Improvement of the mechanical properties of concretes … · Improvement of the mechanical...

Journal of Engineering Technology Volume 6, Special Issue on Technology Innovations and Applications Oct. 2017, PP. 139-154

139

Improvement of the mechanical properties of concretes and mortars by addition

of polypropylene

Abdelhamid Noufid and Sougrati Belattar

University of Cadi Ayyad, Faculty of Science Semlalia, Mly Abdelah’s Street, BP 2390/40000, Marrakech,

Morocco.

Abstract: The objective of this work is the valuation of polypropylene fibers in the manufacture of

concretes and mortars. The aim is to investigate the impact of adding polypropylene fibers to the

concrete and mortar in order to improve the mechanical properties. In order to measure the

compressive and tensile strengths of concrete at ages of 3 days, 7 days and 28 days and the

characteristics of adhesion of the mortar. The results obtained are compared with an ordinary

concrete and mortar. According to this study an optimal addition of the polypropylene fibers

allows an increase in the mechanical characteristics of the concretes and the mortars. This

increase can reach 12% of the compressive strength at the age of 28 days. In order to enhance this

work, we will try to compare the results with thorough research that has been carried out.

Keywords: concrete, mortar, fibers, compressive strength, tensile strength.

1. Introduction

Polypropylene fiber reinforced concrete (PFRC) belongs to the category of multiphase heterogeneous

composite materials composed of fine aggregates, coarse aggregates, cement and polypropylene fibers.

Because polypropylene fibers possess a good strengthening and toughening feature, which are able to improve

plain concrete’s mechanical performance, PFRC has been widely used and in engineering construction (e.g.,

applied in aqueduct structures to prevent cracks causing water leakage) [1-2]. Concrete is frequently used in

buildings, water structures, bridges and highways [3]. Relatively low tensile strength and poor resistance to

crack opening and propagation are the main disadvantages of conventional concrete [4-7]. To deal with this

kind of problem, additions are used, as is the case with steel or polypropylene fibers. There are several types

of fibers in both families, but this paper focuses on the addition of fibers in concrete and mortar

polypropylenes to assess the impact of these fibers on the characteristics. In parallel, for the mortar, many

researchers have treated the use of fibers in mortar to improve its thermal and / or mechanical properties,

including polypropylene fibers [8-9]. Moreover, studies have revealed that the main role of these fibers in

composites is the ‘‘sewing effect’’ [10-11], which consists of a bridging action by fibers across the formed

cracks, thus controlling propagation of cracks and leading to more post-cracking resistance. On the one hand

the addition of fibers to concrete has been shown to enhance the toughness of concrete [12]. On the other hand

a significant improvement of toughness and flexural post-cracking behavior have been observed in reinforced

mortars, regardless of the nature and amount of fibers [13]. As part of the upgrading of polypropylene fiber in


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the field of ready-mix concrete production, an experimental program is launched to highlight the different

characteristics of the polypropylene fiber. For this purpose, two applications are possible:

- Incorporation in concrete

- Incorporation in mortar to be used as a masonry substrate for common elements.

The experimental study carried out in this direction has a dual objective:

- Identify the general characteristics of conventional concretes and mortars by comparing them to

polypropylene fiber concretes.

- Highlight the particular characteristics of polypropylene fiber-based concrete and mortars.

2 Tests on Basic Materials

2.1 Materials

The materials used in this study come from different quarries:

- Crushed gravel GII and crushed sand from BBENSLIMANE quarry/Morocco;

- GI gravels come from the SKHIRAT quarry/Morocco;

- The dune sand comes from the region of KENITRA/Morocco;

- The cement (binder) used is a Portland CPJ 45 cement brand HOLCIM/Morocco;

- The mixing water used is the drinking water of the network;

- Polypropylene fibers as shown in Fig1, whose geometry and properties are provided in Table 1.

Figure. 1. Picture taken of the fibers used in this study

Table 1. Properties of polypropylene

Shape of fiber Length l (mm) Diameter d

(mm)

Aspect ratio l/d Density (g/cm3) Tensile

strength (MPa)

Straight 12

0.022 545

9.1 350 6 273

2.2 Tests and Studies

The different basic materials were subjected to identification. The main results are summarized in Table 2:


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Table 2. Test on aggregates

Aggregate Gravel GII Gravel GI Crushing sand Sand of dune

Sand Equivalent (%) [14] - - 56 89

Coefficient of flattening [15] 27 21 - -

Actual density [16] 2,65 2,66 - -

Absorption coefficient [16] 1,0 1,1 - -

Actual density [17] - - 2,60 2,64

Absorption coefficient [17] - - 1,2 1,0

Los Angeles [18] 17 18 - -

Superficial cleanliness [19] 1.2 0.8 - -

The results obtained show that the materials used meet the performance requirements for the manufacture

of concrete type B2 in the sense of Moroccan standard NM 10.1.008 [20].

