An Experimental Investigation on Effect of Alternate Wetting and Drying on Impact Strength of Fibrous Ferrocement Using Round Steel Fibers
Dr. N. K. Patil, Director, S.B. Group of Institutes, Miraj- Sangli
Dr. K B.Prakash, Professor and Head of Civil Engineering Department, K.L.E. Society's, College of Engineering and Technology, Belgaum.
Water retaining and marine structures are invariably subjected to alternate drying and wetting phenomenon. Ferrocement is considered as versatile construction material and is highly suitable for a variety of structures such as water tanks, marine structures, etc. Ferrocement, which utilizes welded mesh and woven rectangular chicken mesh in its construction, has many applications in the civil engineering field. Ferrocement is extentensivly used in USSR for roofing large spans and it has been reported that about 10 million sq. meters of ferrocement components have been used in various types of structures. Most of the roofs have spans ranging from 24 to 30 meter and thickness is only in the order of 20mm. Use of ferrocement has also been reported from the Eastern Europe for the lining of shafts and tunnels.
Similarly fiber reinforced concrete also has several applications. Besides many advantages, these two materials have limitations also. Because of which these two materials may not become viable wherever high impact and high wear and tear occur. To overcome these limitations and to enhance the strength properties, a material called fibrous ferrocement can be used in the construction. New material fibrous ferrocement, which is a combination of ferrocement and fiber reinforced concrete, shows promising performance in the field.
In this paper, an attempt has been made to study the effect of alternate wetting and drying on impact strength of fibrous ferrocement using round steel fibers. The percentages of round steel fibers were varied as 0%, 0.5%, 1.0%, 1.5%and 2.0%.
During last twenty years, several investigators have studied the behavior of fiber reinforced concrete (FRC) structural elements subjected to various types of loading. Different types of fibers and their efficient techniques of mixing and placing of FRC have been reported. In addition to steel, glass and polypropylene fibers, natural fibers like sisal, coir and bamboo are also employed. FRC has been used in the production of different sizes of manhole covers, industrial floors and apron slab in airports. In most of these applications, steel fibers have been used.1
High-performance fiber reinforced concrete (HPFRC) results from the addition of either short discrete fibers or continuous long fibers to the cement-based matrix. Due to the superior performance characteristics of this category of HPFRC, its use by the construction industry has significantly increased. The strength of the FRC can be measured in terms of its maximum resistance when subjected to compressive, tensile, flexural or shear loads. In field conditions, usually some combinations of these load is imposed, however for evaluation purpose, the behavior is characterized under one type of loading without the interaction of other loads. The strength under each individual type of loading is a useful indicator of the FRC material's performance charac- teristic for design purpose.2
FRC is a composite material essentially consisting of conventional concrete or mortar reinforced by the random disposal of short, discontinuous and discrete fine fibers of specific geometry. The major advantages of fiber reinforced concrete are resistance to microcraking, impact resistance, resistance to fatigue, reduced permeability, and improved strength in shear, tension, flexure and compression. In fiber reinforced concrete various types of fibers such as steel, GI, polymeric, polypropylene, carbon, glass, etc are used to improve the properties of concrete.3 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.4 Addition of steel fibers in concrete mix significantly improved the cracking behavior and ultimate strength of deep beams. Addition of steel fibers in concrete resulted in an increase in beam stiffness.5 In addition to increase in flexural strength, a considerable increase in toughness is also imparted by the fibers.6 The fiber reinforced concrete is also gaining more importance these days especially in the earthquake resistant structures, where ductility plays an important role.7
Among thermal loads, thermal shocks are the most exacting because high-temperature gradient and steam pressure peaks occur in pores. Even in worse case the hot spots which may reduce pressure peaks and tensile stresses, greater than ordinary concrete strength in tension, followed by the explosive spalling of the concrete close to the heated surface.8
The research carried over last 15 years has shown that the addition of relatively small amount of fibers to mortars and concrete can provide significant improvement in many of the engineering properties of these materials. The use of steel fibrous concrete as an overlay or rehabilitation material for bridge decks, highways and airfield pavements is receiving particular keen attention at particular time.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 next together. This phenomenon is called balling effect. The balling effect may be reduced to some effect 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
The fibrous ferrocement, can overcome to some extent the limitations offered by ferrocement and fiber reinforced concrete. Thus in crucial situations fibrous ferrocement will play an important role in many applications.
