High-performance fibre-reinforced heavyweight self-compacting concrete: Analysis of fresh and mechanical properties

Construction and Building Materials 232 (2020) 117230

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High-performance fibre-reinforced heavyweight self-compacting concrete: Analysis of fresh and mechanical properties Farhad Aslani a,b,⇑, Fatemeh Hamidi a, Afsaneh Valizadeh a, Anthony Thanh-Nhan Dang a a b

Materials and Structures Innovation Group, School of Engineering, University of Western Australia, WA 6009, Australia School of Engineering, Edith Cowan University, WA 6027, Australia

h i g h l i g h t s  Development of fibre-reinforced heavyweight self-compacting concretes (FRHWSCCs).  THooked-end steel and polypropylene (PP) fibres used in the FRHWSCCs.  Four different volume fractions of steel and PP fibres were utilized.  Evaluation of fresh and mechanical properties of FRHWSCCs.  The effect of heavyweight aggregate percentages was investigated.  Compressive, tensile, and flexural strength results have been presented.

a r t i c l e

i n f o

Article history: Received 15 April 2019 Received in revised form 22 August 2019 Accepted 11 October 2019

Keywords: Heavyweight concrete (HWC) Self-compacting concrete (SCC) Steel fibre reinforcement Polypropylene (PP) fibre reinforcement Mechanical properties

a b s t r a c t Fibre-reinforcement of most of the concrete technologies, such as heavyweight concrete (HWC) and selfcompacting concrete (SCC), would promote their practical applications through upgrading their mechanical properties, and subsequently, structural performance. This study evaluates the fresh and mechanical properties of fibre-reinforced heavyweight self-compacting concrete (FRHWSCC). Magnetite was used as heavyweight aggregate (HWA). Hooked-end steel and polypropylene (PP) fibres with length of 60 and 65 mm and diameter of 0.75 and 0.85 mm, respectively, were applied as reinforcements. Two different HWA content (75% and 100%), and four different volume fractions including 0.25%, 0.50%, 0.75%, 1.00% for steel, and 0.10%, 0.15%, 0.20%, 0.25% for PP fibres were utilized. To evaluate the fresh properties of FRHWSCC, slump flow test including slump flow diameter and T500mm, and J-Ring test have been performed. Hardened-state density, compressive, tensile, and flexural strength were measured to assess the mechanical properties of FRHWSCC. The obtained results for the fresh properties revealed that despite the negative impact of HWA and fibres on the workability of SCC, the FRHWSCC with both steel and PP fibres were still capable to retain their self-compacting characteristics according to the EFNARC standards. The hardened densities of specimens were above the density threshold for HWC, except for the FRHWSCC containing steel fibre, which showed slight decrease by increasing the steel fibre content to 0.5%, 0.75%, and 1.00% at 75% magnetite content, revealing the void formation within the cement paste. Compressive, tensile, and flexural strength results showed enhancement in the mechanical properties of FRHWSCC by increasing the fibre content, however, for both fibre types, increasing the HWA content impacted the mechanical properties in a negative manner, especially by aging the concrete. Finally, the load-deflection curves analysis confirmed the more ductile failure mode for the FRHWSCC comparing to plain SCC. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction As the most widely used structural material, concrete has been the subject of extensive investigations aiming at reaching to the ⇑ Corresponding author at: Materials and Structures Innovation Group, School of Engineering, University of Western Australia, WA 6009, Australia. E-mail address: [email protected] (F. Aslani). https://doi.org/10.1016/j.conbuildmat.2019.117230 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

superior mechanical properties and durability [1], less negative environmental impact [2,3], resistance against fire [4], ductile behaviour [5], etc. Some of these leading attempts have been led to genesis of new-born concrete technologies to cover up the issues associate with the conventional concrete structures. Green concretes, in which supplementary cementitious materials (SCMs) such as fly ash, granulated ground blast furnace slag (GGBFS), and silica fume, would substitute the cement binder to develop

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eco-friendly sustainable concrete and reduce the extensive CO2 emission generated from the production process of cement [6], high strength and high performance concrete, engineered cementitious composites (ECCs) containing synthetic or non-synthetic fibres to promote ductile behaviour [7,8], self-compacting concrete (SCC), and heavyweight concrete (HWC) are some of the most lucrative concrete technologies in recent years attracting the worldwide attention to not only fabricate sustainable structures with prolonged longevity but also reduce the environmental impacts of construction industry [2]. More recently, combination of these technologies to broaden the range of their applications through creating unique mechanical properties leading to better structural performance, has been received great attentions as well. Undoubtedly, deeper insight into the materials characteristics of each specific concrete type and their combination, stablishing analytical models to predict the structural performance, and evaluating the fresh and hardened-state properties are the fundamentals of developing new concrete materials for each specific demands in the construction industry. Mechanical performance is the dominant factor for developing new structural materials including high strength, high stiffness, high ductility, high toughness, etc. Admittedly, the concept of fibre-reinforced concrete (FRC) has been introduced in 1900 when the asbestos were utilized to reinforce the brittle cement. The most common fibre reinforcements for structural applications include steel fibres, glass fibres, synthetic fibres such as PP and polyvinyl alcohol (PVA), carbon fibres, and natural fibres [1]. However, natural vegetable fibres are not applicable for applications requiring high strength and/or high resistance against elevated temperatures [9]. By substituting a single large crack with a dense network of micro-cracks, and subsequently bridging the micro-cracks within the cement matrix, fibres would transfer the induced tensile stresses from the cement matrix to the fibres, rendering ductile post-cracking behaviour [10]. Besides, fibres not only influence the mechanical properties of the cement but also they would impact the rheology of the cement past based on their type, size, and geometry [11]. HWC fabricated by substituting the normal weight aggregate (NWA) with HWA such as magnetite, barite, hematite, iron ore, etc. is a new class of concrete materials with superior performance for radiation shielding. The density threshold for HWA is 3000 kg
m 3, while concrete must have specific gravity above 2600 kg
m 3 to be categorized as HWC [12]. The electromagnetic shielding effectiveness of HWC turns it into the most suitable structural material for nuclear power plants, where insulation against radioactive radiation is of paramount importance [13]. Moreover, as Aslani et al. [14,67], Gencel [15], and Horszczaruk et al. [16,17] reported, HWC has a better fire performance than the conventional concrete owing to the higher thermal stability of HWA rather than NWA. Nevertheless, as many researches revealed, introduction of HWA into the cement paste would have detrimental impact on the fresh and mechanical properties of concrete due to the relatively higher water absorption of HWA [18], and their highly crystalline microstructure [19]. Hence, the new direction in developing the practical applications of HWC is moving towards fibre-reinforced HWC (FRHWC). However, there are very few available researches in this regard. Tufekci and Gokce [20] explored the impact of steel fibres on the mechanical properties of HWC containing barite and granulated ferrous waste (GFW). They reported that by increasing the steel fibre content from 1% to 3%, the flexural strength would increase proportionally regardless of the curing regime. Moreover, they found that the steel fibre content and type of aggregate would control the energy absorption capacity and toughness of the specimens [20]. Notwithstanding, in another report Tukekci and Gokce [21] reported that the impact of steel fibre reinforcement on linear attenuation coefficient of heavyweight high performance fibre-reinforced concrete is not sig-

