Influence of Replacement of Cement by Micro Silica on Flexural Strength of Fibrous Ferrocement made from GI Fibers

N. K. Patil, Assistant Professor, Dept of Civil Engineering; D.Y. Patil, College of Engineering and Technology,Kolhapur, and Dr. K B.Prakash, Professor and Head of Civil Engineering Department, K.L.E. Society's, College of Engineering and Technology, Belgaum

Applications of ferrocement are vast increasing. Ferrocement is lightweight cement composite. It does not require any special construction technique and skilled labor. It has its cracking resistance, ductility and fatigue resistance higher than those of concrete. In addition, impermeability of ferrocement elements is far superior to that of ordinary concrete. It is used for variety of structures such as prefabricated residential units, marine structures and industrial structures. Similarly, fiber reinforced concrete (FRC) has also wide applications. FRC possesses higher compressive strength and toughness, increased resistance to wear and tear, and higher post cracking strength. But these two materials have some limitations also. These two materials cannot be employed where high vibrations, high tensile forces and high impact are to be resisted.

Fibrous ferrocement, which is a combination of ferrocement and fiber reinforced concrete, shows better improvement in some of mechanical properties, such as toughness, impact resistance. This new composite also shows higher compressive strength, higher tensile and impact strength.

Besides strength properties, the performance against secondary effects like temperature and shrinkage also play an important role in the durability of concrete. Use of pozzolanic materials like micro silica can improve the strength properties, serviceability and durability of concrete. The replacement of cement to some extent by such pozzolanic material can also achieve economy.

In this paper, an attempt has been made to study the effect of replacement of cement by micro silica on the flexural strength of fibrous ferrocement using GI fibers. The percentages of GI fibers were varied as 0%, 0.5%, 1.0%, 1.5%, and 2.0%. The cement was replaced by micro silica in different percentages like 5%, 10%, 15%, 20%, 25%, and 30%.

Introduction

Along with many advantages of concrete, it has deficiencies like low tensile strength, prone to cracking, low post cracking capacity, brittleness, low ductility, not capable to accommodate large deformations and low impact strength. These deficiencies lead concrete to be brittle material with low tensile strength and limited ductility. The contribution of conventional steel reinforcement in RCC structural elements to take care of tensile stresses is limited only in its own plane.¹

Ferrocement is a particular type of reinforced concrete although the extent of investment is relatively less in case of ferrocement construction. Because of similarity in the nature of the ingredients of conventional concrete and ferrocement, the durability characteristics are expected to be similar, even though the mechanical characteristics may be quite different.²

In terms of structural behavior, ferrocement exhibits very high tensile strength-to-weight ratio and superior cracking performance. The distribution of small diameter wire mesh reinforcement over the entire matrix surface provides a very high resistance against cracking. Moreover engineering properties such as toughness, fatigue resistance, impermeability, etc are considerably improved. Sometimes, conventional reinforcing bars in the skeleton form are added to thin wire meshes in order to achieve a stiff reinforcing cage.³

FRC is a composite material consisting of cement, sand, metal, water and fibers. In this composite material, short discrete fibers are randomly distributed trough out the concrete mass. The behavioral efficiency of this composite material is far superior to that of plain concrete and many other construction materials of same cost. Due to this benefit, the use of FRC has steadily increased during last two decades and its current field of application includes airport and highway pavements, earthquake resistant and explosive resistant structures, mines and tunnel linigs, bridge deck overlays, hydraulic structures, rock slope stabilization. Extensive research work on FRC has established that the addition of various types of fibers such as steel, glass, synthetic and carbon, in plain concrete improves strength, toughness, ductility, and post cracking resistance, etc.⁴

When the loads imposed on concrete approach that for failure, cracks will propagate, sometimes rapidly, fibers in concrete provide means of arresting the crack growth. Cosequently, the real advantage of using of fibers in concrete can be seen after matrix cracking. These types of materials are useful if a large amount of energy absorption capacity is required to prevent the failure. Reinforcing steel bars in concrete have the same beneficial effect because they act as long continuous fibers. However short discontinuous fibers have the advantage of being uniformly mixed and dispersed throughout the concrete. Fibers are added to a concrete mix, which normally contains cement, water and fine and coarse aggregate.⁴

