Flashcards
Concrete is produced from
Portland Cement course and fine aggregates and water
The aggregates used in concrete are
Inert mineral ingredients that reduce the amount of cement that would otherwise be needed
Aggregate
Gravel stone sand or other inert material used in concrete these materials may be fine of course
Coarse aggregate
Gravel or stone
Fine aggregate
Sand
Aggregates make up a
Large percentage of the total volume of concrete
Concrete must be Cured in order to reach
It’s proper strength
Curing fresh concrete requires
Correct hydration and temperature control
Improper curing methods will negatively affect
The finished surface of the concrete as well as it’s strength
Hardening of concrete involves a chemical process known as
Hydration
As concrete hardens the gel gives of heat which is known as
Heat of hydration
Because hydration involves water proper curing requires
That the concrete be kept moist until it reaches its desired strength
Moist curing of concrete produces
A stronger concrete
Concrete can be kept moist by several techniques including
Sprinkling it with water, ponding, or covering it with a plastic film
In addition to maintaining proper moisture concrete that is curing must be maintained at the correct temperature ideally between
50-70 degrees F
Concrete that is cured at or above 100 degrees Fahrenheit will
Not reach its proper strength
Concrete cured near freezing temperatures will
Harden more slowly
In massive concrete structures the heat of hydration generated can
Adversely affect the final strength of the concrete
Concrete can be cooled using
Cold water during curing or using chilled water in the mixing process
During the curing process Concrete will
Shrink slightly
Theoretically properly cured concrete continues the to harden indefinitely at a gradual rate however normal design strength is reached
After 28 days
Different types of concrete Can be produced for specific purposes this is accomplished by varying the ingredients or adding chemicals to the concrete mixture these are generally known as
Admixtures
The density of concrete can be reduced by using a lighter weight aggregate such as
Shale or clinker
An admixture known as superplastizer can be used to produce
A mixture that flows more freely
Coloring can also be added to concrete for
Aesthetic or safety reasons
Like masonry concrete is strong in compression but weaker in tension. the ultimate compressive strength can’t be varied from
2500 psi to 6000 psi with the allowable stress used in design reduced by a factor of safety
In most structural designs tensile forces exist and the concrete must be
Reinforced to resist them
The techniques that are used to reinforced concrete are
Ordinary reinforcing
Prestressing reinforcing( pretensioning or post tensioning)
In ordinary reinforcing when the concrete has hardened it adheres to the reinforcing bars because of
Deformations on the surface of the bars
The diameters of standard size reinforcing bars vary from
.375 inches to 2.257 inches
Vertical reinforcing bars are known as
Stirrups and are provided to resist the diagonal tension
Concrete beams are frequently cast in the shape of a
Tee
The wider cross-section area at the top of the Tee beam permits the concrete to
Support a greater load
Reinforcing steel is placed in the bottom of the tee to resist
Tensile force
Although the primary function of placing reinforcing steel in concrete is to resist tensile forces the steel can also be used to support
Some of the compressive forces
More efficient use of concrete is made using a technique known as
Prestressing
The prestressing process requires large loads to be applied to the concrete along
The axis of the beam
Is widely used in concrete structures
Prestressing
Pretensioning. when the steel strands are released the concrete member usually
takes on a slight upward deflection
The reinforcing steel used concrete is essential to the strength of
a reinforced concrete member
As a general rule reinforcing steel should not be cut in the course of firefighting operations unless
It is necessary to rescue trapped victims
The forces that are produced in the prestressing process remain locked in the steel for
The steel for the life of the assembly
Cutting through reinforcing steel with a saw or torch is particularly dangerous in
Posttensioned concrete because the steel is not bonded to the concrete
