EXPERIMENTAL STUDY ON THE BEHAVIOUR OF TORSION IN GEOPOLYMER CONCRETE BEAMS WITH STEEL FIBERS

Manoj Kumar K^1* and Ramasubramani R²

¹M. Tech, Department of Civil Engineering, SRM University, Kattankulathur, Chennai, India

²Assistant Professor, Department of Civil Engineering, SRM University, Kattankulathur, Chennai, India

*Corresponding Author:

Manoj Kumar K
M. Tech, Department of Civil Engineering
SRM University, Kattankulathur, Chennai, India
E-mail: Kakarlamanoj94@gmail.com

Received Date: 17 June, 2017 Accepted Date: 22 August, 2017

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Abstract

Geopolymer concrete is new emerging trend of concrete which is an alternative for Ordinary Portland Cement (OPC). This alumino-silica binder in geopolymer concrete provides high resistance to chemical attack, elimination of water curing, rapid strength gain, and is an Ecofriendly material. Comparative study is carried out between geopolymer and conventional concrete and evaluated in terms of torsion. In this experiment research, four geopolymer beams and four conventional concrete beams with and without fibers have been casted and tested. This study carries out the comparison of conventional concrete and geopolymer concrete precracking, post-cracking behavior and torsional resistance. The ultimate torque resistance of geopolymer concrete is higher than conventional concrete. It is found that steel fibers volume of 1% increases torsional strength and prevents sudden brittle failure. It shows cracking torsional resistanceof geopolymer concrete (GPC) steel fiber beams are higher than conventional beam.

Keywords

Geopolymer concrete, Torsion, Reinforcement, Ductility, Cracks

Introduction

General

Cement is most commonly used a binding material in the construction process. Demand and production for these construction materials have been increasing day by day. Portland cement concrete is highly energy intensive, and it causes large amount emission of CO2 which routes to global warming. This leads to an idea of developing alternative eco-friendly material by using industrial waste materials like ground granulated blast slag (GGBS) and fly ash to prepare GPC. Alkaline activator solution (AAS) activates the alumina and silicon ions to form Geopolymer Binder from the source materials, this binder binds aggregates to produce geopolymer concrete (GPC). It has high compressive strength, stiffness, ductility and corrosion resistance to acid and Sulphate attacks. However, studies have considered that GPC has good potential which can be considered as an alternative to ordinary Portland cement concretes (Menon and Pillai, 2014; Madheswaran, et al., 2015).

An addition of fibers in conventional concrete has become a new tradition in civil engineering constructions to meet their own structural and durability requirements. By adding steel fibers into concrete it significantly increases mechanical properties and also controls crack propagation in post cracking stage which increases ductility and torsional strength. Torsion plays a very vital role in the structural behaviour; it occurs appreciably in many structural elements like spandrel beams, main girders in bridges, eccentrically loaded beams. Torsion has so far given secondary consideration in the design of structures. Torsion usually associates with bending moment and shear force. From previous studies reinforced concrete (RC) beams are found to be deficient in torsional capacity and hence in need of strengthening of materials by incorporating steel fibers to improve tensile properties of material. Thus a clear experimental investigation has been made to improve the torsional capacities of the beam by different percentage of steel reinforcement, change in spacing of stirrups, with and without the addition of fibers are tested and explained in detail (Chalioris and Karayannis, 2009; Patel and Sangle, 2016; Mahadik, 2014).

Experimental Investigation

Material Specifications

Ordinary Portland cement of 53 grade is used for the experimental investigation, satisfying the standard requirements of IS12269-1987. River sand passing through the sieve of 4.75 mm confining to zone-iii according to IS 383-1970 has been used in the experiment. Coarse aggregate used in this experiment are of a 20 mm size and tested as per IS 383-1970 specifications.

