Effect of Rate of Loading on Compressive Strength of Concrete

International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 Available online at www.ijiere.com International Jou...
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International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 Available online at www.ijiere.com

International Journal of Innovative and Emerging Research in Engineering e-ISSN: 2394 – 3343

p-ISSN: 2394 - 5494

Effect of Rate of Loading on Compressive Strength of Concrete Saurah Mali, Tushar Pachpande and Ashwini Jogi Kothrud, Pune, India ABSTRACT: To accurately predict the behavior of a reinforced concrete structure subjected to dynamic loads, effect of change in rate of loading should be accurately determined. It’s found that load rate as per IS specifications is time consuming and tedious and hence generally avoided. Hence, concrete of grade varying from M20 to M40 using loading rates 40 to 1540 kg/cm2/min were tested for above purpose. The ratio of strength to rate of loading was calculated which revealed logarithmic relationships. Suitable correction factor was proposed for determining the true strength of concrete cubes from obtained strength at higher rate of loading. Keywords: Concrete, Rate of loading, Correction Factor, Cube Test, Strength I. INTRODUCTION Compressive strength of concrete is determined by testing concrete cubes of size 150mm x 150 mm x 150 mm. CTM, either electronic or manually operated are used for this purpose. The strength of concrete is tested on site using manually operated CTM. The rate of loading in such cases cannot be accurately determined and it violets the code specifications. The rate of loading applied by manually operated CTM is generally 8 to 10 times greater than the code specifications. It has been observed that strength increase as rate of loading increases. This is not the true strength and hence we get inaccurate results. False strength of concrete proves fatal as it may fail due to various factors in the long terms. Therefore, to ensure the functional performance and safety of structure it is necessary to determine the true strength of concrete. In this project the effect of high rate of loading on compressive strength of concrete has been studied. As it has been observed that strength increases at higher rate of loading, it gives false strength which may prove hazardous. To avoid this, a suitable correction factor has been formulated which will enable us to obtain the true strength of concrete. II. LITERATURE REVIEW 1. Compressive behaviour of concrete at high strain rates by P. H. Bishoff and S. H. Perry (materials and structures/ materiaux et constructions, 1991, 24, 425-450) In the year 1991, Bishoff & Perry [1] conducted experimental technics commonly used for high strain- rates testing of concrete in compression with methods used for recordings and testing stress and strain. The data recorded by them show and indicate the various types of loading rate that will be applied on concrete under various conditions. They also pointed out the vast range of straiun rate applied on concrete (10 -8 under creep to 103 under blast effect). Hence studies are carried out to determine the strength of concrete under higher loading rate. 2. Review of effect of loading rate on concrete in compression by H. C. Fu, M. A. Erki, M. Seckin. (J. Struct. Eng., 1991, 117(12), 3645-3659) In the year 1991 H. C. Fu, M. A. Erki, M. Seckin [3] studied the response of reinforced concrete material and elements to different strain rate. They concluded that increasing rate of loading will result in increase in strength and stiffness of concrete, yield strength of still increases and thus the improved axial and flexural capacity of RC elements may lead to undesirable effects on dynamic response on RC sections. 3. Rate effects and load relaxation in static fracture of concrete by Zdenek P. Bazant and Ravindra Gettu, Page 456-457 & 467 In 1992, Zdenek P. Bazant and Ravindra Gettu [6] published a paper regarding the rate effects and load relaxation in static fracture of concrete. They reposrted an experimental study of the fracture of concrete at various crack mouth opening displacement rates with time. They proposed that the causes for the increase in concrete strength fast loading is the change crack path with rate. They also concluded that the effective critical crack- tip opening decreases with the increase in time to peak load. 20

