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Materials of Construction-Concrete 1

Concrete

Chapter 10

Properties of Hardened Concrete

Wikipedia.org



The principal properties of hardened concrete which are of practical importance can be listed as:

1. Strength

2. Permeability & durability

3. Shrinkage & creep deformations

4. Response to temperature variations


Strength of hardened concrete is its ability to resist strain or rupture induced 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



Among the properties of hardened concrete,

compressive strength is the most important

property of concrete.

Because;

1. Concrete is used for compressive loads,

2. Compressive strength is easily obtained,

3. It is a good measure of all the other

properties.


Factors Affecting the Strength of

Concrete

- W/C ratio,

-Degree of compaction,

- Quality of mixing water,

- Properties of cement,

- Properties of aggregates,

- Type and amount of admixtures,

- Mixing, transportation, placing, and consolidation operations applied to concrete,

- Curing conditions and age.

- Rate of loading, shape and size of the specimen and its moisture condition are also effective.


Effect of Curing Conditions and

Age of Concrete on Strength

The function of water in a concrete mix is:

- to react with cement to form sufficient gel,

- to provide good workability for fresh concrete.

The rate and amount of hydration are directly proportional with the rate and amount of strength development in cement paste and concrete.

After placing, compacting and finishing operations of fresh concrete, the concrete needs to be protected so that a satisfactory moisture content and temperature can be maintained in it for the hydration process to continue at a desired rate.


Curing Methods

Water curing Supplying additional water to the concrete

and prevent moisture loss from concrete.

Sealed curing Prevent moisture loss from the concrete

Effect of weather conditions on

curing

After the placing, compacting and finishing operations of fresh concrete,

the concrete needs to be protected so that a satisfactory moisture

content and temperature can be maintained in it for the hydration

process.

The weather conditions affecting the evaporation rate of some of the

water from fresh concrete also affect strength development of concrete.


Protect the fresh concrete!

In hot weather!

In hot and dry weather conditions!

In windy days!

The optimum temperature for placing fresh concrete is about 15-16 °C.

When the temp. difference between the enviroment and concrete

exceeds 20°C, the ultimate strength of the concrete may become lower

than the desired.


Test methods for determining concrete

compressive strength c:

1. Standard test method

2. Testing core specimens drilled from the

hardened concrete.

3. Non-destructive methods (rebound

hardness)

Standard Test Method


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.


- 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 room or in saturated lime water at 23 ± 1.7°C until the testing day.

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

Standard Test Method-

Preparation of Test Specimens


Standard Test Method- Determination of the

Compressive Strength

The concrete specimen is tested in a suitable testing machine equipped with two steel bearing blocks, the upper one is a spherically seated block. When the machine is on, one of the blocks moves vertically and an axial load is applied to the specimen. The load is applied at a constant rate of 0.6 ± 0.2 MPa/s until the specimen breaks.

The compressive strength of the specimen is calculated as follows:

c = Pmax / A


COMPRESSIVE STRENGTH

15

Concrete strength

classes (EN206-1)


Standard Test Method

The compressive strength of concrete is usually determined at an age of 28 days. The 28-day compressive strength value is generally used in concrete designs.

Generally;

c-3 days 0.45 c-28 days



At least three specimens should be tested; the average of their compressive strengths is found for determining c of a concrete sample on a particular testing day.


The cube strength can be converted to cylinder strength by using the following relation:

cylinder 0.85 cube(200mm)


Tensile Strength (t) of Concrete

t of concrete is the maximum unit stress that the material is capable of resisting under tensile loading.

Although the t of concrete is not considered directly in design (being assumed to equal zero), it is very important since cracking in concrete tends to be the cause of tensile failure.

The t of concrete is approximately 10% of its c



The tensile strength of concrete can be measured by ‘direct

or indirect tensile loading tests’

Direct Tensile Test

Gripping the material is very

difficult.

Dog-bone shaped samples

can be used. But due to

crack formation in the grip

region, unreliable results

may be obtained.



