Scholarly article on topic 'Investigation on Flexural Properties of Hybrid Fibre Reinforced Self-compacting Concrete'

Investigation on Flexural Properties of Hybrid Fibre Reinforced Self-compacting Concrete Academic research paper on "Civil engineering"

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{"self-compacting concrete" / "hybrid fibers" / "steel fibers" / "polypropylene fibers" / "flexural tensile strength."}

Abstract of research paper on Civil engineering, author of scientific article — Małgorzata Pająk

Abstract The paper considers compressive and flexural parameters of self-compacting concrete reinforced with combination of steel and polypropylene fibers. Three volume ratios of steel fibers (0.5%, 1.0%, 1.5%) were mixed with two amounts (0.3%, 0.9%) of two types of polypropylene fibers. The influence of hybrid fibers on the compressive strength of SCC was negligible and comprised in the range of -5 to 11% of the strength of the matrix. Based on the flexural tensile tests on hybrid fibre-reinforced mixes it was noticed that steel fibers play the most important role in enhancement of the mechanical parameters. Meanwhile, in the hybrid mix of polypropylene fibers only slightly improve the toughness, irrespectively of the length of the PP fibers. The highest applied amount of the longest polypropylene fibers indeed improved the flexural parameters of the SCC matrix reinforced with steel fibers but on the other hand such concrete did not satisfy the requirements for SCC in the fresh state.

Academic research paper on topic "Investigation on Flexural Properties of Hybrid Fibre Reinforced Self-compacting Concrete"

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Procedía Engineering 161 (2016) 121 - 126

Procedía Engineering

www.elsevier.com/locate/procedia

World Multidisciplinary Civil Engineering-Architecture-Urban Planning Symposium 2016,

WMCAUS 2016

Investigation On Flexural Properties of Hybrid Fibre Reinforced

Self-Compacting Concrete

Malgorzata Paj^k^*

aSilesian University of Technology, Faculty of Civil Engineering, Department of Structural Engineering, Akademicka 5, 44-100 Gliwice, Poland

Abstract

The paper considers compressive and flexural parameters of self-compacting concrete reinforced with combination of steel and polypropylene fibers. Three volume ratios of steel fibers (0.5%, 1.0%, 1.5%) were mixed with two amounts (0.3%, 0.9%) of two types of polypropylene fibers. The influence of hybrid fibers on the compressive strength of SCC was negligible and comprised in the range of -5 to 11% of the strength of the matrix. Based on the flexural tensile tests on hybrid fibre-reinforced mixes it was noticed that steel fibers play the most important role in enhancement of the mechanical parameters. Meanwhile, in the hybrid mix of polypropylene fibers only slightly improve the toughness, irrespectively of the length of the PP fibers. The highest applied amount of the longest polypropylene fibers indeed improved the flexural parameters of the SCC matrix reinforced with steel fibers but on the other hand such concrete did not satisfy the requirements for SCC in the fresh state. © 2016 The Authors. Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of WMCAUS 2016

Keywords: self-compacting concrete, hybrid fibers, steel fibers, polypropylene fibers, flexural tensile strength.

1. Introduction

Self-compacting concrete (SCC) is a new type of a cement-based material, which due to its high deformability and resistance to segregation fills the framework in a natural manner and consolidates under its own weight with no need for vibration. Among its advantages over the traditionally vibrated concrete, which are widely discussed in [8], the SCC reduces the overall costs of the structure and ensures more space for designers because it can be used even in

* Corresponding author. Tel.: +48 32 237 22 88. E-mail address: malgorzata.pajak@polsl.pl

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of WMCAUS 2016

doi:10.1016/j.proeng.2016.08.508

very complicated framework with a congested reinforcement. The composition of SCC is based on the same ingredients as in conventional concrete, however, they are applied in different proportions [1].

As any other cement-based materials, the SCC has a brittle nature, therefore to improve its tensile mechanical properties as well as the behaviour under the impact different kinds of fibers can be applied. Fibers are spread uniformly in the matrix, which prevents or delays initiation and propagation of matrix cracking. This supplement changes large single cracks into a system of multiple smaller cracks, which is desired from safety and durability point of view [2]. According to the material and geometrical parameters like: diameter, length, aspect ratio, longitudinal profile and cross-sectional shape, the fibers enhance the mechanical parameters of the matrix under tension and flexure [4,5]. Among the fibers available in the market the best performance can be observed for steel fibers. However, steel fibers cannot ensure the required safety of the concrete structures to their users during fire, which has nowadays become an important aspect. The process of destruction of concrete under high temperature can be delayed by addition of the polypropylene fibers (PP), [3].

