Mahmoud Mohammed Hassan

4th year Major Chemistry

Supervisor

Dr. Safaa El-Gamal

Associate Professor in Physical Chemistry

Concrete Strength

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Outlines

Introduction

1. What’s the Concrete?…………………………………………………..…..2

2. What’s Compressive Strength of Concrete? ….…………………….........4

3. Why is Compressive Strength Determined? ……….………………..….4

4. Factors affecting Compressive Strength ……….………………..……….6

1. Quality of Raw materials ………………...……………………………..........…….6

2. Water/Cement Ratios …………………………………………………………..……7

3. Coarse/fine aggregates ……….………………………………………………….8

4. aggregate/Cement ratio ………..……………….………..……………….….……..8

5. Age …………………………………………………………………………………….……..9

6. Compaction ………………………………………….……………...……………..…….10

7. Temperature …………………………………………..………………….…………….11

8. Relative Humidity …………………………………….……………………….………12

9. Curing ………………………….…………………………….…………………………....12

5. Measurement of Strength ……………………………………………………..13

6. Summary ……………………………………………………………………...19

7. Comparison between Modern and Classic Method …………………………..20

8. Conclusion ……………………………………………………...…………….22

9. References ……………………………………...…………………...……….23

2

Introduction

Concrete is basically a mixture of two components: aggregates and paste. The

paste, comprised of Portland cement and water, binds the aggregates (usually

sand and gravel or crushed stone) into a rocklike mass as the paste hardens

because of the chemical reaction of the cement and water (Fig. 1-1).

Supplementary Cementitious materials and chemical admixtures may also be

included in the paste.

Aggregates are generally divided into two groups: fine and coarse. Fine

aggregates consist of natural or manufactured sand with particle sizes ranging up

to 9.5 mm (3⁄8 in.); coarse aggregates are particles retained on the 1.18 mm

(No. 16) sieve and ranging up to 150 mm (6 in.) in size. The maximum size of

coarse aggregate is typically 19 mm or 25 mm (3⁄4 in. or 1 in.). An intermediate-

sized aggregate, around 9.5 mm (3⁄8 in.), is sometimes added to improve the

overall aggregate gradation.

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The paste is composed of cementitious materials, water, and entrapped air or

purposely entrained air. The paste constitutes about 25% to 40% of the total

volume of concrete. Fig. 1-2 shows that the absolute volume of cement is usually

between 7% and 15% and the water between 14% and 21%. Air content in air-

entrained concrete ranges from about 4% to 8% of the volume.

Since aggregates make up about 60% to 75% of the total volume of concrete,

their selection is important. Aggregates should consist of particles with adequate

strength and resistance to exposure conditions and should not contain materials

that will cause deterioration of the concrete.

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A continuous gradation of aggregate particle sizes is desirable for efficient use of

the paste. Throughout this text, it will be assumed that suitable aggregates are

being used, except where otherwise noted.[1]

1. What’s the Compressive Strength of Concrete?

Concrete mixtures can be designed to provide a wide range of mechanical and

durability properties to meet the design requirements of a structure. The

compressive strength of concrete is the most common performance measure used

by the engineer in designing buildings and other structures.

The compressive strength is measured by breaking cylindrical concrete specimens

in a compression-testing machine. The compressive strength is calculated from the

failure load divided by the cross-sectional area resisting the load and reported in

units of pound-force per square inch (psi) in US Customary units or megapascals

(MPa) in SI units. Concrete compressive strength requirements can vary from 2500

psi (17 MPa) for residential concrete to 4000 psi (28 MPa) and higher in

commercial structures. Higher strengths up to and exceeding 10,000 psi (70 MPa)

are specified for certain applications.

2. WHY is Compressive Strength Determined?

Compressive strength test results are primarily used to determine that the concrete

mixture as delivered meets the requirements of the specified strength, ƒ´c, in the

job specification.

Strength test results from cast cylinders may be used for

1. quality control, acceptance of concrete, or for estimating scheduling

construction operations such as form removal or for evaluating the adequacy

of curing and protection afforded to the structure. Cylinders tested for

acceptance and quality control are made and cured in accordance with

procedures described for standard-cured specimens in ASTM C 31 Standard

Practice for Making and Curing

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2. Concrete Test Specimens in the Field. For estimating the inplace concrete

strength, ASTM C 31 provides procedures for field-cured specimens.

Cylindrical specimens are tested in accordance with ASTM C 39, Standard

Test Method for Compressive Strength of Cylindrical Concrete Specimens.

3. A test result is the average of at least two standard-cured strength specimens

made from the same concrete sample and tested at the same age. In most

cases strength requirements for concrete are at an age of 28 days.

