1 Scope
NOTE This specification covers the materials, placement, compaction, and quality control of aggregate base course (ABC) used as the structural layer between the prepared subgrade and the overlying slab, pavement, or hardscape. (1.1)
NOTE ABC is a load-spreading and load-supporting element of the assembly — it distributes wheel and slab loads over a wider area of subgrade than the surface course alone could span, it provides a uniform, free-draining platform on which concrete or asphalt can be placed to plan grade and surface tolerance, and it interrupts capillary rise of moisture from the subgrade into the surface course. (1.2)
NOTE This standard addresses material sources and gradation, subgrade acceptance prior to base placement, geotextile separation between soft or fine-grained subgrades and the base course, placement of the base course in lifts, moisture conditioning, compaction to the specified density, proof rolling of the completed base, surface tolerances, and field testing for density, moisture, and gradation. (1.3)
NOTE Predecessor work — site clearing, mass grading, structural fill, and subgrade preparation — is covered by
Earthwork; successor work — slabs on grade and concrete or asphalt paving placed atop the completed base course — is covered by
Cast In Place Concrete and the project paving specification.
(1.4) NOTE A correctly placed and compacted base course is one of the lowest-cost insurance policies on a project, while a defective base course is one of the most expensive defects to remediate because it lies beneath the finished surface and cannot be repaired without demolition of that surface. (1.5)
NOTE Coordination between scopes is critical: the Contractor placing ABC inherits whatever subgrade the earthwork Contractor leaves behind, and the Contractor placing slabs or pavement inherits whatever base surface the ABC Contractor leaves behind. (1.6)
1.7 All work under this specification shall conform to the recommendations of the geotechnical investigation report prepared for this project.
1.8 Where this standard and the geotechnical report conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
1.9 The aggregate base thickness shown on the contract drawings is design-specific and is not subject to substitution; the Contractor shall not reduce base thickness to compensate for a high subgrade or to economize on material.
2 Referenced Standards
2.1 Materials, testing, and execution shall comply with the latest adopted edition of the following standards.
| Standard |
Title |
| ASTM D2940/D2940M |
Standard Specification for Graded Aggregate Material for Bases or Subbases for Highways or Airports |
| AASHTO M 147 |
Standard Specification for Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses |
| ASTM C136/C136M |
Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates |
| ASTM C117 |
Standard Test Method for Materials Finer than 75-μm (No. 200) Sieve in Mineral Aggregates by Washing |
| ASTM D4318 |
Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils |
| ASTM D2419 |
Standard Test Method for Sand Equivalent Value of Soils and Fine Aggregate |
| ASTM D2487 |
Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) |
| ASTM D1557 |
Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft³) |
| ASTM D698 |
Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft³) |
| ASTM D6938 |
Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods (Shallow Depth) |
| ASTM D1556/D1556M |
Standard Test Method for Density and Unit Weight of Soil in Place by Sand-Cone Method |
| ASTM D2216 |
Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass |
| ASTM D1883 |
Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils |
| ASTM C131/C131M |
Standard Test Method for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine |
| ASTM C88/C88M |
Standard Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magnesium Sulfate |
| ASTM D5821 |
Standard Test Method for Determining the Percentage of Fractured Particles in Coarse Aggregate |
| AASHTO T 99 |
Standard Method of Test for Moisture-Density Relations of Soils Using a 2.5-kg Rammer and a 305-mm Drop |
| AASHTO T 180 |
Standard Method of Test for Moisture-Density Relations of Soils Using a 4.54-kg Rammer and a 457-mm Drop |
| AASHTO M 288 |
Standard Specification for Geosynthetic Specification for Highway Applications |
2.2 Where project documents, adopted codes, and referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
3 Submittals
3.1 Action Submittals
3.1.1 The Contractor shall submit the following for the Engineer of Record's review prior to delivering aggregate base course to the site:
- Source qualification for each proposed aggregate source: location of the quarry or processing facility, parent rock or source description, and certification that the source has produced acceptable base course on projects of similar scope within the past 24 months
- Gradation analysis per ASTM C136/C136M and ASTM C117 for each source, with results plotted against the specified gradation envelope and identifying the producer's target gradation and tolerances
- Atterberg limits per ASTM D4318 (liquid limit, plastic limit, plasticity index) on the material passing the No. 40 sieve
- Sand equivalent value per ASTM D2419 on the material passing the No. 4 sieve
- USCS classification per ASTM D2487
- Modified Proctor moisture-density curve per ASTM D1557 (or AASHTO T 180) for each source, identifying maximum dry density and optimum moisture content
- Percentage of fractured particles per ASTM D5821 where the specified gradation requires crushed material
- Los Angeles abrasion loss per ASTM C131/C131M
- Sulfate soundness per ASTM C88/C88M
- California Bearing Ratio (CBR) per ASTM D1883 where a CBR criterion is specified
- Geotextile product data, including AASHTO M 288 classification, mass per unit area, apparent opening size (AOS), permittivity, and grab tensile strength, where a separation geotextile is shown on the drawings or required by the geotechnical engineer
- Proposed source change notification protocol: any change in aggregate source mid-project requires re-submittal and re-approval before delivery
☐ Source qualification
☑ Gradation analysis (ASTM C136 / C117)
☐ Atterberg limits (ASTM D4318)
☐ Sand equivalent (ASTM D2419)
☐ USCS classification (ASTM D2487)
☐ Modified Proctor curve (ASTM D1557)
☐ Fractured particle count (ASTM D5821)
☐ L.A. abrasion (ASTM C131)
☐ Sulfate soundness (ASTM C88)
