Mechanical Properties of Rubberized Concrete with Recycled Aggregate and Polypropylene Fiber

Mechanical properties of recycled aggregate concrete containing crumb rubber and polypropylene fiber

F.M. Zahid Hossain

^a

, Md. Shahjalal

^b

, Kamrul Islam

^c

, Mohammad Tiznobaik

^a

, M. Shahria Alam

^a,⇑

aSchool of Engineering, The University of British Columbia, Kelowna, BC, Canada

bDepartment of Civil Engineering, Military Institute of Science and Technology (MIST), Dhaka, Bangladesh

cDepartment of Civil, Geological and Mining Engineering, Ecole Polytechnique de Montreal, Montreal, QC, Canada

h i g h l i g h t s

Performance of rubberized concrete with RCA and polypropylene fiber is studied.

Properties of rubberized concrete with RCA and polypropylene fiber are experimentally investigated.

Concrete with 30% RCA, 5% CR and 2% fiber shows the highest toughness/ductility.

Mechanical properties of the developed concrete are compared with code equations.

a r t i c l e i n f o

Article history:

Received 15 January 2019 Received in revised form 5 July 2019 Accepted 20 July 2019

Available online 29 July 2019 Keywords:

Recycled coarse aggregate Crumb rubber

Compressive strength Flexural strength Toughness Ductility

a b s t r a c t

Scrap tire-derived crumb rubber (CR) in concrete, along with recycled coarse aggregate (RCA) and polypropylene fiber, constitute a way to reusing and recycling waste material and providing green and sustainable concrete structures. This study investigates the effect of substituting recycled constituents such as RCA and CR as a partial replacement of coarse aggregate and fine aggregate, respectively, along with polypropylene fiber addition into the concrete mixture. Fifteen different mixes are considered, with 10% and 30% RCA, 5% and 10% CR, and 1% and 2% fiber content. This study focuses on the experimental investigation of concrete combining RCA, CR, and fiber and evaluates its compressive strength, splitting tensile strength, and flexural strength at different ages. The compressive strength, splitting tensile strength, and flexural strength decrease as the CR content increases, but increase with the increasing fiber content. With regard to toughness and ductility, the effect of fiber is greater than that of RCA and CR, with each being found to increase with the incremental addition of fiber. It is also observed that the beams with fiber show failure in a more gradual manner. Finally, as a general recommendation in the interest of sustainability and environmental concern this paper suggests the use of rubberized concrete with RCA and polypropylene fiber for any structural purpose subjected to further investigations.

Ó2019 Elsevier Ltd. All rights reserved.

1. Introduction

Every year a huge amount of waste concrete has been produced from demolished structures due to the new construction of infras- tructure. Every year, approximately 450 million tonnes and 200 million tonnes of C&D (Construction and Demolition) waste are produced in the EU and China, respectively [1,2]. In the United States in 2014, concrete represented 70% of total construction and demolition (C&D), with 15.9% of all C&D being concrete from buildings, 29.5% concrete from roads and bridges, and 24.9% con-

crete from other structures[3]. On the other hand, many western countries like Canada are facing difficulties with maintenances and reuse of the structural debris due to the need to make the green environment and keep it pollution free and reduce carbon emission[4]. Moreover, the growing demand for concrete through- out the world is so pronounced that the concrete industry is facing tremendous challenges to satisfy it with the production of natural aggregates alone. The gravel for these aggregates is generally col- lected by excavating mountains, mining, and breaking river gravel.

These non-renewable natural resources will soon be exhausted, which is a significant concern for the construction industry. Many research has been conducted to produce an environment-friendly green concrete by replacing a percentage of natural aggregate with Poly-Ethylene Terephthalate (PET), Recycled Granulated Steel, https://doi.org/10.1016/j.conbuildmat.2019.07.245

0950-0618/Ó2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.

E-mail addresses:kamrul.islam@polymtl.ca(K. Islam),mohammad.tiznobaik@

ubc.ca(M. Tiznobaik),shahria.alam@ubc.ca(M.S. Alam).

Contents lists available atScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t

Recycled Coarse Aggregate (RCA) and FRP Scrap Aggregate, etc[4–

6]. So, to reduce the carbon footprint and for better environmental conservation, these waste materials can be used as a partial replacement of either coarse or fine aggregate. Notably, there is no significant effect on the mechanical properties of concrete when replacing 25–50% of the natural aggregate with RCA, which indi- cates that RCA can be used to make a greener environment with an ultimate goal of using it as an accepted structural material [4,7–9]. At the same time, 60–70% of recycled concrete can be used as sub-base during pavement construction[10]. RCA is a promising construction material, but its mechanical behavior must be better understood before it can be produced and used at a large scale because RCA has inferior properties compared to Natural Coarse Aggregate such as higher absorption capacity, lower density, higher Los Angeles abrasion capacity[11]. The properties of RCA significantly influence the properties of recycled aggregate con- crete, where variations of sources and impurities are the primary controlling factors influencing the material gradation, specific gravity, shape, and texture of aggregates.

To improve the toughness, shock absorption capacity and fati- gue performance a more popular approach is to use crumb rubber (CR) as a partial replacement of fine aggregate[16–19]. Every year a massive number of used tires are being disposed of and that is creating a waste management problem. As per the recently pub- lished 2017 U.S. Scrap Tire Management Summary, 4189.19 thou- sand tons of net tires were generated in the US in the year of 2017 [12]. Of this, 43.0% was diverted to tire-derived fuel, 8.0% was used for civil construction projects, and the remainder ended up in land- fills or stockpiles. Only 315.98 thousand tons were diverted to civil construction projects. Moreover, a recent study mentioned that one billion tires reach their service life every year worldwide [13]. Therefore, the use of scrap tires in civil construction projects, especially in aggregate production, provides a double benefit to us by reducing natural resources demand as well as mitigating a waste management problem. One of the potential uses of CR might be the replacement of natural aggregates in concrete and cement mortar[14]. The concept of rubberized concrete has garnered pop- ularity among researchers due to its enhanced mechanical proper- ties in terms of impact resistance and energy absorption capacity [15]. However, some researchers have observed poor mechanical properties of rubberized concrete compared to conventional con- crete[16]. It was observed that the strength and modulus of elas- ticity of concrete decrease with the increasing replacement level of CR, but significantly improves its energy absorption capacity[17–

19]. Though using rubber as a replacement of natural aggregates in concrete is a feasible option for utilizing waste rubber tires, but past studies suggested to use rubberized concrete in the non- structural application rather than main structural components because of its low compressive strength[20–22]. Thomas et al.

