Some Studies on Flexural Behaviour of Pozzolanic (Rice Husk Ash) Fibre Reinforced Ferrocement

Some Studies on Flexural Behaviour of Pozzolanic (Rice Husk Ash) Fibre Reinforced Ferrocement

*Dr. K. Lakshmi Pathi

* Senior Lecturer in Civil Engineering,

Dept. of Civil Engineering, Govt. Polytechnic for Women, Hindupur-515201, A.P.

**Dr. V. Bhaskar Desai

** Professor, Dept. of Civil Engineering,

J.N.T.U. College of Engineering, Anantapur 515002. A.P..

Abstract:- Now a days due to many civil engineering constructions through out the globe the usage of OPC is very large and so the raw material for OPC becoming scarce. Because of this main reason in the present days the usage of Pozzollanic cement has gained momentum. This paper presents a brief study on the flexural behaviour of fibre reinforced ferrocement elements with partial replacement of cement by pozzollanic material like rice husk ash. Nearly 360 specimens were cast and tested with the variables such as different percentages of replacement of cement by pozzolanic material addition such as rice husk ash addition, different percentages of steel fibre, number of wire mesh layers and different span to depth ratios (a/d) etc,. From the results it is observed that with the increase in percentage of fibres and RHA the compressive strength of mortar, first crack load, ultimate load in flexure, flexural stress at first crack load, flexural stress at ultimate load and energy absorption increase up to certain extent and afterwards get decreased. Also the above strength parameters are found to increase with number of wire mesh layers. More so the above strengths are found to decrease with the increase in a/d ratio except the flexural stresses at first crack load and ultimate load. Besides the paper presents the behaviour of load deflection variation and crack pattern for number of variables studied. Finally an analytical model has been proposed for Mcr and Mu with the inclusion of the most of the variables used in the present investigation.

Key words: RHA, Span to Depth Ratio (a/d), Number of Mesh Layers (N), Volume Percentage of Fibres (Vf).

INTRODUCTION

In the present investigation an attempt has been made to study the behaviour of ferrocement elements with the addition of natural agricultural waste such as rice husk ash which is other wise posing serious disposal problem. Ferro cement is not a special type of cement; it is a composite material made up of cement mortar, wire mesh and/or skeletal steel reinforcement. When fibres with some aspect ratio are added to this Ferro cement, fibre-reinforced Ferro cement is obtained. When pozzolanic material like rice husk ash is added to this, pozzolanic fibre reinforced ferrocement is obtained. Ferrocement constructions in building industry began almost 60 years back. In late nineties, Chien Hung Lin & Shyh ming Perng1 studied the flexural behaviour of concrete beams with welded wire fabric as shear reinforcement. Also in late nineties, Behaviour of Ferro cement beams under shear was studied by M.A.Al-Kubaisy and P.T. Nedwell2. S.K.Kaushik3 et.al

(+) conducted experimental investigations on Ferro cement plates using super plasticized fly ash mortar. M.Mazloom, A.A.Ramezanianpour and J.J.Brooks4 studied the effect of silicafume on mechanical properties of high strength concrete. Sheela.S and Ganesan.N5, studied on the Flexural Behaviour of Polymer Modified Ferrocement Structural Elements. Sheela.S and Ganesan.N6, conducted study on the behaviour of Polymer Modified Channel Shaped Ferrocement Elements

1. MATERIALS & EXPERIMENTAL PROGRAMME

   1. Materials

In this investigation ordinary Portland of 43 Grade cement

,Rice Husk Ash, Crimpled Steel Fibres and Natural River Sand were used.

2.2 Experimental Programme

In this investigation 360 specimens of size 860x200x30mm were cast with varying volume percentages of crimpled steel fibres (flat 30mm length) .i.e. 0, 0.5, 1.0,1.5 and 2.0

,with increasing number of chicken mesh layers 1,3 and 5,and with varying percentages of replacement of cement by pozollanic material (rice husk ash ) .i.e.0,5,10,15 and

1. Constant water cement ratio of 0.5 was adopted. A constant vibration of 10 seconds was applied for all castings.

   Fig. 1 : Elevation of Loading Arrangement

   d=30mm

   1. DISCUSSION OF TEST RESULTS

      Here the experimental results were analyzed and discussed. The variation of ultimate load in flexure of ferrocement specimens with the partial replacement of cement by rice husk ash for constant percentage of fibres, constant number of mesh layers and for different a/d ratios is presented in fig-3.