nificant. Tobbala [22] studied the impact of addition of nano-ferrite on the mechanical properties and gamma ray shielding effectiveness of hooked-shape steel fibre-reinforced HWC containing barite aggregate. He reported that addition of steel fibre up to 1.5% accompanied with 2% nano-ferrite enhanced the compressive strength and splitting tensile strength by 41% and 101%, respectively. SCC is another innovative type of concrete representing unprecedented technical characteristics in terms of high flowbility, resistance against segregation and bleeding, high strength, and filling capacity [23,24]. SCC is capable of flowing under its own weight without settlement, segregation, or bleeding, and filling the remote corners. Therefore, it is an appropriate structural material for structures with complex geometry or highly congested reinforcement [25,26,56–65]. To produce SCC, concrete must have sophisticated stability since after casting into the place, segregation may occur, owing to the low viscosity and pseudo-plastic behaviour of concrete [24], hence, superplasticizer (SP), viscosity modifying agent, and SCMs such as fly ash, GGBFS, natural pozzolans etc. are essential elements to reduce the water/binder ratio, increasing viscosity, and reducing the production cost of SCC [27]. Regarding the impact of fibres on the SCC characteristics, Mastali and Dalvand [28] studied the impact of silica fume and recycled steel fibre on the fresh and mechanical properties of SCC. They observed no segregation due to the uniform distribution of steel fibre within the past, however, the workability of the SCC reduced. Moreover, addition of 0.75% recycled steel fibre resulted in the lowest flowability but the highest rate of compressive, splitting tensile, and flexural strength gaining. El-Dieb [29] also studied the steel fibre-reinforced ultra-high-strength SCC. He found that the mechanical properties of the ultra-high-strength SCC improved by increasing the steel fibre content, in particular, by increasing the fibre content from 0.08% to 0.52%, improving in the splitting tensile strength did change from 92% to 111% comparing to the control sample without steel fibre. However, he reported that the steel fibres did not affect the durability of the SCC in the sulphate environment. Sahmaran et al. [30] pioneered the impact of hybrid steel fibres (straight and hooked-end) on the mechanical properties of SCC. They reported sufficient workability and moderate viscosity for SCC through addition of 60 kg
m 3 fibre content. Moreover, SCC containing only straight steel fibre revealed the highest compressive strength while those specimens containing hybrid fibres had the highest splitting tensile strength after 28 and 56 days. Akcay and Tasdemir [31] also studied the mechanical properties of SCC reinforced with hybrid fibres (high strength straight steel fibre, normal strength hooked-end steel fibre, and high strength hooked-end steel fibre) with different lengths (6 and 30 mm), and volume fraction of 0.75% and 1.5%. They noticed that the flowability of the SCC would reduce by increasing the fibre content and the main influencing factor was the fibre geometry. Furthermore, they reported that the high-strength long steel fibre would promote the toughness and ductility of SCC more than the normal strength steel fibre. Beigi et al. [32] explored the combined impact of nanosilica and fibre reinforcements on the fresh and mechanical properties of SCC. They reported that fibres would reduce the workability of SCC while nanosilica deteriorated the durability of SCC owing to its high activity. Considering the mechanical properties, they observed initial increase in the compressive strength of SCC reinforced with steel fibre up to 0.3% reaching to a peak value and then followed by a decline, while for tensile and flexural strengths linear increase by increasing fibre content was observed. For SCC reinforced with PP fibre, they observed a decrease in the compressive strength, whereas both tensile and flexural strengths promoted by increasing the fibre content. Moreover, both fibre types enhanced the toughness of the SCC, of which 20–30 and 5 times higher toughness were recorded for SCC reinforced with

F. Aslani et al. / Construction and Building Materials 232 (2020) 117230

steel and PP fibres, respectively. Gencel et al. [33] also explored the mechanical properties of SCC reinforced with PP fibre and found that uniform dispersion of fibres would prevent losing of the essential workability for SCC by inclusion of PP fibre up to 9 kg
m 3. Enhancing the compressive, tensile, and flexural strengths by utilizing much lower monofilament PP fibre content than steel fibre content was also reported. Evidently, as the results of literature review pointed out, despite the relatively extensive researches regarding the impact of fibre reinforcements on the performance of both HWC and SCC, exploring the impact of metallic and synthetic fibres, such as PP, nylon, etc. on the properties and structural performance of heavyweight SCC (HWSCC) has not been developed yet. Herein, the main objective of this study is to develop high-performance magnetite-based FRHWSCC containing two different types of fibres, including hooked-end steel fibre and PP fibre representing rigid and flexible behaviour, respectively, and evaluate its fresh and mechanical properties. For that, two different replacement ratio of 75% and 100% have been chosen to evaluate the impact of HWA content on the characteristics of HWSCC. Simultaneously, steel fibre with volume fraction of 0.25%, 0.50%, 0.75%, and 1.00%, and PP fibre with volume fraction of 0.10%, 0.15%, 0.20%, and 0.25% have been added to the HWSCC to produce FRHWSCC. Thereafter, the prepared FRHWSCCs containing steel and PP fibres were subjected to the fresh and mechanical tests.

2. Experimental study 2.1. Materials The general purpose cement (GPC) containing up to 7.5% limestone mineral additive was utilized in this study in accordance with AS3972 [34] and AS2350 [35] standards. Grade 1 fly ash in accordance with AS 2350 [35] and AS 3583[36], GGBFS in accordance with AS 3582 [37], and silica fume in accordance with ASTM C1240 and AS3583 [36] were used as SCMs to improve the compressive strength and workability, and decreasing the permeability of the concrete. However, silica fume would limit the passing ability of SCC [38]. Table 1 represents the chemical compositions of cement and SCMs. 10-mm coarse and less than 4-mm fine NWA were used for this study. Fine 45/50 silica sand was also used in the mix design as a portion of the fine aggregates. Magnetite aggregate with five different particle size were utilized as HWA to

Table 1 Chemical compositions of cement and SCMs. General Purpose Cement Chemical Composition CaO SiO2 Al2O3 Fe2O3 SO3 MgO Na2O Total Chloride

Fly Ash Chemical Composition 63.40% 20.10% 4.60% 2.80% 2.70% 1.30% 0.60% 0.02%

CaO SiO2 Al2O3 Fe2O3 SO3 MgO Na2O K2O SrO TiO2 P2O5 Mn2O3 Total Alkali

Ground Granulated Blast Furnace Slag Chemical Composition

Silica Fume

S SO3 MgO Al2O3 FeO MnO Cl Insoluble Residue Content

Silicon as SiO2 Sodium as Na2O Potassium as K2O Available Alkali Chloride as Cl Sulphuric Anhydride Sulphate as SO3