(2016) [23] experimentally investigated the abrasion resistance of normal and high strength rubberized concrete where rubber particles were used as a partial replacement of fine aggregate up to 20%. They reported the effectiveness of rubberized concrete against abrasion compared to control concrete.

As concrete has a tendency to experience brittle failure mecha- nism, therefore, the inclusion of fiber in concrete mixes improves the ductility of concrete[24]at the same time reduces the worka- bility. Limited amount of research has been conducted targeting a comparative study of mechanical properties combining recycled coarse aggregate, CR, and fiber[25–28]. In those studies, research- ers used steel fiber and found that the coupling effect of CR and sil- ica fume enhances the compressive strength, ductility, and toughness of concrete while the increased content of CR signifi- cantly reduces the compressive strength and fracture toughness.

They concluded that the inclusion of silica fume has a significant effect on the mechanical properties of rubberized fiber-based con-

crete. This underscores the need for further research evaluating the feasibility of using RCA, CR, and fiber as a substantive step toward more sustainable construction. This study thus investigates possi- ble correlations in the mechanical properties of different mixes of RCA, CR, and fiber to optimize the composition of this mixed aggre- gate for future construction projects.

Generally, RCA possesses a specific gravity much lower than NCA. Recent studies show that the attached mortar content in the RCA decreases the specific gravity of RCA[29,30]. Furthermore, NCA shows a lower absorption capacity compared to RCA at only approximately 0.3%. The absorption capacities of RCA ranges from 3.2% to 12% because of the attached mortar [30]. The higher absorption capacity of RCA makes the concrete less workable in comparison with NCA[29,31]. Another group of researchers found that the addition of additional 5–10% water content in concrete made with RCA resulted in the same workability as NCA. CR also affects slump value. In many previous studies, it was observed that with increasing the percentage of CR in the concrete mixture, the slump value began to decrease, indicating that the use of CR in con- crete reduces its workability due to the high amount of absorbed water in it[32–35]. Another reason behind this is that, as CR is a porous material and can’t mix properly with the other ingredients of concrete, so it requires more water to overcome the inter- particle friction[36].

Concrete containing RCA has been found to show higher air con- tent than does concrete containing NCA [29], entailing that increasing rubber percentage in concrete results in increased air content in concrete matrix due the rubber’s property of trapping air[32,33], which decreases the compressive strength of the con- crete[37,38]. Alam et al. (2013)[4]found that concrete made with RCA showed a 15% reduction in compressive strength for 25%

replacement. In a study of RCA concrete by Limbachiya et al.[9], it was found that concrete compressive strength didn’t change sig- nificantly up to 30% replacement, while RCA replacement exceed- ing 30% reduced the strength. Meanwhile, according to a study by Alam et al.[4], concrete made with RCA increased the tensile strength by 34% for 25% RCA replacement and decreased it by 16% for 50% RCA replacement and for the flexural strength it was decreased by 15.8% for 25% RCA replacement. For CR, Meherier [36] reported that when the percentage of CR was more than 20%, there was a significant reduction in concrete compressive strength, but it showed better strain capability compared to the control concrete specimens. In that study, he also found that the values of splitting tensile strength and flexural strength decreased with an increase in CR percentage. However, that study suggested not to go more than 10% CR replacement to get a comparable result with the control concrete specimens. Soroushian et al. [39]

reported that 1% volume replacement of polypropylene fiber increased the flexural strength by 21% and the compressive strength by 23%. Meanwhile, studies by Konig [40] and Najimi et al.[24]found improvement in the tensile strength and ductility by incorporating fibers in concrete mixes, while the incidence of spalling is reduced[41].

This study focuses on the experimental investigation of con- crete combining RCA, CR, and fiber and evaluates its compressive strength, splitting tensile strength, and flexural strength at differ- ent ages. The measured experimental properties of fiber-based rubberized recycled concrete were used to check whether the existing design standards developed for normal aggregate concrete is applicable to this new type of concrete.

2. Research significance

Over the last few decades, researchers have investigated the mechanical and durability properties of recycled aggregate con- crete. However, in most cases, they failed to attain enhanced

mechanical properties in terms of compressive strength, flexural strength, toughness, and ductility compared to the control con- crete. Inclusion of fibers in concrete mixtures increases the ductil- ity and inclusion of rubber particles increase the energy absorption capacity of concrete. Effect of fiber and rubber on concrete have been investigated independently by other researchers. To the authors’ knowledge, a limited amount of research has been con- ducted to date to investigate the short- and long-term mechanical properties of concrete combining RCA, CR, and fiber. The present study thus thoroughly investigates the short- and long-term mechanical properties of recycled aggregate concrete containing CR and polypropylene fiber, which will pave the way towards sus- tainable construction. Moreover, it is important to know whether the existing design guidelines available for normal aggregate con- crete are applicable to recycled aggregate concrete containing CR and polypropylene fiber. This would eventually help the construc- tion industry in sustainable development without destruction of natural resources.

3. Experimental program 3.1. Materials and mixtures

A detail description of the ingredients used in this study to pre- pare the concrete mixtures is presented below (SeeFig. 1).

3.1.1. Cement

Portland Composite Cement (PCC) is used in this study. PCC is the most widely used cement in Bangladesh. The chemical compo- sitions and properties of PCC are shown inTable 1. The normal con- sistency of cement is measured using ASTM C187-16 [42], and setting time are measured using ASTM C191-18[43].

3.1.2. Natural fine aggregate (NFA)

A locally available well graded natural sand with a nominal maximum grain size of 4.75 mm is used in this study. The specific gravity and water absorption capacity of the sand are measured according to ASTM C128-15[44], while the unit weight is mea- sured according to ASTM C29-17[45]. Sieve analysis and gradation of sand are done using ASTM C136-14[46]standard. The physical

properties of sand are summarized inTable 2.Fig. 2(b) depicts the gradation curve of NFA and it falls within the ASTM designated range.