      Variation of Ultimate Load Vs Rice Husk Ash %

      a=150MM

      660mm

      Fig. 2 : Loading Pattern for a/d=5

      3 .TESTING

      The specimens were tested under two point loading as shown in fig.1 through a pre calibrated 5 tonnes proving ring .Three dial gauges were used as shown in fig.1 to measure deflections. During testing all specimens were tested with three span to depth ratios 5, 8 and11. A span to

      1500

      1375

      1250

      Ultimate Load N

      1125

      1000

      875

      750

      625

      500

      375

      250

      125

      0

      a/d=5

      a/d=8

      a/d=11

      0 5 10 15 20 25

      Rice Husk Ash %

      depth ratio (a/d) is defined as the ratio of distance between the loading point and support point of the specimen to the depth of the specimen. More details of a sample loading are shown in fig 2. The specimens were tested up to failure.

      4 . DISCUSSION OF CRACK PATTERN

      For almost all the specimens it was commonly observed that the crack initiation was mostly at the bottom. As the load was increased further already formed cracks got widened and progressed towards the top edge of the specimen.

      Also immaterial of a/d ratio, all most all the specimens with one layer of wire mesh and with 0% of fibres failed suddenly due to poor ductility with out any clear warning. As the number of wire mesh layers was increased the ductility got increased and specimens failed by showing sufficient warning. However with the addition of fibres the ductility was additionally increased and the decrease in the crack width was noticed.

      Regarding the failure of specimens for lower a/d ratios, the cracks were found to form in the zone between the load point and mid point of the specimen .For higher values of a/d ratios the cracks were formed with in the vicinity of midpoint.

      Regarding the effect of Pozzolanic material content, it was observed with naked eye that for higher percentage of pozzolonic addition, cracks were found to be widened when compared with those specimens with lower percentage of pozzolonic material addition.

      Fig:3:Variation of Ultimate Load Vs Rice Husk Ash %

      From this it may be observed that, with the increase in rice husk ash content the ultimate load in flexure increases slowly and marginally up to an optimum content (around 5%) and afterwards decreases. Similar behaviour was also observed with respect to first crack load .

      Secondly the variation of ultimate load in flexure with number of wire mesh layers for a given percentage of fibre content for constant % of rice husk ash and constant a/d ratios are shown in fig-4.

      Variation of Ultimate Load Vs Number of Mesh layers (N)

      1500

      1375

      1250

      Vf=0

      Vf=0.5 Vf=1.0 Vf=1.5

      Vf=2.0

      Ultimate Load N

      1125

      1000

      875

      750

      625

      500

      375

      250

      125

      0

      0 1 2 3 4 5

      Number of Mesh layers (N)

      Fig:4:Variation of Ultimate Load Vs Number of Mesh Layers

      From these figure it is seen that with the increase in number of wire mesh layers and fibre content the ultimate load in flexure is found to increase.

      Thirdly the variation of ultimate load in flexure with the percentage of fibres for constant number of mesh layers constant % of rice husk ash and constant a/d ratio is presented in fig-5.

      In this investigation load deflection variations (P. diagrams) for number of variables were also studied. The sample load deflection variations are presented in figs 7 ,8 and 9.

      1500

      1375

      1250

      Ultimate load N

      1125

      1000

      875

      750

      625

      500

      375

      250

      125

      0

      1500

      1375

      1250

      1125

      Ultimate load N

      1000

      875

      750

      625

      500

      375

      250

      125

      0

      RHA=0

      RHA=5

      RHA=10 RHA=15 RHA=20

      N=1 N=3

      N=5

      0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

      Deflection mm

      0 0.5 1 1.5 2 2.5

      Volume % of Fibres (Vf)

      Fig:7:Variation of Ultimate Load Vs Deflection

      Fig:5:Variation of Ultimate Load Vs Volume % of Fibres

      From this figure it may be observed that as the percentage of fibres is increased the ultimate load in flexure is increased up to optimum content and there after it gets decreased.

      Fourthly the variation of ultimate load in flexure with the a/d ratios for constant number of wire mesh layers for a given percentage of fibre content for constant % of rice husk ash are shown in figs-6

      Ultimate Load N Vs Span to Depth Ratio (a/d)

      1500

      1375

      RH=0

      Ultimate Load N

      1250

      RH=5

      1125

      1000

      RH=10 RH=15

      RH=20

      875

      750

      625

      500

      375

      250

      125

      0

      0 2 4 6 8 10 12

      Span to Depth Ratio (a/d)

      Fig:6:Variation of Ultimate Load Vs Span to Depth ratio.

