Mechanical properties of natural fibers reinforced sustainable masonry

Construction and Building Materials 24 (2010) 1536–1541

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Mechanical properties of natural fibers reinforced sustainable masonry C. Juárez b,*, B. Guevara a, P. Valdez b, A. Durán-Herrera b a b

Academic Group on Concrete Technology, Facultad de Ingeniería Civil-UANL, Mexico Center for Innovation, Research and Development in Engineering and Technology (CIIDIT), Mexico

a r t i c l e

i n f o

Article history: Received 2 July 2009 Received in revised form 19 January 2010 Accepted 2 February 2010 Available online 1 March 2010 Keywords: Natural fibers Lechuguilla PET Masonry Cracking Cement

a b s t r a c t This research evaluates the physical and mechanical properties of Portland cement masonry blocks reinforced with lechuguilla natural fibers, that were lightened with 2-l bottles of polyethylene terephthalate. A concrete mix was designed for a target compressive strength of 16 MPa at 28 days, and slump of 70 mm. Masonry concrete blocks with dimensions of 730  340  130 mm were produced for two different fiber lengths (25 and 50 mm) and with fiber contents of 0.25%, 0.50%, 0.75% and 1.0%. Based on the obtained results, it was found that as the aspect ratio decreases the compressive strength increases and that the use of natural fiber (Vf = 0.5–0.75%) improves masonry post-cracking features, showing a ductile behavior and generating a uniform cracking pattern in the longitudinal sides of the blocks. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, globalization and free trade agreements around the world favor an increase in the production of consumer goods. This generates an endless production of contaminating industrial byproducts that require storage, transport and final disposal expenses. Reuse and recycling for a sustainable development are among the main concerns of governments around the world. Mexico is not the exception. In order to solve this problem, the issue of industrial contamination has generated actions in different fields of knowledge. From the researcher’s point of view, reusing by-products as primary components of innovative construction materials turns out to be an attractive alternative for sustainable development. Masonry is a housing construction material widely used around the world, particularly in Latin America. In Mexico, masonry has a long and outstanding use in construction. A good example would be the different pre-Hispanic buildings that are still standing, which amaze for their beauty and constructive quality. Also, many of the Colonial public and religious buildings distributed around the country are still in use as proof of the validity of the adopted solutions. At the present time, more than 90% of single-family residences and apartment buildings in our country are built using load-bearing walls. Thus, masonry is a better construction alternative in Mexico, as well as an auto-construction procedure, that allows low-income population to aspire to dignified housing [1]. * Corresponding author. Tel.: +52 81 8352 4969; fax: +52 81 8376 0477. E-mail address: cjuarez@fic.uanl.mx (C. Juárez). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.02.007

On the other hand, the incorporation of natural fibers such as lechuguilla(Agave lechuguilla) into the masonry concrete matrix improves its physical and mechanical properties, such as stress cracking, abrasion and toughness [2]. These fibers are natural and low-cost to produce compared to steel or polymer fibers. Natural fibers require less extraction power, even when a mechanical extraction is required [3]. This is particularly appealing to Latin American countries where the fiber is widely available in opposition to a serious housing and infrastructure shortage. Juarez et al. [4] studied the effect of natural fibers on mechanical properties of construction Portland cement based composite materials. In this case, natural fibers were used to provide a greater cracking control and obtain a ductile flaw after cracking. It is also important to point out that lechuguilla may be susceptible to volume change due to humidity variations. This could dramatically affect adherence between the fiber and the cement paste. Polyethylene terephthalate (PET) waste bottles were the additional component used in this new construction material. This is a medium term non-degradable plastic element under normal conditions, since there is no other known organism that could consume its relatively large molecules. Therefore, recycling processes are economically the best option to reduce PET waste [5]. On the other hand, thanks to their light weight and easy storage, the use of PET bottles has extraordinarily increased worldwide since they have replaced glass bottles as beverage recipients [6]. Several studies have tried to find a significant application for this waste material. The studies include its use in geotechnical materials for light embankments [7], also as a partial substitute

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of the fine aggregate in concrete compounds without reducing the compressive and flexural strengths of the composite [8]. The current increase of PET use and the contamination problems generated by its storage, or even worse, by frequently used incineration, account for the need to study PET usefulness to create added-value products [9]. The purpose of this research project was to provide alternative solutions to the problem through the study of the mechanical properties of new construction materials such as fiber-reinforced concrete masonry lightened with waste PET bottles.