International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 4. Fracture of rock: Effect of loading rate by Ravindra Gettu, Zdenek P. Bazant & Shang-Ping Bai In 1993, Ravindra Gettu, Zdenek P. Bazant & Shang-Ping Bai [7] published a paper on effect of loading rate on rock. They proposed that the fracture in all material is rate-sensitive. The paper concluded that the strength of rock increases with an increase in the loading rate. In case of concrete, they concluded that for higher time to peak load, the brittleness of failure increases. 5. Experimental Investigations of loading rate effects in reinforced concrete columns by W. Ghannoum, V. Saouma, G. Houssmann, K. Polkingorane, M. Eck & D. H. Kang (J. Struct . Engg., 2012.138: 1032-1041) In the year 2012, W. Ghannoum, V. Saouma, G. Houssmann, K. Polkingorane, M. Eck & D. H. Kang [8] studied loading rate effect on RC columns. They studied the behaviour of RC under effect of seismic loading. The single concrete column prototype was chosen to investigate effects of varying loading rates on behaviour of the specimen. The dynamic loading effects on column behaviour were observed in all tests. They concluded that at the observed increase in strength at higher loading rate, the increase in strength is significant. And by ignoring such an increase, the codes are currently erring on the conservative side in strain assessment for short duration seismic loading. 6. Strain rate induced strength enhancement in concrete: much ado about nothing? by Leonard Schwer, 7th europian LSDYNA conference M-I-03, 2009 In 2009, Leonard Schwer [4] of Windsor, California, USA conducted unconfined compression test simulations on concrete cylinder models. The strain-rate loading was increased from 10/sec to 100/sec and graph of axial load v/s time is plotted. The graph indicates behaviour of specimens under different loading rates, each of the clearly indicates that the axial load increased with increment inn loading rate. 7. Effect of loading rate on the fracture behaviour of high-strength concrete by G. Ruiza, X. X. Zhangb, R. C. Yu, E. Poveda, R. Porras and J. del Viso, Applied Mechanics and materials Vols. 24-25, 2010, p. 179-185 In 2010, G. Ruiza, X. X. Zhangb, R. C. Yu, E. Poveda, R. Porras and J. del Viso [2] studied the fractured behaviour of high strength concrete by subjecting it to increased strain rate based on experimental data prepared by them, a graph of load-displacement curve was plotted. It was clearly indicated by the graph that the peak load increases with increasing load rates and the displacement remains almost constant. 8. Strain rate effect on performance of reinforced concrete member by Otani, Shunsuke, Takashi, Kaneko and Hitoshi Shiohara, Proceeding of FIB Symposium on concrete structures++++++ in seismic regions. May 2003.5, p. 367-371 In 2003, Otani, Shunsuke, Takashi, Kaneko and Hitoshi Shiohara [5] conducted experiments on four pairs of RC beams to study its behaviour under various static and dynamic loading conditions. Loading gauge and strain gauge actuators were used for testing the specimens. From the experimental data, it was concluded that at the strain rate expected during earthquake, the flexural resistance of beams can increase from 7% up to 20%. III. METHODOLOGY The project has been divided into following three stages in order to obtain the required objective: 1. Market Survey & Literature Review 2. Experimental Work 3. Analysis of data obtained 1. MARKET SURVEY: Through market survey, it’s been observed that many institutes and land laboratories do not follow the prescribed rate of loading and thus do not comply with I.S. Codes. There is no availability of pace rate adjuster. Also manually operated CTM was majorly found on construction sites. As the time required to test the concrete cubes at 140 kg/cm2/min is high, it is not followed on site. As concrete grades ranging from M20 to M40 are usually used, they were selected for the project. It is also observed that the time taken to test a cube is 20 seconds to 1 min which generates the rate of loading as 900 kg/cm2/min to 1400 kg/cm2/min. 2. EXPERIMENTAL WORK: The experimental work consisted of the following stages: i. Casting of concrete cubes ii. Testing of concrete cubes iii. Analysis of data iv. Formulation of correction factor v. Application of correction factor 3. Analysis of data obtained: Though the analysis of the above data, results are obtained which are explained further. 21

International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 IV. CORRECTION FACTOR Definition: A factor used to reduce the amount of deviation in a measurement to obtain the accurate value is called ‘correction factor’ [9] Necessity: To correct the strength obtained at higher rate of loading, the correction factor shall be such that it will reduce the obtained strength at rate of loading greater than 140 kg/cm2/min and increase it at loading rate less than 140 kg/cm2/min. hence, the correction factor will be a divisor term and hence it will vary between 0 and 1.6 as the strength increases by a maximum of 50-60% of the true strength. As the equation obtained for curves of various grades are different, the correction factor shall not remain the same for each grade. Correction factor from each equation shall be computed separately for each grade from the equations or directly from the graph of correction factor. V. RESULTS 1. COMPUTATION OF CORRECTION FACTOR a. Following table shows the correction factors computed for M20 grade of concrete. These corrections factors were calculated by taking the strength obtained at 140 kg/cm2/min as datum. Table 1: Correction Factors for grade M20 Correction