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 round steel fibers were obtained by cutting steel wires of 1 mm diameter. The aspect ratio adopted for fibers was 40. The different percentage of round steel 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 wire. 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 alternate wetting and drying on impact strength of fibrous ferrocement, the impact test specimens of dimensions 250x250x35 mm were cast. The required size of welded mesh and chicken mesh were first cut according to the mould size for impact strength test. The chicken mesh was tied to the welded mesh using steel wires at regular intervals. This formed cage of (1WM+1CM) with specific surface area = 49345 mm2. In the similar fashion the cages of (1WM+2CM) and (2WM+2WM) were prepared with specific surface areas 60914 mm2 and 49345 mm2 respectively. Four methods of impact test are described in the literature.12 Out of these methods drop weight method was used owing to its simplicity. A steel ball weighing 20 N was dropped from the height of 1.5 m over the specimen, which was kept on the floor. The number of blows required for complete failure was noted. The impact energy was calculated as follows. Impact energy = w x h x n, ( N-m ), where w = weight of the ball = 20 N, h = height of fall = 1.5 m, n = number of blows required for complete failure.
Now the fibrous mortar was prepared by mixing separately, the required percentage of round steel fibers into the cement mortar. This cement mortar was placed inside the moulds 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 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 and were kept in the open atmosphere for three days and then they were immersed in water for three days. This constituted one cycle of alternate drying and wetting. The specimens were subjected to 25 such cycles of alternate wetting and drying and were tested for impact strength. The test specimens for reference mix were tested after 150 days of curing.
Table 1 gives the impact strength test results for fibrous ferrocement made from round steel fibers with and without subjecting them to alternate wetting and drying. The tables also indicate the percentage decrease in the impact strength of fibrous ferrocement when subjected to twenty-five cycles of alternate wetting and drying. In tables WM represents welded mesh and CM represents chicken mesh. Table 2 gives best fit equations and R2 values for graphs in fig. 1. Table 3 gives Multiplying factors for different percentages of round steel fibers. Table 4 gives variables C&D for fibrous ferrocement with different specific surface areas. Table 5 gives predicted (from equation 7) and observed value of impact strength for fibrous ferrocement without subjected to alternate and drying. Table 6 shows predicted (from equation 8) and observed value of impact strength for fibrous ferrocement when subjected to alternate and drying.
The figures 1 gives the variation of impact strength for fibrous ferrocement made from round steel fibers with and without subjecting them to alternate wetting and drying. Figures 2 to 4 show some photographs during experimentation.
Impact energy of fibrous ferrocement can be expressed as
σcuff = σcu F - - - - - - - (1)
where σcu is impact energy 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
(σcuff / A1) = σcu F1
[σcuff / (A1 x σcu )] = F1 - - - - - - - (2-a)
Similarly
[σcuff / (A2 x σcu )] = F2 - - - - - - - (2-b)
[σcuff / (A3 x σcu )] = 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 experim- ental values and statistical approach following best fit exponential equations are obtained for multiplying factors F1, F2 and F3, where vf is volume fraction of fibers.
F1 = 22.349 e0.2903vf, with R2 = 0.8948 - - - - - - - (3-a)
F2 = 19.978 e0.2666vf, with R2 = 0.8596 - - - - - - - (3-b)
F3= 13.40 e0.2808vf, with R2 = 0.919 - - - - - - - (3-c)
Factors F1, F2 and F3 can be used to find impact energy 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,
σcuff = σcu x A x C x eDvf - - - - - - - (4)
Where C and D are variables, A is specific surface area of fibrous ferrocement and vf 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 = 624.43 A2 – 273.79 A + 34.339 with R2 = 1 - - - - - - - (5)
D = 49.133 A2 – 7.466 A + 0.5391 with R2 = 1 - - - - - - - (6)
Thus equation (4) can be used to predict impact energy 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 without subjecting to alternate wetting and drying are shown in Table 5.
Using statistical approach 'Average' which is average of the absolute deviations of predicted values from observed values.
Average ∑(X1+X2+…..+Xn) / n, where n is number of readings, X1, X2, X3…..Xn are readings under consideration.
