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Structural performance of corrosion damaged reinforced concrete beams with glass fibre reinforced polymer laminates.

Introduction

One of the major deterioration processes in reinforced concrete is reinforcement corrosion made possible by carbonation of concrete or by chloride penetration to steel level. A conceptual model [1] to estimate the service life of corroding reinforced concrete is based on corrosion initiation and corrosion propagation time periods. The effects of rebar corrosion on the structural characteristics of RC beams are well documented [2, 3]. Umoto subjected Several small RC beams reinforced with a single 16mm rebar to accelerated corrosion using an impressed current, and found that rebar corrosion has pronounced effect on the structural strength of RC beams than it does on the yield strength of the rebar. This result was attributed to the loss of bond between the rebar and concrete due to cracking in concrete resulting from the expansive forces of the corrosion products.

All the specimens were tested in flexure after corrosion and the results showed that un corroded beams failed in flexure; while corroded beams exhibited a shear-bond failure at a load range of 67-95% as compared to un corroded beams. It was concluded that the critical point in strength degradation occurs when longitudinal cracks form along the steel bars [4].Umoto et al suggest that increasing the concrete cover decrease diffusion of chloride ions, but also to prevent crack formation along the reinforcing bars.

Al-Suleiman et al. [5] performed a total of 54 pull-out tests of 10, 14 and 20mm which were corroded to various degrees ranging from 0 to 14% mass loss. The author suggest that bond strength increased up to 50, 33 and 25%,respectively, for the 10, 14 and 20mm rebar's at about 1% corrosion. Beyond 1% corrosion, bond strength decreased, dropping below the bond strength of un corroded rebar's only when the first surface corrosion cracks were detected.

Lee et al. [6] studied the characteristics of four RC beams subjected to accelerated corrosion and subsequently repaired with CFRP laminates externally bonded to the tension face. Specimens were 200X250X2400 mm, and were reinforced with three 13mm rebar at both top and bottom, and with 6mm stirrups at 50mm spacing. The specimen with the tension rebar corroded to 10% mass loss achieved an ultimate strength of 85% that of a un corroded specimen, and experienced a failure due to loss of bond, evidenced bond splitting longitudinal cracks. Two corroded specimens were repaired with CFRP laminates after corrosion, and obtained an ultimate strength of 141% and 143% that of an un corroded specimen and experienced failure due to tensile rupture of the CFRP laminates.

A promising application of composite materials is in the strengthening and repair of reinforced concrete (RC) structures. Many researchers have shown that concrete repair using FRP laminates is very successful in restoring or increasing the strength of concrete members.

A further promising aspect of FRP repair is the prevention of deterioration due to rebar corrosion by confinement of the concrete member [7, 8]. By strengthening concrete members with FRP laminates, concrete spalling and cracking caused by the expansive forces of the corrosion products may be delayed or even prevented.

The results of the different studies discussed above strongly suggest that the corrosion cracking around the steel rebar is a fundamental component contributing to the loss of structural strength. This implies that if corrosion cracking can be prevented, or at least decreased, a certain degree of structural strength may be maintained in a corroding RC beam. This research paper derives such a relationship based on experimental data.

Experimental Programme

Details of the test program are given in Table 1.A total of four beams were subjected to accelerated corrosion (at 10% and at 25%). Following the corrosion phase, UDC GFRP laminates having 5mm thickness were bonded to the soffit of the beams. One of the specimens was kept as a control specimen, and was neither strengthened, nor does corroded (control).

Fig.1 shows the reinforcement details of the beam specimens. It consisted of two 10mm diameter bars at the top, and two 12mm diameter bars at the bottom. Shear reinforcement consisted of 8mm diameter stirrups at 150mm spacing. The bottom reinforcing steel was extended 50mm beyond the end of concrete face, for the purpose of making external electrical connections.

[FIGURE 1 OMITTED]

A 10%, A25%, UDC5 refers to degrees of corrosion damage at 10%, 25% and UDC 5mm thick respectively.

Material Properties

The specified 28-day compressive strength Concrete used was 29.7 Mpa with a maximum aggregate size of 20mm, and a w/c ratio of 0.48. Uni--directional cloth glass fibre reinforced polymer (GFRP) sheets with 5mm thickness used for this investigation had a tensile strength of 451.5Mpa, an elastic modulus of 17365.38 Mpa, and an Ultimate elongation of 2.6%.

Accelerated Corrosion

All specimens were subjected to accelerated corrosion. Fig.2 represents the accelerated corrosion setup. The four specimens were placed in a tank where 3.5% NaCl solution was used as an electrolyte. The solution level in the tank was adjusted to slightly exceed the concrete cover plus reinforcing bar diameter to ensure adequate submersion of the longitudinal reinforcement. The specimens were incorporated with a direct current power supply with an output of11Amps; thereby achieving theoretical steel weight loss of 10% and 25%.