☐ CBR (ASTM D1883) — where required
☐ Separation geotextile product data (AASHTO M 288) — where required
3.1.2 Base course placement shall not begin until the action submittals are reviewed and returned.
3.1.3 Submittal review does not relieve the Contractor of responsibility for compliance with contract documents.
3.2 Closeout Submittals
3.2.1 Prior to substantial completion the Contractor shall provide the following:
- Field testing reports for all density, moisture, and gradation testing performed on the placed base course, indexed to location, lift, and test date, and signed by the testing technician
- As-built record of finished top-of-base elevations sufficient to verify conformance with the surface tolerance requirements of this specification
- Certification from the testing agency or the geotechnical engineer of record that the placed base course conforms to the requirements of this specification and the geotechnical report
- Material certifications for each aggregate source delivered, including any source changes that occurred during construction
☐ Field testing reports (density, moisture, gradation) indexed and signed
☑ As-built record of finished top-of-base elevations
☑ Conformance certification from testing agency or geotechnical engineer
☑ Material certifications for each aggregate source delivered
4 Quality Assurance
4.1 Geotechnical Engineer of Record
4.1.1 The Owner's geotechnical engineer of record shall observe subgrade conditions before base course is placed.
4.1.2 The geotechnical engineer of record shall observe proof rolling of the subgrade and, where required, of the completed base.
4.1.3 The geotechnical engineer of record shall evaluate any differing site condition reported by the Contractor and shall direct adjustments to materials or compaction procedures when warranted.
4.1.4 The geotechnical engineer's authority to halt base course operations that do not conform to this specification or to the geotechnical report is binding on the Contractor.
4.1.5 The geotechnical engineer shall provide written observation reports for each significant event, including subgrade acceptance, geotextile placement where used, and final base acceptance.
4.2 Independent Testing Agency
● Owner-retained testing agency (independent of Contractor)
○ Contractor-retained testing agency with Owner approval — acceptable only for non-structural applications
4.2.1 Compaction and gradation testing shall be performed by a qualified independent testing agency retained by the Owner.
4.2.2 Testing for compaction acceptance shall not be performed by the Contractor or by an agency under the Contractor's direction.
NOTE The testing entity must be independent of the party whose work it accepts. (4.2.3)
4.2.4 The testing agency shall be experienced in aggregate base testing, equipped with calibrated nuclear density gauges or sand-cone equipment, and capable of providing results within the reporting times required by this specification.
4.3 Pre-Construction Conference
4.3.1 Prior to beginning base course placement the Contractor shall participate in a pre-construction conference attended by the Contractor's superintendent responsible for base course, the aggregate supplier's representative, the testing agency, the Owner's geotechnical engineer of record, and the Engineer of Record.
4.3.2 The conference shall review the source qualification submittal and approved gradation, the specified compaction requirements and testing program, the procedure for source changes mid-project, the procedure for subgrade acceptance, the procedure for handling failing density tests, lift thickness limits, moisture conditioning expectations, weather restrictions, and the proof rolling procedure for the completed base.
4.4 Compaction Reference — Standard vs. Modified Proctor
● ASTM D1557 (Modified Proctor) — required for all structural applications
○ ASTM D698 (Standard Proctor) — non-structural areas only, with geotechnical engineer approval
4.4.1 Modified Proctor (ASTM D1557 / AASHTO T 180) shall be used as the compaction reference for all aggregate base course placed beneath foundations, slabs on grade, pavements, and other structural surfaces.
4.4.2 Standard Proctor (ASTM D698 / AASHTO T 99) shall not be used for base course acceptance except where the geotechnical report explicitly substitutes Standard Proctor for a particular non-structural application and the Engineer of Record concurs in writing.
