steel fibres

PERFORMANCE OF CONCRETE BLENDED WITH STEEL FIBERS

OBJECTIVE

To increase the compressive strength, tensile strength and ductility of concrete by

adding steel fibers to ordinary concrete

WORKABILITY

According to IS:6461-1973 workability is defined as that property of freshly mixed

concrete or mortar which determine the ease and homogeneity with which concrete can

be mixed, placed, compacted and finished.

Workability is broadly defined as the ease with which concrete can be

compacted 100% having regard to made of compaction and place of deposition.

Every job requires a particular workability. A concrete which is considered

workable for mass concrete foundation is not at all workable for concrete to be used in

roof construction. Similarly, a concrete considered workable for thick sections is not

workable required to be used in thin sections. Therefore, the word workability assumes

full significance of the type of work, thickness of sections, extent of reinforcement and

mode of compaction.

Factors affecting Workability

Workable concrete is the one which exhibits very little internal friction between

particle and particle or which overcomes the frictional resistance offered by the

formwork surface or reinforcement with just the amount of compacting efforts. The

factors helping concrete to have more lubricating effect to reduce internal friction for

helping easy compaction are as follows:-

1. Water content

2. Mix proportions

3. Size of aggregates

4. Shape of aggregates

5. Surface texture of aggregates

6. Grading of aggregates

7. Use of admixtures

Water content

Water content in a given volume of concrete will have significant influence on

workability. The higher the water content per cubic meter of concrete, the higher will be

the fluidity of concrete, which is one of the most important factors affecting workability.

The damaging effect of more water in concrete is reduction in compressive strength. In

case where all steps to improve workability fail, only as a loss recourse the addition of

more water should be considered. More water can be added, provided a

correspondingly higher quantity of cement is also added to keep the w/c ratio content,

so that the strength remains the same.

Mix proportions

Aggregate/cement ratio is an important factor influencing workability. The higher

Aggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity of

paste is available for providing lubrication per unit surface area of aggregate and hence

the mobility of rounded aggregate of some volume. A reduction of inter particles

frictional resistance offered by smooth aggregates also contributes to higher workability.

Size of aggregates

The bigger the Size of aggregates, the less is the surface area and hence less

amount of is required for wetting the surface and less matrix or paste is required for

lubricating the surface to reduce internal friction. For a given quantity of water and

paste, bigger the Size of aggregates will give higher workability. The above, of course

will be true within certain limits.

Shape of aggregates

Shape of aggregates influences workability in good measures. Angular,

elongated or flaky aggregate makes the concrete very harsh when compared to

rounded aggregate or cubical shaped aggregates. Contribution to better workability of

rounded aggregate will come from the fact that for the given volume of concrete or

weight it will have less surface area and less voids than angular or flaky aggregate. This

explains the reasons why river sand and gravel provide greater workability to concrete

than crushed sand and aggregate.

Surface texture

The influence of surface texture on workability is again due to the fact that the

total surface area of rough texture aggregate is more than the surface area of smooth

rounded aggregate of same volume from the earlier discussion it can be inferred that

rough texture aggregate will show the poor workability and smooth or glassy texture

aggregate will give better workability the reduction of inter particle frictional resistance

offered by smooth resistance also contribute to the higher workability .

Grading of aggregate

This is one of the factors which will have maximum influence on workability. A

well aggregate is the one has least amount of voids in a given volume. Other factors

being constant, when the total voids are less, excess paste is available to give

lubricating effect. Hence better the grading, less is the void content and higher the

workability.

Slump test

Slump test is the most commonly used method of measuring consistency of

concrete which can be employed either in laboratory or at site of the work. It is not a

suitable method for very wet or very dry concrete. It does not measure all factors

contributing to workability nor is it always representative of the placability of the

concrete. However it is used conveniently as a control test and gives or indication of the

uniformity of concrete from batch to batch. Repeated batches of the same mix, brought

to the same slump, will the same water content and water cement ratio provided the

weights of aggregate, cement and admixture or uniform and aggregate grading is wit in

acceptable limit. Additional information of workability and quality of concrete can be

obtained by observing the manner in which in concrete slumps. Quality of the concrete

can also be further assessed by giving a few tamping or blows by tamping rod to the

base plate. The deformation shows the characteristics of the concrete with respect to

tendency for segregation.

The apparatus for conducting the slump test essentially consist of a metallic

mould in the form of frustum of a cone having the internal dimension as under.

Bottom diameter : 20 cm

Top diameter : 10 cm

Height : 30 c

True slump

In this case mix subsides uniformly and cohesively. This type is obtained for

rich mixtures which have proportion of fine aggregates.