The major advantages of fiber reinforced concrete are resistance to microcraking, impact resistance, resistance to fatigue, reduced permeability, improved strength in shear, tension, flexure and compression.⁵ Fibers influence the mechanical properties of concrete or mortar in all modes of failure, especially those that induce fatigue and tensile stress. The strengthening mechanism of fibers involves transfer of stress from matrix to the fiber by interfacial shear or by interlock between the fibers and the matrix. ⁶ Addition of steel fibers in concrete mix significantly improves the cracking behavior and ultimate strength of deep beams. Addition of steel fibers in concrete results in an increase of beam stiffness.⁷ In addition to increase in flexural strength, a considerable increase in toughness is also imparted by the fibers.⁸

The fiber reinforced concrete is also gaining more importance these days especially in the earthquake resistant structures, where ductility plays an important role.⁹

In spite of many advantages of FRC, it has some limitations also. It cannot be employed where high impact, high vibration and high wear and tear is expected. Many problems have to be faced during construction of FRC, especially when quantity of fiber used is more. The fibers if put in bulk along with other ingredients do not disperse but nest together. This phenomenon is called balling effect. The balling effect may be reduced to some extent by mixing the fibers and other ingredients in dry form and then adding water. The fibers placed in concrete, may block the discharge port. Since the flow of FRC is low, the FRC has to be placed near to the place where it is to be used finally.¹⁰

The fibrous ferrocement, which is a combination of fiber reinforced concrete and ferrocement, can overcome most of the limitations of the FRC and ferrocement. And it can be used with assurance where high impact, high vibration and high wear and tear are expected. In this new material advantages of both FRC and ferrocement are combined.¹⁰

Research Significance

Eventhough, performance of fiber reinforced concrete in airfield, pavements, industrial floors and machine foundations is satisfactory, it suffers from some limitations. It cannot be employed where high impact, high vibrations and high wear and tear are expected. Similar to fiber reinforced concrete the ferrocement also have many advantages and its applications are rapidly increasing. The major limitation in ferrocement is that the percentage of reinforcement cannot be increased beyond certain limit. This limitation affects the strength of ferrocement and it cannot be employed where high impact or high load are expected.

The fibrous ferrocement, which is combination of ferrocement and fiber reinforced concrete, can overcome to some extent the limitations offered by ferrocement and fiber reinforced concrete and even improve some of the mechanical and strength properties of ferrocement.

Replacing cement by pozzolanic material like micro silica in fibrous ferrocement, not only its strength gets enhanced but serviceability and durability characteristics can also be improved. The replacement of cement by micro silica can achieve economy too. Thus fibrous ferrocement can be effectively used as an economical construction material.

Experimental Programme

The main aim of this experimentation is to study the effect of replacement of cement by micro silica on the flexural strength of fibrous ferrocement produced with GI fibers. The percentages of GI fibers were varied as 0%, 0.5%, 1.0%, 1.5%and 2.0% by volume fraction of total mix. The cement was replaced by micro silica in different percentages like 5%, 10%, 15%, 20%, 25% and 30%.


Ordinary Portland cement of 53 grade and locally available sand with specific gravity 2.64 and fineness modulus 2.91 was used in experimentation. To impart additional workability a superplasicizer (Algisuperplast – N), 0.5 % by weight of cement was used. The GI fibers were made available by cutting GI binding wires of diameter one mm. The aspect ratio adopted for fibers was 40. The different percentages of GI fibers used in the experimentation were 0%, 0.5%, 1.0%, 1.5% and 2.0% by volume fraction. The welded mesh (WM) used in the experimentation was having square opening of 25 mm with 2.5 mm diameter. The chicken mesh (CM) used was having a hexagonal opening with 0.5 mm diameter. The cement mortar with a proportion of 1:1 was used with a water cement ratio of 0.4.

To study the effect of replacement of cement by micro silica on flexural strength of fibrous ferrocement, the flexural test specimens of dimension 100x100x500 mm were cast and tested under two point loading on UTM of 1000 kN capacity. The required size of welded mesh and chicken mesh were first cut according to the mould size for flexural test test. The chicken mesh was tied to the welded mesh using GI binding wires at regular intervals. This formed cage of (1WM+1CM) with specific area 49345 mm². Similarly cages of (1WM+2CM) and (2WM+2WM) were prepared with specific surface areas 60914 mm² and 98690 mm² respectively.

Now the fibrous mortar was prepared by mixing the required percentage of GI fibers into the cement mortar prepared by replacing cement by micro silica with varied percentage as mentioned above. This cement mortar was placed inside the mould in which cages were kept. All moulds were kept on the table vibrator and sufficient vibration was given to compact the mortar. The specimens were finished smooth after vibration.


