Strength Tests on Hardened Concrete

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Chapter 5

TESTS ON HARDENED CONCRETE

This chapter describes the results of the test programmed to establish the mechanical properties of the conventional concrete as well as GGBFS added concrete with different percentage to the weight of the cement. Concrete mixes detailed in the proceeding chapter. Mixing of ingredient of concrete is done for the mix proportion for M50 grade of concrete mixes by adding GGBFS with different percentages 0%,10%,20%,30%,40%,50% and 60%.

5.1 Preparation of test specimen

Cube specimen of size15cmx15cmx15cm,cylinder specimen of length20cm and diameter 10cm,and prism of size 50cmx10cmx10cm were casted. The ingredients of concrete were mixed thoroughly in the mixer till uniform consistency was achieved. The specimens were compacted on a vibrating table .The specimens were demolded after 24 hours of casting and cured for 7,14 and 28 days.In the experimental work total 189 specimens were casted which includes 63 cubes,cylinders and prisms.

Table-10Number of specimens casted

Replacement of ggbfs in %

Number of cubes

Number of cylinders

Number of prisms

7days

14days

28days

7days

14days

28days

7days

14days

28days

0

3

3

3

3

3

3

3

3

3

10

3

3

3

3

3

3

3

3

3

20

3

3

3

3

3

3

3

3

3

30

3

3

3

3

3

3

3

3

3

40

3

3

3

3

3

3

3

3

3

50

3

3

3

3

3

3

3

3

3

60

3

3

3

3

3

3

3

3

3

Total-63

Total-63

Total-63

5.2 TESTS ON HARDENED CONCRETE

Following tests were conducted to determine the mechanical properties of the concrete

Compressive strength

Split tensile strength

Flexural strength

Ultra sonic pulse velocity

5.2.1 Compressive strength of concrete

Among all the test of the concrete compressive strength test is the upmost important which gives idea about all the characteristics’ of concrete, and it has a definite relationship with all the other properties of concrete i.e these properties are improved with the improvement in compressive strength. The size of the mould is usually 15cmx15cmx15cm.concrete cubes are tested for 7, 14 and 28days as per IS: 516-­1959 (Part 5).Rate of application load is 1.40KN/cm2/min.

Compressive strength =

Table-11 Compressive strength of concrete for various percentages of GGBFS in N/mm2

Replacement of GGBFS , %

7Days compressive strength

Mpa

14Days compressive strength Mpa

28

Days cube compressive strength Mpa

0

39.50

46.44

50.68

10

37.60

45.33

51.67

20

35.66

47.80

52.55

30

34.12

43.33

49.07

40

33.63

39.36

45.95

50

31.89

36.21

42.17

60

30.85

34.50

39.70

5.2.2 Split tensile strength

Cylinder Splitting Tension Test: This is also sometimes referred as, “Brazilian Test”. This test was developed in Brazil in 1943. At about the same time this was also independently developed in Japan.

The test is carried out by placing a cylinder specimen horizontally between the loading surfaces of a compression testing machine and the load is applied until failure of the cylinder, along the vertical diameter.

Split tensile strength=

Where:

P = load applied to the cylinder

D = diameter of the cylinder

L = Length of the cylinder

The loading condition produces a high compressive stress immediately below the two generators to which the load is applied. But the larger portion corresponding to depth is subjected to a uniform tensile stress acting horizontally. It is estimated that the compressive stress is acting for about 1/6 depth and remaining 5/6 depth is subjected to tension.

In order to reduce the magnitude of the high compression stresses near the points of application of the load, narrow packing strips of suitable material such as plywood are placed between the specimen and loading planets of the testing machine. The packing strips should be soft enough to allow distribution of load over a reasonable area, yet narrow and thin enough to prevent large contact area. Normally, a plywood strip of 25 mm wide, 3mm thick and 30cm long is used.

The main advantage of this method is that the same type of specimen and the same testing machine as are used for the compression test can be employed for this test. That is why this test is gaining popularity. The splitting test is simple to perform and gives more uniform results than other tension tests. Strength determined in the splitting test is belived to be closer to the true tensile strength of concrete, than the modulus of rupture. Splitting strength gives about 5 to 10% higher value than the direct tensile strength.

