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MIX DESIGN, CASTING AND TESTING RC BEAM AND CONTROL SPECIMENS CARDIFF UNIVERSITY

Shainal Sutaria

c1059965

In the experiment the mix design of concrete was tested by the slump test and vebe test to test the

workability of concrete and while testing was done to determine the strength of concrete. In this

experiment specific amount of fine aggregate coarse aggregate and water was added together and

was added into the slump compacted in three layers and the value of slump was measured by a

scale then the vebe test is done and time taken to fill the under layer with concrete is noted. The

concrete is then casted into the beams and other specimens compacted through different ways and

then the strength of concrete is determined by testing them after 7 days. Appropriate graphs weremade to determine the behaviour of concrete while it failed. Results found out that the slump was

0.47 which was workable while vebe test showed .Finally the strength of concrete can be found out

by this way which would assure which type of concrete should be used.

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Contents

Mix design, casting and testing RC beam and control specimens ........ Error! Bookmark not

defined.

1.Introduction .............................................................................................................................. 2

2.Theory ....................................................................................................................................... 3

3.Procedure: ................................................................................................................................. 4

1.Introduction

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To determine the workability of concrete by the slump test and the vebe test by mixing fine

and coarse aggregate with water to obtain a workable concrete mix. Casting this mixed

concrete into casting beams and different specimens while compacting them in different

ways and then determining the strength of concrete after 28 days of the mix. Thus

observing the cracks while tensile or compression load is applied on the concrete.

2.Theory

2.1Mix of concrete

Mixing of concrete is done to determine whether the concrete is workable or not i.e. the easy with

which the concrete is mixed, applied, compacted and transported. The mix of concrete depends on

the amount of fine aggregate coarse aggregate is added and water is mixed as well as other factors

as size and shape of the aggregates the shapes of the aggregates; cement strength and the

water/cement ratio.

Too much water destroys the concrete and makes it sloppy while too less of water makes the

concrete stiff and makes it hard to work with. Class of cement is also important while determining

the mix A class cement makes the strength of concrete high hence such type is used in dams bridges

where there is lot of load applied. Slump test and vebe factor test are used to determine the

workability of concrete. A slump between 30-60 is considered to be good on the w/c=0.8

2.2 Casting of concrete

Casing of concrete is done so that the strength of concrete can be measured. The fully workable

concrete in placed in beams , cubes and cylinders. This concrete mould is placed to find the quality

of concrete under specific curing conditions for 28 days .The quality of concrete also depends upon

the no. Of days it is kelp in curing conditions before it becomes suitable to use. The strength of a 28

days year old concrete is higher than the strength of a 7 day concrete block

Bonding with the steel is prevented by coating with releasing agent (Oil). The trowelled surface of

cube is covered with a rubber mat with identification marks, or the entire mould is sealed. After 16 -

24 hours, the cube sample is removed from the mould and cured under water at about 27C, until

testing.

2.3 Testing of concrete

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In this we determine the compressive and tensile strength by testing cube, cylinder and beam

samples which is 28 days old with characteristic strength of 40 N/mm2 and measure its

stiffness by static mean.

Stress is the amount of internal force per unit area.

All materials deform when subject to external actions such as loads or temperature changes.

The deformation, i.e. change in shape is measured by the strain at a point.

Linear strain=

Procedure:

Mix the fine aggregate , coarse aggregate and water using the concrete mix machine Transfer this concrete into the slump cone in three different layers, between each

layer compact the concrete using the tamping rod with 25 strokes of it

After the concrete is properly placed scrape up the extra concrete in the cone andtake out the cone within 3 to 6 seconds time

Measure the slump in the concrete with the help of a scale(mm)

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Switch on the vibrator and clock simultaneously and watch for the cement paste tocover the underside of the disc. Switch off the clock the moment this occurs.

Switch off the vibrator and the time that records are known as the V-B seconds anddegree of workability of concrete.

Now this fully workable concrete is taken to beams, cubes and cylinders and each ofthem is filled with concrete and compacted by machines such that neither

segregation nor excessive laitance occurs.