In the preparation of geopolymer concrete, cement is replaced by fly ash and GGBS completely in different proportions. In this study 50% of GGBS, 50% of fly ash and AAS is used to form geopolymer concrete meeting the requirement of IS 12089:1987. GGBS cement concrete has a higher ultimate strength than concrete made with Portland cement. Concrete made with GGBS continues to gain strength over time. Alkaline activator solution is a combination of solutions of hydroxides and alkali silicates, with distilled water. The main objective of AAS is to activate the geopolymer source materials such as GGBS and Fly ash. The mix proportions for GPC can be done in conventional cement concretes mixer – such as pan mixer. Crimped type of steel fibers is used in this experiment having a diameter of 0.5 mm and length of 30 mm. Crimped steel fibers have good bonding with concrete and show good elastic nature (Prashant, 2015; Rajamane, Basics of Geopolymer Technology; IS 456: 2000, 2000; IS 10262-2009; IS 3812-1981).

Mix proportions

The In this experimental study, mix design is calculated for both conventional and geopolymer concrete for an M40 grade as per IS10262-2009 and ACI 211 respectively, while mix proportion for binding in geopolymer concrete is taken as 50% of fly ash and 50%of GGBS. GPC mix was made with several trails. The details of mix ratios for both conventional concrete and geopolymer concrete are given below in Table 1.

Table 1. Mix proportions for CC and GPC

Specimen and reinforcement details

Eight Beams are casted as per the Codal provisions of IS 456-2011, having dimensions of 150 x 230 mm and which is having effective span of 1500 mm. The beam is designed to fail in torsion. Thermo mechanically treated bars (TMT) of Fe 500 Grade and characteristic compressive strength of 40 MPa were used. Reinforcement details of specimens are given in the Table 2 (Figures. 1-4).

Figure 1: Plan view of beam.

Figure 2: Model-1.

Figure 3: Model-2.

Figure 4: Cantilever details.

Table 2. Reinforcement details

Experimental set-up

All the beams were whitewashed to aid the visual crack pattern on the beam. Testing was conducted on a load frame having capacity of 40 tonnes. Hydraulic jack having the capacity of 25 tonnes load capacity is placed on Indian standard steel beam (ISMB) 175 which is placed diagonally on top of the cantilever portions. Deflections were measured using dial gauges which are fixed at the bottom of cantilever portions of the beams, with a least count of 0.01 mm.

The load was constantly applied through hydraulic jack. ISMB steel section placed below the hydraulic jack transfers the loads equally to its edges. Beams were tested at a constant increase in the rate of static loading till it reaches the ultimate torque. Testing of beams was explained in detail, and schematic diagram of the experimental setup is shown in (Figure. 5). The testing arrangement and the crack pattern of a specimen are shown in (Figure. 6) and (Figure. 7) respectively. The tested specimens of both CC and GPC beams are shown in (Figure. 8).

Figure 5: 3D view of experimental setup.

Figure 6: Test arrangements of specimen (a) and (b).

Figure 7: Crack pattern on specimen.

Figure 8: Tested specimens (a) CC-Beams (b) GPC beams.

Tests on hardened concrete

Cubes and Cylinders are casted having dimensions of 150 × 150 × 150 mm and 150 × 300 mm respectively. They are tested at particular interval of time to obtain the compressive strength and split tensile strength; results are listed in the given Table 3 below and (Figure. 7).

Table 3. Compression and split tensile test results

Results and Discussion

In Before the discussion of experimental results, the author feels that it is necessary to know the facts of torsional problems. We know that longitudinal steel or transverse steel all cannot increase the ultimate torque of a beam. Torsional strength of the beam mainly depends upon two contributions, i.e., concrete contribution and steel contribution, so to change these traditional methods conventional concrete is completely replaced with geopolymer concrete and in steel contribution, change in the percentage of reinforcement and steel fibers were added to avoid sudden brittle failure and the experimental results were compared with control beams.

All the eight specimens were tested, and the experimental results of conventional concrete and geopolymer concrete are compared, graphs are drawn for Torque ( T), Twist per unit length (V). It was observed that beams of geopolymer concrete show good strength than conventional concrete beams and whereas beams with fibers show an increase in torsional strength than beams without fibers. It can be found that torsional strength performances of steel fibers specimens clearly increases due to the addition of 1% volume of fibers compared to control specimens (Table 4) and (Figures. 9 and 10).

Figure 9: Torque versus twist for CC1, CC2 and GPC1, GPC2.

Figure 10: Torque versus twist for CC3, CC4 and GPC3, GPC4.

