International Journal of Theoretical and Applied Mechanics.

ISSN 0973-6085 Volume 12, Number 4 (2017) pp. 845-856

© Research India Publications

http://www.ripublication.com

Flexural Behavior Of Fly Ash-Slag Based Reinforced

Geopolymer Concrete Slabs Cured At Room

Temperature

Mahantesh NB 1, Amarnath. K2, Raghuprasad. B K3

1 Alliance University,Bangalore 2 & 3 The Oxford College of Engineering, Bangalore

Abstract

The Alkali activated low calcium fly ash & slag based reinforced structural

components are being popularized in the most consumed sector of concrete

construction industry. Slag by providing early age strength to geopolymer

concrete(GPC) fulfills the actual need of consumers of cast in situ

applications. The present research work outlines the efficacy of slag in

reinforced geopolymer flexural components. Reinforced GPC slabs using low

calcium fly ash & slag at 70:30 proportion, reinforced with HYSD steel bars

and cured at room temperature are tested by applying monotonically

increasing transverse loads. Slabs were subjected to central point load (CPL)

and UDL conditions. The computed and measured load vs. displacement

curves are found to be in close agreement. The flexural behavior of reinforced

GPC slabs follow distinct stages similar to OPC based RCC components.

Keywords:Geopolymer concrete, flexural behavior, Fly ash, GGBS, alkaline

solution, crack width.

1.0 PREVIOUS RESEARCH WORK.

From the early research groups of 21st century it was observed that low calcium fly

ash based geopolymer concrete (GPC) develops strength in proportion to the amount

of heat or steam provided during its early stage of polymerization. Although fly ash

based GPC has appreciable structural skills but heat/steam curing requirement has

become the major limiting factor in further developing the in-situ applications of

reinforced geopolymer concrete structural elements. Concluding investigations from

Vijay Rangan et al[3],[4], [18] & Radhakrishna[2] it is observed that fly ash - slag

846 Mahantesh NB, Amarnath.K, Raghuprasad.B K

based GPC develops significant early strength and very good structural skills at

ambient curing which are superior to OPC based RCC applications and thus opening a

broader way for in situ applications of Reinforced Geopolymer Concrete

2.0 MATERIAS AND MIX PROPORTIONS USED.

Fly ash used in this work collected from Raichur thermal power plant in Karnataka

has sp.gr 2.15,Silicon dioxide (SiO2) 61.98%,Aluminium oxide (Al2O3)

26.06%,calcium oxide(Cao) 3.05%. Slag is procured from Jindal Steel Plant Bellary-

Karnataka has sp.gr 2.62, Silicon dioxide (SiO2) 33.88%,Aluminium oxide (Al2O3)

18.02%,calcium oxide(Cao) 34.98%. M-Sand ,crushed from granite stone, having

Sp.gr 2.45 , Fineness Modulus (F.M) 2.70 & River

Sand of sand stone origin having F.M 2.62 confirming to Zone III of IS 383-1970 are

used. Coarse aggregates of granite origin of sizes 20mm,12.5mm & 4.75mm having

water absorption 0.5% by weight at room temperature (16 to 28 degree).

Sodium hydroxide of 97% purity and sodium silicates with Na2O=14.7%,

SiO2=29.412%, water = 59.9% by mass are used to form Alkaline Activator Solution

using ratio Na2Sio3/NaoH = 2.5 . Alkaline Activator Solution (AAS) is prepared 24

hours before mixing of concrete. Molarity of the NaoH solution (SHS) is determined

by the relation M=0.25PD , where M is the molarity of NaoH solution , P is

concentration of sodium , D is the density of SHS for P. Therefore to get 1liter of SHS

of 8,10 and 12 Molarity (255+745) ,(306+694) and (354+646) of ( Sodium

Hydroxide pallets in gms + water in gms) are added respectively. Sulphonated

naphthalene based super plasticizer i.e Conplast SP430 DIS distributed by FOSROC

chemicals is used. Reinforcement is of Fe500 grades.

