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Impact study on ferrocement slabs reinforced with polymer mesh.

Introduction

Ferrocement is a form of reinforced concrete that differs from conventional reinforced or pre- stressed concrete primarily by the way of dispersed and arrangement of reinforcement [1]. Ferrocement, in spite of several advantages like cost-effectiveness and proven technology for adoption in developing countries, apprehensions about its long-term performance, especially with respect to corrosion of reinforcement has been raised [2]. The processes involved in the corrosion of reinforcement in ferrocement elements and the factors that affect durability, which are unique to ferrocement have also been identified and reported [3,4]. Two accepted methods to overcome the above problem i.e. protective coating and ensuring impermeability may not offer the best solution. To reliably insure long- term service life, corrosion must not only be delayed but it should be completely eliminated. This can be achieved by using materials which are not susceptible to corrosion, such as stainless steel, fiber reinforced polymer or plastic reinforcement [5]. In this context, recent developments in polymer technology and its pervasive influence in the construction industry raises a ray of hope. For example, commercially available polymer meshes, grids etc, have found extensive applications in Civil Engineering. However, the potential of such polymer materials, as an alternate form of reinforcement has not been fully investigated and reported.

Based on past experience, existing information's and broad--based knowledge, International Ferrocement Society (IFS) committee 10, Thailand has published Ferrocement Model Code (FMC) in January 2001. In this ferrocement has been defined now to reflect the current and future developments of ferrocement as: "a type of reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of relatively small wire diameter mesh. The mesh may be made of metallic or other suitable materials. The fineness of the mortar matrix, and its composition should be compatible with the opening and tightness of the reinforcing system it is meant to encapsulate. The matrix may contain discontinuous fibers" [6].

Although polymer material in the form of mesh were used in Civil Engineering applications in various forms like in tunnel lining, geo-technical engineering etc. but its potential use in ferrocement is not made. In this orientation, a series of comprehensive studies have been initiated by the author in the above for the utilization of polymer mesh as reinforcement in ferrocement elements and to ascertain the potential of the above material. In this paper, the impact characteristics like energy absorbed, failure pattern, residual impact strength of ferrocement slab specimens reinforced with polymeric meshes (layers ranging from one to three) have been investigated and the results compared with slab specimens reinforced with chicken mesh and reported.

Experimental Investigations

Materials

Ordinary Portland cement (53 grade) was used for the entire investigations. The physical properties of the above cement tested according to standard procedure and it conforms to the requirements of IS: 12269-1989[7]. Locally available river sand conforming to Zone II of IS: 383-1970 [8] was used as fine aggregate. The above grading of sand also conforms to the recommendations of ACI Committee 549R97[1] and its physical properties are given in Table 1. Potable tap water available in Pondicherry Engineering College (PEC) campus was used for casting and curing specimens.Two types of meshes which are commercially available, namely (i) chicken mesh (hexagonal mesh) and (ii) P.R. mesh (diamond mesh) i.e., polymer mesh were used. The physical properties of the two chosen meshes obtained from the firms are given in Table 2. A view of the chosen meshes is shown in Figure 1-2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Matrix Proportion and Properties

Sand--cement ratio 2.0 and water--cement ratio 0.43 was adopted for the present study. The above values correspond to the average values of the ACI recommendations (ACI 549), which also satisfies the required slump of fresh mortar stipulated in ACI 549 and FMC. The various strength properties of the above mortar mix, such as, compressive strength, flexure strength, split tensile strength, and water absorption were determined by standard I.S. test procedures at 3,7 and 28 days of normal curing. The test results obtained are given in Table 3.

Preparation of Specimens

Ferrocement slab specimens 250mm x 250mm x 25mm were cast. The above size was chosen based on the ease of handling and on a review of published literature. Chicken mesh and polymer mesh were used at different locations of the slab specimens as in shown in Figure 3. The mortar mix for the above specimens was prepared in a pan mixer available in the laboratory and spread to a depth of 6mm initially and compacted. Then the required type and layer/(s) of reinforcement was placed and the mortar applied over the mesh, leveled and compacted to the required thickness. The slab specimens were demoulded after 24 hours and water cured for 28 days before testing.