4. Design engineers us the specified strength ƒ´c to design structural elements.

This specified strength is incorporated in the job contract documents. The

concrete mixture is designed to produce an average strength, , ƒ´c , higher

than the specified strength such that the risk of not complying with the

strength specification is minimized.

To comply with the strength requirements of a job specification both the following

acceptance criteria apply:

1. The average of 3 consecutive tests should equal or exceed the specified

strength, ƒ´c

2. No single strength test should fall below ƒ´c by more than 500 psi (3.45

MPa); or by more than 0.10 ƒ´c when ƒ´c is more than 5000 psi (35 MPa)

It is important to understand that an individual test falling below ƒ´c does not

necessarily constitute a failure to meet specification requirements. When the

average of strength

tests on a job are at the required average strength, ƒ′cr, the probability that

individual strength tests will be less than the specified strength is about 10% and

this is accounted for in the acceptance criteria.

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When strength test results indicate that the concrete delivered fails to meet the

requirements of the specification, it is important to recognize that the failure may

be in the testing , not the concrete. This is especially true if the fabrication,

handling, curing and testing of the cylinders are not conducted in accordance with

standard procedures. See CIP 9, Low Concrete Cylinder Strength. Historical

strength test records are used by the concrete producer to establish the target

average strength of concrete mixtures for future work. [2]

3. Factors affecting Compressive Strength

Intrinsic Production related

properties of the raw material Compaction

mix proportions External Environment

age Curing

1. Quality of Raw Material

Cement : provided the cemment conforms with the appropriate standard and it has

been stored correctly (i.e in dry Conditions) it should be suitable for use in

Concrete.

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Aggregate : we have already looked at the influence of aggregates on concrete

strength in point 4 of factors

Water : Frequently the quality of the water is covered by a clause stating "the

water should be fit for drinking" "this criterion through is not absolute and refrence

should be made to BS 3148 : Tests for water for making Concrete"

2. Water/Cement Ratios

The water/cement ratio versus strength relationship has already been covered. The

higher the w/c ratio the greater the initial spacing between the cement grains and

the greater the volume of residual voids not filled by hydration products.

A lower water cement ratio means less water, or more cement and lower

workability. However, if the workability becomes too low the concrete becomes

difficult to compact and the strength reduces.

For a given set of materials and environmental conditions the strength at any age

depends only on the water/cement ratio, providing full compaction can be

achieved.

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3. Coarse/fine aggregates

A. If the proportion of fines is increased in relation to the coarse aggregate, the

overall aggregate surface area will Increase

B. If the surface area of the aggregate has increased, the water demand (for a

constant workability) will also Increase

C. Assuming the water demand has increased, the water/cement ratio will

Increase

D. Since the w/c ratio has increased, the compressive strength will Decrease

4. Aggregate / Cement ratio

A. If the volume remains the same and the proportion of cement in relation to

that of sand is increased the surface area of the solid particles will Increase

B. If the surface area of the solids has increased, the water demand (for a

constant workability) will Stay the Same

C. Assuming an increase in cement content for no increase in water demand,

the water/cement ratio will Decrease

D. If the water/cement ratio reduces, the strength will Increase

The influence of cement content on workability and strength is an important one to

remember and can be summarised as follows:

For a given workability an increase in the proportion of cement in a mix has

little effect on the water demand and results in a reduction in the

water/cement ratio.

This reduction in water/cement ratio leads to an increase in strength.

Therefore for a given workability an increase in the cement content results in

an increase in strength.

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5. Age

The degree of hydration is synonymous with the age of the concrete provided the

concrete has not been allowed to dry out or the temperature is too low

A. Before the water is added to the mix the cement grains exist in an inert state.

However, after the cement has been mixed with the water, hydration

products form rapidly on the surface of the cement grain producing a high

rate of strength gain, as shown.

B. After a time the hydration products become so dense that it is difficult for

the water to get to the core of the Relative unhydrated cement grain.

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C. The rate of reaction, and curing therefore the rate of gain of strength then

reduces.

D. In theory, providing the concrete is not allowed to dry out, then it will

always be increasing in strength albeit at an ever reducing rate. For

convenience and for most practical applications it is generally accepted that

the majority of the strength has been achieved by 28 days.

E. For a typical Portland cement, the approximate relative proportions of the 28

day strength achieved at other ages is shown.

6. Compaction

Any entrapped air resulting from inadequate compaction of the plastic concrete

will lead to a reduction in strength.

Can you recall what the approximate relative strength would be if there was 10%

air trapped in the concrete?

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The relative strength range is assumed to be in the range of 35-45% of the

theoretical strength for 10% voids.