3.1.3. Natural coarse aggregate (NCA)

The maximum aggregate size of 19 mm is used as NCA in this study. The specific gravity and water absorption capacity of the NCA are measured according to ASTM C127-15[47], while the unit weight is measured based on ASTM C29-17[45]. The abrasion test is also conducted following ASTM C131-14[48]. Sieve analysis and gradation of NCA are done using ASTM C136-14 [46] standard.

Fig. 2(a) represents the gradation curve of NCA which clearly shows that the coarse aggregate is within the ASTM upper and lower bound range.

3.1.4. Recycled coarse aggregate (RCA)

The recycled concrete used in this study is collected from a demolished building and crushed manually with a maximum aggregate size of 19 mm. As the recycled concrete was collected from an old demolished building, lots of unwanted materials were mixed with it. Several screening, sieving, and washing were done to remove these impurities. The aggregate was sieved to discard particle size smaller than 4.75 mm. The material characterization

Fig. 1.Aggregate used in concrete: (a) Recycle Coarse Aggregate (b) CR (c) Polypropylene Fiber.

Table 1

Properties of Portland Composite Cement (PCC).

Compound (%Mass) Normal consistency (%) Setting time (min)

Initial Final

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O LOI 26.7 130 180

20.60 4.74 3.28 64.82 1.84 2.4 0.21 0.38 1.73

Table 2

Physical properties of aggregates.

Variables Natural coarse

aggregate

RCA Sand CR

Apparent specific gravity 2.73 2.64 2.47 –

Bulk specific gravity (SSD) 2.71 2.51 2.41 1.18

Bulk specific gravity (OD) 2.70 2.44 2.36 –

Absorption capacity (%) 0.48 3.23 1.8 1.3

Fineness modulus 2.51 2.45 2.39 3.49

Loose condition unit weight (kg/m³)

1477.02 1557.54 1455.44 – Compact condition unit weight

(kg/m³)

1525.57 1658.71 1579.25 –

Loose condition % of voids 45.1 35.9 38.3 –

Compact condition % of voids 43.3 31.7 33.0 –

Abrasion value (%) 14.58 40.5 – –

of the RCA is performed using the same procedure as for the NCA.

The gradation curve of the RCA is presented inFig. 2(a).

3.1.5. Crumb rubber (CR)

A maximum CR size of 4.75 mm is used as a fine aggregate. The gradation curve of CR and NFA are shown inFig. 2(b). From the fig- ure, it is clear that NFA fits within the ASTM range but CR does not fit. It happens because CR is produced commercially and supplied by a local company. However, because of using CR as NFA replace- ment, the combined gradation of both the materials are considered to fit within the standard specified. The fineness modulus, absorp- tion capacity, and specific gravity for CR are presented inTable 2.

3.1.6. Polypropylene fiber (PP Fiber)

A Japanese private company provided the polypropylene fiber for this study. The specifications for the polypropylene fiber are presented inTable 3.

3.2. Mixture proportions

Fifteen different concrete mixes are prepared for this study to support comparative analysis. A target compressive strength of 25–35 MPa (i.e., 28 days concrete strength) is commonly used in designing reinforced concrete structures in Bangladesh depending on the category of construction. For this reason, the designed com- pressive strength was fixed at 30 MPa to provide more realistic and comparable evidence to the concrete industry. The total portland cement content and the effective water-cement ratio were kept constant at 427.1 kg/m³and 0.38, respectively. As a result, the vari- ations are in the percentage of natural coarse aggregate, RCA, sand, CR, cement, and polypropylene fiber. The coarse and fine aggre- gates used in the concrete are in saturated surface dry (SSD) condi- tion. The percentage of RCA replacement is 10% and 30% which are replaced in weight basis. From the previous study, it was shown that RCA replacement up to 30% had no significant effect on con- crete strength, but thereafter there was a gradual reduction in strength as the RCA content increased[4,8,9,36]. Some researchers have mentioned not to exceed 30% replacement level to maintain standard requirements for 5% absorption capacity of aggregates [49]. This is why the RCA replacement level is considered as 10%

and 30%. Given that the specific gravity of CR is very low compared

to sand, they are proportioned in volume basis. CR is used in the replacement of sand in 5% and 10% proportions, and polypropylene fiber is added in the concrete mixture at 1% and 2% by volume of concrete. These proportions are considered because Meherier (2016)[36]suggested not to exceed the CR content by more than 10% to get comparable results with the controlled concrete speci- mens. In a recent study, Ravi and Karvekar (2014)[50]showed the mechanical and durability properties of concrete containing various fibers including polypropylene fiber, galvanized iron fiber, steel fiber, and high-density polyethylene fiber. They used up to 1.5% fiber content in the concrete mixture. Xie et al. (2018)[28]

mentioned in their study that optimum steel fiber content is 1–

1.5% by volume in concrete. In this study, 1% and 2% polypropylene fiber content is used to see the effect of fiber in the concrete mix- ture. No water-reducing admixture is used in the mix design.

Table 5 summarizes the concrete mix proportions of different materials for 15 different combinations. The specimen identifica- tion description is shown in Table 4. For example, Batch 6 (R10C5F2) means 10% RCA replacement, 5% CR replacement along with 2% fiber and Batch 10 (R30C0F1) means 30% RCA replacement, 0% CR replacement along with 1% fiber.

3.3. Specimens

A total of 270 cylinders are cast for the compressive strength test at 7, 28, and 56 days, for the splitting tensile strength test at 28 and 56 days, and for the unit weight of hardened concrete cylin- ders at 28 days. The mold size of the cylinders is 100 mm diame- ter200 mm height. For fifteen different concrete mixes, a total

of 60 small size beams with mold size of

100 mm100 mm500 mm are cast to quantify the flexural strength at 28 days.