According to Faraday's law,

[DELTA]w = [A.sub.m]*l*t/Z*F

Where, [DELTA]w = mass loss due to corrosion, [A.sub.m] = atomic mass of iron (55.85 g), I = corrosion current in amps, t = time since corrosion initiation (sec), Z = valency (assuming that most of rust product is due to Fe [(OH).sub.2], Z is taken as 2), F = Faraday's constant [96487 coulombs (g/equivalent)]

Thus, by knowing the original mass of the rebar and the total current of the mass loss, the duration of corrosion activity can be determined.

[FIGURE 2 OMITTED]

The specimen were prepared for GFRP lamination by removing all loose materials on the soffit of the rectangular beam by applying wire brush and roughened with a surface grinding machine. Two component room temperature curing epoxy adhesive was used for bonding the laminates.

The laminated specimens were cured for a period of 7 days.

Test Procedure

The beams were tested under two point loading in a loading frame capacity of 750KN. The deflections were measured at midspan and load points using mechanical dial gauges of 0.01mm accuracy. The crack widths were measured using crack deflection microscope with a least count of 0.02.

The curvature measurement was also done using dial gauges placed over the compression face of the beam at near to the support points. The deflections, curvature and crack width were measured at each load stage. The loading was continued until failure. The details of test set up are shown in Fig.3

[FIGURE 3 OMITTED]

Test Results and Discussion

The test results on the load and deflection properties of the specimens are reported in Table 2.

The first crack loads were obtained on visual examination only. At this load level, the load carrying capacity of GFRP laminated Corrosion damaged RC beams exhibited an increase up to 36.32%, but for corroded specimens (A10% and A25%) decreased by an average of 41% with respect to the control specimens.

The service loads were obtained from the ultimate loads with the usual partial safety factors. At this stage, the load carrying capacity of the corroded beam specimens reduced by 27% and 34% for 10% and 25% degrees corrosion damage respectively, On the other hand, the performance was greatly enhanced by 86% at 10%and79%at 25% mass loss,respectively compared to the control beam, due to the addition of GFRP laminates.

The yield loads were obtained (by inspection) corresponding to the stage of loading beyond which the load-deflection response was not linear. At this load level, the corroded strengthened specimens exhibited an increase up to 133% compared to the control specimen. However the strength decreased by an average of 37% for corroded un strengthened specimens.

The Ultimate loads were obtained corresponding to the stage of loading beyond which the beam would not sustain additional deformation at the same load intensity. At this load level, the corroded unstrengthened specimens decreased by an average of 27% at 10% mass loss and 37% at 25% mass loss respectively, compared to the control specimen, but the strength increased for 10% level of corrosion with UDC material as 86.2% and 79% at 25% mass loss. Based on the test results, it was found that GFRP Laminates have beneficial effects even at the corrosion-damaged stage.

The deflection capacity is defined as the deflection of the beam at failure. The load deflection responses of the specimens are shown in Figs.4-5. The effects of corrosion on flexural behavior were: The deflection at first crack load level of corroded specimens decreased by an average of 41% at 10% mass loss and 30% at 25% mass loss, respectively compared to the control specimens, the yield load level of the corroded specimens reduced by an average of 8% at 10% mass loss and 13% at 25% mass loss respectively; the service load level of the corroded specimens exhibited a decrease of 24% and 15% at ultimate load level of the corroded beams compared to the control beam.

It is clear from Table2 that the corroded-GFRP strengthened specimens had lesser width when compared to the control specimen. The deflection ductility performance of the corroded strengthened specimens was improved by an average of 60% when compared to the control specimen. But in the case of corroded unwrapped beams, the ductility values got reduced marginally.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Conclusion

Based on the test results the following conclusions are drawn.

(1) GFRP laminates properly bonded to the tension face of RC beams can enhance the Ultimate strength substantially. The UDC GFRP strengthened beams exhibit an increase of up to 86% in Ultimate strength for 10% steel mass loss, compared to the control specimen and up to 79% in Ultimate strength for 25% steel mass loss.

(2) The deflection got reduced at all load levels in GFRP strengthened beams. At the ultimate stage, UDC GFRP laminated beams exhibit a decrease of 19% at 10% mass loss and 20% at 25% mass loss, when compared to the corroded control beam.

(3) UDC GFRP laminated beams show enhanced ductility. The increase in deflection ductility was found to be 62% at 10% mass loss and 58% at 25% mass loss.

References

[1] Pritpal S Mangat and Mahmovel S. Elgarf, 1999, Flexural strength of Concrete beams with corroding reinforcement, ACI structural journal, pp 149-158.

