Thermal segregation is the non-uniform temperature distribution in hot mix asphalt (HMA) during transport and placement, where cooler areas compact less, resulting in localized low-density zones with higher air voids that are prone to raveling, cracking, and moisture damage. It creates a distinctive patchy distress pattern. Covers causes, detection (thermal imaging during paving; infrared; visual), and prevention.
Thermal segregation is defined as a non-uniform temperature distribution across the uncompacted mat of hot mix asphalt (HMA) during paving operations. It is a construction-related defect distinct from aggregate segregation, though both produce similar distress symptoms in the finished pavement. The universally accepted quantitative definition, established through research by Sebesta, Scullion, and others at the Texas Transportation Institute and validated by the National Center for Asphalt Technology (NCAT), identifies thermal segregation as a temperature differential exceeding 14°C (25°F) between the hottest and coldest zones within the uncompacted mat immediately behind the paver screed.
The mechanism by which thermal segregation causes pavement damage is fundamentally linked to the temperature-dependent viscosity of asphalt binder and the cessation temperature concept. As HMA cools, the asphalt binder increases in viscosity, reducing the lubricity required for aggregate particles to rearrange under roller compaction. The cessation temperature — commonly accepted as 80°C (175°F) for conventional HMA mixtures — is the temperature below which the binder becomes too viscous for further particle rearrangement and density gain, regardless of the number of roller passes applied.
In a thermally segregated mat, cold zones cool to cessation temperature significantly faster than the surrounding hot mat. The roller operator establishes a compaction pattern based on the general mat temperature, typically monitoring the hottest areas. By the time the roller reaches the cold spots, those areas may already be below cessation temperature. The result is inadequate compaction — the cold zone retains higher air voids, lower density, and reduced mechanical properties compared to the properly compacted hot zones.
NCAT research by Fernandez Cerdas (2012) on 28 Alabama paving projects demonstrated a statistically significant negative correlation between thermal segregation magnitude and in-place density. The study found that cold spots consistently exhibited air void contents 2–4% higher than adjacent hot spots, correlating directly with reduced fatigue life. Bending beam fatigue testing showed that cold-spot specimens failed in significantly fewer cycles than hot-spot specimens, with the initial stiffness being the parameter most affected by the higher air void content in thermally segregated areas.
Thermal segregation does not originate from a single source but from a combination of factors in the HMA production, transport, and placement process. Understanding each causal factor is essential for designing effective prevention strategies.
HMA is loaded into haul trucks at production temperatures typically between 120°C and 175°C (250°F to 350°F) , depending on the binder grade and mixture type. During transport, the HMA mass transfers heat to the surrounding environment through the truck bed walls and the exposed top surface. The outer layer of the HMA load — approximately 25–75 mm (1–3 in.) thick — cools rapidly, forming a temperature crust around a significantly hotter interior core. This crust is typically 15–30°C (27–54°F) cooler than the core temperature upon arrival at the paving site, depending on haul distance, ambient temperature, wind speed, and truck bed insulation.
When the truck discharges into the paver hopper, the cold crust material enters first (from the top of the load) and last (from the bottom corners of the bed). This end-of-load effect is the most commonly cited cause of thermal segregation, as the coldest material flows into the paver at the beginning and end of each truckload, creating a cyclic pattern of cold spots at regular longitudinal intervals in the mat.
When the paver stops — whether due to truck exchange, material shortage, or operational delays — the HMA remaining in the paver hopper, auger chamber, and screed assembly continues to lose heat to the ambient air, the machine components, and the underlying surface. Research published in the ASCE Journal of Materials in Civil Engineering on the effects of paver stoppage on temperature segregation demonstrates that a 5-minute stoppage can create a cold zone extending 3–6 m (10–20 ft.) behind the restart point. The restart process compounds the problem: the first material laid after a stoppage has been sitting in the hopper cooling, while the paver’s augers and screed are also cold, drawing additional heat from the first few meters of material.
Wind speed has a disproportionately large effect on HMA cooling rate compared to ambient temperature alone. MnDOT’s PaveCool modeling software and NAPA’s cold weather compaction guidelines (QIP-118) document that wind speeds under 10 knots (11.5 mph) have a minor effect, but as wind speed increases, the convective heat transfer coefficient at the mat surface rises dramatically. A 25 km/h (15 mph) wind can double the cooling rate of an exposed HMA mat compared to still air conditions at the same ambient temperature. This effect is most pronounced on thin lifts (less than 50 mm) where the volume-to-surface-area ratio is low.
