Strengthening and rehabilitation of reinforced concrete beams with

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 2, No 1, 2011

© Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN 0976 – 4399

Received on July 2011 published on September 2011 127

Strengthening and rehabilitation of reinforced concrete beams with opening

Subhajit Mondal 1 , Bandyapadhya J.N 2 , Chandra Pal Gautam 3 1, 3 Post graduation Student, Civil Engineering Department

2 – Professor, Civil Engineering Department, Indian Institute of Technology, Kharagpur, India

[email protected]

ABSTRACT

In modern building construction opening in beams are more often used to provide passage for utility duct and pipes. As a result storey height and material cost can be reduced. However providing an opening in the beam causes crack around opening reduces stiffness and also leads to more complicated structural response. In this paper the use of Glass Fiber Reinforced Polymer (GFRP) to strengthen and rehabilitate are discussed. In this experiment 10 beams, one solid as reference beam and other nine beams categorized as beams with openings, strengthened beams and rehabilitated beams are tested. The effect of FRP on the deflection pattern, cracking, strain near vicinity, initial crack load, and ultimate failure load are discussed. This investigation may help the designer to provide sufficient opening in the beams without reducing its carrying capacity furthermore, help to understand the behavior of retrofitted beams with openings.

Keywords: Reinforced Concrete Beam, Rectangular Opening, GFRP, Strengthening and Rehabilitation

1. Introduction

An opening into beams changes their simple behaviour to a complex one and it will cause serviceability problem. Cracks that develop near the opening seriously reduce the load carrying capacity of the beams. Thus, it becomes necessary to study the effect of openings on the beams so that it can be provided as structural elements without compromising their carrying capacities. Furthermore, the effect of opening must be considered in the designing process of beams with openings.

Numerous investigations have been carried out on beams with openings to predict the behavior of beams, crack propagation, effect of opening size and shapes, on the other hand Fiber Reinforced Polymer (FRP) can play a major role in strengthening and retrofitting of strength deficient and degraded structures.

Opening with height of 0.6 of beam depth may reduce the beam capacity by 75 percent 1 . Furthermore, it is found that shear failure occurs at the opening chord of strengthened opening due to combination of shear failure of concrete and bond failure of FRP sheets glued to the concrete. Experimental investigations using fully wrapped shows that CFRP substantially increase the shear capacity of beams without stirrups 13 .

External reinforcement increases the ultimate strength from 60 to 150 percent and the orientation of fibers influences the shear strength contribution 9 , also Flexural strengthening

Strengthening and rehabilitation of reinforced concrete beams with opening Subhajit Mondal, J.N. Bandyapadhya , Chandra Pal Gautam

International Journal of Civil and Structural Engineering 128 Volume 2 Issue 1 2011

by FRP can induce shear failure 10 . Furthermore, shear strengthening of beams can change the mode of failure 5 .

Unfortunately only a limited number of investigations have been carried out in the past to establish the efficiency of GFRP sheet on strengthening and retrofitting of beams with openings. Carbon fiber reinforced polymer (CFRP) strengthening of beams with small opening at shear zone can change their mode of failure 1 .

In this research GFRP sheets are used in the opening zone of beams to strengthen and rehabilitation. The aim of the study is to investigate the effect of opening on the load carrying capacity, deflection behavior of beams with different size of opening, strain distribution in the vicinity of opening, strengthened the beam opening and the efficiency of FRP to increased the shear capacity of damaged beams with openings. After the investigation it is found that GRFP can increase the carrying capacity effectively for small openings only and it is unable to increase the carrying capacity effectively for large openings.

This paper will guide the user to use GFRP in a beam with opening and it will give the idea of importance of GFRP used for the rehabilitation of damaged beam. Results of this experiment may guide to formulate the design guide lines of beams with openings and strengthen and rehabilitated beams.

2 Experimental program

2.1 Experimental Set up

Figure 1: Experimental Set up Figure 2: Reinforcement Details

The experimental program consists of ten beams of which one beam is solid having no opening and the other nine beams are divided into three groups having three beams in each group. The sizes of the opening are 100 mm width x 100 mm in the first group of three beams, 200 mm x 100 mm in the second group of three beams and 300 mm x 100 mm in the third group of three beams. OB1, OB2 and OB3 are the first beam each of the three groups with opening and without any GFRP layer. The three beams SB1, SB2 and SB3 are the second beam of each of the three groups with opening and strengthened with isotropic GFRP lamination at the opening zone. Similarly, RB1, RB2 and RB3 are the third beam of each of the three groups with opening and rehabilitated with GFRP lamination at the opening zone after they develop initial crack in the absence of GFRP lamination. The letters O, S and R represent beams with openings, strengthened beams and rehabilitated beams respectively, while the numbers 1, 2 and 3 indicate the width of opening 100, 200 and 300 mm,



respectively. The height of each opening is fixed at 100 mm. The opening is provided in the shear zone at a distance of 200 mm from the support. The reference solid beam without opening is designated as RSB and has no number in the suffix. All ten beams are subjected to two point loads at an increment of 1 tone applied at a distance of one third of the span from the two supports.