Average ratio = 1.002, means average percentage deviation of predicted values from experimental values is (1.002-1) x 100 = 0.2%, which is within acceptable limits. Since predicted values are on higher side, suitable multiplying factors which can be used in Equation (4) is = 1 - 0.002 = 0.998= 1.00. Hence final equations to predict flexural strength of fibrous ferrocement without subjected to alternate wetting and drying is as follows.
σcuff = σcu x A x C x eDvf - - - - - - - (7)
Where σcuff = Impact energy of fibrous ferrocement without subjected to alternate wetting and drying
σcu = Impact energy of ferrocement (1WM+1CM).
A= Specific surface area of ferrocement in m2.
C & D = variables to be obtained from equations (5) & (6).
vf = Percentage volume fraction of fibers.
The average percentage reduction in impact strength for fibrous ferrocement as observed from table 1 is 12.29% when subjected to alternate wetting and drying. The multiplying factor = 1-0.1229 = 0.877 =0.88 can be used in equation 7 to predict impact strength of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying.
σcuff1 = 0.88 σcu x A x C x eDvf - - - - - - -(8)
Where σcuff 1 = Impact energy of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying.
σcu = Impact energy of ferrocement (1WM+1CM).
A= Specific surface area of ferrocement in m2.
C & D = variables to be obtained from equations (5) & (6).
vf = Percentage volume fraction of fibers.
Equation 8 can be used to predict impact energy of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying. Predicted values of impact energy of fibrous ferrocement after subjecting to twenty five cycles of alternate wetting and drying are shown in Table 6.
Average ratio = 1.0047, means average percentage deviation of predicted values from experimental values is (1.0047 - 1) x 100 = 0.47%, which is within acceptable limits. Since predicted values are on higher side, suitable multiplying factors which can be used in Equation (8) is = 1 - 0.0047 = 0.995= 1.00.
Hence equations 8 can be used to predict impact energy of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying.
Fibers act as crack arrester and distribute stresses over large area resulting increase in energy absorption capacity due to impact. Therefore more percentage of fibers or more percentage of welded mesh and chicken mesh enhance impact strength of fibrous ferrocement.
2) It has been observed that percentage decrease in the impact strength of fibrous ferrocement produced with (1WM+1CM) is 14.47%, 13.36%, 12.17%, 12.10%, 11.81% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of round steel fibers in it respectively when it is subjected to alternate wetting and drying. And the percentage decrease in the impact strength of fibrous ferrocement produced with (1WM+2CM) is 12.61%, 12.48%, 12.38%, 12.20%, 10.69% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of round steel fibers in it respectively when it is subjected to alternate wetting and drying. Similarly the percentage decrease in the impact strength of fibrous ferrocement produced with (2WM+2CM) is 15.57%, 12.87%, 11.38%, 10.27%, 9.95% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of round steel fibers in it respectively, when it is subjected to alternate wetting and drying.
3) Percentage reduction in impact strength of ferrocement when subjected to alternate wetting and drying is compensated by increase in strength due to incorporation of round steel fibers in it. 1 to 1.5 % fiber content seems to be optimum to regain the reduced strength of ferrocement when subjected to alternate wetting and drying.
The decrease in impact strength of fibrous ferrocement subjected to alternate wetting and drying may be due to the fact that the alternate wetting and drying may induce contraction and expansion respectively, which may result in micro cracks in concrete. The atmospheric agencies can enter through these micro cracks and may cause rusting of reinforcement and even of fibers. These induced cracks may bring down the impact strength of fibrous ferrocement.
Dr. K B.Prakash, Professor and Head of Civil Engineering Department, K.L.E. Society's, College of Engineering and Technology, Belgaum.
Water retaining and marine structures are invariably subjected to alternate drying and wetting phenomenon. Ferrocement is considered as versatile construction material and is highly suitable for a variety of structures such as water tanks, marine structures, etc. Ferrocement, which utilizes welded mesh and woven rectangular chicken mesh in its construction, has many applications in the civil engineering field. Ferrocement is extentensivly used in USSR for roofing large spans and it has been reported that about 10 million sq. meters of ferrocement components have been used in various types of structures. Most of the roofs have spans ranging from 24 to 30 meter and thickness is only in the order of 20mm. Use of ferrocement has also been reported from the Eastern Europe for the lining of shafts and tunnels.
