Strength and Durability Evaluation

Technical note

Development of rubberized geopolymer concrete: Strength and durability studies

Salmabanu Luhar

^a,⇑

, Sandeep Chaudhary

^b

, Ismail Luhar

^c

aDepartment of Civil Engineering, Malaviya National Institute of Technology, Jaipur 302017, Rajasthan, India

bDiscipline of Civil Engineering, Indian Institute of Technology Indore, Simrol, Indore 453552, India

cWater Resources Department, India

a r t i c l e i n f o

Article history:

Received 10 November 2018

Received in revised form 24 January 2019 Accepted 27 January 2019

Available online 7 February 2019

Keywords:

Waste rubber tyre Geopolymer concrete Fly ash

Compressive strength Split tensile strength Flexural strength Modulus of elasticity Pull off and abrasion resistance

a b s t r a c t

Nowadays, an exigency has gone through the roof for cementitious material. Unfortunately, the present process of production of Ordinary Portland Cement (OPC) is found associated with pessimistic impacts on environments because of an emission of immense magnitude of CO2– a primary Green House Gas (GHG).

This has, on the whole, pushed around concrete scientists and construction industries to hunt for inno- vative, sustainable, durable, user plus eco-benign and of course, cost-efficient substitutes for current bin- ders and construction materials. Quite recently, Geopolymer binders produced by a synthesis of Silica and Alumina rich pozzolanic precursor – like Fly Ash, with alkali solution as an activator through the process of Geopolymerization have become known as luminously promising option to conventional cement. This study includes not only development of rubberized Geopolymer concrete but also investigates on its strength and durability criteria. What’s more to add, at this time, natural sand is over exploited for con- struction and industry creating a scarcity for it, which in turn, resulted into a swift escalation of its cost. In the current study, the waste rubber tire fibres are employed as a partial substitute of fine aggregates. The study reveals that the application of waste rubber tire fibre as a replacement of Sand is not merely cost- effective but also user- and eco-benevolent appropriate corridor for developing rubberized Geopolymer concrete sans compromising its sustainability. This paper is a scientific approach for complying the per- formance evaluation of strength studies such as Compressive Strength, Flexural Strength, Split Tensile Strength, Modulus of elasticity; Pull off strength and durability parameter like abrasion resistance of fly ash based rubberized geopolymer concrete and compared the results with OPC rubberized concrete.

The outcomes of comparison have unearthed that Rubberized Geopolymer Concrete (RGPC) enjoys supe- riority in context of all the above parameters to its counterpart Rubberized OPC-Concrete.

Ó2019 Elsevier Ltd. All rights reserved.

1. Introduction

Global development of infrastructures and constructions to cope with the mushrooming world population is driving to a gigan- tic exigency of concrete. Concrete has been revolutionized since Romans and its use is highest as a construction material on the earth [1–3]. It requires essentially Ordinary Portland Cement (OPC) as a binder and hence, the demand for OPC is also pushy.

However, the production process for OPC is quite in the wrong as it not only takes place at elevated temperature consuming high energy in form of burning up of natural restricted mineral coal resources but also emits CO2into atmosphere bringing bad news of heating the earth and polluting of air. CO2is a primary Green

House Gas (GHG) responsible for world concerning gigantic dilemma of global warming [4–8]. The aggregates sought-after for manufacturing concrete and mortar and raw materials for OPC are also degrading day by day due to haphazard mining and no stringent rules for it in some countries. These altogether forms the core reasons to twist the arm of researchers, scientists and engineers to search for alternative construction materials that should be essentially sustainable, durable, user and eco-friendly and more significantly economically affordable.

A solution to all these predicaments is hopefully lies with inno- vative Geopolymeric construction materials. Geopolymers are an inorganic materials produced at low temperature in alkali medium through the process of Geopolymerization – a synthesis which is as analogous as geo-synthesis of natural rocks, whereby Aluminium and Silicates rich precursors react in an exothermic way with alkali activators to give rise to Geopolymers[4–9]. They not only exhibit https://doi.org/10.1016/j.conbuildmat.2019.01.185

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

⇑Corresponding author.

E-mail address:ersalmabanu.mnit@gmail.com(S. Luhar).