0.40% 2.40% 5.70% 12.60% 0.80% 0.10% 0.01% 0.20%

3

replace the NWA, of which 6–10 mm and 4–6 mm substituted the coarse aggregate while 2–4 mm, 1–2 mm and 0.5–1 mm replaced the fine aggregates. Particle size distribution of aggregates is shown in Fig. 1 and their properties are given in Table 2. Chemical admixtures were also incorporated including SP, high-range water reducing agent (HRWRA) and viscosity modifying admixture (VMA). Both SP and HRWRA were supplied by BASF. The types of SP, HWRA and VMA used were comply with the standards in AS 1478.1 [39]. Both SP and HRWA would improve the flowability of fresh concrete by reducing its viscosity and are essential to achieving the rheological properties required for SCC, however, overuse of them would result in mixture segregation and bleeding. VMA can improve homogeneity of the mixture and the segregation resistance of the concrete, hence, it is essential for SCC production. Dramix steel fibres Grade 3D was the type of steel fibres used in this study. The fibres have an aspect ratio of 80, length of 60 mm, diameter of 0.75 mm and feature hooked-end geometry. They comply with the standards in ASTM A820 [40]. Macro synthetic PP fibres were the other type of fibres used in this experimental study with a length of 65 mm, aspect ratio of 76.5, and diameter of 0.85 mm. They conform to the standards given in EN 14889-2 [41]. Table 3 summarizes the properties of steel and PP fibres.

2.2. Mix designs It has been proved that the absolute volume approach, developed by the American Concrete Institute for the normal concrete, is the most suitable approach to produce HWC [42,43], therefore, in this study the HWSCC mix designs were prepared based on the absolute volume approach. For the experimental program, a SCC control mix, named CSCC, was initially designed using 583 kg
m 3 of binder content with water to binder ratio (w/b) of 0.45 [66]. Thereafter, to compare the effect of HWA content, two different mix series were prepared based on the CSCC with different HWA content, including Series I and Series II. Series I involves the replacement of the coarse and fine NWA with 75% heavyweight magnetite aggregate while Series II involves the total replacement of NWA with magnetite aggregate. The Series I containing eight specimens based on the two fibre types and the 4 vol replacement fractions for each. The steel fibre-based mixes contained 0.25%, 0.5%, 0.75% and 1.00% fibre whilst PP fibre-based mixes had 0.10%, 0.15%, 0.20% and 0.25% fibre content. The mix code M75ST0.25 represents a mix with 75% HWA replacement and 0.25% steel fibre replacement. Likewise, Series II contains eight samples and the mix code M100PP0.15 denotes a mix with 100% HWA replacement and 0.15% PP fibre replacement. The mix design proportions for Series I and II are given in Tables 4 and 5.

2.3. Samples preparation To conduct the hardened properties tests, 100  200 mm cylindrical moulds and 100  100  450 mm prism moulds were used to fabricate specimens. The compressive, splitting tensile strength and load-deflection behaviour tests required 12 cylindrical samples whilst the flexural strength tests required 3 prism samples per mix. The samples were prepared by pouring the concrete into the moulds and allowing to set for 24 hr. Once set, the samples were demoulded and placed in a curing room with constant humidity and temperature of 20 °C for 7 and 28 days.

3.30% 50.40% 31.50% 10.40% 0.10% 1.10% 0.30% 0.50% <0.10% 1.90% 0.50% 0.20% 0.60%

Chemical Composition 98% 0.33% 0.17% 0.40% 0.15% 0.83% 0.90% Fig. 1. Particle size distribution of silica sand, NWA and HWA.

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F. Aslani et al. / Construction and Building Materials 232 (2020) 117230

Table 2 Properties of silica sand, NWA, and HWA. Silica Sand (AFS 45/50) Chemical Composition 99.86%

Fe2O3 Al2O3 CaO MgO Na2O K2O TiO2 MnO

0.01% 0.02% 0.00% 0.00% 0.00% 0.00% 0.03% <0.001%

Physical Properties

Apparent particle density Dry particle density SSD particle density Water absorption Moisture content

2.76 t
m

3

2.65 t
m 2.69 t
m 1.40% 2.50% 0.60%

3 3

2.5. Hardened properties The density, compressive strength, splitting tensile strength, and flexural strength of prepared SCCs were evaluated for hardened-state properties. Before testing, dry density of each specimen was recorded in accordance with AS 1012 [46]. Samples were then tested on the Baldwin Universal Testing Machine and in accordance with AS 1012 [46].

4–10 mm 0.01% <0.001%

Moisture content Flakiness index

0.5% 24.0%

3. Result and discussion

47.50%

Magnetite Aggregate Chemical Properties Fe Si C Mn

The fresh properties of prepared SCCs without and with fibres and heavyweight magnetite aggregate were investigated using the slump flow and J-Ring tests that conformed to guidelines and specifications within the ENFARC standards [44,45]. The slump flow test, and the associated T500mm test, assessed the flowability of the SCC whilst the J-Ring test evaluated the passing ability. These tests measured the slump flow diameter, T500mm, and the J-Ring height difference. Both tests were conducted using an Abrams cone which conformed to AS1012 [46].

Natural Crushed Aggregates 0–4 mm

SiO2

Loss on ignition Water content (at 150 °C) AFS number

2.4. Fresh properties

3.1. Fresh properties

Physical Properties >95.5% 2.20% 0.50% 2.20%

Hardness Specific gravity

5.1 4600 kg
m

3

Table 3 Properties of steel and PP fibres. Steel Fibres Physical Properties Length (mm) Diameter (mm) Aspect ratio (l/d) Specific Gravity Tensile Strength (N/mm2) Young’s Modulus (N/mm2)

PP Fibres Physical Properties 60 0.75 80 7.85 1.225 210,000

Length (mm) Diameter (mm) Aspect ratio (l/d) Specific Gravity Strength (MPa) Energy Absorption Rate (J) Melting Point

65 0.85 76.5 0.92 550 500 170 °C

3.1.1. Slump flow and J-Ring The results of slump flow test including T500mm and flow diameter, and J-Ring test including the diameter and step height are represented in Table 6 and Fig. 2. As it can be seen, by substituting HWA in the mix and introducing fibres, the slump flow diameter of all samples decreased, following by an increase in the required time to reach to the diameter of 500 mm, comparing to the CSCC which had spreading diameter of 680 mm, and reached to the diameter of 500 mm in less than 1 s. According to European guidelines for plain SCC, spreading diameter in the range of 600– 700 mm is sophisticated since slump flow diameter lower than 600 mm would result in insufficient flowability while for those with slump diameter higher than 700 mm, the risk of segregation and bleeding would increase [44]. Therefore, all the FRHWSCC specimens revealed sufficient spreading diameter, except M75PP0.20 and M100ST0.75 which their slump diameters were less than 600 mm. The results of T500 mm are in accordance with the slump flow diameter. By decreasing the flowability, the