      1500

      1375

      1250

      ultimate load N

      1125

      1000

      875

      750

      625

      500

      375

      250

      125

      0

      Variation of ultimate load of Vs Deflection

      Vf=0 Vf=0.5 Vf=1.0

      Vf=1.5

      Vf=2.0

      0 50 100 150 200 250 300 350 400

      Deflection (X0.001)mm

      Fig:8:Variation of Ultimate Load Vs Deflection

      Variation of ultimate load Vs Deflection

      1500

      1375

      1250

      ultimate load N

      1125

      a/d=5

      a/d=8

      a/d=11

      1000

      875

      750

      625

      500

      375

      250

      125

      0

      From these figures it is seen that with the increase a/d ratio the ultimate load in flexure is found to decrease.

      The variation of flexural stresses with the number of variables is observed to be more or less same as that with the first crack and ultimate loads. How ever their variation gets increased with the increase in a/d ratio.

      0 50 100 150 200 250 300 350 400

      Deflection (X0.001) mm

      Fig:9:Variation of Ultimate Load Vs Deflection

      From the Fig.7, load-deflection variations studied in this investigation it is seen that with the increase in rice husk ash content up to more or less optimum content, the curves

      were observed to become steeper and steeper and afterwards the curves were showing reverse trend.

      From the Fig.8, it is seen that with the increasing volume percentage of fibres the P- variation is increasing up to optimum content and there after the variation gets decreased. Besides from Fig.9 with the increase in a/d ratio, the curves are found to become flatter and flatter. From this it is also observed that with the increase in number of wire mesh layers, the P- variation also gets increased.

      Finally energy absorpted by the ferro cement specimens have been calculated as the area under P- diagrams. Sample diagram is presented in Fig:10.

      Y =-1.99045+445.85987 X+0 X2

      M /f -bd2 = 0.000257 +0.000653 ar + 0.001168 Vf + 0.000522 a/d 2.8X10-5 Pz —————(2)

      u cu

      Regression coefficient: 0.941, S.D=0.000589

      Where

      Mcr = Cracking moment or First crack moment, Mu = Ultimate moment,

      cu

      f – = Compressive strength of cement mortar cube for 28

      days curing,

      b = Breadth of the specimen, d = Depth of the specimen,

      375

      Ultimate load N

      250

      125

      0

      R2=0.99873

      Area=159.375 N-mm N=1,Vf=0,a/d=8,RHA=5%

      0.0 0.2 0.4 0.6 0.8 1.0

      Deflection mm

      Fig:10 Area under the Ultimate Load Vs Deflection (Energy absorbed by the specimen)

      ar = Ratio of area of contact of hexagonal chicken mesh wire layer to the unit

      plan area of wire mesh layer, Vf = Volume percentage of fibres, a/d = Span to depth ratio,

      S.D = Standard deviation.

      From these equations generally it is possible to calculate Mcr and Mu values for known values of ar ,Vf and a/d and for the type of chicken mesh adopted in this investigation and for the type of crimpled steel fibres used in this investigation,

      CONCLUSIONS

      Its behaviour and variation are observed to be same as the P- variation as discussed above. From the study it is also noticed that more number of wire mesh layers and fiber addition up to certain extent increase the ductility, load carrying capacity etc.

   2. PROPOSED REGRESSION MODEL

Here an attempt has been made to formulate a regression model incorporating most of the variables studied in this experimental study.

In the present investigation the assumed dependent variables are M /f -bd2 and M /f -bd2. Independent

On the basis of limited experiments conducted in this investigation the following tentative conclusions seem to be valid.

1. The compressive strengths of mortar specimens increase with the addition of fibres up to some extent called optimum content (1.5%) and there after decrease.

2. Both the first crack and ultimate loads increase with the partial pozollanic material addition up to some extent called optimum content and after wards decrease. They increase with increase in volume percentage of fibres, number of wire mesh layers and decrease with increasing a/d ratio

3. Both the first crack and ultimate loads in flexure

   u cu cr cu

   variables are 1) Ratio of area of contact of hexagonal

   chicken mesh wire layer to the unit plan area of wire mesh layer (ar) 2) Volume percentage of fibres (Vf) and 3) Span to depth ratio (a/d).