Table-12 Tensile strength of concrete for various percentages of GGBFS in N/mm2

Replacement of GGBFS , %

7Daystensile strength

Mpa

14Daystensile strength Mpa

28Daystensile strength Mpa

0

3.95

4.95

5.10

10

3.76

4.56

5.17

20

3.58

4.38

5.30

30

3.21

3.96

4.34

40

3.07

3.35

3.85

50

2.93

3.12

3.72

60

2.80

2.98

3.58

5.3.5 Flexural strength of concrete

Flexural strength is one of the measure of compressive strength of concrete.It is a measure of an unreinforced concrete slad or beam to resist failure in bending.It is measured by loading 500x100x100mm concrete beam.The flexural strength is expressed ac modulus of rupture and it is determined by standard test method ASTM C-78(third point loading)or ASTM(centre point loading).Flexural strength of concrete is dbout 10-20% of the compressive strength of the concrete depending on size,type and volume of the coarse aggregate used.The flexural strength determined by third point loading is lower than the strength determined by centre point loading.

Modulus of rupture =

Where,

P=load applied

L=Length of the prism

B=Width of the of the prism

D=Depth of the prism

Table-13 Flexural strength of concrete for various percentages of GGBFS in N/mm2

Replacement of GGBFS , %

7Daysflexural strength

Mpa

14Days flexural strength Mpa

28 Days flexural strength Mpa

0

5.18

5.54

5.94

10

5.01

5.62

6.03

20

4.89

5.71

6.15

30

4.66

4.93

5.86

40

4.49

4.77

5.45

50

4.21

4.45

5.34

60

4.14

4.38

5.19

5.3.6 Ultra sonic pulse velocity

Ultrasonic pulse velocity is one of the non destructive testing method which provides information about the uniformity of concrete, cavities, cracks and defects .The pulse velocity in a material depends on its density and its elastic properties which in turn related to the compressive strength and quality of concrete. The test is carried according to IS 13311(Part1):1992.The experiment’s specific equipment determines the travel time of an ultrasonic wave through the concrete specimen between the transmitter and reciver placed on two opposite sides of the sample .By means of the determined travel time the wave’s velocity can be determined.

C:\Users\MOON\Desktop\ggbs\new

Fig-11 Pulse velocity test

Table-14 pulse velocity results

Replacement of GGBFS , %

Pulse velocity (km/sec)

General condition

Quality classification

0

6.422

Excellent

Very Good

10

4.534

Excellent

Very Good

20

4.901

Excellent

Very Good

30

4.602

Excellent

Very Good

40

4.102

Excellent

Very Good

50

4.544

Excellent

Very Good

60

4.689

Excellent

Very Good

Chapter 6

RESULTS AND DISCUSSIONS

6.1COMPRESSIVE STRENGTH

Graph-1 Compressive strength of concrete at7, 14 and 28 days

For concrete the main criteria to know the mechanical properties is compressive strength ,in this case compressive strength test is conducted for the concrete containing various percentages of ground granulated blast furnace .Here the 7 and 14 days compressive strength of the GGBFS concrete is less than the conventional concrete. But the 28 days strength of concrete containing 20% of GGBFS is more as compared to conventional concrete .i.e optimum percentage of GGBFS for compression is 20% as shown in the graph.

6.2 SPLIT TENSILE STRENGTH

Graph-2 Split tensile strength of concrete at 7,14 and 28 days

It is very difficult to measure the tensile strength of the concrete directly,so it measured indirectly by placing the cylinder specimen horizontally and then applying the compression load..Here the 28 days tensile strength of concrete containing 20% of GGBFS is more as compared to conventional concrete as shown in graph

6.3 FLEXURAL STRENGTH

Graph-3 Flexural strength of concrete at7,14 and 28 days

One of the main criteria to know the mechanical properties of concrete is flexural strength ,in this case test is conducted for the concrete containing various percentages of ground granulated blast furnace. Here the 7 and 14 days flexural strength of the concrete incorporating GGBFS for all dosages is less than the conventional concrete. But the 28 days strength of concrete containing 20% of GGBFS is more than the conventional concrete .

as shown in the graph

6.4 STATIC MODULUS OF ELASTICITY

Modulus of elasticity of concrete is a key factor for estimating the deformation of buildings and members, as well as a fundamental factor for determining modular ratio, which is used for the design of section of members subjected to flexure. It is frequently expressed in terms of compressive strength.

Table-15 Comparison of codal provision for static modulus of elasticity Ec in N/mm2

Replacement of GGBFS , %

Asper Measured value, Ec

Asper IS-456 code

Asper ACI:318 code

Asper New Zealand code NZS:3101

Asper Euro code EC:02

Asper BS:8110

0

25998.43

35594.94

30141.86

28038.78

34208.69

20010.14

10

26203.50

35940.92

30437.82

28246.33

34393.74

20010.33

20

26428.39

36245.68

30694.43

28426.30

34553.99

20010.51

30

25263.30

35024.99

29662.20

27702.39

33908.34

20009.81

40

24105.80

33893.21

28702.25

27029.17

33305.60

20009.20

50

23602.66

32469.22

27497.98

26184.60

32546.62

20008.43

60

23101.53

31503.96

26678.93

25610.19

32028.81

20007.94

Comparisons of static modulus of elasticity obtained experimentally and that obtained from using empirical expressions given design code of various country for both conventional concrete and GGBFS concrete is presented in the above table.