3 Experimental Program on Concrete

3.1 Composition Study

Based on the results of Table 2, we will proceed to the formulation of the concrete using the method of

“DREUX-GORISS” [21]. Thus, on the basis of the "DREUX - GORISSE" method, the quantities of the

concrete mixture are shown in Table 3.

Table 3. Concrete formulation

Sample Granular class % of mixture Actual density Dosage

(Kg/m3)

GII 8.3/25 44 2.65 840

GI 6.3/16 18 2.66 345

Crushed sand 0/3.15 12.7 2.60 238

Sea sand 0/0.4 25.3 2.64 481

Cement CPJ 45 350

Mixing water - - - 185

3.2 Experimental Program

Three mixes were made with the same amounts of the mixture mentioned in the Table 3 by varying the level of

fibers in each mixture:

- Mixture B1 with 0% of fibers (witness concrete);

- Mixture B2 with a dosage of 900 g / m3, a percentage by volume of 0.10 %;

- Mixture B3 with a dosage of 1200 g / m 3, a percentage by volume of 0.15 %.


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The fiber introduced is a polypropylene fiber 12 mm long. The three blends were subjected to the

characterization tests in the fresh and cured state.

Figure. 2. A mix of concrete with fibers at the time of mixing

4 Experimental Program on Mortar

The mortar object of the study is composed of sand (dune sand), cement and drinking water in the proportions

defined in Table 4.

Table 4. Composition of mortars

Materials Quantity (g)

Sand 1350

Cement 4450

Water 300

Three mixes were made with the same amounts of the mixture mentioned in the Table 9 by varying the

level of fibers in each mixture:

- A mixture M1 with 0% of fibers (witness mortar) ;

- A mixture M2 with a dosage of 600 g / m3, a percentage by volume of 0.07 % ;

- A mixture M3 with a dosage of 900 g / m3, a percentage by volume of 0.10 %.

The fiber used is a 6 mm long polypropylene fiber.

5 Results and Discussion

5.1 Results of Tests on Concrete

Fresh Tests. After the pouring of the mixes, fresh concrete tests were carried out, namely the test for

slumping, LCPC (Central laboratory for bridges and highways) maneuverability and density [22]. All the

results obtained, in the fresh state, are shown in Figs 3 and 4:


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Figure. 3. Maneuverability tests for the three mixes studied

Figure. 4. Air occlus and density mass for the three mixes

The presence of the fibers in the concrete has the effect of reducing its maneuverability. Thus, for a

concrete test with 30 mm of collapse, the fiber concrete at 900 g / m3 has a lower slump (25 mm). This

collapse is reduced to 23 mm for the concrete dosed at 1200 g / m3. This remark remains valid for the LCPC

maneuverability test whose time t (s) increases with a larger fiber dosage.

Hardened Tests. The hardened concrete undergoes a crushing program, namely a compression test at the age

of 3, 7 and 28 days and a tensile test at the age of 28 days. We represent the results in Tables 5, 6 and 7.

Figure. 5. Hardened concrete without fiber


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Table 5. Tests on hardened concrete without fiber

Ref specimen Age (days) Weight (kg) Density

(Kg/m3)

Compressive

strength (MPa)

Tensile

strength (MPa)

B1/1

/2

/3

3

15.361

15.370

15.368

2.39

2.39

2.39

15.0

15.5

15.0

-

-

-

/4

/5

/6

7

15.586

15.572

15.566

2.42

2.42

2.42

19.0

18.5

19.0

-

-

-

/7

/8

/9

28

15.627

15.641

15.636

2.43

2.43

2.43

28.0

28.0

28.0

-

-

-

/10

/11

/12

28

15.638

15.647

15.656

2.43

2.43

2.43

-

-

-

2.5

2.5

2.6

Figure. 6. Hardened concrete containing fibers 900 g/m

3

Table 6. Tests on hardened concrete: Fiber concrete (900g/m3)


(Kg/m3)

Compressive

strength (MPa)

Tensile

strength (MPa)