Improper truck loading at the HMA plant can initiate thermal segregation before the truck leaves the facility. When HMA is discharged from the storage silo into the truck bed in a single large mass, large aggregate particles tend to roll down the pile and collect at the base and corners of the truck bed. This creates both aggregate segregation and, critically, differential cooling rates — the finer material at the center of the pile retains heat better than the coarse material at the edges. Standard industry practice recommends loading in three smaller drops: one at the front of the bed, one at the rear, and one in the center. This method produces a more uniform temperature distribution and reduces aggregate segregation.
Haul distances exceeding 30–45 minutes (one way) significantly increase the risk of thermal segregation unless the truck beds are insulated and tarped. The rate of heat loss from the HMA follows Newton’s Law of Cooling and is proportional to the temperature differential between the mix and the ambient environment. Longer hauls in cold weather (below 10°C / 50°F) can result in crust temperatures falling below the minimum placement temperature specified for the mixture, forcing the contractor to reject the load or — worse — place material that cannot be adequately compacted.
Multiple transportation agencies have established standardized thresholds for classifying thermal segregation severity. These thresholds are fundamental to quality control specifications and acceptance criteria.
| Classification | Temperature Differential | Compaction Impact | Corrective Action Required |
|---|---|---|---|
| None | < 14°C (< 25°F) | No significant compaction impairment | None |
| Moderate | 14°C – 28°C (25°F – 50°F) | Localized density reduction; 1–3% higher air voids | Process adjustment; evaluate with density profiles |
| Severe | > 28°C (> 50°F) | Significant density reduction; > 3% higher air voids; full-depth compaction failure | Suspend operations; remove and replace affected area |
The 14°C (25°F) threshold originates from the SWUTC/15/600451-00111-1 study on infrared thermography applications, which established that differentials below this value produce statistically insignificant density variations. The 50°F threshold used by TxDOT for the severe classification is based on research showing that differentials exceeding this magnitude consistently produce mat areas where compaction cannot achieve the minimum specified density regardless of roller effort.
Research by Willoughby et al. (2001) for the Washington State Department of Transportation (WSDOT Report 476.1) confirmed that temperature differentials greater than 25°F can potentially cause compaction problems, establishing the foundation for many current specifications. The study evaluated multiple paving projects and correlated thermal imaging data with core density results to validate these thresholds.
The cessation temperature is not a fixed value but depends on mixture characteristics:
The detrimental effects of thermal segregation on pavement performance are well-documented through field studies, laboratory testing, and long-term pavement monitoring. The root cause of all these distresses is inadequate compaction resulting in elevated air voids within the cold zones.
The direct consequence of thermal segregation is a reduction in in-place density of 1–4% in cold zones compared to adjacent properly compacted areas. In a typical HMA pavement specification requiring 92–96% of laboratory maximum specific gravity (Gmm — Rice density), a cold zone achieving only 88–91% Gmm represents a substantial increase in interconnected air voids. The target air void content for newly constructed HMA is typically 4–7%. Thermally segregated cold zones commonly exhibit 8–12% air voids, which is the critical range where water permeability increases exponentially.
The relation between air voids and pavement durability follows an established pattern: for every 1% increase in air voids above the target, the fatigue life decreases by approximately 10% and the mixture’s resistance to moisture damage decreases proportionally. This relationship was derived from the Strategic Highway Research Program (SHRP) studies and validated by subsequent NCAT research.
Raveling — the progressive loss of aggregate particles from the pavement surface — is the most characteristic distress pattern associated with thermal segregation. In cold zones where the binder is too viscous to coat and bond aggregate particles effectively during compaction, the mechanical interlock between particles is insufficient to resist traffic abrasion. The raveling typically appears as isolated patches that correspond to the cold zones identified by thermal imaging during construction. These patches progressively deepen as traffic removes more aggregate, creating surface depressions that accelerate the next distress mechanism.
Thermally segregated zones are more susceptible to both fatigue (alligator) cracking and thermal cracking. The higher air void content reduces the mixture’s tensile strength and fracture resistance. Under repeated traffic loading, the cold zone reaches its fatigue limit sooner than the surrounding properly compacted material, producing a localized pattern of interconnected cracks that may be the first visible sign of pavement distress. The cracking typically initiates at the bottom of the HMA layer in the cold zone and propagates upward (bottom-up fatigue cracking), although thin overlays may exhibit top-down cracking.