[FIGURE 3 OMITTED]

Impact Test Set-up

Toughness of a material, defined as the ability to absorb energy, is generally evaluated by the 'impact test'. Toughness which depends on the strength and ductility of material, is determined in two ways, namely, (i) by measuring the strain or deformation under an impact load and (ii) by determining the energy required to cause rupture/complete failure of the specimen. Several methods have been reported to evaluate the impact characteristics of concrete/cement composites. Of them, the simplest and widely used test is the drop-weight test, which can be used to evaluate the relative performance of composites [9]. Reported work on the impact behaviour of ferrocement slabs relate to the use of conventional reinforcement (chicken mesh and M.S. skeletal) and drop-weight method (instrumented/ordinary falling weight) followed by charpy-type impact test [10-15]. Hence, in the present study drop-weight method was selected and used to study the impact characteristics of slab specimens. The actual experimental test set-up used is shown in Figure 4 & Figure 5.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Testing of Specimens

All the ferrocement slab specimens were surface dried and white washed to obtain a clear picture of cracks, when subjected to impact load/test. The impact test on the specimens was conducted under two conditions, namely, (i) over a sand bed of 100mm thick contained in an aluminum box of plan area 600 mm x 600 mm and conveniently positioned below the impact test set--up and (ii) the slab was simply supported in all the four sides of a fabricated stand and held firmly to the floor. The schematic diagram of Impact Test Loading is shown in Figure 6. A steel ball of weight 1 kg was allowed to fall freely from a constant height of 300mm through a guide at the center of the slab for all the specimens with the above support conditions, till the formation of first crack at the bottom of the specimen. The width of the (first) crack and number of blows required to cause the first crack were noted. Then the process was continued further, till the crack propagated further and appeared at the top surface of the specimen. At that point, the width of the final crack and the corresponding number of blows were noted. The impact energy absorbed in Joules was computed using the relation (n x W x h x 9.81), where as n = No. of blows, W = weight in kg (1 kg), h = drop height in meters (0.30m).

[FIGURE 6 OMITTED]

Results and Discussions

Impact Energy

The impact energy absorbed / required to cause the first crack, the impact energy absorbed / required to cause the final crack / failure of the ferrocement slab under sand bed condition is given in Table 4. The ratio of energy absorbed up to failure of specimen to the energy absorbed at initiation of first crack, defined as the 'residual impact strength ratio' (Irs). The Irs value for various ferrocement slab were computed and given in Table 5. As the entire impact energy is utilized fully by the specimens when they are tested under simply supported conditions, 'Irs' is relevant for the above case and hence calculated and reported for the test data pertaining to the above support condition only. It can be seen that the impact energy absorbed by the polymer mesh reinforced ferrocement slab were lower and it may be attributed to the poor bonding between the matrix and polymer mesh, there by causing poor transfer of load/energy. Hence the above aspect needs further investigation with a view to enhance the energy absorption characteristics and to determine the corresponding applications.

Effect of Support Condition

Energy absorbed Vs mesh location and no. of mesh layer location in simply supported condition and sand bed condition relations are shown in Figures 7-10. It is observed that there is no variation in the (impact) energy absorbed by the ferrocement slab when the reinforcement (chicken mesh/polymeric mesh) is positioned at the bottom or center as a single layer and this result / characteristic is also seen in the case of two layered specimens. The above phenomenon is found to be irrespective of the support condition adopted for the impact strength study of ferrocement specimens. On the other hand, three layered specimens [mesh reinforcement located at top--bottom--center (T-B-C)] have shown higher energy absorption than two-layered specimens and the trend is also independent of the support condition and the type of mesh reinforcement. It is found that the residual impact strength ratio (Irs) lies in the range of 4.75 to 5.00 and 2.50 to 3.00 when chicken mesh and polymer mesh are used as reinforcement in two or three layers. In terms of the maximum energy absorbed for the above support condition it can be seen from the results (Table 4) that polymer mesh reinforced ferrocement slabs absorb only 53-60% of the maximum energy absorbed by chicken mesh reinforced specimens, considering the corresponding location of reinforcement and the number of layers of reinforcement (i.e. 2 and 3 layers). Almost the same trend is exhibited when the specimens are tested under sand bed condition.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Failure Pattern

