302 - Aggregate Base Courses
GENERAL DESCRIPTION
Definitions of Terms
Aggregate: Aggregate refers to the material used as the foundation of infrastructure. It typically consists of sand, gravel, crushed stone, or recycled materials like crushed concrete.
Approach: An approach is the section of roadway leading up to a bridge, culvert, or a main roadway. It is designed to provide a smooth transition.
Automated Machine Guidance (AMG): Automated machine guidance is the process of automatically adjusting the motion of a machine with an onboard computer that obtains its position from global positioning systems, robotic total stations, lasers, or combinations of similar methods while referencing the Contractor’s model developed for the project.
Booking Grade: Booking grade is the process of documenting verification checks of the grade to ensure elevation and slope are within the project specifications.
Concrete Crushing Plant: A concrete crushing plant is a facility where concrete rubble is crushed and processed into aggregate for reuse in construction projects.
Dense-Graded Aggregate: Dense-graded aggregate is a mixture of aggregates with an even distribution of different size aggregates.
Fine-Graded Aggregate: Fine-graded aggregate is a mixture of aggregates with a uniform distribution of small size aggregate particles passing through a #4 sieve.
Open-Graded Aggregate: Open-graded aggregate is a type of aggregate material characterized by a high porosity and void content, allowing water to drain through freely. It is commonly used in drainage applications to prevent water accumulation on road surfaces.
Salvaged Aggregate: Salvaged aggregate refers to granular material salvaged from the project site. The material is tested to meet specific gradation requirements prior to re-use on the project.
Segregation: Segregation in construction refers to the separation of different sizes or types of aggregate particles within a mixture. It can occur during handling, transport, or placement of materials and can lead to inconsistencies in the composition and performance of the final product.
Shoulder: The shoulder is the portion of the roadway adjacent to the main driving lanes. It provides additional space for emergency stopping, vehicle breakdowns, and pedestrian use, and also helps to support the road edge.
Stone: Stone refers to a naturally occurring mineral material, typically used as aggregate in concrete, asphalt, and road base layers. Stones vary in size and composition, with common types including granite, limestone, and basalt.
Subbase: Subbase is a layer of material placed beneath the road surface or pavement layers and roadway base course. It provides additional support, drainage, and stability to the pavement structure, helping to distribute loads and prevent uneven settlement.
Subgrade: Subgrade is the existing ground or soil surface upon which the road or pavement section is constructed. It serves as the foundation for the road layers above and must be properly prepared and compacted to ensure stability and durability.
General Overview
The aggregate base course serves as a crucial foundation layer for a road. Made of crushed stone, gravel, or recycled concrete, it is compacted and graded to specific standards and is typically placed between the subbase and paved surface. The purpose of the aggregate base course is to evenly spread out the weight of traffic, preventing damage to the pavement surface. Additionally, it stabilizes the subgrade beneath the road, helping to resist settling, moisture damage, and frost heave. It also aids in draining water away from the pavement, which is essential in areas with heavy rain or freezing conditions.
MATERIALS
All materials must meet acceptance requirements in the Materials Source Guide and be listed on the Contractor-provided Material Source Lists in the project files.
Description of Materials
Section 902 of the Standard Specifications for Construction. Aggregate base courses in Michigan typically consists of:
- Natural Aggregate: Limestone, basalt, granite, etc.
- Recycled concrete
- Slag aggregate
Loss by wash is limited to 8% after placement except when crushed material is used, in which case the loss by wash limit is 10%.
When crushed concrete is used in the aggregate base course, it cannot be within 100 feet of a waterbody. Project documents typically include details in the Notice to Bidders for areas where the use of crushed concrete is restricted.
Materials must come from the qualified product list and are subject to onsite testing to confirm the material meets the specifications.
The Contractor may elect to use an onsite crushing plant to recycle existing concrete pavement. If the Contractor requests this, coordination with the MDOT Construction Field Services Division is required to test the plant’s product.
Pictures of Materials
EQUIPMENT
The following equipment is typically used for construction of an aggregate base course:
- Bulldozer
- Excavator
- Grader
- Single or dual drum rollers
- Trimmer
- Spreader
- Water truck
- Concrete crushing plant (potential on large projects)
Description of Equipment
Aggregate base course material is typically delivered to the project by a truck and rough graded into place using a bulldozer. Fine grading is performed with a trimmer, bulldozer, or grader, which typically utilizes AMG to achieve the correct cross-section. The aggregate is then compacted with a roller to achieve the required density. A water truck is sometimes used to increase the moisture content of the aggregate during compaction.
Pictures of Equipment
PRECONSTRUCTION
Preconstruction meetings are held for the Engineer and Contractor to discuss and understand installation requirements, stockpile management, placement of the aggregate base course, and acceptance/payment.
