This is demonstrated by the Indiana DOT’s move to use/implement the LWD tests for base layer characterization, and by a major National Cooperative Highway Research Program NCHRP project
Introduction
Problem Statement
For approximately the last fifteen years, Wisconsin Department of Transportation
(WisDOT) construction specifications have been transitioning from “method” specifications to
WisDOT’s current base aggregate specifications rely on construction method terms like “Standard Compaction” to guide quality construction, but lack explicit “performance” criteria Ambiguous and subjective terms such as “appreciable displacement,” “soft,” and “spongy” are used to assess foundation preparation, leading to inconsistent base layer stiffness This variability can cause hot mix asphalt (HMA) pavement performance issues, highlighting the need for clearer, performance-based specifications to ensure durable and high-quality pavement foundations.
Flexible pavement design includes unbound granular layers (as defined by WisDOT SS
Enhancing the cost-effectiveness of pavement structures depends on the consistency and correlation of the engineering properties of pavement materials with performance specifications Implementing base aggregate specifications rooted in compaction performance criteria can significantly improve pavement design accuracy This approach helps reduce construction costs and delays caused by base failures, ultimately leading to more durable and reliable pavements.
Many state highway agencies are adopting performance-based specifications for base aggregates, prompting consideration of WisDOT’s feasibility to transition to this approach Implementing performance-based specifications could lead to significant cost savings in HMA expenditures and enhance pavement performance, making it a strategic move for WisDOT to improve infrastructure durability and reduce long-term maintenance costs.
Research Objectives
This research will develop comprehensive technical engineering and cost analysis to enable WisDOT management to objectively assess the feasibility of transitioning to new specification philosophies for base aggregate materials It will also provide technical recommendations for implementing a performance-based base aggregate specification that employs performance criteria such as minimum and uniform stiffness measurement parameters, aligning with modern technology and mechanistic principles.
2 empirical pavement design guide (MEPDG) pavement design input parameters Furthermore, these criteria should be consistent with other pavement layer performance-based specifications.
Background
Controlling the construction quality of unbound granular base layers is essential for long-lasting pavements with minimal distress and optimal ride quality Various QC/QA methods, including density-based and modulus-based approaches, are available to ensure material integrity These methods may involve spot tests, such as in-place density measurements, or continuous characterization of the base layer While spot tests are commonly used, they often focus on density measurements which do not directly indicate load-bearing capacity Most state highway agencies require laboratory compaction tests, like AASHTO T 99 or T 180, to determine target densities and ensure proper compaction of aggregate base layers.
Moreover, spot-test methods include the modulus-based methods using portable devices such as the light weight deflectometer (LWD), dynamic cone penetrometer (DCP) and GeoGauge
Continuous monitoring of the constructed aggregate base layers quality can also be achieved using the intelligent compaction technique
While spot tests and continuous measurements are widely used in pavement quality control, some U.S highway agencies, including Wisconsin, still rely on observation and personal judgment for assessing base layer quality This subjective approach uses terms like “appreciable displacement,” leaving the determination of a well-compacted base to the field engineers’ discretion, which can result in inconsistencies Such methods may lead to non-uniform base constructions, increasing the risk of pavement distress and early deterioration Therefore, QC/QA programs focus on ensuring durable, high-quality pavements through laboratory and field testing of base materials and constructed layers, establishing objective standards for construction quality and long-term performance.
Organization of the Report
This comprehensive report is structured into seven chapters, beginning with Chapter One, which outlines the research problem and objectives Chapter Two provides an in-depth literature review and synthesis of relevant studies, ensuring a solid theoretical foundation Chapter Three discusses the research methodology employed, detailing the approaches used for data collection and analysis Chapters Four and Five offer a detailed analysis of field and laboratory testing programs, including critical evaluations of the results This organized structure ensures clarity and coherence while enabling effective insights into the research findings.
A framework for base layer construction specifications is presented in Chapter Six The conclusions and recommendations are provided in Chapter Seven
Background
Significance of Unbound Base Layers for Pavement Performance
Unbound aggregates are commonly used as base course layers in flexible pavement construction worldwide, including in the U.S Their primary function is to support traffic loads from the asphalt concrete surface layer and effectively distribute these loads to underlying pavement layers or the subgrade As a crucial intermediate component, unbound aggregate layers play a vital role in ensuring pavement stability and enhancing overall performance.
The performance of unbound aggregate materials in base course layers relies on both the properties of individual particles and their interaction within the aggregate matrix Key particle characteristics influencing this performance include size, shape, texture, angularity, durability, specific gravity, absorption, toughness, and mineralogical composition Additionally, the properties of aggregates within the matrix—such as shear strength, stiffness, density, resistance to permanent deformation, permeability, and frost susceptibility—play a crucial role in ensuring the stability and durability of the base layer Understanding these properties is essential for optimizing aggregate performance and ensuring the longevity of pavement structures (Saeed et al., 2001).