It was observed during testing that the ferrocement specimens exhibited localized failure at the point of contact of the drop-weight and there is no fragments got detached from the specimens as the various layers of the mesh reinforcement helped to hold the different fragments together unlike the case of plain slab (with out any reinforcement) where the fragments got detached/ separated and fallen into pieces. It can be thus inferred that meshes used as reinforcement play a major role in not only improving the impact energy absorption but also help to retain / hold the various fragments together, after full damage has occurred to the specimens due to impact loading. Typical crack pattern observed after impact test is given in Figure 11. The initial crack width formed was found almost same in both chicken and polymer mesh reinforced ferrocement slabs. The final crack width in polymer mesh reinforced ferrocement was found more than that of chicken mesh reinforced ferrocement however crack width reduced as the number of layers of mesh reinforcement increased.

Conclusions

Following are the salient conclusions based on the experimental investigations reported in this paper.

1. Polymer mesh reinforced ferrocement slab specimens absorb only about 50% of the impact energy absorbed by chicken mesh reinforced slab specimens tested under identical conditions. The above phenomenon is the same for specimens tested under sand bed condition and simply supported condition.

2. The lower energy absorption may be attributed to poor bonding between the polymer mesh and the matrix of the ferrocement, which has to be properly addressed and investigated.

3. The residual impact strength ratio (Irs) lies in the range of 4.75 to 5.00 and 2.50 to 3.00 for specimens reinforced with chicken mesh and polymer mesh, respectively and considering two and three layers of the above reinforcement.

4. Identical failure patterns were observed in ferrocement slab specimens and it is independent of the type of reinforcement (i.e. chicken mesh/polymer mesh) adopted.

5. Higher energy absorption found in sand bed condition can be advantagesly used for applications like small channel lining, open terrace roof finish, pavement slab etc. under sand bed / fully supported condition.

References

[1] ACI 549R -97, "State-of-the- Art Report on Ferrocement."

[2] Ramesht, M.H., 1995, "Effect of Corrosion on Flexural Behaviour of Ferrocement", Journal of Ferrocement, 25 (2), pp105-113.

[3] Ambalavanan,R., 2000, "Corrosion of Ferrocement Elements & Use of Blended Cements," The Master Builder, pp18-20.

[4] Mansur, M.A; Paramasivam, P., Wee,T.H., and Lin,H.B., 1996, "Durability of Ferrocement-A case study," Journal of Ferrocement, 26 (1), pp11-19.

[5] Naaman, A.E., 2000, "Ferrocement and Laminated Cementitious Composites," Techno press 3000, P.O Box 131038, Ann Arbor, Michigan 48105, USA, pp372.

[6] "Ferrocement Model Code," 2001, Building Code Recommendations for Ferrocement (IFS 10-01) Reported by IFS committee IU.

[7] I.S: 12269-1987, "Specifications for 53 grade ordinary Portland cement," Bureau of Indian Standards, New Delhi.

[8] I.S: 383-1970, "Specifications foe coarse and fine aggregates from natural sources for concrete," Bureau of Indian Standards (BIS), New Delhi.

[9] Balaguru, P.N., and Shah, S.P., 1992, "Fiber Reinforced Cement Composites," McGraw Hill Inc., New York, pp530.

[10] Khan, M.B., Ong, K.C.G., and Paramasivam, P., 1999, "Behaviour of Ferrocement Slabs Under Low-Velocity Projectile Impact," Journal of Ferrocement, 29 (4), pp255-265.

[11] Arif, M., Pankaj, and Kaushik, S.K, 1998, "Experimental Studies on Fatique and Impact Characteristics of Ferrocement Plates," Journal of Ferrocement, 28 (3),pp 247-256.

[12] Grabowski, J., 1985, "Ferrocement under Impact Loads," Journal of Ferrocement, 15 (4), pp 331-341.

