THE INTERLOCK

Over two thousand years ago, the Romans developed a simple system for building roads.  Some of these roads are still in use today.  The system consisted of a well compacted base  of limestone (or lime and gravel), covered with a tight fitting cut stone, which produced an excellent and economical roadway that remained virtually maintenance free in all types of weather. 

The Romans used natural cut stone for their surface; today’s modern paving stone is manufactured of high strength concrete in a modern pre cast concrete plant, under controlled conditions which produce units in many shapes and colors.

The first concrete pavers weren’t produced until the mid 1940’s. It started in Holland where all the roads need to be flexible because Holland 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, laid in sand, perform far better than concrete in environments that move and shift.  Before the paver was made from concrete, either real stone or a clay product had to be employed.

Individual cut stones or clay bricks were used for roadways up until the mid 1940’s when the first concrete pavers were manufactured in Holland.  Since Holland is under sea level and has a constant problem of a failing infrastructure they had to use a segmental paving stone and a concrete paver turned out to be more economical and much stronger than chipped stone or bricks.

The first concrete pavers were shaped just like a brick, 4” by 8” 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.

The first production of concrete pavers in North America was in Canada, in 1973. Due to their success, paving stone manufacturing plants began to open throughout the United States working their way from East to West.

Since pavers weigh so much it becomes economically prohibitive to transport them long distances, over 200 to 300 miles. So today most metropolitan areas have at least one or two manufacturing plants, in some cases like Los Angeles as many as five. 

HOW PAVERS WORK, THE INTERLOCKING SYSTEM

The pavers POL uses are individual pre-cast concrete units. They are manufactured with state of the art paving stone machines located at various manufacturers we use in Southern California. This specialized manufacturing process ensures an exacting quality controlled environment that produces pavers of higher strength and durability than normal concrete. Per industry specifications, pavers must meet a minimum average compressive strength of 8,000 psi (pounds per square inch). This can be compared to a range of about 1,500 – 2,500 psi for poured in place concrete. That means a paver is 4 times stronger than a concrete driveway or patio.

The pictures below show pavers being produced.  They 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 cause the water to set the cement and create a bind between the aggregate.

When the mold is moved away, the pavers are in their finished state.

Check out these landscape lighting ideas for your Los Angeles property.

Pavers are said to have zero slump.  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, it is turned upside down and the cone is 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.

Since pavers do not slump at all they have a zero slump, the more the slump, the weaker the concrete.  If one has a very wet mix, it could 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 aggregate or gravel (the larger the pieces the stronger). Concrete made from cement and sand (called mortar) is not as strong as concrete made with a ¾ inch gravel or aggregate.

Concrete delivered in a truck needs to be fairly wet to get it out of the truck.  Typical concrete from a concrete truck will come in at about 2,000 psi.  Paving stones minimum compressive strength is 8,000 psi.  That means a paving stone will resist breaking with a 1 inch square rod applying the pressure of 8,000 pounds.

HOW PAVERS WORK, THE INTERLOCKING SYSTEM

Regardless of which paver shape, color or laying pattern you choose, they are ALL interlocking paving stones. The term “interlock” comes from the friction of joint sand between paver to paver. Interlock is the inability of one stone to move independently of the other stones.  In other words, it can not break away.  This interlock provides pavement superiority in a multitude of ways. The pavers distribute traffic loading over a greater surface. The pavers remain flexible and can withstand minor and major movements of the native soils. Maintenance and repairs (if required) are simplified because no mortar is used in the installation process.

The basic system works like this: The pavers are placed on one inch of bedding sand over a compacted aggregate base. The thickness of the base will vary depending on its application. Pavers and sand are retained using edge restraints. Commonly used edge restraints include concrete, plastic and aluminum edge restraints. The pavers are compacted into the bedding sand using a vibratory plate compactor. Sand is then swept and compacted into the joints, filling them and creating “interlock”.

Upon completion of this compacting process, the area is ready for immediate use. Unlike asphalt and concrete, no curing time is required for interlocking concrete paving stones, because they have already been cured at the manufacturing plant.  The sand between the joints causes the interlock. It’s important that irregular angular shaped sand is used.  Imagine two pieces of sand paper being rubbed together between your hands, they would not move.  Now flip the sand paper over so the backs are facing each other, now they will slide.

The picture below shows pavers being lifted.  Notice that the whole surface lifts, not just one paver.  If one paver did move, that one paver would eventually cause a rut.  The fact that it does not move demonstrates the sand holding the pavers together. 

Garden wall Installation 2

The Flexibility of Interlocking Pavers

Interlocking pavers have the unique ability of transferring loads and stresses over large areas of paving, by means of a “bridging” action between the individual units. Such spreading of the load allows heavier weights and traffic over sub bases which normally would require concrete that is heavily reinforced with steel.  The key word that describes a paving stone surface is flexibility as compared to a rigid concrete surface.

Because interlocking pavers are manufactured in a variety of shapes and colors, they allow the designer infinite possibilities to comply with specific criteria of projects as well as the latitude of interesting designs.