STUDY ON STRENGTH OF BEAMS RETROFITTED USING FERROCEMENT JACKETS

 

ABSTRACT

l Our main attention in this dissertation is the use of ferrocement as retrofitting material.

l This disseration is carried out by casting cubes, cylinders, prisms to evaluate compressive strength, tensile strength and flexural strength of concrete.

l The dissertation carried out the study on the strength of stressed beams retrofitted with ferrocement with different orientation.

l In that two control beams are tested to failure to find safe load carrying capacity to allowable deflection as per IS 456-2000 and six beams are stressed upto 75% of the safe load and these beams are retrofitted with different orientation wire mesh as 0⁰, 45⁰ and 60⁰.

INTRODUCTION

l In this developed world RCC structures are suffer from damage and distress before their service period is over, such structure required immediate attention to bring the structures into its functional use again.

 

l There are various types of retrofitting techniques used to bring such structure into its functional use again, apart from which plate bonding technique is considered as the best.

 

l The plate bonding technique consists of plate of different material such as CFRP, GFRP, ferrocement etc.

 

l Ferrocement sheets are most commonly used or retrofitting material these days due to their easy availability, economy, durability, and their property of being cast without needing significant formwork.

 

l The National Disaster Mitigation Agency (NMDA), Government of India also accepted the use of ferrocement as retrofitting material.

 

l The behaviour of ferrocement in flexure depends upon various parameters such as mortar, type of wire mesh, orientation of wire mesh etc.

ØFERRORCEMENT

     Ferrocement is a composite material consisting of rich cement mortar matrix uniformly reinforced with one or more layers of very thin wire mesh with or without supporting skeletal steel.

 

v Compressive Strength:

            Compressive strength of ferrocement is more in use of welded wire mesh due to its lateral restraint provided by welded transverse wires.

            Kameshwara Rao and Kamasundra (1986) (13) studied the behaviour of ferrocement in compression it indicates that compressive strength depends upon specific surface area of the composite.

 

v Flexural Strength:

            Mansur and Paramasivam (1986) (14) studied the behaviour and strength of ferrocement in flexure. It was found that the ultimate moment increase with increase matrix grade and increasing volume fraction of reinforcement.

 

v Shear Strength:

            Venkata Krishna and Basa Gouda (15) (1988) found that shear strength depends upon mortar, strength of wire mesh, volume fraction and shear span

 

v Impact Resistance:

          Impact strength is defined as the energy absorbed by the specimen when struct by a swinging pendulum dropped from a constant height.

          Shah and Key (1972) (16) tested ferrocement slabs using impact tester. It indicated that the higher the specific surface of the meshes and the higher the strength of the mesh, the lower the damage due to impact loading.

 

EXPERIMENTAL PROGRAMME

v Test Programme

            To carryout the investigation, Eight Prototype Underreinforced Beams of size 2300X130X230mm(lxbxd) reinforced with 2-12mmΦ and 2-10mmΦ in compression are cast using M20 grade concrete.

 

            Out of these Eight beams two are used as control beams (Type – A) and tested to failure to find out the safe load carrying capacity corresponding to the allowable deflection as per IS: 456-2000 i.e. Span/250. The other six beams are then stressed to 75 percent of the safe load  obtained from the testing of the control beams and are then retrofitted with 20mm thick ferrocement jackets made with 1:2 cement sand mortar and w/c ratio 0.40. The jackets are reinforced with single layer of 40 mm x 40 mm square welded wire mesh. The three wire mesh orientation viz., 00, 450, 600 degree are used in the ferrocement jackets.

 

v Test for compressive strength of concrete (IS: 516-1959):

     Compressive strength is calculated by dividing load by area of specimen.

Fc =P/A

Where  

Fc = Cube compressive strength in N/mm2.