Graph-4 results of modulus of elasticity as per different codes for 0% replacement of GGBFS

Graph-5 results of modulus of elasticity as per different codes for 10% replacement of GGBFS

Graph-6results of modulus of elasticity as per different codes for 20% replacement of GGBFS

Graph-7 results of modulus of elasticity as per different codes for 30% replacement of GGBFS

Graph-8 results of modulus of elasticity as per different codes for 40% replacement of GGBFS

Graph-9 results of modulus of elasticity as per different codes for 50% replacement of GGBFS

Graph-10 results of modulus of elasticity as per different codes for 60% replacement of GGBFS

Graph-11 results of modulus of elasticity as per different codes

The fig shows that the static modulus of elasticity predicted by Indian code IS:456-2000 and euro code EC:02 are higher than those predicted by American code( ACI:318) ,New Zealand code( NZS:3101) and British code (BS:8110).

Fig also shows that experimentally measured modulus of elasticity is higher than the British code (BS:8110) and comparatively lower than all other design codes.

Table -16 Constants for empirical relationship between static modulus of elasticity and compressive strength (C1 for cube compressive strength)

Replacement of GGBFS , %

Asper Measured value,Ec

Asper IS-456 code

Asper ACI:318 code

Asper New Zealand code NZS:3101

Asper Euro code EC:02

Asper BS:8110

0

3652.20

5000.71

4234.03

3938.59

4805.27

2810.81

10

3655.65

5001.26

4234.72

3930.06

4784.24

2783.80

20

3658.80

5002.66

4234.98

3931.50

4766.63

2760.39

30

3611.71

5001.10

4234.50

3987.52

4839.40

2856.50

40

3570.03

5000.80

4234.44

4034.59

4963.87

3018.15

50

3541.82

5000.45

4234.32

4062.49

5010.90

3081.20

60

3551.82

5000.20

4233.11

4086.36

5083.29

3175.46

Table-17 Constants for empirical relationship between static modulus of elasticity and compressive strength (C2 for cylinder compressive strength)

Replacement of GGBFS , %

As per Measured value, Ec

As per IS-456 code

As per ACI:318 code

As per New Zealand code NZS:3101

As per Euro code EC:02

As per BS:8110

0

4075.44

5590.40

4734.40

4403.69

5329.25

3142.73

10

4076.04

5591.22

4734.85

4384.88

5312.34

3112.22

20

4083.24

5591.70

4735.28

4385.95

5299.85

3086.30

30

4063.40

5590.35

4733.65

4420.84

5410.40

3193.52

40

4031.94

5589.97

4733.90

4367.92

5492.33

3300.14

50

3985.65

5589.80

4733.40

4361.10

5612.35

3444.62

60

3910.90

5589.40

4732.99

4360.20

5683.51

3550.30

Based on the regression analysis of the experimentally obtained test results, the proposed correlation of the modulus of elasticity and compressive strength of cylinder and cube for conventional and GGBFS based concrete are given below

For cube compressive strength:

Ec=C1 fc

For cylinder compressive strength:

Ec=C2 fc

Where,

EC is the static modulus of elasticity at 28 days in Mpa.

is the cube compressive strength of concrete at 28 days Mpa.

is the cylinder compressive strength at 28 days in Mpa.

C1, C2 Constants given in table

6.5 MODULUS OF RUPTURE

Modulus of rupture is defined as a material's ability to resist deformation under load The Modulus of rupture represents the highest stress experienced within the material at its moment of rupture.

Table -18 Comparison of codal provision for flexural tensile strength concrete fr in N/mm2

Replacement of GGBFS , %

Asper Measured value, fr

Asper IS-456 code

Asper ACI:318 code

Asper New Zealand code NZS:3101

Asper Euro code EC:02

Asper Canadian code of practice CSA

0

5.94

4.983

3.947

3.820

4.162

3.820

10

6.03

5.031

3.986

3.857

4.216

3.857

20

6.15

5.074

4.019

3.890

4.265

3.890

30

5.85

4.903

3.885

3.759

4.074

3.759

40

5.45

4.606

3.676

3.567

3.783

3.567

50

5.32

4.545

3.601

3.485

3.680

3.485

60

5.19

4.410

3.494

3.380

3.534

3.380

Comparisons of flexural strength or modulus of rupture obtained experimentally and that obtained from using empirical expressions given design code of various country for both conventional concrete and GGBFS concrete is presented in the above table

