Interlock Paving - It Was Good Enough For The Romans - Is It Good Enough For Us?

Interlock Paving: It Was Good Enough For The Romans Is It Good Enough For Us?

Table of Contents What are Interlocking Pavers?

Spoilt for Choice for Material Myriad combinations of Shape, Size and Color Environmentally Friendly Understanding the Science How to Install Interlocking Pavers Conclusion

What are Interlocking Pavers?

Interlocking pavers have been used by engineers for hundreds of years to build the roads and pathways in our towns and cities. The old cobbled streets and courtyards that are preserved in the old quarters of many European cities are a testimony to the durability and the elegance of natural stone interlocking pavers. But they have a history that extends way back beyond even then to Roman times. For Cobbles or natural stone was often used in the past to build roads or ornate courtyards with the cobbles placed in intricate patterns on a bed of sand. The secret to the durability and strength of this method was that instead of using mortar to lock the stones together the gap between the stones was filled with sand. This allowed the stones to lock but still move and flex under pressure making the completed structure very strong and resistant to ground movement such as freezing in winter. But importantly it also allowed designers to create wonderful designs by placing the stones or bricks in intricate patterns. This feature made using interlocking paving essential in the ornate designs required in the courtyards and gardens of the palaces and stately homes of the period. Today interlocking pavers come in many materials, shapes and colours and the patterns available to a designer are limited only by imagination. Also, the variety of material means that they are no longer used in just roads, driveways and heavy duty constructions but makes them suitable for a vast range of possible uses such in patios, pool decking, ornate internal flooring, or garden landscape features.

Spoilt for Choice for Material The variety of interlocking pavers available today is vast but they are typically categorized by the material used as this will often determine the suitable usage as well as the shape and the colour of the paving as this may determine the possible choice of patterns. Typically interlocking paving available today comes in a variety of materials, such as concrete, brick, cobblestone, natural rubber, bluestone, flagstone or travertine. Therefore it is necessary to select the correct type of pavers that suits the intended use and design. Concrete, for example, is a cheap, durable and easy laid option for driveways as it is easily cut to size.

Also, as it is manufactured rather than mined so it comes in a wide range of uniform sizes, shapes, thicknesses and colours. Traditional clay bricks are another manufactured form of paving used in driveways that is more expensive than concrete but they have the advantage of a more aesthetic and naturally classic appearance. The major benefit of both these types of paving is that they are uniformed size so can be laid in precise patterns and are importantly easily replaced should they be damaged. On the other hand there are a variety of natural stone pavers that are also popular. Bluestone for example is used in the construction of ornate driveways due to its toughness, elegance and beauty.

It is a type of sandstone so it can be cut into a wide range of shapes and sizes to create elegant patterns however it is its natural bluish hue that makes it so attractive.

Flagstone is another attractive natural stone paver but it is most commonly found with a grey or reddish hue. However flagstone comes in thinner and irregular sizes, which makes it more suitable for light duties such as in a patio or garden landscaping but not driveways. Also due to its irregular natural shapes it is often best suited to patterns that are irregular and fitted together much like a jigsaw puzzle. This however has the downside that if damaged they are not readily replaceable like brick, concrete or bluestone pavers. Travertine is another outdoor paver suited to landscaping, patios and swimming pool decking. Again being a natural stone it has pleasing aesthetic qualities as well as being durable. However for an all-round material suited to both heavy duty use such as in driveways or in elegant designs and landscaping we return to the traditional granite cobblestone. Cobbles are extremely durable as well as having an attractive appearance which makes them perfect for low-maintenance projects where long-levity and durability are more important than price.

Myriad combinations of Shape, Size and Colour Interlocking paving come in a wide variety of shapes but for intricate patterns there is a basic requirement for uniform size, texture and colour. As a result concrete and brick tend to be preferred for this type of role as they are easily manufactured in a wide variety of shapes and colours. The most common shapes are rectangle, square, hex, trihex, zig-zag and dumbbell and they will also come in a full range of colours. This vast selection makes concrete pavers an ideal solution for any project that requires pavers being strong, low-maintenance, cost effective as well as ornate.