3.0 SPECIMEN DETAILS AND LOAD TESTING:

The size ,reinforcement details ,compressive strength of GPC of slab concrete and

other relevant details of specimens are listed in Table 2 Three types of slabs with

aspect ratios 1, 1.5, and 2 were cast with 10 mm clear cover, compacted and cured at

room temperature 16 degrees at night and 28 degrees Celsius during peak day time.

Maximum temperature outside the room was 36 degrees during peak day time and 24

degrees celsius at night.

The specimen were tested under point load and udl system generated through 50Mton

self straining loading frame with electrically operated hydraulic jack. One

displacement transducer (LVDT) connected to multichannel digital automatic data

logger was fixed at the center bottom of the slab to measure the maximum deflection.

Digital load measuring system comprises of electronic load cell connected to

Flexural Behavior Of Fly Ash-Slag Based Reinforced Geopolymer Concrete… 847

automatic data logger. The automatic data logger is connected to computer interface

system with advanced menu driven software enabling automatic measuring, recording

and storing of load Vs displacement out puts in to computer.

Table 1: Mix Design

SN Materials Weight

kg Specifications

1 Fly ash 276 70% of total fly ash

2 GGBS (30%) 120 30% of total fly ash

3 20mm to 12mm size

CA 451 35% of total CA

4 12mmto 4.75mm CA 451 35% of total CA

5 4.75mm & down sizes 389 30% of total CA

6 River sand 111 20% of total FA

7 M-sand 444 80% of total FA

8 Sodium Hydroxide of

8M 45 97% purity (26.20%)

9 Sodium

Silicate(Na2sio3) 113 Na2O14.7%,SiO229.4%

10 Super plasticizer 3.6 SP430DIS (1.5%)

11 Extra water 4.0 Potable water

NOTATIONS; FA : Fine Aggregates , CA: Coarse Aggregates

Fig 1 : Typical Reinforcement Details And Stress Strain Diagrams

L=975 mm

B=650 mm

08mmdia - 04# 08mmdia - 06#

b

D

Section A-A

N A

Strain Diagram

Xu

d-Xu

d

ecu

esu

Stress Diagram

ecu :strain at top edge of concrete

Stress Strain Diagram at 0.67fck- Non Linear Cracked Phase

d : effective depth

d

0.67fck

T=fsu.Ast

fsu : Ultimate tensile stress in steel

C=0.67fck.b.Xu

0.416 Xu

848 Mahantesh NB, Amarnath.K, Raghuprasad.B K

4.0 FLEXURAL STRESS STRAIN BEHAVIOR OF CONCRETE AND STEEL

Among the most widely used and easily available aggregates , Granite holds the first

place having Young’s Modulus varying from 10 – 70 GPa - far above the values of

sand stone rocks i.e 1- 20 GPa . When Manufactured sand of granite origin as fine

aggregate mixed with coarse aggregates of granite origin , with fly ash : slag at 70:30

the resulting composite of granite based geopolymer concrete develops better

structural properties. The relations between compressive strength and flexural

strength, modulus of elasticity i.e., for fly ash:slab at 70:30 closely follow the

expressions fcr=0.7√fck [2] and Ec= 5000√fck after 28 days of ambient

curing.[18],[19]

All slabs are analyzed using conventional elastic theory for the applied loads and

provided boundary condition using geometrical & material properties like

compressive strength, steel strength etc. listed in Table 2. The flexural stress strain

relations of top compressive concrete follow distinct stages similar to OPC based

RCC flexural components. The basic flexural compressive stress strain relations

proposed by popovics with modified curve fitting factor suggested by Ganesan [5]

used to predict the flexural behavior of compression concrete. The failure loads

corresponding to first appearance of tension crack in concrete, yielding of tension

steel and peak compressive stress corresponding to 0.67fck and 0.85fck are

determined

Typical Slab analysis and structural design output after numerical computations are

described in following graphs using load deflection curves, stress and strains

developed at top edge of compressive concrete, stress and strains developed in steel .