The investigations presented in the paper are focused on the simultaneous influence of the steel as well as polypropylene fibers on the compressive and flexural mechanical parameters of the SCC matrix. The analysis will help to answer the question whether a separate influence of each type of fibers on the SCC matrix overlaps and the synergy effect appears.

2. Experimental investigation

The composition of the mixes is presented in Table 1 and the detailed description of its preparation can be found in [6]. In the present paper the hybrid mixes were prepared with the use of steel and polypropylene fibers. Three volume ratios of steel fibers: 0.5%, 1.0% and 1.5%, which is the dosage weight of 39.25, 78.5 and 117.8kg/m3, were tested. The examined steel fibers were 35mm long with a sector of the circle cross-section and corrugated longitudinal shape (Table 2). The considered polypropylene fibers had two lengths: 19mm and 38 mm (Table 2) and were added in the amounts of 0.3% and 0.9%, which is the dosage of 2.7 and 8.1 kg/m3, respectively. The different combinations of steel and polypropylene fibers can be found in Table 3, where the rheological parameters of hybrid fibre-reinforced self-compacting concrete (HFR-SCC) including slump-flow and L-box tests are also summarized.

The mechanical properties of the HFR-SCC mixes were tested in compressive and flexural tensile tests at the age of 28 days. The compressive tests have been carried out in 3000kN hydraulic compression testing machine on cubes with dimensions of 150*150* 150 mm. For every mix 6 specimens have been tested with a constant loading rate. The flexural tensile parameters were tested in three-point bending tests on three beams for each mix with dimensions of 100x 100x400mm (Fig. 1). The mid-span deflection increased constantly with the rate of deflection equal to 0.2mm/min, until the deflection reached 2 mm.

3. Test results and discussion

The extensive studies on the influence of only polypropylene fibers on the mechanical parameters of the SCC matrix has been performed by the authors in [6].

3.1. Properties of mixes in the fresh state

In general, fresh parameters decrease with an increase in the amount of fibers, which is a well-set knowledge [7]. The mixes prepared with the use of only steel fibers satisfied the requirements for the SCC in the fresh state. All the hybrid mixes conformed to the demanded range of slump flow test (550^850mm), except for those containing 0.9% of the 38mm PP fibers. The passing ability of all hybrid mixes investigated in L-box tests, where 0.9% of both investigated PP fibers was applied, was not satisfied. Considering the summary amount of fibers it can be concluded that for the Vf > 1.4% the hybrid mixes did not satisfy the requirements for the SCC. This trend was observed without regard for superplasticizer amount. In all hybrid mixes the increase in porosity in comparison to plain SCC was observed.

3.2. Compressive strength

The compressive strength of the self-compacting matrix was equal to 85.7 MPa (Fig. 2). As expected, the application of hybrid fibers did not have any significant influence on the compressive strength of the SCC matrix (Fig. 2a), which can be attributed to the fact that the fibers influence mostly the post-cracking behaviour of the matrix [2]. An increase in the compressive strength was noted when the steel fibers were added up to 1.0%, for higher amount it decreased. The maximum strength reduction of 7% was observed for the matrix reinforced only with PP fibers (Fig. 2b). The value of fc of hybrid mixes varied in the range of -5^11% of the compressive strength of the control mix. The compressive strength reduction can be generally explained by the fact that the fibers are some perturbation which causes higher amount of voids in the matrix. The highest observed decrease of fc in case of application of the PP fibers can be also attributed to the instability of the mix in the fresh state [8].

Tab. 1. Composition of HFR-SCC mix.

Cement Natural sand CEM I 42.5R (0-2 mm) (kg/m3) (kg/m3)

485 749 467.7 / 467.7 203 48.5 0.5, 1.0, 1.5 0.3, 0.9 17.2 1.6 0.38

Tab. 2. Properties of fibers.

