Highway Materials and Laboratory Evaluation

Introduction

Highway performance depends on the interaction of materials, traffic, climate, drainage, construction methods, and maintenance. A pavement may fail even when its structural design is theoretically adequate if the soil is poorly characterized, aggregates are unsound, bitumen is unsuitable, or asphalt production is uncontrolled. Laboratory evaluation therefore provides the evidence needed to select materials, establish acceptance criteria, control construction, and diagnose defects.

Highway materials are not judged by a single test. Their suitability is assessed through a combination of sampling, testing, specification limits, engineering judgment, and field verification. The relevant standards may be issued by national agencies, ASTM, AASHTO, EN, or equivalent bodies. Because test methods and acceptance limits vary by jurisdiction, laboratory results must always be interpreted against the specification named in the contract.

Learning Objectives

By the end of this chapter, students should be able to:

1. Explain the functional requirements of soils, aggregates, bitumen, asphalt mixtures, cementitious materials, and recycled materials.
2. Develop representative sampling and sample-preparation procedures.
3. Describe major laboratory tests and interpret their results.
4. Apply fundamental equations used in soil, aggregate, and asphalt evaluation.
5. Compare test results with specifications and make defensible material decisions.
6. Recognize sources of error, limitations, sustainability issues, and quality-control responsibilities.

1. Material performance and specification philosophy

A pavement material must satisfy both immediate construction requirements and long-term performance requirements. Important properties include strength, stiffness, durability, permeability, workability, adhesion, resistance to moisture, resistance to temperature effects, and compatibility with adjoining layers.

Specifications commonly use three approaches:

• Method specifications, which prescribe equipment, procedures, and construction operations.
• End-result specifications, which require achieved properties such as density, strength, or smoothness.
• Performance-related specifications, which connect measured properties to predicted field performance, such as rutting, fatigue, or thermal cracking.

A test result is meaningful only when the sample is representative, the method is followed, equipment is calibrated, and the result is compared with the correct specification category. For example, an aggregate intended for a heavily trafficked surface course may require higher abrasion resistance and lower flakiness than aggregate used in a lower base layer.

2. Sampling, preparation, and quality assurance

2.1 Representative sampling

Sampling is often the largest source of laboratory error. A sample should represent the material lot, stockpile, production period, or pavement section from which conclusions will be drawn. Sampling plans should define the lot size, number of increments, locations, frequency, test quantities, and procedures for reducing samples.

Materials should be sampled from multiple increments rather than from a single convenient point. Stockpiles may segregate by particle size, so increments should be taken from different locations and depths using safe procedures. Conveyor-belt sampling, when feasible, can provide better representation than surface sampling. Soil samples should distinguish disturbed samples, suitable for classification and compaction, from undisturbed samples, needed for some strength or consolidation investigations.

2.2 Sample reduction and conditioning

A bulk sample is reduced using a mechanical splitter or quartering method. Hand selection of particles must be avoided because it introduces bias. Samples may require drying, washing, sieving, heating, mixing, aging, or moisture conditioning. The preparation temperature and duration can substantially affect bitumen and asphalt results.

A laboratory report should identify the material source, date, sampling location, lot, sample condition, test method, equipment, operator, raw observations, calculations, and acceptance decision. Chain-of-custody records are particularly important when results may support payment, rejection, or dispute resolution.

2.3 Quality control and quality assurance

Quality control (QC) consists of actions taken by the producer or contractor to control materials and processes. Quality assurance (QA) consists of independent verification that specified requirements are being achieved. Laboratories should use calibration, reference materials, duplicate tests, control charts, standard operating procedures, and proficiency testing.

For repeated measurements, the sample mean is:

where x̄ is the arithmetic mean, xᵢ is an individual result, and n is the number of results. The mean is appropriate for summarizing repeated measurements when the observations are comparable and extreme errors have been investigated. It must not be used to conceal nonrepresentative or invalid results.

The sample standard deviation is:

where s has the same units as xᵢ. The denominator n − 1 is used for estimating population variability from a sample. A specification may require individual results, an average, or a statistical acceptance plan; these are not interchangeable.

3. Soils and subgrade materials

Soil forms the foundation of the pavement system. Its classification and engineering behavior influence drainage, compaction, frost susceptibility, swelling, bearing capacity, and pavement thickness.