2.2 Materials properties

The concrete mix used for all the beams is designed for a M30 with a water cement ratio of 0.37. Main reinforcing bars of Fe 415 of dia. 8 mm at top and 12 mm at bottom are used. Rectangular closed stirrups of 6 mm mild steel bars although except in the opening zone, where ‘U’ shape stirrups are used. Glass fiber reinforced polymer of thickness 0.32 mm is used for strengthening the beams. Epoxy adhesive is used to attach the GFRP to the beam surface. The resin is a 9:1 mixture of Araldite CY 230 and hardener HY 951. Average cross sectional dimension of FRP along with epoxy is 0.94 x 26. 92 mm. The property of FRP and Epoxy are presented in Table 1. Properties of GFRP are determined as per ASTM guidelines.

Figure 3: Typical Load versus Deflection plot of FRP Sample

Table 1: Properties of Material

Material Young Modulus (MPa)

Max. Load (N)

Tensile stress at Yield (MPa)

Tensile Strain (%)

GFRP 3310 1834 63.23 2.62 Epoxy 1242 4075 1.09

3. Experimental Results and Discussion



Available experimental data shows that shear failure of FRP strengthened beams are generally by two modes: first mode FRP ruptures and second mode debonding of FRP from concrete surface. Sidebonded FRP strips fails in second mode only and also most of the beams with FRP Ujackets fails in second mode 11 . Some beams strengthened by U jacketing failed due to FRP rapture 4 . Beams with FRP wrapping shows the failure in FRP only. To avoid the debonding and premature failure of beams, FRP are wrapped around the opening keeping at least 80 mm from the opening side.

Table 2: Presentation of Test Result

Opening Concrete Initial Ultimate Size Strength Crack load

(W x H) (MPa) load (kN) (kN)

Beam

(mm) RSB 44 86 156 OB1 100 x 100 42 64 104 OB2 200 x 100 42 62 96 OB3 300 x 100 39 40 95 SB1 100 x 100 45 62 126 SB2 200 x 100 40 59 104 SB3 300 x 100 41 60 98 RB1 100 x 100 43 40 120 RB2 200 x 100 42 61 105 RB3 300 x 100 43 60 92

Table 3: Change in Strength and Mode of Failure

Beam Ri Ru Gu Ru Ri Mode (%) (%) (%) of failure

RSB Flexural OB1 25 33 7 Shear OB2 28 38 10 Shear OB3 53 39 14 Shear SB1 29 19 21 8 Shear SB2 31 33 4 2 Shear SB3 30 37 3 7 Shear RB1 53 23 15 30 Shear RB2 29 32 9 3 Shear RB3 30 41 3 10 Shear

Strength of beams with openings:

As shown in the Figure 5, the strength and stiffness both reduced in the beams with openings. The ultimate loads of beams with openings and solid beam are evaluated to find out the influence of opening at the shear zone. It can be seen that reduction in ultimate load (Ru) increased as the opening size increase and vary to 6 percent as we increase opening size for three times. For the initial crack load the variation is 28% as we increase the opening for three times. The Figure 4(a), 4(b) and 4 (c) shows the ultimate failure pattern of solid beam



and beam with opening. The column six of Table 3 indicates the failure pattern of solid beam and beam with opening. The failure mode changes due to small size opening also. This shows an important difference in behavior of solid beam and beam with opening.

Figure 4: Crack patterns of Beam (a) RSB (b) OB1 (c) OB2 (d) SB1 (e) SB3

Figure 5: Load versus Deflection at Central Point of Beam

Strength of solid beams

As shown in the Figure 6, strength of the strengthened beam increase for small opening. The beams with large opening do not show much effectiveness. The Table 3, show the strength of strengthen beam increased 21 percent for the beam with small opening only. Increase in strength for large opening varying three to four percent. For such cases the GFRP does not



show much effectiveness. All the three strengthened beams fails due failure at shear. The Single layer GFRP increased the load carrying capacity but it is not effective to change the failure pattern. Table 3, indicates the failure pattern of solid beam and beam with opening. The Figure 4(d) and 4(e) shows failure pattern of strengthened beam.


