Optimization of Coconut Fiber in Coconut Shell Concrete and ... - MDPI

materials Article

Optimization of Coconut Fiber in Coconut Shell Concrete and Its Mechanical and Bond Properties Anandh Sekar *

and Gunasekaran Kandasamy

Department of Civil Engineering, Faculty of Engineering and Technology, SRM Institute of Science and Technology, Chennai 603203, India; [email protected] * Correspondence: [email protected] Received: 13 August 2018; Accepted: 11 September 2018; Published: 14 September 2018

Abstract: Coconut shell concrete is one of the recently established lightweight concretes. This paper discusses the optimization of adding coconut fibers in both coconut shell concrete and conventional concrete. Coconut fibers at different aspect ratios of 16.67, 33.33, 50, 66.67, 83.33, and 100 and volume fractions of 1, 2, 3, 4, and 5% were tried. The maximum compressive strength was attained at an aspect ratio of 83.33 and volume fraction of 3% for conventional concrete, and aspect ratio 66.67 and volume fraction 3% for coconut shell concrete. Flexural strength increased by 30.63% (conventional concrete) and 53.66% (coconut shell concrete) on the addition of coconut fibers. Similarly, the split tensile strength increased by 19.44% and 30%, respectively. The number of blows needed for failure of specimen in impact resistance test was more for concrete mixed with fibers. The experimental bond stresses were higher than the theoretical values recommended by IS 456: 2000 (Indian Standard) and BS 8110 (British Standard). This study shows that the addition of coconut fiber enhances the properties of both conventional and coconut shell concrete. Keywords: coconut shell; aggregate; coconut fiber; mechanical properties; bond properties

1. Introduction In general, concrete is strong in compression and weak in tension. Plain concrete possesses a very low tensile strength, limited ductility, and little resistance to cracking. Internal microcracks are inherently present in the concrete and its poor tensile strength is due to propagation of such microcracks, eventually leading to the brittle failure of the concrete. Conventionally the tensile properties of concrete members are improved by the reinforcement with steel. However this does not increase the inherent tensile strength of concrete itself. In plain concrete, microcracks develop even before loading. After loading, the microcracks propagate, and stress concentration results in additional cracks where there are minor defects in the concrete. The development of such microcracks is the main cause of inelastic deformations in concrete. It has been recognized that the addition of small, closely spaced, and uniformly dispersed fibers to concrete would act as crack arresters [1]. Recent works on the use of different fibers in concrete indicates that the fibers enhance the properties of concrete [2–7]. Type of fibers used in concrete matrix influences the structural responses and also in design [8,9]. Coconut shell has been recently established as viable material in the production of concrete and its properties as an aggregate and in structural and non structural members have been published [10–18]. This paper explores the effects of adding coconut fiber to coconut shell concrete. The mechanical properties such as compressive, flexural, splitting tensile strengths, and impact resistance after the addition of coconut fiber are analyzed and results are reported. Since bonding between the concrete and the reinforcements is also equally important for any new combination of concrete, bond tests were also conducted. Coconut fibers were also added to conventional concrete (CC) and the properties were studied for comparison. Materials 2018, 11, 1726; doi:10.3390/ma11091726

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2. Materials Used Materials 2018, 11, x FOR PEER REVIEW

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Ordinary Portland cement (OPC) conforming to the Indian Standard IS 12269: 1987 [19] was used 2. Materials Used throughout this study. River sand of maximum size 4.75 mm, obtained from the local river Palar and Ordinary Portland cement (OPC) conforming to the Indian Standard IS 12269: 1987 [19] was conforming toused grading zone III as specified in IS 383: 1970 [20] was used as fine aggregate. Crushed throughout this study. River sand of maximum size 4.75 mm, obtained from the local river coconut shellsPalar (Figure 1) and conventional stones of ainmaximum size 12.5 mmaggregate. were used as coarse and conforming to grading zone III as specified IS 383: 1970 [20] wasof used as fine Crushed coconut shellsthe (Figure 1) and conventional stones of a maximum used size of 12.5 mm were used aggregates. Table 1 illustrates physical properties of aggregates in this study. as coarse aggregates. Table 1 illustrates the physical properties of aggregates used in this study.

Figure 1. Coconut shell aggregate.

Figure 1. Coconut shell aggregate. Table 1. Physical properties of aggregates.

Table 1. Physical properties of aggregates. Physical and Mechanical Properties Crushed Stone Aggregate Maximum size (mm) 12.50 Physical and Mechanical Properties Crushed Stone Aggregate Water absorption (%) 0.25 Maximum size (mm) 12.50 Specific gravity 2.67 Water absorption (%) 0.25 Fineness Modulus 6.76 Specific gravity 2.67 Bulk density (kg/m3) 1620 Fineness Modulus 6.76 Crushing value 18.44 1620 Bulk density (kg/m3 ) (%) Impact 15.70 Crushing valuevalue (%) (%) 18.44

Impact value (%)

15.70

Coconut Shell River Sand 12.50 Shell 4.75 Coconut River Sand 23.50 12.50 4.75 1.15 2.61 23.50 6.55 3.72 1.15 2.61 655 1695 6.55 3.72 2.62 - 1695 655 4.72 2.62 -

4.72

-

Coconut fibers used were procured from the local coconut industries (Shanmugam Traders, Kancheepuram, India) (Figure 2a). Different aspect ratios (16.67, 33.33, 50, 66.67, 83.33, and 100) and 2%, 3%, 4%, 5%, and 6%) of coconut fibers were tried in order to achieve the Coconut volume fibersfractions used (1%, were procured from the local coconut industries (Shanmugam Traders, maximum compressive strength. The optimum aspect ratio and volume fraction that gave the Kancheepuram, India)compressive (Figure 2a). Different ratios (16.67, 33.33, 50,microscope 66.67, 83.33, maximum strength was used aspect for further studies. Scanning electron (SEM) and 100) and images showed average of coconut fiber was 0.6 mm. A typical image isto achieve the volume fractions (1%, 2%,that 3%,the4%, 5%,diameter and 6%) of coconut fibers were triedSEM in order shown in Figure 2b.

maximum compressive strength. The optimum aspect ratio and volume fraction that gave the maximum compressive strength was used for further studies. Scanning electron microscope (SEM) images showed Materials 2018, 11, x FOR PEER REVIEW 3 of 15 that the average diameter of coconut fiber was 0.6 mm. A typical SEM image is shown in Figure 2b.