Designation SF PP19 PP38

Length [mm] 35 ± 3.5 19 38

Tensile strength [MPa] 800 ±120 570^660 570^660

Width/Depth [mm] 2.63 ± 0.33/0.7 ± 0.07 - -

Effective diameter 1.28 - -

Aspect ratio (length/effective diameter) 27 - -

Fibre shape corrugated straight straight

Acid/Alkali Resistance - Excellent

Coarse aggregate

Steel fibers Polypropylene

(2-8 mm) / Water Silica fume „ Superplasticizer Stabilizer W

(8-16 mm) (kg/m3) (kg/m3) (%) * ! (kg/m3) (kg/m3)

(kg/m3)

3 by volume (%) by volume

Tab. 3. Properties of fresh HFR-SCC mix

Vf of fibers [%] Slump flow test L-box Density

Mix Steel polypropylene ■ S SFD [mm] T500 [s] (H2/H1) [kg/m3]

19mm 38mm

SCC - - - - 805 1.5 1 2.370

0.5SF 0.5 - - 0.5 825 1 0.89 2.407

0.5SF+0.3PP19 0.5 0.3 - 0.8 740 1.5 1 2.473

0.5SF+0.9PP19 0.5 0.9 - 1.4 565 7 0.44 2.400

0.5SF+0.3PP38 0.5 - 0.3 0.8 740 1.5 0.9 2.408

0.5SF+0.9PP38 0.5 - 0.9 1.5 56.0 H=70 3 0 2.440

1.0SF 1.0 - - 1.0 825 1 0.94 2.413

1.0SF+0.3PP19 1.0 0.3 - 1.3 790 1 0.81 2.433

1.0SF+0.9PP19 1.0 0.9 - 1.9 595 3.5 0.57 2.380

1.0SF+0.3PP38 1.0 - 0.3 1.3 720 1.5 0.77 2.460

1.0SF+0.9PP38 1.0 - 0.9 1.9 53.0 H=105 3 0 2.470

1.5SF 1.5 - - 1.5 820 1 1 2.440

1.5SF+0.3PP19 1.5 0.3 - 1.8 750 1.5 0.7 2.393

1.5SF+0.9PP19 1.5 0.9 - 2.4 565 9.5 0.33 2.400

1.5SF+0.3PP38 1.5 - 0.3 1.8 720 3 0.58 2.464

1.5SF+0.9PP38 1.5 - 0.9 2.4 47.0 H=140 3 0 2.490

Fig. 1. View of the setup.

— 85.7

□ Plain SCC □ Sf

□ ST»€L3PP1S ■ SF +0L9PP13

□ SF-KL3PP38 ■ 5F+0-W38

¿■--Ft

V, of steel fibers [WJ

□ Plain SCC

m M" m o to o u>

ov N o CO s ™

o aa as

V,of polypropylene fibers [K]

Fig. 2. Variation of the compressive strength of SCC containing: a) steel and hybrid fibers,

b) polypropylene fibers.

3.3. Flexural behaviour

The average load-deflection curves evaluated from flexural tests of SCC reinforced with steel, polypropylene and hybrid fibers are presented in Figure 3a, 3b and 4, respectively. The paper considers first-peak strength, flexural tensile strength and toughness (Fig. 5). Flexural toughness per unit volume under flexure was determined as the area under the load-deflection (F-5) curve to the deflection equal to L/150 = 2mm.

The flexural tensile strength of the SCC was improved by the steel as well as polypropylene fibers (Fig. 3). The proportional increase in fts and toughness could be observed for matrix reinforced with only steel fibers. Meanwhile, when PP fibers were applied the flexural parameters were less predictable. For 0.9% of 38mm PP fibers the specimens failed when achieving peak load and no post-peak behaviour was noted.

The base point for analysing the hybrid mixes results are the load-deflection curves obtained with the use of steel fibers with PP fibers as a supplement. Referring to Figure 4, the efficiency of the polypropylene fibers in the mix containing steel fibers was rather low and decreased with the increase in SF volume ratio independently form the length of PP fibers. The exceptions where the increase in toughness was observed are the mixes containing 0.9% of 38mm long PP fibers. However, these mixes did not satisfy the requirements for the SCC in the fresh state. The addition of steel fibers caused a pronounced increase of fts of plain SCC (4.96MPa), which was proportional to the increase in Vf (Fig. 3a, 5a). When the PP fibers were added to this mixes, the reduction of fts was noted as well as the toughness enhancement.

-SCC —0.5SF -•-1.0SF

/^X/X. 0.5SF 1.0SF x 1.5SF

v V \>< 1.5SF

Deflection [mm]

—SCC —0.3PP13 -o 0.9PP19

° 0.3PP3S -Q-0.9PP3S

Deflection [mm]

Fig. 3. Load-deflection curves flexural tests of SCC reinforced with: (a) SF; (b) PP fibers.