Table 4 Series I mix proportions. Series I

CSCC

M75 ST0.25

M75 ST0.50

M75 ST0.75

M75 ST1.00

M75 PP0.10

M75 PP0.15

M75 PP0.20

M75 PP0.25

Binder (kg
m 3) GP Cement Fly Ash GGBFS Silica Fume Total Cementitious Content Water (l
m 3)

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

Aggregates (kg
m 3) Fine Silica Sand 45/50 Natural Aggregate (accompanied with < 4 mm HWA) 0.5–1 mm HWA 1–2 mm HWA 2–4 mm HWA Natural Aggregate (accompanied with 10 mm HWA) 4–6 mm HWA 6–10 mm HWA Total Aggregate

360 1050 0 0 0 900 0 0 2310

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

360 262.5 367.5 367.5 367.5 225 472.5 472.5 2895

Admixtures (l
m 3) SP HRWRA VMA

2.5 0.625 1

3.75 0.625 1.5

4.25 0.625 2

4.25 0.625 1.5

6.25 0.625 7.75

3.625 0.625 0.375

4.25 0.625 1.375

5.5 0.625 2.25

7.25 0.625 3.75

Fibres (kg
m 3) Steel PP

0 0

19.5 0

39 0

58.5 0

78 0

0 0.9

0 1.35

0 1.8

0 2.25

5

F. Aslani et al. / Construction and Building Materials 232 (2020) 117230 Table 5 Series II mix proportions. Series 1

CSCC

M100 ST0.25

M100 ST0.5

M100 ST0.75

M100 ST1.00

M100 PP0.10

M100 PP0.15

M100 PP0.20

M100 PP0.25

Binder (kg
m 3) GP Cement Fly Ash GGBFS Silica Fume Total Cementitious Content Water (l
m 3)

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

300 150 100 33 583 262.35

360 1050

360 0

360 0

360 0

360 0

360 0

360 0

360 0

360 0

0 0 0 900

490 490 490 0

490 490 490 0

490 490 490 0

490 490 490 0

490 490 490 0

490 490 490 0

490 490 490 0

490 490 490 0

0 0 2310

630 630 3090

630 630 3090

630 630 3090

630 630 3090

630 630 3090

630 630 3090

630 630 3090

630 630 3090

Admixtures (l
m 3) SP HRWRA VMA

2.5 0.625 1

3.75 0.625 1.25

4.25 0.625 2

5 0.625 1.25

7.0375 0.625 14.0625

3.25 0.625 0.5

4 0.625 1

5 0.625 1.875

6 0.625 1.25

Fibres (kg
m 3) Steel PP

0 0

19.5 0

39 0

58.5 0

78 0

0 0.9

0 1.35

0 1.8

0 2.25

Aggregates (kg
m 3) Fine Silica Sand 45/50 Natural Aggregate (accompanied with < 4 mm HWA) 0.5–1 mm HWA 1–2 mm HWA 2–4 mm HWA Natural Aggregate (accompanied with 10 mm HWA) 4–6 mm HWA 6–10 mm HWA Total Aggregate

Table 6 Fresh properties test results. Series

Mix ID

Slump flow diameter (mm)

J-Ring flow diameter (mm)

T500mm (s)

J-Ring Height Difference (mm)

Control

CSCC

680

605

<1

8

Series 1

M75ST0.25 M75ST0.50 M75ST0.75 M75ST1.00 M75PP0.10 M75PP0.15 M75PP0.20 M75PP0.25

625 605 620 600 625 610 595 620

520 510 475 425 505 550 535 530

3.5 4 4.2 4 3.5 4.1 4.2 4

2.6 2.7 3 4.1 1.5 0.7 1.6 2

Series 2

M100ST0.25 M100ST0.50 M100ST0.75 M100ST1.00 M100PP0.10 M100PP0.15 M100PP0.20 M100PP0.25

600 630 565 660 650 625 630 610

505 490 485 495 515 510 530 555

4 4.5 6 2.8 2.5 3 4.5 3.2

2 2.5 3 2.8 1 2.9 3 1

required time to reach to the diameter of 500 mm after eliminating the mould would increase, indicating a growth in the viscosity. The obtained results for T500mm for all the samples are in the range of 2.5–6 s. As suggested by EFNARC, the T500mm for plain SCC must be in the range of 2–5 s [44], therefore, except for M100ST0.75 with T500mm equal to 6 s, all the samples showed T500mm in line with the available standard. However, it must be considered that the EFNARC standards for the slump diameter and T500mm are for plain SCC. As results indicated, both HWA and fibres would deteriorate the flowability of SCC. Thus, to explain the observed trend, the amount of heavyweight magnetite aggregate, fibre content, and admixture dosage are the key contributing parameters. Magnetite aggregate has higher water absorption capacity than normal weight silica sand, hence, upon incorporation of HWA in the cement paste, the flowability of the mix would decline [15]. As former studies revealed, addition of plastic fibres would reduce the flowability of the fresh cement

paste owing to their high surface area and formation of network structure within the past, which not only will increase the viscosity of the paste but also restrict its segregation and flow [7]. Steel fibres with hooked ends may cause blocking of particles during flow, and therefore deteriorate the flowability [30]. However, in both Series I and Series II some specimens with higher fibre content represented better flowability than those with lower fibre content (i.e. M75ST0.75 and M75PP0.25 in Series I and M100ST0.5, M100ST1.00 and M100PP0.20 in Series II). This is probably due to the SP and VMA dosage since the admixture dosage was altered frequently in order to meet SCC fresh properties criteria. Increasing the SP dosage will enhance the workability of the fresh concrete without increasing the water content and therefore is an essential admixture for FRC [24,7]. However, at large binder content and high w/b ratio (more than 0.4), by increasing the VMA dosage in the magnetite-based HWC, the relative water absorption would increase which deteriorate the flowability of

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The observed trend is due to the fact that the presence of fibre in the cement paste cause congestion and blockage of large aggregates around reinforcement leading to the promotion of the passing ability of the FRHWSCCs. Nevertheless, in Series II, the specimens containing the highest content of steel and PP fibres, i.e. M100ST1.00 and M100PP0.25, revealed lower passing ability than M100ST0.75 and M100PP0.20, respectively, due to the higher VMA dosage, and therefore, more homogenous paste [48]. Comparing the obtained results for HWSCC containing rigid steel fibre with those for HWSCC containing flexible PP fibre, steel fibre-reinforced HWSCC (STFRHWSCC) served higher passing ability while PP fibre-reinforced HWSCC (PPFRHWSCC) showed better flowability. This could be related to the malleable nature of PP fibres while steel fibres are not capable to bend and deform around reinforcement, increasing the chance of aggregate blockage [30]. However, it is not possible to accurately compare the STFRHWSCCs with PPFRHWSCCs since the rheological properties of cement paste in the presence of flexible fibre would be completely different with rigid fibres owing to the bending behaviour of the plastic fibre [11]. 3.2. Hardened properties

Fig. 2. Slump and J-Ring flow diameters of STFRHWSCC and PPFRHWSCC containing (a) 75% and (b) 100% HWA.