3.4. Testing procedures

Concrete was prepared with a mixture machine. Before pouring the materials into the mixture machine the polypropylene fiber is mixed uniformly with the cement whereas, the CR is mixed with the sand manually by hand. A glove is used so that the materials could not stick to the hands. Then the NCA and RCA are poured into 0

20 40 60 80 100

1 10 100

Percent Fine (%)

Sieve Opening (mm)

ASTM Upper NCA RCA ASTM Lower

(a)

0 20 40 60 80 100

0.1 1 10

Percent Fine (%)

Sieve Opening (mm)

ASTM Upper NFACR ASTM Lower

(b)

Fig. 2.Gradation curve of (a) natural coarse aggregate (NCA) and Recycled coarse aggregate (RCA); (b) sand and CR.

Table 3

Specifications for polypropylene fiber.

Variables Polypropylene fiber

No. of denier 03

Specific gravity (g/cm³) 0.91

Fiber length (mm) 12

Tensile strength (MPa) 480

Elastic modulus (GPa) 7.0

Table 4

Specimen identification description.

Batch identifying name: RxCyFz Description

R RCA

x % of RCA replaced – NCA

C CR

y % of CR replaced – sand

F Polypropylene fiber

z % of polypropylene fiber

the machine. After that, sand, cement, and water are added respec- tively. Freshly prepared concrete is used for the slump test and air content test. ASTM C143-15[51] standard is used for measuring the slump value of concrete mixes, and ASTM C231-17[52]is used for the air content measurement of fresh concrete samples. The specimens are cured for 24 h initially with the mold in a moist room and then demolded them and kept the specimens under fresh water at a controlled laboratory temperature in a constant condition before taken out for testing. The compressive strength test is performed according to ASTM C39-18[53]. The rate of the load is 0.25 ± 0.05 MPa/s. For 56 days concrete cylinders two strain gauges are attached at the mid-height of the cylinders with a data logger to measure the longitudinal and transverse strain. Before testing both surfaces of each cylinder are leveled with sulfur cap- ping to eliminate the eccentricity of loading. The splitting tensile strength test is performed according to ASTM C496-14[54]. The flexural strength of the concrete prisms is tested according to ASTM C78[55] and ASTM C1609[56]. The flexural strength test is performed using a displacement rate of 0.15 mm/min. To observe the unit weight of hardened concrete cylinders, ASTM C642-13[57]standard is adopted in conducting the test. The tests set up for compressive strength test and flexural strength test are shown inFig. 3.

4. Experimental results and discussion 4.1. Fresh concrete properties

The slump value of different concrete mixes is shown inFig. 4. It is observed that the control concrete mixture has a higher slump value of 110 mm compared to the other 14 combinations. This

indicates that the control mixture provides the highest workability of the specimens under study. Batch-15 (R30C10F2) shows the low- est slump value because this combination is made with the highest replacement level of RCA, rubber, and fiber. It is observed that, for a fixed RCA replacement, with the incremental replacement level of CR the slump value decreases. For 10% RCA replacement when the fiber is 1%, the slump value decreases by 40.9% for 5% CR and by 54.5% for 10% CR. The same trend is observed for the fixed 10%

RCA replacement with 2% fiber, and also for the 30% RCA combina- tions. As CR is a porous material that does not blend properly dur- ing mixing, therefore, it requires more water to overcome inter- particle friction. It is also observed that the slump value decreases dramatically due to the increased fiber content. Fiber creates higher interlocking between aggregates that reduces its workabil- ity. For 10% RCA replacement, when CR is 0% then the slump value decreases by 20% for 1% fiber and by 58.2% for 2% fiber. The same trend is also observed for 10% RCA and 5% CR and for 10% RCA Table 5

The proportion of aggregates for concrete mixtures per cubic meter.

Batch No. Batch code Natural coarse aggregate (kg) Fine aggregate (kg) RCA (kg) CR (kg)

1 R0C0F0 990.2 678.7 00.0 00.0

2 R10C0F0 891.2 678.7 99.0 00.0

3 R10C0F1 891.2 678.7 99.0 00.0

4 R10C0F2 891.2 678.7 99.0 00.0

5 R10C5F1 891.2 662.1 99.0 16.6

6 R10C5F2 891.2 662.1 99.0 16.6

7 R10C10F1 891.2 645.5 99.0 33.2

8 R10C10F2 891.2 645.5 99.0 33.2

9 R30C0F0 693.2 678.7 297.0 00.0

10 R30C0F1 693.2 678.7 297.0 00.0

11 R30C0F2 693.2 678.7 297.0 00.0

12 R30C5F1 693.2 662.1 297.0 16.6

13 R30C5F2 693.2 662.1 297.0 16.6

14 R30C10F1 693.2 645.5 297.0 33.2

15 R30C10F2 693.2 645.5 297.0 33.2

3@150 mm = 450mm

(a) (b)

Fig. 3.Test setup (a) compression test (b) quasi-static flexural test.

0.0 1.0 2.0 3.0 4.0

0 20 40 60 80 100 120

Air content (%)

Slump (mm)

Slump Air content

Fig. 4.Sump value and air content of different concrete mixtures.

and 10% CR, as well as for 30% RCA combinations. During the test, it is observed that the effect of fiber on slump value is more signifi- cant than CR and RCA. Slump value decreases drastically with increasing the level of fiber content. On the other hand, slump value decreases with increasing the percentage of RCA because of its higher absorption capacity and increased surface roughness.

For air content, it is seen fromFig. 4that, air content in concrete increases with the increasing level of RCA. In the case of CR, air content also increases with the increasing level of CR. For 10%

RCA, when fiber is 1% then the air content is 1.54%, 2.69%, and 3.05% for 0%, 5%, and 10% replacement levels of CR, respectively.

This trend is also observed for 10% RCA and 2% fiber as well as for 30% RCA combinations. Adhered mortar with RCA and CR together entraps air in the concrete mixture and create air bubbles in the mixture, resulting in higher air content. Previous studies by the authors have found similar results for recycled concrete[4,8]. It is also seen that with incremental increases of fiber content, the air content decreases (Batch-3 & Batch-4, Batch-5 & Batch-6, Batch-7

& Batch-8, Batch-10 & Batch-11, Batch-12 & Batch-13 and Batch- 14 & Batch-15). For the fresh density of concrete, it is seen that RCA, rubber and fiber content in mixture reduce the fresh density of the samples than the control mix. As the CR and fiber are low in specific gravity, the addition of these materials in the mixture slightly reduce the concrete weight. Batch-14 (R30C10F1) shows the lowest density because, in this combination, the RCA and rub- ber contents are higher.

