically a concern only with CRCP.

Problem: Roughness, allows moisture infiltration leading to erosion of base/subbase support, cracks will spall and disintegrate. 

Possible Causes: Can indicate a localized construction defect such as inadequate consolidation.  In CRCP, it can be caused by steel corrosion, inadequate amount of steel, excessively wide shrinkage cracks or excessively close shrinkage cracks.

Repair: Full-depth patch.

Reactive Aggregate Distresses

Severe crazing

     

Description: Pattern or map cracking (crazing) on the PCC slab surface caused by reactive aggregates.  Reactive aggregates are those that either expand or develop expansive by products when introduced to certain chemical compounds. 

Problem: Roughness, an indication of poor aggregate - will eventually lead to PCC slab disintegration.

Possible Causes: This type of distress is indicative of poor aggregate qualities.  Most commonly, it is a result of an alkali-aggregate reaction.

Repair: Partial-depth patch for small areas of scaling or slab replacement for large areas of scaling.

Shrinkage Cracking

Shrinkage cracks on brand new slabs

Severe shrinkage cracking

   

Description: Hairline cracks formed during PCC setting and curing that are not located at joints.  Usually, they do not extend through the entire depth of the slab.  Shrinkage cracks are considered a distress if they occur in an uncontrolled manner (e.g., at locations outside of contraction joints in JPCP or too close together in CRCP). 

Problem: Aesthetics, indication of uncontrolled slab shrinkage.  In JPCP they will eventually widen and allow moisture infiltration.  In CRCP, if they are allowed to get much wider than about 0.5 mm (0.02 inches) they can allow moisture infiltration (CRSI, 1996).

Possible Causes: All PCC will shrink as it sets and cures, therefore shrinkage cracks are expected in rigid pavement and provisions for their control are made.  However, uncontrolled shrinkage cracking can indicate:

Contraction joints sawed too late.  In JPCP, if contraction joints are sawed too late the PCC may already have cracked in an undesirable location.

Poor reinforcing steel design.  In CRCP, proper reinforcing steel design should result in shrinkage cracks every 1.2 - 3 m (4 - 10 ft.).

Improper curing technique.  If the slab surface is allowed to dry too quickly, it will shrink too quickly and crack.

High early strength PCC.  In an effort to quickly open a newly constructed or rehabilitated section to traffic, high early-strength PCC may be used.  This type of PCC can have a high heat of hydration and shrinks more quickly and to a greater extent than typical PCC made from unmodified Type 1 portland cement.

Repair: In mild to moderate severity situations, the shrinkage cracks can be sealed and the slab should perform adequately.  In severe situations, the entire slab may need replacement.

 

Spalling

Linear crack spalling Spalling onset Spalling from a bad construction joint

 

Description: Cracking, breaking or chipping of joint/crack edges.  Usually occurs within about 0.6 m (2 ft.) of joint/crack edge.

Problem: Loose debris on the pavement, roughness, generally an indicator of advanced joint/crack deterioration

Possible Causes: Possible causes are (AASHTO, 1993):

Excessive stresses at the joint/crack caused by infiltration of incompressible materials and subsequent expansion (can also cause blowups).

Disintegration of the PCC from freeze-thaw action or "D" cracking.

Weak PCC at a joint caused by inadequate consolidation during construction.  This can sometimes occur at a construction joint if (1) low quality PCC is used to fill in the last bit of slab volume or (2) dowels are improperly inserted. 

Misalignment or corroded dowel.

Heavy traffic loading.

Repair: Spalling less than 75 mm (3 inches) from the crack face can generally be repaired with a partial-depth patch.  Spalling greater than about 75 mm (3 inches) from the crack face may indicated possible spalling at the joint bottom and should be repaired with a full-depth patch.

Construction

Flexible Pavement Plant Operation

HMA production is the first step in construction. The basic purpose of an HMA plant is to properly proportion, blend, and heat aggregate and asphalt to produce an HMA that meets the requirements of the job mix formula (JMF) (Roberts et al., 1996).  There are two basic types of HMA plants commonly in use today: the batch plant, and the drum mix plant.  Batch plants produce HMA in individual batches while drum plants produce HMA in a continuous operation.  Each type of plant can produce the same types of HMA and neither type of plant should impart any significant plant-specific HMA characteristics.  The choice of a batch or drum mix plant depends upon business factors such as purchase price, operating costs, production requirements and the need for flexibility in local markets; both can produce quality HMA.  This section gives a brief overview of batch and drum mix plants. More detailed information on plant operations can be found in: 

Transportation Research Board (TRB). (2000). Hot-Mix Asphalt Paving Handbook 2000. Transportation Research Board, National Research Council. Washington, D.C.

Roberts, F.L., Kandhal, P.S., Brown, E.R., Lee, D.Y., and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Research and Education Foundation. Lanham, MD.

 

Batch Plants

Batch plants, which produce HMA in individual batches, are the older of the two types of HMA production facilities.  HMA was originally made in batches; it was not until the 1970s that drum plants became a popular HMA production option.  Currently about 70 percent of all operational HMA plants in the U.S. are batch plants while about 95 percent of all newly manufactured plants in the U.S. are drum plants (Roberts, et al., 1996).  This means that as older batch plants are retired they are more than likely to be replaced by new drum plants, which can provide greater mobility and production capacity.  Typical batch quantities range from 1.5 to 5 tons of HMA.  Figure 7.18 shows the basic components of a batch plant and their functions.

Drum Plants

Drum plants, which produce HMA in a continuous manner, generally offer higher production rates than batch plants for comparable cost.  Typical production rates for drum plants vary between about 100 tons/hr up to over 900 tons/hr depending upon drum design.  Figure 7.19 shows the basic components of a drum plant and their functions.