The unique characteristics of aggregate particles—such as shape, angularity, and texture—are crucial for their ability to interlock within base course layers, providing essential structural stability to support traffic loads Properly constructed aggregate bases achieve dense packing and strong particle interlock, which enhances shear strength and stability while reducing permanent deformation by minimizing void spaces Conversely, instability in base layers leads to lateral movement of aggregates, resulting in pavement distress and reduced lifespan (Barksdale, 2001).
The particle and matrix properties of aggregate particles and unbound base layers are crucial factors that influence the performance of flexible pavements Proper construction of these layers ensures optimal pavement durability and longevity When these materials are correctly prepared and installed, flexible pavements are expected to perform effectively, providing reliable and long-lasting road infrastructure.
Poor and inadequate construction of base course layers can lead to premature pavement distress and deterioration Flexible pavement issues such as fatigue cracking, rutting, corrugations, depressions, and frost heave are often caused by the poor performance of unbound aggregate base layers (Saeed et al., 2001) Table 2.1 summarizes the common distresses in flexible pavements linked to substandard base layer performance and highlights the contributing factors responsible for these issues.
Saeed et al (2001) identified various distress types caused by poor performance of unbound base course layers, with fatigue cracking being a primary concern Fatigue cracking manifests as fine, longitudinal hairline cracks parallel to the wheel path, resulting from repeated traffic loading Excessive bending strains in the asphalt surface, often due to high flexibility in the aggregate base, contribute to these cracks, especially when the base thickness is inadequate or its properties degrade over time Key factors influencing fatigue cracking include low elastic modulus, improper gradation, high fines content, elevated moisture levels, poor particle angularity and surface texture leading to weak interlocking, and degradation from repetitive loads and freeze-thaw cycles.
Rutting is caused by permanent deformation in one or more layers or at the subgrade, typically resulting from consolidation or lateral movement of materials under load It manifests as a longitudinal depression in the wheel path and may become noticeable during and after rainfall Inadequate shear strength in the base layer allows particles to move laterally under wheel loads, leading to a reduction in base thickness, while low density causes settlement of the base Contributing factors include low shear strength of the aggregate base, insufficient compaction indicated by low density, improper gradation, high fines content, elevated moisture levels, lack of particle angularity and surface texture, and deterioration due to repeated loading and freeze-thaw cycles.
Depressions in pavement are caused by inadequate initial compaction or non-uniform material conditions, leading to localized deformations under load Unlike rutting, depressions are confined to specific areas where the pavement foundation is "soft," typically due to under-compaction or inability to reach target density Key factors contributing to this distress include low density of base material and a weak foundation, which compromise the pavement's structural integrity Proper compaction and quality materials are essential to prevent such localized subsidence and ensure long-term pavement durability.
Frost heave manifests as an upward bulge in the pavement surface, often accompanied by surface cracking such as alligator cracking and the formation of potholes This phenomenon occurs when ice lenses develop within the base or subbase layers during freezing temperatures, as moisture is drawn upward through capillary action During spring thaw, significant amounts of water are released from the frozen ground, further contributing to pavement damage and deterioration Proper understanding and management of frost heave are essential for maintaining the integrity and longevity of road infrastructure in cold climates.
Table 2.1: Flexible pavement distresses and contributing factors (Saeed et al 2001)
Distress Description of Distress Base Failure Manifestation Contributing Factors
Fatigue cracking begins as fine, longitudinal hairline cracks that run parallel to each other within the wheel path and in the direction of traffic As the damage progresses, these cracks interconnect, forming sharp-angled, polygonal pieces commonly known as “alligator cracking.” Over time, the cracks widen, leading to spalling and the presence of loose pavement pieces This type of cracking occurs exclusively in areas subjected to repeated traffic loading, indicating significant structural distress.
A lack of base stiffness leads to high deflection and strain in asphalt concrete surfaces under repeated wheel loads, causing fatigue cracking Alligator cracking typically occurs in areas subjected to frequent wheel traffic, where a highly flexible base results in excessive bending strains in the asphalt surface Insufficient base thickness can also contribute to surface distress by unable to properly support loads Additionally, changes in base properties over time can diminish its load-bearing capacity, increasing the risk of pavement failure.
Low modulus base Improper gradation High fines content High moisture level Lack of adequate particle angularity and surface texture Degradation under repeated loads and freeze-thaw cycling
Rutting manifests as a longitudinal surface depression in the wheel path, often becoming noticeable during and after rainfall Pavement uplift may be observed along the edges of the rut This phenomenon results from permanent deformation in one or more pavement layers or the subgrade, primarily caused by material consolidation and lateral movement under load, leading to persistent surface distress.