Potholes form when a localized area of pavement weakens under traffic and the material fails catastrophically. Thermally segregated zones are prime initiation points for potholes because they combine low density, high permeability, and weak aggregate interlock. Water enters the high air void structure, and freeze-thaw cycling in cold climates accelerates the deterioration. The characteristic pothole that develops from thermal segregation is typically small (0.1–0.5 m²), round or oval, and surrounded by sound pavement, creating a distinctive isolated pothole pattern as opposed to the continuous pothole development seen in structurally failed pavements.
The high air void content in thermally segregated cold zones provides pathways for water to penetrate the pavement structure. Water trapped within the air voids generates pore pressure under traffic loading, weakening the bond between asphalt binder and aggregate — a phenomenon known as stripping. The loss of adhesion between binder and aggregate accelerates all other distress mechanisms and significantly reduces the pavement’s remaining service life.
Accurate detection of thermal segregation requires a combination of real-time temperature monitoring during construction and post-construction evaluation of the compacted mat.
Handheld thermal (infrared) cameras are the most basic tool for thermal segregation detection. The test procedure described in TxDOT Test Method Tex-244-F specifies the equipment requirements and methodology. A compliant thermal camera must:
The procedure requires the operator to walk alongside the paver at a distance of approximately 1 m (3–4 ft.) from the mat edge, maintaining a consistent angle to capture the full mat width. The maximum baseline temperature is determined from the first 6 m (20 ft.) of mat behind the paver. The minimum temperature is recorded continuously over a 45 m (150 ft.) test section. The difference between these values determines the segregation classification.
Advanced thermal imaging systems provide real-time, full-width temperature profiles of the mat behind the screed. The MOBA Pave-IR system — developed through TxDOT research and subsequently commercialized — is the most widely deployed system. Its specifications per Tex-244-F include:
The system collects, displays, saves, and analyzes temperature data in real time. It determines low and high temperatures using the statistical 1st percentile and 98.5th percentile, respectively, eliminating outlier readings. Output files include longitudinal temperature profiles tied to station numbers and GPS coordinates, enabling precise identification of segregation locations for subsequent evaluation or remedial action.
The infrared temperature bar — also known as the Pave-IR system — is a transverse bar-mounted array of infrared sensors attached to the rear of the paver screed. Development through TxDOT Project 0-4577 produced Generation 1, 2, and 3 versions, each improving sensor density, data acquisition rate, and software analysis capability. The bar typically contains 8–16 infrared sensors spaced at 300 mm (12 in.) intervals across the mat width, collecting temperature readings at increments of 150–300 mm (6–12 in.) along the paving direction. Data collection and processing software allow the paving crew to identify suspected segregated areas in real time and make operational adjustments before the material is compacted.
Visual identification of thermal segregation after compaction is possible but requires experienced inspectors. The characteristic appearance includes:
Visual identification is unreliable for moderate segregation (25–50°F differential) and is most effective for severe cases (>50°F). The visual appearance can be confused with aggregate segregation, binder-rich areas, or differential aging, making thermal imaging the preferred detection method.
Ground penetrating radar is an emerging technology for detecting thermal segregation in completed overlays. TxDOT research project 0-4577 developed recommendations for GPR-based segregation detection using surface dielectric measurements. For coarse-graded mixtures, locations with surface dielectrics outside ± 0.8 of the mean value should be investigated. For dense-graded mixtures, the threshold is ± 0.4 of the mean. The RadSeg software package enables rapid analysis of GPR data, and three-channel GPR systems can collect data over both wheel paths and the centerline in a single pass.
Preventing thermal segregation requires addressing each causal factor in the HMA production, transport, and placement chain.
The single most effective transport measure is the combination of insulated truck beds and mandatory tarping. The insulation layer — typically 25–50 mm (1–2 in.) of fiberglass or polyurethane foam encapsulated between the truck bed metal panels — reduces heat loss through the bed walls by 50–70%. Tarping eliminates convective heat loss from the top surface of the HMA load and prevents wind-induced cooling. Studies by the National Asphalt Pavement Association (NAPA) and multiple state DOTs have demonstrated that tarping alone can reduce crust temperature differential by 8–14°C (15–25°F) on a typical 30-minute haul.
