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
concreteQC 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.
