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It is somewhat of a balancing act to develop a pervious concrete mix that places easily, performs well, and has good durability. Finding this balance requires the producer to develop mix designs for each of his aggregates and have the ability to identify the telltale signs of problems with pervious concrete mixtures that may affect proper placement.
Generally problems with pervious concrete mixtures are related to water content, mixture proportions, or admixture dosing. Some material-related issues can be remediated by changing construction practices, but this article only covers suggested changes from the concrete producer’s perspective.
Void content
First and foremost, pervious concrete is defined by the ability to drain water. Percolation of water through pervious concrete is achieved through a series of interconnected voids that are termed the porosity. The void spaces in the concrete that create this porosity are interconnected and rapidly permeable to water movement. The voids in this mixture are measured by density (ASTM C1688), but the relationship to permeability is reflected by the infiltration test (ASTM C1701).
Producing pervious concrete with a specific Design Void Content (DVC) allows the producer to control the quality of the mix by determining the design unit weight for a corresponding DVC. The primary considerations when determining a mix design are:
Strength for loading
- Freeze/thaw resistance and also durability against winter maintenance operations, such as plowing and the use of deicers
- Porosity to produce the desired permeability and maximize the required maintenance intervals
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Workability
The goals of any pervious concrete placement are a smooth and durable surface with the desired void content. These are obtained through proper mix proportions that result in a workable mix. The workability, strength, and freeze/thaw durability of pervious concrete are primarily controlled by:
Aggregate angularity and gradation
- Cementitious volume
- Sand or fiber content
- Water-cementitious materials ratio (w/cm)
- Admixtures
- Environmental factors such as mixing time, water temperature, and mixer/truck moisture state
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When the ready-mix truck arrives at the jobsite, the workability must be checked and adjusted if needed. One method to measure the onsite workability is the inverse slump flow test. The flow behavior of pervious concrete through a slump cone closely mimics the discharge ability from the concrete truck. To perform this test:
Fill an inverted slump cone with fresh concrete (do not rod or compact) to roughly level with the top surface.
- With one even motion, lift the slump cone to approximately knee level. If required, lightly tap the cone to initiate flow.
- If concrete flows from the cone, then it will discharge easily from the truck chute. If the material sticks in the cone and requires substantial energy (vigorous shaking) to free the concrete, then the concrete is too stiff. A mixture that is too stiff will be difficult to discharge from the truck, have high porosity, low strength, and high raveling potential.
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Mix constituents
Aggregate makes up the largest volume of material in pervious concrete, and because cementitious content is dependent on aggregate angularity, aggregate selection has the largest influence on the unit weight. The size of the aggregate will vary depending on the application and use of the pervious concrete pavement being constructed. Both rounded aggregate (gravel) and angular aggregate (crushed stone) can be used, however, angular aggregate has more surface area than rounded aggregate and will accommodate more cementitious material.
The use of good quality, clean, well-graded crushed aggregate results in pervious concrete pavement with improved structural properties, reduced raveling, and improved permeability. In freeze/thaw climates, coarse aggregate should have a specific gravity greater than 2.5 and absorption less than 2.5%.
It has been shown that the addition of fine aggregate may increase durability; however, the fines may reduce the flow rate of water through the pervious concrete. This practice should be done with caution and only when working with clean, uniformly graded aggregate. When working with w/cm greater than 0.33, adding fine aggregate may increase strength and density, but in some cases, the addition of fine aggregate in a pervious concrete mix that already contains fines naturally, may require placing at a lower w/cm and may result in a pavement that never reaches its desired properties.
Synthetic fibers (micro and macro) now are common in pervious mixes. When fibers are used, the mix responds with decreased porosity, increased unit weight, and increased compressive and flexural strength. Although the porosity is decreased, the permeability is maintained or increased by the additional connectivity of the hydraulic channels created by the fibers. Also, while many factors control freeze/thaw durability in pervious concrete, the addition of fibers causes a large improvement in freeze/thaw resistance without additional sand.
