Concrete Edited(1)

7/31/2019 Concrete Edited(1)

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BTEC Higher National Diploma in Civil Engineering

Edexcel International UK

Page 1.

Slump Test

Introduction

Concrete slump test (or simply the slump test) is an in situ test or a laboratory test used to

determine and measure how hard and consistent a given sample of concrete is before curing.

Purpose

The goal of the test is to measure the consistency of concrete. Many factors are taken into account

when satisfying requirements of concrete strength, and to make sure that a consistent mixture of

cement is being used during the process of construction. The test also further determines the

workability of concrete, which provides a scale on how easy is it to handle, compact, and cure

concrete. Engineers use the results to then alter the concrete mix by adjusting the cement-water

ratio or adding plasticizers to increase the slump of the concrete mix.

Suitability of Slump Test:The slump test is suitable only for the concrete of high or medium workability.

Procedure

This test is carried out with a mould called slump cone whose top diameter is 10cm, bottom

diameter is 20 cm and height is 30 cm. the test may be performed in the following steps:

1. Place the slump mould on a smooth flat and non-absorbent surface.

2. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add

the required quantity of water.

3. Place the mixed concrete in the mould to about one-fourth of its height.

4. Compact the concrete 25 times with the help of a tamping rod uniformly all over the area.

5. Place the concrete in the mould about half of its height and compact it again.

6. Place the concrete upto its three fourth height and then upto its top. Compact each layer 25

times with the help of tamping rod uniformly. For the second subsequent layers, the tamping rod

should penetrate into underlying layers.

7. Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its

top.

8. Remove the mould immediately, ensuring its movement in vertical direction.

9. When the settlement of concrete stops, measure the subsidence of the concrete in millimeters

which is the required slump of the concrete

Slump Test Results

Trial Slump (mm)

1 175

2 165

3 185


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Page 2.

CUBE TEST

For new concrete this usually involves casting specimens from fresh concrete and testing them for various

properties as the concrete matures. The concrete cube test' is the most familiar test and is used as thestandard method of measuring compressive strength for quality control purposes. Concrete beam

specimens are cast to test for flexural strength and cast cylinders can be used for tensile strength.

Specimens for many other tests can be made at the same time to assess other properties, eg. Dryingshrinkage, thermal coefficient, modulus of elasticity.

150 mm moulds should be filled in three approximately equal layers (50 mm deep). A compacting bar

is provided for compacting the concrete. During the compaction of each layer with the compactingbar, the strokes should be distributed in a uniform manner over the surface of the concrete and each

layer should be compacted to its full depth. During the compaction of the first layer, the compactingbar should not forcibly strike the bottom of the mould. For subsequent layers, the compacting bar

should pass into the layer immediately below. The minimum number of strokes per layer required to

produce full compaction will depend upon the workability of the concrete, but at least 25 strokes will

be necessary except in the case of very high workability concrete. After the top layer has beencompacted, a trowel should be used to finish off the surface level with the top of the mould, and the

outside of the mould should be wiped clean.

Results

Slump test

Trial 1 2 3

Slump (mm) 175 165 185

Cube compression test results

Identification 1 2

Length (mm) 152 150

Width (mm) 150 150

Height (mm) 149 150

Weight (kg) 8.164 8.330

Failure (kN) 5.442 403.8

Observations Failure mode satisfactory Failure mode satisfactoryCalculations

Density

Density = Weight/ Cross sectional Area

Cube 1

8.164/3.3972*10^-3 = 2.403156*10^-3

Cube 2

8.330/3.375*10^-3 = 2.468*10^-3

Compressive Strength

Compressive Strength = Failure Load / Cross sectional Area * 10^-6


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Page 3.