Construction and Building Materials 204 (2019) 740–753

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

excellent thermal, fire, and freeze thaw resistance but also con- sume six times less energy and nine times less CO2emissions. That means, no high energy and high temperature reactions are essen- tial any more through providing 21st century modern construction materials of Geopolymeric origin. Not only that, they devour diverse profuse waste, otherwise filling landfills creating health hazards, making it more economical[10–12]. Therefore, this pre- sent era researchers are inclined towards this user and eco- benign construction technology. Beforehand, crumb rubber was utilized as partial replacement of fine aggregate in geopolymer composites[13–16]. For instance; Bashar et al.[13], have produced rubberized geopolymer interlocking bricks by employing crumb rubber as fine aggregate. Moreover, Wongsa et al. [14] have reported the mechanical and thermal properties of lightweight geopolymer mortar integrating crumb rubber as fine aggregates.

Yahya et al.[15]have explored the seawater resistance of fly ash based geopolymer concrete incorporating crumb rubber as coarse aggregates. Not merely that, Park et al.[16]have also investigated the compressive strength of rubberized fly ash-based geopolymer concrete using crumb rubber as partial substitute of fine aggregate.

A variety of wastes having diverse origin will get a systematic solution for their disposal management through their incorpora- tion with Geopolymer concrete manufacturing which would other- wise contaminating air, soils, surface and sub surface waters in landfills. As we are all aware that the tyres of vehicles and carts are thrown away by automobile and transporting agencies in a titanic quantity in either open spaces filling fertile land as complex wastes responsible for different pollutions and root cause for dis- eases. An estimated 1000 million tyres reach the end of their useful lives every year and 5000 millions more are expected to be dis- carded in a regular basis by the year 2030. According to the EURO- STAT report, 1,246,447 tonnes End of life vehicles waste was generated in UK in 2016. Every year, End of life vehicles generate between 8 and 9 million tonnes of valuable waste in the Commu- nity.[6,17,18]. Roughly, 0.6 million tonnes of scrapped tyres are reported to have been in landfills. Their burning up also creates toxic smell and emissions of lethal gases with restrictions of law in some of the countries.

What’s more, their complex structure is a challenge to their bio- degradation. Consequently, their systematic consumption for good cause is only the best way for this colossal waste management.

Analogously, piles of enormous Fly ash are also plentifully accessi- ble as industrial by-products from thermal power stations. It is cementitious pozzolonic material which is useful to develop diverse Geopolymer mortars and concretes. It is found o fill fertile spaces causing great trouble to health of mankind and environ- ments too. Global coal ash production totals more than 390 million tons per year. Out of which, merely 15% is currently utilized. There- fore, its consumption in a systematic way will lend a hand to its management.

The scope of present manuscript is to develop rubberized geopolymer concrete and to comply the performance evaluation of strength studies such as Compressive Strength, Flexural Strength, Split Tensile Strength, Modulus of elasticity, Pull off strength and durability parameter like abrasion resistance of fly ash based rubberized geopolymer concrete and compared the results with OPC rubberized concrete.

2. Materials

Class-F Fly ash complying with IS 3812[19]was used as a precursor. The speci- fic surface area 428 m²/g was found.Fig. 1, 2andTable 1shows energy dispersive spectrometer (EDS), The X-ray diffraction patterns (XRD) and chemical composition of fly ash[6], respectively. The texture of fly ash demonstrated inFig. 3 [6].

River sand was used as fine aggregate to prepare fly ash based rubberized geopolymer concrete. The fineness modulus, specific gravity and water absorption were of 2.56, 2.61 and 0.5%, respectively. The river sand complies zone-II as per IS:

383-1970[20]specifications.

Napthaline sulphonate based superplasticizer was used to achieve the desired workability for geopolymer concrete.

Rubber tyre fibres acquired from the mechanical grinding unit of rubber tyre waste were employed as partly replacement of the fine aggregates, as illustrated inFig. 4. Looking to the physical attributes, rubber tyre fibres taken in use were roughly width is 2–4 mm and length is up to 22 mm[6], specific gravity of 1.09 and aspect ratio of 8–10[6,7]. The rubber tyre fibres have substituted the fine aggregate in proportion of 10%, 20% and 30% by weight. The particle size of the rub- ber tyre fibres is within Zone II, as per IS:383-1970[20], as demonstrated inFig. 5.

Chemical composition of rubber tyre fibres are also displayed inFig. 6.

Fig. 1.EDS spectrum of fly ash[6,7].