   Applying multiple regression model analysis taking all the experimental values studied in this chapter into account, following regression equations are proposed for cracking moment (Mcr) and ultimate moment (Mu).

   cu

   Mcr /f -bd2 =-3.7X10-5+ 0.000266 ar + 0.000364 Vf + 0.000336 a/d 1.5X10-5 Pz ———-(1)

   Regression coefficient: 0.90, S.D=0.000429

   increase with the addition of fibres up to some

   extent called optimum content (1.5%) and after wards decrease for a given number of mesh layers and for a given a/d ratio.

4. Both the first crack and ultimate loads in flexure increase with the increase in number of mesh layers, for a given a/d ratio and for a given volume percentage of fibres.

5. The first crack load and ultimate loads in flexure are found to increase with the decrease in a/d ratio for a given volume percentage of fibres and for a given number of wire mesh layers.

6. The flexural stress values at first crack load and ultimate load are found to increase with the addition of fiber up to some extent called optimum content (1.5%) and after wards decrease.

7. The flexural stress values at first crack load and ultimate load are found to increase with the increase of number of mesh layers and a/d ratio.

8. With the increasing volume percentage of fibres up to optimum content (1.5%) the energy absorption of specimen increases and there after gets decreasd.

9. It may be observed that with the increasing number wire mesh layers the energy absorption of specimen gets increased.

10. It may be observed that with the increasing a/d ratio the energy absorption of specimen gets decreased..

11. From the P- diagrams it may be observed that with the increase in rice husk ash content up to more or less optimum content, the curves were observed to become steeper and steeper and afterwards the curves were showing reverse trend. It is also observed that with the increasing volume percentage of fibres and number of mesh layers the curves are found to become steeper and steeper. Also with the increase of a/d ratio the variation is found to become flatter and flatter.

12. Thus from the present investigation it has been observed that certain natural agricultural wastes which is posing serious disposal problem can be effectively used as pozzolanic material in ferrocement industry there by decreasing the content of costly binding material cement.

13. From load-deflection (P-) diagrams, in general it is observed that as the load increases deflection also increases. Up to first crack the P- diagrams are more or less straight and after wards the curvature gets changed. Beyond the first crack, the rate of increase of deflection is more compared to the un cracked range.

14. From the P- diagrams it may be observed that with the increasing volume percentage of fibres up to optimum content (1.5%) the P- variation is increasing and afterwards the variation gets declined for higher volume %ge of fibres beyond the above optimum content.

15. Besides from the P- diagrams it may be observed that with the increase in a/d ratio, the P- variation gets decreased.

16. More so from the P- diagrams it may be observed that with the increase in number of wire mesh layers the variation is maximum for the specimens with maximum number of wire mesh layers.

17. Finally analytical models have been proposed for Mcr and Mu incorporating most of the variables used in the present investigation .The regression coefficient for the analytical models is almost unity. Hence these models are supposed to predict the Mcr & Mu values for any given set of variables with in the frame work of present investigation with satisfactory accuracy.

18. Higher volume fraction of reinforcement in the form of chicken mesh provides more effective control of cracking and also improves the strength properties of the specimens.

19. Also introduction of fibers increases ductility, crack control and load carry capacity of the member of the ferrocement.

20. Thus the combination of chicken mesh and fibers improves load carrying capacity of the specimen in flexure and in addition improves the crack arresting mechanism

REFERENCE :

1. Chien Hung Lin and Shyh ming Perng : Studied the flexural behaviour of concrete beams with welded wire fabric as shear reinforcement in 1998.

2. Al-Kubaisy M.A. and Needwell P.J.: Behaviour and strength of ferrocement rectangular beams in shear, Journal of ferrocement, Vol.29, No.1, January, 1999.

3. Kaushik S.K., Pankaj, Akhtar S. and Arif M.: Experimental investigations on Ferro cement plates using super plasticized fly ash mortar. Journal of Ferro cement Vol.32 No.2, April 2002.

4. Mazloom M., Ramezanianpour A.A. and Brooks J.J: Effect of silica fume on mechanical properties of high-strength concrete. Cement and concrete composites 26 (2004).

5. Sheela.S and Ganesan.N, "Flexural Behaviour of Polymer Modified Ferrocement Structural Elements", Proceedings of the International Seminar on Civil and Infrastructure Engineering ISCIE '06, University Teknologi Mara, Malaysia, 13-14, June 2006

6. Sheela.S and Ganesan.N, " Behaviour of Polymer Modified Channel Shaped Ferrocement Elements", 5th Asian Symposium on Polymers in Concrete, September 11&12, 2006, SERC, Chennai-600113, India, pp.195-203