305 - HMA Base Crushing and Shaping
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.
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.
Bulking: Bulking refers to the increase in volume when existing HMA is crushed. Dense-Graded Aggregate: Dense-graded aggregate is a mixture of aggregates with an even distribution of different size aggregates.
Salvaged Aggregate: Salvaged aggregate is made of recycled materials recovered from demolished structures or construction waste. These materials are processed and reused as aggregate in new construction projects, minimizing waste and reducing the need for virgin materials.
Salvaged Crushed Material: Salvaged crushed material is existing gravel, crushed stone, or selected excavated materials. It may be used for aggregate base or aggregate shoulder.
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.
Surplus and Unsuitable Material: Surplus and unsuitable material is excess material produced by the project that cannot be incorporated into the project and for which the Contractor will be paid to remove.
General Overview
Hot mix asphalt (HMA) base crushing and shaping is an operation where the existing HMA pavement is turned into an aggregate base for new pavement, providing a sustainable and cost-effective method for reusing old asphalt pavements. This technique involves breaking up the existing HMA surface, along with a shallow layer of the underlying gravel base, into a uniform, specified size. Specialized machinery crushes and blends this material onsite. The resulting material is then graded and compacted in place to the dimensions, limits, and profile shown on the plans. Sometimes, additional aggregate may be added to meet project design requirements and any surplus material may be used elsewhere onsite or removed. Any unsuitable crushed material must be removed from the site. Once the material has been graded and compacted, a new HMA pavement is placed on top.
MDOT’s HMA Crushing and Shaping video has further information and visuals.
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
In the event additional base material is needed to construct the profile as shown on the plans, a dense graded aggregate, such as 21A, 21AA, or 22A, is required.
Photos of Materials
Pictures of Materials
EQUIPMENT
The following equipment can be used for HMA crushing and shaping operations:
- Pavement crusher
- Front end loader
- Bulldozer
- Grader
- Roller
- Water injection system
- Water truck
- Transit or laser level
- Leveling/grade pole
- Cold milling machine (as approved by Engineer)
Description of Equipment
A self-propelled rotary reduction pavement crusher is one of the main pieces of equipment used for this operation. The machine, which includes an enclosed rotary drum with teeth, is designed to crush the existing HMA pavement and mix it with the underlying aggregate base. The drum height must adjustable to the depth of mixing called out in the project documents and have the capability of injecting water into the mixing chamber to facilitate better density compaction.
The crushing machine being used by the Contractor will affect the quantity of additional crushed material produced through bulking, as well as particle size and gradation. The amount of bulking of the crushed material has historically been approximately 20% of the crushing depth.
A cold milling machine may be used in specific situations and is subject to Engineer approval. Final trimming is typically completed by a grader. If the Contractor elects to use AMG, the Inspector should ensure the Contractor is familiar with the requirements detailed in the special provision. If AMG is not being implemented on the crushing operation, the equipment must be able to reference and average the existing surface.
A water truck is typically used to wet the material and prevent dust.
Photos of Equipment
PRECONSTRUCTION
Preconstruction meetings are held for the Engineer and Contractor to discuss and understand installation requirements, stockpile management, equipment being used, 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.
- Review the specifications and any included special provisions for the work.
- Compare the plan quantities to the locations called out to perform the work.
- Verify whether the project will require milling a portion of the existing HMA surface prior to crushing and shaping.
- Verify a surveyor has provided stakes for the work. This is especially critical in horizontal curves where superelevation corrections may be called out.
- If AMG is used, verify the plan is in place and review the work with the Contractor.
- Review the plans, soil borings, and the Special Provision for Maintenance of Traffic. There may be restrictions, staging requirements, or provisions for porous soils. 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 stockpiled materials are protected from weather and sunlight as required by the specifications and manufacturer recommendations.
- Verify equipment to be used meets the project requirements.
- Obtain photographs of material tickets, storage method, and location.
- Hold an onsite meeting with the Contractor to discuss:
- The construction methods that will be used to complete the work.
- Whether excess crushed material will be placed on the existing shoulder. If this is required, verify all inspection work associated with preparing the shoulder has been completed and accepted prior any excess material being place on the shoulder.
- Equipment that will be used.
- Required traffic control measures, proximity of traffic to the work area, and maintaining driveways 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 excavated 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
Crushing
Crushing should be performed in stages if the depth exceeds 12 inches. The initial stage is crushing the HMA pavement. Subsequent stages include windrowing the crushed HMA pavement on the shoulder, then crushing the underlying layer and mixing it with the windrowed material. An alternative is to mill any excess HMA pavement from the grade and pay for it as Cold Milling HMA, Surface.