(a)

(b)

Figure 2. (a) Coconut fibersused usedin inthis this study; study; and SEM Image of coconut fiber.fiber. Figure 2. (a) Coconut fibers and(b) (b)Typical Typical SEM Image of coconut

3. Experimental Procedure For both CC and coconut shell concrete (CSC), a compressive strength of 25 N/mm2 at 28 days was targeted. Mix ratio used for CC was 1:2.22:3.66:0.55 with a cement content of 320 kg/m3, and for CSC, it was 1:1.47:0.65:0.42 with a cement content of 510 kg/m3 [11,12]. These mix proportions were used as controlled mixes without the addition of coconut fibers. Then, coconut fibers were added at different aspect ratios of 16.67, 33.33, 50, 66.67, 83.33, and 100 and volume fractions of 1%, 2%, 3%,

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3. Experimental Procedure For both CC and coconut shell concrete (CSC), a compressive strength of 25 N/mm2 at 28 days was targeted. Mix ratio used for CC was 1:2.22:3.66:0.55 with a cement content of 320 kg/m3 , and for CSC, it was 1:1.47:0.65:0.42 with a cement content of 510 kg/m3 [11,12]. These mix proportions were used as controlled mixes without the addition of coconut fibers. Then, coconut fibers were added at different aspect ratios of 16.67, 33.33, 50, 66.67, 83.33, and 100 and volume fractions of 1%, 2%, 3%, 4%, 5%, and 6% to find the optimum mix that produced the maximum compressive strength. Cubes of size of 100 × 100 × 100 mm were tested according to IS 516: 1959 [21] to determine the compressive strength of concrete. To determine the flexural resistance, the standard size of the specimen used was 100 × 100 × 500 mm. This test was conducted by the application of four points loading as per ASTM C78-84 PL (American Society for Testing and Materials) [22]. The flexural strength was calculated as bd 2 , where P is maximum load applied in N, L is supported length of specimen in mm, b is the width of specimen in mm, and d is the depth of specimen in mm. To determine the split tensile strength, the standard size of the specimen used was 100 mm in diameter and 200 mm height. This test was conducted as per ASTM C496-90 [23]. The split tensile 2P strength was calculated as πDL , where P is maximum load applied in N, D is cross sectional diameter of the specimen in mm, and L is the length of specimen in mm. To determine the impact resistance, the size of the specimen used was 152.4 mm in diameter and 63.5 mm thick. This test was conducted as per the method developed by ACI committee 544.1R-82 (American Concrete Institute). During the impact test, the number of blows was counted till the first crack appeared (initial crack) on each specimen and counting was continued till the specimen was broken (final crack) into a number of pieces. Though the tests were conducted at the age of three days, seven days, and 28 days, only the 28 days result is presented. To study the bond stress, a pull out test was conducted as per IS 2770 (Part-I 1987) [24], and the bond strength was calculated. The size of the specimen used was 100 mm in diameter and 200 mm height. Both plain and deformed bars having diameters of 8, 10, 12, and 16 mm were used in CC and CSC mixes and tested for bond strength with and without addition of coconut fibers. The bond stress F was calculated as ι = πdl where ι is the bond stress in N/mm2 , F is the applied load in N, d is the nominal bar diameter in mm, and l is the effective bonded length (150 mm). The bond stress reported is the average of three specimens in each case. The line diagram of the universal testing machine (UTM, Fyne Spavy Associates, Miraj, India) modified for testing the bond strength is shown in Figure 3. Table 2 illustrates the different test parameters considered, number of specimen used for each parameter, and the age of specimen during testing. Table 2. Experiments and number of samples used in this study. Sl. No

Test Parameter

1

For compressive strength test—optimization

For CC—31 mixes; 9 cubes in each mix; total of 279 cubes. For CSC—25 mixes; 9 cubes in each mix; total of 225 cubes.

Number of Specimens

3 days, 7 days and 28 days 3 days, 7 days and 28 days

Age during Testing

2

Flexural strength test

For CC—6 mixes; 9 beams in each mix; total of 54 beams. For CSC—6 mixes; 9 beams in each mix; total of 54 beams.


3

Split tensile test

For CC—6 mixes; 9 cylinders in each mix; total of 54 cylinders. For CSC—6 mixes; 9 cylinders in each mix; total of 54 cylinders.


4

Impact strength test

For CC—6 mixes; 9 specimens in each mix; total of 54 specimens. For CSC—6 mixes; 9 specimens in each mix; total of 54 specimens.


5

Bond strength test

For CC—4 mixes; 48 specimens in total. For CSC—4 mixes; 48 specimens in total.

28 days 28 days

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Table 2 illustrates the different test parameters considered, number of specimen used for each 4 of 14 parameter, and the age of specimen during testing.

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Figure 3. An arrangement for testing bond specimen in UTM. Figure 3. An arrangement for testing bond specimen in UTM.