NOTE Modified Proctor delivers more compaction energy and produces a higher maximum dry density value, so a base course tested against Standard Proctor and declared passing may be substantially less dense than the same base course tested against Modified Proctor, and the resulting structural performance will be inadequate. (4.4.3)
NOTE Specifying or testing against the wrong Proctor reference is one of the most consequential errors in earthwork and base course acceptance. (4.4.4)
4.5 Field Density Test Method
Nuclear gauge per ASTM D6938
Sand cone per ASTM D1556/D1556M
As directed by geotechnical engineer
NOTE The nuclear gauge method (ASTM D6938) is the standard production testing method for aggregate base because it is fast, non-destructive, and allows the same location to be re-tested after additional compaction if the first test fails. (4.5.1)
NOTE The sand cone method (ASTM D1556/D1556M) is the reference method for calibration and for locations where nuclear gauge use is restricted. (4.5.2)
4.5.3 The testing technician shall perform gauge standard counts at the beginning and end of each testing session.
4.5.4 The testing technician shall correlate nuclear gauge readings to sand cone results at the start of base course production and whenever the apparent density-moisture relationship suggests gauge drift.
5 Materials
5.1 Aggregate — General Requirements
● Crushed stone
○ Crushed gravel (minimum percent fractured faces required)
○ Crushed slag
○ Recycled concrete aggregate (RCA) — Engineer of Record approval required
5.1.1 Aggregate base course material shall consist of clean, hard, durable, sound particles of crushed stone, crushed gravel, or crushed slag, free of organic matter, soft or weathered particles, clay lumps, vegetable matter, and other deleterious substances.
5.1.2 The aggregate shall be processed to conform to the gradation, plasticity, fines content, and durability requirements specified herein.
NOTE The intent of this specification is to deliver a well-graded, dense-graded aggregate that compacts to a high density with positive interparticle interlock between angular crushed faces, drains free of standing water, and provides uniform structural support across the full footprint of the overlying assembly. (5.1.3)
5.1.4 Recycled concrete aggregate (RCA) and reclaimed asphalt pavement (RAP) blends may be permitted as base course materials where allowed by the Engineer of Record, the geotechnical engineer, and the Authority Having Jurisdiction, and where the material is processed and tested to conform to the same gradation, plasticity, and durability requirements as virgin crushed stone.
NOTE RCA tends to have higher absorption, lower density, and the potential for tufa formation in drainage paths, so its use shall be evaluated on a project-specific basis. (5.1.5)
5.1.6 RAP-blended base shall be evaluated for binder content and its effect on compaction and stability.
5.2 Gradation
ASTM D2940/D2940M base course (1-1/2 in. maximum)
ASTM D2940/D2940M subbase (2 in. maximum)
AASHTO M 147 Grading A (2 in. maximum)
AASHTO M 147 Grading B (2 in. maximum)
AASHTO M 147 Grading C (1 in. maximum)
AASHTO M 147 Grading D (1 in. maximum)
AASHTO M 147 Grading E (1 in. maximum)
AASHTO M 147 Grading F (3/4 in. maximum)
State DOT specification — identify on drawings
○ 2 in.
● 1-1/2 in.
○ 1 in.
○ 3/4 in.
5.2.1 Aggregate base course shall be dense-graded, with a smooth and continuous distribution of particle sizes from the maximum size down through the fines.
5.2.2 Open-graded materials with little or no fines are not aggregate base course and shall not be substituted for ABC under this specification.
NOTE Dense gradation produces high density, high stiffness, and high stability — the coarse particles carry load by skeletal interlock and the fines fill the voids between coarse particles so the assembly behaves as a solid, while open-graded drainage layers serve a different purpose and require a separate specification. (5.2.3)
5.2.4 Maximum aggregate size shall be selected to suit the placed lift thickness and the surface tolerance requirements of the overlying assembly.
5.2.5 The largest particle shall not exceed two-thirds of the compacted lift thickness, and shall not exceed one-half of the total base thickness where the base is placed in a single lift.
NOTE A smaller maximum size (3/4 in. or 1 in.) is preferred immediately beneath thin concrete topping slabs and architectural slabs where a coarse particle near the surface can telegraph through the slab as a hard spot or contribute to surface cracking, while a larger maximum size (1-1/2 in. or 2 in.) is appropriate for thick pavement sections where structural stiffness is the controlling criterion. (5.2.6)
5.3 Fines Content
● 8 percent (typical dense-graded base, well-drained applications)
○ 10 percent (typical dense-graded base, moderate exposure)
○ 12 percent (cohesive-bound base, dry climates only)
○ Per geotechnical engineer
NOTE The percentage of material passing the No. 200 sieve (75 μm) is the single most important secondary parameter of an aggregate base course, after gradation. (5.3.1)
NOTE Fines fill voids and contribute to compacted density, but excessive fines — particularly plastic fines — produce a base that holds water, becomes weak when saturated, pumps under traffic, and loses strength catastrophically when subjected to freeze-thaw cycling. (5.3.2)
5.3.3 The fines content shall be controlled at the source and verified at delivery.
5.4 Plasticity
○ Non-plastic
○ PI not greater than 4
● PI not greater than 6
○ PI not greater than 10 (dry climate, non-frost-susceptible applications only)
● LL not greater than 25
○ LL not greater than 30
NOTE Plasticity indicates the presence and behavior of clay-sized fines: a non-plastic or low-plasticity material drains and dries, retains strength when wet, and resists frost heave, while a plastic-fines material holds water, swells, weakens, and pumps. (5.4.1)
5.4.2 The plasticity index of the material passing the No. 40 sieve shall be determined per ASTM D4318 on every approved source.