Inadequate shear strength in the base layer can lead to lateral particle displacement under wheel loads, causing a reduction in the base thickness along the wheel path Rutting may also occur due to base consolidation from inadequate initial compaction, weakening the pavement structure Over time, deterioration of base properties caused by poor durability or frost effects can contribute to rut formation, compromising roadway stability and safety.
Low shear strength Low density of base material Improper gradation
High fines content and high moisture levels in pavement materials can compromise durability and strength, leading to increased risk of surface issues A lack of adequate particle angularity and surface texture reduces the pavement’s ability to resist shear forces, resulting in accelerated degradation Repeated loading and freeze-thaw cycles accelerate material breakdown and weaken the pavement structure over time Depressions are localized low areas caused by settlement of the foundation soil or improper compaction in subgrade or base layers, which can lead to surface roughness and pose a risk of hydroplaning when filled with water.
Inadequate initial compaction and nonuniform material conditions can lead to additional volume reduction under load, compromising structural stability Poor durability and frost effects can cause material condition changes, resulting in localized densification that may ultimately lead to fatigue failure Proper compaction and durable material management are essential to prevent volume loss and ensure long-term performance.
Low density of base material
Factors Affecting Construction /Compaction of Aggregate in Base Layer… 9
The construction of aggregate base course layers involves spreading the aggregate materials in lifts of specified thickness and compacting them using rollers under controlled moisture conditions Compaction is the process of densifying the aggregate by reducing void spaces between particles through mechanical energy Water acts as a lubricant during compaction, facilitating particle movement and reorientation to achieve a tightly packed, dense aggregate matrix This dense, well-compacted layer ensures strong particle interlocking, which significantly enhances the overall performance of the pavement structure.
Aggregate base layers play a crucial role in enhancing pavement performance by reducing deformation and settlements, ensuring long-term durability They increase shear strength, thereby providing essential structural stability for roads and pavements Additionally, these layers improve the bearing capacity of granular bases, supporting heavy loads effectively Properly designed aggregate bases also help control undesirable volume changes caused by frost action, swelling, and shrinkage, contributing to the overall integrity and longevity of the pavement (Holtz, 1990).
Table 2.3: Selected aggregate characterization tests (After Saeed et al., 2001)
Test Method Test Reference Test Parameter
Sieve Analysis T 27, T 11ᵅ Particle Size Distribution Atterberg Limits T 89, T 90ᵅ PL, LL, PI
Understanding the properties of construction materials is essential for ensuring quality and durability Specific gravity and absorption (T 84, T 85ᵅ) are key parameters that influence material performance in different environments The moisture/density relationship (T 99, T 180ᵅ) helps determine optimal compaction levels to achieve maximum dry density, which is crucial for stability Evaluating flat and elongated particles (D 4971ᵇ F or E) aids in assessing the shape and texture of aggregates, impacting workability and strength Additionally, the percent uncompacted void content (TP 33ᵅ) indicates the porosity within the material, affecting its durability and compaction behavior Particle shape and texture (D 3398ᵇ) are vital for understanding material characteristics that influence compaction and bonding, ensuring the structural integrity of construction projects.
Static Triaxial Shear T 296ᵅ C, , shear strength Repeated Load Triaxial Deviator stress California Bearing Ratio T 193ᵅ CBR Stiffness Repeated Load Triaxial ** Resilient modulus
Tube Suction Test * Dielectric constant Index Method * F categories
This article assesses the durability and degradation characteristics of aggregates through various standardized tests The Los Angeles Abrasion test measures the percentage loss of material passing the #12 sieve, indicating resistance to abrasion Aggregate Impact Value (BS 812) evaluates the percentage loss passing the BS 2.40 mm sieve, reflecting toughness against impact forces The Aggregate Crushing Value (BS 812) assesses the percentage loss of material passing the same sieve, indicating strength under crushing loads The Micro-Deval Test (TP 58-99ᵅ) measures the percentage loss passing the #16 sieve, evaluating the resistance to wear and degradation Gyratory degradation tests before and after gradation provide insights into the aggregate’s stability under gyratory forces Lastly, the durability of the aggregate is determined through the Sulfate Soundness test (T 104ᵅ), calculating the weighted average loss to assess sulfate-related weathering resistance.
Aggregate Durability Index T 210, T 176ᵅ Durability index a: AASHTO reference test method b: ASTM reference test method c: British reference test method
*: No test method is currently available
**: Test method is developed in this research
According to the Manual for Highway Construction (AASHTO, 1990), key factors influencing soil compaction include moisture content, gradation, and compaction effort Molenaar and Niekerk (2002) examined how gradation, composition, and degree of compaction affect the mechanical properties of unbound base course materials made from recycled concrete and masonry Their research focused on different levels of compaction, evaluated using the single point Proctor density (SPPD), and concluded that these factors significantly impact the stability and performance of base materials in highway construction.
