The strength of the natural subgrade below is often not durable enough to suit long term pavements. Because of this, limestone aggregate bases typically granular in nature and with good gradation are placed and mechanically compacted to increase their density. The overall finished product is a durable, uniform, and long term substrate and base for the support of the pavement section and traffic above.
If deemed suitable for soil type/condition, the subgrade is graded, shaped, and compacted prior to aggregate base. Gradation is the amount and size of aggregate which make up an aggregate base material. The most common aggregate mixture for pavement base meets the following gradations for amount of crushed limestone material passing through different size sieves:
100% passing 2”
70-100% passing 1”
50-90% passing ¾”
30-60% passing 4.75mm
9-33% passing 600 micrometers
0-15% passing 75 micrometers.
Due to its specially engineered gradation, this material is most suitable for achieving the right balance of aggregate sizes to reach a compacted density and strength that is the most durable as a pavement base . Other gradations are also fairly common which are similar to the above but varied slightly.
Aggregate bases thicknesses for new pavements typically range between 4-12” or more depending on the pavement material being specified above, engineers design requirements, and purpose of the future pavement. Asphalt pavements tend to require a thicker base (6-10″) than concrete pavements to maintain a longer pavement life as they are flexible and transfer the loads in a more localized fashion. Concrete pavements often require a slightly less aggregate thickness (4-8”) as concrete has higher compressive, tensile, and flexural strength and can transfer the load over more area below.
Aggregate base requiring compaction is typically placed in lifts varying from 4-8”. Lift thickness is commonly driven by weight of the rollers that will be used for compaction. A rule of thumb for compaction effectiveness is:
Greater than 12 ton vibratory Rollers – 8″ max lifts
10-12 Ton vibratory Rollers – 6″ max lifts
Non Vibratory rollers – 4″ max lifts
It will make the most sense to compact in equal lifts if the above maximums exceed the design thickness.
For mass aggregate placement, the typical crew will consist of a foreman, a laborer, and 1-2 operators. The most common order of operations is :
- Rough placement with dozer
- Further gradng by grader (optional and project specific)
- Roll/compact material
- Repeat for multiple lifts
- Fine grade with grader and/or smaller equipment (skidsteer loader, handwork)
- Roll/compact one last time
Material placement and spreading means might consist of a ‘spreader box’ attachment to a piece of equipment, paving machine, dozer, and roller. Graders may be used as well but are aimed for at the finish grading phase.
For smaller areas; loaders, backhoes, skidsteers, excavators, or hand placement may be most efficient. Each lift is typically watered to achieve its optimum moisture content then compacted based on the maximum dry-density of the material, which is usually between 90-100% of maximum compaction available. in roads and high-load vehicle application, 97-100% possible compaction (per the materials dry density) is common. For driveways, parking lots, and other light load applications, 92-96% compaction is commonly considered sufficient.
It’s important not to compact or pave on an overly saturated or ‘spongy’ subgrade as not only can rutting occur during material placement, but a spongy or ‘segregated’ subgrade under aggregate base is not conducive to a long term durable pavement section.
Also, poorly-draining, overly moist, high-fine clay and other cohesive soils under aggregate bases tend to retain moisture, becoming freeze/thaw vulnerable and generally unstable long term. In these situations of unsuitable subgrades; time and sunlight or undercut/replacement may be needed to rectify the poor subgrade. Also in these situations, having good drainage (and even perimeter underdrains) becomes critical to pavement durability and long-term success.
Aggregate base can actually be over-compacted if the roller takes too many passes (often because the foreman or operator is having a hard time getting passing compaction tests). Over compacting causes segregation and fines to sink down below, creating ‘bony’ areas of larger aggregates. Oversaturation (and eventual fissuring) can be expected for overcompacted areas with segregation and high fines. Both of these scenarios result in a decrease in density and stability of the base.
Once rough placement and rough compaction is complete, finish grading will commence where a grader and a roller will get final aggregate elevations as close as possible to desired grades. The grader will have it’s blade set to a predermined grade and cross-slope (if required) and will scrape the surface at peaks in the rough stone grade to achieve final finish grades, often +/- 0.5″ or closer. More roller passes may be needed after finish grade is believed to be achieved.
The placement of aggregate base is critical for many reasos:
-Grading is critical as many pavement placement operations are dependent on the aggregate base grades being very accurate, particularly asphalt paving done without grade reference systems (governed by paver wheels/tracks only). Having accurate grades is crucial for assuring the proper design longitudinal and transverse cross slopes are achieved with the final pavement material. This is also important for concrete crews as they too have a screed which follows the existing grade below or the top of the adjacent forms nearby. The aggregate grades below will be reflected in the final pavement elevations.
A paving crew foreman should be cognizant of the grades under him being correct, but overall should have to worry very little about grade issues so he can focus his attention on proper installation of paving material and uniform pavement thickness. He should be able to trust the skill and experience of the aggregate base crew before him.