3. Manufacturing process for geopolymer concrete

The key divergence among mixing geopolymer concrete and OPC concrete is of binder material. In the first one, fly ash that is rich in silicon and aluminium oxides reacts with the alkaline solu- tion to produce geopolymeric bonds between the aggregates and other unreacted materials. In the mix design of geopolymer con- crete, the total aggregates constituted 75% by mass of the concrete, which is similar to OPC concrete, i.e. 75–80%. Fine aggregates cover 35% of the total aggregate content. The density of geopolymer con- crete was analogous to that of OPC concrete, i.e. 2500 kg/m³. The mass of fly ash, sodium silicate, and sodium hydroxide necessitated in the mix was calculated by means of parameters such as the alka- line solution to fly ash ratio. To manufacture rubberized geopoly- mer concrete, waste rubber tyre fibre particles accessible in the vicinity were employed to replace 10%, 20%, and 30% of the fine aggregates. The test specimen of rubberized geopolymer concrete is cured with heat curing while the rubberized cement concrete

cured with water curing. The steps involved in manufacturing geopolymer concrete are represented inFig. 7.

As per this study, the workability of control OPC concrete is found to have slump value of 100 mm, whereas that of control geopolymer concrete it is sticky stiff type with a slump value of 250 mm.

4. Mix design proportioning and testing parameters.

4.1. OPC concrete mix design

The proportions for the OPC concrete mix were calculated based on IS 10262-2009[21]. The volume of aggregate used in the OPC concrete was in the range 75–80% by mass. The fine aggregate was employed as 35% of the total aggregate. The mixture propor- tion of OPC concrete is listed inTable 2. Waste rubber tyre fibres were used in concrete as a partial replacement for the fine aggre- Fig. 2.XRD image of fly ash[6,7].

Table 1

Physical properties and chemical composition of fly ash^*[6,7].

Sr. No. Test Unit IS-3812 Specification[19] Fly ash

1 Fineness – Specific Surface by Blaine’s Permeability Method m²/kg >320 428

2 Lime Reactivity N/mm² >4.5 6.60

3 Moisture Content % 2 0.23

4 Autoclave Expansion % 0.8 0.024

5 Chemical Analysis

Loss on Ignition % 5 0.94

SiO2+ Al2O3+ Fe2O3 % 70 min. by mass 92.26

SiO2 % 35 min. by mass 58.88

MgO % 5 max. by mass 1.64

SO3 % 3.00 max. by mass 0.74

Na2O % 1.5 max. by mass 0.50

Total Chlorides % 0.05 max by mass 0.025

*Provided by manufacturer.

742 S. Luhar et al. / Construction and Building Materials 204 (2019) 740–753

gate. The mixture proportions of OPC concrete are analogous to those of the geopolymer concrete mixture, except for the water content (seeTable 2).

4.2. Geopolymer concrete mix design

The geopolymer concrete mix design was finalized, according to the past studies of Rangan[22], by considering parameters such as NaOH concentration, alkaline liquid ratio, alkaline liquid to fly ash ratio, quantity of aggregate, and water content. The chief dissimi- larity between mix design of geopolymer and OPC concrete is the

Fig. 3.SEM image of fly ash (magnification of 10000)[6].

Fig. 4.Rubber tyre fibres[6,7].

Fig. 5.Particle size analysis of sand and rubber fibres[6].

Fig. 6.Chemical composition of rubber fibres[6].

binder. In geopolymer concrete, a source material such as fly ash, which is rich in silica and alumina, reacts with an alkaline liquid to form geopolymeric bonds between the aggregates, geopolymer paste, and other unreacted materials that constitutes geopolymer concrete [23]. In geopolymer concrete, the aggregate taken was 75–78% of the entire mix by mass. This value is similar to that used in OPC concrete. Fine aggregate constituted 35% of the total aggre- gate. The average density of the geopolymer concrete was similar to that of OPC concrete, i.e. 2500 kg/m³[24].

4.3. Concrete mix proportioning for geopolymer concrete

The following constituents were derived from Taguchi method for mix design of each

mix:

1. Ratio of alkaline liquid to fly ash by mass: 0.4 2. Ratio of sodium silicate to sodium hydroxide: 2.5 3. Concentration of sodium hydroxide solution: 14 M 4. Admixture dosage: 2%

5. Additional water content: 5%

6. Curing temperature: 90°C 7. Curing time: 48 h 8. Rest period: 1 day

Based on this mix design process, the final mass of the con- stituents was calculated as described inTable 2.

