Pavement Success Relies on a Properly Prepared Base

By Bob Cramer and Nicole Rein, Pave Tech

Measuring Excavation: It is important to excavate to the proper depth, measure with a level and measuring tape as shown here. Photos courtesy of Pave Tech

When creating a beautiful segmental pavement to last a lifetime, the biggest mistake you could make is not properly preparing the base. Although all components in a segmental pavement work together, none are as important as the base. Just as you wouldn?????t want to build a house on a weak foundation, the same is true with segmental pavements. Future pavement success relies on a properly prepared base.

You need to start by excavating the area and preparing the soil sub-grade. The sub-grade is equally important to the base because all imperfections that are not fixed in the sub-grade will show through to the base. For a residential walkway or patio, 4” is the minimum depth of the compacted base, more if you are in poor soils. The base material should be crushed 3/4” minus, which means that the base material is composed of particle sizes that range from 3/4” diameter all the way down to fine dust and a mix of all sizes in between. If the compacted base depth was specified to be 6”, the total required excavation would be 9” from top of pavement (6” base, 3/4” of bedding sand after compaction and 2 3/8” for the paver). The base would then need to extend beyond the edge of the pavement by 6” as well. Always extend the base a distance equal to the base depth.

 

Hand Excavation: Using a flat head shovel helps keep the sub-grade smooth and flat.

Most of the excavation can be done with a skid steer loader. Always use a sharp, smooth bucket on the skid steer loader, especially in clay soils and hand dig within 24” of a marked utility line. For small projects, or when a skid steer can not be used, use a flat shovel to be able to ???skim?????(R) the soil. The final excavation should be smooth and flat.

Now that the soil has been excavated, it needs to be compacted. A compactor with a minimum of 5,000 lbs centrifugal force should be used to compact the loose material from the excavation. If any areas were excavated too deeply, do not put soils back in to fill the low spots. Fill these areas with base material and then compact. In order to make sure that the pavement moves uniformly during freeze thaw cycles and under loads, a uniform base thickness is required.

Rake Base Material: When spreading loose base material, use a rake with a smooth, solid head.

After the soil has been compacted, it is time to install a woven geotextile fabric. Woven geotextiles are good insurance for clay soils and when dealing with new construction. In clay soils, it prevents the clay from pumping up into the 3/4” minus base material and degrading the strength. On new construction jobs, it can help prevent future settling. Only use geotextiles between the soil and the base. Make sure you extend the geotextiles up the sides of the excavation. When stretched tightly, it will provide a safety net in case the sub-grade settles. The fabric will support the weight of the base, bedding sand, pavers and the traffic on top of the pavers and spread loads over a larger area.

Once the fabric has been laid, spread the first ???lift?????(R) of base material. The thickness of the lift (layer) of loose base material is determined by the type of compaction equipment being used. A compactor with a minimum of 5,000 lbs centrifugal force will effectively compact about 3” of loose base material at a time. Larger compactors are able to do thicker lifts of base material at a time. In order to see where the final lift should stop, measure down from the top of the final pavement 3” (3/4” bedding sand after compaction and 2 3/8” for pavers). Make a mark on the side of the excavation with a line using a marker or crayon.

Check Base Level: The base should be smooth and flat with no more than 3/8" gap anywhere along the straight edge and the base.

Moisture is very important to achieve proper compaction of base material. Water works as a lubricant to move the base particles together. Pick up a hand full of the base material on the job site and squeeze it together. If it falls apart when you open your hand, it is too dry. If it stays clumped together, there is enough moisture. If water runs out, there is too much water.

After the base material is spread out and raked smooth, compact the first lift by starting around the outside and work in circles toward the center. Always overlap each pass by half the plate compactor?????s width. Repeat this a second time, again starting on the outside. After the second compaction is complete, start compacting back and forth and diagonally. When finished with the base, it should be very smooth and flat. If you were to put a straight edge flat on the surface, there should be no more than 3/8” (1cm maximum) gap anywhere along the straight edge and the base.

The Proctor density test defines the maximum density of a soil at an optimum moisture content. This diagram illustrates the relationship between percent moisture and density for a particular soil sample.

Edge restraints are designed to be installed before the sand and pavers are laid. With the edging in place before you start screeding, you are able to use Pave Edge as a guide to screed off of, saving time and labor by eliminating over screeding. Also, setting the edge restraint first will help create a smooth curve, and is also a good time for the homeowner to approve the final layout. Edge restraints should never be installed on top of the bedding sand, and it must always be spiked in directly on the base material.

Once the edging is installed, the bedding sand needs to be screeded. The bedding sand should be a coarse, washed concrete sand. The best way to screed sand is with 1” outside diameter metal pipes directly on top of the compacted base. Using a screeding tool, pull across the top of the pipes to strike off the sand. Fill and trowel voids. Only screed as much sand as you can cover with pavers that day. Do not attempt to level any area or surface irregularities with the bedding sand. This will result in an uneven final surface and unwanted settling. Once the bedding sand has been screeded, it is time to start laying pavers.

Sand Screed: Make sure your screeding tool is flat and straight.

Following these steps will provide your installation with a properly prepared base to build upon. The more time and effort you put into the preparation of the base, the better the project will look and the longer it will last. But as important as the base is, all components of a segmental pavement will work together to keep it looking beautiful for years to come.

For more details on installation instructions please visit www.pavetech.com

Building Blocks

15: Years, the approximate longevity of asphalt (when installed on driveways). Application takes one to two days, and vehicles use can occur after one week.

25 to 30: Years, the expected durability of concrete. De-icing salts can deteriorate surfaces, however.

20 to 100: years, the expected durability of bricks or stone. Installation time is about two weeks for 1,000 square feet of flagstone, Ten days for brick. Source: Welcome Wagon