3.1 Particle-size distribution

Particle-size analysis uses sieving for coarse particles and sedimentation methods for fine particles. The grading curve indicates the proportions of gravel, sand, silt, and clay. Well-graded granular materials generally have a broad range of particle sizes and can achieve high density, whereas poorly graded or gap-graded materials may have less efficient packing, although special gap gradations can be deliberately designed.

The coefficient of uniformity is:

and the coefficient of curvature is:

D₁₀, D₃₀, and D₆₀ are particle diameters corresponding to 10%, 30%, and 60% passing, respectively, on the particle-size distribution curve. These parameters are useful for granular-soil classification but do not alone establish pavement suitability. Fines, plasticity, mineralogy, drainage, and compaction must also be considered.

3.2 Atterberg limits

For fine-grained soils, the liquid limit (LL) is the water content separating liquid and plastic behavior, while the plastic limit (PL) is the water content separating plastic and semisolid behavior. The plasticity index is:

All three quantities are normally expressed as percentages by dry soil mass. A high PI often indicates greater volume change, workability problems, and sensitivity to moisture, but the interpretation depends on soil mineralogy and the governing classification system.

3.3 Moisture content and specific gravity

Water content is calculated as:

where M𝑤 is the mass of water and Mₛ is the mass of dry soil. Equivalently, if Mwet is wet mass and Mdry is dry mass, M𝑤 = Mwet − Mdry and Mₛ = Mdry.

Specific gravity of soil solids is:

where ρₛ is the density of soil solids and ρ𝑤 is the density of water at the reference temperature. Specific gravity is dimensionless.

3.4 Compaction

Laboratory compaction tests establish the relationship between moisture content and dry density for a specified compactive effort. The maximum dry density (MDD) and optimum moisture content (OMC) are determined from the compaction curve. Field compaction is commonly evaluated by:

where ρ𝑑,field is the in-place dry density and ρ𝑑,max is the laboratory maximum dry density obtained using the specified method and effort. The equation is valid only when the field material and laboratory reference are comparable.

The dry density from wet bulk density is:

where ρ is wet bulk density and w is water content expressed as a decimal, not a percentage. For example, if ρ = 2.10 Mg/m³ and w = 0.08, then ρ𝑑 = 2.10/1.08 = 1.944 Mg/m³.

3.5 Strength and bearing tests

The California Bearing Ratio (CBR) compares the penetration resistance of a compacted soil with that of a standard crushed-rock material. It is reported as:

where pₛ is the measured pressure at a specified penetration, commonly 2.5 or 5.0 mm, and pₛₜ is the standard pressure at the same penetration. The relevant penetration is normally the one producing the governing specified result, with anomalous curves investigated according to the test standard.

CBR is empirical. It is affected by compaction, moisture, soaking, surcharge, drainage, and loading rate. It should not be treated as a fundamental elastic modulus or transferred between conditions without justification.

4. Aggregates

Aggregates constitute most of the volume of a flexible pavement. Their particle shape, grading, surface texture, strength, cleanliness, and durability influence stability, skid resistance, permeability, fatigue, and deformation.

4.1 Gradation and shape

Gradation is determined by sieve analysis and reported as percentage passing or retained. A designed gradation should provide adequate packing while retaining the void structure needed for binder and mixture performance. Excessive fines may increase binder demand and moisture sensitivity; excessive coarse material may reduce cohesion and workability.

Important shape measurements include flakiness, elongation, angularity, and surface texture. Flat or elongated particles may orient under traffic, break during compaction, and reduce interlock. Angular, rough particles generally improve mechanical interlock but may increase mixing and compaction demands.

4.2 Specific gravity, absorption, and moisture

Bulk specific gravity and apparent specific gravity are used with absorption and mixture volumetric calculations. Aggregate absorption is:

where MSSD is the saturated-surface-dry mass and Mdry is the oven-dry mass. Highly absorptive aggregate can remove effective binder from an asphalt mixture and may require special conditioning or rejection.

4.3 Strength and durability

Common aggregate tests include Los Angeles abrasion, Micro-Deval abrasion, aggregate crushing or impact value, soundness, polished stone value, and resistance to fragmentation. No single test represents all field mechanisms. Abrasion tests assess resistance to degradation under specified laboratory actions; soundness tests simulate deterioration associated with crystallization or freezing processes; polished-stone tests address skid resistance in relevant applications.