Table 4. Torsional strength at cracking and at ultimate for all eight specimens

Cracking characteristics

Crack patterns are observed while testing. Initial crack is observed at either of the wider faces of the beam. As the applied torque increases spiral, cracks are observed at 450 and spread over the test region.

Load at which the primary crack is determined is called Cracking load. The torque versus twist values are calculated and shown in the Table 4. It was observed that beams with steel fibers have less crack pattern when compared to beams without fibers.

Pre-cracking behaviour and analysis

For both CC and GPC beams, the torsional strength and the behaviour of beams was resisted by concrete in both CC and GPC beams. The measured values of torque and twist are shown in Table 5. The graphical representation of torque versus twist for both CC and GPC beams are shown in (Figures. 5 and 6) respectively. From the obtained graphs it shows that, before initial cracking starts, both cases of torque versus twist are linear, which shows that both CC and GPC beams were an inelastic condition. After cracking, the curves show a non-linearity. However, the behaviours of CC beams are similar to GPC beams as shown by graphical representation.

Table 5. Experimental stiffness comparison

Table 6. Comparison of experimental results with theoretical prediction

Post cracking behaviour and analysis

After the initial crack starts, both cases show that torque versus twist is non-linear. It can be observed that, after cracking, it formed a truss action in which reinforcement’s act as a tensile link and concrete as compression diagonal. Since cracks are formed inclined and propagated nearly at 450, it can be confirmed that applied torque develop pure torsion and not associated with any bending and shear effects. Also, it is interesting to know that ultimate torque resistance of GPC beams was more compared to CC beams while ultimate torque resistance of conventional beams and GPC beams with steel fibers show more resistance. This may be due to good materialistic properties of geopolymer concrete, and in the case of fibrous beams extends its angle of twist in turn resistance to its ultimate torque.

Torsional reinforcement

Even Torsional reinforcement plays a vital role in the torsional resistance capacity of the beam. Minimum torsional reinforcement in beams is necessary to ensure that the beam does not fail at cracking. Hsu suggested that to avoid the sudden failure, minimum volumetric torsional reinforcement should be provided greater than 1%, and specimen should fail at torque more than 1.2 times the cracking loading. In this study, it is provided greater than 1% for all the beams. All the beams failed at torque more than 1.2 times the cracking loading. Stiffness comparison (Tables 5 and 6).

Torsional resistance

The The total volumetric reinforcement (%) ratios provided are 1.15%, 2.12% for CC1, CC2, GPC1, GPC2 and CC3, CC4, GPC3, GPC4 respectively. When Compared to CC1 and CC2 torsional resistance of CC2 increases about 12% due to the addition of fibers. Likewise, when compared to GPC1 and GPC2 torsional resistance of GPC2 increases by 16%. Similarly, when compared to CC3 and CC4 torsional resistance of CC4 increases by 12%. In the same way when compared to GPC3 and GPC4 torsional resistance of GPC4 increases by 16%.

Conclusion

Experimental studies on the torsional behaviour of OPC and GPC beams are carried out. The parameters investigated include torsional resistance, torque versus twist of reinforced OPC and GPC beams. Comparative studies have been carried out with the addition of steel fibers.

Based on the experimental investigation when compared between OPC and GPC beams it was observed that,

1. The torque versus twist characteristics for GPC and OPC beams were found to be comparable and GPC beams have increased torsional resistance than OPC beams.

2. From this experimental investigation, it was observed that beams should have minimum torsional reinforcement and concrete materials should have good mechanical properties to show good torsional resistance.

3. By incorporating steel fibers improved mechanical properties and reduction in the crack growth can be achieved. Crack resistance for beams with fibers is also increased.

4. Considering a good improvement in flexural strength and ductile behaviour of concrete beams it is worthy to use steel fibers in high strength concrete mix.

Future Scope

GPC has wider applications which even involve a construction of marine structures and precast units. GPC is more eco-friendly and provides a faster mode of construction when compared to OPC. Considering the environmental issues involved in the manufacturing of OPC, Geopolymer concrete can be one of the suggestible alternatives for replacing OPC.

References

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2. IS 456: 2000. (2000). Indian standard plain and reinforced concrete – code of practice, BIS, New Delhi.
3. IS 10262-2009. Guide lines for concrete Mix Design.
4. IS 3812-1981. Specifications for fly ash.
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