Concrete (Portland Cement Concrete)

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Concrete

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heterogeneous mixture of fine and coarse aggregates, water, cement,
air, and admixtures.

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Heterogeneous

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consisting of dissimilar ingredients or constituents.

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Unit volume of fresh concrete:

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Cement
Aggregates
Air

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Cement

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is a man made material that binds the aggregates together. Cement is the
"glue" that holds the constituents of concrete together. A cement paste is formed
by adding water to cement. When mixed with water, the cement undergoes a
time-dependent exothermic reaction. The rate of the chemical reaction is
dependant on several factors.

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Aggregates

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consist of sand and gravel or crushed stone. Aggregates are divided
into fine aggregates (natural or manufactured sand with particle sizes up to 3/8 in)
and coarse aggregates (aggregates with particles retained on a No. 16 sieve and
ranging up to 6 in.). Aggregates are used in concrete as a cheap filler material.
Aggregates are strong in compression and occupy volume that would otherwise
need to be filled by cement paste. For economy, a concrete mix should include as
much aggregates as possible.
Aggregates should not react with the cement. It is also important that the
aggregate have a good distribution of sizes so that the volume of voids is
minimized, and therefore the amount of cement paste required is
minimized.

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Air

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gets trapped in the concrete during mixing. In addition, in some cases airentraining
admixtures are added to the concrete to purposefully increase the air
content of the concrete.

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Concrete Mix Design:

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purpose of a concrete mix design is to assign specific values to the percent
volume of each main ingredient. The volume of each ingredient is selected so that
the concrete has the desired properties.

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There are two stages that need to be considered when determining the properties
of concrete:

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1. Freshly-mixed stage
2. Hardened (cured) stage

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1. Freshly Mixed Properties:

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Workability
Finishability

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Workability

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how easily the concrete will flow. This is measured with a
slump test.

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Finishability

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the ability to put a nice surface on the concrete.

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Hardened (Cured) Properties

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Strength
Durability

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Slump Test

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is used to measure the
workability of fresh concrete. In a slump
test, a 12" tall metal cone-shaped mold is
filled with concrete. The mold is then
lifted straight up. The amount that the
fresh concrete drops is referred to as the
concrete slump. With a larger slump, the
concrete is easier to place.

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Air Entrainment

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Instead of adding water to increase the workability of
concrete, chemical admixtures are sometimes added to cause air entrainment.
Entrained air differs from entrapped air. Air entrainment produces millions of tiny
air bubbles in the concrete. Chemically what happens is that hydrocarbons in the
air-entraining agent form negative and positive ions on the air bubble.

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Bleeding

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is the tendency for water to rise to the surface of freshly placed
concrete.

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Air entrainment makes the concrete weaker BY...

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(≈ 15%), however it significantly
increases the workability and durability of concrete. The increased durability is
highly desirable in freeze-thaw conditions.

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Finishability

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For most applications it is desirable to have a small amount of bleed water to get a
nice smooth finish on the concrete. The aggregate settles shortly after placement
which causes a slightly thicker layer of cement paste at the top of the concrete. A
small amount of bleed water allows the concrete to have a smooth finish applied to
the top surface.

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How concrete Strengthens

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As concrete "cures" the cement particles react with the water in an exothermic
reaction. This exothermic reaction called hydration.

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Hydration

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is a chemical reaction consuming water.

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The strength of concrete is typically controlled by the water/cement ratio (W/C).
The lower the ratio,

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the stronger the concrete.

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Cement Factor

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# sacks of cement/ cubic yards of concrete

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The compressive strength of normal strength concrete is typically in the range of

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3-6 ksi.

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High strength concrete typically ranges from

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7-10 ksi.