3.2.1. Hardened density Fig. 3 represents the obtained results for the hardened-state density of the specimens. By substituting NWA with HWA, the density of the specimens increased as all the prepared samples showed higher density than the CSCC. Moreover, specimens in Series II with 100% magnetite aggregate represented higher densities comparing to Series I, which was predictable. In general, the density threshold for concrete to be considered as HWC is 2600 kg
m 3 [12]. Hence, all the prepared SCCs in Series II can be considered as HWSCC, however, in Series I only M75ST0.25, M75PP0.10, M75PP0.15, and M75PP0.25 have density above 2600 kg
m 3. Incorporation of synthetic fibres would increase the air entrainment leading to a reduction in the density [49]. Rigid steel fibre are able to move the large aggregate, and subsequently, increasing void space within the paste [11]. These factors could be attributed to the lower density for specimens with higher PP and/or steel fibre in Series I. Furthermore, the average density in Series I is 2601 kg
m 3 while in Series II it equals to 2673 kg
m 3 indicating that there is only a marginal difference between density of specimens containing 75% magnetite aggregate with those containing 100% mag-

HWSCC [47]. These could well explained the observed exceptions in both series. Likewise the slump flow diameter, all the samples showed lower J-Ring diameter than the control sample in the range of 425–550 mm. The step height results revealed that the CSCC and FRHWSCCs in both series had step height below 10 mm. According to the workability limits established by EFNARC for plain SCC, the J-Ring height must be in the range of 0–10 mm [44] which is satisfied by all the prepared mixes. By increasing the HWA, the step height decreased as the samples with 75% magnetite content (Series I) showed higher J-Ring height rather than specimens containing 100% magnetite (Series II) indicating that by increasing the HWA the passing ability of SCC would decrease. Notwithstanding, increasing the fibre content resulted in an increase in the passing ability of the specimens. By normalizing the obtained results based on the step height of CSCC, HWSCC containing the highest steel fibre content, M75ST1.00, and HWSCC containing the highest amount of PP fibre, M75PP0.25, reached to 0.5125 and 0.25 respectively. Similarly, in Series II, by increasing the fibre content, the J-Ring height and passing ability of the specimens increased.

Fig. 3. Hardened-state densities.

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netite, which is possibly due to the difference between the specific gravity of fine and coarse magnetite aggregate [50]. According to Yang et al. [50] substituting the NWA with fine magnetite aggregate causes slight increase in the density of concrete, while coarse magnetite aggregate increases the density of the concrete more significantly. Herein, as it can be seen in Fig. 1, due to the broad range of particle size distribution for magnetite coarse aggregate, the difference in the density of specimens with 75% and 100% magnetite aggregate is not noticeable. 3.2.2. Compressive strength The 7-day and 28-day compressive strengths of STFRHWSCCs and PPFRHWSCCs are shown in Fig. 4. The failure modes of the concrete specimens for compressive strength testing are given in Figs. 5 and 6. The control sample showed 7-day and 28-day compressive strengths equal to 38 and 54 MPa respectively. For STFRHWSCC in Series I, the obtained early age and 28-day compressive strengths were in the range of 39.6–41.3 MPa, and 42.5– 56.4 MPa, respectively. The highest 7-day compressive strength belonged to M75ST0.50, which was 8.7% higher than the CSCC,

Fig. 4. 7-day and 28-day compressive strengths of HWSCC with 75 and 100 HWA containing (a) steel fibre and (b) PP fibre.

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while the highest 28-compressive strength was almost 4.5% higher than that for the CSCC recorded for M75ST0.75. Overall, all the STFRHWSCCs in Series I showed better early age compressive strength than the CSCC, except the M75ST0.25 showing 7-day compressive strength almost equal to the control sample. Nevertheless, only M75ST0.75 represented higher 28-compressive strength than CSCC. For STFRHWSCCs in Series II, the obtained early age and 28-day compressive strengths were in the range of 29.6–45.2 MPa, and 38.2–55.4 MPa respectively. The M100ST75 revealed 18.9% and 3.5% higher early age and lateral compressive strength than the CSCC respectively, but for the other samples in this category, the obtained data indicated lower 7-day and 28day compressive strength than the control sample. As it can be seen, most of the samples revealed decreasing trend in their strength after 28 days by substituting 75% of the NWA with HWA. In addition, total replacement of the NWA with HWA would decline both the initial and the 28-day compressive strength. Three main factors may explain the decreasing trend by incorporating HWA: 1) the highly crystalline microstructure of HWA containing weak lamella [19], relatively higher water absorption of HWA than NWA [15,12], and segregation [47]. The relatively higher water absorption of magnetite aggregate would prevent the propagation of the hydration products, meaning that in the absence of HWA the microstructure of the cement would improve reaching to higher compressive strength. Mostofinejad et al. [51] and GonzálezOrtega et al. [19] also reported similar findings, however, as Mostofinejad et al. [51] revealed, the declining trend in the compressive strength of the barite-based HWC comparing to cement containing limestone NWA would be more significant in the w/b ratio less than 0.4. Moreover, it is probable that the lower increment rate in the 28-compressive strength than the 7-day compressive strength has also originated from the high water absorption capacity of HWA. By reducing the available free water for hydration of cement particle, HWA would inhibit the propagation of calcium-silicate hydrate (C-S-H) gel, and consequently leading to lower compressive strength comparing to cement pate without HWA. Furthermore, the cement paste containing magnetite aggregate will be susceptible to bleeding and segregation owing to the high water absorption and weight of the aggregate [12]. Segregation and bleeding would result in the void formation, especially in the vicinity of the interfacial transition zone (ITZ) between the cement paste and the HWA reducing the bond strength between the aggregate and cement paste [52], which is more evident at higher replacement level of NWA with HWA. Similar declining patterns in the compressive strength of HWSCC by increasing the magnetite aggregate content has been also reported by [14]. Considering the negative impact of HWA on the compressive strength of the HWSCCs, the observed improvement in the early age strength of M75ST0.50, M75ST0.75, M75ST1.00, and M100ST0.75, as well as in the 28-day strength of M75ST0.75 and M100ST0.75, as compared to the control SCC mix, can be contributed to the steel fibres. By bridging the micro-cracks and transferring the imposed compression stresses from the cement matrix to the fibres, steel fibres would enhance ductility of the concrete and improve the strength of the concrete after occurrence of cracks [22,53,21]. It must be also noted that the maximum increase in the compressive strength of the STFRHWSCCs at any age has been obtained through addition of 0.75% steel fibre, regardless of the magnetite aggregate content. However, for barite-based HWC, Tobbala [22] reported that the maximum increase in the compressive strength was obtained for the mixes containing silica fume, up to 2% nano-ferrite and up to 1.5% steel fibres. For the normalstrength and high-strength concrete, Koniki and Prasad [53] reported that the optimum amount of hooked-end steel fibres is 1% and only marginal improvement in the compressive strength of both concrete type was reported.