2.1.3. Fibers The natural fiber used is a member of the ‘‘A. lechuguilla” family from northern Mexico. The fiber has a density of 1.38 and an absorption rate of 92.3%. It is 25 and 50 mm long (Lf) with aspect ratios of 167 and 330, respectively. Fiber volume fractions (Vf) used were 0.25, 0.50, 0.75 and 1.0 with respect to total volume of the concrete mixture. These fibers have been previously characterized [12]. Table 4 shows the physical properties of the fiber. 2.1.4. PET waste bottles Additionally, six PET bottles with a diameter of 100 mm and a height of 320 mm, were used in the masonry blocks to make them lighter. These bottles were washed with drinking water and confined within the concrete matrix. The PET has a density of 1.39, a compressive strength between 26 to 48 MPa and flexural strength of 145 MPa. The PET bottles were distributed within the block as shown in Fig. 1.

2. Experimental program 2.1. Materials 2.1.1. Cement Blended Portland Cement CPC 30R was used, which meets Mexican Standard specification NMX C 414-2004 [10], with a relative density of 3.02. Table 1 shows its chemical composition.

2.2. Mixture composition Mixtures were tested in order to improve concrete proportions. The purpose was to obtain low cement consumption and a target compressive strength of approximately 15 MPa, at an age of 28 days. Concrete consistency was measured through the slump test [13]. Table 5 shows the proportions of the final mixture. Finally, cylindrical samples and masonry blocks were manufactured adding the natural fiber in the following ratios: 3.45, 6.90, 10.35 and 13.80 kg/m3 which corresponds to a Vf of 0.25, 0.50, 0.75 and 1.0, respectively.

2.1.2. Aggregates Crushed limestone aggregate from the State of Coahuila was used. Table 2 shows the physical properties of the aggregates, and the grading can be seen in Table 3. Gravel and sand used met the NMX C-111-ONNCCE-2004 Standard [11]. Grading of the aggregates improved the workability of the mixture with natural fibers. Previous studies have shown that grading with a maximum size of less than 10 mm is adequate when fibers are used as reinforcement of the cement matrix, since they allow a uniform distribution and reduce the agglomeration effect [2].

2.3. Manufacturing and testing methods 2.3.1. Concrete compressive strength Twenty-one 100 mm in diameter and 200 mm high nominal concrete cylinders without fiber were manufactured for each test mixture. The cylinders were cured inside a room with a relative humidity of 95% and a temperature of 23 °C until they were used for testing. Both the manufacturing and curing processes were carried out using the procedure indicated by the ASTM C 19207 Standard [14]. Compressive strength was tested at 1, 3, 7,

Table 1 Chemical composition of Portland cement. Material

Chemical composition (%) SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

Portland cement

17.55

4.70

1.77

64.74

1.23

0.37

Table 2 Physical properties of aggregates. Aggregate type

Sand Gravel

Properties (kg/m3) Bulk density

Dry weight

Saturated surface dry condition

Absorption (%)

Specific weight

Humidity (%)

Fineness modulus

1651 1453

2617 2666

2660 2674

1.63 0.30

2660 2674

0.10 0.05

2.71

Table 3 Grading of the aggregates. Aggregate type

Passing,% at a mesh (mm) 0.15

0.30

0.60

1.18

2.36

4.75

9.50

12.50

Sand Gravel

8.40

22.40

35.60

63.70

98.80 7.57

100.00 44.99

100.00 99.20

100.00

Table 4 Properties of lechuguilla fiber. Material

Natural fiber

Physical and mechanics properties Diameter (mm)

Average length (mm)

Absorption (%)

Specific weight

Tensile strength (MPa)

Elongation (mm)

Porosity (%)

0.16–0.26

451

92.3

1.38

275–627

6–14

21–25

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Fig. 1. Detail of concrete masonry blocks lighter with PET bottles.

14, 28, 56 and 90 days. Concrete cylinders were tested accordance with the ASTM C 39/C39 M-03 Standard [15].

3. Results and discussion 3.1. Tensile strength of fiber

2.3.2. Splitting tensile strength Thirty concrete cylinders (100 mm diameter; 200 mm high) were made in triplicate for each established Lf (fiber length) and Vf of natural fiber. The fiber was washed only with drinking water to avoid adverse reactions during the hydration process of the cement, and after, dried at laboratory conditions for at least 72 h. The splitting tensile strength was tested at 28 days old, according to the ASTM C 496–96 Standard [16].