4.2. Compressive strength

The summary of the hardened concrete compressive strength test results after 7, 28, and 56 days are presented inFig. 5. It is observed that 7 days compressive strength of concrete for all batches (except batch-7 and batch-9) is higher than that of the control concrete mix. Approximately 61% of the designed concrete compressive strength is achieved after 7 days for control specimen.

From the batches of control, R10C0F0and R30C0F0it is seen that the compressive strength increases for 10% RCA replacement but decreases for 30% RCA. The effect of higher compressive strength for R10C0F0may be attributed to better/optimal internal curing pro- vided by the higher amount of absorbed water by RCA, and thus the water to cement ratio is lower at the interfacial transition zone.

Thus an improved bond is created between the attached mortar on the surface of the RCA and new cement paste. But for 30% RCA, this effect is faded away due to the higher amount porosity introduced to the system by RCA in R30C0F0. This strength reduction may hap- pen due to the multiple layers of ITZ. Huda and Alam[8]also found that as the percentage of RCA replacement increases the compres- sive strength decreases for RCA replacement 30% and more. From Batch-3 (R10C0F1), Batch-5 (R10C5F1) and Batch-7 (R10C10F1), and Batch-4 (R10C0F2), Batch-6 (R10C5F2) and Batch-8 (R10C10F2) it is seen that at a fixed RCA and fiber content with the incremental

replacement of CR, the compressive strength decreases. Previous research also found a similar trend[28,36]. CR creates a weaker bond within the aggregate and cement paste. It has a very low water absorption capacity resulted in a higher amount of free water in the concrete mixture, thus weakens the transition zone.

Moreover, CR has a similar effect in concrete due to its much lower strength compared to NFA. Also, CR has lower elastic modulus and it is hydrophobic in nature which increases the porosity resulting in a lower concrete strength[28]. In addition, various impurities (i.e., sulfur and zinc) are present in the CR, which result in a poor bonding in the cement matrix[36]. Same trend is also followed for 30% RCA combinations (Batch-10, Batch-12, Batch-14 and Batch-11, Batch-13, Batch-15) On the contrary, it is seen that, for a fixed RCA and CR replacement level, the compressive strength increases with an increasing proportion of fiber which can be observed between Batch-3 (R10C0F1)& Batch-4 (R10C0F2), Batch-5 (R10C5F1) & Batch-6 (R10C5F2), Batch-7 (R10C10F1) & Batch-8 (R10C10F2), Batch-10 (R30C0F1) & Batch-11 (R30C0F2), and Batch-12 (R30C5F1) & Batch-13 (R30C5F2).

Fiber can make a bridging effect between the cement paste and aggregate which leads to improving the strength of concrete. How- ever, batch-14 (R30C10F1) and batch-15 (R30C10F2) do not follow this trend. This is due to the fact that in these combinations the RCA replacement level and CR replacement level is both relatively high, and for a large amount of rubber replacement level, the effect of fiber is less significant. And for this reason, Batch-15 shows the lowest compressive strength of the batches investigated. From Batch-6 (R10C5F2) and Batch-7 (R10C10F1), it is seen that compres- sive strength decreases more for Batch-7 than for Batch-6 because, for Batch-7, the rubber percentage increases while the fiber con- tent decreases. Same trend is seen for Batch-4 (R10C0F2), Batch-5 (R10C5F1); Batch-11(R30C0F2), Batch-12 (R30C5F1) & Batch-13 (R₃₀C₅F₂), Batch-14 (R₃₀C₁₀F₁). Batch-11 (R₃₀C₀F₂) shows the high- est compressive strength which is 27% more than the control mix- ture at 56 days as this combination does not contain any CR and the fiber percentage is higher.

FromTable 6it is observed that rubber-based concrete shows high Poisson’s ratios due to its soft nature and enhanced energy absorption capacity. The Poisson’s ratio increases with the increas- ing replacement level of rubber, which signifies better deformabil- ity and energy absorption capacity. On the contrary, it is seen that the Poisson’s ratio decreases with the incremental increase of fiber percentage, which means that fiber resists deformation. For R10C0F0and R30C0F0, the Poisson’s ratio is found to increase along with increasing RCA replacement level. Concrete cylinders with CR and fiber do not exhibit brittle failure like control specimens.

The CR can experience higher deformation under compressive loading condition due to its inherent mechanical properties com- pared to sand. Fiber is effective in preventing cracking because it creates a strong bond between the aggregate and cement paste and holds the aggregate particle together to resist cracking. So, lar-

0 10 20 30 40 50

Compressive strength (MPa)

7 days 28 days 56 days

(a)

0 10 20 30 40 50

Compressive strength (MPa)

7 days 28 days 56 days

(b)

Fig. 5.Variation in compressive strength of different concrete mixtures for (a) 10%, and (b) 30% RCA combinations.

ger deformation, higher energy absorption, and more impact resis- tant structures can be a focus for future studies with rubber and fiber combination.

4.3. Splitting tensile strength

The results of splitting tensile strengths at 28 days and 56 days for different concrete mixes are shown inFig. 6. It is seen that Batch-4 (R10C0F2) shows the maximum result both at 28 days and 56 days which are 20.7% and 17.8% more than the control respectively. Batch-14 (R30C10F1) shows the lowest tensile strength both at 28 days and 56 days which are 22% and 10.1% less than the control respectively. From Batch-2 (R10C0F0) and Batch-9 (R30C0F0), it is seen that with the increasing amount of RCA the splitting ten- sile strength decreases which are 2.4% and 3.4% less, respectively, compared to the control mixture. In the case of CR, the splitting tensile strength of concrete decreases with the increasing level of CR. For 10% RCA, when the fiber content is 1%, the values of tensile strength are 3.16 MPa, 2.82 MPa, and 2.79 MPa for 0%, 5%, and 10%

replacement levels of CR, respectively. This trend is also observed for 10% RCA and 2% fiber as well as for 30% RCA combinations both at 28 days and at 56 days. CR has a lower elastic modulus, and it is fragile in tension. Bonding between CR and cement paste is thus prone to break suddenly, creating a weak interfacial transition zone. On the other hand, due to the good tensile capacity of fiber, the splitting tensile strength increases with increasing level of fiber in the concrete mixture. This may attribute to the presence of a large amount of fiber along the fracture plane before splitting which creates a strong bond between aggregate and cement paste.

