When Not To Use Stone Dust
By Steve Pirie (Sales, Plymouth Quarries)
This article is not meant to be a complete step by step instruction for constructing a paver walkway, patio or driveway. (Plymouth Quarries has detailed instruction booklets available.)
Instead, the main focus will be on one aspect of the process - the proper materials used beneath the pavers that make up the base and the setting bed.
Of all the procedures in the construction of paver driveways, walkways and patios, one of the most misunderstood is what materials are best used as the base and the setting bed.....
A typical paving project consists of the following, after excavation is done: the base (the first layer which supports and keeps everything above it level), the setting bed (a thin layer that goes on top of the base and contacts the pavers, aiding in final leveling), the pavers (the item which consumes the most time in choosing the color, texture and shape.)
Certainly, the choice of a good quality paver (one that is dense enough to resist absorption of water and strong enough to hold up to the traffic to which it will be subjected) is important. But if the base fails to do its job, everything above it will be affected.
ICPI (Interlocking Concrete Paver Institute), the North American trade association representing the interlocking concrete paving industry was founded in 1993. Since then, it has been in the forefront by adopting standards for every aspect of the industry. One of ICPI's goals, taken from their website is, "to improve paver installation through education." Part of this educational process has been ICPI-sponsored seminars, classes, certification programs and a series of technical sheets available through their website HYPERLINK "http://www.icpi.org" www.icpi.org .
From their Tech Spec Number 10 they state, "Aggregate base materials should conform to that used under asphalt. If no local, state or provincial standards exist, then the requirements for aggregate base in ASTM D 2940 are recommended." This standard is quite technical, but simply put, it refers to products like "crusher run" or "processed gravel" which are a mixture of larger crushed stone (approx. ¾") and progressively smaller crushed stone down to the smallest particles called "fines" (about the size of a grain of sand). The mixture that makes up this material does two things: 1) it is highly compactable and 2) it drains well. A base that is not fully compacted will probably sink. A base that does not drain well will retain moisture and likely heave when freezing temperatures arrive. These are things you don't want happening after the construction of a walkway, patio or driveway (whether you are a do-it-yourselfer or a professional contractor).
That brings us to "stone dust" which is a by product of crushing stone. What has stone dust been used for? Answers.com shows in their definition, "(mining engineering) inert dust spread on roadways in coal mines as a defense against the danger of coal-dust explosions; effective because the stone dust absorbs heat." An article in The Progressive Review (published on the Web) says that "rock dust, a quarrying bi-product" has been the focus of research that claims "rock dust can replace the minerals that have been lost to the earth over the past 10,000 years..." Mixed with compost, it has created soils so rich they are "capable of producing cabbages the size of footballs, onions bigger than coconuts and gooseberries as big as plums." (So, if you want to keep your walkway from exploding or you wish to harvest carrots the size of a man's arm from your patio...by all means, use stone dust.)
Yes, stone dust has been used and continues to be used as both the base and setting bed in many patios, walkways and driveways. Armed with the knowledge I've gained from ICPI seminars, from paver manufacturers as well as my personal experience, I wandered around the Web looking for articles and blogs that preached "the evils of stone dust." I've got to admit, an overwhelming majority called for the use of stone dust. Yet, I am not surprised.
Stone dust, being a by-product of crushed stone, had no intended use in the beginning. Through sheer innovation, uses were found for lowly stone dust....in agriculture (for its mineral content), as an ingredient in earlier forms of mastic (due to its ability to hold moisture, keeping it from drying/setting too quickly), a polishing medium (for soapstone) and, of course, as a base material (for brick pavers).
Let's look at the characteristics of stone dust used as a supporting base material in paving projects. Its fine texture (almost powdery) along with tiny bits of stone (mostly 1/8"or less) would make it appear to be the perfect material for compacting.....
Actually, stone dust is more difficult to compact fully than crusher run, the aggregate base material mentioned earlier. Any base material, if not fully compacted, is likely to settle. Most contractors and do-it-yourselfers, who have worked with both stone dust and crusher run, would agree that crusher run is the winner for compacting. Once compacted, walking directly on it leaves no footprints and working on it barely disturbs it.
