At the 2006 World of Concrete Mega Demos, construction of a section of pervious concrete drew a lot of interest from the huge audience. Pervious concrete pavement has the unique ability to allow water to flow through easily, which results in parking lots that have no runoff from rainfall or snowmelt. Due to new Environmental Protection Agency rules that regulate stormwater runoff, developers are using pervious concrete and saving money by avoiding the cost of constructing retention ponds and elaborate drainage systems. Pervious concrete is bound to become more popular and represents a potentially lucrative market for concrete contractors.
Pervious concrete is a mixture of portland cement, coarse aggregate, water, and admixtures. Because the mix contains little or no sand, the pore structure has many voids, allowing water and air to pass through. Unfortunately, this lack of sand in pervious concrete also results in a very harsh mix that negatively affects mixing, delivery, and placement. Common experiences with traditional pervious concrete include:
Difficulty getting the mix out of the truck
- The need to add water onsite (inconsistent mix quality)
- A short working time (minimal workability life)
- Difficulty placing and compacting the mix
Through laboratory and field experience, Degussa Admixtures has identified a system of admixtures that helps producers and contractors overcome these issues. The pervious concrete admixture system consists of a combination of Poly-Heed 1020 or 1025, DELVO, and Rheomac VMA 362. When pervious concrete is placed in areas exposed to freezing and thawing conditions, an air-entraining admixture should also be used. These admixtures accomplish the following:
PolyHeed 1020 or 1025: Acts as a cement dispersant, allowing water reduction and efficient cement hydration to maximize the strength of pervious concrete mixes. Strength is sometimes a challenge because pervious concrete has a high void content, which negatively affects strength.
DELVO: Since pervious concrete has a very low water content, the cement tends to hydrate quickly, shortening the working time. DELVO extends the workability window and the life of fresh pervious concrete mixes, reducing or eliminating the need to retemper.
Rheomac VMA 362: Since pervious concrete is a harsh mix, Rheomac VMA 362, a viscosity-modifying admixture, adds body and helps lubricate the mixes. The result is better flow and faster discharge time from a truck and easier placement and compaction. In addition, the use of Rheomac VMA 362 is an insurance policy to help prevent paste drain down and has been shown to increase compressive and flexural strength in low compaction mixes.
Structural and Site Design Specifications, Mix Design, and Test Methods
The combination of these products has significantly improved the mixing and handling of pervious concrete. As with all concrete, admixture dosages can be varied to achieve the desired properties for the application. The table contains suggested starting point mixture proportions for different pervious concrete applications. These proportions, however, can and should be modified to achieve the desired level of performance.
One thing that dictates the mix design is the compaction method. Compaction is a critical step in placing pervious concrete. Because pervious mixes contain very little water, adequate compaction is needed to achieve the desired compressive strengths. Mix designs will vary depending on the method of compaction used during placement.
Low compaction is compaction by means of a hand roller with little or no ballast, or with a roller screed. High compaction is compaction by means of a heavy hand roller with ballast (filled with water or sand), or with a high-density paver.
Proper mixing can also become an issue with pervious concrete. Our limited experience indicates that mobile mixers (volumetric mixers) may not mix pervious concrete uniformly. For example, the slab placed by a mobile mixer at the World of Concrete clearly showed wet and dry areas. Whether placed by ready-mix truck or mobile mixer, it should be determined beforehand that the mixing vessel is suitable for uniformly mixing pervious concrete.
Success in the field is assured by performing a trial mix and slab placement using the materials and equipment that will be used on a project. Properly trained contractors are also critical for a successful project.
—Mark A. Bury and Christine Mawby are the product line managers for pervious concrete for Degussa Admixtures, Inc.
Advocating Pervious Concrete
Most of the available info about pervious concrete mix design includes disclaimers about what won't work and a presumption about what ought to work. During the past three years I've researched, inquired about, batched, placed, and followed up on pervious concrete activity from literally every source I can find in this country.
As a manufacturer of machinery, we supply our clients with tools and equipment appropriate for the project. That's the easy part. The main object of my work is to be an advocate to our clients in helping them obtain a proper mix and install it properly. Pervious concrete has unique properties and requires a close working relationship between ready-mix producer and contractor. Certain issues become apparent when we consider pervious concrete performance.