Similarly fiber reinforced concrete also has several applications. Besides many advantages, these two materials have limitations also. Because of which these two materials may not become viable wherever high impact and high wear and tear occur. To overcome these limitations and to enhance the strength properties, a material called fibrous ferrocement can be used in the construction. New material fibrous ferrocement, which is a combination of ferrocement and fiber reinforced concrete, shows promising performance in the field.
In this paper, an attempt has been made to study the effect of alternate wetting and drying on impact strength of fibrous ferrocement using round steel fibers. The percentages of round steel fibers were varied as 0%, 0.5%, 1.0%, 1.5%and 2.0%.
Introduction
Ferrocement is a composite material made of cement, steel, wire mesh, sand and water and has unique combination of high strength and stiffness. Because of its several advantages ferrocement has vast potential in developing countries. Both cement and steel consumptions are low and ferrocement can be fabricated to almost any shape. It is more durable than most of the woods and it is less costly than steel. Ferrocement construction does not need heavy machinery and highly skilled labors. Besides these advantages of ferrocement, it has some limitations also. It has low resistance against impact, vibration and wear and tear. Therefore, it cannot be employed where these forces are predominant.1, 10During last twenty years, several investigators have studied the behavior of fiber reinforced concrete (FRC) structural elements subjected to various types of loading. Different types of fibers and their efficient techniques of mixing and placing of FRC have been reported. In addition to steel, glass and polypropylene fibers, natural fibers like sisal, coir and bamboo are also employed. FRC has been used in the production of different sizes of manhole covers, industrial floors and apron slab in airports. In most of these applications, steel fibers have been used.1
High-performance fiber reinforced concrete (HPFRC) results from the addition of either short discrete fibers or continuous long fibers to the cement-based matrix. Due to the superior performance characteristics of this category of HPFRC, its use by the construction industry has significantly increased. The strength of the FRC can be measured in terms of its maximum resistance when subjected to compressive, tensile, flexural or shear loads. In field conditions, usually some combinations of these load is imposed, however for evaluation purpose, the behavior is characterized under one type of loading without the interaction of other loads. The strength under each individual type of loading is a useful indicator of the FRC material's performance charac- teristic for design purpose.2
FRC is a composite material essentially consisting of conventional concrete or mortar reinforced by the random disposal of short, discontinuous and discrete fine fibers of specific geometry. The major advantages of fiber reinforced concrete are resistance to microcraking, impact resistance, resistance to fatigue, reduced permeability, and improved strength in shear, tension, flexure and compression. In fiber reinforced concrete various types of fibers such as steel, GI, polymeric, polypropylene, carbon, glass, etc are used to improve the properties of concrete.3 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.4 Addition of steel fibers in concrete mix significantly improved the cracking behavior and ultimate strength of deep beams. Addition of steel fibers in concrete resulted in an increase in beam stiffness.5 In addition to increase in flexural strength, a considerable increase in toughness is also imparted by the fibers.6 The fiber reinforced concrete is also gaining more importance these days especially in the earthquake resistant structures, where ductility plays an important role.7
Among thermal loads, thermal shocks are the most exacting because high-temperature gradient and steam pressure peaks occur in pores. Even in worse case the hot spots which may reduce pressure peaks and tensile stresses, greater than ordinary concrete strength in tension, followed by the explosive spalling of the concrete close to the heated surface.8
The research carried over last 15 years has shown that the addition of relatively small amount of fibers to mortars and concrete can provide significant improvement in many of the engineering properties of these materials. The use of steel fibrous concrete as an overlay or rehabilitation material for bridge decks, highways and airfield pavements is receiving particular keen attention at particular time.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 next together. This phenomenon is called balling effect. The balling effect may be reduced to some effect 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
Even though performance of fiber reinforced concrete in airfield, pavements, industrial floors and machine foundations is satisfactory, it suffers from some limitations. Balling effect limits the percentage of fibers in FRC. 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 the percentage of reinforcement. The 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, can overcome to some extent the limitations offered by ferrocement and fiber reinforced concrete. Thus in crucial situations fibrous ferrocement will play an important role in many applications.