4. Results and Discussion Table 2. Experiments and number of samples used in this study.

4.1. Properties of Fresh and Hardened Concrete Without Fibers Sl. No Test Parameter Number of Specimens Age during Testing The slump was 10 mm for both CC and CSC mixes without coconut fibers. The density of fresh For CC—31 mixes; 9 cubes in each For compressive CC and CSC mixes were found as 2520 kg/m3 and 1957 kg/m3 , respectively. Densities of hardened mix; total of 279 cubes. 3 days, 7 days and 28 days 1 of CC andstrength concrete CSC were 2510 kg/m3 and 1952 kg/m3 at three days; 2515 kg/m3 and 1955 kg/m3 For CSC—25 mixes; 9 cubes in each 3 days, 7 days and 28 days at seven days;test—optimization and 2527 kg/m3 and 1968 kg/m3 at 28 days, respectively. Compressive strengths of CC mix; total of 225 cubes. and CSC mixes were 19.6 N/mm2 and 18.3 N/mm2 at three days; 21.9 N/mm2 , and 20.4 N/mm2 at For CC—6 mixes; 9 beams in each seven days; and 30.1 N/mm2 and 25.6 N/mm2 at 28 days, respectively. mix; total of 54 beams. 3 days, 7 days and 28 days 2 Flexural strength test Forand CSC—6 mixes; 9 beams in each 3 days, 7 days and 28 days 4.2. Properties of Fresh and Hardened CC CSC with Fibers mix; total of 54 beams. Since fibers were added on volume basis, aspect did not in influence the densities of the mixes For CC—6 mixes;ratio 9 cylinders used. Therefore, the slump values were eight–10 CC and six–10 3mm for7CSC and no each mix; totalmm of 54for cylinders. days, daysmixes and 28 days 3 Split tensile test casting of specimens and compaction. Both the slump and density difficulties were faced during For CSC—6 mixes; 9 cylinders in 3 days, 7 days and 28 days decreased with increase in the percentage of fibers. The fibers play a role in influencing the workability each mix; total of 54 cylinders. and density of both CC and CSC. Fresh (zero day) and hardened concrete densities of both CC and For CC—6 mixes; 9 specimens in CSC mixes at three, seven and 28 days for different volume fractions (1, 2, 3, 4, 5, and 6%) irrespective each mix; total of 54 specimens. 3 days, 7 days and 28 days strength of the4 aspectImpact ratios are giventest in Figures 4 and 5. For CSC—6 mixes; 9 specimens in 3 days, 7 days and 28 days each mix; total of 54 specimens. For CC—4 mixes; 48 specimens in total. 28 days 5 Bond strength test For CSC—4 mixes; 48 specimens in 28 days total.

mixes used. Therefore, the slump values were eight–10 mm for CC and six–10 mm for CSC mixes and no difficulties were faced during casting of specimens and compaction. Both the slump and density decreased with increase in the percentage of fibers. The fibers play a role in influencing the workability and density of both CC and CSC. Fresh (zero day) and hardened concrete densities of Materials 1726 mixes at three, seven and 28 days for different volume fractions (1, 2, 3, 4, 5, 5 of 14 both CC2018, and11,CSC and 6%) irrespective of the aspect ratios are given in Figures 4 and 5. 2525

Density (kg/m3)

2500 2475

Zero day 3 days

2450

7 days 28 days

2425 2400 1

2

3

4

5

6

Fiber Volume Fraction (%) Figure mixes. Materials 2018, 11, x FOR PEER REVIEW Figure 4. 4. Densities Densities of of conventional conventional concrete concrete mixes.

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1975

Density (kg/m3)

1960 1945

Zero day 3 days

1930

7 days 28 days

1915 1900 1

2

3 4 5 Fiber Volume Fraction (%)

6

Figure 5. Densities of coconut shell concrete mixes.

4.3. Optimization of of Coconut Coconut Fibers Fibers in 4.3. Optimization in CC CC and and CSC CSC Mixes Mixes Thirty one one mixes for CC Thirty mixes for CC and and 25 25 for for CSC CSC with with different different aspect aspect ratios ratios and and volume volume fractions fractions were were tested for compressive strength. For each mix, nine cubes were cast, of which, three cubes each were were tested for compressive strength. For each mix, nine cubes were cast, of which, three cubes each tested at three, seven, and 28 days and the average was reported. Table 3 shows the results for tested at three, seven, and 28 days and the average was reported. Table 3 shows the results for CC CC mixes. The maximum compressive strength corresponding to a particular aspect ratio and volume mixes. The maximum compressive strength corresponding to a particular aspect ratio and volume fraction 6 shows thethe results of compressive strength test for allfor CCall mixes. fraction isishighlighted highlightedininbold. bold.Figure Figure 6 shows results of compressive strength test CC 2 The maximum compressive strengthstrength of 43.8 N/mm was achieved for aspect of 83.33 and volume 2 was achieved mixes. The maximum compressive of 43.8 N/mm forratio aspect ratio of 83.33 and fraction 3% (Figure 6).(Figure This value is 45.51% as compared compressive of CC volume of fraction of 3% 6). This valuemore, is 45.51% more, to as the compared to thestrength compressive 2 ). mix without coconut fibers (30.1 N/mm 2 strength of CC mix without coconut fibers (30.1 N/mm ). Table 3. Results of CC mix with different aspect ratios and volume fractions.

