Concrete TechnologyConcrete Technology

Properties of Hardened Concrete

Lecture 17

Eng: Eyad HaddadEng: Eyad Haddad

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Introduction:

Concrete is a highly complex heterogeneous material whose response to

stress depends not only on the response of the individual components but

also upon the interaction between those components.

Strength of concrete may not be

the most important characteristic of

concrete; durability, volume stability,

and impermeability may be equally

significant. However, strength has

become universally accepted as the

most important indication of

concrete quality.

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Factors that effect concrete strength may be divided

into four categories:

(1) constitute materials,

(2) methods of preparation,

(3) curing procedures, and

(4) test conditions.

We have already discussed methods of preparation and curing.

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Compressive strength

The compressive strength of concrete is the most common measure for

judging not only the ability of the concrete to withstand load, but also the

quality of the hardened concrete. Test results obtained from compressive

strength tests have proved to be sensitive to changing mix materials and

mix proportions as well as to differences in curing and compaction of test

specimens.

There several reasons for this:

1) it is assumed that the most important properties of concrete as directly

related to compressive strength;

2) concrete has little tensile strength and is used primarily in

compression;

3) structural design codes are based on compressive strength;

4) the test is relatively simple and inexpensive to perform.

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ASTM Cylinder Test

The normal compressive specimen in America is a cylinder with length

to diameter ratio of 2:1. If the slump is more than 3 inches, concrete is

consolidated by rodding; if the slump is less than 1 inch, the concrete is

consolidated by vibration. Poorly compacted cylinders will have lower

strength.

If the specimen is to be rodded, it should be filled in three equal layers,

each rodded 25 times with 16mm diameter steel rod with a rounded end.

If specimens are to used for quality control the cylinders must be stored

at 15 to 27C for the first 24 hours in such a way that moisture loss is

prevented. The cylinder are then removed and stored in a standard moist

room or in saturated lime water (23C) until tested.

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ASTM Cylinder Test (cont.)

Capping cylinder reduces the effects of concentrated stresses under

loading.

Testing should be done as soon as capping is completed. Sulfur caps

lose strength and pourablity with used and therefore should not be

reused more than five times.

Determination of compressive strength using ASTM C39 states

tolerances for the testing machine. Since strength is dependent on

loading rate, the specimen should be loaded at a controlled rate of 1.4 to

3.5 kg/cm2/s or a deformation rate of 1.2mm/min.

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Cube Test

Cube test, standard in Great Britain and Germany. BS 1881: Part 108: 1983.

Filling in 3 layers with 50 mm for each layer (2 layers for 100 mm cube).

Strokes 35 times for 150 mm cube and 25 times for 100 mm cube. Curing at

20±5 0C and 90% relative humility. The loading rate is 2.3 kg/cm2/s.

Compressive strength is defined as:

fc’= P /A ( N/mm2 or MPa)

where

P = load to failure, N

A = cross-sectional area, mm2

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Cube Test (cont.)

Characteristic strength (fck) is defined as the val ue for the compressive

strength of concrete, below which not more than 5% of the valid test

results obtained on cubes of concrete of the same grade should fall.

For example, a concrete with characteristic strength of 30 MPa has a 95%

probability of achieving 30 MPa and more, and a 5% probability of being

less than 30 MPa.

A valid test result is the average result obtained from the testing of

three test specimens of concrete.

Specified strength normally refers to the character istic strength and is

indicated on design drawings or project specifications.

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Cube Test (cont.)

Target strength (fct) is the compressive strength that is aimed at to ensure

that the concrete meets the characteristic strength requirement. It is

obtained by using the formula:

fct = fck + 1,64 x standard deviation

where the standard deviation (SD) is dependent on the degree of control at the

concrete production facility.

Test specimens are crushed between two

plates in a hydraulic press.

The rate of load application influences the

compressive strength results and is specified

at a uniform rate of 0,3 MPa/s ± 0,1 MPa/s.

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Satisfactory modes of failure

are shown in the below

Figure:

Note: All four faces are cracked approximately equally, generally with little damage

to the faces in contact with the plates. The shape of the crushed specimen is a

good indication of whether the test was conducted in accordance with the

specification.

An unsatisfactory failure, as illustrated in the Figure below, may indicate that

the plates are not parallel, the cube is not square or the faces of the cube

are not flat.

The concrete areas in contact with the plates must be plane, parallel to each

other and at right angles to the y-axis of the specimen. An unsatisfactory

failure may give a suspect result, and indicates a deviation from standard

procedures

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Unsatisfactory mode of failure

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Factors Affecting the Measured

Compressive Strength:

Compression tests assumed that a pure state of uniaxial loading.

However, this is not the case, because of frictional forces between the

load plates and the specimen surface.

As specimen length to diameter ratio decreases the end effects are

more important resulting in higher apparent compressive strengths.

As l/d decreases below a value of 2 the strength increases. At ratios

above 2 the effect is more dramatic. Also, this phenomena is significant in

high-strength cement.

Specimen size is important for the simple fact that as the specimens

become larger it is more likely to contain an element that will fail at a low

load.

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Modulus of elasticity:

The modulus of elasticity of a material is defined by the slope of the stress-the slope of the stress-

strain curvestrain curve. The higher the elastic modulus, the more resistant the material is

to deformation. Concrete is not a perfectly elastic material and therefore the

stress-strain curve indicates a varying elastic modulus (the slope of the

tangent). Typical stress-strain curve for concrete

Young’s modulus or initial tangent modulus

is the initial linear part of the curve.

Tangent modulus is the slope of the tangent at

an arbitrary strain.

)(

)(tan

cC Strain

cStressE

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Modulus of elasticity: (cont.)

Secant modulus or static modulus of elasticity is the strain corresponding to a given

stress.

The value is normally determined by testing, where the stress is equal to one third

of the compressive stress. The value depends on the rate of load application.

According to ACI Building Code 318, with a concrete unit weight between 1441 and

2482 ton/m3, the modulus of elasticity can be determined from:

Where:

Ec = elastic modulus,

Wc = unit weight of concrete (lb/ft3),

f’c = the 28-day compressive strength of standard cylinders

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Modulus of elasticity: (cont.)

The splitting tensile strength can be obtained using the following formula:

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Modulus of elasticity: (cont.)

Indirect tension test (split cylinder test or Brazilian test) BS 1881: Part 117:1983.

Specimen 150 x 300 mm cylinder. Loading rate 0.02 to 0.04 MPa/s

ASTM C496-71:

Specimen 150 x 300 mm cylinder. Loading rate 0.011 to 0.023 MPa/s

The splitting test is carried out by applying compression loads along two axial

lines that are diametrically opposite. This test is based on the following

observation from elastic analysis. Under vertical loading acting on the two

ends of the vertical diametrical line, uniform tension is introduced along the

central part of the specimen.