[2] Almusallam AA, Al-Gahtani AS, Aziz AR, Dakhill FH, Rasheeduzzafar A,1996, Effect of reinforcement corrosion on flexural behavior of concrete slabs. Journal of Materials in civil Engineering 8(3): 123-7.

[3] Lee HS, Tomosawa F, Noguchi T, 1996. Effects of rebar corrosion on the structural performance of singly reinforced beams. Durability of building materials and components; 571-580.

[4] Uomoto T, Tsuji K, Kakizawa T, 1984,. Deterioration mechanism of concrete structures caused by corrosion of reinforcing bars. Transactions of the Japan Concrete Institute 6:163-70.

[5] Al-Sulaimani GJ, Kaleemullah M, Basunbul IA, Razeeduzzafar A, 1990, Infuence of corrosion and cracking on bond behavior and strength of reinforced concrete members, ACI Structural Journal 87(2):220-31.

[6] Lee HS, Tomosawa F, Masuda Y, Kage T, 1997, Effect of CFRP sheets on flexural strengthening of RC beams damaged by corrosion of tension rebar.Proceedings of Third International Symposium on Non metallic(FRP) Reinforcement for concrete structures 1:435-42.

[7] ACI Committee 318 (1999), Building Code Requirements for Structural Concrete and Commentry, American Concrete Institute, Farmington Hills, Michigan, USA

[8] Sherwood EG, Soudki KA, 1998,. Durability of concrete beams repaired by carbon fibre reinforced polymer laminates subjected to accelerated rebar corrosion. Proceedings of the CSCE Annual Conference, 3b, Halifax, Nava Scoutia, p.663-72.

[9] Wang Wenwei, Li Guo, 2006, Experimental study and analysis of RC beams strengthened with CFRP laminates under sustaining load, International Journal of Solids and structures 43, 1372-1387.

[10] Sherwood EG, Soudki KA,1998,Repair of corroded RC beams with carbon FRP sheets. Proceedings of the fifth International conference on Composites Engineering.ICCE/5, Las Veegas, Nevada, p.819-20

[11] Norris. T. Saadatmanesh. H. Mohammad. R. E., 1997, Shear and flexural strengthening of R.C beams with carbon fiber sheets, ASCE, Journal of Structural Engineering 123(7), 903-911

[12] M.A. Shahawy, M. Arockiasamy, T. Beitelman and R. Sowrirajan, 1996, Reinforced Concrete rectangular beams strengthed with CFRP laminates. Composites: Part B 27B, 225-223

[13] ACI committee 222, 1996, Corrosion of metals in concrete. ACI222R-96, 30pp.

(1) A. Leema Rose. (2) K. Suguna and (2) P.N. Ragunath

(1) Research Scholar, Department of Civil & Structural Engg, Annamalai university, Annamalai nagar, Chidambaram-608 001, Tamilnadu, India, E-mail:leema_arose@yahoo.com

(2) Professor of Structural Engineering, Annamalai University, Annamalai nagar, Chidambaram--608 001, Tamilnadu, India
Table 1

               Level of      Type of       GFRP
Specimen*    Corrosion (%)     GFRP    Thickness (mm)

Control      No Corrosion       --          0
A 10%             10            --          0
A 10 UDC 5        10            UDC         5
A 25%             25            --          0
A 25 UDC 5        25            UDC         5

A10%, A25%, UDC5 refers to degrees of corrosion damage at 10%, 25%
and UDC 5mm thick respectively.

Table 2: Test Results.

Specimen    First Crack Stage      Yield Stage

           Load    Deflection   Load     Deflection
           (kN)       (mm)      (kN)        (mm)

Control    26.98      2.36       51.50      8.43
A10%       19.62      0.83       34.34      7.75
A25%       12.26      2.2        29.43      7.25
A10UDC5    36.78      1.85      120.17     10.00
A25UDC5    34.34      1.07      110.36       10

Specimen    Service Load        Ultimate Stage
                 Stage                                Maximum
                                                       Crack
           Load    Deflection   Load     Deflection    width
           (kN)       (mm)      (kN)        (mm)        (mm)

Control    47.41       6.5      71.123       40         1.2
A10%       34.34       7.75     51.5         36         1.24
A25%       31.09      10.56     46.59        32         1.3
A10UDC5    88.29       5.63     132.43       75         0.88
A25UDC5    85.03       4.88     127.53       70         1.00

Table 3: Ductility Indices.

Specimen   Deflection   Curvature   Energy Ductility
           Ductility    Ductility

Control       4.74         7.87           7.87
A10%          4.41         6.64           6.64
A25%          4.64         8.40           8.40
A10UDC5       7.5         13.43          13.43
A25UDC5       7.00        11.10          11.10
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Article Details
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Author:Rose, A. Leema; Suguna, K.; Ragunath, P.N.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2009
Words:2368
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