As for drainage, stone dust's powdery nature inhibits drainage. It absorbs moisture, holds on to it and drains very slowly. If freezing temperatures are present while the stone dust is still moist, the base will probably move (frost heaves). Crusher run, on the other hand, drains much better. The smallest particles, called fines, are more granular (not powdery). This, combined with the varying sizes of crushed stone, make a strong compacted base with enough voids to let water pass through more easily.
The setting bed is discussed in ICPI's Tech Spec Number 2: "Bedding sand under concrete pavers should conform to ASTM C 33... This material is often called concrete sand. Masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust....as they often do not compact uniformly and can inhibit lateral drainage of moisture..." Concrete sand (coarse granular sand) used as the setting bed (in 1" thickness or less) will prevent these potential problems. Also, in one of the final steps, that of leveling/compacting the pavers into the setting bed (whether using a mechanical compactor or by hand with a rubber mallet) the vibration it produces brings some of the setting bed material up into the joints, helping to lock in the pavers. Concrete sand is better suited to make this happen.
So why is stone dust still used as the base and/or setting bed in so many paver projects? First, stone dust is mostly used in smaller jobs, around the home and privately owned businesses. Large commercial jobs and government-associated jobs will have strict codes/standards to follow. You won't find stone dust as the base for asphalt highways, parking lots and driveways (or, at least, you shouldn't). The same is true for large commercial and government paver jobs. Some form of "aggregate base" will be used because testing and long-term experience has shown it to be the strongest, most reliable material in those applications.
Another reason stone dust is still popular is due to lack of education directing smaller contractors and homeowners to a better alternative. Remember, ICPI is less than 20 years old and has mainly benefitted paver manufacturers and companies involved in big projects. The information that they have gathered and published is slowly, but surely reaching those with smaller jobs to do. Besides, stone dust has been used long before ICPI has been around to enlighten the masses. And, old habits are hard to break.
What about the many jobs where stone dust was used and no problems have developed?
The key word is "many." (It doesn't mean "all the jobs" or even a "majority of the jobs.") There's no way of knowing what has happened to some of these jobs. If the stone dust were fully compacted (with a great amount of effort) on top of well-draining, stable sub-soil and the walkway, patio or driveway did not get heavy use, it is possible to have a problem-free result for several years. But I will speculate that, had the same job been done using crusher run instead of stone dust, the result would be problem-free for several years plus a few years more.
It is not uncommon to have customers come to Plymouth Quarries to get more stone dust to re-do a project that failed over time. So many people take it in stride, like painting a house every few years, they expect to re-construct their walkway or patio as a matter of course. Over 20 years ago I had a walkway professionally installed using stone dust (I confess, I didn't know about crusher run then). Every few years I'd be re-leveling at least part of it. It was a pain, but I expected it. About 3 years ago I had the opportunity to work alongside another professional, re-constructing my walkway once again. This time crusher run was used. While compacting it, I could immediately tell the difference; in the end it was solid. I'm confident that, with the crusher run in place, the walkway will stay in place.
Your project doesn't have to be huge to benefit from using materials that are specified for "big" jobs that ASTM (American Society for Testing and Materials, formed in 1898) recommends. And it certainly doesn't hurt to cash in on their years of expertise. And...currently crusher run and concrete sand average 35% less expensive than stone dust.
Using one particular material over another for a paver project does not guarantee how long it will last before problems occur. Sub-soil conditions and drainage, appropriate depths of base materials, proper compaction of base materials, loads that the materials are subjected to, the use of edge restraints.....all these factors enter into it. The more information, skill and experience the installer has (whether a homeowner or a contractor) the better the end result will be. For the homeowner - always get references and see completed projects before hiring anyone. For contractors - attend classes, read current trade literature (whenever possible), don't be afraid to ask questions or consider advise given. All these things will help to make your job easier and better.