Fig. 2 shows the distribution of the ultimate tensile strength for lechuguilla fibers. From 160 individual fibers samples tested under direct tension, the 83% resulted with strengths between 200 and 600 MPa. Only 4% of the natural fibers presented strengths below 200 MPa. Based on the previous results, the natural fiber could be an appropriate reinforcement for the cement matrix of the masonry. 3.2. Concrete compressive strength

2.3.3. Masonry compressive strength For the two studied Lf and the five studied Vf, and using the PET waste bottles as a lightening element, ten concrete mixture were made to cast in total twenty 730  340  130 mm lightened concrete blocks (two per each mixture). After demoulding, the blocks were cured during the first 7 days through continuous humidification, after that the blocks were covered with a plastic membrane. Each sample was tested under compression after 28 days.

Table 6 shows the results of the compressive strength for the optimized concrete mixture to obtain the target strength of 15 MPa, the obtained 28 days compressive strength was slightly higher (16.3 MPa). After 28 days, compressive strength increased 9% at an age of 90 days.

Table 5 Proportion of concrete mixture. Mixture

Mix proportion (kg/m3) Cement

Water

Sand

Gravel

W/C ratio

Slump (mm)

Definitive

200

200

872

950

1.00

70

60 45

40

33 29

30

25

20

Table 6 Development of concrete compressive strength.

13

Age (days)

7

10 0

3

1

0

3

Fig. 2. Ultimate tensile strength for 160 testing fiber [12].

1100-1200

1000-1100

900-1000

800-900

700-800

600-700

500-600

400-500

300-400

200-300

100-200

Tensile strength (MPa)

Compressive strength (MPa) Definitive mixture

1

0 0-100

Frequency

50

1 3 7 14 28 56 90

8.1 10.8 12.4 14.8 16.3 17.6 17.8

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(a) Lf = 25 mm

2.5

Lf = 50 mm

2.0 1.5 1.0 0.5

0.75

2.15

2.00

1.54

3.3. Strength of fiber-reinforced concrete samples 3.3.1. Splitting tensile strength Fig. 3 shows the mechanical behavior of samples with different fiber volumes and lengths. It is evident that for Lf = 50 mm, splitting tensile strength decreases as fiber volume increases. On the other hand, samples with Lf = 25 mm show the same trend. That is to say, an increase in the aspect ratio of the fiber has an adverse effect on tensile strength. However, the percentages of 0.25, 0.50 and 0.75 with a lower aspect ration, Lf = 25 mm, were better distributed in the mixture and therefore splitting tensile strength was higher compared to the Lf = 50 mm of the samples. 3.3.2. Masonry compressive strength Compressive strength of fiber-reinforced masonry was obtained according to the Mexican supplementary technical standard for design and construction of masonry structures, NMX-C-036 [17]. The ultimate compressive strength fp* was determined, considering in the calculation the gross area of each individual piece.

ð1Þ

where fp is the average compressive strength of the individual pieces, related to the gross area in mm2. cp is the compressive strength coefficient of variation of the pieces. It was considered to be equal to 0.35 for hand-made pieces. Fig. 4a shows that masonry with Vf = 0.75% and Lf = 25 mm presented the highest ultimate compressive strength, which is 10% higher with respect to the pieces without fiber. The combination of Vf < 1% and a low fiber aspect ratio leaded to an increased strength. On the other hand, Fig. 4b shows the results for Lf = 50 mm, which presented a different behavior from the previous analysis due to a decrease in strength as fiber Vf and Lf increases. An increment in the aspect ratio of the fiber was a significant influence because it decreases the ultimate compressive strength of the masonry and the concrete mixture loses its consistency in fresh stage. Also, fibers tend to form agglomerations affecting compacting. Fig. 5 shows the effect of natural fiber on the post-cracking behavior of cracked pieces. When fiber content increases, the ratio of the ultimate compressive strength versus the cracked load also increases, unlike fiber-less masonry where the compressive strength is only 9% higher than the load cracked. A ductile postcracking behavior is obtained when masonry is reinforced with natural fiber, since the addition of fiber provided the capacity to support more than three times the cracking load. This indicates that the fiber-less blocks show a fragile behavior and could

1.67

1.50 1.00 0.50 0.00 0.25

0.50

0.75

1.00

Volume of fibre, %

(b)

Fig. 3. Effect of natural fiber in splitting tensile strength.

fp 1 þ 2:5cp

1.95

1.93

1.00

Compresión strength fp*, MPa

0.50

2.50 2.00

1.93

1.92 1.55

1.50

1.24 0.95

1.00 0.50 0.00 0.00

0.25

0.50

0.75

1

Volume of fibre, % Fig. 4. Ultimate masonry compressive strength for different Vf: (a) Lf = 25 mm and (b) Lf = 50 mm.

