Interlocking concrete pavers

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Interlocking Concrete Paver Driveway

An interlocking concrete paver is a type of paver. This special type of paver, also known as a segmental paver, has emerged over the last couple of decades in the United States as a very popular alternative to brick, clay or concrete.

Segmental pavers have been used for thousands of years. The Romans built roads with them that are still there. But it was not until the mid-1940s that pavers began to be produced out of concrete. It started in the Netherlands where all the roads are made to be flexible because the country is below sea level and the ground shifts, moves and sinks. Poured concrete is not an option because it will crack. Individual units not set in concrete placed in sand perform far better than concrete. Before the paver was made from concrete either real stone or a clay product had to be employed.

The first concrete pavers were shaped just like a brick, 4” by 8” (10cm x 20cm) and they were called Holland Stones and still are today. These units turned out to be far more economical to produce and were exceedingly strong.

How Are Interlocking Pavers Made?[edit]

Pavers are made from a very dry mix of gravel, sand, cement and color. Since there is very little water, the mixture is moved on conveyor belts and then funneled into the paver mold. A press is inserted into the mold and vibrated at the same time. The high pressure and the vibration causes the water to set the cement and creates a bind between the aggregate. When the mold is moved away, the pavers are in their finished state.

Pavers are said to have zero slump. The more the slump, the weaker the concrete. A slump test is done in a laboratory to test the strength of concrete. It is done by placing concrete in an 8” tall cone, which is then turned upside down and removed. The amount the concrete slumps from the original 8” is measured. For instance, if it now measures 6”, it is said to have a 2” slump. If one has a very wet mix, it would not hold up at all and would run all over the table.

The strength of concrete is achieved in three main ways.

  1. The amount of water (the less water, the stronger).
  2. The amount of cement in the mix (the more cement, the stronger).
  3. The amount and size of aggregate (the bigger, the stronger).

Concrete delivered in a truck needs to be fairly wet to get it out of the truck. Typical concrete from a concrete truck will produce a surface with a load strength of about 2,000 pounds per square inch (14,000 kPa).

A surface is tested for strength by placing a point load onto it and exerting a direct force until it fails. The pressure is calculated, and that is the failing point. Pavers need to withstand a minimum of 8,000 pounds per square inch (55,000 kPa) to meet industry standards. Usually when tested they far exceed this minimum. That means a paving stone driveway is at least four times stronger than a regular concrete driveway.

How Interlocking Pavers Interlock[edit]

When one thinks of interlocking paving stones one often gets the idea of dog bone shape stones that fit together like a jigsaw puzzle. That is not the case at all. The interlocking comes from the fact that each stone can not move independently from its neighbor. In fact there are two important aspects of an interlocking paving stone job that keeps the pavers interlocked.

Pavers Being lifted to demonstrate interlock

The first is the sand. As simple as this might seem the sand creates the interlock. Not any sand though, it must be a sharp angular sand (also called Joint Sand and Polymeric Sand) that will lock together. If you filled the joints between the pavers with silica sand, each particle is perfectly round, it’s like putting ball bearing between the pavers and they will then be able to move independent of the stone next to them. Pavers are made with a small ridge on the side of each one so there is space for the sand.

The picture below shows pavers being lifted. Notice the whole surface lifts, not just one paver. The fact that the whole surface moves together demonstrates the sand holding the pavers together.

The second important aspect is the edge restraints (the border that is holding the pavers together). Since they are sitting in sand not mortar if the edge is not held firmly in place they could start to spread apart. This would create larger and larger opening between the pavers that would eventually be so wide that there would no longer be friction between the stones.

With the second condition in place you have now created interlock.

Installation of Interlocking Concrete Pavers[edit]

A sieve is used to test soil and base material.

The correct installation of interlocking concrete pavers

Step 1: Demolition – remove old driveway or patio. Go down a minimum of 7”.

Step 2: Compact the sub base – the dirt that will be directly under the base material.

Step 3: Install the base – Class II Road Base is used. This is composed of various sizes of gravel. See the picture of the sieve. A sieve is used to test soil and base material. There is a wire mesh at the bottom of each pan. At the top the mesh will allow ¾” rock through, the next one down will be ½”, then ¼” all the way to the last pan that will allow only “fines”. A fine is described as a particle that can pass through a #200 mesh. A #200 mesh has 350 openings per square inch. This is actually like a fine cloth. Water will not pass through a #200 mesh but fines will. So these particles are very small!

Certified base has correct quantities of all the above sizes so that it will compact to at least 95%. Imagine stepping on a field of marbles—they would move. Or even stepping in sand at the beach—not very firm. That’s because they are all the same size. Since certified base comes with the right proportions, the end result will be a very firm sturdy base.

Loose soil.jpg
Compact soil.jpg

The base should be compacted and at least twice as shown below.

1st recommended compacting pattern for compacting pavers
2nd recommended compacting pattern for compacting pavers

To achieve the desired level of compaction, 98% base should be compacted in “lifts”. A lift is a layer of material, base, or dirt to be compacted. For base, the lifts should be no more than 4” thick. The compactor should be a very heavy duty machine that produces a minimum of 5,000 pounds per square inch (34,000 kPa). Smaller machines will never achieve the desired 98% compaction.

Step 4: The next step is the sand bedding. (this is angular sand as mentioned above) –a one inch sand bed is screeded in place, the pavers are laid on top.

Step 5: Cutting the pavers – the edges are cut with a dry saw and each fit precisely in place.

Step 6: Install restraints – Since pavers are set in sand, they need to be held together by something. What holds them together is the borders or the outside perimeter. This outside edge is referred to as a restraint. Once they are finally held in place, they can not move yet the entire surface is flexible. As compared to a poured in place concrete surface which is rigid. As a word, flexible, best describes the interlocking concrete surface whereas rigid would describe a concrete surface.

There are several types of restraints as shown below.

Restraints.gif

Step 7: Compacting the pavers – Once the restraints are in place, the next step is to compact the pavers in doing so, the pavers are pushed into the bedding sand and the sand is forced up between the pavers.

Step 8: Sand between the pavers – Once this is done more sand, either the same bedding sand or joint sand that comes in bags, is swept between the pavers and the pavers are compacted again. In using a vibrating plate compactor, all the joints between the pavers are filled.

References[edit]