In accordance with Subsection 305.03.B of the Standard Specifications for Construction, 1 to 2 inches of existing aggregate base must be crushed with the HMA pavement. No particle sizes should exceed 4 inches in the largest dimension after crushing with 95% of the particles having a maximum size of 1-1/2 inches.
Shaping
Grading the crushed material should be referenced to either grade stakes or the existing pavement surface elevation at the centerline to attain plan grade or cross-section. Excess crushed material should be windrowed on the shoulder for re-use or removal by the Contractor. The crushing depth and grading requirements should be monitored to guard against excessive generation of material. If additional material is required to obtain the section required, the Contractor must use a dense-graded homogeneous material.
Excess windrowed material should be removed from the shoulder as soon as possible. The Contractor must cut a sufficient number of drainage trenches through the windrowed material to avoid ponding of runoff and saturation of the base. Upon removal of excess material, the shoulder will be graded to a safe, usable condition.
Compaction of this material type is moisture sensitive and adequate moisture must be present. Final trimming with an automated grade and slope control apparatus is accomplished after compaction requirements have been met. The Contractor is responsible for the condition of the crushed base and for maintaining the base in an uncontaminated, smooth, compacted condition, true to line, grade, and cross-section until placement of the next layer.
INSPECTION & TESTING
Inspection
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
- 10-foot straight edge
- 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:
- Observe crushing of the existing HMA pavement for conformance to the project documents.
- Confirm the particle size is uniform in thickness with 95% to 100% of the material being 1.5 inches or smaller. The maximum size allowed is 4 inches.
- Verify the crushing depth is in accordance with the project specifications.
- Inform the Contractor of any oversized particles. These particles must be removed, or re-crushed, using a slower crusher speed or maintenance of crushing machine teeth.
- Document replacement of drum teeth and note the crushed uniformity prior to and after the changing of the teeth.
- Ensure the Contractor is taking measures to prevent dust during crushing and shaping operations. A water truck is typically used to wet the material and prevent dust.
- If there are any drainage structures in the area, ensure they are protected from runoff if a rain event were to occur.
- Crushing and shaping operations should be halted if a rain event that could cause the material to become destabilized is expected.
- 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, 98% of the maximum unit weight with a moisture content not greater than optimum is required as established with:
- A T-180, as detailed in the Density Testing and Inspection Manual.
- Michigan cone density testing procedures in areas where 100% new aggregate is placed.
- 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 use onsite.
- Complete quality assurance verification of the trimmed grade and document results of these checks. Possible checks include:
- Cross-section checks.
- Ensure the surface is accurate to the cross slope specified in the plans.
- Confirm milling depths are in accordance with the plan if the project details include HMA milling prior to base crushing, to ensure adequate existing material is available for the crushing operation.
- Level to within 1/2-inch with 10-foot straightedge every 50 feet minimum, or more often in superelevated and transition sections.
- Longitudinal final grade checks.
- Parallel to the alignment, ensure the surface is level to within 1/2-inch with 10-foot straightedge every 50 feet minimum, or more often in superelevated and transition sections.
- If AMG is used by the Contractor, a surveyor with a Total Station should be utilized to check surface elevations of the crushed material.
- Depth checks.
- Cross-section checks.
- Verify all undulations or variations in grade are corrected prior to paving.
Testing
The crushed material should be tested to confirm compliance with Subsection 305.03.B of the Standard Specifications for Construction and the Density Testing and Inspection Manual. 95% of the crushed material should have a maximum particle size of 1-1/2 inches, and the remaining 5% should contain no particles larger than 4 inches.
The material should be compacted to at least 98% of the maximum unit weight and maintained until the HMA surface is placed. The Inspector should ensure the minimum number of required density tests are being performed per the Density Testing and Inspection Manual.
MEASUREMENT, DOCUMENTATION & PAYMENT
Measurement and Payment
HMA Base Crushing and Shaping is measured based on the plan width of the roadway. The item is paid for in square yards.
If the plans call for salvaging or removal of materials, payment is based on loose measure cubic yards.
Documentation
The Daily Work Report (DWR) should show all computations, measurements, and supporting documentation needed to verify the quantity submitted for payment. Density and gradation reports are primarily used for acceptance.
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 results.
LOCAL AGENCY PROJECTS
If this item is being completed for a Local Agency, review the plans and contract documents to determine if the agency has additional requirements or procedures.
RAIL PROJECTS
-Reserved-