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Strength and ductility of GFRP wrapped corrosion-damaged concrete columns.

Introduction

Corrosion of steel reinforcement plays a significant role in the deterioration of concrete structures, especially those in aggressive environments. The most important causes of corrosion in steel reinforcement are the ingress of chloride ions and carbon dioxide. The formations of rust products lead to cracking and spalling of cover concrete. Also the reduction in cross-sectional area of steel bar may lead to a loss in structural integrity of the reinforcing steel. This damage results in reduction in service life and ultimate capacity of the member.

The repair and rehabilitation of existing structures is becoming very popular in the construction field. Advanced composite materials show a very high potential for cost effective application in the rehabilitation of structures around the world. Fibre reinforced polymer (FRP) composites have been considered for their high strength, good fatigue life, and low maintenance light weight, ease of transportation and handling cost. In addition to strengthening a concrete member, the fibre reinforced polymer materials are more resistant to corrosion and provide a protective barrier to concrete from aggressive environment.

The application of FRP on concrete columns has been investigated by many researchers (Mirmiran et al 1997, Toutanji 1999). They reported that FRP confined concrete increased the strength and ductility of the column. It was also reported that FRP reduce or slow reinforcement corrosion by confining the concrete and providing a surface layer with reduced permeability. Fewer projects have been addressed on FRP for rehabilitation of corrosion-damaged concrete structures (Pantazopoulou et al 2001, Debaiky et al 2002). They demonstrated that the application of FRP wrapping was found to be more effective in reducing the corrosion rate and also retarded the post- repair corrosion rate. The experimental procedure adopted for accelerated corrosion was impressed current on reinforcements. This test method was accepted by many researches (Wootton 2003, Neale 2005, and Belarbi 2007). Issac Wootton (13) studied on lollipop specimens to investigate the corrosion of steel reinforcement and the specimens were wrapped with carbon fibre reinforced polymer sheets. They also concluded that wrapping the cylinders lowered the corrosion potential and reduction in mass loss of steel reinforcement. Also on increasing the number of wrapping layers from one to two provided an effective increased confinement. This research study is intended to evaluate the effect of GFRP wrapping on the performance of corrosiondamaged concrete columns.

Experimental Program

The columns were subjected to accelerated corrosion and then tested under uni-axial compression. Three columns were tested without wrapping and the remaining column specimens were wrapped with UDC of 3 mm and 5 mm thickness. Table 1 summarizes the experimental program.

Specimen Layout

All the specimens were cast in asbestos pipe mould of 150 mm of internal diameter and 900 mm in height. The columns were cast with concrete having a strength of 63.24 MPa. The properties of concrete mixture are given in Table 2.

The specimens were provided with six bars of 8 mm diameter for longitudinal reinforcement and 6 mm diameter steel ties were provided at a spacing of 115 mm c/c. The longitudinal bars were kept protruding for a length of 75 mm beyond the column face towards inducing accelerated corrosion. The schematic representation of test specimen is shown in Fig.1.

[FIGURE 1 OMITTED]

Accelerated Corrosion Process

Each column was subjected to an accelerated corrosion process by applying a direct power supply with an output of 32 V and 11 amps. Fig.2 represents the accelerated corrosion set up.

[FIGURE 2 OMITTED]

The columns were immersed in 3.5 % sodium chloride to two-third height in a high density polyethylene tank. The specimens were kept immersed in the solution for 24 hours, to ensure full saturation condition. The reinforcing steel bar was connected to the positive terminal of the external DC source to act as anode and negative terminal was connected to a thin stainless steel hollow pipe with perforated holes. This was made to increase surface area and oxygen diffusion. The various degrees of corrosion are shown in Table 1. The mass loss due to corrosion is related to the current consumed and time for corrosion can be estimated by using Faraday's law.

[DELTA]w = [A.sub.m] * I * 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 [96487coulombs (g/equivalent)]. During the test period, corrosion activity in all the specimens was monitored by measuring the corrosion potential according to the procedures outlined in ASTM C 876-[91.sup.3]. The probability of active corrosion is based on specific ranges of potential of steel reinforcement with respect to standard reference electrode.

Repair Procedure

Glass fibre reinforced polymer was used to repair the corrosion damaged columns. The fibres were oriented in a single direction. The mechanical properties of GFRP wrap material are summarized in Table 3.

The column surface was made rough and cleaned with an air blower to remove all dirt and debris. A primer was first applied on the concrete surface for the GFRP application. An iso-phthalic polyester resin was applied to the concrete surface as a base coat. The dry fibrous reinforcement material was placed on the resin and the wrapped surfaces were pressed with a rubber roller to ensure a good contact between layers and proper distribution of resin. The wrapped columns were cured for a period of 7 days.

