Shear behavior of RC Deep beams retrofitted with CFRP Sheets

Shear behavior of RC Deep beams retrofitted with CFRP Sheets

Alexin M S PG Scholar

Department of Civil Engineering Saintgits College of Engineering Kottayam, India

Pinky Merin Philip Assisstant Professor Department of Civil Engineering Saintgits College of Engineering Kottayam, India

Abstract RC deep beams are structural members widely used in numerous structural applications like bridges, water tanks, foundations, silos, bunkers, oshore structures, and tall buildings. Deep beams are beams having shear span to depth ratio less than 1. Carbon Fiber Reinforced Polymer (CFRP) is a strong and light fiber reinforced plastic containing carbon fibers. Shear behavior of RC deep beams retrofitted with CFRP Sheet enhances the load carrying capacity. The main concern of this paper is to investigate experimental behaviour of retrofitted reinforced concrete deep beams by CFRP sheets.

Keywords Deep beams, Carbon Fiber Reinforced Polymer (CFRP) Sheets, Retrofitting, Shear behavior

1. INTRODUCTION

   A beam is considered as deep beam, if the depth of beam is in relation to the span of the beam. For simply supported beams, it behaves as deep beam when the ratio of its effective length

   1. to overall depth (D) is less than 2.0 and that for continuous beam is when the ratio is less than 2.5. In deep beams effective length is defined as the centre to centre distance between the supports or 1.15 times the clear span, whichever is less. Types of deep beams are classified as: (1)Simply Supported Deep Beams, (2)Continuous Deep Beam and (3)Deep Beams with and without openings. Deep beams can be widely used in many situations where ordinary beams or other structural members cannot be applied such as in bridges where long spans are required, in large halls or building where no column is to be used and in R. C. C. side walls of water tanks. Pile caps can also behaves as deep beams when the required span is lesser. In some cases; Raft foundation also contains deep beams: Bunkers and Silos of such structures may act as deep beam. For decades, methods of design and analysis for concrete members reinforced with normal strength steel have been developed. Lately reinforcing steel with enhanced strength than conventional steel has become commercially available.

      The strut and tie modelling method is a widely accepted approach for R.C deep beams. However there are major differences among various design code Applications for this method with respect to reinforcement tie influences on the capacity of adjoining concrete struts. Furthermore, each design code provides altered limits on the maximum allowable stresses in the ties. Since high performance reinforcement continues to get wider reach in industry practices; it is

      necessary to validate existing design approaches for the mechanical properties of these new materials. The replacement of the smatched structural elements is very difficult and uneconomical process and the replacement of a particular structural element from an existing structure also creates hazard to the integrity of other connecting structural members. To reinstate the strength required of the smatched structure retrofitting is the solution. Retrofitting can be applied in two ways:

      • Global Retrofitting

      • Local Retrofitting

   In Global Retrofitting, the entire structure is retrofitted to fulfil the serviceability requirements. It involves the design and analysis of the entire structure as per the specifications given in standard codes. In Local Retrofitting only selected member of the structure is either strengthened or replaced.

   From the past studies conducted; it has been shown that externally bonded CFRP can be used to enhance the flexural, shear and tensional capacity of RC beams. The application of FRPs for the repair of accessible concrete structures has grown very speedily over the past few years. The CFRP can replace steel with overall cost savings in the order of 25%.

2. EXPERIMENTAL INVESTIGATION

   1. Experimental Sample

      Experimental study is done to examine the shear behavior of RC deep beams retrofitted with CFRP Sheets. Deep beams of size 450mm x 200mm x 1000mm is used and is designed for 280kN. M30 grade concrete is used and the grade of steel used is Fe500.

      Details of the reinforcement in the deep beam are shown in Fig 1. 2 No.s of 16mm TOR steel bars at the bottom and 2 No.s of 12mm TOR steel bars at the top are given as main reinforcements. 2 legged 8mm stirrups at 240mm centre to centre are given as vertical shear reinforcements and 2 No.s of 8mm TOR steel bars are laid as horizontal shear reinforcements.

      Fig 1.Reinforcement details

      Fig 2. Casted Specimen

   2. Material Characteristics

      According to IS 10262:2009, mix proportion obtained for casting specimen is 1:1.97:3.84 by weight and water cement ratio was 0.38 by weight. Ordinary Portland cement, Msand and crushed gravel of 20mm maximum size are used. Ceraplast, a super plasticizer is used as admixture.

      Carbon fiber reinforced polymer (CFRP) sheets with 0.10 mm thickness is used for wrapping as two layers. The mechanical properties of the CFRP material having elastic modulus of 230 GPa, tensile strength of 3500 MPa. The epoxy resin and hardener are used as adhesive.

   3. Experimental Setup

      Shear strength test of reinforced concrete deep beam is carried on loading frame of 50T capacity. Single point loading is applied. Dial gauges with 0.01mm and maximum range of 10mm were used to note the deflection values and a strain gauge is used to calculate the strain values.

      Fig 3.experimental setup

      Fig 4. Retrofitted test beam Setup

3. RESULTS AND DISCUSSIONS

   • Load v/s deflection of the beams are compared and is given in fig 9.

   • The load v/s strain of the beam are also compared as shown in fig 14 and 15.

   • The 28 days cube compressive strength obtained is 32.4MPa.

   • Initial crack obtained is 182kN and the ultimate load obtained is 322kN. Mode of failure obtained is shear compression failure and is shown in the fig.5.

   • Ultimate load of the retrofitted specimen obtained is 415kN

     Fig.5. Failure Pattern

     Fig 6.Retrofitted beam after failure

     Fig 7.Load v/s deflection graph of control specimen

     Fig8. .Load v/s deflection graph of retrofitted specimen

     Fig 9.Combined Load v/s Deflection graph

     Fig 10.Load v/s Strain graph at the compression face

     Fig 11. Load v/s strain graph at the tension face

     Fig 12. Load v/s strain graph of retrofitted specimen at the compression face

     Fig 13. Load v/s Strain graph of retrofitted specimen at the tension face

     Fig 14.Load v/s Strain at the compression face

     Fig 15. Load v/s strain at the tension face

4. CONCLUSIONS

Based on experimental studies, the following can be concluded:

• Retrofitting the deep beams with CFRP sheets gives an 28.8% increase in the ultimate load carrying capacity.

• Retrofitted specimen gives deflection less compared to the control specimen.

• For ultimate load, retrofitted specimen shows less strain compared to the control specimen.

REFERENCES

1. Y. W. Choi and H. K. Lee Shear behavior and performance of deep beams made with self compacting concrete, International Journal of concrete structures and materials, Vol. 6, pp. 65-78 (August-2012)

2. Dr. Pandurang S. Patil Experimental study of behaviour of RC deep beams Interational Journal of emerging technology and advanced engineering Vol. 4, Issue 7, July 2014, pp.285-289

3. Jamal A Khudair Shear behaviour of self compacting deep beams strengthened with CFRP sheets IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE), Volume 11, Issue 2 Ver. II (Mar- Apr. 2016), PP 69-77

4. Mohammed Rasheed Retrofit of RC deep beams with different shear reinforcement by using CFRP civil and environmental researcp224-5790,ISSN Volume 8, no.2 ,2016

5. Lalin Lam and Qudeer Hussain Behavior of RC Deep Beams Strengthened in Shear using Glass Fiber Reinforced Polymer with Mechanical Anchors International Journal of Engineering Research and Applications (IJERA) Vol. 3, Issue 4, september 2015, pp.213-223

6. SOO yean SEO et.al structural behaviour of RC deep beams with headed longitudinal reinforcement13th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6, 2004 Paper No. 58