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Fig. 5. Compressive test failure mode of (a) M75ST0.25, (b) M75ST0.50, (c) M75ST0.75, (d) M75ST1.00, (e) M100ST0.25, (f) M100ST0.50, (g) M100ST0.75, and (h) M100ST1.00.

Fig. 6. Compressive test failure mode of (a) M75PP0.10, (b) M75PP0.15, (c) M75PP0.20, (d) M75PP0.25, (e) M100PP0.10, (f) M100PP0.15, (g) M100PP0.20, and (h) M100PP0.25.

For PPFRHWCSCCs in Series I, the initial compressive strengths in the range of 37.8–49.3 MPa were obtained, of which the highest one belonged to the M75PP0.25, almost 29.8% higher than the

CSCC. Regarding the 28-day compressive strength, all specimen revealed compressive strength in the range of 42.05–61.58 MPa, and M75PP0.15 showed the highest compressive strength which

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was 14.3% higher than the CSCC sample. The PPFRHWSCCs in Series II, the 7-day and 28-day were obtained in the range of 37.85–48.68 and 47.10–54.2 MPa respectively. In this category, the highest early age and 28-day compressive strengths were obtained for M100PP0.25, which showed 28.1% and 0.36% improvement in the 7-day and 28-day compressive strength, respectively, comparing to the CSCC. Similar to the observed pattern for STFRHWSCCs, due to the presence of HWA, the increasing rate of 28compressive strength is less than that for 7-day. According to the results, it seems that the 0.25 PP content would result in the highest initial and lateral compressive strength for magnetite-based HWSCC. In general, the PPFRHWSCCs followed an obvious trend, in which lower HWA content resulted in higher compressive strength and all the specimen in Series I revealed higher compressive strength than the control sample, except M75PP0.20 which could be a result of improper mixing, non-uniform fibre dispersion, and air content [49]. Notwithstanding, Akça et al. [54] reported that incorporation of PP fibre would not influence the compressive strength of concrete containing recycled aggregate noticeably. Awal and Mohammadhosseini [55] explored the impact of addition of waste carpet fibre on the mechanical properties of concrete and reported that the carpet fibres, mainly consisting PP and nylon fibre, at volume fraction of 0.25%, 0.50%, 0.75%, 1.00%, and 1.25% would decrease the 28-compressive strength of the ordinary Portland cement concrete by 6%, 7.5%, 11.15%, 18.06%, 11%, 23% respectively. However, the failure mode of the concrete specimen was ductile and the reduction in the compressive strength of the concrete was not significant. Void formation and weak interfacial bonds between carpet fibre and cement were suggested as the main factors lessening the compressive strength of the concrete [55]. In another report, Nam et al. [68] explored the impact of addition of nylon and polyvinyl alcohol (PVA) fibre on the mechanical properties of recycled fine aggregate concrete and no significant improvement in the compressive strength was observed. Similar to rigid steel fibres, flexible PP fibres would bridge the micro-cracks within cement paste, and subsequently, enhance the post-cracking behaviour of the cement past. Moreover, polymeric fibres tend to bend and occupy empty spaces in the cement past, thus, would result in more compact microstructure with better mechanical properties [11]. Nevertheless, filling voids within the cement matrix will lengthen the water paths to reach to the unhydrated cement particle, postponing the C-S-H propagation [49]. This phenomenon accompanied with the presence of the HWA in the cement paste would well explain the lower rate of strength gaining at 28-day age. Comparing the impact of steel and PP fibres on the compressive strength of HWSCC, it must be noted that comparison between the results obtained for the compressive strength of STFRHWSCCs with PPFRHWSCCs is not so straight forward due to the rigid and flexible nature of the two different fibre types, resulting in the lack of any obvious trend between the obtained results. However, in both Series I and II, the range of 7- and 28-day compressive strength for PPFRHWSCCs are higher than those for STFRHWSCCs, which is more evident from the data obtained for the Series II. Nevertheless, as standard deviations revealed, more scattered data are recorded for the PPFRHWSCCs in both series. To explain the reason, it seems that the flexible nature of the PP fibre plays a crucial role. PP fibres are able to bend and fill more voids within the cement paste rather than steel fibre. Therefore, more compact microstructure of PPFRHWSCCs could be attributed to their higher compressive strength comparing to the STFRHWSCCs. 3.2.3. Tensile strength The results of splitting tensile strength test are represented in Fig. 7 and the failure mode of specimen under tensile test are shown in Figs. 8 and 9. The 7-day and 28-day tensile strength of

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Fig. 7. 7-day and 28-day splitting tensile strength of HWSCC with 75 and 100 HWA containing (a) steel fibre and (b) PP fibre.

the control sample were 3.35 and 4.75 MPa, respectively. Considering the STFRHWSCCs in series I, the obtained results of early age tensile strength were in the range of 3.745–4.45 MPa, and after 28 days, the obtained results were in the range of 3.86–5.2 MPa. For STFRHWSCCs in Series II, the 7-day and 28-day tensile strengths were in the range of 2.7–4.23 MPa and 3.44–5.26 MPa, respectively. The observed trend in both series revealed that by increasing the steel fibre content, the splitting tensile strength of specimen would growth. However, similar to the compressive strength, by levelling up the magnetite content from 75% to 100%, the tensile strength decreased for most of the mixes due to the formation of porous ITZ between HWA and cement paste which deteriorate the concrete strength, except for M100ST0.75 comparing to M75ST0.75. Overall, all the STFRHWSCCs in Series I revealed higher early age tensile strength than the CSCC, while in Series II, only specimens with higher fibre content, i.e. M100ST0.75 and M100ST1.00 represented better 7-day tensile strength than CSCC by 41.2% and 26.2%, respectively. Regarding the 28-day tensile strength, in both

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Fig. 8. Splitting tensile test failure mode of (a) M75ST0.25, (b) M75ST0.50, (c) M75ST0.75, (d) M75ST1.00, (e) M100ST0.25, (f) M100ST0.50, (g) M100ST0.75, and (h) M100ST1.00.