5.4.3 The plasticity index shall be retested whenever the visual or testing data suggest a source change.
5.5 Sand Equivalent
○ 35
● 40
○ 45
○ Not required on this project
NOTE The sand equivalent test (ASTM D2419) is a rapid field-applicable measure of the relative proportion of plastic fines and dust to acceptable sand-sized particles in the fine fraction of the aggregate, where a high value indicates clean, sand-dominated fines and a low value indicates clay-dominated fines. (5.5.1)
NOTE The sand equivalent test is particularly useful as a production-control test because results are available in approximately one hour, unlike Atterberg limits which require multi-day specimen preparation. (5.5.2)
5.5.3 The geotechnical engineer may direct sand equivalent testing in place of or in addition to plasticity index testing as a production-control measure.
5.6 Durability — Abrasion and Soundness
○ 40 percent
● 45 percent
○ 50 percent
● 12 percent (sodium sulfate)
○ 18 percent (magnesium sulfate)
○ Not required — non-freeze-thaw exposure
5.6.1 The aggregate shall be resistant to mechanical breakdown during handling, placement, and compaction, and to weathering breakdown over the service life of the assembly.
5.6.2 Mechanical durability shall be measured by the Los Angeles abrasion test (ASTM C131/C131M) and weathering durability shall be measured by sulfate soundness (ASTM C88/C88M).
NOTE A high L.A. abrasion loss indicates an aggregate that will degrade under roller compaction, producing additional fines during placement and altering the in-place gradation from the as-tested gradation, which is a common cause of compaction tests that pass at the time of placement but lose density and develop pumping behavior shortly thereafter. (5.6.3)
NOTE Sulfate soundness predicts the resistance of the aggregate to freeze-thaw breakdown over time and is critical in climates where the base course experiences saturated conditions during freezing weather. (5.6.4)
5.7 Fractured Particles
○ Not required (crushed stone source — fracture inherent)
● 75 percent with two or more fractured faces
○ 90 percent with two or more fractured faces
○ 100 percent with one or more fractured faces
NOTE For crushed gravel sources, the percentage of fractured particles per ASTM D5821 is the principal means of ensuring that the rounded river-run particles have been broken to expose angular faces that interlock under compaction. (5.7.1)
5.7.2 Crushed-stone sources are inherently 100 percent fractured by the crushing process and do not require fractured-particle verification, but the source-qualification submittal shall describe the crushing process so the verification can be waived appropriately.
5.8 California Bearing Ratio
Not specified — gradation and plasticity governance only
CBR 60 minimum
CBR 80 minimum
CBR 100 minimum
Per pavement designer / geotechnical engineer
NOTE CBR is a strength-based acceptance criterion used by pavement designers and by military and FAA specifications. (5.8.1)
5.8.2 Where the pavement section design is based on a specific assumed CBR of the base course, that CBR shall be verified by laboratory testing of the source material at the specified compaction.
5.8.3 CBR testing alone shall not be used as an acceptance basis and shall always be paired with gradation, plasticity, and density requirements, because a material can exhibit acceptable CBR while failing other parameters that govern long-term performance.
5.9 Separation Geotextile
○ Required — woven monofilament per AASHTO M 288 Class 1
○ Required — nonwoven needle-punched per AASHTO M 288 Class 2
● Required where directed by geotechnical engineer in the field
○ Not required — subgrade confirmed adequate
○ 0.43 mm (No. 40 sieve)
● 0.60 mm (No. 30 sieve)
○ Per geotechnical engineer based on subgrade gradation
5.9.1 Where the subgrade is fine-grained (silt, clay, or silty/clayey sand) and saturated or near-saturated conditions are anticipated, a separation geotextile shall be installed between the subgrade and the aggregate base course to prevent migration of subgrade fines upward into the base voids under cyclic loading.
NOTE Without separation, subgrade fines pump into the base voids over time, reducing base permeability, contaminating the base gradation, and eventually destabilizing the assembly. (5.9.2)
5.9.3 The need for a separation geotextile shall be determined by the geotechnical engineer based on the subgrade soil type, anticipated moisture conditions, and the importance of the overlying assembly.
