Behaviour of Reinforced Concrete Beams Strengthened by CFRP Wraps

IJSTE - International Journal of Science Technology & Engineering | Volume 2 | Issue 11 | May 2016 ISSN (online): 2349-784X

Behaviour of Reinforced Concrete Beams Strengthened by CFRP Wraps MohammadShahrukh G. Surti PG Student L.J. Institute of Engineering & Technology

Vaibhav Doshi Assistant Professor L.J. Institute of Engineering & Technology

Abstract The paper presents an experimental activity carried out on RC beams strengthened in flexure and shear with different configurations of CFRP laminates. Currently many building and other structure are deteriorated due to age or poor construction. There are many techniques used for retrofitting purpose like jacketing, fiber reinforced polymer (FRP) wrapping etc. This research is basically to strengthen and redesign existing structure. This research work includes Strengthening of full size beam (150mm*150mm*1000mm). Flexure or shear-critical beams were provided with CFRP longitudinal sheets like single layer, double layers etc or U-wraps, respectively. Flexure – shear shear critical beams were provided with shear or combined systems. Concrete beams reinforced internally with steel and externally with carbon FRP. Laminate applied after t he concrete had cracked were tested under two-point bending. Result show that FRP is very effective for flexural strengthening. Different type of wrapping of the FRP laminate combined with adhesive bonding is effective in anchoring the laminate. Keywords: Carbon fiber, Moment carrying capacity, Retrofitting, Strengthening, Wrapping ________________________________________________________________________________________________________ I.

INTRODUCTION

A structure is designed for a specific period and depending on the nature of the structure and surrounding environment, its life varies. Depending on usage of structure life of the structure are decided. During the life span of structural deterioration happens. The deterioration can be mainly due to environmental effects, which includes corrosion of steel, gradual loss of strength with ageing, repeated high intensity loading, variation in temperature, freeze-thaw cycles, contact with chemicals and saline water and exposure to ultra-violet radiations. Now to compensate deterioration, structure should be either replaced or properly retrofitted. As complete replacement or reconstruction of the structure costs very high compared to strengthening or retrofitting, so generally it is preferred to retrofit or strengthened the structure. Retrofitting works are generally done by some techniques like FRP wrapping, Jacketing, near surface mounting reinforcing etc. Till date mainly Glass fibers, carbon fibers, basalt fiber, aramid fiber etc. are used for retrofitting works. Carbon fibers can be used for the same purpose. Carbon fiber has very high tensile strength and is also very lightweight. When it bonded to the exterior of a concrete column, beam, or slab, it can add significant strength without adding weight that would increase the load on foundations and other structural members. Carbon fiber can be bonded to concrete, masonry, steel and wood structures using a specially formulated structural epoxy to increase the load carrying capacity and ductility. II. PHYSICAL AND MECHANICAL PROPERTIES

Physical Properties Properties Carbon Fibre has High Strength to Weight Ratio (also known as specific strength). Carbon Fibre is very rigid. Carbon fibre is very rigid. Carbon fibre reinforced plastic is over 4 times stiffer than Glass reinforced plastic, almost 20 times more than pine, 2.5 times greater than aluminium. Carbon fibre is Corrosion Resistant and Chemically Stable. Although carbon fibre themselves do not deteriorate measurably, Epoxy is sensitive to sunlight and needs to be protected. Carbon fibre is Electrically Conductive. Carbon Fibre has good Tensile Strength and fatigue resistance. Carbon fibre can be made to feel quite soft to the hand and can be made into or more often integrated into protective clothing for firefighting. Carbon fibre low coefficient of thermal expansion. Carbon Fibres are brittle, in other words carbon fibre does not bend much before failing. Carbon is also very good resistor of ultra-violate rays. They do not degrade under ultra-violate rays. Carbon Fibre is Relatively Expensive. The low maintenance requirement of carbon fibre is a further advantage. Mechanical Properties Properties Mechanical properties of Carbon fibres are as given in following Table 1.