The cubes and the cylinders are placed on the vibrating table and are compacted bymeans of vibration while the beams are

Cast two 100mm cubes, two 100x200 cylinders and two 100x100x500 beams. Cast one 100x150x2000 RC beam. Load one of the 150mm cubes to failure in the compression testing machine and

record the failure load

Load the concrete beam to failure. Record failure load. Measure the spacing of thesupports.

Note the loading and deflection measurement arrangements and take the Initialdeflection readings.

Load the beam in increments until the first crack appears, pause and observe thebehaviour.

Continue loading until failure. Carefully note the crack pattern and the mode offailure.

Calculations and results:

Testing of different concrete specimens

Compression of cube:

Maximum load-438 KN

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Compressive strength at failure=438*1000/(100*100)

= 43.8 KN

Cylinder:

Max. Load =116.98 KN

Applied load- 0.05 KN/sec

Total strength of the cylinder:

=2*116.98*1000/(3.14*200*100)=3.76KN

RC Beam:

Beam length=1.4 m

Max. Load- 13.78 KN

= Pl /b

= 13.87*1000*400/(100*100*100)

= 5.548 KN

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Observations and discussion:

1. Explain why it is necessary to design a mix to have a higher mean strength thancharacteristic strength.

As the concrete strength varies when the total characteristic strength of concreteis measured there is a certain percentage of concrete which is defective( for this

case 5%) which reduces the total strength of concrete so a margin is added which

is k*standard deviation so that the mean strength is always higher than the

characteristic strength.

2. Highlight the areas, and parameters used in the mix design which are most prone to

error/variability.

The most important aspect in the mix design is the w/c ratio ,if in the mix design thewater content is more the concrete tends to become more sloppy while if the water

content is low the concrete the concrete becomes hard to work with (in this case its

0.47)other areas are the aggregate sizes the cement strength(42.5) and other

different aspects

3. Mention any factors, not included in the mix design process which may affect the results. A lot of human errors occurs if the amount of coarse aggregate cement and water are

not used in the right amount also during the casting time the concrete is not properly

compacted it may result into concrete having really less compressive or tensile

strength.

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4. Comment on the difference between the design and measured slump values and vebetimes. Also comment on the overall accuracy of these two workability tests.

The design value of the slump test was measured to be 30-50 mm while it measuredto be 43 mm for us and vebe time the design value is measured to be 6-12 sec which

measured to be 8 sec which was inside the range. Thus the concrete was decided tobe highly workable and could be used to cast concrete in different specimens.

5. Comment on the accuracy of the construction of the RC beam. Suggest reasonabledimensional tolerances.

The RC beam had dimensions as shown in the sheet. The dimensions obtained of the rc

beam are correct

The tensile strength of concrete is only about 10 percent of the compressive strength.

Because of this all Rc structures are designed on the assumption that concrete doesnt

have tensile forces. Wherever tension occurs it is likely that the cracking of concretetakes place.When the compressive or shearing forces exceed the strength of concrete,

then steel reinforcement must again be provided, bit in these cases it is only required in

supplement the load-carrying capacity of the concrete,

The graph depicts that the loading gradually occurs at some point concrete doesnt

accept any more load which is know as the peak point and after some point the

failure load occurs . This describes a typical stress strain graph .The beam also retains

its original position after loading occurs.

Refrences

0

5

10

15

20

25

30

0 20 40 60

Series1

LOADKN

Disp(mm)

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1. Building Research Establishment, Design of normal concrete mixes (revised

edition), BRE/DOE, 1988.

2. BS1881 Part 102. Method for the determination of slump.

3. BS1881 Part 104. Method for the determination of vebe time.4. Neville, A.M., Properties of Concrete, 1995.

5. Eurocode 2: Design of concrete structures. (European standard ENV 1992)

6. Reinforced and prestressed concrete by Kong and Evans

7. Reinforced concrete design to EC2 by Mosley and Bungey

.