6 Subgrade Preparation and Acceptance
6.1 Subgrade Predecessor Work
6.1.2 The base course Contractor inherits the subgrade in the condition left by the earthwork Contractor, and shall not begin base course placement until the subgrade has been formally accepted by the geotechnical engineer.
6.2 Subgrade Acceptance
● Required on all subgrades to receive base course
○ Required only where directed by geotechnical engineer
○ Not required — subgrade compaction testing alone is sufficient
6.2.1 Before base course is placed on any area, the subgrade beneath that area shall be observed and accepted by the geotechnical engineer of record.
6.2.2 Acceptance shall confirm that the subgrade has been compacted to the density and moisture conditions required by Earthwork and the geotechnical report, that the subgrade is at the elevation shown on the drawings within the tolerances of the predecessor specification, and that the surface is free of soft spots, ruts, loose material, standing water, frost, or organic debris. 6.2.3 The Contractor shall provide the geotechnical engineer at least 24 hours notice before a subgrade area will be ready for observation.
6.2.4 Proof rolling of the subgrade is described in Earthwork and shall be completed and accepted by the geotechnical engineer before base course is placed. 6.2.5 Areas that fail proof rolling shall be undercut, stabilized, or otherwise remediated by the earthwork Contractor before base course placement begins.
6.2.6 The base course Contractor shall not attempt to bridge soft subgrade areas with base course material, because placing base course over a yielding subgrade transfers the problem upward into the base and ultimately into the surface course.
6.3 Subgrade Protection
6.3.1 After the subgrade has been accepted and before base course is placed, the subgrade surface shall be protected from disturbance.
6.3.2 Construction traffic shall be restricted to designated haul paths.
6.3.3 Rain that softens the subgrade shall be addressed by re-compaction or, where damage is severe, by re-acceptance through the geotechnical engineer.
6.3.4 The Contractor shall not place base course on a subgrade that has been disturbed or saturated since acceptance without confirming with the geotechnical engineer that the subgrade still meets the original acceptance criteria.
6.4 Separation Geotextile Installation
6.4.1 Where a separation geotextile is required, it shall be installed on the accepted subgrade in continuous panels with overlaps not less than 12 inches in the direction of base placement, or sewn seams per the manufacturer's instructions.
6.4.2 The geotextile shall be installed without folds, wrinkles, or tension that would prevent intimate contact with the subgrade.
6.4.3 Damaged or torn panels shall be replaced or patched with overlapping new material; small punctures from foot traffic during installation are acceptable, but tears and cuts longer than 6 inches shall be patched.
6.4.4 Aggregate shall be placed on the geotextile by dumping ahead onto previously placed material and pushing forward, and shall not be dumped directly on uncovered geotextile because of the risk of tearing.
6.4.5 The minimum cover over the geotextile before any construction traffic shall be 8 inches of compacted base, or the manufacturer's recommendation if greater.
6.4.6 Compaction equipment shall not turn sharply on geotextile-covered surfaces with less than 12 inches of cover, because the wheel scrubbing action can damage the fabric.
7 Placement
7.1 Delivery and Stockpile Management
7.1.1 Aggregate base course material shall be delivered to the site in vehicles that protect the material from contamination and from segregation during transport.
7.1.2 Stockpiles on site shall be managed to prevent segregation of coarse and fine particles, contamination by subgrade soil or organic material, and excessive moisture loss or gain.
7.1.3 Stockpiles shall not be built up by end-dumping into a conical pile because this causes severe segregation as large particles roll to the base of the cone; stockpiles shall be built in horizontal layers using a loader or by walking the dumping point across the stockpile face.
7.1.4 If the aggregate has segregated between delivery and placement, it shall be re-blended by passing through a mechanical blender, by re-mixing with a loader bucket, or by spreading in a thicker layer and re-mixing with a motor grader before final spreading.
7.1.5 Material that cannot be re-blended to a uniform gradation shall be rejected.
7.2 Spreading
4 in.
6 in.
8 in. (with heavy vibratory roller, geotechnical engineer verification)
Per geotechnical engineer
7.2.1 Aggregate shall be spread to a uniform loose lift thickness using a motor grader, an asphalt paver, or a similar mechanical spreader.
7.2.2 End-dumping followed by dozer spreading is not acceptable for the final spreading operation immediately beneath structural surfaces, because dozer spreading produces inconsistent thickness and induces segregation.
7.2.3 Spreading shall be performed on the accepted subgrade or on the previously compacted lift, never on a frozen or saturated surface.
7.2.4 Compacted lift thickness shall not exceed the depth to which the available compaction equipment can transmit effective compaction energy.
