If you are a homeowner and you’d like to do a little area of pervious concrete, aka porous or permeable concrete, you are going to have to make your own mix, because pervious concrete is not available in bags yet. Fortunately the basic ingredients for pervious concrete are simple. They are the same as regular concrete, rock, sand, cement & water, but minus the sand.
The basic pervious concrete mix is:
3 parts rock
1 part loose cement
just enough water
The rock is generally a 3/8″ pea gravel that should be readily available from your local rockery. But really almost any rock will do as long as it is clean, meaning no or little dirt or fine sand is in it. Round or angular rocks are both okay. For aesthetic purposes the smaller the rock the nicer the final product will look. Though a word of caution here, because the smaller your rock is, the more angular and/or uniform in size it needs to be or your concrete won’t be pervious. We have mastered this process with our PerkTop™.
The cement portion refers to any type of Portland cement, type I-II, type III-V, etc. Get whatever they have at your rockery or local hardware store. When you are filling your container, do not pack the cement or you will end up with too much in your mix as cement does compact quite a bit.
The water portion is the trickiest part of your pervious concrete mix to get right. Don’t feel bad if it takes you a couple tries to get the hang of the what the right amount of water is, because even the professionals have a hard time with this one. Unlike regular concrete, the correct amount of water to add to pervious concrete lies within a very narrow range, if you add a little too much you could end up with impervious concrete and if you add too little you might get raveling (loose rocks on the surface) later. The general rule of thumb to know if the water amount is correct is to make a ball (wearing gloves of course) and the ball should hold together and have a nice shine to it. Note: If your rock is pretty wet to begin with, you probably will need to add very little water to the mix.
When we mix our pervious concrete by hand, I like to always be in the shade. Pervious concrete is very moisture sensitive and once you’ve worked so hard to get that moisture just right, you don’t want it drying out before you have a chance to finish placing it. Again pervious concrete is not like regular concrete, pervious concrete will dry out very fast and you generally can’t retemper it (add water) to bring it back like you might for regular concrete.
Now that you know how to make your own porous / pervious / permeable concrete, whatever you want to call the stuff, check back in later to find out how to place and cure it.