Cleanliness tests identify clay coatings, deleterious materials, organic matter, and excessive dust. Sand equivalent and methylene blue procedures are examples of tests used to assess the nature of fine particles, although their interpretation depends on the local specification.

5. Bituminous binders

Bitumen is a temperature-sensitive, viscoelastic binder. Its behavior changes with temperature, loading time, aging, and chemical composition. Binder selection must consider climate, traffic speed and volume, mixture type, and compatibility with aggregate.

5.1 Conventional binder tests

Penetration measures the depth, in tenths of a millimetre, that a standard needle enters a binder under specified load, time, and temperature. Softening point indicates the temperature at which a standardized binder specimen reaches a defined deformation condition. Ductility measures elongation under specified conditions. These tests are useful for classification and routine control but do not fully describe modern pavement performance.

Viscosity measures resistance to flow and supports selection of mixing, pumping, and compaction temperatures. Flash point identifies a temperature-related safety property and must not be confused with a recommended operating temperature.

5.2 Aging and rheology

Short-term aging occurs during heating and mixing; long-term aging occurs gradually in service through oxidation and volatilization. Aging generally increases stiffness and may improve rut resistance while increasing brittleness and cracking risk.

Modern performance grading uses rheological measurements such as complex shear modulus and phase angle. In oscillatory testing, the complex modulus is represented as:

where G′ is the elastic-storage component, G″ is the viscous-loss component, and i is the imaginary unit. The magnitude |G*| indicates total resistance to shear deformation, while the phase angle δ is related to the balance between viscous and elastic response. These measurements are valid for the specified temperature, frequency, geometry, and linear-viscoelastic test range.

Binder tests should also consider elastic recovery for modified binders, separation during storage, compatibility with additives, and low-temperature cracking behavior. A binder that performs well in one climate or traffic regime may be unsuitable in another.

6. Asphalt mixtures

An asphalt mixture consists primarily of aggregate, binder, and air voids, with possible mineral filler, reclaimed asphalt pavement, fibers, polymers, or other additives. Performance depends on volumetric structure and mechanical response, not merely on binder content.

6.1 Volumetric properties

Theoretical maximum specific gravity, Gmm, represents a mixture with no air voids. Bulk specific gravity, Gmb, represents the compacted specimen including air voids. Air voids are calculated as:

where Vₐ is air-void content in percent. This equation assumes that Gmm and Gmb are measured on a consistent mass and volume basis.

Voids in mineral aggregate (VMA) are the intergranular void space in a compacted aggregate structure, including air voids and effective binder. Voids filled with asphalt (VFA) are:

where VMA and Vₐ are expressed as percentages of mixture volume. VFA should be interpreted with VMA and air-void requirements; a high VFA alone does not guarantee durability.

Effective binder content excludes binder absorbed into aggregate pores. If binder content by total mixture mass is Pb and aggregate content is Ps, the relationship is Pb + Ps = 100%. Exact effective-binder calculations depend on the adopted definitions of aggregate bulk specific gravity, effective specific gravity, and absorption; therefore, the applicable mix-design standard should be followed rather than combining incompatible conventions.

6.2 Stability, deformation, and cracking tests

Marshall stability and flow, gyratory-compactor measurements, wheel-tracking tests, indirect tensile strength, moisture susceptibility tests, fatigue tests, and fracture tests are used for different design and control purposes. Marshall stability is not a universal measure of field rutting resistance, and gyratory density is not by itself a complete performance prediction.

Moisture susceptibility is often evaluated by comparing conditioned and unconditioned strength. A tensile strength ratio may be expressed as:

where Sconditioned and Sunconditioned are tensile strengths after the specified moisture conditioning and control procedures. The result is method-dependent and should be considered with stripping observations, aggregate-binder compatibility, drainage, and field history.

6.3 Construction control

Asphalt quality control includes aggregate stockpile management, binder-temperature control, plant calibration, gradation and binder-content checks, mixture temperature, haul time, paver operation, rolling pattern, joint construction, and in-place density. Density may be measured by cores, nuclear gauges, electromagnetic devices, or other approved methods. Gauge readings require calibration and correlation with reference measurements.

