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Pervious Concrete
Mix
Design and Properties
MAIN INGREDIENTS OF PERVIOUS
CONCRETE:
(1)
Portland / Blended
Cements
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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.
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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.
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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%.
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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.
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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
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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).
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Aggregates can be either rounded or angular (i.e.,
pebbles or crushed stone). Typically, higher strengths are achieved with the rounded
kind.
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Fine aggregates such as sand are used in minimal
amounts because they fill the air voids (which creates conventional concrete).
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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
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Too little in a mixture leads to aggregates that are
dry and do not place or compact well.
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Too much water makes the concrete mixture soupy
(paste flows off the aggregates) and eliminates air voids (fills in the spaces between the aggregates).
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As a general
rule, water that is drinkable is suitable for use in concrete.
(4)
Admixtures
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Admixtures are chemicals which are added to the
mixture to provide the concrete with special properties.
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Retarders /
hydration-stabilizing admixtures are those that lengthen cement's rapid setting time.
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Air-entraining
admixtures are those that reduce freeze-thaw damage, and are therefore used in climates where
this is a concern.
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There are also proprietary
admixture products which facilitate the placement and protection of pervious pavements.
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ConcreteAnswers for Architects, Engineers and
Developers:
Concrete
Answers.org
Pervious
Pavement.org
GreenConcrete.info
Concrete
Buildings.org
SelfConsolidating
Concrete.org
FlowableFill.org
GreenRoofTops.org
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