Determining how thick to make pervious (permeable, porous, no-fines) concrete depends on many factors. Typically pervious concrete is poured the same or a little thicker than ordinary concrete. Some common thicknesses for pervious concrete are 4″ (101mm) for pedestrian applications, 6″ (152mm) for vehicular and 8″ (203mm) or more for heavy vehicular applications like bus stops and high-use roads. However these are general rules of thumb and since every site is unique may be adjusted thinner or thicker accordingly.
Before we consider the specifics of how thick to place or specify pervious concrete, let’s first consider the why. Primarily we are trying to avoid cracking. Cracking in concrete pavement is its main cause of failure or undesirability. Potentially exacerbating the problem for pervious concrete, is that steel (at least non-water proofed steel), like rebar and welded wire mesh, cannot be used as it would rust and deteriorate fairly quickly. A primary function of steel reinforcement in concrete pavement is to hold the pavement together and minimize displacement when it does crack (and essentially all concrete cracks:).
There are many types and reasons for cracking in concrete, but arguably the largest factor, at least for the most deleterious cracking, is the thickness of the pavement. Often the thicker the pavement the less chance it will crack. This is so because the modulus of rupture decreases (flexural strength increases) as the pavement gets thicker. For instance, doubling the pervious concrete thickness could decrease up to threefold the modulus of rupture.
However, making the pervious concrete much thicker than is needed isn’t a good solution. It would be expensive, since more material and more excavation would be required. And it wouldn’t be as environmental, since more raw materials would be consumed, including lots of carbon dioxide producing cement.
Therefore getting the thickness of pervious concrete just right is very important!
Some of the factors that go into determining how thick to make the pervious concrete pavement are how much and what type of traffic it will receive, the type and thickness of the base and subgrade it is over, the mix design, and the spacing of the control joints.
For most municipal, commercial and other high use pedestrian or bike only areas, specifying the pervious concrete to be at least 4″ (101mm) thick is recommended. On a residential application that receives only occasional foot traffic, 3 1/2″ (89mm), is generally sufficient. For most parking lots and other low speed vehicular pavements, specifying the pervious concrete to be at least 6″ (152mm) thick is recommended. Again for residential and occasional use, making a driveway a tad thinner, at 5 1/2″ (140mm), is okay. If frequent bus, rv or other heavy vehicle use is anticipated for the pavement, then bumping the pervious concrete thickness up to 8″ (203mm) is better.
The type and depth of the base material and the condition of the subgrade the pavement is over, also play an important role in determining how thick to pour the pervious concrete. See our “Pervious Concrete Base Material and Depth” to determine how it impacts the design thickness of the pavement.
The mix design of the pervious concrete itself could also play a part in how thick it should be made. If the pervious concrete mix is designed with low modulus of rupture (high flexural strength) or low modulus of elasticity (more ‘flexible’) it could be placed thinner than the above recommendations. Generally the extra material cost and attention these mixes require don’t justify it, however there are several applications where being able to make thinner pervious concrete can be very useful, like trying to minimize disturbance around sensitive tree roots or on tight podium or other restrictive areas where one simply can’t pour to the full desired depth.
Conversely if a pervious concrete mix is weaker than normal, has a high modulus of rupture (low flexural strength), then designing the pavement to be thicker than the recommendations would be important. For instance for the 2008 Beijing olympics, a lot of the pervious concrete that was installed for it, was placed using a very coarse, 1 1/2″ (38mm), rock with a low amount of cement as a very thick base. Then a more traditional 3/8″ (9mm) pervious concrete with high amounts of color was placed 1 1/2″ thick over this extra thick, coarse base.
Fortunately, pervious concrete is less prone to cracking to begin with than ordinary concrete. Drying shrinkage in pervious concrete is generally about half (1/2) of ordinary concrete. Drying shrinkage, which occurs in all concrete (portland cement-based), is a major contributor to cracking. It is not entirely clear why pervious concrete shrinks so much less. A factor could be that pervious concrete is typically cured for at least 7 days under plastic, but while this almost certainly mitigates another common form of cracking from plastic shrinkage, it’s probably not the main reason for the reduction in drying shrinkage.
This leads us to our final factor for consideration of how thick to make pervious concrete pavement, ‘joints’. Joints aka ‘control’ or ‘contraction’ joints are cuts either tooled or saw-cut into concrete to a depth at least a 1/4 the thickness of the pavement. The idea is when the concrete cracks from shrinkage or other factors that it will crack in this weakened area and therefore not be an unsightly eyesore in the field of the concrete. Because pervious concrete shrinks less the joints could be made less frequently (spaced further apart) than is recommended for ordinary concrete. However we generally still prefer to place our joints on our pervious concrete slabs at the minimum recommended of ‘2’ times the thickness, so for 4″ (101mm) thick pervious concrete we place joints around 8′ (2.4m) on center or less and for 6″ around 12′ on center or less. If fewer joints are desired, an option is to make the pervious concrete thicker than it otherwise would need to be.
Crack-free emergency vehicle access (EVA) PerkTop pervious concrete path poured 6″ thick.