Shear slump

In this half of the cone shear of along on inclined plane. This is so for leaner

mixtures such as 1:6 or 1:8 and where slump requirement is higher. The slump is

measured from level of the cone to center of sheared plane.

Collapse slump

In this type of slump the concrete just collapse and spread over a large area.

This occurs in very wet mixes.

Lamination of slump test

The slump test has no relation to the useful internal work. For very low degree

workability the mix gives zero slumps and the text is in effective. It is also ineffective for

low mixes. For low workability concrete compaction factor test is useful.

Compaction factor test

The compaction factor test was developed by the road research laboratory

UK; the test was works on the principle of determining the degree

of the compaction achieved by standard amount of work done by following the concrete

to fall through a standard height. The degree of compaction is measured by density ratio

that i.e. radio of actual density achieved in test to density of some concrete fully

compacted.

The compaction factor test apparatus consist of to hopper vessels provided

with hinged doors at bottom. A cylindrical vessel is placed below the hopper. In an

experimental procedure the concrete after mixing is placed in an upper hopper than its

hinged bottom door is opened to allow the concrete to fall in to second hopper. The

second hopper is opened next and concrete falls in to the cylinder. Excess concrete in

cylinder is struck of and cylinder is then weighted. The cylinder is then emptied and

refilled in three layers of concrete. Each layer being compacted by giving 25 blows. The

excess concrete is then struck of and cylinder is weighted again. The radio of un

compacted concrete and hand compacted concrete gives the compaction factor.

Workability of different condition

Workability is dependent on the proportions of the ingredient materials as

well as on their individual characteristics. The degree of workability required for proper

placement and consolidation of concrete is governed by dimension and shape of the

structure and by spacing and size of reinforcement. Small complicated section with

much reinforcement steel or either embedded parts require concrete of high.

Si.no. Type of construction Slump

Max(cm) Min(cm)

1 Reinforced foundation walls and footings 10.0 5.0

2 Unreinforced footings caissons & substructure walls

7.5 2.5

3 Reinforced slabs beams and columns 12.5 7.5

4 Building columns 12.5 5.0

5 Bridge decks 7.5 5.0

6 Pavements 5.0 2.5

7 Side walls drive ways & slabs on ground 10.0 5.0

8 Heavy mass construction 5.0 5.0

Table 4.3.1

Significance of compressive strength

Test to determine are undoubtedly the most common type made to evaluate the

properties of hardened concrete. The reasons are mainly

1. The strength of concrete in compression has in most cases a direct influence

on the load carrying capacity of both plain and reinforced concrete structure

2. Of all the properties of hardened concrete, those concerning strength can

usually by determine early.

3. By means of correlation with other more complicated test, the result of the

strength can be used as qualitative indication of other important properties of

hardened concrete.

Factor affecting compressive strength

The factors affecting the compressive strength of the hardened concrete are

as follows

1. The ratio of cement to mixing water

2. The ratio of cement to aggregate

3. Grading, surface structure, shape, strength and stiffness of aggregate

particles

4. Degree of compaction

5. Maximum size of aggregate

In the above factors it can be further inferred that water cement ratio primarily

affect the strength directly, where as other factor indirectly affect the strength by

affecting the water cement ratio.

Strength of the concrete primarily depends upon on the strength of the cement

paste. The strength of the paste increases with cement content and decreases with air

content. The relation between water cement ratio and strength of the concrete shown in

fig

It can be seen from the graph that low water cement ratio could be used when

the concrete is vibrated to achieve higher strength, where as comparatively higher water

cement ratio is required when concrete is hand compacted or un compacted.

INDIAN STANDARD SPECIFICATIONS

COMPRESSION TEST SPECIMENS

Making and curing of specimens

IS : 516-1959 “Indian standard methods of tests for strength of concrete”

specifies the procedure for making and curing compression test specimens of concrete

of the quantities of materials making and curing of specimens and test conditions are

possible. The method is specially suitable mix proportions.

Preparations of materials

All materials shall be brought to room temperature, preferably 27degree Celsius

+3 degree Celsius before the tests.

Samples of aggregate and cement, on arrival at the laboratory shall we

thoroughly mixed dry either by hand or n a suitable mixture in such a manner as to

ensure the greatest possible blending and uniformity in the material care being taken to

avoid the intrusion of foreign matter .

Proportioning

The proportion of the materials including water in concrete mixes used for

determining the suitability of the material available shall be similar in all respects to

those to be employed in the work where the proportions of the ingredients of the

concrete as used on the site to be specified by volume. they shall be calculated from the

proportions be weight used in the test specimens and the unit weights of the materials.

Weighing

The quantities of cement size of aggregates and water for each batch shall be

determined by weight to an accuracy of 0.1% for the total weight of the batch.