7. Cementitious materials and concrete-related pavement materials

Cementitious materials include Portland cement, blended cements, supplementary cementitious materials, cement-treated base, lean concrete, and concrete pavement mixtures. Their performance depends on cement chemistry, water content, aggregate quality, curing, temperature, and cracking control.

7.1 Cement and fresh mixture tests

Routine cement evaluation may include fineness, setting time, soundness, density, and compressive strength of standardized mortar. Fresh concrete or cement-treated material may be evaluated for temperature, consistency, air content, unit mass, and workability. Workability tests indicate consistency under a defined procedure; they do not directly measure durability or long-term strength.

The water-to-cementitious-material ratio is:

where M𝑤 is the mass of mixing water and M𝑐𝑚 is the mass of cement plus permitted supplementary cementitious materials. Lower w/cm commonly increases strength and reduces permeability when adequate compaction and curing are achieved, but excessively low values may impair workability.

Compressive stress is:

where P is the maximum applied load and A is the loaded cross-sectional area. Units must be consistent; for example, newtons divided by square millimetres gives megapascals. Test age, specimen size, curing, loading rate, and failure mode must accompany the reported strength.

7.2 Durability and pavement application

Important durability concerns include freeze-thaw action, sulfate attack, alkali–silica reaction, carbonation, chloride ingress, abrasion, shrinkage, and thermal cracking. Cement-treated bases require control of cement content, moisture, mixing uniformity, compaction, curing, strength, and cracking. Excessive strength can sometimes increase reflective-cracking risk; material selection must therefore reflect the function of the layer rather than maximize strength alone.

8. Recycled and alternative materials

Recycled materials can conserve aggregates, reduce landfill demand, and lower embodied emissions, but they require careful characterization. Common examples include reclaimed asphalt pavement (RAP), reclaimed asphalt shingles, recycled concrete aggregate, crushed glass, steel or copper slag, industrial by-products, and recycled polymers.

RAP evaluation includes stockpile uniformity, gradation, moisture, binder content, recovered-binder properties, and aggregate quality. RAP binder may be aged and stiff; the final mixture must account for the degree to which RAP binder blends with virgin binder. High RAP contents may require softer or rejuvenated binder, reduced production temperature, staged heating, or performance testing.

Recycled concrete aggregate may contain adhered mortar, increasing absorption and variability. It can affect water demand, density, shrinkage, and durability. Slag and other industrial materials require investigation of expansion, leaching, volumetric stability, and environmental compatibility.

Environmental evaluation should consider life-cycle impacts, transport distance, energy use, emissions, worker exposure, and end-of-life recovery. A recycled material is not automatically sustainable if it causes premature failure or requires energy-intensive processing.

9. Interpretation of specifications and decisions

A laboratory result should be classified as conforming, nonconforming, or requiring investigation. A result near a limit should trigger review of sampling uncertainty, test precision, lot variability, and the consequences of acceptance. Engineers should distinguish between a true material deficiency and a testing or sampling error.

Specifications may define acceptance using individual values, averages, moving averages, statistical quality levels, or pay factors. When a test method has repeatability and reproducibility limits, small differences between laboratories may not be practically significant. Corrective action may include resampling, retesting, process adjustment, material blending, removal and replacement, or engineering assessment.

A professional decision should answer four questions:

1. Was the sample representative and traceable?
2. Was the correct method and equipment used?
3. Does the result satisfy the exact contractual requirement?
4. What is the likely consequence for pavement performance and public safety?

10. Illustrative case: variable RAP in an asphalt surface course

Background

A contractor proposes increasing RAP from 20% to 35% in a surface-course mixture to reduce aggregate consumption and material cost. The project specification requires compliance with gradation, binder content, air voids, VMA, moisture susceptibility, and rutting criteria.

Problem

Initial tests show that the RAP stockpile varies considerably. One sample contains aged, stiff binder and excess fine material. Plant-produced mixtures meet average binder content but show inconsistent compaction and a reduction in low-temperature cracking resistance.

Analysis

The laboratory reviews sampling records and finds that only one surface increment was taken from the stockpile. Additional belt samples show that the original sample was not representative. RAP is then fractionated, and each fraction is tested for gradation, moisture, binder content, and recovered-binder properties. Trial mixtures are produced at 20%, 30%, and 35% RAP using controlled plant conditions.