B2/1

/2

/3

3

15.362

15.377

15.356

2.40

2.40

2.40

15.0

16.5

17.0

-

-

-

/4

/5

/6

7

15.483

15.471

15.496

2.41

2.40

2.41

20.0

19.5

19.0

-

-

-

/7

/8

/9

28

15.651

15.661

15.648

2.43

2.43

2.43

30.0

30.5

29.5

-

-

-


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/10

/11

/12

28

15.672

15.663

15.656

2.43

2.43

2.43

-

-

-

2.8

2.6

2.6

Figure. 7. Hardened concrete containing fibers 1200 g/m

3

Table 7. Tests on hardened concrete: Fiber concrete (1200 g/m3)


(Kg/m3)

Compressive

strength (MPa)

Tensile

strength (MPa)

B3/1

/2

/3

3

15.452

15.463

15.448

2.40

2.40

2.40

19.0

19.5

18.5

-

-

-

/4

/5

/6

7

15.520

15.531

15.538

2.41

2.41

2.41

22.5

22.5

22.0

-

-

-

/7

/8

/9

28

15.441

15.456

15.463

2.40

2.40

2.40

31.0

31.5

32.0

-

-

-

/10

/11

/12

28

15.445

15.459

14.470

2.41

2.41

2.41

-

-

-

2.7

2.8

2.7

We represent in Figs 8 and 9 the average of the results at the age of 3, 7 and 28 days:


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Figure. 8. Compressive strength at the age of 3, 7 and 28 days

Figure. 9. Tensile strength at the age of 28 days

Thus, there is an improvement in the compressive strength between the ordinary concrete and the fiber

concrete, this improvement being 2 MPa for the concretes measured at 900 g / m 3 (approximately 7%) and 3.5

MPa for the concrete measured at 1200 g / m3 (about 12%). On the similar trend, there is an improvement in

flexural strength (respectively 6 % and 8 %). Similar studies have been conducted to confirm these results.

Indeed, Afroughsabet V & Ozbakkaloglu T [23] recorded an increase in compressive and tensile strengths by

addition of PP fibers: this increase reached 8% for compressive strength and 13% for resistance to bending at

the age of 28 days.

Correlation between flexural and compressive strength. In this paragraph, the correlation between the compressive

strength and the tensile strength is studied; The BAEL code [24] speaks of this correlation and gives a correlation law

between the compressive strength and the tensile strength in accordance with equation (1). According to Fig 10, there is a

good linear correlation between the compressive and tensile strength (correlation coefficient : R2 = 0.979). The slope of

this correlation is almost the same with respect to the BAEL 91 code, but a difference of 0.3 MPA between the additional

parameters, i.e. 0.9 MPa instead of 0.6 MPa.

28 280.06* 0.6c tf f ; For 2840 60cMPa f MPa (1)

With:

- 28cf : Compressive strength at the age of 28 days;


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- 28tf : Tensile strength at the age of 28 days;

Figure. 10. Correlation between compressive and tensile strengths

5.2 Results of Tests on Mortar

Study of Behavior in the Fresh State. Measurement of the start and end of setting time was carried out

according to the European standard (EN 196-1) [25]. The main results obtained are given in Table 8.

Table 8. Start and end of cement setting

Mortar Start of setting End of setting

M1 (witness) 4 hours 5 hours 30 min

M2 (600 g/m3) 3 hours 35 min 5 hours 10 min

M3 (900 g/m3) 3 hours 35 min 5 hours 15 min

There is thus a decrease in the start and end time of setting for the fiber mortar in comparison with the

ordinary mortar. This reduction is approximately 20 minutes for mortars at 600 g/m3 and 15 min for mortars at

900 g / m3.

Study of Behavior in the Hardened State. The different mixtures were introduced into 4 × 4 × 16 cm3

prismatic molds. The made-up prisms were subjected to bending and compression tests at 3, 7 and 28 days .

The main results obtained are given in Figs 10 and 11. The measurements were carried out in accordance with

the European standard EN 196-1 test standard.


148

Figure. 11. Flexural strength at the age of 3,7 and 28 days

Figure. 12. Compressive strength at the age of 3, 7 and 28 days

According to these results, it can be said that the best results are obtained in the case of M2 (fiber dosage of

600 g / m3). Thus, the optimum dosage of use of the polypropylene fibers is 600 g / m 3. Silva E R et al [26]

studied several types of fibers, found an increase or decrease in flexural and compression strengths which

reached between -8.6% to 40.4% for the compressive strength and between 1.7% and 40.7% for the flexural

strength. This depends on the morphology, geometry and percentages of the fibers, but in our case, it is

interesting to study the percentage of fiber presence.