4.4. Testing procedure

Various strength, durability, and non-destructive tests were performed as per Indian standards and ASTM codes (seeTable 3).

5. Test methods 5.1. Strength properties

After heat curing of the geopolymer specimen, strength proper- ties were appraised according to Indian standards. Comprehensive details of the testing program are described below.

Fig. 7.Steps involved in the manufacture of geopolymer concrete.

Table 2

Mix design proportion (per m³).

Mixes Fly ash kg/m³

Fine Aggregate kg/m³

Cement kg/m³

Coarse Aggregate kg/m³

NaOH kg/m³

Na2SiO3

kg/m³

Admixture kg/m³

Water Content kg/m³

Rubber Fiber

20 mm 10 mm

GP-0 446.43 656.25 – 731.25 487.5 51.02 127.55 8.93 22.32 0.0

GP-10 446.43 629.35 – 731.25 487.5 51.02 127.55 8.93 22.32 26.9

GP-20 446.43 602.55 – 731.25 487.5 51.02 127.55 8.93 22.32 53.80

GP-30 446.43 575.64 – 731.25 487.5 51.02 127.55 8.93 22.32 80.71

CC-0 – 656.25 446.43 731.25 487.5 – – 8.93 160.71 0.0

CC-10 – 629.35 446.43 731.25 487.5 – – 8.93 160.71 26.9

CC-20 – 602.55 446.43 731.25 487.5 – – 8.93 160.71 53.80

CC-30 – 575.64 446.43 731.25 487.5 – – 8.93 160.71 80.71

744 S. Luhar et al. / Construction and Building Materials 204 (2019) 740–753

5.1.1. Compressive strength test

As per IS:516-1959[25], cube specimens of size 100 mm were cast for compressive strength tests. All test specimens were fin- ished with a steel trowel after casting. The specimens were covered with sheets soon after finishing to minimize the moisture loss.

After a period of 48 h, the covers were taken and the geopoly- mer specimens stored at room temperature until the test. The OPC specimens were prepared and cured as conventional method.

Compressive strength tests were carried out after 3, 7, 28, 90, and 365 days. An automated compression testing machine (CTM) was used to test all specimens, as shown inFig. 8. Specimens were placed in the CTM such that the load was applied gradually at a rate of 140 kg/cm²/min on smooth surfaces until the ultimate load resistance was reached. The average of three specimen values was considered to be the compressive strength. The compressive strength was calculated as a ratio of ultimate load resistance to the cross-sectional area of the specimen.

Compressive Strength N=mm²

¼P A where

P = Failure load of cube (kN) A = Area of cube (100100) (mm²)

5.1.2. Split tensile strength test

This test was first developed in Brazil in 1943. The split tensile strength was measured as per IS:5816-1999 [26]. This test was used to determine the tensile strength of geopolymer concrete.

Strength was measured 28, 90, and 365 days after the specimens were cast. Cylindrical specimens of 150 mm diameter and 300 mm height were used for the test. The test was performed by placing a cylindrical specimen horizontally between load sur- faces in the CTM. A gradual load was applied along the longer face of the specimen, as demonstrated in Fig. 9, until the specimen failed. The average of three tests was considered to be the split ten- sile strength of that particular specimen type. The split tensile strength was calculated as:

Split Tensile Strength N=mm²

¼ 2P

p

^LD

where

P = Failure load of cylinder (kN) L = Height of Specimen (300 mm) d = Diameter of Specimen (150 mm)

Fig. 8.Compression testing machine. Fig. 9.Testing of split tensile strength.

Table 3 Type of test.

Properties Test Testing Age (days) Code Size of test specimen

Strength Properties Compressive Strength 3, 7, 28, 90, 365 IS:516-1959 100100100 mm size cubes

Split Tensile Strength 28, 90, 365 IS:5816-1999 150 mm diameter and 300 mm height

Flexural strength 28, 90, 365 IS: 516-1959 100100500 mm beams

Pull-off Strength 28 ASTM 1583-04 –

Modulus of Elasticity 28 IS:516-1959 150 mm diameter and 300 mm height

Durability Properties Abrasion Resistance 28 IS:1237-1980 100100100 mm size cubes

5.1.3. Flexural strength test

Flexural strength tests were performed as per IS:516-1959[25]

specifications. The three-point loading method was adopted, whereby the bearing surfaces of the supporting and loading rollers were cleaned and loose sand or other material was removed from the surface of the specimen. Specimens of size 100100500 mm were used for this test, as per IS:516-1959 [25]specifications. The test specimen beam was placed in such a way that the load was applied on the uppermost surface of the beam along two lines 13.3 cm apart. The axis of the specimen was aligned with the axis of the loading device. A gradual load was applied at a rate of 180 kg/cm²/min until the specimen failed.