7. Temperature

The rate of the hydration reaction is temperature dependent.If the temperature

increases, then the reaction also Increases

This means that a concrete kept at a higher temperature will gain strength more

quickly than a similar concrete kept at a lower temperature.

However the final strength of the concrete kept at the higher temperature will be

lower. This is because the physical form of the hardened cement paste is less well

structured and more porous when hydration proceeds at a faster rate.

This is an important point to remember because temperature has a similar. but

more pronounced: detrimental effect on permeability as we will see later

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8. Relative Humidity

In the topics 'Fresh Concrete' and 'Cement' we emphasised the importance of

moisture to the hydration reaction.

If the concrete is allowed to dry out, the hydration reaction will stop. The hydration

reaction cannot proceed without moisture. If you have forgotten about this then

review topics 3 and 5 on Cement and Fresh Concrete respectively.

a. 100% Relative humidity (water).

b. Moist Air.

c. Dry Air.

The three curves shown represent the strength development of similar concretes

exposed to different conditions.

9. Curing

It should be clear from what has been said that the detrimental effects of storage in

a dry environment can be reduced if the concrete is adequately cured to prevent

excessive moisture loss.

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5. Measurement of Strength

1. Concrete suffers from one major drawback compared with materials like

steel and timber

2. Its strength cannot be measured prior to it being placed.

3. We have to wait some time (usually 7 or 28 days) for it to harden before we

can measure its strength.

4. Before looking in any detail at strength tests it is worth mentioning some

important points, some of which have been highlighted before with regard to

the testing of aggregates and plastic concrete. The tests must be performed in

exactly the same way every time.

5. The sample of concrete used must be truly representative of the batch from

which it is taken. The sample is usually taken at the same time as that for the

workability tests.

6. The test must be a measure of the "inherent" or "potential" strength of the

concrete as controlled only by:

7. Mix Proportions

8. Quality of Materials

9. Other influencing factors, i.e. Degree of Compaction. Age and Storage

Conditions must be kept constant: they must not influence the result. You

cannot blame the supplier of the concrete for a lower than expected strength

if this is a result of poor compaction or inadequate curing on the part of the

contractor!

10. Only a brief summary of the strength tests will be given here, further details

can be found in the appropriate standards as indicated.

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11. After fully compacting the concrete into the moulds the specimens are kept

under controlled conditions 20°C± 2°C. The relative humidity must not be

less than 90% (the cubes are usually kept in water -relative humidity 100%).

12. The specimens are tested at a fixed age (usually at 7 and 28 days) in

compression to failure, the load being applied at a controlled rate

Further details on test procedures can be found in BS 1881 Testing concrete

13. It is very important to realise that the strength obtained from this test is

likely to be greater than that achieved from the same concrete when placed

in the structure. This is because the cube/cylinder will be more thoroughly

compacted and will be stored under more favourable conditions than the

concrete in the structure.

14. The cube/cylinder strength is used to calculate the characteristic strength

of the concrete which is used for design purposes.

15. The expected differences between the cube/cylinder strength and the in-situ

strength are allowed for in design by the use of a partial safety factor

16. Because of the nature of the material it is far more difficult to test for tensile

strength than it is for compressive strength.

17. There are two tests commonly in use (see BS 1881), the splitting tension

test and the flexural test. In both cases the requirements for sample

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preparation, storage conditions and age of testing are the same as those for

the compressive strength test.

Splitting tension test: A cylinder of dimensions shown is tested on its side in

compression. The load P induces a tensile stress across the diameter.

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18. Concrete is a composite material made from natural and sometimes artificial

materials: it is not surprising, therefore, to find that its properties are

variable.

19. Compressive strength tests taken from one batch or several batches of

nominally the same concrete will not yield the same results.

20. The spread of results is assumed to approximate to the normal distribution

curve as shown.

21. The extent to which the results are spread about the mean is a measure of

the degree of control achieved during the production process.

22. The better the control the smaller the spread (i.e. the lower the variability)

and vice versa.

23. In statistical terms the magnitude of this variability is measured by a factor

called the Standard Deviation (SD). The lower the SD the better the control.

24. The standard deviation for concrete production can vary between

approximately 3.0-12.0 Nimm2 with a value of 5.0 1,1/mm2 being

considered as good.

25. When designing a structure, the designer will specify that the concrete

should have a given strength. When attempting to produce concrete to a

specific mean strength, tests on samples show that the actual strength

deviates from the mean. The amount of deviation depends on how closely

the mixing process is controlled and the strength of the individual materials

in the mix.