Study of Behavior in the Hardened State. Principle: The adhesion strength is measured as the maximum

tensile stress per direct charge perpendicular to the surface of the coating mortar applied to a support. The

tensile force is applied to a defined traction pad bonded to the test surface of the mortar. The force of adhesion

is the ratio between the breaking load and the corresponding surface [27].The test consists of bonding the

tensile pellets to the center of the surface of the specimens by means of a resin-based adhesive, for example an

epoxy resin or a methyl methacrylate resin. By means of the pulling apparatus, the tensile load is applied

perpendicularly to the test surfaces by means of the pellets. The load is applied without impact and at a uniform

speed. Using a velocity such as that stress increases in a range of 0.003 N / mm2.s to 0.100 N / mm2.s

depending on the expected bond strength so that failure occurs between 20 s and 60 s. The breaking load is

recorded. Adhesion strengths are calculated from the equation (2) at 0.05 N / mm2 [27]:


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uu

Ff

A (2)

With:

- fu is the adhesion force (N / mm2);

- Fu is the breaking load (N);

- A is the test surface of the cylindrical specimen (mm2).

Forms of rupture giving reliable results are shown in Figs. 12 to 14. When rupturing forms such as those

illustrated in Figs. 13 and 14 are obtained, that is to say without breaking at the mortar interface / Support, the

results should be considered as values at the lower limit. These values must be taken into account when

calculating the average value of the adhesion force.

Figure. 13. Break at the interface between the mortar and the support: The test value is equal to the adhesion

force

Figure. 14. Break in the mortar: The adhesion strength is higher than the test value


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Figure. 15. Breaking of the support: The adhesion strength is greater than the test value

Figure. 16. Test area for non-fiber mortar

Figure. 17. Test area for a mortar of 600 g/m

3 fiber


151

Figure. 18. Test area for a mortar of 900 g/m

3 fiber

The results obtained are summarized in the Table 9.

Table 9. Adhesion tests on mortar and mortar with fiber

Samples Charge (daN) Adhesion (MPa) Average Type of rupture

Mortar

N=°1 30.64 0.35

0.33

Adhesive failure

N=°2 34.46 0.40 Adhesive failure




Mortar with

fiber (600 g/m3)

N=°1 12.21 0.15

0.17

Cohesive rupture





Mortar with

fiber (1200

g/m3)

N=°1 62.39 0.70

0.32

Adhesive failure

N=°2 31.69 0.35 Cohesive rupture




It is then noted that:

- For witness mortar (0g/m3), they have an adhesive strength of 0.33 MPa (arithmetic average) with

adhesive failure.

- For mortars with a dosage of 600 g/m3 of fiber, bond strength of 0.17 MPa (arithmetic average) is

observed with adhesive failure mode in the majority of cases.

- For mortars with a dosage of 900 g/m3 of fiber cement, an adhesion strength of 0.32 MPa (arithmetic

average) with a cohesive rupture mode is noted.


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The comparison of the different values obtained shows that with the addition of the polypropylene fiber,

there is an improvement in the adhesion of the mortar to the support since the mode of rupture is in most cases

cohesive.

6 Conclusions

We conclude with the following points:

At the concrete level, the incorporation of the polypropylene fibers in the concrete has improved

the mechanical performance of the concretes studied, in particular the compressive strength, this

improvement is approximately 7% for a dosage of 900 g / m 3 and of 12% for a dosage of 1200

g/m3. Conversely this improvement reaches 6% for the tensile strength in the case of a dosage of

900 g / m3 and 8% for a dosage of 1200 g / m3. These results are confirmed in comparison with

similar studies that have been carried out in this direction.

At the mortar level, the incorporation of the polypropylene fibers has reduced the initial setting time

for mortars. This reduction is approximately 20 minutes for mortars at 600 g and 15 min for mortars

at 900 g / m 3. As to regard of the resistances, the optimum dosage for better results in terms of

compressive and flexural strengths is 600 g / m 3, being a percentage of 0.07 % of volume.

The study of the adhesion of the mortars to the support shows that the incorporation of the fibers

improves the adhesion of the mortars to the support since one passes from an adhesive rupture for a

mortar without fibers to a cohesive rupture for a fibered mortar.

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