Strength of rehabilitated beam

As mentioned earlier the three strengthened beams are wrapped with FRP layer before applying any load on them. The respective loads at initial crack and ultimate failure load (Wi and Wu ) as obtained from the experimental investigation are furnished in Table 2 and table 3. The gain in strength of strengthened beam is calculated as [(strength of strengthened beams with openings – strength of beams with same opening without FRP layer) / (strength of beam with opening without FRP layer.)]. Figure 7 shows the load versus deflection of rehabilitated beam.

It is seen that Ri and Ru are progressively increasing except for the Ri value of SB3, such a trend of monotonic increase of both Ri and Ru values indicates the increasing influence of opening size of three beams. It is conjectured that human error of noting the value of Wi in case of SB3 is the main reason of deviation from the normal trend.

It is worth mentioning that all the three beams have the initial crack at the flexural zone though they fail in shear ultimately. The increasing value of (Ru Ri) from 8.7 to 6.95 indicates the effect of opening prominently. While at the smallest opening this value is negative indicating lower value of Ru than the corresponding value of Ri . Further the lower value of Ru of SB1 is due to the value of ultimate strength of SB1 closed to that of solid beam. This shows the effectiveness of wrapping in case of small opening.

The effect of wrapping as observed from the calculated Gu values are seen to be the maximum of 21.15% for the SB1 having smallest opening size. The gain abruptly decreases to 4.16% and 3.15% respectively for SB2 and SB3 clearly reflecting the influence of bigger opening in such gains.




Deflection Pattern of Beams with Openings

Figure 8, shows a typical deflection curve of all three beams with openings. It is observed that solid beam (SB) undergoes less deformation than beams with openings (OB1, OB2 and OB3) as shown in Figure 5. The deflections of beams OB1, OB2 and OB3 are more both near the opening and at the mid span.

A comparative study of the location of maximum deflection of the solid beam and beams with openings shows that location of the maximum deflection of beams with openings shifts from the mid span (location of maximum deflection of the solid beam) to a point which is in between centre of the beam and centre of opening. The beam with FRP wrapping shows more deflection at middle point than those at other two points. This indicates that due to FRP wrapping, flexural stiffness and the shear resistance of beams with openings increase.

Figure 8: Typical Deflection of Beam with Opening

Ultimate load versus opening size

Figure 9, shows the ultimate load with increasing opening of the three category of beams viz., beams with openings only, strengtehned beams and rehabilitated beams.It is observed that solid beam carried an ultimate load of 156 kN. While all the beams carry arounnd 100 kN for the opening size of 200 mm to 300 mm. For opening upto 100 m strengthened beam caries maximum load of 126 kN followed by rehabilitated beams (120 kN) and finally beams with 100 opening nearly 100 kN.



Figure 9: Variation of Ultimate Load with Opening Width

Failure of Beams

In these experimental investigation four types of beams are tested viz. solid beam, beams with openings, strengthened beams and rehabilitated beams. It is worth mentioning that the reference solid beam fails in flexure where as other three types of beams have shear failure in all the cases. Figure 4 presents the failure patterns of OB1, OB2, SB1 and SB3 respectively. The OB2 beam develops diagonal cracks at the top and bottom corners around the opening. With the propagation of these cracks the beam finally fails in shear (Figure 4(c)). In case of strengthened beams diagonal cracks also developed in the top and bottom corners around the opening though the beams are wrapped with FRP layer. This FRP layer have a confining effect which helps to increase the failure load to some extent however this beams also displays debonding of FRP layer leading to their tearing along the diagonal cracks and beams finally fails in shear.

Contribution of GFRP in Shear Strengthening

Existing research on beam with opening show that shear force carried by the bottom chord Vb (Kennedy et al. (1992))

b b b

b b t t

A I V V A I A I

= +

Where = Crosssectional areas of bottom and top chords. = bw x hb

Atw = bw x ht, Moments of inertia of top and bottom chords, about centroidal axes respectively. V= Total shear force, =Shear force at bottom chord and top chord Now shear force carried by top chord will be Vt , where

Vt = V Vb

Now for strengthened and rehabilitated beam the shear force carried by GFRP Vgfrp, where Vgfrp =Total shear force – (Vb + Vt)).



Therefore, this experiment may lead to calculate the shear capacity of the beam and to understand the contribution of GFRP to strengthened and rehabilitated the beam with opening. From this investigation it is found that GFRP contributes significantly for in shear strengthening for beam with small opening only for both cases strengthen and rehabilitated. Efficiency of GFRP for rehabilitated is less than strengthened beam.