Truck loading protocol is equally important. Three-drop loading — one drop at the front, one at the rear, one in the center — minimizes aggregate segregation and produces a more thermally uniform load. Each drop should be approximately one-third of the total load volume.
The material transfer vehicle (MTV) is the most effective equipment-based solution for thermal segregation prevention. The MTV receives HMA from haul trucks, temporarily stores the material in an agitated, heated hopper, and transfers it to the paver via a conveyor system. The MTV performs three critical functions:
NCAT research on 28 Alabama projects found that remixing operations were a key factor in reducing high temperature differentials. Projects using MTVs consistently demonstrated smaller temperature differentials and higher in-place densities compared to projects with direct truck-to-paver discharge.
Some paver manufacturers offer remixing devices — augers or paddles installed in the paver hopper or auger chamber — that provide limited temperature blending of the HMA before it exits the screed. While less effective than MTVs, these devices can reduce moderate temperature differentials by 5–10°C (9–18°F). The effectiveness depends on the specific design and the degree of temperature non-uniformity in the incoming material.
Proper logistics management can eliminate many causes of thermal segregation:
Implementing real-time thermal monitoring as a process control tool allows the paving crew to identify temperature uniformity problems immediately and make corrections. The thermal imaging system displays color-coded temperature maps on a cab-mounted monitor, alerting the operator when the mat temperature differential approaches the 25°F threshold. The system enables:
Airport asphalt pavement construction introduces additional considerations for thermal segregation management due to the higher performance requirements and different construction conditions compared to highway paving.
The Federal Aviation Administration’s standard specifications for airport construction (AC 150/5370-10H, Item P-401 for bituminous pavement) establish quality control requirements that implicitly address thermal segregation. The specification requires:
While the FAA specification does not explicitly mandate thermal profiling, the density requirements create a quality control framework that exposes thermal segregation when it occurs. Airport paving projects commonly specify 92–96% of Gmm for the surface course, and any cold zone falling below this threshold triggers investigation and corrective action.
Airport paving presents unique challenges for thermal segregation control:
For airport projects, thermal segregation prevention should include:
Inspecting an existing pavement suspected of having thermal segregation requires a systematic approach combining thermal imaging, density testing, and visual assessment.
The following protocol is adapted from TxDOT and FAA inspection guidelines for evaluating thermally segregated pavements:
Step 1 — Thermal Survey: For new construction, review the thermal profile records from the paving operation. For existing pavements, conduct a thermal survey using a handheld thermal camera on a hot day (to maximize temperature contrast) or during the early morning when surface moisture highlights permeability differences.
Step 2 — Density Testing: Extract 100 mm (4 in.) diameter pavement cores from identified cold zones and adjacent hot zones for comparison. The density difference between cold and hot zones should not exceed 2% of Gmm for acceptable construction. Cores should also be tested for air void content using saturated surface-dry (SSD) method per ASTM D2726.
Step 3 — Visual Distress Survey: Document the extent and severity of raveling, cracking, and other distresses in the identified cold zones. The characteristic patchy pattern of distress — isolated areas of raveling or cracking surrounded by sound pavement — strongly suggests thermal segregation as the root cause.
Step 4 — Permeability Testing: Field permeability testing using devices such as the falling-head field permeameter (ASTM PS 129) can identify cold zones, as areas with air voids above 8% typically exhibit significantly higher permeability than well-compacted areas.
TxDOT’s Test Method Tex-207-F (Segregation Density Profile) provides a standardized procedure for evaluating areas identified as having severe thermal segregation. The procedure requires extracting cores at a minimum of five locations across the affected area and comparing the density to the project specification requirements. If the average density falls below the specified minimum, the area is considered unacceptable and must be removed and replaced.
| Parameter | Acceptable | Marginal | Unacceptable |
|---|---|---|---|
| Cold zone density vs. hot zone density | ≤ 1.5% lower | 1.5–3.0% lower | > 3.0% lower |
| Cold zone air voids | ≤ 7% | 7–10% | > 10% |
| Field permeability | < 1 × 10⁻⁵ cm/sec | 1–5 × 10⁻⁵ cm/sec | > 5 × 10⁻⁵ cm/sec |
| Visual distress at 2 years | None | Minor raveling | Raveling with cracking |
Various transportation agencies have incorporated thermal segregation requirements into their standard specifications. These specifications typically take one of two forms: thermal profile requirements that establish direct limits on mat temperature differential, or density-based specifications that indirectly penalize thermal segregation through compaction requirements.