Prior to the start of construction, the Inspector should perform the following:
- Review the plans to verify the location of the work.
- Verify the existing subgrade and/or sand subbase are in sound condition and have been accepted. A proof roll or geotechnical analysis may be conducted to determine condition of existing subgrade. If poor soils are encountered, coordinate with the Engineer for testing and/or removal of the soils.
- Verify project control points are established and matching between the Contractor and the inspection staff.
- Verify the AMG plan is in place and review the work with the Contractor.
- If AMG is not used, verify a surveyor has provided grade stakes for the work.
- Verify the subgrade and subbase are at the required elevation and the cross-slope is in accordance with the project specifications.
- Review the plans and the Special Provision for Maintenance of Traffic. There may be restrictions or staging requirements for installation of the aggregate. Installation restrictions are typically associated with staging requirements on a project and access to side streets and driveways.
- Verify all materials are approved for use and meet specifications, and verify that stockpiled materials are protected from weather as required by the specifications and manufacturer.
- Obtain photographs of material tickets, storage method, and location.
- Review the Stockpile Management Plan if crushed concrete produced from an onsite crushing facility is used.
- Hold an onsite meeting with the Contractor to discuss:
- The construction methods that will be used to complete the work.
- Required traffic control measures, proximity of traffic to the work area, and maintaining driveways and other access points during construction.
- Communication required if impacting residents or businesses during installation.
- Residents and businesses should be contacted if there will be disruptions to services. Coordination by the Contractor with impacted users may be required if disruption is necessary.
- Review of local ordinances.
- Disposal of material and material stockpile locations. The Contractor should provide authorization letters to the Engineer noting material stockpile and disposal locations.
- Soil erosion and sedimentation control measures that will be utilized to minimize soil erosion and subsequent sedimentation.
- Methods for securing the site during work operations and at the end of each workday.
CONSTRUCTION
The subbase or existing grade should be accepted prior to the aggregate base course being placed. The grade should not be frozen, soft, rutted, or generally unstable prior to placement of the aggregate base course.
After placement, the aggregate base layer will be compacted to a uniform thickness no less than 3 inches and no more than 8 inches. If less than 3 inches of material will be placed, the Contractor must blend the new material with the existing aggregate base so 6 inches of material will be compacted. The finished surface layer thickness will be shaped to within ±1/2-inch of the crown and the grade shown on the plans.
The Contractor should take care not to rut or distort the subbase material or the aggregate base during spreading/placing; the layer must remain smooth, compacted, and uncontaminated, especially after the grade has been booked. The required line, grade, and cross-section should be maintained until placement of the next layer. If any deformities occur, the Contractor must restore any subgrade, subbase, or aggregate base that is damaged. In the case of the aggregate base becoming contaminated, the contaminated area should be removed and replaced at no cost to the contract.
Final grading is performed using a trimmer, grader, or bulldozer generally equipped with AMG. Material that is trimmed during final grading needs to be stockpiled and retested for gradation before it can be placed again.
The Contractor should perform the necessary compaction to meet the density requirements of the aggregate base course. If density testing continually fails due to a low moisture content, the Contractor is allowed to use a water truck to increase the moisture content of the aggregate up to, but not above, the optimum moisture content. A moisture content closer to optimum may improve the ability to obtain density. A change in rolling pattern or equipment are other means the Contractor can use to achieve density.
If concrete or HMA trucks are allowed on the aggregate base course during pavement installation, it should only be to back onto the grade from the haul road, unload, and get back off the grade. Where possible, truck paths should be varied so as not to overlap. Do not use the finished grade as a haul road for empty trucks.
INSPECTION & TESTING
Inspection
Grade checks are required every 50 feet across the section to ensure all layers are within ±1/2-inch. Depth checks are required to be performed periodically to ensure the aggregate base conforms to the plan thickness for items paid by the square yard. The Inspector should utilize, at a minimum, the following tools to perform the required inspection:
- Measuring wheel
- 100-foot tape measure or longer
- 25-foot tape measure
- Chaining pin
- 6-foot stick ruler
- 4-foot level
- Working plans
- Total Station for documenting grade checks if AMG is used by the Contractor
- String line or laser level if traditional survey stakes are used
- Camera
The Inspector should take the following steps during construction:
- Obtain approved drawings and specifications for review. Lay out and discuss with the Contractor areas where crushed concrete is not permitted, if applicable.
- Observe delivery, placement, and compaction of the material for conformance with the Density Testing and Inspection Manual.
- Perform a moisture test on the aggregate material if payment is based on weight.