4.4. Flexural strength

The results of the quasi-static flexural strength are presented in Fig. 7. Most of the concrete mixes show lower flexural strength

than the control mix. The flexural strength decreases with the increasing level of RCA (Batch-2 & Batch-9). At a fixed fiber content with incremental increases in CR replacement level, the flexural strength decreases. CR creates a weaker bond within the aggregate and cement paste, weakens the ITZ (Interfacial Transition Zone), and does not react uniformly with the matrix. Thus, it can be con- cluded that the flexural strength decreases with incremental increases in the replacement level of CR. On the contrary, it is seen that, for a fixed CR replacement level, the flexural strength increases due to increasing the percentage of fiber inclusion in the concrete mixture. This occurs because fiber creates a good bridge between the aggregate and cement paste. After the beam was broken, it is seen that the bridging fibers are projecting out- ward and perpendicular to the cross-section. Regarding Batch-6 (R10C5F2) and Batch-7 (R10C10F1), it is seen that flexural strength decreases for Batch-7 than for Batch-6 because for Batch-7 the rub- ber percentage is increased while at fiber content is decreased. The same trend is seen for Batch-4 (R10C0F2), Batch-5 (R10C5F1); Batch- 11 (R30C0F2), Batch-12 (R30C5F1) and Batch-13 (R30C5F2), Batch-14 (R₃₀C₁₀F₁). Batch-11 (R3°C₀F₂) shows the highest flexural strength at 8.6% more than the control. In this combination, there is a high percentage of fiber which produces a strong bond between aggre- gate and cement paste, and there is no CR present. Batch-7 (R10C10F1) shows the lowest result, 16.2% lower than the control.

In this combination, the percentage of CR is higher, while the fiber percentage is lower.

The toughness of a given batch is determined based on the load versus deflection curve. According to ASTM C1609, the toughness, T^D150,indicates, the area under the load versus deflection curve up to a deflection of L/150 (= 3 mm, where the effective span of the beam is 450 mm). FromTables 7 and 8it is seen that for all batches tough- ness value is more than the control concrete batch. The toughness value increases due to the increasing the percentage of fiber in the concrete mixes because fiber can form a bridge inside the densified Table 6

Mechanical properties of different concrete mixtures at 56 days.

Batch ID Peak stress (MPa) Axial strain at peak stress Transverse strain at peak stress Ultimate strain Poisson’s ratio

R0C0F0 35.02 0.00278 0.000684 0.00294 0.246

R10C0F0 36.23 0.00201 0.000386 0.00305 0.181

R10C0F1 37.12 0.00340 0.000670 0.00370 0.197

R10C0F2 37.94 0.00561 0.000976 0.00581 0.174

R10C5F1 32.70 0.00011 0.000032 0.00021 0.291

R10C5F2 36.02 0.00439 0.001070 0.00552 0.244

R10C10F1 31.17 0.00267 0.000839 0.00323 0.314

R10C10F2 35.91 0.00101 0.000266 0.00120 0.263

R30C0F0 33.31 0.00264 0.000660 0.00310 0.250

R30C0F1 36.38 0.00585 0.001825 0.00619 0.312

R30C0F2 44.56 0.00238 0.000508 0.00270 0.213

R30C5F1 35.21 0.00408 0.001340 0.00427 0.328

R30C5F2 35.70 0.00444 0.001010 0.00463 0.227

R30C10F1 32.11 0.00340 0.001140 0.00382 0.335

R30C10F2 30.95 0.00518 0.001242 0.00542 0.240

0 1 2 3 4 5

Splitting tensile strength (MPa)

28 days 56 days (a)

0 1 2 3 4 5

Splitting tensile strength (MPa)

28 days 56 days (b)

Fig. 6.Splitting tensile strength of various concrete mixtures for (a) 10%, and (b) 30% RCA combinations.

concrete matrix on either side of the crack and is able to resist load against a larger deflection after it has reached its ultimate failure point. Thus, the load versus deflection graph extends far beyond what is shown in Figs. 8–10. Fiber also creates a strong bond between aggregate and cement paste and acts as a barrier against crack propagation. On the contrary, it is seen that toughness value increases with an increasing replacement level of CR up to 5% and then decreases for 10% CR (Batch-3, Batch-5, Batch-7; Batch-4,

Batch-6, Batch-8; Batch-10, Batch-12, Batch-14 & Batch-11, Batch- 13, Batch-15). Batch R30C5F2shows the highest toughness value.

From the load versus deflection curve, the ductility of various batches is calculated according to the following equation:m=^d_d^U

Y, wheredU= deflection at ultimate strength (i.e., deflection value when the failure crack forms and applied load starts to drop), anddY= deflection up to linear potion of the load-displacement curve (i.e., deflection when the first crack forms in the beam).

Table 9 shows the ductility value of the various batches. It is observed that, with increasing level of fiber ductility increases, indicating that rubberized concrete with fiber will give ample warning before failure. Ductility increases with an increasing replacement level of CR up to 5% and then decreases for 10% CR.

4.5. Comparison with the standard code of practices

Various design guidelines exist for predicting the mechanical response of concrete using the compressive strength. In the pre- sent study, design guidelines from the ACI, fib, and Euro-code (EC2), CSA are used to predict the different response parameters of the concrete specimens (Figs. 11 and 13). These predicted behaviors are then compared with the experimental results. The ratio between the experimental and predicted values (n) is calcu- lated for the purpose of comparison, as presented inFigs. 12 and 14, where an value greater than 1 indicates an over-prediction, while a value less than 1 indicates an under-prediction of the result.

0 1 2 3 4

Flexural strength (MPa)

(a)

0 1 2 3 4

Flexural strength (MPa)

(b)

Fig. 7.Flexural strength variation in different concrete mixtures for (a) 10%, and (b) 30% RCA combinations.

Table 7

Summary of flexural response (for 10% RCA).

