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Pervious Concrete

Mix Design and Properties

 

 

MAIN INGREDIENTS OF PERVIOUS CONCRETE:

 

(1)     Portland / Blended Cements

  • Supplementary cementitious materials (SCMs) such as fly ash, pozzolans, and slag can be added to the cement. These influence concrete performance-- setting time, rate of strength development, porosity, permeability, etc.

  • The key to high-performance concrete is the use of SCMs. Silica fume, fly ash, and blast furnace slag all increase durability by decreasing permeability and cracking.

  • Silica fume is a byproduct of silicone production. It consists of superfine spherical particles which significantly increase the strength and durability of concrete, and is used frequently to produce concrete for high-rise buildings that exceeds 20,000 psi compressive strength. Silica fume can replace cement in quantities of 5-12%.

  • Fly ash is the waste byproduct of burning coal in electrical power plants. It used to be landfilled, but today a significant amount is used in cement. Fly ash can replace 5-65% of the Portland cement.

  • Blast furnace slag is the waste byproduct of steel manufacturing. It imparts added strength and durability to concrete, and replaces anywhere from 20-70% of the cement in the mix.

(2)     Course Aggregates

  • Aggregates are kept to a narrow gradation in order to minimize surface roughness, as well as for aesthetic reasons (size 89 has been used extensively).

  • Aggregates can be either rounded or angular (i.e., pebbles or crushed stone). Typically, higher strengths are achieved with the rounded kind.

  • Fine aggregates such as sand are used in minimal amounts because they fill the air voids (which creates conventional concrete).

  • As in the laying of dense concrete, the aggregates must be close to a saturated, surface-dry condition--  therefore, water control is important in concrete mixtures.

(3)     Water

  • Too little in a mixture leads to aggregates that are dry and do not place or compact well.

  • Too much water makes the concrete mixture soupy (paste flows off the aggregates) and eliminates air voids (fills in the spaces between the aggregates).

  • As a general rule, water that is drinkable is suitable for use in concrete.

(4)     Admixtures

  • Admixtures are chemicals which are added to the mixture to provide the concrete with special properties.

  • Retarders / hydration-stabilizing admixtures are those that lengthen cement's rapid setting time.

  • Air-entraining admixtures are those that reduce freeze-thaw damage, and are therefore used in climates where this is a concern.

  • There are also proprietary admixture products which facilitate the placement and protection of pervious pavements.

 

 

ConcreteAnswers

 for Architects,

 Engineers and

 Developers:

 

ConcreteAnswers.org

PerviousPavement.org

ConcreteStreets.org

GreenConcrete.info

ConcreteBuildings.org

SelfConsolidating

Concrete.org

FlowableFill.org

GreenRoofTops.org

 

 

 

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