Cube 1

544.2*10^3/(152*150) = 23.868642

After 7 days, 67% strength must be regained

23.86/.67 = 35.61MPa

Cube 2

403.8*10^3 (150*150) = 17.9466

After 7 days, 67% strength must be regained

17.9466/ .67 = MPa

The target strength of the experiment was aimed at 43.1 MPa. The results displayed strengths of 35.61and 26.78 for Cube 1 and 2 respectively. Therefore, the experiment was not successful for it did not

fulfill the required strength


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DISSCUSION

1. Describe the purpose of the concrete mix design.

Mix design can be defined as the process of selecting suitable ingredients of concrete and

determining their relative proportions with the object of producing concrete of certain minimum

strength and durability as economically as possible. Concrete mix design plays an important role in

the production of concrete used for a variety of purposes. There are three stages in concrete's life

that need to be considered when developing the mix: the fresh concrete, the newly completed

concrete, and everything after that for the rest of its life. As it is known, a wet easy-to-place

concrete is not likely to be a durable concrete. But if you order the world's most durable concrete

and find it impossible to place, then that's no good either. So it is important to figure out what

concrete mix would produce the most durable and which carries the best workability for the

specified purpose.

2. Comment on the specifications for a mix design.

The specification of a concrete mix must therefore define the materials, the strength, workability

and durability to be attained. The materials have been detailed one by one below:

Cement:

In mix design the most important property of a type of cement is its influence on the strength of

the concrete. The DOE method, incorporates empirical data, which relate concrete strength to

the free water/cement ratio and the coarse aggregate type for concrete made from different

cements. It is the decision of the guy who mixes to decide in what proportions the cement must beadded for certain specified works.

Fine aggregate

The fine aggregate may be either crushed or uncrushed. Uncrushed aggregates are usually

smoother than crushed aggregates and in comparison with concrete made with crushed fine

aggregate, concrete containing uncrushed fine aggregate will generally have superior workability.

Coarse aggregate

The type (crushed or uncrushed) and maximum size of the coarse aggregate is important in mix design andshould be specified. The density should be also be specified if known. The coarse aggregate comprises theportion of the aggregate, which has a large particle size.

The DOE method was developed for aggregates conforming to BS 882 and the coarse aggregate is defined as

containing a high proportion of particles retained on a 5mm (0.197 in.) sieve. Coarse aggregate can also becrushed and uncrushed. Uncrushed aggregates are usually smoother than crushed aggregates and in

comparison with concrete made with uncrushed coarse aggregate, concrete containing crushed coarse

aggregate will generally have superior strength and inferior workability, although the coarse aggregate has

considerably less influence on the workability than the fine aggregate.

Water


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Page 5.

In this design water utilized must be pure water - in fact it has been noted that the water used

must carry all properties of drinking water.

Strength

Generally a concrete mix is required to provide a specified strength. The most common measure of concretestrength is the compressive strength, determined in either a cube test or a cylinder test. (Cube tests as per

BS and Cylinder tests as per US Standards)

The DOE Method assumes that the major factors, which determine the strength of a mix, are:

The free water/ cement ratio The coarse aggregate type The cement properties

Durability

Tests for durability are not normally specified for concrete. Instead conditions are set for the mix,

which are intended to provide the required durability.

Workability

The required workability and the method of testing must be discussed and agreed upon for a

designed concrete mix.


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Page 6.

3. Discuss the importance of the property of "Workability" stating the factors affecting it.

Workability is often referred to as the ease with which a concrete can be transported, placed and

consolidated without excessive bleeding or segregation.

In the case of concrete, consistence is sometimes taken to mean the degree of wetness; within limits, wet

concretes are more workable than dry concrete, but concrete of same consistence may vary in workability.

Because the strength of concrete is adversely and significantly affected by the presence of voids in the

compacted mass, it is vital to achieve a maximum possible density. This requires sufficient workability for

virtually full compaction to be possible using a reasonable amount of work under the given conditions.Presence of voids in concrete reduces the density and greatly reduces the strength: 5% of voids can lower

the strength by as much as 30%.