Fig 2 a) Stress Vs Strain in Compressive

Concrete-Slab No.6

Fig 2 b) Stress Vs Strain in Tension steel -

Slab No. 6

Flexural Behavior Of Fly Ash-Slag Based Reinforced Geopolymer Concrete… 849

5.0 STRENGTH AND DEFORMATION BEHAVIOR

The measured Load vs Deflection relations at center bottom of slabs are represented

in figure (3) to (8).The flexural stiffness of RGPC decreases with the gradual increase

in applied loads similar to OPC based RCC flexural elements. Till the appearance of

first crack in tension concrete, whole section including reinforcements are effective

in producing linear elastic behavior with noticeably low profile deflections.

The appearance of first crack and further loading will noticeably reduce the effective

depth and will increase the crack depth and width making concrete in tension less

effective and therefore tension resistance offered by concrete is ignored. However the

membrane action developed is taken into account by considering effective moment of

inertia recommended by Indian RC designer.

The composite continues to be in linear elastic stage till yielding of tension steel.

Numerical computations indicate significant shift in neutral axis once the tension steel

yields. The deflections based on new effective section indicate the loss of flexural

stiffness similar to OPC based sections. The close agreement between measured and

calculated deflections indicate a healthy bond strength between reinforcement and

tension concrete justifying the negligence of tensile strength in concrete .

The specimen is said to have failed structurally when compression concrete reaches

0.67fck while it has already crossed yield strength of tension steel as all slabs were

under reinforced. Further loading at post failure stage develop significant deflections

which are slightly deviating from calculated ones based on effective moment of

inertia.

The numerical computations for flexural deflections do not include shear deflections

which are estimated to be less than 0.5% of flexural deflections.

All the specimens behaved to produce strain hardening flexural deflections.

Comparison of load Vs deflection for CPL and UDL from Fig (3) to Fig (8) , it is

observed that the calculated deflections for Central Point Loads differ more than the

measured ones while for UDL there is marginal difference. Thus confirming the

sensitiveness of Geopolymer Concrete for point loads

6.0 DUCTILITY INDICES

The performance of structural elements is well appreciated , when their ability to

absorb and dissipate energy by post elastic deformations subjected to several cycles of

loading are naturally imbedded at low cost. Reinforced Geopolymer Concrete

(RGPC) develop significant ductility along with steel reinforcement.

850 Mahantesh NB, Amarnath.K, Raghuprasad.B K

Ductility indices of flexurally deflected RGPC elements are compared with calculated

ones. The Ductility Index(calculated ) = ∆u / ∆y where ∆u & ∆y are measured

deflections corresponding to computed yield load Fy & ultimate load Fu.

Similarly Ductility Index (measured ) = ∆um/ ∆y , ∆um is the maximum deflection the

component under maximum applied load Fum. Ductility Index(calculated) represents

the minimum ductility the GPC will develop and Ductility Index (measured)

represents maximum ductility the GPC will develop.

The Average Displacement Ductility is the average ductility between these two values

which is more probable to develop under Normal Quality Control during GPC

production.

Fig 3a): Load Vs Deflections Curves of Slab 1

Under CPL

(Measured & Calculated)

Fig 3b): Crack Patterns slab 1

Fig 4a): Load Vs Deflections Curves of Slab

2Under CPL

(Measured & Calculated)

Fig 4b): Crack Patterns slab 2

Flexural Behavior Of Fly Ash-Slag Based Reinforced Geopolymer Concrete… 851

Fig 5a): Load Vs Deflections Curves of Slab 3

Under CPL

(Measured & Calculated)

Fig 5b): Crack Patterns slab 3

Fig 6a): Load Vs Deflections Curves of Slab 4

Under UDL

(Measured & Calculated)

Fig 6b): Crack Patterns slab 4

Fig 7a): Load Vs Deflections Curves of Slab 5

Under UDL

(Measured & Calculated)