The 35% mixture meets the volumetric limits but has poor moisture-conditioned strength and greater cracking susceptibility. The 30% mixture meets all requirements when combined with a compatible virgin binder and improved RAP stockpile management.

Approach and outcome

The contractor segregates RAP by size, increases sampling frequency, calibrates the RAP feeder, adjusts virgin binder grade, and verifies production through plant and field tests. The 30% RAP mixture is accepted after confirmatory testing and a controlled trial section. The trial section demonstrates acceptable density, surface texture, and early performance.

Learning points

The case illustrates that average binder content cannot replace representative sampling or performance testing. Recycled-material decisions must consider variability, aged binder interaction, moisture, volumetrics, cracking, rutting, and production control. Sustainability and quality are compatible when the entire material system is evaluated rather than a single test result.

11. Limitations, safety, ethics, and contemporary practice

Laboratory tests simplify field conditions. A specimen may not reproduce traffic wander, seasonal moisture, freeze-thaw cycles, construction defects, aging, or multi-axial stress. Correlations such as CBR-to-pavement thickness or laboratory rutting-to-field rutting should be used only within validated ranges.

Laboratory safety requires ventilation for solvents and fumes, heat-resistant equipment, guarding of rotating and crushing machinery, electrical protection, chemical labeling, appropriate personal protective equipment, and procedures for spills and waste. Bitumen heating and solvent extraction require special attention to fire and inhalation hazards.

Ethically, engineers must report unfavorable results, preserve raw data, avoid selective retesting, disclose uncertainty, and prevent conflicts of interest from influencing acceptance decisions. Falsified or manipulated test records can endanger road users and undermine public procurement.

Digital laboratories increasingly use automated sieving, image analysis, intelligent compaction, laboratory information-management systems, and machine-learning models. AI can identify trends, detect anomalous results, optimize mixture designs, and support predictive maintenance. It cannot replace representative sampling, validated test methods, engineering accountability, or the need to explain decisions. Models must be trained on relevant materials and checked for bias and extrapolation error.

Best-practice checklist

• Define the material lot and sampling plan before testing.
• Use recognized standards and record the exact edition and method.
• Calibrate equipment and document maintenance.
• Preserve sample identity and chain of custody.
• Report raw data, calculations, conditioning, and deviations.
• Use duplicate or referee testing when results are disputed.
• Interpret properties together rather than relying on one index.
• Link laboratory acceptance to construction control and field performance.
• Consider durability, safety, environmental effects, and life-cycle value.
• Use AI and automation as decision-support tools, not substitutes for professional judgment.

Chapter Summary

Highway materials must be evaluated as parts of an integrated pavement system. Representative sampling, correct preparation, reliable laboratory methods, and specification-based interpretation are essential. Soil evaluation includes grading, plasticity, moisture, compaction, and bearing behavior. Aggregate evaluation addresses gradation, shape, cleanliness, strength, absorption, abrasion, and durability. Bitumen requires consideration of temperature sensitivity, aging, viscosity, and rheology. Asphalt mixtures are assessed through volumetric properties, strength, rutting, cracking, moisture susceptibility, and field density. Cementitious materials require control of composition, water-to-cementitious ratio, workability, strength, curing, and durability. Recycled materials can provide environmental and economic benefits when their variability and performance effects are rigorously evaluated. Laboratory professionals must also manage uncertainty, safety, ethics, documentation, and the responsible use of digital technologies.

Key Terms

Air voids: The volume of air in a compacted asphalt mixture, expressed as a percentage of total mixture volume.

Atterberg limits: Water-content limits used to describe the consistency and plasticity of fine-grained soil.

Bitumen: A temperature-sensitive hydrocarbon binder used primarily in asphalt mixtures.

CBR: An empirical index comparing soil penetration resistance with a standard crushed material.

Compaction: Mechanical densification of soil or asphalt mixture by reducing air voids.

Gradation: Distribution of particle sizes in an aggregate or soil.

Maximum dry density: The greatest dry density achieved at a specified laboratory compactive effort.

Moisture susceptibility: Tendency of an asphalt mixture to lose performance because of water action.

OMC: Optimum moisture content corresponding to maximum dry density for a specified compaction effort.

Plasticity index: Difference between liquid limit and plastic limit.

Quality assurance: Independent activities verifying that specified quality requirements are achieved.

Quality control: Producer or contractor activities used to control materials and processes.