Environmentally Friendly However, interlocking pavers are not just strong and ornate they are also more environmentally friendly than solid concrete or tarmac surfaces. This is because the interlocking design is permeable to rainwater as it can seep through the gaps between the stones into the ground. This importantly prevents heavy rainfall from gathering in pools or running off into the street creating further drainage problems by overloading the storm drains. In addition the run-off from an impervious surface such as asphalt or concrete may contain harmful chemicals and oil residue that will be diverted into the storm drains via the curbs and culverts polluting the water supply. However it is not just the rain run-off that can cause problems as traditional asphalt surfaces get very hot during summer when subjected to direct sunlight. So they heat up quickly during the day and cool down again slowly overnight. This in effect creates what are known as urban heat islands with artificially high ambient temperatures. Interlocking paving can reduce the effects of overheating by using palecoloured pavers as these naturally reflect surface sunlight and therefore they will absorb less heat and thus reduce the ambient temperature. It has also been noted that by using palecoloured pavers urban street lighting energy levels can be reduced as the pavers reflect the light which then requires less electricity be expended to provide the same amount of light.

Understanding the Science One of the reasons that interlocking concrete pavers is preferable to a solid concrete surface is the way that the interlock works as it makes them many times stronger. The science behind this is down to the fact that it is the deliberate gaps left between the pavers that gives the entire structure its strength and stability. Because the pavers despite being laid touching one another do not sit flush is due to the texture of the surfaces with all those little pits and holes.

These irregularities in the surface texture provide friction against movement. Also when the sand in poured into the gap between the touching pavers the sand finds its way into the pits and holes on the surface of the pavers and this creates even more friction. Also as the sand is compacted and forced into the gaps it not only holds the pavers together but it starts to hold all the sand together. Now, should the pavers be restrained by a perimeter edge that prevents them spreading then the sand becomes even more compacted providing even greater resistance to movement. As a result the individual grains of sand cannot move without a tremendous amount of force being applied in order to overcome this friction.

This is why using sand is much better than mortar as the sand performs the interlock which provides the entire structure with tremendous strength and stability. It is this interlock function that allows pavers that are not actually physically touching along their entire surface to remain stable and highly resistant to movement even when strong forces are applied such as a car accelerating on a driveway. But the interlock also provides along with formidable strength great flexibility. This is due again to the gap and the sand within the gap allowing the stones in summer or winter to bend or flex as there is no actual solid structure.

How to Install Interlocking Pavers In order to reap the benefits of interconnecting pavers it is essential that they are installed correctly and that means by diligently preparing the underlying foundation. This is an essential step as they require a compact and stable base. The depth of the base is an important consideration as it will vary depending on the type of use, such as a patio, footpath or driveway. In general for light use such as a patio or footpath there will need to be a well-compacted base of 3-4 inches to support pedestrian footfall. For a driveway that will support light traffic and vehicles the compacted base will need to be a greater depth of around 4-6 inches. For even heavier vehicles this might need to be up to 8 inches deep.

Whatever is appropriate the base should be well compacted and graded to provide sufficient drainage. This might require an eight to a quarter of an inch gradient per foot to provide sufficient drainage but either way it should provide an even and sound foundation for laying the bedding sand. The bedding sand will need to be around 1 inch deep to provide for levelling and to provide the stability necessary for holding the pavers in place. A good rule of thumb is to allow for 1 cubic yard of fine concrete sand per 300 square feet of paving.

Once the foundations and the bedding sand have been prepared the next step is to install the edge restraints. These are physical restraints that will prevent the spread or lateral movement of the pavers. The edges should therefore be strong enough to resist such forces so suitable edging material are concrete or ideally the side of a house though there are also commercial heavy-duty vinyl restraints available. Once all the perimeter edges have been installed it is finally time to lay out the pavers in the pattern you have selected. The most common is the herringbone pattern but there are literally thousands in online catalogues to choose from.

Once all the pavers have been laid then they will need to be covered in sand in order to fill the gaps between them and create the interlocking that holds them together. Then they will need to be compressed to bed them firmly and securely into the sand bed in order to level them. This is done using a vibrating plate compactor for projects covering large areas as a way to evenly spread the force and to avoid damaging individual pavers. For small areas of paving a rubber mallet can be used to compress and even up the pavers to get everything level. The final step is to once more spread fine masonry sand over the pavers and work them once again with the compactor or the mallet to work the sand into the gaps. This is a crucial step as it is very important that all the gaps are filled with sand as that is what performs the interlocking function and keeps everything stable and strong.

Conclusion As we have seen it is the sand between the rough surfaces of the stones that creates the interlocking through introducing the high friction that restricts movement of the stones against one another, which effectively locks them in place. However that is also ironically the one weak point of the interlocking paver design in so much as over time the sand leaks away through exposure to the elements such as rain and wind. With the loss of sand comes a loss of strength and cohesion therefore the sand must be regularly replenished to maintain the overall structural integrity. However that is a single flaw in an otherwise near perfect engineering structure that has stood the test of time over the centuries.

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