Experimental Programme
The main aim of this experimentation is to study the effect of alternate wetting and drying on the impact strength of fibrous ferrocement using round steel fibers. Both the fiber percentage and the reinforcement percentage were varied in the study.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 round steel fibers were obtained by cutting steel wires of 1 mm diameter. The aspect ratio adopted for fibers was 40. The different percentage of round steel 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 wire. 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 alternate wetting and drying on impact strength of fibrous ferrocement, the impact test specimens of dimensions 250x250x35 mm were cast. The required size of welded mesh and chicken mesh were first cut according to the mould size for impact strength test. The chicken mesh was tied to the welded mesh using steel wires at regular intervals. This formed cage of (1WM+1CM) with specific surface area = 49345 mm2. In the similar fashion the cages of (1WM+2CM) and (2WM+2WM) were prepared with specific surface areas 60914 mm2 and 49345 mm2 respectively. Four methods of impact test are described in the literature.12 Out of these methods drop weight method was used owing to its simplicity. A steel ball weighing 20 N was dropped from the height of 1.5 m over the specimen, which was kept on the floor. The number of blows required for complete failure was noted. The impact energy was calculated as follows. Impact energy = w x h x n, ( N-m ), where w = weight of the ball = 20 N, h = height of fall = 1.5 m, n = number of blows required for complete failure.
Now the fibrous mortar was prepared by mixing separately, the required percentage of round steel fibers into the cement mortar. This cement mortar was placed inside the moulds 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 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 and were kept in the open atmosphere for three days and then they were immersed in water for three days. This constituted one cycle of alternate drying and wetting. The specimens were subjected to 25 such cycles of alternate wetting and drying and were tested for impact strength. The test specimens for reference mix were tested after 150 days of curing.
Test Results
| Table 1: Impact strength test results | |||||||||
| Percentage of round steel fibers | Impact energy (N-m) with (1WM+1CM) ) having specific surface area 49345 mm2 | Impact energy (N-m) with (1WM+2CM) having specific surface area 60914 mm2 | Impact energy (N-m) with (2WM+2CM) having specific surface area 98690 mm2 | ||||||
| without subjecting to alternate wetting and drying (Ref.mix) | after subjecting to 25 cycles of alternate wetting and drying | Percentage reduction in impact energy as compared to ref.mix. | without subjecting to alternate wetting and drying (Ref.mix) | after subjecting to 25 cycles of alternate wetting and drying | Percentage reduction in impact energy as compared to ref.mix. | without subjecting to alternate wetting and drying (Ref.mix) | after subjecting to 25 cycles of alternate wetting and drying | Percentage reduction in impact energy as compared to ref.mix. | |
| 0.0 | 2322 | 1986 | 14.47 | 2544 | 2223 | 12.61 | 2831 | 2390 | 15.57 |
| 0.5 | 3224 | 2793 | 13.36 | 3523 | 3083 | 12.48 | 3797 | 3308 | 12.87 |
| 1.0 | 3631 | 3189 | 12.17 | 4013 | 3516 | 12.38 | 4261 | 3776 | 11.38 |
| 1.5 | 4030 | 3542 | 12.10 | 4170 | 3661 | 12.20 | 4771 | 4281 | 10.27 |
| 2.0 | 4291 | 3784 | 11.81 | 4555 | 4068 | 10.69 | 5096 | 4589 | 9.95 |
Table 1 gives the impact strength test results for fibrous ferrocement made from round steel fibers with and without subjecting them to alternate wetting and drying. The tables also indicate the percentage decrease in the impact strength of fibrous ferrocement when subjected to twenty-five cycles of alternate wetting and drying. In tables WM represents welded mesh and CM represents chicken mesh. Table 2 gives best fit equations and R2 values for graphs in fig. 1. Table 3 gives Multiplying factors for different percentages of round steel fibers. Table 4 gives variables C&D for fibrous ferrocement with different specific surface areas. Table 5 gives predicted (from equation 7) and observed value of impact strength for fibrous ferrocement without subjected to alternate and drying. Table 6 shows predicted (from equation 8) and observed value of impact strength for fibrous ferrocement when subjected to alternate and drying.
The figures 1 gives the variation of impact strength for fibrous ferrocement made from round steel fibers with and without subjecting them to alternate wetting and drying. Figures 2 to 4 show some photographs during experimentation.