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6. 6. Summary

1. The strength of hardened concrete.

For a given set of materials the compressive strength of a concrete increases

with a reduction in the water/cement ratio, or an increase in the degree of

hydration which is synonymous with an increase in age provided the water

in the concrete does not evaporate.

Concretes kept at higher temperatures will have higher early strengths but

lower later strengths than similar concretes kept at lower temperatures.

The tensile strength of concrete is approximately ten times less than its

compressive strength.

2.Testing methods.

Compressive strength is determined by crushing cubes or cylinders to

failure.

Tensile strength is measured by the splitting test or the flexural test.

The spread of strength results of samples taken from nominally the same

concretes is assumed to approximate to the normal distribution curve.

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7. Comparison between Modern and Classic Method

Concrete Compressive Strength Testing with Press and Rebound

Hammer

Advantages of testing the compressive strength of concrete with two

methods:

1. The NDT method with the rebound hammer

2. the classical method by crushing specimens.

A. The Classical Method

By crushing specimens in the compressive testing machine we obtain the

compressive strength as the test result. Therefore, it is a direct test method which is

globally standardized.

Together with the modulus of elasticity, the compressive strength is the most

important property of concrete.

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B. The Rebound Hammer Test

This non-destructive testing (NDT) technique is an indirect method because a

rebound value is measured and not the compressive strength. It complements the

classical method in a perfect way because:

• The method does not damage the structure like the classical method, where cores

must be taken for the evaluation of the compressive strength.

• Tests can be done in-situ on the whole surface of a concrete structure, hence the

quality of the entire structure is tested but not only the quality of a few specimens

crushed with the classical method.

• It is a fast, inexpensive and easy to perform method using a light and portable test

equipment.

SilverSchmidt Rebound Hammer

The SilverSchmidt is the first integrated concrete test hammer featuring true

rebound value and unmatched repeatability. Up to five conversion curves can be

installed on the instrument. The rebound value is independent of the impact

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direction, thus it is especially practical when testing in different directions, e. g. on

circular tunnel linings.

European Standards

The following European standards refer to the rebound method:

• EN 12504-2: Non-destructive testing – Determination of rebound number.

• EN 13791: Assessment of in-situ compressive strength in structures and precast

concrete components.

8. Conclusion

• The rebound value can be measured discretionary, whereas the number of

crushed specimens is limited.

• The combination of both methods is the best and most reliable procedure to

determine the compressive strength of concrete structures.

• The procedure is described in EN 13791. A conversion curve must be created to

convert the rebound value - obtained with the NDT rebound method - to compres-

sive strength measured with the classical method.

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References

[1] Design and Control of Concrete Mixtures , 40 EDITION , Published by PCA

[Portland Cement Association ]

The authors of this engineering bulletin are:

Steven H. Kosmatka, Managing Director, Research and Technical Services, PCA

Beatrix Kerkhoff, Civil Engineer, Product Standards and Technology, PCA

William C. Panarese, former Manager, Construction Information Services, PCA

ISBN 0-89312-217-3 (pbk. : alk. paper) , PCA R&D Serial Number SN2561

[2] THE MATURITY METHOD: FROM THEORY TO APPLICATION

By N.J. Carino and H.S. Lew , Building and Fire Research Laboratory , National

Institute of Standards and Technology , Gaithersburg, MD 20899-8611 USA

Reprinted from the Proceedings of the 2001 Structures Congress & Exposition

,May 21-23, 2001, Washington, D.C., American Society of Civil Engineers,

Reston, Virginia, Peter C. Chang, Editor, 2001, 19 p.

[3] 1. ASTM C 31, C 39, C 617, C 1077, C 1231, Annual Book of ASTM

Standards, Volume , 04.02, ASTM, West Conshohocken, PA, www.astm.org

2. Concrete in Practice Series, NRMCA, Silver Spring, MD,www.nrmca.org

3. In-Place Strength Evaluation - A Recommended Practice,NRMCA

Publication 133, NRMCA RES Committee, NRMCA, Silver Spring, MD

4. How producers can correct improper test-cylinder curing, Ward R. Malisch,

Concrete , Producer Magazine, November 1997, www.worldofconcrete.com

5. NRMCA/ASCC Checklist for Concrete Pre-Construction Conference,

NRMCA, Silver , Spring, MD

6. Review of Variables That Influence Measured Concrete Compressive

Strength, David N. Richardson, NRMCA Publication 179, NRMCA, Silver

Spring, MD

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7. Tips on Control Tests for Quality Concrete, PA015, Portland Cement

Association, Skokie, IL, www.cement.org

8. ACI 214, Recommended Practice for Evaluation of Strength Tests Results of

Concrete, American Concrete Institute, Farmington Hills, MI, www.concrete.org