Batch R0C0F0

(Control)

R10C0F0

(B-2)

R10C0F1

(B-3)

R10C0F2

(B-4)

R10C5F1

(B-5)

R10C5F2

(B-6)

R10C10F1

(B-7)

R10C10F2

(B-8)

Toughness, T^D150(N-mm) 1827 2387

(30.6%)

3553 (94.5%)

5207 (185%)

3828 (109.5%)

5337 (192%)

3178 (74%)

5134 (181%)

Flexural Toughness Factor (FTF) (MPa) 0.27 0.36 0.53 0.78 0.57 0.80 0.48 0.77

Equivalent Flexural Strength ratio, R^D150(%) 7.79 10.29 15.17 21.88 19.16 25.67 16.16 25.16

Table 8

Summary of flexural response (for 30% RCA).

Batch R0C0F0

(Control)

R30C0F0

(B-9)

R30C0F1

(B-10)

R30C5F1

(B-12)

R30C5F2

(B-13)

R30C10F1

(B-14)

R30C10F2

(B-15)

Toughness, T^D150(N-mm) 1827 3477

(90.3%)

5689 (211.3%)

6277 (243.5%)

10,179 (457%)

5242 (186.9%)

5640 (208.7%)

Flexural Toughness Factor (FTF) (MPa) 0.27 0.52 0.85 0.94 1.53 0.79 0.85

Equivalent Flexural Strength ratio, R^D150(%) 7.79 16.29 25.24 28.13 40.22 23.61 24.25

0 2 4 6 8

0 0.5 1 1.5 2 2.5 3 3.5

Load (kN)

Deflection (mm)

0 2 4 6 8

0 0.5 1

Load (kN)

Deflection (mm)

Control R₁₀C₅F₂ R₃₀C₅F₂

Fig. 8.Load vs deflection response of beams under various replacement level of RCA (for 5% CR and 2% fiber).

0 2 4 6 8

0 0.5 1 1.5 2 2.5 3 3.5

Load (kN)

Deflection (mm) (b)

0 2 4 6 8

0 0.5 1 1.5 2 2.5 3 3.5

Load (kN)

Deflection (mm) (a)

0 2 4 6 8

0 0.5 1

Load (kN)

Deflection (mm)

Control R₃₀C₀F₁

R₃₀C₅F₁ R₃₀C₁₀F₁

0 2 4 6 8

0 0.5 1

Load (kN)

Deflection (mm) Control R₃₀C₀F₂ R₃₀C₅F₂ R₃₀C₁₀F₂

Fig. 9.Load vs deflection response of beams under various replacement level of CR for (a) 30% RCA and 1% fiber, and (b) 30% RCA and 2% fiber combinations.

0 2 4 6 8

0 0.5 1 1.5 2 2.5 3 3.5

Load (kN)

Deflection (mm) (b)

0 2 4 6 8

0 0.5 1 1.5 2 2.5 3 3.5

Load (kN)

Deflection (mm)

(a)

0

2 4 6 8

0 0.5 1

Load (kN)

Deflection (mm) Control R₃₀C₀F₀ R₃₀C₀F₁ R₃₀C₀F₂

0 2 4 6 8

0 0.5 1

Load (kN)

Deflection (mm) Control R₃₀C₅F₁ R₃₀C₅F₂

0 2 4 6 8

0 0.5 1 1.5 2 2.5 3 3.5

Load (kN)

Deflection (mm) (c)

0 2 4 6 8

0 0.5 1

Load (kN)

Deflection (mm)

Control R₃₀C₁₀F₁ R₃₀C₁₀F₂

Fig. 10.Load vs deflection response of beams under various fiber content for (a) 30% RCA and 0% CR, (b) 30% RCA and 5% CR, and (c) 30% RCA and 10% CR combinations.

Table 9

Ductility value of various batches.

Batch R0C0F0

(Control)

R10C0F0

(B-2)

R10C0F1

(B-3)

R10C0F2

(B-4)

R10C5F1

(B-5)

R10C5F2

(B-6)

R10C10F1

(B-7)

R10C10F2

(B-8)

Ductility 2.30 1.31 1.62 1.75 1.78 2.00 1.73 1.87

Batch R0C0F0

(Control)

R30C0F0

(B-9)

R30C0F1

(B-10)

R30C0F2

(B-11)

R30C5F1

(B-12)

R30C5F2

(B-13)

R30C10F1

(B-14)

R30C10F2

(B-15)

Ductility 2.30 1.15 1.21 1.29 2.43 2.80 1.27 1.31

4.6. Hardened concrete density

The results of the hardened concrete density of various concrete mixtures are shown inFig. 15. From the figure, it is observed that there is a considerable weight difference between the concrete specimens from different batches. The control concrete specimens show a slightly higher density compared to the recycled rubber- ized fiber-based concrete. Although the density of the 10% RCA is very similar to that of the control, for 30% RCA it is found to decrease by 3.6%. It is also observed from R10C10F2 and R30C10F2

that when rubber and fiber content is fixed, the density decreases with increasing level of RCA by 2.7% and 6.2%, respectively com- pared to the control one. From, Batch-3 (R10C0F1), Batch-4 (R10C0F2) and Batch-14 (R30C10F1), Batch-15 (R30C10F2), it is observed that the density also decreases with an increasing level of fiber. From Batch-4 (R10C0F2) and Batch-6 (R10C5F2), and Batch- 0

2 4

Splitting tensile strength (MPa)

ACI 318-14 fib2010 EC2

Fig. 11.Comparison of splitting tensile strength with different codes.

0.0 0.5 1.0 1.5

Ratio between experimental and calculated values

ACI 318-14 fib2010 EC2

Fig. 12.The ratio between the experimental and the calculated values (n) of splitting tensile strength using available guidelines for concrete.

0 2 4 6 8

Flexural strength (MPa)

ACI 318-14 ACI 209-92

fib2010 CSA A23.3-14

Fig. 13.Comparison of flexural strength with different codes.

0.0 0.5 1.0 1.5

Ratio between experimental and calculated values

ACI 318-14 ACI 209-92 fib2010 CSA A23.3-14

Fig. 14.The ratio between the experimental and the calculated values (n) of flexural strength using available guidelines for concrete.

2200 2300 2400 2500 2600

Unit Weight (Kg/m3)

Fig. 15.Harden density of concrete specimens.

