Slabs In Demand
Each year, the Interlocking Concrete Pavement Institute (ICPI) conducts a sales survey of U.S. and Canadian manufacturers to estimate sales from the previous year. The most recent survey estimates total concrete paver sales for 2014 at around 600 million sf, about an 11% growth from 2013. (This area does not include paving slabs.) What’s interesting is where growth occurred as a percent of all concrete segmental paving units sold. For permeable interlocking concrete pavers, it was 3.6 to 4% and for paving slabs 14.6 to 16.5%. Paving slab sales were about four times higher than pavers used in permeable applications. Both products exhibit double-digit percent growth each year.
Paving slabs are units requiring two or more hands to install, whereas pavers only require one hand. Some manufacturers’ literature mislabels paving slabs “pavers.” Over the years, this has led to confusion among some designers thinking that paving slabs can withstand vehicular traffic similar to smaller interlocking concrete pavers. The absence of paving slab design and construction guidelines for vehicular areas may have contributed to this confusion.
Paving slabs are units generally 12 x 12 inches or larger used mostly in residential and commercial at-grade pedestrian and vehicular applications, and on roofs for commercial buildings. The paving slab industry came into existence about the same time as the concrete paver industry in the last quarter of the previous century. Fast-forward 40+ years to the present and we are seeing development of paving slab design and construction design guidelines by ICPI. Development of guidelines is proceeding with publication expected later in 2016.
A parallel activity is the development of an ASTM segmental concrete paving slab product standard. Approval is expected in 2016 as well. This will complement the Canadian paving slab standard CSA A213.1 Precast Concrete Paving Slabs first issued in 1972. Like the Canadian standard, the draft ASTM standard includes a requirement for flexural strength since they primarily fail in bending and not from compressive forces.
As done previously for interlocking and permeable interlocking concrete pavements, ICPI is developing design charts for paving slabs that include inputs for soil strength, base and axle loads. There will be certain size and thickness of units applicable for axle load ranges, provided that certain base materials and strengths are designed and constructed. Maximum axle loads will be significantly lower than for interlocking concrete pavements and permeable interlocking concrete pavements.
The guidelines also will address structural design of concrete paving planks subject to vehicular loads, those thin, long (12 to 24 in.) units in vogue, especially among landscape architects. Preliminary conclusions from finite element modeling research sponsored by the ICPI Foundation points to the need to support these units under vehicular traffic with a cement-stabilized aggregate base.
The future of paving slabs might include life-cycle cost analyses and life-cycle assessments of environmental impacts compared to cast-in-place concrete and asphalt.
While the initial costs of more attractive and durable paving slabs will likely be higher, maintenance and repair costs are likely lower as paving slabs are removed and reinstated after underground repairs, i.e., no wasted discarding of the pavement surface. The possibilities from ICPI tools, commensurate with high-quality, manufactured paving units, continue to expand the product pallet for the industry, designers and users.