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Behaviour of Reinforced Reinforced Concrete Concrete Beams Beams Strengthened Strengthened by CFRP CFRP Wraps (IJSTE/ Volume 2 / Issue 11 / 057)

Table – Table – 1 1 Mechanical Properties Technical data

300 g/m 2

Weight per unit area of sheet (g/m 2)

330

Elastic modulus modulus (KN/mm2)

260

Tensile strength (N/mm 2)

3900

Fibre weight (g/m 2) (main direction)

300

Density (g/cm (g/cm 3)

1.8

Elongation Elongation at rupture rupture (%)

1.55

Design thickness thickness (Fibre weight/density) weight/density) (mm)

0.176

III. MATERIAL USED IN EXPERIMENTS

Carbon Fibre Carbon fibre has very high tensile strength and is also ver y lightweight. When it bonded to the exterior of a concrete column, beam, or slab, it can add significant strength without adding weight that would increase the load on foundations and other structural members. Carbon laminate is a stiff composite plate use to enhance the flexural and moment enhancement for structure. It is available in Rolls of 100 m, 150 m, or cut to size. An unwinding reel is available upon request. Special dimension upon Request. Its elastic modulus is 260 kn/mm 2 and tensile strength is 3900 n/mm 2. Epoxy Epoxy resin are generally low molecular weight pre-polymers capable of being processed under a variety of conditions. Two important advantage of these resin over unsaturated polyester are: First they can be partially cures and stored in the state and they exhibit low shrinkage during cure. It is a 100% solids low viscosity epoxy resin able to cure in the presence of moisture and at temperatures as Low as 2 2 ̊ C. The chemical resin has high chemical and corrosion resistance, good mechanical and thermal properties. It has two components, A – A – resin resin and B – B – hardener. hardener. Ratio of the components by weight is 100 parts of component B to 50 parts of component A shown in fig 1. Mixing is done thoroughly for 5 min with low speed mixer at 400 rpm until components are thoroughly dispersed. The properties of epoxy are mention in Table 2.

Fig. 1: Epoxy adhesive applying on beam Table - 1 Epoxy properties Properties (unit) Value Density (kg/1) (kg/1) 1.10 + 0.01 Mix Ration Ration (Resin (Resin : Hardener) Hardener)

50:100

Packing size (kg)

15 Kg

Tensile Strength (N/mm2)

22

Flexural Strength (N/mm2)

52

Initial Initial tackiness tackiness (hours) (hours) Final Set (Days) Bond strength strength in concrete concrete

1.0 1 (Days) Failure in concrete concrete

Coverage

0.25 – 0.35 0.35 kg / sq meter


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Concrete Concrete used in this experiment are of M25 grade general concrete. No any admixture is used. IV. MIX DESIGN

The specified design strength of concrete is 25MPa at 28 days. The specific gravity of Fine Aggregates (FA) and Coarse Aggreg ates (CA) is 2.61 and 2.84 respectively. The standard deviation can be taken as 5MPa. Ordinary Portland cement was used of 53grade. Coarse aggregate is found to be Absorptive to the extent of 1% and free surface moisture in sand is found to be 2%. According to IS10262-1982 clause 3.3 Table no 3. The mixing water content calculated is 178 kg/m 3. Mix proportion of M25 grade is shown in Table 3 & 4. Table - 2 Mix proportion Fine Aggregates Coarse Aggregates

Water

Cement

178

360 kg

697 kg

1213 kg

0.49

1

1.94

3.37

Table - 3 Concrete mix design quantities Grade of concrete: M25 Coarse aggregate(20mm): 2.65 Type of exposure: Mild Coarse Aggregate(10mm): 2.65 Sp. Gravity of cement: 3.15 Maximum Water Cement Ratio: 0.4 95 Fine Aggregate: 2.65