The average of three specimens was considered as the flexural strength of that concrete type. The test setup used for this test is shown in Fig. 10. The flexural strength of the beam specimens was calculated as:

Flexural Strength N=mm²

¼ PL BD² where,

P = Failure Load of Beam L = Span of Beam (400 mm) b = Width of Beam (100 mm) d = Depth of Specimen (100 mm) 5.1.4. Modulus of elasticity test

The slope of the stress-strain curve with a proportional limit of the material is known as the modulus of elasticity. Tests were per- formed at CTAE, Udaipur as per IS:516-1959[25] specifications.

Cylindrical specimens of 150 mm diameter and 300 mm height were used for this test. A cylindrical specimen was placed verti- cally between two frames of a compressometer, as shown in Fig. 11. Spacers were used to hold the frames in the proper posi- tion. A dial gauge was attached to the compressometer to show the change in length of the specimen when subjected to a compres- sive load. The compressive load was applied gradually to the test specimen at a rate of 140 kg/cm²/min over three loading- unloading cycles. The average of three cycles was used to calculate the modulus of elasticity according to the following formula.

Es¼ ð

r

²

r

1Þ ð€0:000050Þ

where,

r

2= stress corresponding to 40% of ultimate load,

r

1= stress corresponding to a longitudinal strain, and

e

2= longitudinal strain produced by stress

r

2.

5.1.5. Pull-off strength test

The pull-off test determines the tensile strength of concrete near to the prepared surface or cover zone of the concrete. In this test, 50 mm diameter steel discs were bonded to the concrete sur- face with the help of epoxy adhesive material one day prior to test- ing. The test setup is illustrated inFig. 12. The force required to pull off the concrete surface was recorded as the pull-off load. The pull- off strength was calculated as the pull-off load per unit area. The average of three specimens was considered to be the pull-off strength of the specimen.

5.1.6. Abrasion resistance test

This test was conducted as per IS:1237-1980[27]specifications on 100100100 mm specimens. Abrasion resistance refers to the depth of wear. Fig. 13 shows the arrangement of this test.

Firstly, specimens were dried at 110°± 5°C for 24 h in a drying chamber and weighed to the nearest 0.1 g to give W1. Twenty g of abrasive aluminium powder was evenly spread over the grind- ing path of the disc on an abrasion testing machine. The specimen was fixed in a holding device under a 300 N load. The grinding disc

Fig. 10.Testing of flexural strength.

Fig. 11.Testing the modulus of elasticity.

Fig. 12.Pull-off test setup.

746 S. Luhar et al. / Construction and Building Materials 204 (2019) 740–753

was then put in motion at 30 revolutions per minute and the abra- sive powder was continuously spread over the grinding path. After every 22 revolutions, the disc was stopped, the abrasive powder was removed from the grinding path, and a fresh 20 g of abrasive powder was placed in the path. At every 22 revolutions, the spec- imen was turned about the vertical axis by 90°. This process was repeated nine times, thereby giving a total of 220 revolutions. After the abrasion test, the specimen was reweighed to the nearest 0.1 g.

Thus, the depth of wear was determined as T¼W1W2XV1

W1XA

where

T = Average loss in thickness in mm;

W1= the initial weight of the specimen in grams;

W2= the mass of specimen after abrasion in grams;

V1= the initial volume of the specimens in mm³; A = the surface area of the specimen in mm² 6. Results and discussion

6.1. Compressive strength test

The compressive strength of geopolymer concrete measured at 3, 7, 28, 90, and 365 days is shown inFig. 14. It can be seen that, as the percentage of waste rubber tyre increases from 0 to 30%, the compressive strength decreases at all ages. Decline in compressive strength is ascribed to lesser stiffness of the alternative material as compared to the adjacent fine aggregate. In fact, as the rubber fiber contents increased, the voids are generated due to lesser bonding between rubber fiber particles at higher content, which result in lower compressive strength. Parallel results were depicted from past studies also[28,29].

