Pervious Concrete Mix Designs | Stonemont Solutions

Pervious concrete mixes are paving mixes designed to have enough interconnected voids so that water can easily penetrate through the concrete and into the subgrade where it can be absorbed into the soil. Property that was once used for managing storm water runoff can potentially be reduced, leaving more useable land for other purposes. Rather than allowing runoff to discharge directly into streams or into a sewage treatment system; water is allowed to replenish the ground water system in a fashion similar to how the land absorbed water before it was developed. The subgrade thickness can be adjusted to hold varying amounts of water for absorption into the soil depending on the native soils ability to percolate.

Some areas like Florida see wide spread use of pervious pavements because they have significant storm water events and soils that allow water to percolate through them very easily. The use of pervious pavements is a desirable alternative to dealing with copious volumes of storm water runoff. Not all soils are suitable for the use of pervious pavements so the use of this product may vary from one area to the next. Void contents in pervious pavements may vary in design from 15 to 25 percent which may make them less suitable for heavy truck traffic. However; the use of aggregates specifically designed for pervious concrete and the use of admixtures to enhance the properties of the mix have greatly extended the suitability of this product for various applications. If designed, produced, placed, and maintained properly; pervious pavements can have a substantial service life. Many communities are now choosing pervious pavements as an integral part of their storm water management system.

Creating a pervious mix design is now a straight forward process in Stonemont software. The main requirement is that the aggregate component must have a unit weight (jigged) quality test attached to it. This is necessary to calculate the amount of coarse aggregate in the mix. If project specifications or standard practice is to use alternative methods of consolidation such as a proctor hammer, just remember to save the results under the unit weight (jigged) quality test for the process to work properly.

The first step is to open Plant Mixes and create a new final or trial mix. On the Design Tab, for Concrete Type choose Pervious. Aggregate Entry will default to CVB/Fine Mass (Dry) and Coarse Bulk Volume defaults to 1. This means that the total mass of coarse aggregate in the mix will be computed using the jigged Unit Weight and a coarse bulk volume of 1, or in other words 100% of the volume of the mix.

For a typical ACI 211 mix design the unit weight represents the total mass of material that will fit into one cubic foot using a typical method of consolidation. To create a mix with a reasonable amount of workability, only a percentage of that mass is used depending on the nominal maximum size of the aggregate and fineness modulus of the sand. Once that mass has been factored to one cubic yard, a volume for that mass is then calculated. For pervious mix designs we want 100% of the mass that will fit into one cubic yard with an appropriate method of consolidation. A volume of the mass is then calculated. The difference between the volume of one cubic yard and the total volume of aggregate that will fit into one cubic yard is the void content between the aggregate particles.

The coarse aggregate volume or mass will typically not change unless there are changes to the aggregate properties that would effect the jigged unit weight. For pervious mixes, the fine aggregate mass is adjusted to make changes to the void content of the mix. Be sure to fill out the Water and Cementitious Design contents for the mix. The Air content is grayed out because it will be calculated as the void content of the mix once all of the components of the mix are added.

On the Proportions Tab, use the Component Manager to drag-and-drop your components onto the mix design form.

Add the Air component to the component grid of the mix. Notice that it contributes to 100% of the mix because there are no other components in the mix yet.

Choose the Coarse aggregate to be used in the pervious mix and add it to the component grid. The mass and volume are calculated and subtracted from the air (void) content of the mix. At this point the percent of air should match the calculated void content of the Coarse aggregate.

Drop in the appropriate water component to the component grid. The air (void) content is reduced by the volume of water since it is partially filling the voids in the aggregate.

Add a cement component to the component grid. The air (void) content is reduced by the volume of cement since it is partially filling the voids in the aggregate.

If the mix is to include a supplementary cementitious material, add it now and adjust the percent of component accordingly. Notice how this effects the percentage of air (voids) in the mix.

Continue to add any admixtures to the mix as needed for the mix design.

Once all the components are added to the mix, review the air (void) content to see if it is appropriate for the mix design that you are trying to create. If the air (void) content is too high, add the fine aggregate component that you would like to use to reduce the air (void) content and enter the amount of sand you would like to use. Notice that the air (void) content will be lowered accordingly. This is an excellent way of estimating the effects of adding sand to a pervious mix. However, be aware that the actual addition of sand to a pervious mix may not have the same effect as what is calculated. It is always recommended that trial batches be performed, especially with pervious mix designs, as their anticipated charateristics may vary from actual performance until fully understood.

Components can be added to the mix in any order. I chose to enter them in this way to better illustrate the relationship of the components in a pervious mix. Creating a pervious mix is as straight forward as creating other concrete mix designs in concreteQC. The approach to pervious mix designs in StonemontQC allow the user to make changes to any of the components in the mix and quickly evaluate the effects those changes.

For more information on how easily you can create pervious concrete mix designs using concreteQC contact Stonemont Solutions, Inc.

Michael Rodriguez
Stonemont Solutions, Inc.