NOTE Vibratory smooth-drum rollers in the 10 to 12 ton class can typically compact 6 inches of dense-graded aggregate uniformly, while heavier rollers (15 ton and up) can compact 8 inches, and lifts that exceed equipment capability produce a dense surface crust over a loose interior that may pass surface density testing but will deform and lose density over time under load. (7.2.5)
7.2.6 Where total base thickness exceeds the maximum single-lift compaction capability, the base shall be placed in multiple lifts of approximately equal thickness, with each lift fully compacted and tested before the next is placed.
7.2.8 Where the as-built subgrade elevation is high relative to design, the Contractor shall lower the subgrade rather than reduce the base course thickness.
8 Compaction
8.1 Moisture Conditioning
Within -2% to +2% of optimum (Modified Proctor)
Within -2% to +1% of optimum (frost-susceptible or low-plasticity aggregate)
Within -3% to +2% of optimum (dry climates, granular base)
Per geotechnical engineer
8.1.1 Aggregate base course shall be compacted at a moisture content within the range specified herein relative to the Modified Proctor optimum moisture content.
NOTE Dense-graded aggregates with non-plastic or low-plasticity fines have a relatively narrow workable moisture window — too dry and the fines do not lubricate particle re-arrangement, leaving voids; too wet and pore pressure develops under the roller and the material pumps rather than densifying. (8.1.2)
8.1.3 If the base material is too dry as delivered or after spreading, it shall be wetted uniformly using water trucks with spray bars, and surface sprinkling without mixing shall not be accepted because it does not bring the full lift to optimum moisture.
8.1.4 If the base material is too wet, it shall be aerated by motor grader passes or by allowing surface evaporation before compaction.
8.1.5 The Contractor shall not attempt to compact material that is outside the moisture window in the expectation that density testing will pass, because the typical result is a base that meets density at the time of testing but loses density rapidly under traffic and slab loads.
8.2 Compaction Equipment
☑ Vibratory smooth-drum roller, 10 ton minimum operating weight
☐ Pneumatic-tired roller (final rolling, surface tightness)
☐ Plate compactor (confined areas, edges)
☐ Jumping jack (vibratory rammer) — adjacent to structures only
NOTE Vibratory smooth-drum rollers are the primary compaction equipment for aggregate base course because the combination of static weight and dynamic vibration is effective on dense-graded granular materials. (8.2.1)
8.2.2 The minimum static weight of the primary roller shall be 10 tons; lighter rollers may be used only with the geotechnical engineer's approval and only on thin lifts (4 inches or less) over already-compacted base.
8.2.3 The vibratory frequency and amplitude shall be matched to the lift thickness — high amplitude for thick lifts, low amplitude for thin final passes that risk over-vibration and surface particle breakdown.
8.2.4 Pneumatic-tired rollers may be used for a final non-vibratory finishing pass to produce a tight, kneaded surface texture before placement of the overlying course.
NOTE Plate compactors and jumping-jack tampers are used in confined areas where the smooth-drum roller cannot operate — adjacent to manholes, light pole foundations, building edges, and at the perimeter of curbs. (8.2.5)
8.3 Compaction Adjacent to Structures
○ 2 ft
● 3 ft
○ 5 ft (sensitive or new concrete structures)
8.3.1 Compaction equipment shall not operate within 3 feet of any in-place foundation wall, retaining wall, manhole, light pole base, buried utility, or other rigid structure without the approval of the Engineer of Record.
NOTE Heavy vibratory equipment near rigid structures transmits dynamic loads that can crack concrete, displace freshly placed reinforcement, or damage flexible pipe. (8.3.2)
8.3.3 In the zone within 3 feet of any rigid structure, base course shall be compacted with hand-operated vibratory plate compactors or jumping-jack tampers in lifts not exceeding 4 inches.
8.3.4 The Contractor shall verify by a test strip that the hand equipment achieves the required density in confined zones before relying on that equipment for production work.
8.4 Compaction Density Requirements
8.4.1 Base Course Beneath Pavements and Slabs
95100
9598100
Default: 98 percent of Modified Proctor (ASTM D1557) maximum dry density
8.4.1.1 Aggregate base course placed beneath flexible pavements, rigid pavements, and structural slabs on grade shall be compacted to a minimum of 98 percent of Modified Proctor maximum dry density unless the project geotechnical report or pavement design specifies a different value.
NOTE ASTM D2940 suggests 98 percent as the minimum average density for base courses, and 98 percent has been adopted across most state DOT and FAA specifications as the standard requirement for highway and airport base. (8.4.1.2)
8.4.1.3 Where the pavement section is designed on the assumption of 100 percent compaction — typical for heavy-duty truck routes, airport pavements, and crane pads — 100 percent of Modified Proctor shall be specified and achieved.
8.4.2 Subbase or Lower Lift
92100
9598
Default: 95 percent of Modified Proctor (ASTM D1557) maximum dry density
8.4.2.1 Where a separate aggregate subbase layer is placed beneath the base course, the subbase may be compacted to 95 percent of Modified Proctor maximum dry density.
