Hydration
Begins
After the aggregates, water, and the
cement are combined, the mixture remains in a fluid
condition for about four to six hours which permits
transporting, placing and finishing in its final
location, then the mixture starts to harden. All Portland
cements are hydraulic cements that set and harden through
a chemical reaction with water. During this reaction,
called hydration, crystals radiate outwards from cement
grains and mesh with other adjacent crystals or adheres
to adjacent aggregates. The building up process results
in progressive stiffening, hardening, and strength
development. Once the concrete is thoroughly mixed and
workable it should be placed in forms before the mixture
becomes to stiff. During placement, the concrete is
consolidated to compact it within the forms and to
eliminate potential flaws, such as honeycomb and air
voids.
Proportioning
The proportioning of a concrete mix
design should result in an economical and practical
combination of materials to produce concrete with the
properties desired for its intended use, such as
workability, strength, durability and appearance.
The ready mixed concrete producer may
independently select the material proportions to provide
the performance you need or may receive instructions
through the job specifications, such as minimum cement
content, air content, slump, maximum size of aggregate,
strength, and others.
Regardless of the source of
instructions, there are established methods for selecting
the proportions for concrete for each batch. The
Standard Practice for Selecting Proportions for Normal,
Heavyweight, and Mass Concrete (ACI 211.1-91)
published by the American Concrete Institute Committee
211 is often referenced for selecting concrete
proportions.
Here are the
basics of a good concrete mix:
-
Cement and water combine chemically to bind the sand
and aggregate together. Fly ash or other cementitious
materials, which enhance concrete properties may
supplement some of the cement. The key to quality
concrete is to use the least amount of water that can
result in a mixture that can be easily placed,
consolidated and finished.
-
Fine and coarse aggregates make up about 70% of the
concrete volume and impart volume stability to the
concrete. Concrete aggregates are required to meet
appropriate specifications and in general should be
clean, strong and durable.
-
Admixtures, are generally products used in
relatively small quantities to improve the properties
of fresh and hardened concrete. They are used to modify
the rate of setting and strength development of
concrete, especially during hot and cold weather. The
most common is an air-entraining agent that develops
millions of tiny air bubbles in concrete, which imparts
durability to concrete in freezing and thawing
exposure. Water reducing admixtures enable concrete to
be placed at the required consistency while minimizing
the water used in the mixture, thereby increasing
strength and improving durability. A variety of fibers
are incorporated in concrete to control cracking or
improve abrasion and impact resistance.
|
