Pervious concrete, also known as no-fines, gap-graded, permeable, or enhanced porosity concrete, represents an innovative solution for managing and treating stormwater runoff. When employed in pavement applications, it has the ability to efficiently capture and retain stormwater, allowing it to infiltrate the ground and replenish groundwater resources.
Its low or absent fine aggregate content (like sand) and careful control of proportions of water and cementitious materials characterize pervious concrete. The cement paste envelops and bonds the aggregate particles, forming a network of highly permeable, interconnected voids that facilitate rapid water drainage.
- Cementitious Materials: Pervious concrete primarily relies on conventional Portland or blended cements. For specific properties, supplementary materials like fly ash and slag can be added.
- Coarse Aggregate: Fine aggregates are excluded, and coarse aggregates are precisely graded for minimal particle packing. Common sizes adhere to ASTM C33/C33M standards, including 7, 8, 67, and 89.
- Water Control: Managing water content is crucial. The right amount should provide a subtle sheen without excessive runoff. Typically, a water-to-cement ratio between 0.27 and 0.34 is used.
- Admixtures: Chemical admixtures, such as retarders or hydration stabilizers, are common due to pervious concrete’s rapid setting time. Air-entraining admixtures may be used in freeze-thaw-prone regions.
Construction and Considerations
Due to its distinctive material properties, pervious concrete comes with specific construction demands. Essential elements of constructing pervious concrete encompass the following:
Batching and Mixing:
Pervious concrete possesses unique characteristics that demand precise control over the mixture composition. Keeping the water content within a narrow range to ensure sufficient strength and permeability is essential, as it prevents the paste from flowing off the aggregates and closing the voids.
Achieving the correct water content results in a mixture with a noticeable sheen. To maintain consistent mixture proportions, you must conduct a unit weight test. Testing and experience determine the proportions of aggregate and cement. Construction teams use conventional concrete mixing equipment with an extended duration of mixing.
Transporting and placing pervious concrete demands careful consideration due to its low water content. Its extremely low slumps can result in slower discharge from transit mixers compared to traditional concrete. Transit mixers with wider discharge openings typically speed up the unloading process.
Ensuring that the mix is completely discharged within one hour after initial mixing is essential. The use of chemical additives, such as retarders or hydration-stabilizing agents, can extend discharge times to 11½ hours or even longer. Additionally, construction teams should take factors like high temperatures and windy weather into account.
Placement and consolidation
Placement and consolidation of pervious concrete mixtures require careful planning, especially regarding site access because these mixtures are typically not suitable for pumping. In most cases, construction teams employ fixed-form construction methods for laying pervious concrete. Smaller projects may use hand-held straightedges or vibrating screeds, while larger projects may go with A-frame structures and low-frequency vibrating screeds.
To ensure proper consolidation, it’s essential to roll the concrete with a steel roller. Delays in consolidation can lead to issues due to the rapid hardening and high evaporation rates of pervious concrete. It is generally recommended to complete the consolidation process within 15 minutes of placement.
Pervious concrete pavements require a different finishing approach compared to traditional pavements. Traditional methods like floating and troweling tend to close the voids on the surface, which goes against the intended purpose of pervious concrete in most cases. Essentially, the final surface finish is achieved during consolidation, resulting in a rougher surface texture that can enhance traction.
In pervious concrete construction, it’s common to incorporate joints to manage the formation of random cracks. These joints are typically formed using a specialized compacting roller-jointer and should reach a depth between one-fourth and one-third of the slab thickness. Due to the short setting time and accelerated shrinkage in pervious concrete, it’s important to install these joints shortly after consolidation.
Curing Pervious concrete
Curing and protection are important for pervious concrete due to its open structure and relatively rough surface, which exposes more cement paste surface to evaporation compared to conventional concrete. Initiating curing as soon as possible after placing, compacting, and jointing the concrete is essential. Best practice dictates beginning curing within a maximum of 20 minutes following these procedures.
Typically, construction workers achieve curing by applying thick plastic sheeting, usually around 0.15mm in thickness, to cover all exposed surfaces. They should apply this plastic sheeting no later than 20 minutes after pouring the concrete and should keep it in place for at least 7 days.
Inspection and Testing
The American Concrete Institute (ACI 2010) has outlined a set of recommended inspection and testing procedures that construction teams should carry out during the construction of pervious concrete pavements. Additionally, there is a specification for constructing pervious concrete (ACI 2008). Typically, acceptance testing for pervious concrete focuses on density (ASTM C1688) and thickness (ASTM C42).
Over time, accumulations of sand, dirt, vegetation, and other debris can occupy the voids in pervious concrete, reducing its porosity and potentially affecting its functionality. Therefore, periodic maintenance may be necessary to clear surface debris and restore the infiltration capacity. Two common maintenance methods are pressure washing and power vacuuming (ACI 2010).
Pervious Concrete – Typical Properties and Characteristics
– Pervious concrete mixture is notably stiff in comparison to traditional concrete.
– Slumps are generally less than 20 mm.
– Occasionally, slumps as high as 50 mm may be used.
– Aggregates tightly adhere to each other.
– Exhibit the characteristic open matrix when placed and compacted.
4. Quality Control
– For quality control and assurance, unit weight or bulk density is preferred.
– Some fresh concrete properties like slump are not meaningful for pervious concrete.
5. Cylinder Strength Tests
– Conventional cast cylinder strength tests hold little value for assessing pervious concrete.
6. Unit Weight
– Unit weights of pervious concrete mixtures are approximately 70% of traditional concrete mixtures.
7. Working Time
– Concrete working time is typically reduced.
– Usually, only one hour between mixing and placing is recommended.
– However, this period can be extended by 1.5 hours using retarders and hydration stabilizers, depending on the dosage.
Hardened Properties of Permeable concrete
|Density and Porosity
|– Properties and proportions of materials used – Compaction procedures during placement
|In-place densities varies from 1600 kg/m³ to 2000 kg/m³ (Similar to lightweight concretes)
|– Materials used – Placement techniques
|Typical water flow rates: 120 L/m²/min to 320 L/m²/min
|– Material properties – Mix combinations – Placement methods – Environmental conditions
|3.5 MPa to 28 MPa (Typical: around 17 MPa)
|– Degree of compaction – Porosity – Aggregate-to-cement (NC) ratio
|About 1 MPa to 3.8 MPa
|– Mixtures and materials used – Material’s low paste and mortar content
|Shrinkage values approximately 200 x 10^-6
Applications of Permeable Concrete
The table below provides a clear overview of the various applications of pervious concrete.
|Residential roads, alleys, and driveways
|Sidewalks and pathways
|Low water crossings
|Subbase for conventional concrete pavements
|Tree grates in sidewalks
|Foundations/floors for greenhouses, aquatic amusement centers, and zoos
|Swimming pool decks
|Pavement edge drains
|Groins and seawalls
|Walls (including load-bearing)
Benefits of Pervious Concrete
- Effective management of stormwater runoff, potentially reducing the need for curbs and storm sewers.
- Reduction in contamination of waterways.
- Groundwater replenishment.
- Efficient land use by eliminating retention ponds and swales.
- Reduced heat island effect due to water’s cooling effect and improved airflow.
- Prevention of surface ponding and hydroplaning.
- Reduced noise from tire-pavement interaction.
Limitations of Permeable Concrete
- Limited suitability for heavy vehicle traffic areas.
- Requires specialized construction practices.
- Longer curing time.
- Sensitive to water content and control during the placement of fresh concrete.
- Lack of standardized test methods.
- Requires careful design considerations for specific soil types (expansive and frost-susceptible soils).
- Potential challenges with high groundwater conditions.