[13] Mathews, M.S., Achutha, H., and Srinivasa Rao, P., 1985, "Impact Test of Ferrocement," Journal of Ferrocement, 10 (1), pp 31-37.

[14] Shah, S.P., and Key W.H., Jr., 1972, "Impact Resistance of Ferrocement," Journal of Structural Division, ASCE, 98(8), pp111-123.

[15] Jagannathan,A., Sundarajan,T.,2006, "Flexural Characteristic of Ferrocement Panels Reinforced with Polymermesh and Polypropylene Fibres," Eight International Symposium and Workshop on Ferrocement and Thin Reinforced Cement Composites, FERRO-8, Bangkok, Thailand, pp 65-75.

A. Jagannathan

Assistant Professor, Department of Civil Engineering

Pondicherry Engineering College, Pondicherry-605 014, India.

E-mail: a_j_nathan1957yahoo.com
Table 1: Physical properties of sand

SI. No. Property Value

1 Specific gravity 2.63
2 Water absorption 1.0%
3 Percentage of voids 41.72
4 Bulk density
 a) Loose 1.5312 kg/lit
 b) Compacted 1.6340 kg/lit
5 Fineness Modulus 2.60

Table 2. Physical Properties of Meshes used in Ferrocement

SI. No Property/Description Chicken Mesh Polymer Mesh

1 Mesh shape Hexagonal Diamond
2 Width (Standard) 0.9 m 1.00 m
3 Mesh size (mm) 9 x 10 9 x 9
4 Thickness at joint 1.00 mm 1.3 mm
5 Diameter of wire 0.71 mm 0.8 mm (average)
6 Available form Roll (45 m) Roll (25 m)
7 Weight 390 g/sq. mt 120 g/sq.mt
8 Colour White Black
9 Specific Gravity 7.82 0.95

Table 3: Properties of Mortar

SI. No. Properties Age of testing (in days)

 3 7 28

1 Cube Compressive 14.20 23.5 37.80
 Strength (MPa) 0
2 Cylinder Compressive 10.50 18.8 30.50
 Strength (MPa) 0
3 Flexure Strength 7.54 10.6 12.71
 (MPa) 5
4 Split Tensile 2.47 3.87 4.62
 Strength (MPa)
5 Water Absorption (%) 7.14 6.04 3.13

Table 4: Impact Energy Absorbed (Sand Bed Condition)

 Energy Absorbed (J)

Mesh
Position Chicken Mesh Polymer Mesh

 Initial Final Initial Final

C 23.54 61.80 14.72 29.43
B 23.54 64.75 17.66 29.43
B-C 14.72 111.83 8.83 41.20
T-B 14.72 117.72 11.77 44.15
T-B-C 17.66 232.50 14.72 111.83

 Crack Width (mm)

Mesh
Position Chicken Mesh Polymer Mesh

 Initial Final Initial Final

C 0.1 2 0.1 4.0
B 0.1 2 0.1 3.0
B-C 0.1 2 0.1 2.0
T-B 0.1 2 0.1 1.2
T-B-C 0.1 1 0.1 1.2

Table 5. Energy Absorbed and Residual Impact Strength Ratio
(Simply Supported Condition)

Mesh Energy Absorbed (J)
Position

 Chicken Mesh Polymer Mesh

 Initial Final Irs Initial Final Irs

C 11.77 26.48 2.25 8.83 17.66 2.00
B 14.71 29.43 2.00 8.83 17.66 2.00
B-C 11.77 55.92 4.75 11.77 29.43 2.50
T-B 11.77 58.86 5.00 11.77 32.37 2.75
T-B-C 14.72 73.58 5.00 14.72 44.15 3.00

Mesh Crack Width (mm)
Position

 Chicken Mesh Polymer Mesh

 Initial Final Initial Final

C 0.1 1.2 0.2 1.2
B 0.1 1.2 0.2 1.2
B-C 0.1 1.0 0.1 1.0
T-B 0.1 1.0 0.1 1.0
T-B-C 0.1 1.0 0.1 1.0
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Author:Jagannathan, A.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Date:Dec 1, 2008
Words:2846
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