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6. STRENGTH OF CONCRETE

Nature of Strength-1

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Since the resistance to failure is due to both cohesion and internal friction, the angle of rupture is not 45° but is a function of the internal friction angle ; the angle which the plane of failure makes with the axis of loading is equal to 45 - /2. The internal friction angle for concrete is approximately 20°.

Time-2

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In practice, it is common to obtain 7-day as well as 28-day compressive strength tests. Thus, it becomes possible to extrapolate the 28-day strength from 7-day (or other) strengths.

These values are obtained for w/c= 0.52 concretes cast into 100 mm cubic molds and cured at

20oC:

Age of Concrete Age Factor

Range Average

3 days 0.39-0.63 0.51

7 days 0.59-0.86 0.72

14 days 0.77-1.00 0.88

28 days (1 month) 1.00 1.00

2 months 1.01-1.25 1.11

4 months 1.03-1.34 1.17

8 months 1.04-1.41 1.23

1 year 1.11-1.52 1.28

2 year 1.12-1.49 1.31

5 year 1.12-1.76 1.39

W/C Ratio vs. Compressive Strength

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Effect of Curing Conditions on Concrete Compressive Strength

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Dmax vs. Compressive Strength

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Specimen deformation and normal stress distribution for (a) hard and (b) soft platens

Influence of L/D Ratio on Concrete Strength

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SIZE EFFECT

As most of the other real materials, larger concrete specimens give lower apparent compressive strength.

However, after a certain large size, “size effect” becomes negligible.

Table 6.2 Factors for Converting Apparent Strength to Standard Strength

Cube specimen dimensions (mm) Factor to convert to 200mm cube strength

80 1.18

120 1.12

150 1.06

200 1.00

250 0.94

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CUBES OR CYLINDERS ?

In different parts of the world, either cylinder specimens (150x300 mm) or cube specimens (200 mm) are used as standard compressive strength specimens. The relationship between cylinder strength and cube strength is given by,

where, k is a constant depending on the cement content of the mix.

Cubes are more practical to test, but for at least research purposes, cylinders are preferred in most countries.

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cylcube k

Concrete Strength Classes for Normal weight and heavy weight concrete – TS EN 206-1

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Concrete Strength Classes for lightweight concrete – TS EN 206-1

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Methods of tension testing: (a) Direct tension; (b) Flexural tension; (c) Split tension.

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Stress distribution in split-cylinder test-1

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Estimation of Concrete Strength by Cores

TS EN 13791: 2010 Assessment of in-situ compressive strength in structures and precast concrete components

TS EN 12504-1:2002 Testing concrete in structures – Part 1: Cored specimens. Taking, Examining and Testing in Compression

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Why Cores are Taken?

To determine the compressive strength of concrete in structure which is still in service and to compare the result with the relevant standards.

• To comment on the correctness of placement, compaction and curing operations that were applied to the concrete.

• To determine whether compressive strength loss happened due to some reasons, etc., overloading, fire, freeze-thaw, other durability problems.

• To investigate the mezo-micro structure of concrete in order to find the evidence of ASR, carbonation, etc. 17

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TS EN 13791 Assessment of in-situ compressive strength in structures and precast concrete components

This standard applies where cores are used to;

- Determine the conformability of concrete strength in the structure,

- Evaluate the old structures that was damaged or in need of modification/redesign

- Solve the conflictions about concrete quality,

Evaluation of the results of indirect test methods (rebound hammer, pull-out, ultrasonic pulse velocity, e.g.) correlated with core strength tests.

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Determination of Compressive Strength

• l/d :1 and d:100 mm core str. = 150 mm standard cube str.

• l/d: 2 and d:100-150 mm core str = 150x300 cylindrical str

• See the relevant standard (TS EN 13791) for statistical evaluation of results.

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• How to take cores from the structure?

1. Cores should be taken at least from 14 day old concretes,

2. Cutting the reinforcement should be avoided while taking cores,

3. H/D ratio should never be less than 1,

4. At least 100 mm or 150 mm diameter cores should be taken, if core diameter is less than 100 mm, number of core samples should be doubled. Cores less than 50mm should never be used for strength determination,

5. Aggregate Dmax / core diameter should be at least 1/3.

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6. Core caps should be cutted parallel to each other,

7. 3 - 5 mm thick capping should be made with a suitable material,

8. Cores are kept under water at room temperature for 2-3 days in order to bring to saturated condition just before the application of strength test,

10. Compressive strength test is applied at a speed of 1.4-3.5 kgf/cm2/sec.

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Factors Affecting Core Strength;

Samples size (diameter and height);

size strength

Presence of reinforcement

reinforcement present strength

Rate of loading; rate strength

Direction of coring; horizontal strength

vertical strength

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Age of conrete; age strength

Moisture conditions of concrete; moisture strength

Other testing conditions..

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