Fig 7b): Crack Patterns slab 5

852 Mahantesh NB, Amarnath.K, Raghuprasad.B K

Fig 8a): Load Vs Deflections Curves of slab 6

Under UDL

(Measured & Calculated)

Fig 8b): Crack Patterns of slab 6

S.No. Details Slab 1& 4 Slab 2 & 5 Slab 3 & 6

1 Span Side Length 1.3m 0.975m 0.8m

2 Size : L mm X B mm X D mm 1300 x 650 x 75 975 x 650 x 75 800 x 800 x 75

3 L/D 17.33 13.00 10.67

4 Self weight in kg 147.65 & 147.25 110.4 & 110.1 111.25 & 111.75

5 Aspect Ratio 2 1.5 1

6 Molarity 8M 10M 12M

7 Curing days 36 26 22

8 fck 57.87 N/mm2

45.96 N/mm2

41.7 N/mm2

9Reinforcement Parallel to

Shorter & Longer Sides8mm-4# & 7# 8mm - 4# & 6 # 8mm - 5# & 5#

10 Reinforcement 0.507% 0.507% 0.515%

11 Yield Stress & Ultimate Stress 533.94 - 587.33 533.94 - 587.33 533.94 - 587.33

12 Test Results - Central Point Load S1 S2 S3

Support Conditions 2SSS 2SSS 2SSS

First cracking load & deflection 11.1 kN - 0.5 mm 15.76 kN - 0.35 mm 17.53 kN - 0.20 mm

Steel Yielding load &deflection 20.07 kN - 6.2 mm 31.6 kN - 5.74 mm 35.6 kN - 2.85 mm

Ultimate load &deflection 23.0 kN - 12.09 mm 36.2 kN - 6.92 mm 41.11 kN - 3.43 mm

Max.applied load & delfection 23.07 kN - 27.6 mm 37.9 kN - 7.6 mm 44.29 kN - 4.4 mm

Crack width at ultimate load 3.7 mm 1.5 mm 1.5mm

Ductility - Cal - Measured - Average 1.55 - 3.09 - 2.32 1.21 - 1.94 - 1.57 1.16 - 2.66 - 1.91

13 Test Results - UDL S4 S5 S6

Support Conditions 4SSS 2SSS 2SSR

First cracking load & deflection 42.84 kN-0.3 mm 26.13 kN - 0.36 mm 57.22 kN - 0.67 mm

Steel Yielding load &deflection 87.83 kN - 1.1 mm 55.04 kN -5.9 mm 128.7 kN - 3.8 mm

Ultimate load &deflection 99.46 kN - 1.7 mm 63.03 kN - 7.13 mm 146.8 kN - 4.4 mm

Max.applied load & delfection 125.86 kN - 3.4 mm 77.39 kN - 13.80 mm 189.27 kN - 9.6 mm

Crack width at ultimate load 1.9 mm 1.3 mm 1.2 mm

Ductility - Cal - Measured - Average 1.20 - 4.45 - 3.26 1.69 - 2.11 - 1.90 1.20 - 1.54 - 1.37

14 Average Ductility of all Specimens Calculated = 1.483 Measured = 2.633 Avearge = 2.058

Notations Used:4SSS- All 4 Sides Simply Supported ,2SSS- Two Shorter Sides Simply Supported and Other Edges free,2SSR- Two Short Edges Partially Restrained and remanining edges free, UDL- Uniformely Distributed Loading , CPL- Central Point load, M- NaoH Molarity

Table 2: Structural Details of Specimen Tested

Flexural Behavior Of Fly Ash-Slag Based Reinforced Geopolymer Concrete… 853

7.0 CRACK WIDTHS AND PATTERNS .

Developed crack widths within the range of strains in tension steel up to 0.87fy/Es

(0.0023 for Fe415 steel having yield stress 533.87 N/mm2) are within the acceptable

limits and are in agreement with calculated ones based on Indian and BS RC

Designers. Crack patterns follow load type and boundary conditions used and are in

consistency with similar OPC based RC elements. Crack widths beyond strain in steel

0.87fy/Es are in excess of calculated ones.