Fig. 9. Splitting tensile test failure mode of (a) M75PP0.10, (b) M75PP0.15, (c) M75PP0.20, (d) M75PP0.25, (e) M100PP0.10, (f) M100PP0.15, (g) M100PP0.20, and (h) M100PP0.25.

series only mixes containing 0.75 and 1.00% fibre content showed improvement as compared to CSCC, in which the M75ST0.75 and M100ST0.75 represented 1% and 3.15% increase, while increasing

in the 28-day tensile strength of M75ST1.00 and M100ST1.00 were 9.5% and 10.7%, respectively. Amongst all, the highest 7-day tensile strength was recorded for mix M100ST0.75, whereas the highest

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28-day tensile strength belonged to M100ST1.00. Similar findings have been reported by Begum and Anjayenulu [69] for fly ashbased SCC reinforced with steel fibre. They reported that by increasing the steel fibre content from 0% to 1%, the splitting tensile strength would increase from 7.51 to 40.67%. However, Papachristoforou and Papayianni [70] reported that the impact of 30 mm hooked-end steel fibre on the splitting tensile strength of concrete made from electric arc furnace slag as HWA, was not significant. Furthermore, it is evident that the rate of increment in the tensile strength would decrease by increasing the concrete curing age, which may be attributed to the presence of HWA with high water absorption capacity. Steel fibres would influence the tensile strength of the concrete by two mechanisms: 1) bridging micro-cracks within cement paste, rendering ductile post-cracking behaviour [22,70], and void formation. Rigid steel fibres are able to move those aggregates which have large dimensions comparing to the length of the fibre, leading to the void formation within the cement matrix [11]. Based on the experimental data, it seems that at lower fibre percentage, which is fibre content less than 0.75% in this study, the void formation is the dominant factor reducing the tensile strength of STFRHWSCC comparing to CSCC. Notwithstanding, at higher fibre content, i.e. above 0.75%, bridging micro-cracks and inhibiting significant crack propagation would be the governing factor resulting in a growth in the tensile strength of STFRHWSCCs, reinforced with 0.75% and 1.00% steel fibre, than CSCC. The results of failure modes of specimens containing steel fibre (Fig. 8) would confirm the void formation at lower fibre content and ductile behaviour at higher fibre content. Considering the PPFRHWSCCs, the obtained result of 7-day tensile strength for both series were in the range of 3.4–3.9 and 3.5– 3.74 MPa, meaning that the early age tensile strength of all mixes showed marginal improvement comparing to CSCC. For 28-day tensile strength, 3.7–5.24 and 3.9–4.4 MPa were recorded for Series I and Series II, respectively, indicating a decreasing trend for the 28-day tensile strength of mixes comparing to the control sample, except for those with 75% HWA and PP fibre content above 0.20%. Based on the obtained data, the impacts of incorporation of PP fibre and HWA are more evident from 28-day tensile strength, in which the tensile strength would increase by increasing PP fibre content, while total substitution of NWA by HWA would reduce the tensile strength, similar to the pattern observed for the STFRHWSCCs. The highest promotion in the early age and 28-day tensile strength were obtained for M75PP0.25 equal to 16.4% and 10.3%, respectively, comparing to the CSCC. As it can be seen in Fig. 9, higher magnetite content would result in more brittle manner. Nevertheless, addition of flexible PP fibre will increase ductility of the HWSCC by bridging the split parts and transferring tensile stress from the cement matrix to the fibres [55]. Moreover, PP fibre would reduce voids within the cement paste owing to the filling impact of PP fibre, which is originated from the bending behaviour of flexible fibre [11]. Awal and Mohammadhosseini [55] also reported significant improvement in the splitting tensile strength of concrete incorporated waste carpet fibre (combination of PP and nylon fibre). Comparing the tensile strength results for STFRHWSCCs and PPFRHWSCCs in Series I, only marginal differences observed between the 7- and 28-day tensile strength of STFRHWSCCs and PPFRHWSCCs, which are not following a clear trend. Likewise, in Series II, no obvious trend could be detected for the obtained results with both fibre types and only slight differences between the samples reinforced with steel fibre with those reinforced with PP fibre were observed. However, the standard deviation of the data for STFRHWSCCs is higher and more scattered data obtained for STFRHWSCCs rather than PPFRHWSCCs.

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3.2.4. Flexural strength The obtained results for flexural strength are represented in Fig. 10. The flexural test failure mode of specimens are shown in Figs. 11 and 12. The obtained data for 7-day and 28-day flexural strength of the control mix were 6.9 and 10.9 MPa, respectively. For STFRHWSCCs in Series I, the early age flexural strengths were in the range of 6.93–7.95 MPa, whereas, for the Series II, 6.63– 7.9 MPa was recorded. For 28-day flexural strength, 9.5–11 MPa for Series I and 10–10.98 MPa for Series II were obtained. Similar to the compressive and splitting tensile strength, by increasing the HWA content from 75% to 100%, the flexural strength of HWSCCs showed a clear drop due to the high crystalline microstructure of HWA containing weak planes [14,19], and formation of porous ITZ [52,50]. In both series, by increasing the fibre content, the flexural strength increased, which is more evident for fibre content above 0.75%. Comparing to the CSCC, the highest promotion in the flexural strength was reached by M75ST0.75, revealing 15.2% and 1.1% improvement in the 7-day and 28-day flexural strength, respectively.

Fig. 10. 7-day and 28-day flexural strength of HWSCC with 75 and 100 HWA containing (a) steel fibre and (b) PP fibre.

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Fig. 11. Flexural test failure mode of (a) M75ST0.25, (b) M75ST0.50, (c) M75ST0.75, (d) M75ST1.00, (e) M100ST0.25, (f) M100ST0.50, (g) M100ST0.75, and (h) M100ST1.00.

Fig. 12. Flexural test failure mode of (a) M75PP0.10, (b) M75PP0.15, (c) M75PP0.20, (d) M75PP0.25, (e) M100PP0.10, (f) M100PP0.15, (g) M100PP0.20, and (h) M100PP0.25.

Regarding PPFRHWSCCs, all the samples represented higher early age flexural strength than the CSCC. Nevertheless, the 28day flexural strength of all the samples were below the control sample. No clear correlation between the fibre content and flexural strength can be extracted from the results, particularly at lower than 0.20% PP content. However, despite the observed trend for the STFRHWSCCs, for PPFRHWSCCs, the highest 7-day and 28day flexural strengths were belonged to the specimens with 100% magnetite content, which could be attributed to the rigid and flexible manner of steel and PP fibres. M100PP0.20 revealed the highest 28-day flexural strength, whereas, M100PP0.25 showed the highest 7-day flexural strength amongst all. The reason behind such observation can be explained as following: by increasing the HWA, the empty space within the cement matrix would increase owing to the void formation between the HWA and the cement paste. Flexible PP fibre tend to fill the generated empty space within the cement matrix due to their bending behaviour [11]. Since the PP fibres are capable of bridging the cracks at the tension zone and inhibit crack propagation by stretching [55], thus, more void occupation by PP fibre would lead to an improvement in the concrete ductility, and hence, more promotion in the flexural strength of Series II than Series I. Awal and Mohammadhosseini [55] also observed improved flexural strength for the concrete containing a combination of PP and nylon fibres, up to 0.75% fibre content. They stated that more than 0.75% fibre content would result