The degree of compaction significantly influences the resilient characteristics, including stiffness and resistance to permanent deformation, as well as the cohesion of the material A higher degree of compaction enhances the material's ability to recover its shape after loading, improving overall durability Additionally, gradation containing a larger proportion of fines tends to exhibit the highest cohesion, contributing to greater stability and strength of the material These factors highlight the importance of optimal compaction and gradation design in improving the mechanical performance of construction materials.
Laboratory aggregate compaction tests, which determine target density for field evaluations, typically utilize impact/dropped load methods such as AASHTO T 99, AASHTO T 180, ASTM D698, and ASTM D1557 Adu-Osei et al (2000) highlighted that vibratory and gyratory compaction methods provide more accurate simulation of field conditions, offering better estimates of modulus and strength in laboratory samples These methods effectively mimic the stress conditions imposed by vibratory rollers during field compaction While ASTM D7382 covers vibratory compaction procedures for granular soils, AASHTO does not provide specific standards for vibratory compaction (Tutumluer, 2012).
Kaya et al (2012) analyzed the impact of impact and vibratory compaction on unbound aggregate base materials, highlighting that impact compaction leads to aggregate crushing and particle breakage, which alters the gradation and increases the optimum moisture content In contrast, vibratory compaction does not cause significant particle crushing or changes in gradation, maintaining the original aggregate structure This comparison underscores the differing effects of compaction methods on the mechanical behavior and stability of aggregate bases.
Although the vibratory compaction method resulted in higher CBR values, the resilient modulus values for specimens prepared using impact compaction were consistently higher, except for one aggregate type (Tutumluer, 2012)
Holubec (1969) discovered that increasing the density enhances the properties of unbound aggregates, particularly those with angular particles, as long as pore pressure remains unchanged during repetitive loading Generally, higher density in granular materials leads to a stiffer aggregate layer, which reduces both resilient and permanent deformations under static and dynamic loads These findings highlight the importance of proper compaction in improving aggregate performance in construction applications, contributing to increased stability and durability of pavement and foundation layers.
Particle size distribution and the amount of fines are crucial for achieving a dense and compact aggregate base layer Arnold et al (2007) demonstrated that optimal gradation for maximum rut resistance can be achieved within a tight grading envelope with minimal tolerance Their study revealed that fine gradations exhibit lower strength when wet and higher strength when dry, with finer gradations also reducing segregation and total voids to enhance post-construction densification The research provided acceptable ranges for particle size variations to optimize rutting resistance, as measured by the repeated axial load test, with particle sizes detailed in SI units.
Table 2.4: Recommended gradation envelope variations around a measured particle size distribution for the repeated triaxial loading sample (Arnold et al., 2007)
Sieve size (mm) Acceptable variation around measured particle size distribution
1.18 the > of ±14% of the measured PSD or ±2
0.600 the > of ±14% of the measured PSD or ±1
0.300 the > of ±14% of the measured PSD or ±1
0.150 the > of ±14% of the measured PSD or ±1
0.075 the > of ±14% of the measured PSD or ±1
Arnold et al (2007) conducted a comprehensive investigation of specifications across 12 countries, focusing on their gradation envelopes The study highlights the variations in upper and lower gradation limits, as illustrated in Figures 2.1 and 2.2, which depict the respective bounds outlined by Arnold et al (2007).
Figure 2.1: Upper gradation limits for base aggregate specifications in 12 countries (Arnold et al., 2007)
Figure 2.2: Lower gradation limits for base aggregate specifications in 12 countries (Arnold et al., 2007)
According to the following equation, the gradation power (n) is used to control the gradation type and packing:
Where: p = percent passing sieve size d
D = maximum particle size and n = number commonly has a range between 0.3 (fine grading) and 0.6 (coarse grading)
Lay (1984) suggested that an n-value between 0.45 and 0.50 indicates optimal packing, while Belt et al (1997) identified an n-value of 0.4 as ideal for maximum resistance to permanent deformation Bennert and Maher (2003) highlighted that aggregate performance varies with material source and gradation, emphasizing that well-graded bases with fines generally offer higher mechanical stability The literature demonstrates that gradation ranges differ across countries due to variations in material properties and physical parameters, as illustrated in Figures 2.1, 2.2, and Table 2.5.
