Base Preparation - A Solid Foundation
The first thing to consider when installing rigid surface like asphalt or concrete over any area is the foundation or base. Without a solid base, it doesn't matter what you do with your surface material; it will simply not hold up. The primary material used to create a solid base is Gravel, primarily a mix of stone and sand, which creates our solid foundation. Depending on the project, different mixes of gravel are used to provide the support needed for the asphalt pavement.
If the existing ground where the pavement is to be placed is insufficient in its strength or water permeability, then it must be excavated and replaced.
The steps to a successful gravel base installation:
1. Excavation to the appropriate depth to fit both the specified amount of gravel, and asphalt or concrete.
2. If the virgin soil is disturbed it can be helpful to pre-compact the area.
2. Back fill the hole with the appropriate gravel material and get a rough grade.
3. Compact the gravel, for thicker gravel bases this may mean multiple layers of compaction to achieve maximum compaction.
4. Add a final thin layer of gravel for fine grading up to the finished elevation and ensure a proper grade.
5. Final compaction of the gravel base, to provide an even, stable surface on which to pave.
1. Why Gravel Works Best for Paving Foundations:
Gravel offers a combination of properties that make it the optimal choice for a solid and stable base:
The stones in gravel serve as barriers for the sand, preventing the smaller particles from shifting when pressure is applied from the top or water flows through the gravel. This helps the base to stay in place and maintain its structural integrity.
The sand in the mixture ensures that water can flow freely under the surface, preventing pooling and the associated issues, such as frost heave or soft spots in the base. This is critical to the long-term performance and durability of the paved surface.
A tightly packed base is essential for withstanding heavy loads and minimizing shifting. The small particles in the gravel mixture, such as very fine sand, natural soil, or recycled concrete, allow for close packing with minimal gaps, providing the necessary strength and stability. Recycled asphalt, with its malleable bitumen content, can also contribute to the packability of the base by conforming to the shapes of the surrounding materials.
2. Understanding Gravel Types:
There are two main "types" of Gravel used as asphalt pavement bases. "A" and "B" gravel, otherwise known as Granular A or B, the primary difference is the maximum size of the stones in the mixture. A-Gravel uses up to 3/4 inch stones, whereas B gravel uses up to 2-inch stones. The larger the stone, the more resistant the base is to movement due to water or pressure from the surface, but the harder it is to have a consistent smooth and fine-graded surface on which to install the asphalt. The quantity and which materials are best for a particular project are determined by the expected use, existing ground conditions, and costs to value.
"A" gravel, also known as Type A or Granular A, is a type of aggregate material commonly used as a base layer for paving projects, such as driveways, parking lots, and roadways. It is composed of a well-balanced mixture of approximately 3/4-inch crushed stones, sand, and sometimes additional packing-friendly materials like recycled asphalt or concrete, clay, or regular earth. This combination of materials offers excellent stability, drainage properties, and packability, making it an ideal choice for creating a solid and stable foundation for asphalt pavements.
The crushed stones in "A" gravel interlock effectively, providing stability and load-bearing capacity, while the sand ensures proper drainage and prevents water pooling. Additionally, the packing-friendly materials contribute to the tight packing of the base layer, further enhancing its strength and durability. Overall, "A" gravel is an essential component in the construction of a high-quality and long-lasting asphalt pavement.
On light-duty driveways or parking lots, which are not high traffic use like roads and do not see the same heavy traffic, 10"-12" of A-Gravel will provide a base more than capable of maximizing the life of the paved surface. In some cases, the soil in the area may be less stable due to high quantities of impermeable clay and may require either a greater amount of gravel or the addition of larger stone over the lower layer of gravel.
3. Determining the Appropriate Base Thickness:
In most light-duty paved surfaces, we want to consider the base down to the 12-inch mark. Often the sub-base is more than stable enough to do what it needs to in order to hold up for as long as the surface can. For concrete a light duty base thickness can vary from 6 inches for primarily pedestrian use to 12 inches for vehicular traffic.
For heavier use, a slightly deeper base to increase the load spread can increase the life expectancy of the pavement or concrete.
While not exhaustive, the two main reasons for a thicker base are heavy traffic and clay deposits.
When we say heavy traffic, we literally mean weight, but also very high dynamic use. If a surface will be driven over by heavy equipment or transport trucks, it is often good to have an engineered sub-base of gravel fill or B-Gravel that goes deeper. The larger stones provide more stability under heavier loads but maintain the same drainage properties of A-Gravel. The most common upper-end specification for medium to heavy-duty parking lots is 12 inches of B-Gravel, topped with 6 inches of A-Gravel.
Clay deposits are areas with so much clay that they will hold too much water and do not act as a stable enough sub-base. This can be over a whole driveway or parking lot, or just in a small section. It is the most common reason for us to go deeper than 10-12 inches in a light-duty application like a driveway. Again, our experts can detect these areas by how the surface moves during construction or has moved in the past, and the solution is easy: dig out enough of the clay and fill the hole with stable gravel to better spread the load over the sub-base.
The use of Gravel in the foundation of paving projects is crucial for creating a solid and stable base that can withstand the test of time. By understanding the properties and benefits of this versatile material, contractors and property owners can make informed decisions and ensure the longevity and performance of their paved surfaces.