In geopolymer concrete, a fast geopolymerization process takes place due to a chemical reaction between the alkaline solution and source material, resulting in 95% compressive strength gain in only 7 days[8,9,30]. After 28 days, the compressive strength of geopoly- mer concrete varies from 30 to 54 MPa depending on the rubber fibre content. The compressive strength depends on the geopoly- meric mechanism developed in the geopolymer paste. After 365 days, the compressive strength had only increased marginally because of the speed of the polymerization process. This is because of the chemical reduction of the geopolymer gel with age and the development of a crystalline structure [31,32]. The compressive strength of geopolymer concrete depends on factors such as NaOH concentration, alkaline liquid ratio, curing temperature, and aggre- gate content. From past studies, it is clear that increasing each of these factors will increase the compressive strength of the geopolymer concrete[32,33].

Fig. 13.Abrasion testing machine.

Fig. 14.Compressive strength of geopolymer concrete.

Fig. 15shows the compressive strength of OPC concrete mea- sured at 28, 90, and 365 days. The compressive strength of OPC concrete depends on the hydration mechanism of cement paste [34]. The hydration process is a long, continuous process whereby the pores of the concrete gradually fill, resulting in strength gains over the course of a year [34]. The compressive strength of OPC concrete decreases when waste rubber fibres are increases. The compressive strength of OPC concrete is less than that of geopoly- mer concrete. Drastically diminution in compressive strength was also monitored with an enhancement in the content of rubber fibres as reported in previous studies[35–40,49].

6.2. Split tensile strength test

Figs. 16 and 17show the split tensile strength of geopolymer concrete and OPC concrete after 28, 90, and 365 days. The split ten- sile strength of all mixes is ranges from 5.34 to 5.49 MPa after 365 days. The geopolymer concrete exhibits higher tensile strength than OPC concrete because of the good bonding between the geopolymer paste and aggregate. Similar observations have been reported in past studies [31,32,41]. The highest split tensile strength was found in the 30% rubber fibre mix after 365 days, and the lowest tensile strength was found in the control

Fig. 15.Compressive strength of OPC concrete.

Fig. 16.Split tensile strength of geopolymer concrete.

748 S. Luhar et al. / Construction and Building Materials 204 (2019) 740–753

geopolymer concrete after 28 days. A gradual increase in split ten- sile strength can be observed as the rubber fibre content increases from 0 to 30%. Similar results have been reported in previous study [24]. In geopolymer concrete, there is a high level of geopolymeric bonding between the geopolymer paste and aggregate; therefore, during testing, when the cylinder was broken in half, none of the aggregate was pulled out, unlike for the OPC concrete. This is a result of the chemical bonding between the alkaline liquid and aggregate[42].

6.3. Flexural strength test

Figs. 18 and 19 show the average flexural strength of the geopolymer concrete and OPC concrete. The flexural strength of

the geopolymer concrete varies from 6.45 to 9.97 MPa, whereas that for OPC concrete varies from 5.35 to 6.86 MPa. The flexural strength increases with age in all mixtures. This proves that the flexural strength of geopolymer concrete is higher than that of OPC concrete. Similar evidence has been reported in previous research[28,43]. The tension properties of geopolymer concrete, such as flexural and tensile strength, are superior to those of OPC concrete because of the better bonding between the geopolymeric paste and aggregate. The flexural strength also increases with per- centage of rubber fibres for both OPC and geopolymer concrete.

Flexural strength of rubberized geopolymer concrete is influ- enced by the inclusion of waste rubber tire. The increase in flexural strength is owing to fibres which provide a better bridge between propagated cracks. Similar observations announced in the previous Fig. 17.Split tensile strength of OPC concrete.

Fig. 18.Flexural strength of geopolymer concrete.

literature that flexural strength is augmented with the boost of rubber content[44–49].

6.4. Modulus of elasticity test

The average modulus of elasticity of the geopolymer concrete and OPC concrete was measured after 28 days. The modulus of elasticity of the geopolymer concrete varied from 20 to 31.5 GPa, and that for OPC concrete ranged from 18 to 27.5 GPa (see Fig. 20). It can be seen that, in all the mixes, the modulus of elas- ticity decreases as the rubber fibre content increases.