Indirect Tensile Test

Indirect tensile tests can be applied in two different loading

mode: Splitting and flexure.



(Splitting tensile strength)



(Flexural tensile strength)

Another indirect tensile test!

P P/2 P/2

h

Standard dimensions:

150 x 150 x 600 mm


Determination of c of Existing Concrete

Structures

Compressive strength of an existing building

could be determined (or predicted) by destructive

and non-destructive tests

Destructive test : Drilled core specimens

Non-destructive test : Rebound hammer

(Schmidt Hammer)


Determination of c of Concrete Using Drilled

Core Specimens

Core specimens are obtained from hardened concrete by

drilling operation.

Detection of steel rebars before

drilling.

Kurtulus C. & Bozkurt A. (2011), Determination of concrete

compressive strength of the structures in Istanbul and

Izmit Cities (Turkey) by combination of destructive and

non-destructive methods. International Journal of the

Physical Sciences Vol. 6(16), pp. 4044-4047


Determination of c of Concrete Using Core

Specimens

Core specimens are removed from hardened concrete by drilling operation.

The diameter of the concrete core specimen depends on the inner diameter of the cylindrical cutting device (usually 10 cm or 15 cm, sometimes 5 cm).

The length of the core specimens depends on the thickness of the member from which it is cut.

If the core specimen is too long, it is shortened to have a l/d ratio of 2.0. Core specimens having 1.0<l/d<2.0 can be used for compressive strength testing purposes. If l/d<2.0 the strength found by test should be multiplied with the following correction factors:


Determination of concrete compressive strength by testing core specimens is useful in finding the strength of concrete that is present in a structure.

As is known, the strength of the concrete found by the standard method may be different from its strength in the structure.

The operations applied to the concrete in the structure such as placing, consolidation and curing may lead to differences in the strength. Moreover, factors such as chemical attack, repeated loads, freezing and thawing and fire may have caused a decrease in the quality of the concrete in the structure.

Core testing method provides the possibility of finding the actual quality of the concrete in the structure.


Estimation of c by Rebound Hammer

(Schmidt Hammer)

This method is universally used because of its simplicity. The test is based on the principle that the rebound of an elastic mass which determines the hardness of the surface the rebound hammer applied.

At least 10 rebound readings should be taken over the area to be tested and the average value should be considered as the rebound number. These readings should be taken on points which are approximately 1 cm apart from each other.

The test is sensitive to the presence of coarse aggregate particles and voids immediately underneath the plunger. Therefore, readings on voids and aggregate particles should be avoided.


Generally the relationship between the c of 20 cm cube specimens and the measured rebound numbers are given with the manufacturer of the hammer as follows.


The cube strength can be converted to cylinder strength by using the following relation:

cylinder 0.85 cube(200mm)

The concrete does not break by the application of the rebound hammer, thus, the test is a non-destructive one.

The c values determined by the rebound hammer test are approximate values. There may be variations up to 20% (or even more) between the values determined by such a test and the values found by the standard test method.

Exercise_1

A 200 mm cube concrete specimen was failed under

a uniaxial compressive load of 2000 kN. Predict the

amount of uniaxial compressive loads to crush a

150 mm cube and a 150x300 mm cylinder

specimen of the same concrete. (Assume that;

cyl = 0.8 cube200 and cube200 = 0.95 cube150 )

cube200 = 2000x1000/(200x200)=50 MPa,

cube150 = 50/0.95=52.63 MPa, Pcube150 = 1184 kN

cyl = 50*0.8=40 MPa, Pcyl = 707 kN


Exercise_2

Calculate the maximum size of the aggregate to be used in

concrete designed for the following beam.

Dmax ≤ 1/5 *250 = 50 mm (min. dimension of the mould)

Dmax ≤ ¾ *30 = 22.5 mm (clear spacing betw. reinforcements)

Dmax ≤ 1/3 * 120 = 40 mm (thickness of slab)

Dmax ≤ concrete cover (50 mm)

Dmax should be ≤ 22 mm


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