Aspect Ratio

16.67

% of Fiber 1 2 3 4

Compressive Strength, N/mm2 3 Day Strength 7 Day Strength 28 Day Strength 16.80 22.20 31.40 17.30 23.00 32.70 18.90 24.80 34.60 21.00 26.30 37.00

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Table 3. Results of CC mix with different aspect ratios and volume fractions. Compressive Strength, N/mm2

Aspect Ratio

% of Fiber

3 Day Strength

7 Day Strength

28 Day Strength

16.67

1 2 3 4 5 6

16.80 17.30 18.90 21.00 21.40 20.90

22.20 23.00 24.80 26.30 26.50 26.00

31.40 32.70 34.60 37.00 39.50 37.50

33.33

1 2 3 4 5

19.86 20.53 21.76 18.23 17.60

25.70 26.66 27.40 21.90 20.70

32.50 33.73 34.30 29.20 25.80

20.61 23.45 25.12 21.01 16.30 19.33

22.89 27.33 30.20 27.78 25.81 23.37

29.90 32.13 35.17 33.98 29.52 33.16

26.53 16.71 21.57 14.60 19.33 16.71 17.87 14.60 21.30 17.87 23.90 21.30 15.36 23.90 15.36 14.90 14.90 16.90 16.90 17.40 17.40 17.63 17.63 18.70 18.70 15.61 15.61

28.10 19.23 24.97 16.63 23.37 19.23 23.70 16.63 27.03 23.70 34.50 27.03 23.03 34.50 23.03 21.90 21.90 23.10 23.10 26.10 26.10 27.90 27.90 29.50 29.50 24.00 24.00

35.63 32.01 33.46 27.93 33.16 32.01 30.10 27.93

1 2 Materials 2018, 11,50x FOR PEER REVIEW 3 4 35

66.67

83.33 83.33

100100

4 5 1 2 3 4 5 1 2 3 4 5

1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

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36.97

30.10 43.80 36.97 32.03 43.80 32.03 28.41 28.41

31.20

31.20 33.10 33.10 34.90 34.90 37.10 37.10 32.50

32.50

Compressive strength (N/mm2)

45 40 35 30 25

3 days

20

7 days

15

28 days

10 5 0 16.67 (5%) 33.33 (3%)

50 (3%)

66.67 (1%) 83.33 (3%) 100 (4%)

Aspect ratio & Fiber Volume Fraction (%) Figure fraction (%). (%). Figure 6. 6. CC CC mixes: mixes: compressive compressive strength strength vs vs aspect aspect ratio ratio & & volume volume fraction

Similarly the results of CSC mixes with different aspect ratios and volume fractions are given in Table 4. Figure 7 shows the results of compressive strength test for all CSC mixes. The maximum compressive strength of 30.01 N/mm2 was achieved for an aspect ratio of 66.67 and volume fraction of 3% (Figure 7). This value is 17.22% more compared to the compressive strength of CSC mix without coconut fibers (25.6 N/mm2).

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Similarly the results of CSC mixes with different aspect ratios and volume fractions are given in Table 4. Figure 7 shows the results of compressive strength test for all CSC mixes. The maximum compressive strength of 30.01 N/mm2 was achieved for an aspect ratio of 66.67 and volume fraction of 3% (Figure 7). This value is 17.22% more compared to the compressive strength of CSC mix without coconut fibers (25.6 N/mm2 ). Table 4. Results of CSC mix with different aspect ratios and volume fractions. Compressive Strength, N/mm2

Aspect Ratio

% of Fiber

3 Day Strength

7 Day Strength

28 Day Strength

16.67

1 2 3 4 5

16.20 16.30 16.50 17.70 15.90

20.00 21.30 21.90 22.70 20.70

26.00 26.20 26.50 26.90 24.40

33.33

1 2 3 4 5

16.80 17.00 18.33 17.76 17.66

18.10 19.76 22.84 21.83 19.13

21.30 23.60 27.20 25.30 23.50

50

1 2 3 4 5

13.70 15.60 16.20 12.90 11.40

18.60 19.20 20.40 18.40 18.30

26.00 26.30 26.95 25.91 24.00

66.67

1 2 3 4 5

15.20 16.80 18.90 16.20 15.90

19.90 21.89 24.49 22.14 21.09

25.87 27.80 30.01 26.78 25.10

83.33

1 2 3 4 5

17.00 17.10 17.73 16.61 16.31

22.90 22.98 23.11 21.19 20.03

25.20 26.00 26.40 24.90 24.85


Compressive strength (N/mm2)

35 30 25 20

3 days

15

7 days

10

28 days

5 0 16.67 (4%)

33.33 (3%)

50 (3%)

66.67 (3%)

83.33 (3%)

Aspect ratio & Fiber Volume Fraction (%) Figure fraction (%). (%). Figure 7. 7. CSC CSC mixes: mixes: compressive compressive strength strength vs vs aspect aspect ratio ratio & & volume volume fraction Table 4. Results of CSC mix with different aspect ratios and volume fractions.

Aspect Ratio

% of Fiber 1 2

Compressive Strength, N/mm2 3 Day Strength 7 Day Strength 28 Day Strength 16.20 20.00 26.00 16.30 21.30 26.20