8.0 RESULTS AND DISCUSSIONS

In this research work GPC is prepared by manual mixing and cured at room

temperature 16 to 28 degrees celsius. Manual mixing up to 20 to 30 minutes increases

workability for further concreting activities. Manual mixing and compaction needs

careful observation to ensure normal quality control to enhance the basic strength

properties of concrete similar to OPC based concreting.

Since all slabs were under reinforced , tension failure of the specimens were noticed.

The appearance of first crack was little earlier than the calculated ones indicating

slightly less flexural strength of concrete compared to IS Code i.e fcr=0.7√fck. The

average peak strain in concrete at 0.67fck stress gives 0.002 to 0.0025 for parabolic

stress block suggested by Indian RC code and for 0.85fck stress it gives 0.003 to

0.0035 for rectangular stress block.

The flexural behaviour of all tested elements show distinct stages of behaviour similar

to OPC based RCC flexural elements like appearance of first crack, yielding of

tension steel and peak stress failure of compressive concrete as seen from the Fig (3)

to Fig (8) .

The stress strain behavior of compression concrete in Reinforced Geopolymer

Concrete Sections under increasing flexural stresses are in line with popovics model

with slight modification to curve fitting factor.

Load Vs Deflections of measured ones with calculated values based on effective

moment of inertia are found to differ within acceptable limits. Shear deflections being

very less do not show noticeable deflection profile

From fig (9) & fig (10) , it is observed that the developed crack widths co relate with

calculated crack widths based on IS 456-2000 code. Crack widths are found to be

within acceptable limits at service loads.

854 Mahantesh NB, Amarnath.K, Raghuprasad.B K

9.0 CONCLUSIONS

Following conclusions are drawn based on the above research work

Geopolymer concrete manufactured using low calcium based fly ash with slag can be used for in situ applications of reinforced geopolymer concrete flexural applications.

The flexural behavior of Reinforced Geopolymer Concrete is similar to Conventional RCC using OPC. Indian Code 456-2000 can be used to predict all structural design related output. Especially this seems to be more valid for fly ash: slag at 70:30 proportions.

Low calcium fly ash and slag (70:30) based geopolymer concrete , with coarse aggregates & M - sand of granite origin , With Fe500 grade reinforcement, Displacement Ductility of RGPC could be in the range 1.50 to 2.70. And The average displacement ductility of RGPC could be around 2.10

ACKNOWLEDGEMENTS:

The Authors wish to thank Management of Alliance University Bangalore And The

Oxford College of Engineering - Bangalore for their kind support while investigating

this research work.

Fig 9) Load Vs Crack widths - Central Point

Loads

Fig 10) Load Vs Crack widths : for UDL

Flexural Behavior Of Fly Ash-Slag Based Reinforced Geopolymer Concrete… 855

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856 Mahantesh NB, Amarnath.K, Raghuprasad.B K

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AUTHORS:

Prof.Mahantesh N B,Associtae Professor, Alliance University

Bangalore, has 10 years of Industrial Experience as Design Manager &

20 years teaching experience.He is a research scholar working on

alternate concrete technology .

Dr Amarnath .K, Prof & HOD Civil Dept, The Oxford Engg college

Bangalore (TOCE), has 30 years of experience in Teaching & Industry.

His research areas include concrete technology & tall buildings.He is

actively involved in guiding Ph.D & M.Tech thesis, material testing

and industry related consultations.

Dr Raghuprasad B K is working as Professor at The Oxford

Engineering College Bangalore(TOCE) .Formerly he was working as

Professor at Indian Institute of Science – Bangalore. He has guided

many Ph.D (27) & M.Tech thesis. His Areas of research: Fracture

Mechanics of Concrete, Structural Dynamics, Earthquake Resistant

Design, Finite Element and Boudary Element methods.