Generally a concrete mix is required to provide a specified degree of workability, although there is

no general agreement on how workability should be defined. Workability is often considered to be

a measure of the work needed to compact the wet concrete, but it is also used to quantify the easewith which concrete can be placed, although this depends on other properties such as

cohesiveness. Workability is usually measured by the Slump Test.

Factors affecting workability

Water content or Water Cement Ratio

More the water cement ratio more will be workability of concrete. Since by simply adding water

the inter particle lubrication is increased.

High water content results in a higher fluidity and greater workability. Increased water content

also results in bleeding. another effect of increased water content can also be that cement slurry

will escape through joints of formwork.

ii. Amount and type of Aggregate

More the amount of aggregate less will be workability.

Using smooth and round aggregate increases the workability. Workability reduces ifangular and rough aggregate is used.

Greater size of Aggregate- less water is required to lubricate it, the extra water is availablefor workability

Angular aggregates increases flakiness or elongation thus reduces workability. Roundsmooth aggregates require less water and less lubrication and gretaer workability in a

given w/c ratio

Porous aggregates require more water compared to non absorbent aggregates forachieving sam degree of workability.

iii. Aggregate Cement ratio

More ratio, less workability. Since less cement mean less water, so the paste is stiff.

iv. Weather Conditions


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1. Temperature -If temperature is high, evaporation increases, thus workability decreases.

2. Wind: - If wind is moving with greater velocity, the rate of evaporation also increase reduces the

amount of water and ultimately reducing workability.

Admixtures

Chemical admixtures can be used to increase workability.

Use of air entraining agent produces air bubbles which acts as a sort of ball bearing between particles and

increases mobility, workability and decreases bleeding, segregation. The use of fine pozzolanic materialsalso has better lubricating effect and more workability.

vi. Sand to Aggregate ratio

If the amount of sand is more the workability will reduce because sand has more surface area and morecontact area causing more resistance.

4. Compare the available methods of measuring workability briefly.

Workability, a term applied to many concrete properties, can be adequately measured by three

characteristics:

1. Compatibility, the ease with which the concrete can be compacted and air void removed.2. Mobility, ease with which concrete can flow into forms and around reinforcement.3. Stability, ability for concrete to remain stable and homogeneous during handling and

vibration without excessive segregation.

The following tests have been developed to measure workability:

Standards:

A- Slump test - BS: 1882 part II

B- Compacting factor test - BS: 1881 part II 1970.

C- Vebe test - BS: 188 part II 1970.

Apparatus Required:

1. Slump mold - Its in the form of the frustum of cone with the following dimensions: Bottom diameter 20mm. Top diameter 10mm. Height 300mm.2. Tamping rod.3. Trowel.


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Compacting factor test:

1. The compacting factor apparatus -It consists essentially of two conical hoppers fitted withdoors at the base and placed one above the other, and a 150x300 mm cylinder placed below

the hoppers.

2.

Two plasterers trowels.3. Scoop.4. Tamping rod.5. Balance.6. Compacting rod.

Vebe test:

1. The vebe apparatus - It consists of a vibrating table, a cylindrical pan, a standard slumpcone and a glass or plastic disk attached to a free-moving rod, which serves as a reference

end point.

2. 2. Trowel.3. Tamping rod.4. Stop watch.

Procedure -

A- Slump test:

1. Clean the internal surface of the mold.

2. Place the mold on a smooth, horizontal, rigid and non-absorbentsurface and hold it firmly in

place while it is being filled.

3. Fill it with concrete in three layers, each layer is tamped 25 times with the standard tamping

rod.

4. After tamping the top layer, the mold shall be filled and the concrete struck off and finished level

with the travel.

5. Remove the mold slowly and vertically.

6. Measure the slump (the difference in height between the mold and the highest point of thespecimen).

7. Any slump specimen which collapses or shears off laterally will give an incorrect results and if

this occurs the test shall be repeated with another sample. If it happens again take the results.