TxDOT’s HMA specifications incorporate thermal profiling as both a quality control requirement and an incentive mechanism. Contractors who use thermal imaging systems may receive:
The specification requires corrective action for recurring moderate segregation and mandatory suspension of operations to eliminate severe segregation. Areas with severe thermal segregation must be evaluated using the Tex-207-F segregation density profile procedure.
ALDOT’s thermal profile specification, developed following the NCAT research by Fernandez Cerdas, requires thermal profiling for all major paving projects. Key requirements include:
The FAA specification (Item P-401) uses density as the primary acceptance criterion, which indirectly controls thermal segregation. The specification requires:
A comprehensive thermal segregation specification should include:
Repair strategies for thermally segregated pavements depend on the extent, severity, and age of the distress at the time of detection.
When severe thermal segregation is detected during construction (through thermal imaging or immediately post-compaction density testing), the most appropriate repair is immediate removal and replacement. The affected area should be delineated using the thermal profile data, cut with a saw or milling machine to clean vertical edges, and replaced with fresh HMA at the proper temperature. The replacement material should be compacted with particular attention to the longitudinal and transverse joints.
TxDOT specification requires suspension of paving operations when severe thermal segregation is detected, and the contractor must demonstrate that the process has been corrected before resuming production. The removed material can often be recycled through the HMA plant.
For pavements where thermal segregation has manifested as isolated raveling or potholes within the first 1–3 years of service, partial-depth patching is the most common repair method. The procedure involves:
When thermal segregation distress affects more than 10–15% of the pavement surface area, or when the damage extends through multiple cold zones, milling and overlay is the appropriate rehabilitation. The extent of milling should be determined by coring: typically 50–75 mm (2–3 in.) is sufficient to remove the segregated surface layer. The overlay should incorporate the prevention measures described in Section 6 — particularly MTV use and thermal monitoring — to ensure the replacement surface does not repeat the segregation problem.
The overlay thickness should be designed considering the structural requirements of the specific pavement section. For airport pavements, the overlay must comply with FAA AC 150/5320-6F design procedures.
In rare cases where thermal segregation has caused rapid deterioration of an entire pavement section (commonly when combined with moisture damage and stripping), full-depth reconstruction may be necessary. This is determined through falling weight deflectometer (FWD) testing, coring, and structural evaluation. The reconstruction should incorporate all available prevention measures and enhanced quality control requirements to prevent recurrence.
Pavements with documented thermal segregation should be placed on an accelerated monitoring schedule — annual inspection rather than the standard 3–5 year cycle. The monitoring should track:
This data informs the optimal timing for rehabilitation before distress becomes widespread.
Thermal segregation is a well-understood construction defect in hot mix asphalt pavements that results from temperature differentials exceeding 14°C (25°F) in the uncompacted mat. The mechanism involves the cold zones cooling to cessation temperature (80°C/175°F for conventional HMA) before roller compaction achieves the required density, producing localized areas of high air voids, low density, and reduced mechanical properties. These areas develop characteristic premature distress patterns — raveling, cracking, and potholes — that significantly reduce pavement service life.
Detection relies on thermal imaging technology ranging from handheld cameras to sophisticated paver-mounted systems that provide real-time, full-width temperature profiles with GPS location data. Prevention requires a systematic approach including insulated truck beds with mandatory tarping, material transfer vehicles for remixing and continuous paving, proper logistics to minimize paver stoppage, and real-time thermal monitoring. For airport pavements, the higher performance requirements and challenging construction conditions — particularly night paving and wide passes — demand even more rigorous thermal segregation prevention measures.
Specifications from TxDOT, ALDOT, and other agencies provide established thresholds and corrective action frameworks. The FAA’s density-based specification for P-401 mixtures indirectly controls thermal segregation, and thermal profile requirements are increasingly incorporated into project specifications nationwide. When thermal segregation is detected after construction, repair strategies range from localized patching (for early, isolated distress) to milling and overlay (for widespread surface damage) to full-depth reconstruction (in severe cases with structural deterioration). Early detection through construction-phase thermal monitoring remains the most effective strategy for preventing the pavement performance problems caused by thermal segregation.
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