Splitting tensile strength ACI 318-14[58] fctm,sp= 0.556p

f⁰_C; where fctm,spis mean splitting tensile strength in MPa fib2010[59] fctm= 0.3 fðCmÞ²³; where fctmis mean tensile strength in MPa and fcmis mean

compressive strength in MPa

EC2[60] fctm =

a

^spfctm,sp; where EC2 recommends the value of

a

spas 0.9 Modulus of rapture ACI 318-14[58] f^’r= 0.62p

f⁰_c; where f^’cis concrete compressive strength in MPa

ACI 209-92[61] f^’r= 0.013^pffiffiffiffi

q

:f⁰_c; where

q

is the density of concrete in kg/m³and f^’cis in MPa fib2010[59]

f^’r=^{0:3 f}

0 C 23

afl ; where,

a

^fl=_1þ^0:₀^06h_:06h^07b07 b

and hb= depth of beam (mm); f^’cis in MPa CSA A23.3-14[62]

f^’r= 0.6k ffiffiffiffi f⁰_c q

; wherek= 1.0 for normal density concrete and f^’cis in MPa

Fig. 16.Failure pattern of concrete cylinder under compressive strength test: (a) control, (b) only 30% RCA, (c) RCA, CR and fiber combination.

Fig. 17.Concrete specimen after splitting tensile strength test: (a) only for RCA and (b) RCA, rubber and fiber combination.

(a)

(b)

(c)

Fig. 18.Failure pattern of the concrete beam under flexural strength test: (a) control, (b) only 10% RCA, (c) RCA, CR and fiber combination.

8 (R10C10F2) and Batch-11 (R30C0F2), it is shown that when the RCA and fiber is fixed, then the density for 0% and 5% rubber is almost the same but it decreased dramatically for 10% rubber because rub- ber particle has lower specific gravity which results in a low- density concrete. However, this reduction in density cannot be classified as lightweight concrete. Thus, rubberized fiber-based recycled concrete will have a reduced self-weight, which will reduce the dead load of a structure

4.7. Failure pattern of concrete

The failure pattern of the concrete cylinder under compressive strength test is presented inFig. 16. Most of the concrete cylinder showed cone and shear type failure. However, concrete cylinders with CR and fiber showed slow and gradual failure compared to the brittle failure of the control batch. CR is highly deformable while exposed to compressive stress, as it is a soft material com- pared to sand. Fiber is strong to prevent crack because it creates a strong bridging action between aggregate and cement paste and holds the aggregate particle together to resist cracking. Under splitting tensile strength test, for the control mix and the mixes having only RCA replacement, it was observed that the concrete cracked along well-defined failure planes, whereas the rubberized fiber-based concrete cylinders had irregular failure planes because fiber holds the aggregate and cement paste together to prevent the crack (Fig. 17). The failure pattern of the concrete beam under flex- ural strength test is presented inFig. 18. The control and only RCA replacement specimens split into two pieces suddenly and a sud- den drop in the load-deflection curve is observed. However, the beams with fiber, especially the combinations with 2% fiber did not break completely upon reaching the maximum load where sudden failure like other specimens was not observed. They showed more ductile behavior and the failure was gradual. It can be concluded that concrete containing fiber are more ductile with higher toughness values compared to the control of concrete mixes. Later on, those beams are broken manually to examine the failure surfaces which is shown inFig. 19. It is obvious that, at the point of failure, the fiber forms a bridge within the concrete matrix on either side of the crack.

5. Conclusions

This study thoroughly investigates the mechanical properties of rubberized concrete containing recycled aggregate concrete and polypropylene fiber. The results of this study can be summarized as follows:

Slump value decreases with an increasing replacement level of RCA, CR, and fiber. The effect of fiber on slump value is more sig- nificant than the effects of CR and RCA. The concrete mix which contains maximum RCA, CR, and fiber (30% RCA replacement,

10% CR replacement along with 2% fiber) shows the lowest slump value resulting in 76.4% decrease compared to that of the control concrete mixture.

The compressive strength initially increases at 10% RCA replace- ment level but decreases for 30% RCA. Incorporation of CR in concrete mixes results in the reduction of compressive strength compared to the control concrete, whereas the compressive strength increases with the increasing level of fiber content.

Batch 11 (30% RCA and 2% fiber content without CR) displays the highest compressive strength which is 26.9% higher than the control specimen at 28 days.

At the age of 56 days, the highest peak stress was found for 30%

RCA and 2% fiber content without any CR (44.56 MPa) and the magnitude of the peak strain and ultimate strain were 0.0023 and 0.0027, respectively. The peak stress, peak strain and ulti- mate strain values for the control concrete mix were 35 MPa, 0.0027, and 0.0029, respectively.

The Poisson’s ratio increases with an increase in the level of CR but decreases with the increase in fiber percentage.

Splitting tensile strength decreases with an increase in the level of RCA and CR in concrete mixes. However, the splitting tensile strength increases with the increase in fiber content in the con- crete mixes.

Flexural strength is found to decrease with the increasing level of RCA and CR but increases with the increasing level of fiber.

Batch 11 (30% RCA and 2% fiber content without any CR) shows the highest value which is 8.6% higher than the control concrete mixture.

Batch 13 (30% RCA, 5% CR and 2% fiber) exhibits the highest toughness and ductility compared to the other mixes.

Based on the mechanical property tests, Batch 6 (10% RCA, 5%

CR and 2% fiber) and Batch 13 (30% RCA, 5% CR and 2% fiber) show better performance compared to the control concrete mix in terms of compressive strength, split tensile strength and flexural strength.

Since the application of rubberized concrete is new in the field of construction, further research is required before this product can be established as a safe and viable alternative to natural aggregate- based concrete. Short-and long-term environmental exposure should be considered to evaluate its durability performance. Future study will investigate the effect of impact and shock absorption characteristic of rubberized concrete and thereby further develop these materials as alternatives to natural resources for green and sustainable construction.

Declaration of Competing Interest

Authors of subject manuscript have no conflict of interest to declare.

Bridging

(a) (b) (c)

Fig. 19.Beam specimen after flexural strength test (a) R10C0F0, (b) R30C0F2, (c) R30C5F2.