B- Compacting factor test:

1. Clean the hoppers and the cylinder.

2. Gently fill the upper hopper using a scoop.

3. Open the upper hoppers door.


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Page 9.

4. Open the lower hoppers door to fill the cylinder.

5. Remove the excess concrete from the cylinder using the trowel and then weigh it (to obtain the

weight of partially compacted concrete).

6. Re-fill the cylinder with concrete in 6 layers, each layer should be rammed heavily with thecompacting rod. Then weigh it (to obtain the weight of fully compacted concrete).

C- Vebe test:

1. Clean the apparatus.

2. Place the apparatus on a rigid base.

3. Place the mold in the container and fit the funnel over it.

4. Fill the concrete with four layers, each layer should be tamped 25 times with the standardtamping rod.

5. The top of the concrete shall be struck off level using the trowel.

6. Remove the mold slowly and vertically.

7. Swing the disk to its position and lower it until its just touches the concrete.

8. Measure the slump.

9. Start the stop watch at the instant you turn on the vibrating table.


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Page 10.

5. Describe the factors which affect the compressive strength of the concrete.

Following are the factors that affect the strength of concrete:

Water-Cement ratio Type of cementing material Amount of cementing material Type of aggregate Air content Admixtures

1. Water-Cement ratio:

It is water cement ratio that basically governs the property of strength. Lesser the water cement ratio,greater will be strength.

2. Type of cement:

Type of cement affect the hydration process and therefore strength of concrete.

Amount of cementing material: it is the paste that holds or binds all the ingredients. Thus greater amount ofcementing material greater will be strength.

3. Type of Aggregate:

Rough and angular aggregates is preferable as they provide greater bonding.

4. Admixtures:

Chemical admixtures like plasticizers reduce the water cement ratio and increase the strength of concreteat same water cement ratio. Mineral admixtures affect the strength at later stage and increase the strength

by increasing the amount of cementing material.


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Page 11.

6. Comment on the experimental results and possible reasons for any discrepancy of the

result.

The experimental results for the slump tests were as follows

Trial 1 2 3Slump (mm) 175 165 185

Cube compression test results

Identification 1 2

Length (mm) 152 150

Width (mm) 150 150

Height (mm) 149 150

Weight (kg) 8.164 8.330

Failure (kN) 5.442 403.8

Observations Failure mode satisfactory Failure mode satisfactory

Although the failure mode is satisfactory, the calculated strength after 7 days - 35.61 and 26.78 for

Cube 1 and 2 respectively falls short of the targeted strength.


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7. Comment on the suitability of the DOE method for local conditions.

The British method of concrete mix design, popularly referred to as the "DOE method" is used in

the United Kingdom and other parts of the world and has a long established record. The method

originates from the "Road Note No 4" which was published in Great Britain in 1950.

The DOE method utilizes British test data obtained at the Building Research Establishment, the Transportand Road Research Establishment, and the British Cement Association. The aggregates used in the testsconformed to BS and the cements to BS 12 or BS 4027.

a given strength for concrete made from given coarse aggregate and cement types.

It is assumed that the workability of a concrete mix depends primarily on: The free water content The fine aggregate type and, to a lesser degree, the coarse aggregate type The maximum size of coarse aggregate

On the basis of tests the DOE Method provides a Table from which on can estimate the free water

content, which will provide a given workability for concrete made from given fine and coarse

aggregate types and a given maximum size of coarse aggregate.

It is assumed that the workability of a concrete mix depends secondarily on: The percentage of the fine aggregate as a proportion of the total aggregate content. The grading of the fine aggregate The free water/ cement ratio

The DOE method seems perfectly agreeable to Sri lanka. There has been no reason as to why it

should not. It has been widely used through out the years and is continued to be used until thepresent times and no problems whatsoever have been encountered in using it as a mixing

mechanism.

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