Mathemetical model
Impact energy of fibrous ferrocement can be expressed as
σcuff = σcu F - - - - - - - (1)
where σcu is impact energy 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
(σcuff / A1) = σcu F1
[σcuff / (A1 x σcu )] = F1 - - - - - - - (2-a)
Similarly
[σcuff / (A2 x σcu )] = F2 - - - - - - - (2-b)
[σcuff / (A3 x σcu )] = 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.
|
Table 2: Best fit equations and R2 values for graphs in fig. 1
|
||
|
Combination of
WM + CM |
Best fit polynomial equation for flexural strength depending on percentage of round steel fibers (x)
|
R2
Value |
|
(1WM+1CM)
without alternate wetting and drying |
y = -360x2 + 1656x + 2388
|
0.991
|
|
(1WM+1CM)
with alternate wetting and drying |
y = -334.29x2 + 1538.6x + 2016.9
|
0.9939
|
|
(1WM+2CM)
without alternate wetting and drying |
y = -437.14x2 + 1804.3x + 2619.4
|
0.9755
|
|
(1WM+2CM)
with alternate wetting and drying |
y = -334.29x2 + 1526.6x + 2286.9
|
0.973
|
|
(2WM+2CM)
without alternate wetting and drying |
y = -342.86x2 + 1777.7x + 2894.6
|
0.9935
|
|
(2WM+2CM)
with alternate wetting and drying |
y = -334.29x2 + 1742.6x + 2430.9
|
0.9963
|
Using experim- ental values and statistical approach following best fit exponential equations are obtained for multiplying factors F1, F2 and F3, where vf is volume fraction of fibers.
|
Table 3: Multiplying factors for different percentages of round steel fibers
|
|||
|
% of round steel fibers
|
F1
|
F2
|
F3
|
|
0
|
20.2655
|
17.9861
|
12.3539
|
|
0.5
|
28.1377
|
24.9176
|
16.5693
|
|
1.0
|
31.6899
|
28.3720
|
18.5941
|
|
1.5
|
35.1722
|
29.4820
|
20.8197
|
|
2.0
|
37.4501
|
22.2379
|
22.2379
|
F1 = 22.349 e0.2903vf, with R2 = 0.8948 - - - - - - - (3-a)
F2 = 19.978 e0.2666vf, with R2 = 0.8596 - - - - - - - (3-b)
F3= 13.40 e0.2808vf, with R2 = 0.919 - - - - - - - (3-c)
Factors F1, F2 and F3 can be used to find impact energy 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,
σcuff = σcu x A x C x eDvf - - - - - - - (4)
Where C and D are variables, A is specific surface area of fibrous ferrocement and vf 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 = 624.43 A2 – 273.79 A + 34.339 with R2 = 1 - - - - - - - (5)
D = 49.133 A2 – 7.466 A + 0.5391 with R2 = 1 - - - - - - - (6)
Thus equation (4) can be used to predict impact energy 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 without subjecting to alternate wetting and drying are shown in Table 5.
Using statistical approach 'Average' which is average of the absolute deviations of predicted values from observed values.
Average ∑(X1+X2+…..+Xn) / n, where n is number of readings, X1, X2, X3…..Xn are readings under consideration.
 |
 |
|
Figure 2: Drop weight testing of impact specimen
|
Figure 3: Round steel fibers with aspect ratio 40
|
Average ratio = 1.002, means average percentage deviation of predicted values from experimental values is (1.002-1) x 100 = 0.2%, which is within acceptable limits. Since predicted values are on higher side, suitable multiplying factors which can be used in Equation (4) is = 1 - 0.002 = 0.998= 1.00. Hence final equations to predict flexural strength of fibrous ferrocement without subjected to alternate wetting and drying is as follows.
σcuff = σcu x A x C x eDvf - - - - - - - (7)
Where σcuff = Impact energy of fibrous ferrocement without subjected to alternate wetting and drying
σcu = Impact energy of ferrocement (1WM+1CM).
A= Specific surface area of ferrocement in m2.
C & D = variables to be obtained from equations (5) & (6).
vf = Percentage volume fraction of fibers.
