Asphalt pavement distresses can take many forms, from minor cracking and raveling to severe potholes and unraveling. Understanding the root causes of asphalt failures helps prevent them through proper designs, materials selection, construction, and maintenance. This guide provides a detailed overview of major asphalt pavement failure types, what causes them, and how to avoid these issues.
Overview of Asphalt Pavement Failures
Asphalt pavements deteriorate over time from traffic loads, environmental factors, materials issues, and construction flaws. Common failure modes include:
Major Causes of Asphalt Deterioration
Asphalt pavement distress can occur due to a variety of factors related to design, construction, materials, traffic loading, and environmental conditions over time. Understanding the major causes of asphalt deterioration is critical for engineers, contractors, and owners to maximize pavement life. As my experience as a road construction project manager I will discuss the three primary causes – insufficient thickness/strength, poor drainage, and low-quality materials – in detail.
Insufficient Thickness and Strength
A key cause of premature asphalt failure is inadequate structural thickness and strength to support traffic loads throughout the intended design life. Asphalt pavement structures are engineered systems designed to distribute stresses and resist distress under repetitive traffic loading. If the thickness or strength of any layer is undersized, the pavement will experience accelerated fatigue cracking, rutting, and other damage.
Several factors can lead to insufficient thickness and strength:
Overestimating Design Life
According to the Pavement Design CD 226 Design for new pavement construction (formerly HD 26/06) engineers calculate required pavement thicknesses based on estimated traffic volumes over the design life. If this analysis overestimates the design life, the structure will be under-designed for the actual loading experienced earlier in the service life. For example, a parking lot designed for 20 years but needing rehabilitation in 10 would indicate the thicknesses are inadequate.
Underestimating Traffic Volume
Design traffic volumes are projected using growth rates and distributions for different vehicle classes. If actual traffic exceeds these estimates, the pavement will experience higher stresses and more damage than anticipated. Continued growth from development or heavier vehicles than assumed are examples.
Inadequate Structural Layer Thicknesses
The total asphalt structure consists of several layers – surface, intermediate, and base. If the thickness of any layer is insufficient, stresses may exceed its strength capacity leading to cracking, rutting, and fatigue damage. For example, too thin a surface layer could cause top-down cracks, or a thin base could rut under loads.
Weak Subsurface
The subsurface beneath the asphalt structure affects stability and support. Soft subgrades or granular bases allow excessive deformation under loads. Inadequate compaction during construction leads to settlement. If subsurface conditions are weaker than designed, the pavement deteriorates more rapidly.
Table 1 summarizes the key factors related to insufficient thickness and strength:
Table 1. Factors Leading to Insufficient Thickness and Strength
| Factor | Description | Effects |
|---|---|---|
| Overestimated Design Life | Actual traffic exceeds design projections earlier than anticipated | Premature fatigue cracking and rutting |
| Underestimated Traffic Volume | More loads than designed for over life | Overstressed layers exceed capacity sooner |
| Inadequate Layer Thicknesses | Specific layers thinner than required | Cracking, rutting in under-designed layers |
| Weak Subsurface | Inadequate compaction or soft soils | Deformation, settlement, cracking |
Poor Drainage
Excess moisture exposure is another primary cause of asphalt deterioration. Proper drainage, from initial construction to maintenance over time, is essential to prevent water from infiltrating and damaging the pavement structure. Major factors enabling moisture damage include:
Lack of Crowns/Slopes
Effective drainage requires roadway and parking lot profiles to shed water laterally. Without sufficient crown or cross slope, water pools on the surface and infiltrates into the asphalt. This ponding leads to saturation and pavement distress.
Missing/Clogged Drainage Conduits
Edge drains, catch basins, and storm sewers help direct water off or underneath pavements. If these conduits are omitted, clog over time, or overflow they no longer divert water properly. This retention allows moisture to soften subgrades and damage pavements.
Erosion of Shoulders
Unstable aggregate shoulders next to roadways erode over time. This prevents proper sheet flow drainage off the edge. Water infiltrates the pavement edge instead, accelerating deterioration in this vulnerable area.
Ponding at Distresses
Depressions, deformations, and cracks hold water on the pavement surface. These low spots generate additional moisture damage immediately around them, exacerbating the deterioration. Proactive maintenance is needed to restore effective drainage.
Table 2 summarizes the drainage-related factors enabling moisture damage of asphalt pavements:
Table 2. Factors Leading to Moisture Damage
| Factor | Description | Effects |
|---|---|---|
| Lack of Crowns/Slopes | Insufficient cross slope for drainage | Ponding water saturates pavement |
| Missing/Clogged Drains | Inadequate edge drainage over time | Water ingress through edges |
| Eroded Shoulders | No edge support allows moisture beneath | Saturation and softening at edges |
| Ponding at Distresses | Cracks/deformations hold water on surface | Additional moisture damage at distresses |
Low-Quality Materials
The quality of materials used during construction also has a major influence on asphalt pavement durability. Both aggregates and asphalt binder need proper characteristics and durability. Common deficiencies include:
Poor Aggregate Durability
Aggregates comprising over 90% of asphalt volume resist abrasion and polishing from traffic. Weak aggregates like certain limestones break down, leading to raveling, potholes, and surface erosion. Durable aggregates are essential.