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4.4. Flexural Strength Figures 8 and 9 illustrate the results of flexural strength tests on both CC and CSC mixes, respectively. In the case of CC mixes, maximum flexural strength of 5.10 N/mm2 (11.64% of Materials 2018, 2018, 11, 11, xx FOR FOR PEER PEER REVIEW REVIEW of 15 15 Materials 99 of compressive strength) was attained at 28 days for the aspect ratio 83.33 with a 3% volume fraction. This is an increase of of 30.43% compared N/mm22).2 ).For For CSC mixes, maximum This is an increase 30.43% comparedtotoCC CCwithout withoutfiber fiber (3.91 (3.91 N/mm CSC mixes, maximum 2 22 (24.62% flexural strength of 6.50 N/mm of compressive strength) attained at days 28 days for aspect the flexural strength of 6.50 N/mm (24.62% of compressive strength) waswas attained at 28 for the aspect ratio 83.33 with 3% volume fraction. This is an increase of 53.66% to CSC without fiber (4.23 ratio 83.33 with 3% volume fraction. This is an increase of 53.66% to CSC without fiber (4.23 N/mm2 ). 22). The experimental results at 28 days were compared with the theoretical formula TheN/mm experimental results at 28 days were compared with the theoretical formula recommended by IS √ 456: 2000 [25] 0.7 √𝑓𝑐𝑘 , where 𝑓𝑐𝑘 = compressive strength of concrete. In all the 456:recommended 2000 [25] 0.7 by IS f ck , where f ck = compressive strength of concrete. In all the cases, experimental 𝑐𝑘 𝑐𝑘 cases, experimental values were found to be higher than the theoretical values. Though flexural is values were found to be higher than the theoretical values. Though the flexural strengththe of concrete strength of concrete is related to its compressive strength, the addition of coconut fibers (aspect related to its compressive strength, the addition of coconut fibers (aspect ratio) had a different effect ratio) had a different effect on the flexural strength of both CC and CSC mixes (Figures 8 and 9). The on the flexural strength of both CC and CSC mixes (Figures 8 and 9). The flexural strength increase flexural strength increase increase aspect ratio, strength whereas did the compressive strength did not with increase in aspect ratio,with whereas theincompressive not show such a clear relation. show such a clear relation. Additionally, though the compressive strengths of CSC mixes were lower Additionally, though the compressive strengths of CSC mixes were lower compared to CC mixes, the compared to CC mixes, the CSC exhibited higher flexural strength. This is due to fibrous nature of CSC exhibited higher flexural strength. This is due to fibrous nature of the coconut shell. The same the coconut shell. The same trend was observed in the earlier studies on coconut shell concrete trend was observed in the earlier studies on coconut shell concrete [10,26]. [10,26]. [10,26].

Flexural (N/mm22)) strength (N/mm Flexural strength

6 5 4 3 days

3

7 days

2

28 days

1

As per IS 456

0 0 (0%) 16.67 (5%)33.33 (3%) 50 (3%) 66.67 (1%)83.33 (3%) Aspect ratio & Fiber Volume Fraction (%) Figure8.8.Flexural Flexural strength strength test Figure test results resultsof ofCC CCmixes. mixes.

Flexural (N/mm22)) strength (N/mm Flexural strength

7 6 5 4

3 days

3

7 days

2

28 days As per IS 456

1 0 0 (0%) 16.67 (4%)33.33 (3%) 50 (3%) 66.67 (3%)83.33 (3%) Aspect ratio & Fiber Volume Fraction (%) Figure 9. Flexural strength test results of CSC mixes. Figure 9. Flexural strength test results of CSC mixes.

4.5. Split Tensile Strength Figures 10 and 11 illustrate the results of split tensile strength tests on both CC and CSC mixes, respectively. In case of CC mixes, maximum split tensile strength of 4.30 N/mm22 (9.81% of compressive strength) was attained at 28 days for the aspect ratio 83.33 with a 3% volume fraction.

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4.5. Split Tensile Strength

Split tensile strength (N/mm2)

Figures 10 and 11 illustrate the results of split tensile strength tests on both CC and CSC mixes, respectively. In case of CC mixes, maximum split tensile strength of 4.30 N/mm2 (9.81% of compressive Materials 2018, 11, x FOR PEER REVIEW 10 of 15 strength) was attained at 28 days for the aspect ratio 83.33 with a 3% volume fraction. This is an increase of 19.44% compared to CC without fiber (3.6 N/mm2 ). For CSC 2mixes, maximum split tensile This is an increase of 19.44% compared to CC without fiber (3.6 N/mm ). For CSC mixes, maximum strength of 3.90 N/mm2 (14.77% of 2compressive strength) was attained at 28 days for the aspect ratio split tensile strength of 3.90 N/mm (14.77% of compressive strength) was attained at 28 days for the 83.33 with 3% volume fraction. This is an increase of 30% compared to CSC without fiber (3 N/mm2 ). aspect ratio 83.33 with 3% volume fraction. This is an increase of 30% compared to CSC without fiber This shows that the use of coconut fiber in CC and CSC mixes augments the split tensile strength of (3 N/mm2). This shows that the use of coconut fiber in CC and CSC mixes augments the split tensile the concrete. These results also emphasize the significant role of the aspect ratio. strength of the concrete. These results also emphasize the significant role of the aspect ratio.

5 4 3 3 days 2

7 days 28 days

1 0 0 (0%)

16.67 (5%) 33.33 (3%) 50 (3%) 66.67 (1%) 83.33 (3%) Aspect ratio & Fiber Volume Fraction (%) Figure 10. Split tensile strength test results of CC mixes.

Split tensile strength (N/mm2)

4

3 3 days

2

7 days 28 days

1

0 0 (0%)

16.67 (4%) 33.33 (3%) 50 (3%) 66.67 (3%) 83.33 (3%) Aspect ratio & Fiber Volume Fraction (%) Figure 11. 11. Split test results results of of CSC CSC mixes. mixes. Figure Split tensile tensile strength strength test

4.6. Impact Resistance 4.6. Impact Resistance Figures Figures 12 12 and and 13 13 illustrate illustrate the the results results of of impact impact resistance resistance tests tests at at 28 28 days days on on both both CC CC and and CSC CSC mixes, respectively. In general, the impact resistance increases with concrete strength. There mixes, respectively. In general, the impact resistance increases with concrete strength. There is is an an optimum optimum value value of of impact impact resistance resistance in in normal normal concrete concrete beyond beyond which which any any increase increase in in strength strength does does not not play play any any role role in in impact impact resistance resistance both both at at first first crack crack and and at at final final crack crack [27]. [27]. Literature Literature also also shows shows

that the failure of conventional concrete having compressive strength of around 45 N/mm2 requires only 10–22 blows [27]. This is reinforced in this study as the CC mix without fibers have compressive strength 30.1 N/mm2 that failed at 18–23 blows. As the aspect ratio increased, the number of blows required for initial cracks and final failures of CC with fibers that also increased. The CSC mixes also behaved in a similar manner. However, in general, the impact resistance of CSC mixes both with and

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Impact resistance (No.of blows)

that the failure of conventional concrete having compressive strength of around 45 N/mm2 requires only 10–22 blows [27]. This is reinforced in this study as the CC mix without fibers have compressive strength 30.1 N/mm2 that failed at 18–23 blows. As the aspect ratio increased, the number of blows required for initial cracks and final failures of CC with fibers that also increased. The CSC mixes also behaved in a similar manner. However, in general, the impact resistance of CSC mixes both Materials 2018, 11, x FOR PEER REVIEW 11 of 15 with and without fibers is higher compared to CC mixes with and without fibers. This increase in impact resistance CSCmay mixes be the duefibrous to the fibrous nature of aggregate the CS aggregate and its high resistance of CSC of mixes be may due to nature of the CS and its high impact impact resistance. resistance.

