Structural Materials.pdf

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Structural Materials


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CONCRETE MATERIALS

Concrete is a mixture of paste and

aggregates (sand and rock). The paste,composed of cement and water, costs the

surface of the fine (sand) and coarse

aggregates ( rock) and binds together into

a rock-like mass known as concrete.


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Unit weight :Normal concrete density = 23.6 kN/m3


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Aggregate

The total aggregate (fine + coarse) are used inconcrete as filler and generally occupy 60% to75% of the concrete volume (70% to 85% byweight)

Fine aggregate generally consist of natural sandor crushed stone with most particles smaller than4.6 mm size.

Coarse aggregates consist of one or a

combination of gravels or crushed aggregate withparticles larger than 4.6 mm.

( 10 mm < coarse aggregate < 20mm)


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Cement:

Type I : Normal

It is general-purpose cement use in concrete for

making pavements, floors, reinforced, concretebuildings, bridges, tanks, pipes, etc.

It is for all uses where the special properties of

other cement types are not required, such assulfate attack from soil and water, or to anobjectionable temperature rise.


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Type II: Moderate sulfate resistance

It is used where precaution against moderatesulfate attack is important, as in drainage

structures, which may be subjected to a moderatesulfate concentration from ground waters.

It is generates less heat of hydration at a slowerthan type I cement therefore can be used in massstructures such as large piers, heavy abutments,

and retaining walls. Due to less heat generation it can be preferred in

hot weather.


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Type III: High early strength

It is chemically and physically similar to type Icement, except that its particles have been groundfiner.

It provide high early strengths at the early period,usually a week or less.

It is used when forms need to be removed as soonas possible

It used in precast factories It is preferred in cold weather for reduction in the

curing period.


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Type IV: Low heat of hydration

It is used where the rate and amount of heatgenerated from hydration must be minimized.

It develops strength at a slower rate than othercement types.

It is most suitably used in massive concretestructures, such as large gravity dams, where the

temperature rise resulting from heat generatedduring hardening and must be minimized tocontrol the concrete cracking.


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Type V: High sulfate resistance

It is used only in concrete exposed to severesulfate action principally where soils or ground

waters have high sulfate content, levels reachfoundations.

Its high sulfate resistance is due to its low C3Acontent of about 4%

It is not resistant to acids and other highly

corrosive substances It used in off-shore structures


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Mixing water for concrete

Water helps in hydration of cement. It

should be clean and free from dirt and dustor salts. Drinkable water is good for making

concrete.


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Concrete Design Mix (BS 5328 : 1981)


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Admixtures:

To control certain properties of concrete, we addspecial materials, called admixtures, to givedesired properties:

Air-Entraining admixtures: to create voids inconcrete, are used primarily to increaseconcretes resistance to freeing. Improvesdurability under temperature changes, but reducestrength (15%)

Accelerating admixtures: such as calcium chlorideto concrete will accelerate its early strengthdevelopment.


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Retarding admixtures: are used to show the sittingof the concrete and to retard temperate increases.Used in large mass structures, such as dams.

Workability admixtures: used to make concreteeasier to handle and place into molds, without increating water/cement ratio.

Superplasticizers admixtures: made from organic

sulfonates, their use enables engineers to reducethe water content in concrete substantially whileat the same time increasing their slump.


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PROPERTIES OF CONCRETE

The properties of concrete are its characteristics

or basic qualities. The main properties of concrete

are:

Strength: the concrete resists compressive

stresses and not tensile stresses; therefore,

compressive strength is criterion of quality

concrete.

Workability: is used to describe the ease or

difficulty with which the concrete is hadle.


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As water to cement ratio increase, its strengthdecreases its workability increase.

Workability is usually indirectly measured in thefield by the slump test. Workability of concrete (how easily the concrete flows) is an indication ofhow easy it is placed in its molds.

For low w/c, strength is high, but concrete is difficult toplace in molds.

For high w/c, strength is not high.

We can obtain high strength concrete ( using loww/c ration) and high workability by usingadmixtures.


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FACTORS AFFECTING THESTRENGTH OF CONCRETE

1. Water-cement ratio: one of the most important factorsaffecting the strength of concrete. A w/c of 0.5 and 0.7 mayproduce a concrete strength of about 35 MPa and 21 MParespectively.

2. Method of Mixing: the use of mechanical concrete mixersand the proper time of mixing both have favorable effectson strength of concrete. Also, the use of vibrators producesdense concrete with a minimum percentage of voids. A void

ratio of 5% may reduce the concrete strength by about30%.


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3. Method of curing: the curing conditions exercise

an important influence on the strength of

concrete. Both moisture and temperature have a

direct effect on the hydration of cement.

4. Age of the concrete: the strength of the concrete

increases appreciably with age, and hydration ofcement continues for months.


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5. Loading conditions: the compressive

strength of concrete is estimated by

testing a cylinder or cube to failure in afew minutes. Under sustained loads for

years, the ultimate strength of uncrate is

reduced by about 30%.


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STRESS OF CONCRETE

Typical concrete compressive strength,

R/C structures: f c= 20 to 45 MPa

Prestressed concrete: fc = 35 to 55 MPa

High strength concrete: f c = 50 to 150 MPa

(in high rise buildings)


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Tensile strength of concrete

The tensile strength of concrete is relativelylow, the direct tensile strength, for example,

range from about 3 to 5 fc for normal

density concrete.


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