The specimens were demoulded after twenty-four hours of casting and specimens were transferred to curing tank where in they were allowed to cure for 28 days. After 28 days of curing, they were taken out of water, dried and were tested for flexural strength.

Test Result

Tables 1, 2 and 3 give the flexural strength test results for fibrous ferrocement made from GI fibers with and without replacement of cement by micro silica. The tables also indicate the percentage decrease in the flexural strength of fibrous ferrocement with replacement of cement by micro silica. In tables WM represents welded mesh and CM represents chicken mesh. Table 4 gives multiplying factors for different percentages of GI fibers. Table 5 gives variables C & D for fibrous ferrocement with different specific surface areas. Table 6 gives predicted (from equations 8) and observed value of flexural strength for fibrous ferrocement without replacement of cement by micro silica. Table 7 shows final predicted (from equations 8) and observed value of flexural strength for fibrous ferrocement with 25% replacement of cement by micro silica. The Figures 1, 2 and 3 give the variation of flexural strength for fibrous ferrocement made from GI fibers with and without replacement of cement by micro silica. Figure 4 shows predicted (from equations 8) and observed value of flexural strength for fibrous ferrocement with 25% replacement of cement by micro silica. Figures 5 to 8 show some photographs during experimentation.

Mathematical Equation

Flexural strength of fibrous ferrocement can be expressed as

S =σ x F ———————— (1)

where σ is flexural strength of ferrocement (1WM+1CM), and F is multiplying factor which depends on volume fraction of fibers and specific surface of welded mesh (WM) and chicken mesh (CM).

Equation (1) can be rewritten as

( S / A₁) = σ F₁

[S / (A₁ x σ )] = F₁ ————— (2-a)

Similarly

[S / (A₂ x σ )] = F₂ ————— (2-b)

[S / (A₃ x σ )] = F₃ ————— (2-c)

Where A₁, A₂ and A₃ are specific surface areas in m² for fibrous ferrocement with (1WM+1CM), (1WM+ 2CM) and (2WM+2CM) respectively. F₁, F₂ and F₃ are multiplying factors which depend on volume fraction of fibers and specific surface of welded mesh (WM) and chicken mesh (CM) for fibrous ferrocement with (1WM+1CM), (1WM+ 2CM) and (2WM+2CM) respectively.


Using experimental values and statistical approach following best fit exponential equations are obtained for multiplying factors F₁, F₂ and F₃, where v_f is volume fraction of fibers.

F₁= 20923 e^0.1687vf, with R²= 0.9298 ————— (3-a)

F₂= 19.655 e^0.1582vf, with R²= 0.9003 ————— (3-b)

F₃= 14.719 e^0.1724vf, with R²= 0.9021 —-———— (3-c)

Factors F₁, F₂ and F₃ can be used to find flexural strength of fibrous ferrocement for specific surface area of A₁ = 0.049345 m² , A₂ = 0.060914 m² and A₃ = 0.098690 m² respectively.

Now equation (2) can be rewritten in generalized form as follows,

σ_cuff = σ_cu x A x C x e^{D vf} ———— (4)

Where C and D are variables, A is specific surface area of ferrocement composite and v_f is volume fraction of fibers. From equations (3) and (4) it is observed that variable C and D depend on specific surface area of ferrocement composite.

Using equation (3) and statistical approach following best fit polynomial equations are obtained for variables C and D keeping them as dependent on specific surface area (A).

C = -426.83 A²– 62.452 A + 25.048 with R²= 1 -———————— (5)

D = 417.47 A²-46.937 A + 1.4683 with R²= 1 ———————— (6)

Thus equation (4) can be used to predict flexural strength of fibrous ferrocement in which variables C and D can be predicted from equations (5) and (6). Predicted values of flexural strength of fibrous ferrocement are shown in following table.

Using statistical approach 'Average' which is average of the absolute deviations of predicted values from observed values.

Average =Σ(X1+X2+…..+Xn) / n, where n is number of readings, X1, X2, X3…..Xn are readings under consideration.

Average ratio for = 1.001, means average percentage deviation of predicted values from experimental values is (1.001-1) x 100 = 0.1%, which is within acceptable limits. Since predicted values on lower side, suitable multiplying factors which can be used in Equation (4) is = 1 + 0.0037 = 1.001= 1.00.

Hence final equations to predict flexural strength of fibrous ferrocement without 25 % replacement of cement by micro silica.

S = σ x A x C x e^{D vf} ———— (7)

Where S = Flexural strength of fibrous ferrocement without replacement of cement by micro silica.