180 160 140 120 100 80 60 40 20 0

Initial Cracks Final Cracks

0 (0%)

16.67 (5%)

33.33 (3%)

50 (3%)

66.67 (1%)

83.33 (3%)

Aspect ratio & Fiber Volume Fraction (%)

Impact resistance (No.of blows)

Figure 12. Impact resistance test results of CC mixes.

200 180 160 140 120 100 80 60 40 20 0

Initial Cracks Final Cracks

0 (0%)

16.67 (4%)

33.33 (3%)

50 (3%)

66.67 (3%)

83.33 (3%)

Aspect ratio & Fiber Volume Fraction (%) Figure Figure 13. 13. Impact Impact resistance resistance test test results results of of CSC CSC mixes. mixes.

4.7. Bond Bond Properties Properties 4.7. Figures 14 and 15 15 illustrate illustrate the the results results of bond stress stress at 28 days days of of both both plain plain and and deformed deformed bars bars Figures 14 and of bond at 28 with different mixes. with different mixes.

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Bond stress (N/mm22)

10 8 6

8 mm 10 mm

4

12 mm 2

16 mm

0 0 (0%) CC 83.33 (3%) CC 0 (0%) CSC 66.67 (3%) CSC Aspect ratio & Fiber Volume Fraction (%) of CC and CSC mixes Figure mixes. Figure 14. 14. Bond Bond stress stress of of plain plain bars bars with with different different mixes.

12

Bond stress (N/mm22)

10 8 8 mm

6

10 mm

4

12 mm

2

16 mm

0 0 (0%) CC 83.33 (3%) CC 0 (0%) CSC 66.67 (3%) CSC Aspect ratio & Fiber Volume Fraction (%) of CC and CSC mixes Figure mixes. Figure 15. 15. Bond Bond stress stress of of deformed deformed bars bars with with different different mixes.

Theoretical bond strengths were were calculated calculated as as per per IS IS 456: 456: 2000 [25] [25] and and BS BS 8110 8110 [28] [28] (Table (Table5). 5). Table 5. 5. Theoretical bond strength. strength.

Aspect Ratio && Fiber Aspect Ratio Fiber Volume Fraction (%) Volume Fraction (%)&& Type MixMix Type 0 (0%) CCCC 0 (0%) 83.33 (3%) CCCC 83.33 (3%) 0 (0%) CSC 0 (0%) CSC 66.67 (3%) CSC 66.67 (3%) CSC

2 2 Theoretical Strength,N/mm N/mm Theoretical Bond Bond Strength, ISIS456: BS 8110 456:2000 2000 BS 8110 Plain Bar Deformed Bar Plain Bar Deformed Plain Bar Deformed Bar Plain Bar Deformed BarBar 1.53 1.53 2.742.74 1.85 1.85 3.303.30 1.40 2.24 1.40 2.24 1.41 2.522.52 1.41 1.53 2.74 1.53 2.74

In case of of plain plainbars, bars,bars barscome comeout outofof the specimen without forming visible cracks the specimen without forming anyany visible cracks overover the In case the surface. However, in the case of deformed bars, specimens fail suddenly after the formation surface. However, in the case of deformed bars, specimens fail suddenly after the formation of of longitudinal cracks. outcrop exteriorofofthe thedeformed deformedbars barsplays playsaa significant significant role role in longitudinal cracks. TheThe outcrop onon thethe exterior in improving the bond strength. improving the bond strength. In case of CC mixes, the bond strength of specimens with plain bars ranged from 4.12 to 9.02 N/mm² (13.68 to 29.9% of its compressive strength) and in the case of CSC mix, the bond strength of specimens with plain bars ranged from 4.01 to 7.7 N/mm² (15.6 to 30% of its compressive strength).

