Permeable concrete (also called pervious concrete) is made by removing most or all of the fine sand from a standard concrete mix, leaving a network of interconnected voids that water passes through. A well-made permeable slab can drain roughly 3.5 gallons per square foot per minute, over 100 times faster than natural saturated sand. The mix itself is straightforward, but getting the proportions and placement right is critical because small errors can either seal the pores shut or leave the concrete too weak to hold up.
What Makes the Mix Different
Conventional concrete uses a blend of coarse aggregate (gravel), fine aggregate (sand), cement, and water. Permeable concrete strips out nearly all the sand. A typical mix uses about 480 kg of cement per cubic meter of concrete and roughly 1,500 kg of coarse aggregate, with sand making up only about 7% of the total aggregate weight. That minimal sand content is what creates the void structure that lets water flow through.
The water-to-cement ratio is the single most important number to get right. Specifications from major municipalities set the allowable range between 0.27 and 0.35. Research testing multiple ratios found that 0.30 produced the best permeability, while 0.32 gave the highest compressive strength. Staying in this narrow band matters: too much water and the cement paste drains off the stones and pools at the bottom, sealing the voids. Too little water and the paste is too dry to bond the stones together.
Because the mix is so dry compared to regular concrete, chemical admixtures help make it workable. Superplasticizers improve flow in these low-water mixes. Viscosity modifiers keep the paste clinging to the aggregate instead of dripping off. Set retarders buy extra working time, which is important since permeable concrete stiffens faster than conventional mixes. A common approach is combining a superplasticizer with a viscosity modifier to get a mix that flows well and stays put on the stones.
Choosing the Right Aggregate
The size and grading of your coarse aggregate directly controls both the pore structure and the strength of the finished slab. Crushed stone in the 5 to 10 mm range is the most common choice and produces the best balance. Concrete made with 5 to 10 mm aggregate is roughly 1.36 times stronger than the same mix made with 16 to 20 mm stone. Larger aggregates create bigger, better-connected pores and higher permeability, but at the cost of significantly lower compressive and tensile strength.
Using a single, uniform aggregate size (rather than a graded blend of sizes) maximizes the void space between stones. When you mix multiple sizes together, smaller pieces fill the gaps between larger ones, reducing porosity. For most residential and light-duty applications, a single-size crushed stone in that 5 to 10 mm range gives you adequate drainage and reasonable strength. Angular, crushed stone is preferred over rounded gravel because the rough faces interlock better and hold the cement paste.
Building the Sub-Base
Permeable concrete is only half the system. Below the slab, you need a reservoir layer of clean stone that temporarily stores the water draining through the surface and allows it to soak into the underlying soil. This layer is typically made from No. 57 stone (roughly 1/2 to 1-1/2 inches) or No. 2 stone (3/4 inch to 3 inches), which is preferred for its greater structural stability. The stone must be double-washed and completely free of fine particles. Any fines in the reservoir layer will clog the system over time.
The depth of this reservoir depends on three factors: how much stormwater you need to store, how fast the native soil beneath it drains, and how much structural support the pavement needs for its expected traffic load. Local soil conditions and rainfall patterns drive these calculations. Depth to the water table and bedrock also matters, since you need enough separation for the water to infiltrate properly. A geotextile fabric is often placed between the reservoir stone and the native soil to prevent soil migration upward into the stone bed.
Mixing and Placing the Concrete
The mixing process is similar to conventional concrete, but the dry, stiff consistency of permeable concrete changes how you handle it. The finished mix should look like popcorn coated in a thin, glossy layer of cement paste. Each stone should be fully coated but not dripping. If you squeeze a handful and the paste oozes out freely, there’s too much water. If the stones don’t hold together at all, it’s too dry.
Once placed, permeable concrete is struck off (leveled) using a vibratory screed. Manual screeds can work but tend to tear the surface if the mix is stiff, which it usually is. If you use a vibrating screed, keep the vibration frequency low. Too much vibration over-compacts the surface and closes off the voids at the top, which defeats the purpose. After screeding, the slab is consolidated by rolling it with a steel roller that presses the concrete down to the height of the forms. This step is about evening the surface and pressing the stones together, not about densifying the concrete the way you would with conventional paving.
Work quickly. Permeable concrete has a much shorter working window than regular concrete because of its low water content. Having your crew, tools, and curing materials ready before the truck arrives is essential.
Curing With Plastic Sheeting
Curing permeable concrete is different from curing a standard slab. You cannot use water curing (sprinklers or wet burlap) because the open pore structure lets moisture escape too fast. Instead, the standard method is covering the surface with 6-mil clear polyethylene plastic sheeting immediately after finishing.
Cut the sheeting at least 2 feet wider than the slab on each side and have it pre-rolled and staged next to the pour site so you can unroll it quickly. Before laying the plastic, some specifications call for spraying the surface with a construction-grade soy bean oil cure, which reduces surface evaporation and doubles as a bond breaker on forms and tools. Once the plastic is down, seal the edges. Any holes, tears, or cuts in the sheeting need to be taped to prevent moisture loss and air movement underneath.
Leave the plastic in place for a minimum of 7 days, uninterrupted. If you need to saw control joints, remove the sheeting only long enough to make the cuts, then re-secure it for the full 7-day period. Skipping or shortening the cure leads to weak cement paste and a surface that ravels (stones popping loose) under use.
Strength and Practical Limits
Permeable concrete is not as strong as conventional concrete, and that dictates where you can use it. Compressive strength typically falls between 500 and 4,000 psi, compared to 3,000 to 5,000 psi for standard pavement concrete. Optimized lab mixes with carefully controlled proportions have reached significantly higher strengths, but field conditions are less predictable.
This strength range is fine for driveways, patios, walkways, parking lots, and other areas with light vehicle traffic. It is not suitable for roads carrying heavy trucks or high-speed traffic. The porosity that makes it permeable is the same characteristic that limits its load-bearing capacity, so the tradeoff is inherent to the material.
Preventing and Clearing Clogged Pores
Over time, sediment carried by runoff and dust settling from the air will clog the pores at the surface. Without maintenance, a permeable concrete pavement will eventually lose its infiltration capacity completely. The good news is that periodic cleaning restores it.
Research testing multiple cleaning methods found that the most effective approach is pressure washing followed by vacuum sweeping. The pressure level doesn’t need to be extreme: studies found little difference in cleaning effectiveness between 5 and 20 MPa (roughly 700 to 2,900 psi), so a standard pressure washer works. The pressure wash dislodges sediment stuck in the top few millimeters of the pore network, and the vacuum removes the loosened material before it resettles. Vacuum sweeping alone can clean clogging material within about 3 mm of the surface. Pressure washing alone pushes some debris deeper. Combining them gives the best results and extends the pavement’s functional lifespan significantly.
How often you need to clean depends on the site. Pavement next to bare soil or landscaping that generates a lot of sediment will clog faster than a slab surrounded by turf or other stabilized surfaces. Inspecting the surface a couple of times a year by pouring a bucket of water on it gives you a quick visual check. If water pools instead of soaking through within a few seconds, it’s time to clean.