chapter vi properties of hardened concrete...150 × 150 × 750-mm beam according to bs 1881: part...

Chapter VIProperties of Hardened Concrete

Dr. Mert Yücel YARDIMCI

Advanced Concrete Technology - Zongjun Li 1

STRENGTH OF HARDENED CONCRETE




Strength is defined as the ability of a material to resist the stress generated by an external force without failure.

For concrete, failure is frequently identified with the appearance of cracks.

Since the development of a crack is closely related to the development of deformation, in fact, the real criteria of failure for concrete is the limiting strain rather than the limiting stress.

The limiting strain for a concrete is different for different loading conditions and different strength levels.

Limiting strain for concrete

Uniaxial tension Uniaxial compression

100 × 10−6 – 200 × 10−6 4 × 10−3 (for 14 MPa concrete)2 × 10−3 (for 70 MPa concrete)

Materials of Construction-Concrete 4

Strength of hardened concrete is its ability to resist strain or ruptureinduced by external forces.

Center point

(three point)

loading

Four point

loading

Direct tensile

Splitting

tensile

Bending test

Uniaxial

compressive test

Compressive stress

(Compressive strength)

Direct tensile

stress

(Direct tensile

strength)

Splitting tensile

stress

(Splitting tensile

strength)

Flexural stress

(Flexural strength)

Indirect tensile tests



For uniaxial compressive loading mode…

Compressive strength and corresponding tests- Failure mechanism -


In compression, the failure mode is less brittle because considerably more energy isneeded to form and to extend cracks in the matrix. It is generally agreed that in auniaxial compression test on medium- or low-strength concrete, no cracks areinitiated in the matrix below about 40–50% of the failure stress.

Sample size for compressive strength


The c of concrete is found by

conducting compressive strength

tests on 150 mm in diameter and

300 mm long cylindrical

specimens.

150 mm or 200 mm cube

specimens are also used.

Materials of Construction-Concrete 9

- The freshly mixed concrete is placed in the mold in three equal layers.

- Each layer is compacted by 25 strokes of a 16 mm diameter steel rod.

- The top surface is finished by troweling.

- The specimen is kept in the mold for 24 hrs at 16 - 27 °C.

- The specimen is removed from the mold and stored in a moist roomor in saturated lime water at 20 ± 2°C until the testing day.

- The top surface of cylinders are capped with a thin layer (~ 3-5 mm) ofa capping material (mortar, stiff cement paste or sulfur). No capping isnecessary for cubic specimens.

Standard Test Method- Preparation of Test Specimens


Factors affecting the measured compressive strength

• Loading rate

• End condition

• Size effect

Test parameters

Loading rate


In general, the lower the rate of the loading, the lower themeasured compressive strength.

This may be attributed to the fact that deformation generatedby loading needs time to develop. The slow rates of loadingmay allow more subcritical crack growth to occur, thus leadingto the formation of larger flaws and hence a smaller apparentload.

On the other hand, it may be that slower loading rates allow more creep tooccur, which will increase the amount of strain at a given load. When thelimiting value of strain is reached, failure will occur. More likely, theobserved rate of loading effect is due to a combination of these, andperhaps other factors as well.

Loading rate


To make the compression results comparable, a standard load ratehas to be followed.

For a cylinder specimen, ASTM regulates 0.15–0.34 MPa/sec as thestandard loading rate.

For a cube specimen, BSI sets 0.2–0.4 MPa/sec as the standardloading rate.

The recommended loading rate in TS EN 12390-3 is 0.6 ± 0.2 Mpa/s

In the real situation, the loading rate can be transferred to N/sec bymultiplying the area of the specimen under the loading.

End condition


The compression test assumes a state of pure uniaxial compression.However, this is not really the case, because of friction between the ends of thespecimens and the platens of testing machine that make a contact with them.

The frictional force arises due to the fact that, because of the differences in themoduli of elasticity and Poisson’s ratio for steel and concrete, the lateral strain in theplatens is considerably less than the lateral expansion of the ends of the specimen ifthey were free to move.

End condition


The friction between the platens and the cube specimen ends confines amuch greater portion in the specimen than is the case with the cylindricalspecimen, as shown in Figure 5-7. This leads to higher strength values whenmeasured on cubes rather than cylinders. Usually, the ratio between cubestrengths and cylinder strengths is commonly assumed to be 1.25 for normal-strength concrete. However, for higher-strength concretes, the ratio will bereduced.