Mixing of concrete

The concrete shall be mixed by hand, or preferably in laboratory batch mixer, in

such a manner as to avoid loss of water or other materials. Each batch of concrete shall

be such a size as to leave about 10% excess after molding the desired number of test

specimens.

Workability

Each batch of concrete shall be tested for workability, immediately after making,

by one of the methods described in IS 1199-1950. Sufficient care to ensure that no

water or any other materials is lost. The concrete used for the workability tests may be

remixed with the remainder of the batch before the test specimens.

Compacting

The test specimens SHALL be made as soon as possible after mixing , and in

such a way as to produce full compaction of the concrete with neither segregation nor

excessive laitance. The concrete shall be filled into the mould in layers approximately 5

cm deep. Each layer shall be compacted either by hand or by vibration. After the top

layer has been compacted the surface of the concrete shall be finished level with the

top of the mould, using a trowel, and covered with a glass of metal to prevent

evaporation.

Curing

The test specimen shall be stored in a place away from vibration, in moist air or

90% relative humidity and a at a temperature of 27+ or – ½ hours from the time of

addition of water to try ingredients. After this period, the specimen shall be marked and

removed from the moulds and unless required for test within 24 hours, immediately

submerged in clean, fresh water of saturated lime. The solution in which the specimens

are submerged in clean water.

TESTING

IS 516-1959-“methods of tests for strength of concrete” specifies the procedure

as follows for determining compressive strength of concrete.

Apparatus

Testing machine

The testing machine may be of any reliable type, of sufficient capacity for the

tests and capable of applying the load at specified rate of 140kg/cm/min without shock.

The permissible error shall be not greater than +or -2% of the maximum load.

Age at test

Tests shall be made at recognized ages of the test specimens, the most usual

being 7, 14 and 28 days.

Where it may be necessary in obtain the strengths, tests may be made at the

ages of 24 + or -1/2 hrs & 72 + or - 2 hrs. the ages shall be calculated from the the

addition of water to the dry ingredients.

Number of specimens

At least 3 specimens, preferably from different batches, shall be made for testing

at each selected age.

Procedure

Specimens stored in water shall be tested immediately on removal from the

water and while they are still in wet conditions. Surface water and grit shall be wiped of

the specimens and any projecting fins removed. Specimens when received dry shall be

kept in water for 24 hours before they are taken for testing.

Placing the specimens in the testing machines

The cube specimens shall be placed in the machine in such a manner that the

loads shall be applied to the opposite sides of the cubes as cast, that is not top and

bottom. The maximum load applied to specimens shall be recorded and the appearance

of the concrete and any features in type of failure shall be noted.

MIX DESIGN

Scope of investigations

This investigation consists of studies of slump variation, compaction factor

variation and strength variations of concrete mixes with normal coarse

aggregate concrete and recycles coarse aggregate concrete.

For this investigation the following materials are used.

1. Cement (53grade)

2. Sand

3. Water

4. Coarse aggregate

5. Recycle coarse aggregate

To calculate the value of ingredients

Size of cube = 15 cm

Volume of cube = 15*15*15 = 3375 cc

Density of concrete = 2400 kg/m

Density = mass/volume

Mass = 2400*3.375*10 =8.1 kg

Mixed proportion

Cement : sand : coarse aggregate = 1:11/2 :3

Weight of cement required for one cube = 8.1/5.5

= 1.472 kgs

Weight of sand require for one cube = (1.5/5.5)*8.1

= 2.209 kgs

Weight of coarse aggregate for one cube = (3/5.5)*8.1

= 4.418 kgs

Hence for 9 cubes

Weight of cement = 1.472*9

= 13.248 kgs

Weight of sand = 2.209*9

= 19.881

Weight of coarse aggregate = 4.418*9

= 39.672 kgs

Water/cement ratio = 0.45

Weight of water = 0.45*9

= 4.05 lts= 4050 cc.

Steel fibers =0.3%, 0.4%, 0.5%

TEST PROCEDURES

Test procedures for slump test

Apparatus

Slump cone with measuring scale, tamping rod , concrete constituents.

Procedure

Mould are prepared with water cement ratio (by weight)0.5 for concrete of 1:11/:3

1. mix the dry constituents thoroughly to get a uniform colour and then add

water , mix thoroughly.

2. Concrete is filled in the standard slump cone which consists of vessel which is

frustum shaped . it is compacted with in 3 layers.after laying each layer it is

compacted with 25 strokes using a tamping rod of 16 mm diameter 60 cm

long whose striking end is bullet pointed.

3. Level the top surface of the mould with a trowel.

4. remove the top cone immediately raising it slowly and carefully in the vertical

direction.

5. As soon as the concrete settlement comes to shop measure the vertical

settlement of the body of concrete which is called as the slump of concrete.

