Base material / setting bed

Discussion in 'Hardscaping' started by kbremn71, May 7, 2005.

  1. kbremn71

    kbremn71 LawnSite Member
    Messages: 14

    In the past, when laying brick walks, I used only washed sand as a base material. I would excavate 4" to 8" and fill with sand, tamping as I went. Screed a final layer then set the bricks and wash sand in after. These were all clay brick applications in residential settings. Most still look very nice, several years later.

    I am getting ready to install a paver walkway in a more commercial setting and I would like to make sure that the installation will hold up to heavier traffic. I've read the installation guide from the paver mfg (Belgard) as well as many of the treads posted here. I have a few questions that I'd like to verify however.

    First, what is the purpose of the base layer or foundation layer? If I'm looking for a solid compact base layer to set the installation on (the Belgard guide mentions applications that are on concrete slabs), I would think that I could use crusher or stone dust as the base layer in stead of crushed stone and then washed sand as my setting bed? It seems that the crusher dust would level out and be easier to work with than crushed stone would?

    Second, is a geotextile layer necessary in all situations. If the subsoil is well compacted, gravely or sandy and in an area that is not prone to moisture buildup, can it be eliminated?

    Third, what is your experience with the durability with pavers as far as winter de-icing damage. I plan to apply a sealer to the pavers which, according to the mfg, will protect them from salt damage. Is this your experience??
  2. Rex Mann

    Rex Mann LawnSite Senior Member
    Messages: 621

    When we build pavement systems using interlocking concrete paving stones we are trying to create a hard surface, which will last for years. As we build a system each layer is weaker then the previous layer. The pavers are stronger than the concrete sand, the concrete sand is stronger then the aggregate base and the aggregate base is stronger then the soil. The key point to the previous sentence is simply: the soil is the weakest part of the system.

    The role of aggregate is to absorb and displace the loads on the pavement surface over the soil. Without the use of aggregate a failure know as rutting can occur on vehicular applications or the washboard effect on pedestrian applications. These 2-types of failures will show up under the direct path of traffic over a pavement surface. In a driveway rutting will occur coming in and out of the garage stall.

    How do we prevent these failures?

    Using the properly graded aggregate along with the proper thickness and a geotextile, if needed.

    Base material should be any material your local DOT use for its road building. Typically, that is 1-inch down to fines. Fines, any material passing the # 200 sieve, as defined by the ICPI, should range between 5-12 percent.

    Fines play a crucial part in the base. Fines absorb and hold moisture. We need moisture to achieve maximum compaction. The moisture acts as a lubricant, which helps rearrange the aggregate during compaction. To little moisture or not enough will lessen the amount of compaction, which can be achieved.

    In a freeze-thaw environment to much moisture during the cycles can lead to heaving. Using a aggregate base within the range of 5-12 percent will allow for enough moisture to be held for sufficient compaction and allow for adequate draining reducing the amount of moisture held. Those reading this and have taken the ICPI class will remember this as the Moisture Density Curve.

    Geotextiles are generally used in soils, that have are categorized as clay soils according to the USDA soil classification chart. The primary function of a geotextile is to eliminate the soil from migrating up into your aggregate and also keep the aggregate from migrating into the soil. Sometimes the geotextile will be refereed to as a separation layer or material. Because, it separates or keeps separate the soil and aggregate.
    Geotextiles are very cheap insurance against unwanted settling especially in freeze-thaw climates. Many geotextile manufactures claim, that by using there product the thickness of the aggregates base layer can be reduced. This not the case. You can not reduce the thickness of your aggregate because you use a geotextile.

    Pavers, quality ones, will hold up to salts and other deicing materials. My experience is, that you do not have to seal them to prevent damage due to salt or other deicing materials.



  3. YardPro

    YardPro LawnSite Gold Member
    Messages: 3,570

    uh, what he said LOL
  4. kbremn71

    kbremn71 LawnSite Member
    Messages: 14

    Is this on purpose or just the way that it works out? One of the installation guides I read mentioned some variations in the installation process if laying on a slab. Wouldn't this slab make the bottom layer stronger than the setting bed? And if so, what would this do to the final product?

    So what makes 3/4" stone absorb and displace loads better than a more solid layer such as stone dust?

    My local supplier (a road construction company) sells 3/8", 1/2" & 3/4", etc. crushed stone, gravel, stone dust and and washed sand. So are you saying that I should be using "3/4" crushed stone" or should I be mixing it somehow to get more size variation and these "fines" that you refer to. Also, won't the washed or concrete sand, with time, wash down through the foundation layer?

    So lets say I was going to install a 20' x 5' walkway entrance to a building. I excavate down 8" to a good solid sub-base of packed gravelly soil and cover it with geofabric for good measure. Then fill and compact the area with 5" of stone dust. This creates a level hard packed surface. Then I cover this with a 1" layer of sand, level and lay my pavers. I then tamp the pavers and sweep sand into the joints. What should I expect to happen to my walkway in say a year or 5 or 10? Now lets say I doubled the foundation layer to 10" of compacted stone dust. What should I expect now? What if, rather than the gravelly sub-base, I was on packed clay, would this change the outcome?

    I don't ask these questions out of disbelief as it might sound, but rather, I am just trying to get an understanding how the whole walkway/patio system works & behaves and why the recommendations are what they are.
  5. mbella

    mbella LawnSite Bronze Member
    Messages: 1,284

    Kbremn71, take the ICPI course and it will answer all of your questions.
  6. kbremn71

    kbremn71 LawnSite Member
    Messages: 14

    Because the only course listed is this month in Arizona - bit of a drive for me - it being so busy and all...
  7. Rex Mann

    Rex Mann LawnSite Senior Member
    Messages: 621

    I am teaching the Arizona class this Thursday and Friday.

    I was referring to a traditional paver installation, aggregate base, sand and pavers. An overlay can provide a great base, however, in freeze-thaw environments it can crack and heave. Thus, the pavers will mirror the movements of the underlying concrete.

    We use stone of various sizes to aid in compaction and, the stone should be angular as opposed to "round". This will also help with compaction. And, do not forget the 5-12 percent fines.

    This website ICPI.ORG will answer your questions better than I can.



  8. AWJ Services

    AWJ Services LawnSite Platinum Member
    from Ga
    Messages: 4,281

    Call the ICPI and see were the next available test is that is in your area.

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