Uniaxial tensile strength and corresponding tests- Failure mechanism -


Uniaxial tension test is more difficult to conduct for three reasons.

❑ It is difficult to center the loading axis with the mechanicalcentroid.

❑ It is difficult to control the loading process due to the quasi-brittle nature of concrete under tension.

❑ The tension process is more sensitive to a sudden change incross-sectional area, and the specimen-holding devicesintroduce secondary stress that cannot be ignored.


Uniaxial tensile strength and corresponding tests- Failure mechanism -

Why the tensile strength of concrete is much lower than its compressive strength?


The tension strength of concrete is much lower than its compression strength, which can be attributed to the stress concentration generated by the defects in the materials.

The hole (2a x 2b) represents the defect!

Kt is the concentration factor. It can be seen that if a = b, Kt = 3.

Kt depends not only on the geometry of the hole but also on the loading pattern. If the loading is pure shear, Kt can reach 4.

The highest stress occurs at the edge ofthe ellipse and can be expressed as

Relationship between compressive strength and tensile strength


It is known that other mechanical properties of a concrete can be related to itscompressive strength. However, there is no direct proportionality betweentensile and compressive strength.

As the compressive strength of concrete increases, the tensile strength alsoincreases but at a decreasing rate.

The tensile/compressive strength ratio depends on the general level of thecompressive strength; the higher the compressive strength, the lower theratio. The research work done by Price (1951) showed that the direct(uniaxial) tensile/compressive strength ratio is 10 to 11% for low-strength, 8to 9% for medium-strength, and 5 to 7% for high-strength concrete.

The relationship between the compressive strength and the tensile/compressivestrength ratio seems to be determined by the effect of various factors on theproperties of both the matrix and the transition zone in concrete.

Indirect tension test (split-cylinder test or Brazilian test)


The indirect tension test is also called the splitting test or Brazilian test. The standard specimen for the splitting test is a 150 × 300-mm cylinder (BS 1881: Part 117:1983, ASTM C496-71).

The loading rate is 0.02 to 0.04 MPa/sec according to BS, 0.011 to 0.023 MPa/sec according to ASTM C496-71.


Indirect tension test (split-cylinder test or Brazilian test)

Flexural strength and corresponding tests


Flexural strength is also called the modulus of rupture (MOR).

The specimen for a flexural strength test is a 150 × 150 × 500-mm beam according to ASTM C78

150 × 150 × 750-mm beam according to BS 1881: Part 118: 1983.

A beam size of 100 × 100 × 500mm can be used when the maximum size of aggregate is less than 25 mm.


Flexural strength and corresponding tests

?

Valid only the crack occurs inbetween loading points!

Modulus of elasticity


The modulus of elasticity can be measured directly from the initial slope of a specially designed stress–strain curve.


5MPa

0,4*fc

Load and unload cycles 3 times

Wait at the level of 5MPaMeasure deformation (δb) Wait at the level of 0.4*fc stress

Measure deformation (δa).AfterCalculate (δa − δb), the result isdenoted as δ4

Record deformation (δ5)Take off the gages if(δ5 − δ4)<0.003 mm.

Relationship between compressive strength and modulus of elasticity


According to the British Standard for the structural use of concrete (BS 8110: part 2), modulus of elasticity concrete can be related to the cube compressive strength by the expression

when the density of concrete is 2320 kg/m3, i.e., for a typical normal-weight concrete.

If the density of concrete is between 1400 and 2320 kg/m3, the expression for Young’s modulus is

where ρ is the density of concrete in kg/m3.


According to the ACI Building Code 318-83, the relationship between Young’s modulus and compressive strength for normal density concrete is

Relationship between compressive strength and modulus of elasticity

where fc is the cylinder compressive strength.

For concrete with density of 1500 to 2500 kg/m3, the relationship changes to

It should be noted that in all the above equations, MPa is used for strength and stress, and Gpa for Young’s modulus.

What is the E modulus expression of Turkish Standards by means of the compressive strength of concrete?

chapter vi properties of hardened concrete...150 × 150 × 750-mm beam according to bs 1881: part...

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