σ = Flexural strength of ferrocement (1WM+1CM).

A= Specific surface area of ferrocement in m².

C & D = variables to be obtained from equations (5) & (6).

v_f= Percentage volume fraction of fibers.

The average percentage increase in flexural strength as observed from table 1, 2 and 3 is 18.07% with 25 % replacement of cement by micro silica. The multiplying factor = 1+ 0.1807 = 1.18 can be used in equation 7 to predict flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica.

S = 1.18 σ x A x C x e^{D vf} —— (8)

Where S = Flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica

σ = Flexural strength of ferrocement (1WM+1CM).

A= Specific surface area of ferrocement in m².

C & D = variables to be obtained from equations (5) & (6).

v_f= Percentage volume fraction of fibers.

Equation 8 can be used to predict flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica.

Average ratio = 0.999, means average percentage deviation of predicted values from experimental values is

(1-0.00999) x 100 = 0.1%, which is within acceptable limits. Since predicted values on lower side, suitable multiplying factors which can be used in Equation (8) is = 1 + 0.001 = 1.001 = 1.00.

Hence equations 8 can be used to predict flexural strength of fibrous ferrocement with 25 % replacement of cement by micro silica.

Discussions on Test Results

1. It has been observed that the flexural strength of fibrous ferrocement increases in the range of 13 to 95% as compared with ferrocement with increase in percentage of GI fibers in it. This is true for all the fibrous ferrocement specimens produced from (1WM+1CM), (1WM+2CM) and (2WM+2CM). It has been also observed that flexural strength of fibrous ferrocement increases in the range of 7to 20% with increase in amount of the welded mesh and chicken mesh in it goes on increasing.
   
   
   
   
   Fibers act as crack arrester and distribute stresses over large area contributing to flexural strength. Therefore more percentage of fibers or more percentage of welded mesh and chicken mesh enhance flexural strength of fibrous ferrocement. It is observed that 1 to 1.5 % fiber content seems to be optimum.
2. It is observed that there is increase in flexural strength of fibrous ferrocement up to 25 percent replacement of cement by micro silica. After 25 percent replacement of cement by micro silica, strength goes on decreasing. Same trend is observed for fibrous ferrocement produced with (1WM+1CM), (1WM+2CM) and (2WM+2CM).
   
   This may be due to the fact that 25% replacement of cement by micro silica may give rise to higher pozzolanic reaction and it may fill up all the pores making fibrous ferrocement a dense mass.
   
   
   
   
   
3. It is observed that percentage increase in the flexural strength of fibrous ferrocement made from (1WM+1CM), is 15%, 16%, 18%, 19%, 20% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of GI fibers in it respectively, with 25 percent cement replacement by micro silica. It is observed that percentage increase in the flexural strength of fibrous ferrocement made from (1WM+2CM), is 15%, 16%, 18%, 19%, 21% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of GI fibers in it respectively, with 25 percent cement replacement by micro silica. It is observed that percentage increase in the flexural strength of fibrous ferrocement made from (2WM+2CM), is 16%, 18%, 19%, 20%, 21% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of GI fibers in it respectively, with 25 percent cement replacement by micro silica.

Conclusions

• Flexural strength of fibrous ferrocement goes on increasing as the percentage of GI fibers in it goes on increasing. 1 to 1.5 % fiber content observed to be optimum percentage.
• Flexural strength of fibrous ferrocement increases in the range of 13 to 95% with increase in percentage of GI fibers.
• Flexural strength of fibrous ferrocement increases in the range of 7 to 20 % with increase in amount of welded mesh and chicken mesh.
• Fibrous ferrocement with GI fibers shows better performance as compared to ferrocement.
• Suggested mathematical equations can be used to predict flexural strength properties of fibrous ferrocement with 25% replacement by micro silica. Predicted values of flexural strength obtained from mathematical equations agree with experimental values.
• Thus fibrous ferrocement with 25% replacement of cement by micro silica can be used as economical construction material where flexural strength requirement is predominent.

Acknowledgment

The authors would like to thank Dr. S. C. Pilli and Dr. B. D. Dalvi, Principals of KLE's College of Engineering and Technology, Belgaum and D.Y.Patil College of Engineering and Technology, Kolhapur for their encouragement throughout the work. Authors are also indebted to management authorities of both the colleges for their wholehearted support, which boosted the moral of the authors. Thanks are also due to HODs of the civil engineering department and other staff for their kind cooperation.

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