The average percentage reduction in impact strength for fibrous ferrocement as observed from table 1 is 12.29% when subjected to alternate wetting and drying. The multiplying factor = 1-0.1229 = 0.877 =0.88 can be used in equation 7 to predict impact strength of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying.
σcuff1 = 0.88 σcu x A x C x eDvf - - - - - - -(8)
Where σcuff 1 = Impact energy of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying.
σcu = Impact energy of ferrocement (1WM+1CM).
A= Specific surface area of ferrocement in m2.
C & D = variables to be obtained from equations (5) & (6).
vf = Percentage volume fraction of fibers.
 |
|
Figure 4: Failure of impact test specimen
|
Average ratio = 1.0047, means average percentage deviation of predicted values from experimental values is (1.0047 - 1) x 100 = 0.47%, which is within acceptable limits. Since predicted values are on higher side, suitable multiplying factors which can be used in Equation (8) is = 1 - 0.0047 = 0.995= 1.00.
Hence equations 8 can be used to predict impact energy of fibrous ferrocement subjected to 25 cycles of alternate wetting and drying.
|
Table 4: Variables C & D for fibrous ferrocement with different specific surface areas
|
||
|
Combination of WM and CM with specific surface area.
|
C
from equation (3) & (4) |
D
from equation (3) & (4) |
|
(1WM+ 1CM)
A1 = 0.049345 m2 |
22.349
|
0.2903
|
|
(1WM+ 2CM)
A2 = 0.060914 m2 |
19.978
|
0.2666
|
|
(2WM+ 2CM)
A3 = 0.098690 m2 |
13.40
|
0.2808
|
Discussion on Test Results
1) It has been observed that the impact strength of fibrous ferrocement increases in the range of 39 to 85% as compared with ferrocement with increase in percentage of round steel 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 impact strength of fibrous ferrocement increases in the range of 10 to 22% with increase in amount of the welded mesh and chicken mesh in it goes on increasing.|
Table 5: Predicted (from equations 7) and observed value of flexural strength for fibrous ferrocement without subjecting to alternate wetting and drying
|
|||||||||
Percentage of round steel fibers |
Impact energy of fibrous ferrocement with (1WM+1CM) having specific area = 0.049345 m2, without subjecting to alternate wetting and drying
C = 22.35, D = 0.2903, calculated from equations (5) & (6) |
Impact energy of fibrous
ferrocement with (1WM+2CM) ) having specific area = 0.060914 m2, without subjecting to alternate wetting and drying C = 19.98 , D = 0.2664, calculated from equations (5) & (6) |
Impact energy of fibrous ferrocement with (2WM+2CM) having specific area =0.098690 m2,
without subjecting to alternate wetting and drying C = 13.39, D = 0.2804, calculated from equations (5) & (6) |
||||||
|
Predicted
impact energy (N-m) |
Observed impact energy
(N-m) |
Ratio of predicted to observed impact energy
|
Predicted
impact energy (N-m) |
Observed impact energy
(N-m) |
Ratio of predicted to observed impact energy
|
Predicted
impact energy (N-m) |
Observed impact energy
(N-m) |
Ratio of predicted to observed impact energy
|
|
|
0.0
|
2561
|
2322
|
1.10
|
2826
|
2544
|
1.11
|
3068
|
2831
|
1.08
|
|
0.5
|
2961
|
3224
|
0.92
|
3229
|
3523
|
0.92
|
3530
|
3797
|
0.93
|
|
1.0
|
3423
|
3631
|
0.943
|
3689
|
4013
|
0.92
|
4061
|
4261
|
0.95
|
|
1.5
|
3958
|
4030
|
0.982
|
4214
|
4170
|
1.01
|
4673
|
4771
|
0.98
|
|
2.0
|
4576
|
4291
|
1.07
|
4815
|
4555
|
1.06
|
5376
|
5096
|
1.05
|
Fibers act as crack arrester and distribute stresses over large area resulting increase in energy absorption capacity due to impact. Therefore more percentage of fibers or more percentage of welded mesh and chicken mesh enhance impact strength of fibrous ferrocement.