Moisture negatively impacts the performance of unbound aggregate layers in pavement structures by affecting the aggregates through three key mechanisms First, moisture can strengthen aggregates via capillary suction, increasing their stability Conversely, it can weaken the aggregates by reducing friction and causing lubrication between particles, ultimately compromising pavement durability Proper management of moisture is essential to maintain the structural integrity and longevity of unbound aggregate layers.
(3) reduce the effective stress between particle contact points due to increasing pore water pressure, thereby decreasing the strength (Tutumluer 2012)
Table 2.5: Ranges for n-values in the world (after Arnold et al., 2007)
Lower limits Upper limits (coarse side) (fine side)
Tutumluer et al (2009) analyzed the effects of molding moisture content and fines plasticity on the permanent deformation of crushed and uncrushed aggregate materials with 12% passing sieve #200 Their study found that aggregate performance significantly deteriorates when plastic fines are combined with higher molding moisture, leading to increased permanent deformation Specifically, gravel at 110% of the optimum moisture content exhibited notable deformation when mixed with plastic and non-plastic fines, indicating the critical influence of fines characteristics and moisture conditions on aggregate durability.
Figure 2.3: Relative effects of varying moisture content and the plasticity of fines on the permanent deformation behavior of crushed and uncrushed aggregates (Tutumluer et al., 2009)
Characterization of Unbound Granular Base Layers
To ensure durable constructed pavements with minimal distress and optimal ride quality, controlling the quality of unbound granular base layers during construction is essential Implementing effective quality control and assurance methods plays a crucial role in achieving long-lasting pavement performance.
Quality control and quality assurance (QC/QA) of constructed aggregate base layer layers utilize various methods rooted in different concepts, including density-based and modulus-based approaches Some techniques involve spot-test evaluations, predominantly measuring in-place density, which does not directly indicate load-bearing capacity, while others focus on continuous characterization of the base layer Most state highway agencies specify laboratory compaction tests, such as AASHTO T 99 and T 180, to determine the maximum dry density, serving as the benchmark for QC/QA of compacted aggregate base layers Additionally, modulus-based spot-test methods employ portable devices like the Light Weight Deflectometer (LWD), Dynamic Cone Penetrometer (DCP), and GeoGauge to assess material properties on-site efficiently.
Continuous quality monitoring of aggregate base layers is effectively achieved through intelligent compaction techniques, enhancing consistency and performance Despite the availability of spot-test and continuous measurement methods, some U.S highway agencies, including Wisconsin, rely on subjective assessments like “appreciable displacement” to evaluate base layer compaction This reliance on personal judgment by field engineers can result in non-uniform construction, potentially causing pavement distress and early deterioration Implementing objective, data-driven testing methods can improve base layer quality and extend pavement lifespan.
The primary goal of QC/QA is to ensure the construction of high-quality, durable, and high-performing pavements This is achieved through rigorous quality acceptance protocols that adhere to established QC/QA procedures and specifications These procedures involve comprehensive laboratory and field testing of unbound base materials and constructed base layers to verify their compliance and performance, ultimately ensuring the longevity and reliability of the pavement structures.
Currently, pavement design relies on three prominent methods: the Asphalt Institute (AI) method, the 1993 AASHTO method, and the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) Both the 1993 AASHTO guide and MEPDG utilize key material property parameters, including Poisson’s ratio (ν) and resilient modulus (MR) Poisson’s ratio, which measures the ratio of lateral to axial strain, has a relatively minor impact on pavement responses, allowing for the assumption of a standard value in design For untreated granular materials, this ratio typically ranges from 0.30 to 0.40, with a common value of 0.35 being used in practice.
The resilient modulus test evaluates how pavement materials respond under conditions that mimic real-world stress states, using cyclic loads on compacted soils or aggregates This standardized procedure, outlined in AASHTO T 307, involves preparing untreated subgrade and base materials by compacting them in layers within a 6-inch mold, ensuring the specimen's density and moisture content closely mirror field conditions measured by AASHTO T 239, T 191, and T 238 The test simulates repeated traffic wheel loading to assess the material's ability to withstand stresses, providing critical data for pavement design and stability analysis.
If the in-situ moisture content or the in-place density values are not available, then the percentage of maximum dry density and the corresponding optimum moisture content by
To determine the resilient modulus, use AASHTO T 99 or T 180 standards, ensuring proper specimen compaction before mounting it in a triaxial chamber with applied confining pressure The test begins with conditioning load cycles, followed by the application of various deviatoric stress levels The resilient modulus is calculated by averaging the recoverable deformation over the last five deviatoric loading cycles at each confining pressure and deviatoric stress The resulting resilient modulus value, corresponding to the unbound base layer within the pavement system, is selected as the design value following AASHTO T 307 guidelines.