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In case of CC mixes, the bond strength of specimens with plain bars ranged from 4.12 to 9.02 N/mm2 (13.68 to 29.9% of its compressive strength) and in the case of CSC mix, the bond strength of specimens with plain bars ranged from 4.01 to 7.7 N/mm2 (15.6 to 30% of its compressive strength). Similarly for CC mixes, the bond strength of specimens with deformed bars ranged from 5.26 to 11.39 N/mm2 (17.4 to 37.8% of its compressive strength) and for CSC mixes, the bond strength of specimens with deformed bars ranged from 3.98 to 9.57 N/mm2 (15.54 to 37.38% of its compressive strength). Materials 2018, 11, x FOR PEER REVIEW 13 of 15 In the case of CC mixes with coconut fiber, the bond strength of specimens with plain bars ranged from 4.2 toSimilarly 9.08 N/mm (9.5 to its compressive and bars in the casefrom of CSC mixes with for CC2mixes, the20% bondofstrength of specimensstrength) with deformed ranged 5.26 to 11.39the N/mm² (17.4 to 37.8% its compressive and ranged for CSC mixes, the bond strength of 2 (13.8 to coconut fiber, bond strength ofofspecimens withstrength) plain bars from 4.16 to 8.11 N/mm specimens with deformed bars ranged from 3.98 to 9.57 N/mm² (15.54 to 37.38% of its compressive 27.02% of its compressive strength). Similarly for CC mixes with fiber, the bond strength of specimens strength). 2 (12.48 to 26.8% of its compressive strength) with deformed ranged to 11.78fiber, N/mm In bars the case of CC from mixes 5.47 with coconut the bond strength of specimens with plain bars and for CSC mixes with the bond strength specimensstrength) with deformed barsofranged from 4.14 to ranged from 4.2 tofiber, 9.08 N/mm² (9.5 to 20% of itsofcompressive and in the case CSC mixes 2 with coconut fiber, the bond strength of specimens with plain bars ranged from 4.16 to 8.11 N/mm² 10.9 N/mm (13.79 to 33.6% of its compressive strength). (13.8 to 27.02% of its compressive strength). Similarly for CC mixes with fiber, the bond strength of In general, the addition of coconut fibre enhances the bond strength. The experimental results specimens with deformed bars ranged from 5.47 to 11.78 N/mm² (12.48 to 26.8% of its compressive were morestrength) than the theoretical bond strength by both IS 456: 2000 [25] BS 8110 [28]. and for CSC mixes with fiber, therecommended bond strength of specimens with deformed barsand ranged The bond from strength thetodiameter the bar increased, 4.14 todecreased 10.9 N/mm²as (13.79 33.6% of itsof compressive strength). whether a plain bar or deformed In general, the addition of coconut fibreThis enhances the bond The experimental bar, which is similar to earlier studies [10,26]. is because ofstrength. the volume of concreteresults and confining were more than the theoretical bond strength recommended by both IS 456: 2000 [25] and BS 8110 pressure on the steel bars. 4.8.

[28]. The bond strength decreased as the diameter of the bar increased, whether a plain bar or deformed bar, which is similar to earlier studies [10,26]. This is because of the volume of concrete Post-Cracking Behaviour and confining pressure on the steel bars.

Ideally, CC specimens without fibers fail by crushing during compressive test and the cubes do not 4.8. Post-Cracking Behaviour change shape. But the CSC specimens without fibers bulged slightly after the formation of initial cracks. Ideally, CC specimens without fibers fail by crushing during compressive test and the cubes do The addition of coconut fibers delayed the propagation of failure of both CC and CSC specimens. The not change shape. But the CSC specimens without fibers bulged slightly after the formation of initial CSC specimens slowly after the formation cracksofand theofpropagation final failure cracks. bulged The addition of coconut fibers delayed of theinitial propagation failure both CC and of CSC specimens. CSCbespecimens bulged after theofformation initialcan cracks and the was delayed. Hence,The it can concluded thatslowly the addition coconut of fibers enhance the ductility of final failure was delayed. Hence, it can be concluded that the addition of coconut propertiespropagation of concrete, which is an advantage when it is used for earthquake resistant structures. fibers can enhance the ductility properties of concrete, which is an advantage when it is used for The bulging nature of the mix was additionally examined on a cylinder specimen (CSC mix with earthquake resistant structures. coconut fibers). shows themix bulging of the cylinder More studies TheFigure bulging16 nature of the was additionally examinedspecimen. on a cylinder specimen (CSCare mixneeded on with coconut fibers). Figure 16 shows theunder bulgingboth of thestatic cylinder More studiesto areconfirm needed the effect CSC structural elements with coconut fiber andspecimen. dynamic loading CSC structural elements with coconut fiber under both static and dynamic loading to confirm the of coconutonfibers on the ductility of concrete. effect of coconut fibers on the ductility of concrete.

Figure 16. Bulged shape of CSC cylinder during test. Figure 16. Bulged shape of CSC cylinder during test.

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5. Conclusions Both conventional concrete and coconut shell concrete mixes were studied with and without the addition of coconut fiber. In both CC and CSC mixes, as the percentage of fibers increases, workability and density decrease. From the compressive strength test results, it can be stated that the coconut fiber, having an aspect ratio of 83.33 and a volume fraction of 3%, is the optimum that can be used in CC mix to achieve the maximum strength for the mix proportion used in this study. Similarly, coconut fiber having an aspect ratio 66.67 and a volume fraction of 3% is the optimum that can be used in CSC mix to achieve the maximum strength for the mix proportion used in this study. Maximum flexural strengths of CC and CSC mixes added with coconut fibers were 30.63% and 53.66% more than that of CC and CSC mixes without coconut fibers, respectively. The aspect ratio of coconut fibers plays a significant role in the improvement of flexural strength. Additionally, flexural strength increased with the aspect ratio. Maximum split tensile strengths of CC and CSC mixes added with coconut fibers were 19.44% and 30% more than that of CC and CSC mixes without coconut fibers, respectively. Split tensile strength test results also show that the aspect ratio plays a significant role. The impact resistance of CSC mixes both with and without fibers is higher compared to CC mixes. The addition of coconut fiber to both CC and CSC enhances the bond strength. The experimental bond strengths were more than the theoretical values recommended by both IS 456: 2000 and BS 8110 standards. This study proves that addition of coconut fibers enhances the properties of CC and CSC. However, further studies are required on other properties like durability, temperature resistance, and structural behaviour under different types of loading. Author Contributions: The conceptualization of this manuscript was initiated and methodology adopted was also decided by both the authors A.S. and G.K. Test investigations were handled by the author A.S. and supervised by the author G.K. Results analysis were done by the author A.S. and checked by the author G.K. Original draft was prepared by the author A.S. under the guidance of G.K. In the same way review and editing were also done. Funding: This research received no external funding. Acknowledgments: The authors would like to thank the management of SRM Institute of Science and Technology for providing technical support and all those who were directly or indirectly involved in this study. Also thanks to V. Deeptha Thattai, Civil, SRM Institute of Science and Technology for her help in language and other corrections to bring this manuscript to this level. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Shetty, M.S. Concrete Technology Theory and Practice; S Chand & Company Ltd.: New Delhi, India, 2006. Larisa, U.; Solbon, L.; Sergei, B. fiber-reinforced concrete with mineral fibers and nanosilica. Procedia Eng. 2017, 195, 147–154. [CrossRef] Lee, J.H. Influence of concrete strength combined with fiber content in the residual flexural strengths of fiber reinforced concrete. Compos. Struct. 2017, 168, 216–225. [CrossRef] Pikus, G.A. Steel fiber concrete mixture work ability. Procedia Eng. 2016, 150, 2119–2123. [CrossRef] Ganesan, N.; Abraham, R.; Deepa Raj, S. Durability characteristics of steel fibre reinforced geopolymer concrete. Constr. Build. Mater. 2015, 93, 471–476. [CrossRef] Dey, V.; Bonakdar, A.; Mobasher, B. Low-velocity flexural impact response of fiber-reinforced aerated concrete. Cem. Concr. Compos. 2014, 49, 100–110. [CrossRef] Ali, M.; Li, X.; Chouw, N. Experimental investigations on bond strength between coconut fibre and concrete. Mater. Des. 2013, 44, 596–605. [CrossRef] Blanco, A.; Pujadas, P.; Fuente, A.D.L.; Cavalaro, S.H.P.; Aguado, A. Influence of the type of fiber on the structural response and design of FRC slabs. J. Struct. Eng. 2016, 142, 04016054. [CrossRef] Pujadas, P.; Blanco, A.; Cavalaro, S.; Aguado, A. Plastic fibres as the only reinforcement for flat suspended slabs: experimental investigation and numerical simulation. Constr. Build. Mater. 2014, 57, 92–104. [CrossRef]