Compaction factor test

Apparatus

Compaction factor test, trowels, graduated cylinder of 100 ml capacity, balanced

to weight up to 30 kg, tamping rod and iron buckets.

Procedure

1. Keep the compaction factor apparatus on a level ground and Greece all inner

surface of hopper and cylinder.

2. Fastened the hopper door.

3. Weight the empty cylinder accurately and note down the weight as w1 kg.

4. Fix the cylinder on the base with the fly net and bolt in such a way that the

central point of happens and cylinder lie on one line. Cover the cylinder with a

plate.

5. Mixes or to be prepared for water cement ratio 0.4 for each mix take 9 kg

aggregate, 4.5 kg of sand 2.25 kg of cement the mix proceed is as follows.

6. Fill the freshly mixed concrete in upper hopper gently and carefully with hand

scoop without compacting.

7. After two minutes release the trap doors to that the concrete may fall in to the

lower hopper bringing the concrete in to standard compaction.

8. Immediately after the concrete as come to rest open the trap door of lower

hopper and allow the concrete to fall in to the cylinder bringing the concrete in

to standard compaction.

9. Remove the extra concrete above the top of the cylinder by a pair of trowels,

one in each hand, with horizontally sliding from the opposite edges of the

mould, inwards to the center with a sawing motion.

Compressive strength of concrete

Apparatus

I.S. mou77lds tampering rod balance, constituents of concrete ,compression

testing machine.

Procedure

1. For preparing concrete of proportion 1:11/2:3 by weight by weight ,find the

weight of the concrete and volume of test mould. Thus calculate the weight of

each constituent required.

2. After knowing the weights of individual constituents, take constituents and m

ix it with water to cement ratio 0.5. mix it through to have uniform colour .

3. Oil the inner surface of the moulds and fill it with concrete in three layers.

Tamping each layer 25 times by tamping rod.

4. Level the top surface of the mould by using trowel

5. After 24 hrs the specimen removed from the mould and kept in water for

curing.

6. The character compressive strength is noted after the testing of mould and

under compressive strength is noted after the testing of mould under

compressive testing machine after 7,14 & 28 days.

TEST RESULTS

SLUMP CONE TEST

shows the results for slump cone test.

SPECIMEN SLUMP VALUE (CM)

PCC 11.1

0.3% STEEL FIBERS 9.1



Figure 8.1.1 shows the comparison between slump values of concrete with different

proportion of steel fibers

COMPACTION FACTOR TEST

shows compaction factor test results for different proportion of steel fibers.

SPECIMEN COMPACTION FACTOR (%)

PCC 0.89




shows the comparison between compaction factor for different proportions of steel fibers in

concrete.

0

2

4

6

8

10

12

PCC (0 %)

0.30% 0.40% 0.50%

11.1

9.1 7.6

6.8

SLU

MP

(cm

)

PERCENTAGE OF STEEL

SLUMP VALUE

COMPRESSION TEST RESULTS

CURING PERIOD 7 DAYS

SPECIMEN 1 2 3 AVERAGE (Kg/ CM2)

PCC 231.11 231.11 208.88 223.7

0.3% 352.11 342.2 342.2 345.17

0.4% 320 320 324.44 321.48

0.5% 231.11 231.11 240 234.07



PCC 262.22 266.6 280 269.7

0.3% 391.11 422.2 422.2 411.83

0.4% 346.66 413.33 400 386.66

0.5% 351.11 311.11 342.22 334.81

0.86

0.88

0.9

0.92

0.94

0.96

0.89

0.92 0.93

0.95 C

OM

PAC

TIO

N F

AC

TOR

(%

)

PERCENTAGE OF STEEL

COMPACTION FACTOR



PCC 355.55 377.77 377.77 370.36

0.3% 537.7 488.8 493.33 506.61

0.4% 462.22 493.33 488.8 481.45

0.5% 440.77 444.44 440 441.8

COMPARISION CHART

223.7

345.17 321.48

234.07 269.6

411.83 386.66

334.81 370.36

506.61 481.45

441.8

0

100

200

300

400

500

600

PCC 0.30% 0.40% 0.50%

LOA

D (

Kg

/cm

²)

PERCENTAGE OF STEEL

7th day

14th day

28th day

RESULT

RESULT

• Thus optimum compressive strength is obtained when 0.3% of steel fiber is

added to PCC.

• Compressive strength is been hiked by more than 45% by introduction of 0.3%

steel fiber.

CONCLUSION

CONCLUSION

• The experimental program conducted indicated that adding steel fiber with PCC

is an effective measure to enhance the compressive strength.

• Improves bonding strength between materials.

• The experiment indicates with increase in steel fiber content workability

decreases. However the problem can be overcome by adding admixtures like

plasticizers.

steel fibres

Education