By Brian Burton and David R. Smith

After 11 years and the passage of over 50 million sports fans — in one of the harshest climates in North America, interlocking concrete pavers have remained durable and trouble-free at Toronto’s internationally renowned SkyDome stadium.

     According to John Quinn of Ferris and Quinn from Toronto: “Interlocking concrete pavers provided an exceptional upgrade in texture and colour compared to poured concrete for this high-volume pedestrian entrance area.” He also noted that the interlocking concrete pavers, which cover approximately 13,000 square metres, matched the stringent requirements of the City of Toronto urban design guidelines.

     The original paving project began in late 1988 on an accelerated schedule to accommodate the grand opening and first Blue Jays game in June of 1989. According to Quinn, the pavers were one reason the design and construction team was able to meet this important deadline.

     According to SkyDome’s maintenance department, there have been no problems with the concrete pavers over the past 11 years “Believe me, these pavers have taken their share of dirt, chewing gum and stains. This is normal for a pavement exposed to millions of persons each year. The entire surface is pressure washed twice a year and once this process is complete, the installation literally looks as good as the day it was installed,” says Scott Muncaster, managing director of SkyDome’s Engineering and Technical Production Department. An important part of this exceptional long-term performance is the proper installation of the joint sand between pavers.

Joint sand: An important component of ICPs
The sand, which fills the joints between interlocking concrete pavers, plays an essential role in the performance and appearance of the system. The joint sand and the edge restraints installed around the perimeter of the surface, help to create and maintain, “interlock” between the neighboring units.

     The term “interlock” describes the transferring of vertical loads (applied to the surface of the pavement) to a horizontal direction exerted to sides of the pavers. The tightly compacted joint sand and edge restraints prevent the individual units from losing interlock and moving in isolation to each other. When joints are tight and the sand is secure, the result is an incredibly strong — yet flexible pavement system that can be used in a wide range of applications.

     Most ICPs are designed with spacer “nibs” that help maintain a uniform 2 to 4 mm space between units. This space allows fine sand grains, which are applied after placement of the pavers, to completely fill this space. (During construction the pavers are placed and compacted with a vibrating plate compactor. After the first compaction fine, dry joint sand is swept into the joints. The best sand by far for this application is often called mason sand, which is used to make mortar.) Limestone screenings should never be used.

     This sand, without treatment of one form or another, however, has a tendency to “disappear” or erode over time as the result of traffic, wind, manual or power sweeping or cleaning, insects (such as ants), and the combined effects of rain, snow and ice. When the sand is lost from the joint space it is typically replaced by soil or dirt. When this occurs, it creates conditions that will often result in the growth of weeds.

Stabilizing joint sand
Over the past two years, a number of Canadian and U.S. firms have developed products that stabilize and protect ICP pavement systems from experiencing sand loss. These products effectively stabilize joint sand and accelerate the natural weathering processes.

     This sand effectively and permanently binds the sand particles together in a flexible — yet durable state. It really is a breakthrough for the industry because, in the past, weed growth has been one of the field negative aspects of the performance of ICPs in the field. Dr. John Emery, an expert in pavement technology, reports that interlocking concrete pavers are not forgiving when it comes to installation. “Even a minor deficiency, either in selection of materials or installation can cause separation of the pavement units, loss of joint sand and the eventual growth of unwanted vegetation,” he says.

     In addition to the benefits of stabilizing the sand from the erosive forces of wind, water, and other elements, which can lead to loss of sand, an additional benefit is the increased service life and durability it provides to the entire pavement installation. “The growth of weeds is really just the beginning of the deterioration of the entire pavement installation,” reports Emery “Once the process has begun, it’s difficult to control and expensive to repair. It makes much more sense to install joint stabilizer at the time of installation or after a proper cleaning.”

Controlling moss, lichens and algae growth
Under most conditions moss, lichens, and algae should not grow on ICPs unless the area is heavily shaded. If such growth does occur, the area should be treated with a weed killer, applied in accordance with the manufacturer’s instructions. Such products normally take several days to eliminate the problem and are most effective when applied during a spell of dry weather. These products are also more useful if any thick growths are scraped off first andis then applied to the surface. Most treatments leave a residue to discourage the re-growth of the moss and algae, but this will be of limited value only if the environmental conditions are such that the pavers remain damp and in the shade.

Repairing settlement of ICPs
In some cases, the combined effects of frequent freeze-thaw cycles, moisture and the loads from vehicular traffic over time can result in a noticeable settlement of the base, bedding sand, and pavers. A common problem with paving stone installations is settlement of individual pavers or groups of pavers near curbs, site fixtures, walls, or at locations where concrete pavers meet other pavement materials. Settlement is often due to insufficient compaction of base materials. This is especially the case in confined areas difficult to reach with mechanized compaction equipment during installation. Settlement can also occur because of lost bedding sand or base materials as a result of rainfall. Settlement can also occur near downspouts where water flows directly onto the pavers during rainstorms.

     Fortunately, this is not a permanent defect and can be rectified. To repair settlement, the paving units that have shifted, (and those in close proximity) must be removed and set aside for re-instatement later. If the settlement is minor, say fewer than 10 mm, additional bedding sand can be installed; the pavers re-instated at slightly higher elevations than their neighbors and compacted to a level equal with the pavement surface.

     If settlement is greater than 10 mm, remove the bedding sand and fill the depression with base material such as (MTO Granular A) and compact. Then, replace the bedding sand and pavers as previously described. When there are many cut pavers to reinstall, it is a good idea to number the pavers with chalk prior to removal to facilitate easy reinstallment. When reinstalling pavers, keep the joint between pavers tight (2 to 4 mm). After installing the pavers and compacting them, fill the joints with sand and compact the pavers again.

Replacement of damaged pavers
Although cracked or chipped pavers are rare occurrences, accidental breakage occasionally occurs. One of the benefits of ICPs is the ability to remove individual units and replace them with new ones. At the completion of the initial project, the contractor should leave extra pavers at the site for this purpose, thereby avoiding any noticeable variation in colour. (Slight variations in colour may occur from traffic-borne dirt on surrounding pavers.)

     Removal can be accomplished by carefully wedging individual pavers out of position using hand tools such as a metal pry bar or screwdriver. Special tools are now available to remove pavers. Removal can be made easier by removing as much joint sand as possible with a putty knife and/or shop vacuum and wetting the joint sand. Begin prying on the short ends of the paver. The paver will rise a small distance with each pry. When it rises high enough, grasp the unit and pull it free. Once the first paver is removed, additional units can be easily removed if required. In some cases, the stubborn pavers may need to be broken with a cold chisel and maul to enable removal. Always wear protective eye goggles during this procedure. Obviously, a replacement unit will be needed.

Brian Burton is a Member of the Standing Committee for Technical Evaluations for the Canadian Construction Materials Commission (CCMC). He is the author of over 100 published articles on landscaping design and construction. Mr. Burton is also responsible for development of corporate partnerships for Communities In Bloom.

David R. Smith is the Technical Director for the Interlocking Concrete Pavement Institute.