Aggregate connection: Nearly all failures of pervious concrete are from raveled aggregate, which lacks proper adhesion. Is there adequate cement paste coating the aggregate? Is the paste fraction compromised in such a way that coarse aggregates are prone to ravel? These are important questions that can be managed with a good mix that is properly installed.
Cement: We recommend 600 lb/yd3. Fly ash may substitute for up to 50 lb/yd3. Slag cement may substitute for up to 100 lb/yd3.
Water: Clean water is necessary; recycled water could contaminate the batch and possibly compromise the paste. The water volume varies relative to the amount of water-reducing admixture used. Water-reducing admixture is useful but optional. A viscosity modifier designed for pervious concrete is also useful and is recommended. The water-to-cementitious materials ratio should remain between 0.26 and 0.30. The cement paste should have a metallic sheen that should remain throughout the entire placement and compaction process. It is critical that the paste remain moist and flexible as the aggregate is moved and compressed. If the appearance of the paste becomes flat and dull and it starts to hydrate, the pervious concrete will begin to crumble as it is moved or compressed. In order to keep the pervious concrete fresh for at least 90 minutes during delivery, placement, and compaction, a hydration-stabilizing admixture is necessary. Do not substitute a retarder for a hydration-stabilizing admixture, as it does only half the job.
Curing: Water evaporates very rapidly from pervious concrete because the open matrix allows air to flow through the materials. Given the low water-to-cementitious materials ratio, evaporation causes a dangerous loss of moisture. Most failures of pervious concrete can be traced to improper curing. Confilm and curing compound are useful, but there is no substitute for polyethylene sheeting installed quickly and sealed tightly.
Polyethylene sheeting should be at least 2 feet wider than the slab to allow a tight seal at the edges. Adjoining pieces of polyethylene should be connected with adhesive to assure a tight seal. Polyethylene sheeting must be installed in less than 20 minutes after placement. The sheeting should remain in place for seven days, and precautions should be taken to ensure that the polyethylene is not damaged by the wind. If placed in cold weather or if slag cement is used, the concrete should remain sealed for 10 days.
Water Flow: Pervious concrete pavement is a structure that can carry a load while allowing water and air to move through it. Inadequate water flow through the concrete can be caused by improper aggregate gradation, excessive compaction, and clogging from foreign material on the surface.
Aggregate: Don't exceed ½-inch maximum aggregate size because larger rock is just too difficult to place and can create an undesirable surface. In addition, it is important to use smaller coarse aggregate simply for aesthetic appeal. Most people prefer the appearance of 3/8-inch maximum size aggregate. Some ready-mix producers insist that 3/8-inch-minus aggregate is good enough if they leave out the sand. That is not good enough. We want 3/8 inch period! That is to say that when a maximum aggregate size of 3/8 inch is used, do not include anything that will pass a #4 sieve. The use of smaller aggregate will increase the strength but will also compromise the void structure. Crushed aggregate with a lot of sharp fracture may require the use of small amounts of smaller aggregate to yield a proper void structure and a proper density (unit weight). We want a mix with a density (unit weight) of 120 lb/ft3 and a void structure of approximately 20%. A number of other issues become much easier if careful attention is given to aggregate gradation: targeting water content, discharge from the mixer and chute, placement, strike-off, compaction, and stamping. Our reasons to make this process easy go beyond just being easy to perform; ultimately the best way to preclude failures is by being quick enough for proper curing.
Compaction: Some pervious concrete is compacted using riser strips with a compression tube. The time it takes to use riser strips (shims) and a compression tube is dangerous because it delays the time before curing can start. Another compaction method is to use a plate compactor. Plate compactors can also cause delays and tend to over-compact pervious concrete. An efficient compaction method uses a striker that applies shear forces to the surface from the face of a spinning tube. This compresses and consolidates the concrete as it advances over the surface. The spinning tube travels easily in both directions, enabling operators to seed more material on the surface and strike it again. Strike off and compression are simultaneous, and polyethylene sheeting can follow immediately behind the screed—within 5 minutes after placement.
— David C. Mitchell, Bunyan Industries