4. Conclusion

The following conclusions are derived on the basis of testing of one solid beam and three beams with one opening in the shear zone.

(i) FRP wrapping around the opening of the beams with 200 and 300 opening width, shows initial cracks in flexural zone instead of cracks near the opening.

(ii) FRP can be used to strengthen and rehabilitate the beams with small opening only.

(iii)FRP does not show the same efficiency for strengthened and rehabilitated beams.

(iv)Beams with FRP wrapping displays debonding of FRP layer leading to their tearing along the diagonal cracks.

(v) The reduction in initial crack load (Ri) is much influenced with size of opening.

(vi) Reductions in ultimate load carrying capacity (Ru) are not much influenced with the size of the opening in a range in between 200 mm and 300 mm.

(vii) Beams with larger opening the failure are governed by the opening size. FRP does not increase the ultimate load carrying capacity of these beams.

(viii) The beams with openings only shows the maximum deflection at a point which is in between the middle point of beam and middle point of opening instead of maximum deflection at central point of solid beam.

(ix)An unstrengthened beam with100 mm opening width and with a height of 0.38 the beam depth reduce the beam capacity by 33%.

(x) An unstrengthened beam opening with 100 mm opening height and with a width of

0.15 the beam length reduces the beam capacity by 39%.

Further a large number of researches are required to understand the FPP strengthening technique for beam with large opening, rehabilitation of beam using FRP and also to understand their failure mechanism.



Acknowledgement

This study was conducted at Structural Engineering Laboratory, Department of Civil Engineering, Indian Institute of Technology, Kharagpur (India) and I would like to thank the members in the laboratory for providing assistance in specimen fabrication and testing.

5. References

1. Abadlla, H. A., Torkey, A. M., Haggag, H. A. and AbuAmira, A. F. (2003)”Design against cracking at opening in reinforced concrete beams strengthened with composite sheets.” Journal of Composite Structures (Elsevier Ltd.), 60(2), pp 197204.

2. ASTM D7565, Guide lines for Tensile test FRP.

3. Chaallal, A., Nollet, M. J. and Perranton. (1998) “Shear strengthening for beams by externally bonded side CFRP strips.” Journal of Composite Construction, 2(2), pp 111 113.

4. Chajes, M. J., Janusz, T. F., Mertz, D. R., Thomson, T. A. Jr. and Finch, W.W. Jr.(1995) “Shear strengthening of reinforced concrete beams using externally applied composite fabrics.” Structural Journal (ACI), 92(3), pp 295303.

5.Collins, F. and Ropper, H.(1990) Laboratory investigation of shear repair of reinforced concrete beams loaded in flexure, Materials Journal (ACI), 97(2), pp 149159.

6. IS: 102621982 Recommended Guidelines for Concrete Mix Design—Bureau ofIndian Standards, New Delhi.

7. IS: 4562000, Plain and Reinforced Concrete—Code of Practice—Bureau of Indian Standards, New Delhi..

8. J.G. Teng, J.F. Cher, S.T. Smith, FRP – strengthened RC structure by Lam Publishers John Wiley and Sons, Ltd.

9. Kennedy, J.B. and Abdalla, H.A., (1992) “Static response of prestressed girders with openings, Journal of Structural Engg.” ASCE, 118(2), pp 488–504.

10. Michel, J. Chajes, Ted. F. Januszka, Dennis, R. Maertz, Theodore, A., Thomson Jr. and Willam, W. Finch Jr.,(1995) “Shear strengthening of reinforced concrete beams using externally applied composite fabrics” Structural Journal (ACI), 2(3), pp 295303. 11. Talgstan B., (1996)”Plate bonding, strengthening of existing concrete structures with epoxy bonded plates of steel or fibre reinforced plastics, International Journal of Fracture” 82, pp 253266.

12. Teng, J. G, Lam, L. and Chen, J. F.,(2004) ”Shear strengthening of RC beams with FRP composites”, New Materials in Construction (Wiley Interscience),6(3), pp 173–184.

13. Triantafillou, T.C., (1998) “Shear strengthening of reinforced concrete beams using epoxybonded FRP composites”, ACI Structural Journal, 95(2), pp 107–15.



14. Uji, K.(1992) “Improving shear capacity of existing reinforced concrete member by applying carbon fiber sheets”, Transaction of Japan Concrete Institute, 14, pp 253266.

Education

Strengthening and rehabilitation of reinforced concrete beams with