NOTE A separate subbase layer is typical of thick pavement sections, where coarser, lower-cost subbase aggregate is used in the lower portion and finer, higher-quality base aggregate is reserved for the top. (8.4.2.2)
8.4.2.3 The base course immediately beneath the surface shall always be compacted to the higher 98 or 100 percent value.
8.4.3 Confined Areas
8.4.3.1 Confined areas compacted by plate compactor or jumping jack — adjacent to foundations, around manholes, at edges of curbs — shall meet the same density requirement as the open areas.
8.4.3.2 The Contractor shall not relax density requirements simply because the area is small and difficult to compact, because localized soft spots in confined areas propagate as cracks and depressions in the overlying surface course.
8.5 Failing Tests
8.5.1 When a field density test fails, the Contractor shall cease placement of additional base course in the affected area, scarify and rework the failing lift, adjust moisture content if needed, re-compact, and request re-testing at the same location before proceeding.
8.5.2 The geotechnical engineer shall evaluate patterns of repeated failure, because repeated failures in the same area suggest that the underlying subgrade is yielding, that the material moisture is persistently out of range, or that the available compaction equipment is inadequate.
8.5.3 Single isolated failures may indicate testing equipment drift, in which case correlation with sand-cone results shall be performed before continuing acceptance testing.
8.5.4 The Contractor shall not bury, hide, or otherwise conceal failing test results.
8.5.5 All test results, including failures and the corrective action taken, shall be documented and reported.
NOTE A pattern of unreported failures discovered at project closeout will require remediation by core sampling, density verification at depth, and partial removal and replacement at the Contractor's expense. (8.5.6)
9 Proof Rolling the Completed Base
● Required — full coverage of base prior to slab or pavement placement
○ Required — sample areas only, as directed by geotechnical engineer
○ Not required (base density testing alone is sufficient)
○ 1/4 in.
● 1/2 in.
○ Per geotechnical engineer
9.1 Where required, the completed base course shall be proof rolled with a fully loaded tandem-axle dump truck (minimum 20 ton loaded weight) or with another rolling load approved by the geotechnical engineer.
9.2 Proof rolling shall be observed by the geotechnical engineer of record.
9.3 Areas that exhibit more than 0.5 inch of measurable rutting, pumping, deflection, or other unstable behavior shall be undercut, the cause of the instability determined, and the area replaced with properly compacted base over a re-accepted subgrade.
9.4 The geotechnical engineer's written observation of the proof rolling and acceptance of the completed base shall be retained as a closeout document.
10 Surface Tolerances
● Plus or minus 1/2 in. from design elevation (beneath rigid pavement / slabs)
○ Plus or minus 1/4 in. from design elevation (beneath thin overlays or topping slabs)
○ Plus or minus 1 in. from design elevation (beneath flexible pavement)
● Maximum deviation 3/8 in. in 10 ft (beneath rigid pavement / slabs)
○ Maximum deviation 1/2 in. in 10 ft (beneath flexible pavement)
○ Per project specification
10.1 The finished surface of the aggregate base course shall be true to the elevation, cross slope, and surface profile shown on the drawings, within the tolerances necessary to support the overlying course without locally thinning the surface course or producing a wavy finished surface.
NOTE Tolerances tighter than those required by the overlying course are a waste of effort, while tolerances looser than required by the overlying course pass the variability up into the surface. (10.2)
10.4 Areas exceeding tolerance shall be corrected by scarifying, removing or adding base material, and re-compacting.
10.5 Minor high spots shall not be corrected by simply blading off, because doing so removes fines preferentially and alters the surface gradation.
10.6 The base shall be sloped to drain away from foundations and from the building footprint.
10.7 Standing water on the completed base course at the time of slab or pavement placement is not acceptable, and the base surface shall be free-draining and dry of standing water at the time of overlying course placement.
11 Cold and Wet Weather Limitations
11.1 Cold Weather
● 32°F (above freezing) — ambient air
○ 40°F — ambient air, with material and subgrade not frozen
○ Per geotechnical engineer
11.1.1 Aggregate base course shall not be placed on a frozen subgrade or on a previously placed base lift that is frozen.
NOTE Frozen material at the interface will thaw under the overlying compacted lift, creating a layer of low density and low strength that propagates as a settlement defect into the finished surface. (11.1.2)
11.1.3 Where freezing weather is anticipated, the placed base shall be protected from frost between lifts, and insulating blankets or accelerated placement schedules may be required.
11.1.4 Material that has frozen in the stockpile or in transit shall not be placed.
NOTE Frozen lumps within the base material remain dimensionally stable during compaction and pass density testing, then thaw to create voids beneath the surface course. (11.1.5)
11.1.6 Frozen material shall be rejected at delivery or, if discovered after delivery, shall be removed from the stockpile before mixing with unfrozen material.