Oxidized/Hard Asphalt Binder
Asphalt binder acts as the adhesive glue to hold aggregates together. Aged, oxidized, or overly hard binder becomes brittle over time. This leads to premature cracking under loads or thermal changes. Proper binder selection and specification is key.
Lack of Polymer Modification
Polymers blended into asphalt binder improve viscosity, elasticity, and temperature susceptibility. Without modification, the binder is less resistant to rutting and thermal cracking. Polymers should be utilized in critical climate conditions.
Table 3 summarizes material factors contributing to premature asphalt pavement deterioration:
Table 3. Material Factors Affecting Asphalt Durability
| Factor | Description | Effects |
|---|---|---|
| Poor Aggregate Durability | Weak aggregates deteriorate under traffic | Raveling, potholes, and erosion |
| Oxidized/Hard Asphalt | Aged or brittle binder prone to cracking | Thermal and fatigue cracking |
| Lack of Polymer Modification | Binder susceptible to rutting and temperature | Premature rutting and thermal cracks |
Proper asphalt mix design, quality control, and testing is necessary to obtain high-quality aggregates and asphalt binder resistant to deterioration mechanisms. Specifications should ensure suitable durability characteristics.
Environmental Factors Contributing to Failures
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Climate extremes of heat, cold, and moisture
- Extreme heat softens asphalt and causes rutting or shoving failures
- Cold temperatures lead to thermal cracking and brittle fracture of surface
- Moisture damages internal structure and erodes surfaces over time
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UV radiation and oxidation weathering binder over time
- Sunlight UV degrades polymers in asphalt binder, reducing elasticity
- Oxidation from exposure hardens asphalt binder, making it prone to cracking
- Lack of preventive maintenance accelerates weathering deterioration
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Freeze-thaw cycles straining pavement surface
- Water expands inside the pavement when frozen, initiating micro-cracks
- Repeated freeze-thaw weakens the surface, leading to potholes and raveling
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Standing water infiltration through cracks
- Cracks allow water penetration to base and subgrade layers
- Saturated bases lead to pumping, voids, sinkholes, and hydroplaning
- Stripping of asphalt binder from aggregates caused by moisture
Most Common Types of Asphalt Pavement Distress
Major forms of asphalt pavement distress include cracking, surface defects, and deformation failures. Cracking is most prevalent, appearing as fatigue alligator cracks, longitudinal cracks, transverse cracks, block cracks, edge cracks, and reflective cracks. Preventing cracks is key to pavement longevity.
Asphalt Cracking Failures
Cracking represents the most widespread asphalt pavement failure type. Common cracking distresses include:
Fatigue/Alligator Cracking Causes and Prevention
- Caused by traffic loads exceeding pavement capacity
- Prevent with proper structural design and thickness
Fatigue/Alligator Cracking Causes and Prevention
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Caused by traffic loads exceeding pavement capacity
- Repeated traffic loads induce bending stresses in asphalt
- The repetitive flexing initiates microscopic cracks in the surface layer
- Cracks slowly propagate bottom-up as stresses exceed the fracture strength
- This distress typically starts laterally below the wheel paths and then interconnects
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Eventually stresses surpass strength capacity, causing bottom-up cracks
- Over many load cycles, the cumulative stresses fatigue the asphalt
- When the induced stresses exceed the fatigue strength, catastrophic cracks form
- Top-down cracks can also form from prolonged heavy loads exceeding the ultimate tensile capacity
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The interconnected cracking pattern resembles alligator skin
- The parallel cracks underneath the wheel paths eventually connect laterally
- This forms a pattern of adjoining rectangular cracks resembling alligator skin
- As cracks reach the surface, oxidation, and moisture infiltration accelerate damage
Longitudinal Cracking Causes and Prevention
- Parallel cracks from pavement shrinkage and hardening
- Prevent with modified binder, quality construction, and sealing
Longitudinal Cracking Causes and Prevention
Longitudinal cracking refers to the occurrence of cracks that run parallel to the direction of traffic on pavements, roads, runways, and other similar structures. These cracks can vary in length and width and can significantly affect the structural integrity and lifespan of the surface. Several factors can contribute to the development of longitudinal cracking, and there are measures that can be taken to prevent or delay their occurrence.