in lower flexural strength than prism concrete which could be attributed to the increase in matrix porosity and non-uniform fibre distribution. 3.2.5. Load-deflection curves To completely assess the influence of the fibres on the flexural behaviour of the samples, the load vs. deflection curves should also be considered. These curves are based on the 28-day results, given in the Fig. 13. A typical load-deflection curve for the FRC has three main regions: the initial linear region, strain hardening, and strain softening regions. In the initial linear part no crack would be observed in the concrete. In the second region, multiple cracking within the cement matrix will occur, while in the last region, the localized cracks would determine the ductility of the concrete [20]. For STFRHWSCCs, by increasing the fibre content, the peak load occurred at higher deflections and shifted towards higher values, as well as, the strain hardening behaviour of the HWSCCs improved since steel fibre controlled the crack propagation within the cement matrix. At 75% magnetite aggregate, the shape of the curves for higher fibre content revealed higher curve-softening behaviour, meaning that they can withstand higher deflections before fracture, and consequently, promoting the ductile postcracking behaviour for STFRHWSCCs. Increasing the HWA led to lower peak load owing to the crystalline microstructure of the HWA, as well as, segregation which can generate porous ITZ unable

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13

been investigated experimentally. The following results have been drawn from the simultaneous implementation of magnetite HWA and fibres in the SCC:

Fig. 13. Flexural strength test load-deflection curves (a) STFRHWSCCs, and (b) PPFRHWSCCs.

to tolerate higher deflections. The obtained load-deflection curves are in line with Tufekci and Gokce [20] report, in which they revealed that higher peak load and lower post-peak ductility occurred for barite-based and GFW-based HWC by increasing steel fibre from 1% to 3%. For PPFRHWSCCs, by increasing the fibre content, the peak stress occurred at higher deflections, which can be clearly seen for the PPFRHWSCCs with 75% magnetite aggregate. Similar to STFRHWSCCs, by increasing the HWA content, lower peak load and lower softening behaviour have been observed for HWSCCs reinforced with PP fibre. Comparing to specimens reinforced with steel fibre, lower softening behaviour of PPFRHWSCC would suggest the superior performance of steel fibre in trapping cracks and promoting ductile behaviour, rather than PP fibre [71,72]. Nevertheless, the higher peak loads obtained for PPFRHWSCCs rather than STFRHWSCCs, showed improved strain hardening through addition of PP fibre, meaning that PP fibres have better performance to postpone the crack occurrence rather than steel fibre. 4. Conclusions In this study, the feasibility of production of HWSCC reinforced with two different types of fibre including steel and PP fibres has

1. Both HWA and fibres would deteriorate the flowability and passing ability of SCC. Magnetite aggregate has higher water absorption capacity than NWA impacting the flowability of SCC negatively. High surface area of PP fibre, as well as, formation of network structure within the past through addition of PP fibres would increase the viscosity of the SCC. Hooked-end steel fibres are able to trap particles during flow, reducing the flowability of SCC. In general, STFRHWSCC showed higher passing ability while PPFRHWSCC had better flowability, which may be attributed to the flexible nature of PP fibres while steel fibres are not capable to bend and deform around reinforcement, increasing the chance of aggregate blockage. Nevertheless, almost all the FRHWSCCs revealed sufficient workability to be considered as SCC, except M75PP0.20 and M100ST0.75 due to the high fibre content accompanied with high VMA and low SP dosage. 2. Substituting the NWA with HWA would result in higher hardened-state density for all the HWSCC containing 100% magnetite aggregate. For HWSCC containing 75% magnetite aggregate, M75ST0.5, M75ST0.75, M75ST1.0, and M75PP0.20 revealed lower density than the 2600 kg
m 3 threshold for HWC, which is possibly due to the increasing the air content and void space through addition of fibres. Moreover, the difference between density of HWSCCs containing 100% magnetite aggregate and HWSCCs containing 75% magnetite was insignificant due to the broad range of particle size distribution for magnetite coarse aggregate. 3. Substituting the NWA with HWA would reduce the compressive strength of SCC due to the highly crystalline microstructure of HWA, relatively high water absorption, and segregation. However, addition of up to 0.75% steel fibre and up to 0.25% PP fibre resulted in higher 7-day and 28-day compressive strength for FRHWSCC than the CSCC. Moreover, the rate of strength gaining by aging the HWSCC is relatively low owing to the presence of HWA, their high water absorption capacity, and consequently postponing the C-S-H gel propagation. By bridging the cracks and rendering ductile post-cracking behaviour, both fibre types would enhance the compressive strength. 4. Results of tensile strength revealed that increasing the HWA content would reduce the tensile strength, whereas, increasing the fibre content would enhance the tensile strength. For steel fibres, fibre content above 0.75% promoted the tensile strength by increasing the ductility of the HWSCC, while below 0.75%, the tensile strength of HWSCC decreased which revealed the void formation within the cement paste through addition of rigid hooked-end steel fibre. Increasing PP fibre content enhanced the tensile strength of HWSCC since PP fibres not only prevent the crack-propagation but also fill the voids within the cement paste. 0.25% PP fibre represented the highest tensile strength for both series containing 75% and 100% magnetite aggregate. 5. Likewise the compressive and tensile strengths, by increasing the HWA content from 75% to 100%, flexural strength of HWSCCs reduced. However, addition of 0.75% steel, and 0.25% PP fibre would improve the flexural strength. The unexpected better results obtained for PPFRHWSCC containing 100% magnetite and higher PP fibre content can be attributed to the providing more empty space due to the addition of HWA, and therefore more void occupation by PP fibre. 6. The load-deflection curves for STFRHWSCC showed strainhardening and strain-softening behaviour by increasing fibre content. Comparing to STFRHWSCCs, PPFRHWSCCs represented

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higher strain-hardening behaviour and lower strain-softening impact by increasing the fibre content, which suggests the superior capability of PP fibre to postpone the crack occurrence and better crack-propagation control by steel fibre, respectively. For both fibre types, increasing the HWA content resulted in occurrence of peak stress at lower loads. 7. In general, reinforcing HWSCC with steel and PP fibres would result in enhanced mechanical properties for FRHWSCC, without deteriorating the essential workability required for maintaining the self-consolidating characteristics. These improvements are evident in terms of crack-propagation control and tolerating higher deflections (ductile failure mode). Nevertheless, more researches investigating the impact of fibre shape and size, fibre orientation within the cement paste and its impact on the both fresh and hardened properties of HWSCC, and fibre dispersion are required. Exploring the impact of different HWA on the properties of SCC would be of interest as well, in order to determine the most effective HWA type for producing high-performance HWSCC. Moreover, determining the toughness indices and the residual strength factors for more accurate interpretation of the load-deflection curves and the post-cracking behaviour of any specific sample in this study, are of great importance.

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