0.5SF+0.9PP38

—SCC —0.5SF

-♦-0.55F+0.3PP 19 -X-0.SSF4Ü.9PP19 - -0.5SF+0.3PP38 35F+0.9PP38

0.5 t 1.5

Deflection [mm]

30 25 20 15 10 5 0

l.SSFtW

—sec

—1-5SF

-»-1.SSF+Ü.3PP19 -»-1.5SF+0.9PP19 -*- 1.5S F-+0.3PP38 1.5SF+0.9PP38

—1.0SF

-♦-1.0SF+Ü.3PP19 1 0SF+0 9PP38 -J1.0SF+O.3PP19 -Í-1.0SF+0.3PP38 -+ 1.0SF+0.9PP38

0.5 1 1.5

Deflection [mm]

O.S 1 1.5 2

Deflection [mm]

Fig. 4. Load-deflection curves from three-point bending tests of HFR-SCC specimens, where volume ratio of steel fibers is equal to: a) 0.5%; (b) 1.0%; (c) 1.5%.

In case of hybrid mixes no synergy effect, observed by other researchers [1] was noted. Based on the presented test results it can be concluded that in hybrid mixes the steel fibers play the most important role in carrying the loads in flexure, because the load-deflection curves obtained for hybrid mixes are only slightly modified by the addition of polypropylene fibers (Fig. 4). It is connected to the fact that both types of fibers have comparable lengths, so they could probably attract the cracks only at the same level. The polypropylene fibers have much lower stiffness comparing to the steel ones which is the cause of their domination over the PP fibers in the SCC matrix.

4. Summary and conclusions

The following main conclusions can be drawn from the laboratory investigations:

• Analysing the results of the hybrid mixes in the fresh state, considering the summary amount of the fibers, it can be concluded that for the fibers volume ratio higher that 1.4% the mixes did not satisfy the passing ability requirement for the SCC. The mixes containing 0.9% of 38mm PP fibers should be analyzed as conventional concrete;

• A slight influence of hybrid fibers on compressive strength of the SCC matrix was noted;

• A significant and proportional to Vf increase of flexural tensile strength and toughness with the increase of steel fibers volume ratio was observed. The flexural behaviour of the SCC matrix reinforced with only polypropylene fibers was less predictable;

• Steel fibers play the most important role in enhancement of the flexural parameters of hybrid fibre-reinforced SCC mixes. The addition of PP fibers reduced the flexural tensile strength but on the other hand caused a beneficial effect on the toughness of the HFR-SCC. No synergy effect for the investigated HFR-SCC mixes was noted.

С 4 a

□ Лап SCC

OSMLÏPPl»

■ V Hl'tí'-'Vt D5f-HL3FP3» S

■ situeras 3

Vf of steel fibers [К]

О V ir SCC

л Еэи+азррю ■ SWLWP19 ¡ ], ОЬЬОЗРРЗЯ Z «Я-ШЗРРЗ»

*_* Ul

J 25 fîd

Vr of steel fibers [%]

Iя § "

□ feriVJ

ÛSF+03PP19

0 03 1 13

V, of steel fibers [%]

Fig. 5. (a) Flexural tensile strength; (b) toughness; (c) first-peak strength of HFR-SCC.

References

[1] Alberti, M.G., Enfedaque, A., Gálvez, J.C., Cánovas, M.F., Osorio, I.R., 2014. Polyolefín fíber-reinforced concrete enhanced with steel-hooked fibres in low proportions. Materials & Design, 60:57-65

[2] Brandt A.M., 2008. Fibre reinforced cement-based (FRC) composites after over 40 years of development in building and civil engineering. Compos Struct. 86:3-9

[3] Cavdar A., 2012. A study on the effects of high temperature on mechanical properties of fiber reinforced cementitious composites. Composites: Part B. 43:2452-2463.

[4] Naaman, A.E., 2003. Engineered Steel Fibers with Optimal Properties for Reinforcement of Cement Composites. Journal of Advanced Concrete Technology. 1:241-252

[5] M. Paj^k, T. Ponikiewski, Effect of the shape of steel fibres on the mechanical properties of reinforced self-compacting concrete. Cement Lime Concrete. 18 (2013) 335-342

[6] Paj^k M., Ponikiewski T., 2015. The laboratory investigation on the influence of the polypropylene fibers on selected mechanical properties of hardened self-compacting concrete. Architecture Civil Engineering Environment. 3:69-78

[7] Ponikiewski, T., Cygan, G., 2011. Some properties of self-compacting concretes reinforced with steel fibres. Cement Wapno Beton. 16(4):203-208

[8] Widodo S., 2012. Fresh and hardened properties of Polypropylene fiber added Self-Consolidating Concrete. International Journal of Civil and Structural Engineering. 3(1):85-93