Graph-12 results of modulus of rupture as per different codes for 0% replacement of GGBFS

Graph-13 results of modulus of rupture as per different codes for 10% replacement of GGBFS

Graph-14 results of modulus of rupture as per different codes for 20% replacement of GGBFS

Graph-15 results of modulus of rupture as per different codes for 30% replacement of GGBFS

Graph-16 results of modulus of rupture as per different codes for 40% replacement of GGBFS

Graph-17 results of modulus of rupture as per different codes 50% replacement of GGBFS

Graph-18 results of modulus of rupture as per different codes for 60% replacement of GGBFS

Graph-19 results of modulus of rupture as per different codes

From the fig it can be noticed that experimentally measured modulus of rupture is higher than the IS-456 code, ACI:318 code , NZS:3101 code, EC:02 code and Canadian code.

The below table shows the details of empirical relationship between modulus of rupture vs cube compressive strength and modulus of rupture vs cylinder compressive strength respectively

Table -19 Constants for empirical relationship between flexural tensile strength and compressive strength in N/mm2 (C1 for cube compressive strength)

Replacement of GGBFS , %

Asper Measured value, fr

Asper IS-456 code

Asper ACI:318 code

Asper New Zealand code NZS:3101

Asper Euro code EC:02

Asper Canadian code of practice CSA

0

0.8340

0.6998

0.5545

0.5370

0.5846

0.5368

10

0.8388

0.6998

0.5545

0.5365

0.5865

0.5369

20

0.8483

0.6998

0.5545

0.5370

0.5880

0.5371

30

0.8351

0.6998

0.5545

0.5366

0.5816

0.5366

40

0.8240

0.6998

0.5545

0.5380

0.5706

0.5363

50

0.8230

0.6998

0.5545

0.5367

0.5667

0.5360

60

0.8237

0.6998

0.5545

0.5360

0.5608

0.5355

Table-20 Constants for empirical relationship between flexural tensile strength and compressive strength in N/mm2 (C2 for cylinder compressive strength)

Replacement of GGBFS , %

As per Measured value, fr

As per IS-456 code

As per ACI:318 code

As per New Zealand code NZS:3101

As per Euro code EC:02

As per Canadian code of practice CSA

0

0.9330

0.7825

0.6199

0.5999

0.650

0.5999

10

0.9378

0.7825

0.6199

0.5999

0.652

0.5999

20

0.9485

0.7830

0.6199

0.5999

0.657

0.5999

30

0.9336

0.7825

0.6199

0.5999

0.641

0.5999

40

0.9190

0.7825

0.6199

0.5999

0.637

0.5999

50

0.9158

0.7825

0.6199

0.5999

0.634

0.5999

60

0.9209

0.7825

0.6199

0.5990

0.627

0.5990

Based on the regression analysis of the experimentally obtained test results, the proposed correlation of the flexural strength and compressive strength of cylinder and cube for conventional and GGBFS based concrete are given below

For cube compressive strength:

fr =C1 fc

For cylinder compressive strength:

fr=C2 fc

Where,

. is modulus of rupture of concrete at 28 days in Mpa

is the cube compressive strength of concrete at 28 days Mpa.

is the cylinder compressive strength at 28 days in Mpa.

C1,C2 Constants given in table

Chapter 7

CONCLUSIONS

Following conclusions were drawn from this experimental work.

  • The 7 days and 14days compressive strength, Tensile strength and flexural strength of GGBFS concrete is less than the plain concrete but the 28 days strength of the GGBFS concrete with 20% replacement is more than plain concrete further the addition of GGBFS will decrease the strength .i.e optimum replacement percentage of GGBFS by weight of cement is up to 20%.
  • The experimentally measured values of modulus of elasticity of GGBFS Concrete are lower as compared to Indian code IS:456-2000 and euro code EC:02 , American code

( ACI:318) and New Zealand code( NZS:3101) .

  • The static modulus of elasticity predicted by Indian code IS:456-2000 and euro code EC:02 are higher than those predicted by American code( ACI:318) ,New Zealand code( NZS:3101) and British code (BS:8110).
  • The experimentally measured modulus of elasticity is higher than the British code (BS:8110) and comparatively lower than all other design codes.
  • The experimentally measured modulus of rupture is higher than the IS-456 code, ACI:318 code , NZS:3101 code, EC:02 code and Canadian code.
  • The new empirical relations for static modulus of elasticity flexural tensile strength or .modulus of rupture and compressive strength of concrete incorporating different percentage of GGBFS in plain concrete are proposed

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