The resilient modulus is the basic material property, which is defined as the elastic modulus based on the recoverable strain under repeated loads (Figure 2.4)
Where: σd = deviator stress, which is the axial stress in excess of the confining pressure in a triaxial compression test, and εr = elastic strain (recoverable)
This design parameter relates to pavement stiffness, enabling the characterization of pavement materials' response under various conditions and stress states It simulates the effects of moving and repeated wheel loads on the pavement This fundamental material property can be accurately determined through established laboratory testing protocols and assessed in situ using either nondestructive or intrusive testing methods, ensuring reliable performance evaluation.
Titi et al (2012) performed repeated load triaxial tests on Wisconsin crushed limestone aggregates, demonstrating their suitability for construction applications The particle characteristics, including shape, angularity, and texture, indicated high-quality aggregates, as shown in Figure 4.5 Additionally, the aggregates met the Wisconsin DOT's particle size distribution specifications, including size limits and fine content, as illustrated in Figure 4.6.
(a) Shape and duration of repeated load
(b) Stresses and strains of one load cycle
Figure 2.4: Definition of the resilient modulus in a repeated load triaxial test (after Titi et al.,
Dev iato r Lo ad ( kN)
Stress (or Strain ( Deviator stress, d
Figure 2.5: Typical crushed limestone aggregate from Wisconsin (Titi et al 2012)
Figure 2.6: Particle size distribution for a typical limestone aggregate from Wisconsin (Titi et al., 2012)
The repeated load triaxial test results on crushed aggregate demonstrate that the resilient modulus increases with higher bulk stress and confining pressures, indicating strong interlocking within the aggregate matrix According to AASHTO T 307, the resilient modulus varies significantly based on the stress state, highlighting the importance of stress conditions in assessing aggregate performance.
Quintus et al (2009) determined a target resilient modulus of base aggregate at low confining pressure and deviator stress of 6 psi each, to serve as a reference for comparison with field measurements Applying this concept to the data in Figure 2.7, the typical resilient modulus at a bulk stress of 24 psi is approximately 18.5 ksi This value effectively represents the modulus of unbound aggregate layers compacted at maximum density and optimum moisture content, providing a reliable estimate for pavement design and analysis.
Figure 2.7: Resilient modulus test results for typical limestone aggregate at dmax and w opt (Titi et al., 2012)
Eggen and Brittnacher (2004) studied how gradational, regional, and source variations affect the support strength of crushed aggregate base courses They tested the resilient modulus of 37 aggregate sources in Wisconsin to assess the impact of physical characteristics, material type, source lithology, and regional factors Their research highlights how these variables influence the resilient modulus, providing valuable insights for optimizing aggregate selection for enhanced pavement performance.
Re silient Mo dulus, M r (MPa) Resilien tM odu lu s, M r (ps i)
21 with the bulk stress is presented in Figure 2.8 For a typical base course layer bulk stress level of
The resilient modulus values at 24 psi range between 11 and 22 ksi, with an average of 16.5 ksi Eggen and Brittnacher (2004) found no significant difference in resilient modulus between gravel pit and quarry groups, though carbonate quarries tend to produce higher resilient modulus values than Precambrian, felsic-plutonic quarries They observed that variations in the gradation of the base course from a single source influence resilient modulus test results, but these effects are neither consistent nor predictable Additionally, certain physical parameters can impact resilient modulus in specific geologic subsets; however, none of these correlations are strong enough to reliably predict resilient modulus values.
The California Bearing Ratio (CBR) test is conducted according to AASHTO T 193
The Standard Method of Test for the California Bearing Ratio (CBR) involves compacting aggregate within a 6-inch diameter mold to create a specimen approximately 4.6 inches high, using materials with a maximum particle size of 0.75 inches The test can be performed on both soaked and dry specimens, with soaked samples conditioned in water for 96 hours to simulate wet pavement conditions During testing, a 3-inch area plunger penetrates the specimen at a rate of 0.05 inches per minute, and the CBR value is determined from the penetration pressure at 0.1 or 0.2 inches Standard crushed aggregate typically has a CBR of 100%, but high-quality, dense-graded crushed stone frequently exhibits CBR values exceeding 80%, indicating strong load-bearing capacity (Tutumluer, 2012).
Field test methods for characterizing aggregate base layers include nondestructive testing (NDT) techniques, minimally intrusive, and intrusive approaches Recent advancements in NDT technologies—such as ground-penetrating radar (GPR), falling weight deflectometer (FWD), light weight deflectometer (LWD), GeoGauge, and dynamic cone penetrometer (DCP)—have significantly improved the accuracy and efficiency of base course material evaluation Key industry references, including NCHRP Synthesis 382 (Puppala, 2008) and NCHRP Report 626, highlight these technological developments and their impact on pavement engineering.