The modulus of elasticity of geopolymer concrete depends on the geopolymeric microstructure, and is independent of the aggre- gate and source materials. An increase in the rubber fibre content decreases the homogeneity of the geopolymer and OPC concrete,

resulting in a decrease in the modulus of elasticity. The outcomes revels that rubberized concrete has less rigidity as compared to the control concrete, therefore modulus of elasticity of concrete fall with the boost of the contents of rubber fiber.

Similar observations have been made in previous studies[24].

The modulus of elasticity of the geopolymer concrete and OPC con- crete decrease by 36.34% and34.54%, respectively, as the rubber fibre content increases from 0 to 30%.

6.5. Pull-off test

The average pull-off strength was measured after 28 days, and the results are shown in Fig. 21. As the compressive strength increases, the pull-off strength increases. It can be seen that the pull-off strength decreases when rubber fbres are introduced to Fig. 19.Flexural strength of OPC concrete.

Fig. 20.Modulus of elasticity of OPC and geopolymer concrete.

750 S. Luhar et al. / Construction and Building Materials 204 (2019) 740–753

the mix. The results show geopolymer concrete has better pull-off strength than OPC concrete. Both rubber fibre geopolymer and OPC concrete exhibit poor pull-off strength performance, because there is less bonding between the paste and aggregate, which results in a weaker surface layer than in the control geopolymer and control OPC concrete. The pull-off strength of geopolymer concrete decreases from 13.46% to 21.15% and then by 32.69% as the rubber fibre content increases to 10%, 20%, and30%, respectively; the pull- off strength of the OPC concrete reduces from 2.38% to21.42% and then 26.19%, respectively.

6.6. Abrasion resistance test

The abrasion resistance test was carried out at 28 days age according to IS 1237-2009[27]. Abrasion resistance was measured

in terms of depth of wear.Fig. 22shows that the abrasion resis- tance increases as more rubber fibres are added to the mix. As per IS 1237-2009[27], the permissible depth of wear for general purpose tiles and heavy-duty floor tiles is 4.0 mm and 2.5 mm, respectively. From the figure, it is evident that the maximum depth of wear occurs when there are no rubber fibres in the mix. In all mixes, the depth of wear is within permissible limits. It can be con- cluded that rubber tyre fibres could be used with fly ash or cement to make general purpose and heavy-duty floor tiles.

7. Conclusion

From the results reported in this Research, the following con- clusions can be drawn:

Fig. 21.Pull-off strength of OPC and geopolymer concrete.

Fig. 22.Depth of wear for OPC and geopolymer concrete.

1. As the percentage of waste rubber fibres increases, the com- pressive strength decreases at all ages. Geopolymer concrete, gain 95% compressive strength only in 7 days. The compressive strength of OPC concrete is less than that of geopolymer concrete.

2. The geopolymer concrete exhibits higher tensile strength than OPC concrete because of the good bonding between the geopolymer paste and aggregate.

3. The tension properties of rubberized geopolymer concrete, such as flexural and tensile strength, are increased as the percentage of rubber fibres increased. The maximum Flexural strength was observed in 30% replacement of sand by fibres. This is because of the fibres which provide a better bridge between propagated cracks.

4. It can be seen that, in all the mixes, the modulus of elasticity decreases as the rubber fibre content increases. The modulus of elasticity of the geopolymer concrete and OPC concrete decreased by 36.34% and 34.54%, respectively, as the rubber fibre content increases from 0 to 30%.

5. It can be seen that the pull off strength decreases when rubber fibres are introduced to the mix. The results show geopolymer concrete has better pull off strength than OPC concrete.

6. It is evident that the maximum depth of wear occurs when there are no rubber fibres in the mix. In all mixes, the depth of wear is within permissible limits. It can be concluded that rubber tyre fibres could be used with fly ash or cement to make general purpose and heavy-duty floor tiles.

Conflict of interest

None.

Acknowledgments

The authors gratefully acknowledge the financial support for this research by the Department of Science and Technology, New Delhi, under the Women Scientist scheme (sanction number SR/

WOS-A/ET-1016/ 2015) and Material Research Centre (MRC), Malaviya National Institute of Technology, Jaipur for their support in conducting X-ray diffraction, FTIR, TGA/DTA and SEM and XRF analyses respectively.

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