When comparing traditional interlocking base to a contemporary style of base I have noticed a few major problems.
Traditional bases are build up from a compacted sub base, six to eight inches of ¾ clear crushed not rounds compacted every two inches, three inches to four inches of granular “A” compacted twice on two separate layers the last layer is limestone screenings 2” maximum in my opinion deserves a less is more technique when applying this layer.
Limestone screening is a fine product about ¼” of an inch in size with the same limestone fines as ¾” crush. It is used as the final grade on the top of the base for interlocking patios, driveways, and under patio slabs. It is often known as the product used on base ball diamonds.
3/4" Crush Gravel
¾” crush gravel contains limestone fines that when compacted create a solid base. It is used under driveways, interlocking pavers and slabs, patio stones, interlock retaining walls, armor stone walls, etc
Illustration of a traditional interlocking base,
After observing projects constructed with this style of base I have come to the conclusion that less granular aggregate is better. The Canadian climate is harsh on all exterior applications due to the seasonal expiation and contraction. I have found the early spring melts will allow water to seep into the limestone aggregate, or in and under the cracks of the interlocking or wall stone separating the block from the limestone. I have found if heaving will occur, this is the time of year when it will happen. There are many reasons why heaving will occur as well as dips, but the fundamental reason has two components. One being the granular aggregate and the other water, on the edge of raised patios as well as leaks from ease troughs will eventually wash out this granular aggregate. This is just two simple examples of how water can erode an aggregate base. Aggregate on its own has problems one being the inconsistency in the mix of limestone dust to the chip stone. If water and frost does not erode the stones, ants and other bugs surly will. The soft lime dust is perfect for red ants and the only way to get rid of them is boiling water and picking up the stones clean out the nest and re-levelling them.
I believe this traditional base has its place in construction, in my opinion this place is not in a Canadian climate.
Contemporary bases are made up with entirely clear crushed stone and offers superior compaction and drainage. The two main ingredients are three inches of HPB (high performance bedding) and six inches of compacted ¾ clear crushed (not rounds). If you wanted to you could in fact use HPB for the entire base but for the sake of saving money and depending on the square footage to go with the ¾ clear.
High Performance Bedding Aggregate (HPB)
HPB is a fine limestone product about ¼” of an inch in size. All of the limestone fines are washed out of this aggregate. This product is unique because it can be used to lay the larger slab interlock and patio stones, without needing to compact your last layer of base. HPB is 98% compact product when used in a base.
3/4" Clear Gravel
¾” clear gravel is a washed product that is used as a base only when compaction is not possible. It allows for drainage around foundations, behind walls, around and under weeping tile, under decks, drainage ditches, under concrete pads, can be mixed with concrete, etc.
Illustration of a contemporary interlocking base,
I will usually come to the conclusion depending on the subsoil, that a contemporary base will hands down offer more for compaction, drainage, time efficiency and disposal. In my experience with compacting HPB is that if you are only using two – three inches on your final layer of sub base than the only time you will need to tamp is after the pavers or wall stone is laid. Dufferin Aggregates the only Canadian manufacture of the clear washed chips stone HPB says that when you drop this material in place that it will automatically settle to a 90 – 95% rate of compaction. Our ideal compaction is 95 – 98% rating. So when I finish screening out my last two – three inches of HPB before laying my stones, then when I am finished my installation I will secure the outer solider course with Snap Edging and the fill the cracks with polymeric sand. After the sand has filled the gaps I will then do my final tamp. This final tamp will settle the stones no more than 1/4 so make sure to take this into account when you are marking your grades.
As an installer this is a more simple style of base to install and one that I have not yet had to do warranty work on. As a home owner, colder climates play hell with all interlocking exterior finishing’s, this base will save you money and headache in the long term.