Materials 2018, 11, 1726

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

24. 25. 26.

27. 28.

14 of 14

Gunasekaran, K.; Kumar, P.S.; Lakshmipathy, M. Mechanical and bond properties of coconut shell concrete. Constr. Build. Mater. 2011, 25, 92–98. [CrossRef] Gunasekaran, K.; Annadurai, R.; Kumar, P.S. Study on reinforced lightweight coconut shell concrete beam behavior under flexure. Mater. Des. 2013, 46, 157–167. [CrossRef] Gunasekaran, K.; Ramasubramani, R.; Annadurai, R.; Prakash Chandar, S. Study on reinforced lightweight coconut shell concrete beam behavior under torsion. Mater. Des. 2014, 57, 374–382. [CrossRef] Gunasekaran, K.; Annadurai, R.; Kumar, P.S. A study on some durability properties of coconut shell aggregate concrete. Mater. Struct. 2015, 48, 1253–1264. [CrossRef] Jayaprithika, A.; Sekar, S.K. Stress-strain characteristics and flexural behaviour of reinforced eco-friendly coconut shell concrete. Constr. Build. Mater. 2016, 117, 244–250. [CrossRef] Olanipekun, E.A.; Olusola, K.O.; Ata, O. A comparative study of concrete properties using coconut shell and palm kernel shell as coarse aggregates. Build. Environ. 2006, 41, 297–301. [CrossRef] Yerramala, A.; Ramachandrudu, C. Properties of concrete with coconut shells as aggregate replacement. Int. J. Eng. Invent. 2012, 1, 21–31. Gunasekaran, K.; Annadurai, R.; Chandar, S.P.; Anandh, S. Study for the relevance of coconut shell aggregate concrete non-pressure pipe. Ain Shams Eng. J. 2017, 8, 523–530. [CrossRef] Gunasekaran, K.; Pennarasi, G.; Soumya, S.; Richards, N.J. Study for the relevance of coconut shell aggregate concrete flooring tiles. Int. J. Civil Eng. Technol. 2017, 8, 370–379. Bureau of Indian Standards. Specification for 53 Grade Ordinary Portland Cement; IS 12269: 1987; Bureau of Indian Standards: New Delhi, India, 1987. Bureau of Indian Standards. Specification for Coarse and Fine Aggregate from Natural Sources for Concrete; IS 383: 1970; Bureau of Indian Standards: New Delhi, India, 1970. Bureau of Indian Standards. Indian Standard Specification for Methods of Tests for Strength of Concrete; IS 516: 1959; Bureau of Indian Standards: New Delhi, India, 1959. American Society for Testing and Materials International. Standard Test Method for Flexural Strength of Concrete; ASTM C78-84; American Society for Testing and Materials International: West Conshohocken, PA, USA, 1984. American Society for Testings and Materials International. Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens; ASTM C496-90; American Society for Testings and Materials International: West Conshohocken, PA, USA, 1990. Bureau of Indian Standards. Indian Standard Methods of Testing Bond in Reinforced Concrete; IS: 2770-(Part I)1967; UDC 666.982: 620.172.21; Bureau of Indian Standards: New Delhi, India, 1967. Bureau of Indian Standards. Indian Standard Plain and Reinforced Concrete—Code of Practice; IS 456: 2000; BIS 2000; Bureau of Indian Standards: New Delhi, India, 2000. Gunasekaran, K.; Prakash Chandar, S.; Annadurai, R.; Satyanarayanan, K.S. Augmentation of mechanical and bond strength of coconut shell concrete using quarry dust. Eur. J. Environ. Civil Eng. 2016, 21, 629–640. [CrossRef] Swamy, R.N.; Jojagha, A.H. Impact resistance of steel fiberreinforced lightweight concrete. Int. J. Cem. Compos. Lightweight Concr. 1982, 4, 209–220. [CrossRef] Structural use of concrete Part 1. In Code of Practice for Design and Construction; BS 8110; British Standard Institution: London, UK, 1997. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).