Quintus et al., 2009) provided detailed information and data on various technologies applicable for characterizing unbound aggregate base layers
Base Compaction Survey
This article summarizes the findings of a survey assessing the current practices of highway agencies across the U.S and Canada regarding aggregate base construction and quality control/quality assurance The survey results provide valuable insights into industry standards, highlighting key practices and areas for improvement in aggregate base construction and pavement quality assurance Analyzing these responses helps identify trends and best practices among highway agencies, supporting the development of more effective quality management strategies in transportation infrastructure projects.
The research team designed a survey with various questions to obtain the current state of practice of highway agencies in the U.S and Canada on aggregate base construction and
QC/QA The survey questionnaire is presented in Appendix A
The research team conducted the survey through email and phone calls, reaching out to highway agencies to identify qualified engineers The process was challenging and time-consuming, as some engineers couldn't provide answers, requiring us to contact additional engineers within the same agency to gather comprehensive data.
The 50 Departments of Transportation (DOTs) in the U.S and 13 Ministries of
Transportation (MOTs) in Canada were contacted to answer the survey questionnaire Out of the
50 state DOTs, Alaska DOT did not respond to the repeated requests of the research team All Canadian MOTs submitted answers to the survey questionnaire
Forty-nine state Department of Transportation (DOT) agencies participated in the survey, providing valuable insights into their operations The collected data were organized into spreadsheet files to enable efficient analysis and visualization Using Map Viewer software, the survey responses were analyzed to highlight each highway agency's unique responses to various questions, supporting comprehensive data interpretation and graphical presentation.
According to a survey of highway agency engineers, 42% are involved in aggregate materials testing, 32% work on aggregate specifications, 18% participate in construction activities, and 8% are engaged in production, as illustrated in Figure 2.11 These percentages are normalized to total 100%, indicating that many engineers are involved in multiple aspects of aggregate materials.
Figure 2.11: Involvement capacities of highway agencies engineers in aggregate materials
Quality control and quality assurance (QC/QA) for constructed aggregate base layers are primarily conducted using density-based specifications, accounting for 90.3% of implementation In contrast, only 6.5% of highway agencies utilize observation-based specifications, while just 3.2% adopt performance-based standards A pie chart illustrating these percentages, along with a map showing individual state DOT responses, highlights the predominance of density-based QC/QA methods in highway construction.
Approximately 71% of highway agencies utilize ASTM or AASHTO standard procedures to determine target density for assessing in-place compaction, while 17% adopt their own modified procedures based on these industry standards, as illustrated in Figure 2.13.
Figure 2.14 highlights that most highway agencies rely on nuclear density gauges to measure in-place density accurately Additionally, 62.9% of state highway agencies utilize AASHTO or ASTM standards for assessing field moisture content, ensuring consistent and reliable quality control during construction.
37 standard, and 21% using their own modified standard procedure The share of usage for the sand cone method is 14.5%
Most state highway agencies set the acceptance limit for relative compaction at a minimum of 95%, with 93.1% of agencies adopting this threshold These standards ensure optimal pavement quality and longevity, making the 95% compaction level a widely accepted criterion across highway authorities.
Modulus-based QC/QA devices are commonly utilized by highway agencies to assess the quality of aggregate base layer construction According to recent data, 15.2% of agencies employ Falling Weight Deflectometers (FWD), 9.1% utilize Dynamic Cone Penetrometers (DCP), and 1.52% incorporate Light Weight Deflectometers (LWD) for this purpose However, a significant 70% of highway agencies do not implement or utilize any stiffness or modulus-based testing methods, highlighting a potential gap in quality assurance practices within the industry.
Construction of the aggregate base layer is carried out in lifts with various thicknesses
According to Figure 2.17, 43.5% of highway agencies specify a lift thickness of 6 inches, while 29% require an 8-inch lift, highlighting diverse construction practices Only 16.1% of agencies permit a 12-inch lift thickness in aggregate base layer construction, indicating limited adoption of thicker lifts Additionally, all surveyed highway agencies implement strict gradation specifications for aggregates used in base layer construction to ensure quality and durability.
Implementing QC/QA specifications for base layer construction generally does not impact project schedules, with 62.9% of surveyed highway agencies reporting no delays, while only 12.9% indicated it causes project delays (see Figure 2.18) Regarding project budgets, 53.2% of agencies found no effect on costs, whereas 21% reported an increase in the project budget due to QC/QA implementation (refer to Figure 2.19) These findings suggest that adopting QC/QA for base layer construction can be achieved without significantly affecting project timelines or budgets.