Cement paste coats the aggregate particles, providing lubrication for workability, and hardened contact area for load transfer. As more cement paste is incorporated, the mixture becomes more workable, although reducing porosity. With binder-to-aggregate contents of 0.18 to 0.22 by volume (depending on the aggregate gradation and ultimate strength required), the paste completely coats the aggregate particles without occupying too much of the pore space.
Supplementary cementitious materials (SCM) improve the cement hydration chemistry and provide greater durability along with other beneficial properties. Fly ash, slag, and silica fume improve workability and the ultimate strength of pervious concrete, but can have longer times of set, compared with 100% portland cement mixtures. The lower the ambient and/or concrete temperatures, the slower the set times will be. The percentage used also can affect the time of set. Plastic sheeting is not recommended as a cure for pervious concrete pavement containing higher percentages of SCM (>25%) due to erosion of the plastic cement from surface aggregate by condensation under the sheeting.
Admixtures
There are several admixtures used in pervious concrete mixes:
Air entraining
- Internal curing
- Hydration stabilizing
- Viscosity modifying
- Water reducing
Air entrainment affects only the paste content and has been shown to increase the durability (freeze/thaw resistance) of pervious concrete.
Internal curing admixture (ICA) supplies water throughout a freshly placed pervious concrete mix using absorbent polymers. These polymers readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation. Maintaining moisture conditions allows cement hydration to occur so that the potential properties of the mixture may develop. Within the normal dosage range, ICA will generally extend the hydration time of pervious concrete by three to seven days, allowing the paste to gain the needed strength.
Hydration stabilizer (HS) maintains workability by preventing premature cement hydration from heat buildup during mixing. If a standard retarder is substituted for HS, John Kevern writes in his 2011 book, Design of Pervious Concrete Mixtures, test mixtures are required to determine the proper dosage rate for the particular environmental and haul conditions.
Viscosity-modifying admixture (VMA) provides insurance against paste drain down, a condition in which too fluid a cement paste migrates and seals the bottom of the slab, making the pervious concrete functionally useless.
Polycarboxylate-type water reducer (WR) increases workability in low w/cm (<0.30) pervious concrete mixes.
Troubleshooting
Proper yield is the key to troubleshooting pervious concrete mixes as yield can have a large impact on mix proportions. Mix design yield is measured by comparing the mixture density (unit weight) with the theoretical density and should be checked during the mix design process before it is submitted. The difference in density should not be greater than 0.5 lbs./cu. ft. (8 kg/cu. m). This step should not be confused with quality control testing done during placement that usually requires delivered density to be ±5 lbs./cu. ft. (80 kg/cu. m) of the approved mix design.
Under normal conditions, a w/cm less than 0.27 results in failure through surface raveling and w/cm greater than 0.35 causes draining of the cement paste producing an impermeable pervious concrete surface. Admixtures are used to increase the w/cm (up to 0.40) for better hydration of the cementitious material.
Correct aggregate moisture content is paramount for a successful pervious concrete placement. Measured moisture in the aggregate bin may be lost when conveyed to the batching location. An unexpected loss of 1% moisture will reduce a w/cm of 0.30 below 0.27, resulting in a mixture susceptible to excessive raveling.
Dale Fisher is executive director of the National Pervious Concrete Pavement Association in Woodstock, Ga.
Consider an example of a pervious concrete mix with a DVC of 22% designed with 2284 pounds of stone, 550 pounds of cement, and 21 gallons of water (21% paste volume). The design yield is incorrect (not enough aggregate) and expressed by an increase in density of 3 pounds over the theoretical density. The resulting mix has a void content of 17% and actually contains 611 pounds of cement and 23 gallons of water (26% paste volume). This error can be fixed by increasing the aggregate content by 250 pounds and retesting.
Dale Fisher is executive director of the National Pervious Concrete Pavement Association in Woodstock, Ga.
