Placement and finishing

Vibratory screed flattening porous concrete. (CVC, 2012)

Porous concrete being pre-leveled prior to initial consolidation with a vibratory screed. (Photo Source: Washington State Dept of Transportation on Flickr)

Partial covering with plastic and final consolidation with a hand-roller.(Photo Source: CVC, n.d.)

The installation of pervious concrete requires the placement of ready-mix into forms. Unlike permeable pavers, pervious concrete also requires extensive curing time. Pervious concrete has the most particular installation requirements of the various permeable pavement types. It is recommended that contractors with certifications for the installation of pervious concrete, such as National Ready Mix Concrete Association certifications (NRMCA, 2022) are tasked with its installation. See Permeable pavements: Specifications for more information.

Construction Steps:

1. Install 10 mm vertical spacers on the forms.
2. Dampen the sub-base prior to the placement of pervious concrete with a pressure mister. Doing so helps to avoid interfering with the dry mix’s critical moisture content.
3. Direct concrete from the chute of the ready-mix truck onto the subbase.
4. Pre-level the ready mix with rakes.
5. Consolidate the concrete with a vibratory screed. The vibratory screed maintains the void space needed for hydrologic performance. This is the initial consolidation.
6. Apply moisture to the surface of the newly poured concrete with a pressure mister.
7. Remove 10 mm vertical spacers and compact the concrete surface with a hand roller. A 30-cm piece of 4” PVC that is filled with water is recommended for hand rolling. This is final compaction.
8. Hand-tamp the edges of the form.
9. Within 15 minutes of placement, cover the concrete surface with a 6-mm sheet of white plastic.
10. Keep the concrete covered for 7 days without any traffic.
11. Remove the plastic once curing is complete.

Note: Throughout the process of pouring concrete, the moisture content of the ready mix should be periodically adjusted to meet specification.

Key Inspection Points:

• Climate conditions are acceptable for placement. Ideal conditions include the following:
  • Temperature of about 15°C
  • Overcast weather
  • Little or no wind
  • Some humidity
• Moisture of ready mix is certified by an expert or product representative prior to placement.
• Subbase is wetted prior to placement to avoid having moisture drawn out of the concrete by the underlaying material.
• Forms are wide enough to support the vibratory screed.
• Surface of consolidated concrete is wetted prior to laying down plastic cover to avoid sticking.
• Plastic cover is white to reduce the solar load on the pavement.
• Concrete can be formed into a ball, roughly sized as a softball, in one’s hand without falling apart.
• Concrete placement is certified by a product representative.
• Wash-out areas are provided for concrete trucks and other equipment.

Mistakes to Avoid:

• Delivery of excessively wet ready-mix - It is easier to receive ready mix that is on the dry side and add moisture than it is to dry it out. Excess moisture can result in glazing and placing excessively dry concrete can result in raveling.
• Adding moisture with a hose – do not use the hose from the ready-mix truck to add moisture to the subbase or concrete surface. This will separate the rock and cement base of the concrete, resulting in excessive raveling. Only a pressure mister should be used.
• Unacceptable weather conditions – concrete can fail to cure properly if ambient conditions are unsuitable.
• Weighing down plastic – do not weigh down the plastic cover with dirt or other material. Dirt and other materials can fill the voids of the concrete. Instead, plastic can be secured in place at the sides of the LID practice.
• Designated wash-out stations – do not wash concrete trucks and other heavy equipment in or near the infiltration area.
• Insufficient covering time – some raveling will always occur when placing concrete, but raveling can be minimized by ensuring proper covering time.

Credit Valley Conservation

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.