- Complete the required daily verification of compaction efforts of either a proof roll or density testing, whichever was agreed to by the Engineer. If density testing is the agreed upon method, 95% (under concrete) or 98% (under HMA or bridge approaches) of the maximum unit weight with a moisture content not greater than optimum is required as established with:
- The Michigan Cone if natural or slag aggregates are used; or
- The AASHTO T-180 if crushed concrete is used as detailed in the Density Testing and Inspection Manual.
- Review grade after trimming for areas of segregation. If there are areas of significant segregation, notify the Engineer and Contractor for a corrective action plan to be implemented. Areas of significant segregation can cause bigger issues when trucks are placed on the grade for paving material delivery.
- All trimmed material should be either removed from the site or stockpiled for testing to be approved for re-use onsite.
- Complete quality assurance verification of the trimmed grade and document results of these checks. Possible checks include:
- Cross-section checks.
- Top surface elevation checks.
- If AMG is used by the Contractor, a surveyor with a Total Station should be utilized to check surface elevations of the aggregate base.
- Depth checks.
Testing
For evaluation of aggregate materials, use only certified aggregate sampling and testing technicians. Refer to the Procedures for Aggregate Inspection manual and project specifications for testing requirements and frequencies. The Engineer should track the approved testing of installed aggregate and coordinate with the respective testing lab to ensure the required number of aggregate tests for the project are being completed.
For every new source, new day, or suspected change in material, a moisture check needs to be completed. Obtaining moisture readings from a nuclear density gauge is only an approximation and is often used as a guide in the field. Official determination of moisture content will be made from a field sample stored in a plastic bag with the moisture content checked for payment using the oven dry method only if it’s paid for by weight.
Density for each lift needs to be achieved before subsequent layers are built up. The maximum lift thickness permitted is 8 inches after compaction. If HMA or a bridge approach will be placed over the aggregate base course, density needs to achieve a minimum of 98% compaction. For all other cases, density needs to achieve a minimum of 95% compaction. The Inspector should ensure the minimum number of required density tests are being performed per the Density Testing and Inspection Manual.
MEASUREMENT AND PAYMENT
Measurement and Payment
Aggregate base course can be paid by weight, area, volume, or per each area completed. Each load ticket must include the required information. Refer to Section 302.03 of the Standard Specifications for Construction for information required on each ticket.
- By weight: The scale weight of the material will be paid for. If the moisture content exceeds 8%, reductions to the load ticket to remove the excess moisture will need to be made. Final measurement will be in tons.
- By area: The area will be measured by the length and width shown on the plans for the given thickness in square yards.
- By volume: The volume will be measured based on plan dimensions for the aggregate surface course in cubic yards. If loose measure (LM) is noted in the pay item, the hauling units need to be counted, measured, and inspected.
Weighing Materials
Materials paid for by weight are required to be weighed on an approved scale system. There are permanent and portable scale systems.
Permanent Scale System
A permanent scale system is defined as a weighing device that has not been moved from a given location within the last six months. Scale inspections will be conducted by a private scale agency or the Michigan Department of Agriculture and Rural Development. These inspections are valid for one year. The scale owner will provide the Engineer and regional materials personnel written verification that the scale system has been inspected according to specifications.
Portable Scale System
For portable scales, the Contractor will obtain scale approval from a local official sealer of weights and measures, or the Engineer may give approval based on verifying the results of the portable scales by passing the loads over another scale that has an official seal.
Platform Scales
If the platform scales (permanent or portable) are observed in operation, the following are important:
- The scales are operating freely.
- The scales are reasonably clean.
- The approaches are in relatively good condition.
Platform scales need to be balanced at zero and should be periodically checked throughout the day. The Engineer will require a coupled vehicle to be reweighed with each unit being uncoupled at least monthly during its use on the project. The total weight of the uncoupled units will agree with the total weight of the coupled units within ±0.2%.
Tare Weights
Tare weights should be recorded at least twice a day: once in the morning and once in the afternoon. It is good practice to ensure the hauling units are reasonably clean when performing this operation. Aggregates are weighed to the nearest 100 pounds.
Documentation
The Daily Work Report (DWR) should show all computations, measurements, and supporting documentation needed to verify the quantity submitted for payment. Primary supporting documentation includes moisture, grade, and depth checks.
Form 1900, Aggregate Inspection Daily Report, will need to be completed for every sublot of aggregate to track gradation acceptance.
Form 0582B, Moisture & Density Determination - Nuclear Method, is used to record density testing
LOCAL AGENCY PROJECTS
Some local agencies have stricter gradation requirements than MDOT. Refer to the project specifications for aggregate base course to ensure the proper material gradation is being followed.
RAIL PROJECTS
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