RAP: Reclaimed asphalt pavement recovered from existing asphalt surfaces.

Relative compaction: Field dry density expressed as a percentage of a specified laboratory maximum dry density.

Specific gravity: Ratio of a material’s density to the density of water at a reference condition.

TSR: Ratio of conditioned to unconditioned tensile strength used as an indicator of moisture susceptibility.

VMA: Voids in mineral aggregate, including air voids and effective binder volume.

VFA: Percentage of VMA occupied by asphalt binder.

Review Questions

Short Answer Questions

1. Why is representative sampling essential in highway-material testing?
2. Define plasticity index and state its equation.
3. Distinguish maximum dry density from optimum moisture content.
4. What do VMA and VFA describe in an asphalt mixture?
5. State two potential benefits and two risks of using RAP.

Long Answer Questions

1. Explain a complete laboratory evaluation program for a pavement subgrade, from sampling through acceptance.
2. Describe the principal aggregate tests and relate each to a field-performance concern.
3. Explain how binder temperature sensitivity and aging influence asphalt-mixture performance.
4. Discuss the role of volumetric properties in asphalt mix design and identify their limitations.
5. Compare QC and QA responsibilities in a highway-materials laboratory and construction project.

Analytical / Application Questions

1. A soil has LL = 42% and PL = 21%. Calculate PI and explain what the result may imply for subgrade behavior.
2. A compacted layer has wet density 2.10 Mg/m³ and water content 8%. The laboratory maximum dry density is 2.00 Mg/m³. Calculate field dry density and relative compaction, then assess whether a 95% minimum requirement is met.
3. A proposed high-RAP asphalt mixture meets air-void and VMA limits but fails moisture-conditioned strength and low-temperature cracking criteria. Recommend a technically justified investigation and corrective strategy.

Multiple Choice Questions with Answers

Question 1

Which equation defines the plasticity index of a soil?

A. PI = LL + PL

B. PI = LL − PL

C. PI = PL − LL

D. PI = LL × PL

Correct Answer: B. PI = LL − PL

Question 2

A soil has wet density 2.10 Mg/m³ and water content 0.08. Its dry density is approximately:

A. 1.68 Mg/m³

B. 1.94 Mg/m³

C. 2.18 Mg/m³

D. 2.27 Mg/m³

Correct Answer: B. 1.94 Mg/m³

Question 3

Which property is most directly established by a laboratory compaction test?

A. Bitumen softening point

B. Maximum dry density and optimum moisture content

C. Aggregate polished-stone value

D. Concrete setting time

Correct Answer: B. Maximum dry density and optimum moisture content

Question 4

In asphalt-mixture volumetrics, Gmm represents:

A. Bulk specific gravity including air voids

B. Aggregate apparent specific gravity

C. Theoretical maximum specific gravity with no air voids

D. Specific gravity of the binder only

Correct Answer: C. Theoretical maximum specific gravity with no air voids

Question 5

The tensile strength ratio is primarily used to assess:

A. Aggregate particle shape

B. Moisture susceptibility of asphalt mixtures

C. Cement fineness

D. Soil plasticity

Correct Answer: B. Moisture susceptibility of asphalt mixtures

Question 6

Which sampling practice generally reduces bias in a segregated stockpile?

A. Taking one sample from the surface only

B. Selecting visually attractive particles

C. Combining increments from different representative locations

D. Testing only the first delivered load

Correct Answer: C. Combining increments from different representative locations

Question 7

If LL = 50% and PL = 18%, the plasticity index is:

A. 32%

B. 68%

C. 2.78%

D. 0.36%

Correct Answer: A. 32%

Question 8

Which statement about CBR is correct?

A. It is a universal fundamental elastic modulus

B. It is unaffected by moisture conditioning

C. It is an empirical penetration-resistance index

D. It measures bitumen viscosity

Correct Answer: C. It is an empirical penetration-resistance index

Question 9

For concrete, compressive stress is calculated as:

A. P × A

B. P / A

C. A / P

D. P + A

Correct Answer: B. P / A

Question 10

A major concern when increasing RAP content is:

A. Guaranteed reduction in binder stiffness

B. Potential effects of aged binder and stockpile variability

C. Elimination of the need for quality control

D. Automatic improvement in low-temperature cracking resistance

Correct Answer: B. Potential effects of aged binder and stockpile variability