Test set up

All the specimens were tested in a loading frame of capacity 2000 kN. The load was applied monotonically with uniform increments of load. To measure the axial compression of the specimen, two deflectometers were fitted at top and bottom of the specimen. A lateral extensometer was provided at mid-height of the column to measure the lateral strain. Fig. 3 shows the test up and instrumentation provided for the test specimens.

[FIGURE 3 OMITTED]

Test results and Discussion

Table 4 presents the test results for all the columns tested under axial compression.

Ultimate load of columns

It can be seen from Table 4 that the load carried by the corroded columns was significantly reduced due to corrosion damage such as cracking and reduction in cross sectional area of steel reinforcement. The corroded columns lost about 3% and 12% of its load carrying capacity for 10% and 25% degrees of corrosion damage respectively. However, the axial compression capacity of corrosion damaged columns was enhanced by GFRP wrapping. The GFRP wrapped corrosion-damaged concrete columns exhibit an increase of 30% in ultimate strength. The concrete columns wrapped with 3 mm thick UDCGFRP showed an increase in axial load by 22% and those with 5 mm thick UDCGFRP wrap column exhibited an increase of 30% for 10% degree of corrosion damage. For columns subjected to 25% of corrosion damage, UDCGFRP of 3 mm thickness increased the strength level by 32% and 5 mm thick UDCGFRP increased the load capacity by 40%. These results clearly indicate that the effect of GFRP on corrosion damage remarkably improved with increasing wrap thickness.

Stress--Strain Response

Figs.4-6 present the axial compressive stress versus strain of the tested columns. The response of FRP wrapped concrete columns showed a bilinear trend with no descending branch. The initial region of the curves was similar to that of unwrapped RC columns. The point corresponding to the abrupt change in slope of the curves represents the onset of unstable crack propagation. Beyond this point, concrete volume expansion was observed due to extensive propagation of crack. At this stage, the fibre wrap provides an effective confinement to the cracked concrete and thus increased its compressive strength. The lateral strain response was almost closer to a straight line as compared to the axial strain response. This may be due to excessive cracking of the concrete core and hence the column directly depends on the fiber wrap, which is linear-elastic.

It can be seen from Fig.4, that the axial strain decreased due to accelerated corrosion and the ultimate strains were just 2% and 5% respectively for 10% and 25% corrosion damage levels. On the other hand a slight increase was observed in ultimate strains in GFRP wrapped corrosion-damaged concrete columns by 29% to 38%. The change in lateral strain was also observed from Figs. 4-6 and the ultimate lateral strain for the GFRP wrapped corrosion-damaged concrete columns was substantially greater than the unwrapped corroded columns. For specimens subjected to 10% of corrosion damage, UDCGFRP of 3 mm thickness increased the ultimate strain by 55% and the 5 mm thick UDCGFRP increased the lateral strain level by 72%. The columns with 25% level of corrosion showed an increase in ultimate lateral strain by 67% for 3 mm thickness of UDCGFRP. The columns wrapped with UDCGFRP of 5 mm thickness increased the lateral strain by 90%. It can also be inferred that the wrapped columns have more energy absorption capacity before failure. The failure mode in all the wrapped columns consisted of rupture of fibres.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Ductility of Columns

Table 4 summarizes the ductility indices of tested columns. The ductility of GFRP wrapped corrosion-damaged columns increased with increase in wrap thickness. For test specimens subjected to 10% corrosion damage level, 3 mm thick UDCGFRP enhanced the ductility by 98% and 5 mm thick UDCGFRP enhanced the ductility by 110%. The increase in ductility was found to be 114% and 123% for 3 mm and 5 mm thick UDCGFRP for test specimens subjected to 25% corrosion damages level.

Conclusions

This study was mainly focussed on evaluating the effect of glass fibre reinforced polymer wraps on the performance of corrosion-damaged concrete columns under uni-axial compression. The following conclusions are drawn based on the results of the carried out study.

(1) UDCGFRP wrap provided about 30% enhancement in load carrying capacity for specimens with corrosion damage.

(2) UDCGFRP provided the maximum increase in ductility to a level of about 110%.

(3) The strength and the ductility of GFRP wrapped corrosion-damaged concrete column increase with increase in wrap thickness.

(4) The failure mode in all the wrapped columns consisted of rupture of fibres.

References

[1] ACI Committee Report 440.2R, 2002., "State-of-the-art report on Fibre Reinforced Plastic (FRP), Reinforcement for Concrete Structures," American Concrete Institute. Farmington Hills, Michigan

[2] American Concrete Institute(ACI)(1999), "Building Code requirements for Structural Concrete (318-99) and commentary (318R-99)" Farmington Hills, Michicigan

[3] ASTM (1991), "Standard test method for half-cell potentials of uncoated reinforcing steel in concrete". C 876-87.