Correction

Sr No

ROL

Strength

factor

Sr No

ROL

Strength

Factor

1

40

22.22

0.929022668

22

530

28.89

1.207729469

2

40

21.33

0.891861761

23

560

29.33

1.226309922

3

40

23.11

0.966183575

24

610

28.89

1.207729469

4

67.55

24.00

1.003344482

25

633

29.33

1.226309922

5

85.77

22.67

0.947603122

26

720

29.78

1.244890375

6

130

23.56

0.984764028

27

735

30.22

1.263470829

7

135

24.22

1.012634708

28

760

31.11

1.300631735

8

140

24.44

1.021924935

29

820

30.67

1.282051282

9

140

23.56

0.984764028

30

856

31.56

1.319212189

10

140

23.78

0.994054255

31

880

32.22

1.347082869

11

220

24.89

1.040505388

32

915

31.56

1.319212189

12

240

25.33

1.059085842

33

943

32.44

1.356373096

13

250

23.11

0.966183575

34

979

33.33

1.393534002

14

330

25.78

1.077666295

35

1035

32.89

1.374953549

15

340

25.78

1.077666295

36

1150

32.44

1.356373096

16

360

26.22

1.096246748

37

1215

33.56

1.402824229

17

375

26.67

1.114827202

38

1236

32.00

1.337792642

18

410

27.11

1.133407655

39

1356

34.22

1.430694909

19

437

28.00

1.170568562

40

1406

32.89

1.374953549

20

475

28.22

1.179858789

41

1516

34.22

1.430694909

21

515

28.44

1.189149015

Following figure shows graph of rate of loading vs correction factor for M20 grade of concrete. The graph was plotted with respect to the data obtained from above table. The abscissa in the graph represents the rate of loading whereas the ordinate represent the correction factor. The graph gives an equation of curve (y = -6E-11x3-7E-08x2+0.0006x+0.9141). This equation gives the correction factor for particular rate of loading.

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International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 2 y = -6E-11x3 - 7E-08x2 + 0.0006x + 0.9141 R² = 0.9722

1.8 1.6

Correction Factor

1.4 1.2 1

0.8 0.6 0.4 0.2 0 0

200

400

600

800

1000

1200

1400

1600

Graph 1: Rate of loading vs Correction factor of grade M20 b. Following table shows the correction factors computed for M25 grade of concrete. These corrections factors were calculated by taking the strength obtained at 140 kg/cm2/min as datum. Table 2: Correction factors for grade M25 Correction Sr No

Correction

ROL

Strength

Factor

Sr No

ROL

Strength

Factor

1

40

20.44

0.793170353

23

676

32.44

1.258828095

2

40

22.22

0.862242918

24

719

30.67

1.190143578

3

40

24.88

0.965463718

25

745

30.22

1.172681412

4

122.21

27.11

1.051998448

26

745

32

1.241753977

5

130

28

1.08653473

27

786

32.88

1.275902212

6

140

22.66

0.879317035

28

823

32.88

1.275902212

7

140

26.66

1.034536282

29

855

33.77

1.310438494

8

140

28

1.08653473

30

889

31.55

1.224291812

9

170

26.67

1.034924331

31

906

34.22

1.32790066

10

180

27.55

1.069072565

32

906

34.66

1.344974777

11

186

28

1.08653473

33

1024

33.77

1.310438494

12

231

28

1.08653473

34

1065

32.88

1.275902212

13

240

28.88

1.120682965

35

1069

32.88

1.275902212

14

266.6

26.22

1.017462165

36

1110

34.22

1.32790066

15

310

29.77

1.155219247

37

1122

33.77

1.310438494

16

323

29.33

1.13814513

38

1162

36

1.396973225

17

377

29.77

1.155219247

39

1219

35.11

1.362436942

18

444

31.11

1.207217695

40

1238

34.22

1.32790066

19

447

30.22

1.172681412

41

1310

38.22

1.483119907

20

533

28.44

1.103608847

42

1335

37.33

1.448583624

21

560

31.55

1.224291812

43

1463

38.67

1.500582072

22

576

31.55

1.224291812 23

International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 Following figure shows graph of rate of loading vs correction factor for M25 grade of concrete. The graph was plotted with respect to the data obtained from above table. The abscissa in the graph represents the rate of loading whereas the ordinate represent the correction factor. The graph gives an equation of curve (y = 6E-10x3-1E-06x2+0.0012x+0.869 ). This equation gives the correction factor for particular rate of loading.