Figure 7.20: Batch Plant Interactive Picture (click figure to launch)

Figure 7.21: Drum Plant Interactive Picture (click figure to launch)

Video 7.5: Drum Plant Burner

This burner is running on diesel fuel, which results in a bright orange flame.  A natural gas flame would be essentially invisible.  Burners like these can reach temperatures in excess of 760°C (1400°F).  Typical HMA temperatures out of the drum are on the order of 150 - 160°C (300 - 325°F).

Figure 7.22: Infrared View of a Drum Mix Plant

Rigid Pavement Plant Operation

All PCC intended for pavement use is ready-mixed concrete.  Ready-mixed concrete refers to PCC that is delivered to the customer in a freshly mixed and unhardened state (NRMCA, 2002).  Therefore, PCC production involves the batching and mixing of portland cement, aggregate, water and admixtures to form a ready-mixed concrete in accordance with the approved mix design.  There are three basic production methods: truck mixing, central mixing and shrink mixing.  Other methods of PCC mixing do exist but they are not common in pavement applications and are therefore not covered in this Guide.  Truck mixed PCC is dry batched into a concrete mixing truck then blended in the truck either at the batching plant or in route to the job site.  Central mixed PCC is batched and mixed in a central facility and then loaded into a truck for transport to the job site.  Shrink mixed PCC is partially mixed in a plant mixer and then discharged into a truck mixer where the mixing is completed. This section provides a brief overview of truck, central and shrink mixing. More detailed information on plant operations can be found in: 

American Concrete Pavement Association. (1995). Construction of Portland Cement Concrete Pavements. National Highway Institute Course No. 13133. AASHTO/FHWA/Industry joint training. Federal Highway Administration, Department of Transportation. Washington, D.C.

National Ready Mixed Concrete Association (NRMCA). (2002). NRMCA web site, Concrete Basics home page. National Ready Mixed Concrete Association. Silver Spring, MD. http://www.nrmca.org.

Truck Mixed PCC

Truck mixed PCC is usually proportioned by batching the separate ingredients directly into the concrete mixing truck.  The ingredients are usually charged in a certain order to ensure good mixing of all ingredients.  Although the specific order depends upon local practice, mixing plant procedures and possibly job specifications.

Truck mixed PCC (see figure 7.68) is appropriate for all types of pavement construction, but is particularly well-suited for instances that can take advantage of a concrete mixing truck's monitored, continuously agitated storage volume (the mixing drum) and its ability to precisely deliver PCC through its chute (see Figure 7.69).  These situations might include: intersection paving, street paving, pavement repair, urban environments, high traffic areas and staged/phased construction.  Disadvantages of truck mixed PCC are longer load and unload times (due to the nature and opening size of the mixing drum) and the higher operating cost of a concrete mixing truck when compared to an end or bottom dump truck.

Figure 7.68: Loading PCC for Truck MixingFigure 7.69: Using the Mixing Truck Chute to Deliver PCC to a Precise Location

Central Mixed PCC

Central mixed PCC is usually proportioned by batching the separate ingredients into a central plant mixer (see Figures 7.70 through 7.73) where they are completely mixed before discharge into a transport vehicle.  Transport vehicles can be concrete mixing trucks or conventional end and bottom dump trucks depending upon travel distance and other requirements.  About 20 percent of the concrete plants in the U.S. use a central mixer (NRMCA, 2002).  Central mixing plants can either be permanent or mobile (see Figure 7.74) and offer the following advantages (NRMCA, 2002):

High production volume. A high production volume would be on the order of 3000 m3/day (3950 yd3/day). Most central mix drums can mix about 9 m3 (12 yd3) in a single batch and produce fully mixed PCC in excess of 150 m3/hr (200 yd3/hr). Mixing times are on the order of 30 to 90 seconds (ACPA, 1995).

Improved quality control. Since mixing is controlled by a central facility and not by individual truck, PCC quality can be more closely monitored and controlled.

Reduced wear on truck mixer drums. If used, concrete mixing trucks are used as an agitating haul unit rather than a mixing unit. Agitating rotation speeds are much slower than mixing speeds and thus, produce less wear on drum components.

Figure 7.70: Central Mixing PlantFigure 7.71: Tilting Drum Mixer Showing Material Charging Apparatus

Figure 7.72: Aggregate Feed BinsFigure 7.73: Loading a Bottom Dump Truck from a Central Mixing Plant

Figure 7.74: Mobile Central Mixing Plant

 

Shrink Mixed PCC

Shrink mixed PCC is partially mixed in a plant mixer before discharging into a truck mixer.  The short mixing period in the plant mixer reduces the bulk volume of the overall ingredients by allowing fine aggregate, portland cement and water to fill the large void space in bulk coarse aggregate.  Typically, 1 m3 (1.3 yd3) of fully mixed PCC requires about 1.58 m3 (2.07 yd3) of individual ingredients (ACPA, 1995).  Thus, with shrink mixing, more PCC can be loaded into each truck mixer.   Many central mixing plants use a stationary plant-mounted mixer to shrink mix PCC before charging their truck mixers.  The amount of mixing that is needed in the truck mixer varies in these applications and should be determined via mixer uniformity tests. Generally, about thirty turns in the truck drum, or about two minutes at mixing speed, is sufficient to completely mix shrink-mixed concrete (NRMCA, 2002).

References:

1. http://training.ce.washington.edu/wsdot/ 2. http://www.acpa.org/

3. http://www.enggpedia.com