[4] Auyeung, Y., Balaguru, and Chung, L.,2000, "Bond behaviour of corroded reinforcement bars," ACI Mater. J,97(2), 214-220

[5] Belarbi,A., and Bae, S.W., 2007, "An experimental study on the effect of environmental exposures and corrosion on RC columns with FRP composite jackets." Composites, Part (B), 38, 674-684.

[6] Debaiky, A., Green, M., and Hope,B., 2002, "Carbon fiber-reinforced polymer wraps for corrosion control and rehabilitation of reinforced concrete columns," ACI Mater. J, 99(2), 129-137

[7] Mirmiran, A., and Shahawy, M., 1997, "Behaviour of Concrete Columns Confined by Fiber Composites." J. of Struc. Engg. 13(5), 583-590.

[8] Neale, K.W., Demers,M., and Labossiere, P., 2005, "FRP protection and rehabilitation of corrosion damaged reinforced concrete columns," Int. J. Materials and Product Technology., 23(3/4), 348-371.

[9] Pantazopoulou, S. J., Bonacci, J. F., Sheikh, S., Thomas, M. D. A., and Hearn, N. (2001), "Repair of corrosion-damaged columns with FRP wraps," J.Comp. Construc. 5(1), 3-11.

[10] Sangeeta Gadve., Mukherjee, A., Malhotra, S.N.,2009, "Corrosion of steel reinforcements embedded in FRP wrapped concrete." Const. and Building materials, 23(1), 153-161.

[11] Tamer, A., Maaddawy, E., Chahrour, A., Soudki, K.A., 2006, "Effect of fiberreinforced polymer wraps on corrosion activity and concrete cracking in chloride contaminated concrete cylinders." J. of Comp. Construc., 10(2), 139-147.

[12] Toutanji, H.A., 1999, "Stress-strain characteristics of concrete column externally confined with advanced fibre composite sheets." ACI Mater. J., 96(3), 397-404.

[13] Wootton, I., Spainhour, L., and Yazdani, N., 2003, "Corrosion of steel reinforcement in CFRP wrapped concrete cylinders." J. of Comp. Construc., 7(4), 339-347.

J. Revathy *, K. Suguna ** and P.N. Raghunath **

* Research Scholar (E-mail: rrreva@gmail.com)

** Professor of Structural Engineering Department of Structural Engineering, Annamalai University, Annamalai Nagar-608 002, India
Table 1: Experimental Program.

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

NC CON        No Corrosion   --        0
CD 10 CON     10             --        0
CD 10 UDC 3   10             UDC       3
CD 10 UDC 5   10             UDC       5
CD 25 CON     25             --        0
CD 25 UDC 3   25             UDC       3
CD 25 UDC 5   25             UDC       5

* NC, CON and CD refer to no corrosion, control and corrosion damaged
respectively.

Table 2: Properties of Concrete Mixture.

Material                         Quantity in Kg/[m.sup.3]

Cement                           450
Coarse  20 mm                    680
Aggregate10 mm                   450
River sand                       780
Hyperplasticizer--Glenium B233   0.8 % by weight of binder
Silica Fume                      25
Water                            160

Table 3: Mechanical Properties of GFRP.

Type of            Thickness   Tensile    Ultimate     Elasticity
Fibre              (mm)        strength   Elongation   Modulus
                               (MPa)      %            (MPa)

Glass fibre        3           446.90     3.02         13965.63
(uni-directional   5           451.50     2.60         17365.38
cloth)

Table 4: Summary of Test Results.

                                                      Lateral
Specimen   Ultimate   Axial    Axial        Axial     Deflec-   Lateral
           Load       Stress   Deflection   Micro     tion      micro
           (kN)       (MPa)    (mm)         strain    (mm)      strain

NC CON     1025       58.00    3.38         3755.56   3.38      2624.67

CD 10      1000       56.59    3.30         3666.67   3.30      2370.67
CON

CD 10      1220       69.04    4.26         4733.33   4.26      3623.73
UDC 3

CD 10      1300       73.57    4.53         5033.33   4.53      4064.00
UDC 5

CD 25      900        50.      3.21         3566.     3.21      2048.9
CON                   93                    67                  3

CD 25     1190        67.      4.13         4588.     4.13      3420.5
UDC 3                 34                    89                  3

CD 25     1250        70.                   4844.     4.36      3911.6
UDC 5                 74       4.36         44                  0

Table 4: Ductility Indices.

              Deflection
Specimen      Ductility    Energy Ductility

NC CON        1.92         3.07
CD 10 CON     1.90         3.01
CD 10 UDC 3   3.74         5.75
CD 10 UDC 5   3.97         5.82
CD 25 CON     1.74         3.04
CD 25 UDC 3   3.72         5.43
CD 25 UDC 5   3.89         5.61
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Title Annotation:glass fiber reinforced polymer
Author:Revathy, J.; Suguna, K.; Raghunath, P.N.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2009
Words:2397
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