V. TEST PROGRAM AND RESULT

Test Program Table 5 summarizes the general experimental test program. This program consisted of testing twenty-one rectangular beams in order to evaluate the effect of externally bonded CFRP laminates to the different strengthening scheme for the entire beam length. A total of eighteen reinforced concrete beams having different CFRP configurations were fabricated in the laboratory for the strengthening purposes. First beam (designated as CB) was not bonded with CFRP laminates, nine beams (FB-1L, FB-2L, FBML) were bonded with different layers of CFRP laminates (1, 2 layers and mid span layers respectively) and the nine beams (designated as FB-UL, FB-PUL, FB-AL ) were bonded with one layer CFRP laminates (U-shaped edge strips, partially U-shaped strips,45 degree strips).These transverse CFRP laminates provide anchorage for the longitudinal plate, and considered effective in preventing de-bonding failure of laminates from the concrete surface. Table - 4 Test program

Beam designation

Control (not bonded)

1-layer bonded

2-layer bonded

At mid span

Fully Ushaped

Partially U-shaped strips

45 degree strips

CB

FB-1L

FB-2L

FB-ML

FB-UL

FB-PUL

FB-AL

Cured 28 days

Table - 5 Curing specimen details (150x150x1000mm): CB FB-1L FB-2L FB-ML FB-UL FB-PUL 3 nos. 3 nos. 3 nos. 3 nos. 3 nos. 3 nos.

FB-AL 3 nos.

The beams CB, FB-1L, FB-2L, FB-ML, FB-UL, FB-PUL, FB -AL cured for 28 days were tested on the Flexural Testing machine. The following results of failure load were found out for Plain concrete beams, One sided CFRP wrapped concrete beams, Two sided parallel CFRP wrapped concrete beams, Two sided parallel CFRP, at mid span CFRP, U shaped strips CFRP, partially U shaped strips CFRP, 45 degree strips CFRP. The details of the load are thus given in the table 7 below.

Deflection

Table - 6 Deflection and Load result Point load (KN) CB FB-1L FB-2L FB-ML

FB-UL

2

33

33

33

33

33

4

53

53

53

53

53

6

64

64

64

64

64

8

76

84

96

132

113

10

86

92

102

141

124


317


12

91

99

106

145

126

14

86

102

109

149

131

16

61

67

98

121

101

Comparison of normal crack beam at 6mm deflection without wrapping to single layer carbon wrapping, double layer wrapping, at mid span wrapping, u shaped shaped strips wrapping crack beam at 6 mm deflection. deflection. Figure 2 shows the graphical representation of moment carrying capacity(kn-m) versus deflection(mm) graph and Figure 3 shows the graphical representation of point load(KN) versus deflection(mm) graph as below.

Fig. 2: Moment carrying capacity (kn-m) versus deflection (mm) graph

Fig. 3: Point load (KN) versus deflection (mm) graph Result From the above results and graphs following observations are made : Table - 7 Result of moment carrying capacity Sr.No

Beam description

Increase in moment carrying capacity comparing with Plain Concrete beam. (%)

1

Single layer wrapping

12-18

2 3 4

Double layer wrapping At centre span wrapping U shaped strips wrapping

15-30 35-50 60-75


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Also the strength of beam wrapped at tension side was greater than the beams wrapped at two parallel sides. Therefore, from the above results the beam wrapped at tension side gives better results so from economic point of view CFRP wrapped at tension sides is desirable. VI. CONCLUSION

From the above experiment it is clear that with the help of fabric wrapping strength of the existing member increase. For the mentioned size of the beam it is clear t hat if we wrapped the beam with mentioned carbon fabric than mini mum 1.5 times strength increase in the both the case flexure and shear. Moment carrying capacity of single layer beam, double layer beam, at centre and fully u shaped beam is increase by 12-18%, 15-30%, 35-50% and 60-75% respectively. Maximum moment carrying capacity of single layer beam, double layer beam, at centre and fully u shaped beam is 15.3 kn-m, 16.35 kn-m, 19.65 kn-m and 22.35 kn-m respectively. ACKNOWLEDGMENT

For this thesis work I would like to thanks to my guide Mr.Vaibhav Doshi for their sincere guidance and help to carry out my experiments. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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Behaviour of Reinforced Concrete Beams Strengthened by CFRP Wraps

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