Deciphering Planks, Slabs and Pavers
By David Smith, ICPI Technical Director
An increasing number of segmental concrete paving projects include planks, also known as linear pavers. In preparation for eventually developing ASTM and CSA product standards, an initial task for the industry was developing a product definition differentiating planks from slabs and pavers. The intent here is to decrease and eventually remove the interchangeability of these product terms. To achieve this, the following definition for planks was recently provided by the Interlocking Concrete Pavement Institute:
Finished (exposed) face area ≤ to 288 in.2 (0.185 m2)
Length divided by thickness ≥ than 4
Length divided by width ≥ 4
Minimum thickness of 2.36 in. (60 mm)
Minimum length of 11.75 in. (298 mm)
Maximum length of 48 in. (1220 mm)
Minimum width of 2.5 in. (63 mm)
This definition frames a dimensional envelope that distinguishes planks from paving slabs and from interlocking concrete pavers described in ASTM and CSA product standards. To better understand the differences, product definitions for slabs and pavers are provided below for comparison. Compared to ASTM C936, the CSA paver definition allow for slightly larger units in length and width: 11.8 x 11.8 in. or 300 x 300 mm units. Such units need to be at least 75 mm or almost 3 in. to meet the CSA definition of a paver.
The Dividing Lines
In practical terms, what separates planks from pavers are length and thickness. Shorter and thicker units (pavers) see better performance in vehicular traffic because they aren’t as subject to as much bending forces, thereby lowering risk of cracks under tires. A dividing line between planks and pavers is illustrated in the table below with a 3 x 12 x 3.125 in. thick paving unit. Some might think it’s a plank but is not. The unit is a concrete paver that meets C936 and CSA A231.2 definitions.
In addition, its length divided by thickness or aspect ratio is 4, making it acceptable for limited vehicular traffic such as a residential driveway. If the unit was thicker, say 4 in., the aspect ratio would be 3, thereby capable of accepting parking lot and roadway traffic. In contrast, a 3 x 12 in. unit with thickness of 2.36 in. or 60 mm has an aspect ratio of 5, thereby meeting the plank definition. This unit should be used in pedestrian applications only.
The bottom line is that dividing lines define dimensional envelopes. They have implications on whether to apply them in vehicular traffic and how much. The how much question was answered in the winter issue of the Interlock Design magazine’s editorial column. If you’d like to access it now, put www.icpi.org/IDApp into your cell phone and download the new Interlock Design app for Android or Apple devices or visit the archives page at www.icpi.org/interlock-design-archives.
Left: Slabs, Middle: Pavers, Right: Planks