2) It has been observed that percentage decrease in the impact strength of fibrous ferrocement produced with (1WM+1CM) is 14.47%, 13.36%, 12.17%, 12.10%, 11.81% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of round steel fibers in it respectively when it is subjected to alternate wetting and drying. And the percentage decrease in the impact strength of fibrous ferrocement produced with (1WM+2CM) is 12.61%, 12.48%, 12.38%, 12.20%, 10.69% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of round steel fibers in it respectively when it is subjected to alternate wetting and drying. Similarly the percentage decrease in the impact strength of fibrous ferrocement produced with (2WM+2CM) is 15.57%, 12.87%, 11.38%, 10.27%, 9.95% for 0%, 0.5%, 1.0%, 1.5% and 2.0% of round steel fibers in it respectively, when it is subjected to alternate wetting and drying.
3) Percentage reduction in impact strength of ferrocement when subjected to alternate wetting and drying is compensated by increase in strength due to incorporation of round steel fibers in it. 1 to 1.5 % fiber content seems to be optimum to regain the reduced strength of ferrocement when subjected to alternate wetting and drying.
|
Table 6: Predicted (from equations 8) and observed value of
impact energy for fibrous ferrocement after subjecting to alternate wetting and drying |
|||||||||
|
Percentage
of round steel fibers |
Impact energy of fibrous ferrocement with (1WM+1CM) having specific area = 0.049345 m2, without subjecting to alternate wetting and drying
C = 22.35, D = 0.2903, calculated from equations (5) & (6) |
Impact energy of fibrous
ferrocement with (1WM+2CM) ) having specific area = 0.060914 m2, without subjecting to alternate wetting and drying C = 19.98 , D = 0.2664, calculated from equations (5) & (6) |
Impact energy of fibrous ferrocement with (2WM+2CM) having specific area =0.098690 m2,
without subjecting to alternate wetting and drying C = 13.39, D = 0.2804, calculated from equations (5) & (6) |
||||||
|
Predicted
impact energy (N-m) |
Observed impact energy (N-m)
|
Ratio of predicted to observed impact energy
|
Predicted
impact energy (N-m) |
Observed impact energy
(N-m) |
Ratio of predicted to observed impact energy
|
Predicted
impact energy (N-m) |
Observed impact energy (N-m)
|
Ratio of predicted to observed impact energy
|
|
|
0.0
|
2254
|
1986
|
1.13
|
2487
|
2223
|
1.12
|
2700
|
2390
|
1.13
|
|
0.5
|
2606
|
2793
|
0.94
|
2841
|
3083
|
0.92
|
3106
|
3308
|
0.94
|
|
1.0
|
3012
|
3189
|
0.94
|
3246
|
3516
|
0.92
|
3574
|
3776
|
0.95
|
|
1.5
|
3483
|
3542
|
0.98
|
3708
|
3661
|
1.01
|
4112
|
4281
|
0.96
|
|
2.0
|
4027
|
3784
|
1.06
|
4237
|
4068
|
1.04
|
4731
|
4589
|
1.03
|
The decrease in impact strength of fibrous ferrocement subjected to alternate wetting and drying may be due to the fact that the alternate wetting and drying may induce contraction and expansion respectively, which may result in micro cracks in concrete. The atmospheric agencies can enter through these micro cracks and may cause rusting of reinforcement and even of fibers. These induced cracks may bring down the impact strength of fibrous ferrocement.
Conclusion
- Impact strength of fibrous ferrocement increases in the range of 39 to 85% with increase in percentage of round steel fibers.
- Impact strength of fibrous ferrocement increases in the range of 10 to 22 % with increase in amount of welded mesh and chicken mesh.
- There will be reduction in impact strength of fibrous ferrocement in the range of 9 to 16 % when subjected to 25 cycles of alternate wetting and drying.
- Fibrous ferrocement with round steel fibers shows better performance as compared to ferrocement.
- Predicted values of impact strength obtained from mathematical equations agree with experimental values.
- 1 to 1.5 % fiber content seems to be optimum to regain the reduced impact strength of ferrocement when subjected to alternate wetting and drying.
Acknowledgment
The authors would like to thank Honorable Sanjay Bhokare, President, ATS, Miraj and Dr. S. C. Pilli, Principal of KLE's College of Engineering and Technology, Belgaum, for their encouragement throughout the work. Authors are also indebted to management authorities and staff of both the colleges for their wholehearted support, which boosted the moral of the authors.References
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