When preparing a paver, retaining wall, or other project that requires aggregate as a base there are questions as to what aggregate you should use and why. There are different gradations, sizes, shapes, aggregate with sand / fines mixed in, and aggregate without. With all of the options to choose from, gravel suppliers willing to sell you whatever you want, and inexpensive options not always being the best solution. It is important for the longevity of your project to know what aggregate you are using for your application.
Throughout this article we highlight dense graded and open graded aggregates, what they are, various examples of each, and why they are suitable for hardscape applications such as pavers and retaining walls.
Dense Graded Aggregates
Dense Graded Aggregates are gravels that have fines / sand / dust mixed into it with granules that are larger depending on the gradation of that aggregate. These are used in traditional installations of pavers and retaining wall projects and require moisture when compacting. The compaction rate of these gravels are measured using the Proctor Density Test to measure the optimum moisture content to which the gravel can be compacted to its maximum dry density.
Dense graded aggregates are more susceptible to washout, insects, and weed growth due to the nature of having fines mixed in with the larger granules. This makes it not the best option for a project like a raised patio where the gravel will be above the finished grade contained by a retaining wall. But with small openings between the retaining wall blocks, the fines of a dense graded aggregate can washout through the face of the wall causing settling in the future of the patio.
Examples of Dense Graded Gravel
Applications of Dense Graded Base
A dense graded base can be used in any application when it comes to a paver, retaining wall, or hardscape project. It just may not be the best option. We like it best for driveways and retaining walls that do not have a lower area to drain water to.
For driveways where we use a paver that has a thin joint, it is important to use concrete sand as a bedding layer for your pavers to sit on. This is because interlock of those pavers partially comes from the pavers being compacted into the bedding layer which allows the sand to come up through the bottom of the joint to meet the jointing material from the top down. With a thin joint paver, the alternative to concrete sand would be a 1/4″ chip which is too thick for thin joint pavers to compact into so you are unable to achieve full interlock of the paver system.
For retaining wall applications that do not have a lower point to drain water to, a dense graded base is likely the most suitable base option because the drainage pipe in behind the wall that collects and moves water away from behind the wall can be elevated on that base and drained through the face of the wall. This is because a dense graded base does not drain water as fast as the alternative base material (open graded base, which we will discuss next). With open graded base, the drainage pipe is placed within the base because water will flow through the base freely and collect at the bottom of the base. This means you need a lower point in comparison to the drainage pipe that is in the base in the property to be able to drain that water to.
Open Graded Aggregates
Open Graded Base is one alternative to a dense graded base. This is constructed using open graded aggregates that are defined by being clear or clean of fine materials. They are washed so that these fine particles are minimal. This allows water to not be held by the aggregate and will freely drain through this material. The benefits to open graded gravel is that it is freely draining so it will minimize freeze-thaw cycles, insects, and weed growth. They also do not require as much compaction in comparison to dense graded material. The downside is that they are more expensive considering they typically need to be washed / screened in addition to the processing of the gravel.
They also compact differently than dense graded aggregates. Because compaction measured using the Proctor Density Test is measuring moisture content which is held by fine materials, the same test cannot be applied to open graded aggregates that do not have fines holding onto moisture. Instead, the material is rearranged in the compaction process to allow for the angular particles to interlock with one another to achieve compaction. Compaction of other dense materials is removing the air out of the gravel in the compression of those materials, but with open materials there will always be air / void space between the granules. These gravels also require less compaction compared to dense gravels due to their angular and open nature.
Examples of Open Graded Gravel
Open graded base aggregates used in hardscape bases include:
- 3/4″ Angular Crushed Clear Stone
- 1/4″ or 3/8″ Angular Crushed Clear Stone / High Performance Bedding
Applications of Open Graded Base
Due to the drainage characteristics of open graded aggregates, they are best used on projects where there is a lower point to be able to drain water to on the property or where the subsoil beneath the project is sandy. This is because the aggregate allows water to flow freely through it so there needs to be adequate slope on the excavation bottom so water does not pool in the base. Additionally adding a drainage trench towards a lower area on the property away from the base can help move water out of the project area. Open graded gravel is also best used in drainage projects such as French drains because of the amount of void space that it allows for water to flow freely through.
We use these gravels for the majority of our hardscape projects including paver patios, walkways, and retaining walls where there is a lower point in the project to be able to drain to effectively.