P = Cube compressive load causing failure in N

A = Cross sectional area of cube

No. of cubes tested for plain cement concrete are shown in table below

 

v Test for Split Tensile Strength of concrete (IS: 5816-1970)

 

Tensile Strength = fcs = 2F/πdl,

 

Where:

F= fracture compression force acting along the cylinder generatrix,

d= cylinder diameter;

l = cylinder length.

 

     Numbers of cylinder tested for plain concrete are shown in table below

 

v Test for Flexural Strength of concrete (IS:516-1959)

    

     The maximum tensile stress is called modulus of rupture and is computed from the standard formulae.

 

F = M/Z

Where

M = Bending moment at the section where rupture occurs

Z = Section Modulus = I/Y

I = Moment of inertia of the section

Y = Distance from neutral axis = d/2

 

Number of prisms for plain concrete are shown in table below

 

v Testing Methodology

 

            First, the two control beams are tested to failure. The load corresponding to an allowable central deflection of 8.6mm (span/250) is obtained from the load deflection curve as 59.25kN.

 

            The remaining six beams are stressed to 75 percent of this average safe load i.e., 44.44kN.

 

            Subsequently, the retrofitting of beams using different orientaitons of wire mesh in the ferrocement jackets are carried out with cement mortar of thickness 20mm along with wire mesh bonded on three sides for all six beams. After one week of curing the beams are tested again with the same method as the control beams are tested initially and the corresponding results are recorded in the form of load v/s deflection.

DISCUSSIONS

            During the experimental work, close observations have been done to study the following.

 

l The curves shows that with an increase in the load carrying capacity there is a considerable increase in the deflection for all the beams except two control beams.

 

l The percentage increase in the ultimate load w.r.t the control beams is highest for the beams retrofitted with 450 orientation (i.e 39.75%)

 

l The percentage increase in the ultimate deflection w.r.t the control beams in highest for the beams retrofitted with 450 orientation (i.e 24%)

 

 

l   The beams retrofitted with 45⁰ orientation is the best among all the three orientation because of its enhanced maximum load carrying capacity.

 

l   The deflection at the centre at ultimate load is maximum in the case of beams retrofitted with wire mesh at  45⁰ (15.55mm) as compared  to those with wire mesh at zero degree, (13.68 mm) and 60⁰ (14.16mm).

 

l   The ductility ratio is highest in case of beams retrofitted with wire mesh at 0⁰orientation followed by 45⁰ and 60⁰ orientation.

 

l   Retrofitted beams with wire mesh oriented at zero degree are the most efficient of the three orientations as its cost to strength ratio is the lowest at 1.25 as compared to the other two orientation i.e. 0⁰ (1.28) and 60⁰ (1.41).

CONCLUSION

l   The first crack appeared for control beams at early stage and for retrofitted beams the first crack appeared lately.

l   The load carrying capacity of retrofitted beams is more compared to control beams.

l   The beams retrofitted with wire mesh at different orientation do not de-bond when loaded to failure.

l   Wire mesh oriented at 45⁰ has the highest load carrying capacity as compared to the control beams as well as the other orientation of the beams.

l   The failure of the composite is characterized by the development of flexural cracks on the tension side.

l   In case of retrofitted beams it is observed that the spacing of cracks is increased, indicating better distribution of stress when compared to the control beams.

l   Beams retrofitted with wire mesh oriented at zero degree are the most efficient as their cost to strength ratio is lowest.

SCOPE FOR FURTHER STUDY

l   In this present investigation the retrofitting of ferrocement laminates is done on three sides of the specimens. Further work can be carried out by the retrofitting of ferrocement laminate is done only bottom face of the specimens.

l   Further work  can be carried out by using higher grades of concrete i.e., M 25, M 30 etc.,

l   For the present investigation the specimens are stressed to 75% of safe load. Further work can be carried out the specimens are stressed to varying percentage of safe load.

l   Presently the study of RCC rectangular beam is carried out on a limited span of 2.3 m. further work can be extended for greater span.

l   In this investigation only flexural behaviour is studied. Further work can be carried out by combined flexure and shear behaviour.