Causes of Longitudinal Cracking:
1. Fatigue and Aging:
Over time, repeated traffic loads and exposure to weather conditions can cause the pavement to deteriorate and age. This can lead to the development of longitudinal cracks as the material loses its ability to withstand stress and strain.
2. Lack of Proper Drainage:
Inadequate drainage can result in the accumulation of water on the pavement surface, especially during rainy seasons. This trapped water can weaken the pavement structure, leading to the formation of longitudinal cracks.
3. Insufficient Base or Subgrade Support:
Inadequate base or subgrade support can cause the pavement to deform and crack under the weight of traffic loads, resulting in longitudinal cracking.
4. Temperature Changes:
Temperature variations can cause expansion and contraction of the pavement material, leading to cracking. This is particularly common in regions with extreme temperatures, as the constant thermal cycling can weaken and degrade the pavement over time.
Prevention of Longitudinal Cracking:
1. Proper Design and Construction:
Adopting appropriate design and construction techniques can help prevent or minimize the occurrence of longitudinal cracking. This includes ensuring proper pavement thickness, adequate base and subgrade support, and proper mix design.
2. Reinforcement Techniques:
The use of reinforcement techniques, such as adding fibers or grids to the asphalt mix, can enhance the pavement’s ability to resist cracking. These reinforcements help distribute the stress and strain caused by traffic loads, reducing the likelihood of longitudinal cracking.
3. Regular Maintenance:
Implementing a routine maintenance plan that includes regular inspections, crack sealing, and surface treatments can help identify and address potential issues before they become severe. Timely repairs can prevent the progression of longitudinal cracks and extend the pavement’s lifespan.
4. Improving Drainage:
Ensuring adequate drainage systems, such as proper slope and installation of drains or ditches, can help prevent moisture accumulation and subsequent pavement damage.
5. Asphalt Overlay:
Applying an asphalt overlay to the existing pavement surface can help prolong its life and prevent the formation of longitudinal cracks. This overlay provides a new, smooth surface and improves the pavement’s ability to withstand traffic loads and environmental factors.
Overall, effective prevention of longitudinal cracking involves a combination of proper design, construction techniques, regular maintenance, and addressing underlying factors such as drainage issues and inadequate support. By implementing these measures, the occurrence and severity of longitudinal cracking can be minimized, prolonging the life of pavements and reducing the need for costly repairs or reconstruction.
The most common failures in stone mastic asphalt
I would explain stone mastic asphalt and its common failures from the perspective of a civil engineer with 20 years of experience:
As a paving material, stone mastic asphalt (SMA) has been a mainstay on roadways and aircraft taxiways for decades thanks to its durability, rutting resistance, and stability. With over 20 years of designing and analyzing SMA mixes, I have an intimate understanding of their composition and causes of distress.
SMA derives its strength from a high coarse aggregate content – usually over 70% – using hard, crushed stone gradations. This stone-on-stone skeleton is held together by a mastic binder containing fiber stabilizers and increased asphalt content compared to conventional hot mix asphalt.
In my experience, SMA failures primarily result from:
- Insufficient binder film thickness leads to raveling as stones are knocked out under traffic loads. The high void content requires an adequate binder to provide cohesion.
- Poor drainage and moisture damage where water infiltrates the porous SMA and debonds the binder. Timely maintenance of drainage systems is vital.
- Traffic overloads the design capacity, resulting in rutting and channelized cracks despite the enhanced stability from aggregate interlock.
- Construction issues like poor compaction, segregation, and contamination reduce strength. SMA is sensitive to workmanship and requires experience.
With attention to mix design, materials selection, structural analysis, construction quality, and preventive maintenance, SMA can deliver decades of superior performance. Proper edge restraints, tack coats, and joint construction also safeguard against failures. Ultimately, understanding SMA composition along with field experience allows mitigating characteristic distresses.
Stone Mastic Asphalt (SMA)
With over 15 years of experience using SMA on major transportation projects, I am very familiar with its composition and performance advantages.
SMA contains a high percentage of crushed coarse aggregate, typically over 70% by weight. The durable, interlocking stone skeleton provides strength and rutting resistance. The void spaces between aggregate are filled with a mastic binder composed of filler, fibers, and a high asphalt cement content.
Applications and benefits of SMA
The combination of stone-on-stone contact and mastic provides SMA with several benefits:
- Excellent rutting resistance from aggregate interlock and mastic
- High stability and durability under heavy traffic loads
- Good skid resistance and surface texture from coarse aggregates
- Lower traffic noise compared to conventional asphalt mixes
I frequently specify SMA for high-traffic roadways, airport taxiways, and parking areas where durability and deformation resistance are critical. The porous nature also provides good drainage when properly maintained.
With its optimized gradation and binder content, SMA has proven highly effective at combating distresses like rutting, shoving, and surface raveling in my experience. When appropriately designed and constructed, it outperforms standard hot mix asphalt.
