When building an outdoor project and considering different aggregates, there is one common misconception we hear when it comes to self-compacting gravel. You should not hear the word self-compacting aggregate and think that your project will be fine with just laying this material out and achieving a compacted base. Compaction is still necessary to reorientate those granules to achieve compaction, it is just not the same compaction as dense graded materials. Let’s learn a little bit more about the myths around self-compacting materials, open graded aggregates, and dense graded aggregates.
Self Compacting Aggregate
The idea of self-compacting gravel is typically attributed to open graded aggregates. These are aggregates that are clean / clear meaning that they have no fines / sand in the gravel as opposed to a dense graded aggregate which does. Open refers to the void space that this gravel has when installed whereas a dense graded aggregate would have the fines / sand fill in that void space.
Open graded aggregates such as 3/4″ angular clean stone and 1/4″ or 3/8″ angular clean stone or High Performance Bedding are the two aggregates used in an open graded base installation for a paver or retaining wall project. An open graded base installation is beneficial for numerous reasons over a dense graded base, but there are still applications where a dense graded base is best used. The main reasons why an open graded base is beneficial is because the void spaces allow for water to pass freely through the material. This reduces the effects of freeze-thaw cycles that the base would experience from holding onto that water and the potential for washout of the fines / sand that would be present in a dense graded base.
Another main benefit is that these open graded aggregates require less compaction than dense graded aggregates do. Compaction is typically measured on a project that includes the moisture content of a soil / gravel. When measuring compaction of an open graded aggregate, the same measurement does not apply because the moisture flows through the aggregate freely.
However, the main misconception with these open graded aggregates is that they are self-compacting. THIS IS NOT TRUE. They still do require compaction. Just the compaction is more of a vibration to rearrange the angular aggregate in order for them to fit best and to interlock with one another. This differs from dense graded materials as they are compacted to remove void space or air out of them to achieve compaction.
Open graded materials need to be contained otherwise compaction will just move them laterally instead of downward. This is especially seen when compacting smaller chips where running a compactor over them can make it look like it is compacting, but it is also moving it outward. For our open graded base installations, we install a lift of 3/4″ angular clean stone and compact on top of a woven or non-woven geotextile to prevent migration of soils underneath into the base, then lay out a layer of biaxial geogrid, followed by another lift of the same aggregate and compact, and then a bedding layer of HPB which we then lay our pavers on top of. We then compact when installing our jointing compound to consolidate that compound as well as the HPB underneath.
Many believe that these materials are self-compacting or achieve 98% compaction when they are laid out. This is not true. On the How to Hardscape Podcast, we had Dufferin Aggregates talk to us about this exact statement and they claimed anywhere from high 80s to low 90 percent compaction and compaction is still necessary.
Another consideration is the gradation of the materials. Size and shape play an important factor in the performance of the aggregate for paver and retaining wall bases.
Is 3/4″ stone self compacting? No.
Is 1/4″ or 3/8″ stone / HPB self compacting? No.
Is pea gravel self compacting? Definitely not.
Though pea gravel would be considered an open aggregate because it is clear of fines / sand, it is round in shape. This means that the aggregate will not interlock between one another and will instead just roll under a load. In order to be used as a foundation or base for a paver or retaining wall these aggregates need to be angular in shape. You will sometimes find 3/4″ clean stone that has both angular and round materials mixed in it. This is not suitable for the base of your projects.
The importance of compaction remains when thinking about using these open graded gravels for your project. There are many benefits to them over dense graded materials, but compaction will always remain a process that needs to be completed when installing them.