2 1.8 1.6

y = 6E-10x3 - 1E-06x2 + 0.0012x + 0.8697 R² = 0.894

Correction Factor

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

200

400

600

800

1000

1200

1400

1600

Graph 2: Rate of loading vs Correction factors of grade M25

2. PRACTICAL IMPLIMENTATION & VERIFICATION: Correction factors computed are verified for their effective outputs. For this purpose, three cubes of M40 grade were casted. Each of these cube was tested at different rate of loading. Details given below: Table 3: Casting Specifications for verification of correction factor

Cube A

Tested After (Days) 28

Rate of loading used (kg/cm2/min) 140

Strength (MPa) 40.44

Cube B

28

633

45.33

Cube C

28

1024

48.44

Cubes Specimen

Implementation of Correction factor: In order to determine the true strength of concrete cube, the obtained strength shall be divided by suitable correction factor. By knowing the time required in testing, the rate of loading applied during testing can be calculated as shown below: Rate of loading = Strength obtained in kN x 1000 9.81 x 225 x time in minutes The equation of graph corresponding to its grade is given in the results. Substituting the value of ‘x’ that is rate of loading, the corresponding value of ‘y’ that is correction factor can be obtained. On dividing the obtained strength by the correction factor, the corrected strength of the specimen shall be obtained.

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International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4, 2015 Table 4: Implementation of Correction Factor Rate of loading used for testing (kg/cm2/min) 140

Equation for correction factor

Cube B

633

Cube C

1024

Y = 2E – 10x3 – 6E – 07x2 + 0.0006x + 0.891

Cube Specimen Cube A

Correction factor 1

Corrected Strength (MPa) 40.44

1.1299

40.440

1.1978

40.442

Thus, application of correction factor gives us precise results. VI. CONCLUSIONS With variation in rate of loading on concrete specimen, the strength varies proportionately. At higher rate of loading, the compressive strength increases. The increment is from 30% to almost 50% of the original strength. However, at lower rate of loading, the reduction in strength of concrete cube compared to its true strength is insignificant. 5. The equations derived from the graph of rate of loading vs correction factor enables to find the correction factor with respect to rate of loading. 6. These correction factors enables to compute the true compressive strength of concrete. 1. 2. 3. 4.

REFERENCES [1] P. H. Bishoff and S. H. Perry, “Compressive behaviour of concrete at high strain rates”, Materials and structural/ Matriaux et Constructions, 1991, 24, p. 425-450. [2] G. Ruiza, X. X. Zhangb, R. C. Yu, E. Poveda, R. Porras and J. del Viso, “Effect of loading rate on the fracture behaviour of high-strength concrete”, Applied Mechanics and materials Vols. 24-25, 2010, p. 179-185. [3] H. C. Fu, M. A. Erki, M. Seckin, “Review of effects of loading rate on concrete in compression”, J. Struct. Engg. 1991.117(12), p. 3645-3659. [4] Leonard Schwer, “Strain rate iduced strength enhancement in concrete: much ado about nothing?”, seventh europian LS-DYNA conference M-I-03, 2009 [5] Otani, Shunsuke, Takashi, Kaneko and Hitoshi Shiohara, “Strain rate effect on performance of reinforced concrete member”, Proceeding of FIB Symphosium on concrete sturctures in sesmic regions. May 2003.5, p. 367-371. [6] Ravindra Gettu, “ Rate effects and loading relaxation on static fracture of concrete”, ACI Materials Journal, title no. 89, M49, 1992, p. 456-458 [7] Ravindra Gettu, Zednek P. Bazant, Shang-Ping Bai, “ Fracture of rock: Effect of loading rate’’, Engg. Fracture MechanicVolume 45, No.3, 1993, p. 393-398. [8] W. Ghannoum, V. Souma, G. Haussmann, K. Polkinghorne, M. Eck and D. H. Kang, “ Experimental investigations of loading rate of effects in reinforced concrete columns”, J. Struct. Engg., 2012.138, p. 1032-1041. [9] www.google.co.in

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