Figure 2.12: Response of highway agencies on QC/QA of constructed aggregate base layers
Observation based Density based Performance based
Compaction EvaluationData column: ReponsesObservation basedDensity basedPerformance basedNot surveyed
AASHTO T99 and ASTM D698: Standard Proctor compaction test AASHTO T180 and ASTM 1557 Modified Proctor compaction test
Figure 2.13: Methods used by highway agencies to establish target density for aggregate base layer compaction control
AASHTO T 191: In place density by sand cone method AASHTO T 310 and ASTM D6938: In place density and moisture content by unclear gauge AASHTO T 238: In place density by unclear gauge
ASTM D2167: In place density by rubber balloon
Figure 2.14: Methods used by highway agencies to measure in-place density of compacted aggregate base layers
AASHTO T99 AASHTO T180 ASTM D698 ASTM D1557 Agency Modified Method Own Agency Method Control Strip
AASHTO T191 AASHTO T310 AASHTO T238 Agency Modified Method ASTM D2167
ASTM D6938Not surveyedNot applicable
(a) Results by percentage of target density
Agency Modified Test ‐ 95% Agency Modified Test ‐ 98% Agency Modified Test ‐ 100% Own Agency Test ‐ 95%
(b) Results by percentage of target density determined by specific method
AASHTO T 99 and ASTM D698: Standard Proctor compaction test AASHTO T 180 and ASTM 1557 Modified Proctor compaction test
Figure 2.15: Relative compaction limits implemented by state highway agencies for acceptance of constructed aggregate base layers
90% Max density 95% Max density 97% Max density 98% Max density 100% Max density
According to a recent survey, 42% of highway agencies believe there is a need to adopt new methodologies for quality control and quality assurance (QC/QA) in aggregate base layer construction, while 58% consider current methods sufficient, as illustrated in Figure 2.20 The survey also highlights the ASTM and AASHTO standard procedures, detailed in Table 2.9, which are commonly referenced in the industry for aggregate testing and quality standards This data underscores ongoing discussions within the highway construction sector regarding the potential need for updated QC/QA methods to improve infrastructure quality.
Figure 2.16: Methods used by highway agencies for field measurement of stiffness/modulus of aggregate base course layers
Figure 2.17: Aggregate base layer lift thickness required for construction
DCP Geogauge FWD LWD None Benkelman Beam
Figure 2.18: Impact of implementing QC/QA specifications on timelines and project schedules
Figure 2.19: Impact of implementing QC/QA specifications on project budget and cost
No effect Not applicable Depends on test
Increase project cost Reduce project cost
No effectNot applicableDepends on test
Figure 2.20: The need to implement new methodologies for QC/QA of constructed aggregate base layers
Table 2.9: ASTM and AASHTO standard procedures mentioned in the survey
Test AASHTO Specification ASTM Specification
Standard Proctor AASHTO T 99 ASTM D698 Modified Proctor AASHTO T 180 ASTM D1557
Sand Cone AASHTO T 191 ASTM D1556 Nuclear Gauge AASHTO T 238
This article covers key soil testing methods, including AASHTO T 99, which evaluates moisture-density relations of soils using a 2.5-kg rammer with a 12-inch drop It also details AASHTO T 180, another method for determining soil moisture-density relations, employing a 10-lb rammer with an 18-inch drop Additionally, the article explains AASHTO T 191, a standardized in-place soil density test using the sand-cone method for accurate soil compaction assessment These methods are essential for ensuring soil stability and construction quality.
AASHTO T 238 Standard Method of Test for Density of Soil In-Place by Nuclear Methods (Shallow
AASHTO T 310 Standard Method of Test for Density of Soil In-Place by Nuclear Methods (Shallow
Depth) ASTM D698 Standard Test Methods for Laboratory Compaction Characteristics of Soil Using
Standard Effort (12 400 ft-lbf/ft 3 (600 kN-m/m 3 )) ASTM D1557 Standard Test Methods for Laboratory Compaction Characteristics of Soil Using
Modified Effort (56,000 ft-lbf/ft 3 (2,700 kN-m/m 3 )) ASTM D1556 Standard Test Method for Density and Unit Weight of Soil in Place by the Sand-Cone
Method ASTM D2167 Standard Test Method for Density and Unit Weight of Soil in Place by the Rubber
Balloon Method ASTM D4694 Standard Test Method for Deflections with a Falling-Weight Type Impulse Load
Service ASTM D6758 Standard Test Method for Measuring Stiffness and Apparent of Soil and Soil
Aggregate In Place by Electro Mechanical Method ASTM D6938 Standard Test Method for In Place Density and Water Content of Soil and Soil
Aggregate by Nuclear Methods (Shallow Depth) ASTM D6951 Standard Test Method for Use of The Dynamic Cone Penetrometer in Shallow
Pavement Applications ASTM E2583 Standard Test Method for Measuring Deflections with a Light Weight Deflectometer