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C. Compressive Strength of Concrete

is controlled by the proportioning of:

cement,

coarse and fine aggregates,

water (is the chief factor for determining

concrete strength, as shown in Fig.1.11

the lower is the water-cement ratio,

the higher is the compressive strength.),

various admixtures.

Water:

necessary for the proper chemical action in the hardening of concrete,

extra water increases the workability but reduces strength.

C2

2

Compressive Strength of Concrete

a. spliting

b. sliping

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Compressive Strength of Concrete the concrete classes

The concrete of a given strength is identified by its class. A class C20/25

concrete, for example, has a characteristic cylinder crushing strength, at 28

days, fck = 20 N/mm2 and a characteristic cube crushing strength

fck,cube = 25N/mm2.

C20/25

fck(cylinder)

fck,cube(cube)

4

D. Tensile Strength

an important property that greatly affects the extent and size of cracking in

structures.

is a more variable property than compressive strength, and is about 10 to

15% of it.

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D. Tensile Strength

6

E. Stress-Strain Relation

The loads on a structure cause distortion

of its members with resulting stresses and

strains in the concrete and the steel

reinforcement.

To carry out the analysis and design of a

member it is necessary to have knowledgeof the relationship between these stresses

and strains.

Concrete is a very variable material, having a wide range of strengths and stress-strain

curves.

The stress-strain behavior of concrete is dependent on its strength, age at loading, rate

of loading, aggregates and cement properties, and type and size of specimens.

Typical curves for specimens loaded in compression at 28 days using normal testing

speeds are shown in Fig.1.16.

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G. Shrinkage

As concrete hardens there is a reduction in volume.

Shrinkage, broadly defined, is the volume change that is unrelated to load

application.

It is possible for concrete cured continuously under water to increase in volume

(this volume change is known as a swell); however, the usual concern is with a

decrease in volume.

10

G. Shrinkage

Shrinkage is liable to cause cracking of the concrete, but it also has the

beneficial effect of strengthening the bond between the concrete and the steel

reinforcement.

The total shrinkage strain is obtained as a sum of two components, the drying

shrinkage strain (cd) and the autogenous shrinkage strain (ca-). The value of the

total shrinkage strain (cs

) follows from the equation:

The final value of the drying shrinkage strain cd, is obtained from the equation:

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11

G. Shrinkage

12

G. Shrinkage

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H. Creep

creep and shrinkage are time-dependent deformations that, alongside the

cracking, provide the greatest concern for the designer.

concrete is elastic only under loads of short duration. Because of additional

deformation with time, the effective behavior is that of an inelastic material.

Creep, or plastic flow as it is some times called, is the continuous

deformation of a member under sustained load at unit stresses within the

accepted elastic range (say, below 0.5fck).

increased deformation with time.

14

H. Creep

The internal mechanism of creep may be due to any one or a combination of

the following:

(1)crystalline flow in the aggregate and hardened cement paste,

(2)plastic flow of the cement paste surrounding the aggregate,

(3)closing of internal voids,

(4)the flow of water out of the cement gel due to external load and drying.

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H. Creep

The creep deformation of concrete cc(,t0) at time t=, for a constant

compressive stress c with time, may be calculated from the relation:

16

H. Creep

The effects of creep are particularly

important in beams, where the increased

deflections may cause the opening of

cracks, damage to finishes and the

non-alignment of mechanical equipment.

The effect of unloading may be seen from Fig 1.22, where at a certain time t1

,

the load is removed. There is an immediate elastic recovery and a long-time

creep recovery, but a residual deformation remains.

Redistribution of stress between concrete and steel occurs primarily in the

uncracked compressive areas and has little effect on the tension reinforcement,

other than reducing shrinkage stresses in some instance.

The provision of reinforcements in the compressive zone of a flexural member

often helps to restrain the deflections due to creep.