(a) Ration fp*/craked load

0.25

Volume of fibre %

Fp ¼

2.50

0.00

0.0 0.00

5.00 4.00 3.13

3.00 2.00

1.66

2.96

1.89

1.09

1.00 0.00 0.00

0.25

0.50

0.75

1.00

Volume of fibre, %

(b) Ration fp*/craked load

Splitting tensile strength, MPa

3.0

Compresion strength fp*, MPa

C. Juárez et al. / Construction and Building Materials 24 (2010) 1536–1541

5.00 3.98

4.00

3.75

3.00 1.77

2.00 1.09

1.17

0.00

0.25

1.00 0.00 0.50

0.75

1.00

Volume of fibre, % Fig. 5. Effect of Vf in post-cracking behavior masonry: (a) Lf = 25 mm and (b) Lf = 50 mm.

suddenly break. However, ductile post-cracking behavior is considerably improved when adding the fiber.

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(a) 3500

Density, kg/m 3

3000 2500 2000 1500

Lf = 25 mm

1000

Lf = 50 mm

500 0 0.00

0.25

0.50

0.75

1.00

Volume of fibre, %

Reduction of Weight, %

(b)

30

26

Lf = 25 mm

25

Lf = 50 mm

20

18 15

15

11 10

6 5 0

2 0

0 0.00

4

1 0.25

0.50

0.75

1.00

Volume of fibre, % Fig. 7. (a) Density variation for different Vf and (b) effect of Vf in the volumetric weight of masonry.

4. Conclusions

Fig. 6. (a) Vertical cracking on the PET bottles place and (b) cracking pattern on the lateral face of masonry.

Fig. 6 shows the cracking distribution in fiber-reinforced masonry. As expected, cracking first began vertically beside the location of the PET bottles, where the piece shows a weak plane due to the smaller area of the concrete. Nevertheless, the capacity to tolerate load was not affected even after the fiber-reinforced masonry cracked. And even for Vf = 0.75% and Lf = 25 mm, the ultimate compressive strength increased 10% more than the reference. Fig. 6b shows that after cracking on the PET bottles place, there was cracking on the face of the block, showing a cracking pattern by diagonal tension stress up to the bearing fail of the concrete. Fig. 7a shows the volumetric weight variation of masonry in relation to fiber volume. It is evident that as fiber volume increases, volumetric weight tends to decrease in the two lengths of fiber already studied. For Lf = 50 mm and 25 mm, weight reduction percentage was 26% and 15%, respectively, where both cases used fiber-less masonry, see Fig. 7b. A reduction of volumetric weight represents an advantage with respect to handling the piece; however, it is important to evaluate the effect of Vf and aspect ratio of the fiber regarding mechanical properties such as the ultimate compressive strength and ductility, since these characteristics directly affect the constructability with this type of masonry.

1. The lechuguilla fiber shows tensile strength, which allows the reinforcement of the masonry cement composite. 2. The aspect ratio of the fiber is an important factor to be considered since it directly affects in the ultimate compressive strength of the fiber-reinforced masonry. Lower fiber aspect ration leaded to higher compressive strength in masonry pieces. 3. The natural fiber improved the masonry post-cracking performance, for Vf of 0.50% and 0.75% in both fiber lengths. For both fibers lengths, the post-cracking load increased in a range between 0.5 and 2.65 times the first crack load. There was a ductile behavior in comparison to the fiber-less pieces. This is proved when cracking appears in masonry before failure, and by the capacity of the masonry to support additional load after first crack. 4. The reinforcement provided by the fiber in the masonry produced such a distribution of the diagonal tension stress that generated a uniform cracking pattern in the longitudinal sides of the piece. 5. The addition of natural fiber to the concrete matrix reduced the volumetric weight of the masonry piece; however, it adversely affects the ultimate compressive strength in pieces reinforced with Lf = 50 mm. Acknowledgments The authors would like to thank the Universidad Autónoma de Nuevo Leon for its financial support to this research through the

C. Juárez et al. / Construction and Building Materials 24 (2010) 1536–1541

PAICYT CA1055-05 project to buy the materials and equipment required for its development. Also, we would like to thank the Civil Engineering Institute of the School of Civil Engineering of the UANL for providing the infrastructure necessary to carry out the tests. We are grateful to the Professor Konstantin Sobolev, the thesis students and Grant holders who actively participated and enriched the project.

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