11.2 Wet Weather
● Suspend at start of measurable precipitation
○ Suspend at 0.25 in. accumulation
○ Continue with cover and active dewatering
11.2.1 Base course placement shall be suspended during precipitation that raises the placed material above the upper moisture limit, and shall not resume until the material moisture has returned to the workable range.
11.2.2 Material that has been over-wetted by rain after placement and before compaction shall be aerated by motor grader passes or discing until the moisture returns to the acceptable range.
11.2.3 Placed material that has become saturated and pumping shall be removed and replaced.
11.2.4 After heavy rain, the subgrade beneath uncompleted base sections shall be re-inspected by the geotechnical engineer for softening or rutting before placement resumes.
NOTE Saturated subgrade is a more common cause of base course failure than saturated base. (11.2.5)
12 Field Testing and Quality Control
12.1 Testing Frequency
12.1.1 Minimum testing frequencies, in the absence of project-specific geotechnical recommendations, shall be:
- One in-place density and moisture test per 2,500 square feet of base course surface per lift, with a minimum of one test per shift per area placed
- One gradation analysis per 1,000 tons of base material delivered, or one per source per shift, whichever is more frequent
- One Atterberg limits test (or sand equivalent, where substituted) per 1,000 tons of base material delivered, or one per source change
- One Modified Proctor moisture-density curve per source; additional curves whenever the material visibly changes character or compaction test results show unexplained variability
- Testing at every transition between aggregate source, every transition between lift types, and at any location identified by the geotechnical engineer as variable
1 test per 2,500 sf
1 test per 1,500 sf (higher-risk or variable conditions)
1 test per 500 sf (heavy-duty or airport pavement)
Per geotechnical engineer
1 test per 1,000 tons delivered
1 test per 500 tons (high-variability source)
1 test per truck (acceptance testing at delivery)
Per geotechnical engineer
12.1.2 Density, gradation, plasticity, and Modified Proctor testing shall be performed at the minimum frequencies listed for this standard in the absence of project-specific geotechnical recommendations.
12.2 Nuclear Gauge Standardization and Correlation
12.2.1 Nuclear density gauge results shall be standardized at the beginning and end of each testing shift.
12.2.2 The gauge shall be correlated to sand-cone results (ASTM D1556/D1556M) at the start of base course production for each source, and again whenever the apparent density-moisture relationship suggests gauge drift.
NOTE Sand-cone correlation factors greater than approximately 2 to 3 pcf between nuclear and sand-cone results indicate a need for gauge service or re-calibration, and uncorrected gauge drift produces systematic acceptance errors that may not be discovered until the overlying surface course fails. (12.2.3)
12.3 Documentation and Reporting
● Within 24 hours of test (failures by immediate notification)
○ Same day digital delivery (failures by immediate notification)
○ Weekly batch report — not acceptable for structural applications
12.3.1 All field testing data shall be recorded on forms that include project name, test date, test location (station and offset, plan coordinates, or other clear referent), lift number, material description and source, moisture content, wet density, dry density, percent compaction relative to the Modified Proctor reference for that source, and pass/fail determination against the specified criterion.
12.3.2 The testing agency shall provide results to the Owner and Contractor within 24 hours of testing.
12.3.3 Failing tests shall be flagged immediately by direct communication to the Contractor's superintendent and the geotechnical engineer.
NOTE Failing tests reported only in batch reports days later do not allow the corrective-action workflow this specification depends on. (12.3.4)
12.4 Acceptance Records
12.4.1 The closeout package shall contain a complete record of all density tests, gradation tests, and material certifications, indexed by lift and area, with a summary acceptance statement signed by the geotechnical engineer of record (or the testing agency where so directed by the Owner).
NOTE The acceptance record is the project's documentary basis for relying on the base course as a structural element, and an incomplete or missing record exposes the Owner to claims and warranty disputes that no amount of post-construction testing can fully cure. (12.4.2)
13 Warranty
1 year from substantial completion
2 years from substantial completion
13.1 The Contractor shall warrant the aggregate base course operations, including achievement of specified compaction, materials conformance, and surface tolerances, for the project warranty period beginning at substantial completion.
13.2 Warranty obligations include correction of settlement, rutting, pumping, or other defects in the overlying surface course that are attributable to non-conforming base course operations.
13.3 Warranty does not relieve the Contractor of liability for concealed non-conforming work discovered after the warranty period expires.
13.4 Defects in the base course that produce surface course failures discovered after the warranty period may constitute latent defects extending the Contractor's responsibility beyond the warranty term where the defect can be tied to non-conforming materials, inadequate compaction, or undocumented failing tests.