Efficacy of GFRP Wraps on Rehabilitation of Corrosion-Damaged HSC Columns
Corrosion of steel reinforcement in concrete is considered to be the major cause of deterioration in civil infrastructure facilities. This article presents the results of a comprehensive experimental investigation carried out to study the effect of Glass Fibre Reinforced Polymer (GFRP) wrapped high strength concrete (HSC) columns subjected to various levels of corrosion. The columns tested for this investigation were 150 mm in diameter and 900 mm in height. The columns were subjected to an accelerated corrosion inducing 10% and 25% corrosion-damaged levels. The corroded columns were wrapped with different configurations of glass fibre reinforced polymer namely Woven Rovings (WR) and Unidirectional Cloth (UDC) of 3 mm and 5 mm thicknesses. The columns were then tested under uni-axial compression till failure. The test results show a marked enhancement in ultimate strength by 39% and ductility by 123%. The strength and ductility of the GFRP confined corrosion-damaged columns increase with increasing wrap thickness.
IntroductionThere is a large number of concrete structures around the world that have been damaged by extreme environmental conditions, overloading or simply as a result of normal aging. These structures are unsafe to use and urgently need repair for continued use. In metro cities, the emissions of carbon and nitrogen oxide aggravate the situation further by neutralizing the concrete cover. Typically, a reinforced concrete structure requires major restoration work within 15 years of its construction. Corrosion of reinforcing steel bars is one of the main causes of early deterioration of concrete structures. The important causes of corrosion in steel reinforcement are carbonation and chloride penetration. The formation of rust products lead to cracking and spalling of cover concrete. Reinforced concrete undergoing corrosion give the unpleasant appearance and also the reduction in cross-sectional area of steel bar may lead to a loss in structural integrity of the reinforcing steel. Consequently, the safety and serviceability of the concrete structures is affected. Moreover, bond between the steel and the concrete is reduced. The most common methods for repair and retrofit of reinforced concrete columns are concrete jacketing, steel jacketing, and fibre wrapping. Concrete jacketing requires formwork and considerable increase in the weight and cross-section of the column. Steel jacketing is also labor intensive and costly. Recently, Fibre Reinforced Polymer (FRP) proved to be an efficient material in construction world. It offers a high strength, good fatigue life, low maintenance, lightweight, ease of transportation and handling cost. In addition to strengthening a concrete member, the FRP materials are more resistant to corrosion and provide a protective barrier to concrete from aggressive environment. A number of researchers have been devoted to concrete columns retrofitted with FRP and numerous models were proposed. They reported that FRP confined concrete increased the strength and ductility of the column. A limited number of studies have been carried out on FRP for rehabilitation of corrosion-damaged concrete structures. This research study investigates the effectiveness of using Glass Fibre Reinforced Polymer wrapping on the performance of corrosion-damaged high strength concrete columns.
|Figure 1: Specimen Details of RC Columns and Reinforcement Cage in Formwork|
The diameter of the columns was 150 mm and the height was 900 mm. The concrete used for the specimens had a mean compressive cylinder strength at 28 days of 63.2 MPa. The details of the concrete composition are shown in Table 1. The concrete was cast, compacted, finished, demolded, and cured for 28 days in a standard manner. Deformed reinforcing bars with a diameter of 8 mm of strength 450.6 MPa were used as longitudinal reinforcement. Steel bar with a diameter of 6 mm of strength 300.8 MPa were used as ties at a spacing of 115mm. The longitudinal bars were kept protruding for a length of 75 mm beyond the column face towards inducing accelerated corrosion. Fig.1 presents the specimen details of the RC columns.
|Table 1 – Composition of Concrete Mixture|
|Material||Quantity in Kg/m3|
|Coarse Aggregate||20 mm||680|
|Hyperplasticizer – Glenium B233||0.8 % by weight of binder|
Test MatrixThe 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 Woven Roving Glass Fibre Reinforced Polymer (WRGFRP) and Unidirectional Cloth (UDCGFRP) of 3 mm and 5 mm thicknesses. Table 2 summarizes the test program for RC columns.
|Table 2 – Test Matrix for RC Columns|
|Specimen||Level of Corrosion (%)||Type of GFRP||GFRP Thickness (mm)|
|NC CON||No Corrosion||---||0|
|CD 10 CON||10||---||0|
|CD 10 WR 3||10||WR||3|
|CD 10 WR 5||10||WR||5|
|CD 10 UDC 3||10||UDC||3|
|CD 10 UDC 5||10||UDC||5|
|CD 25 CON||25||---||0|
|CD 25WR 3||25||WR||3|
|CD 25 WR 5||25||WR||5|
|CD 25 UDC 3||25||UDC||3|
|CD 25 UDC 5||25||UDC||5|
Accelerated Corrosion ProcessEach 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: Accelerated Corrosion Test Set-up|
The columns were immersed in 3.5% sodium chloride solution in a fiber glass tank such that the reinforcement steel bars were above the NaCl solution. The specimens were kept immersed in the solution for 24 hours, to ensure full saturation condition. The direction of the current was adjusted so that the reinforcing steel bars 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 levels 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.
Δw = (Am.I.t) / (Z.F) --------(1)
where Δw = mass loss due to corrosion, Am = 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)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. The probability of active corrosion is based on specific ranges of potential of steel reinforcement with respect to standard reference electrode.
GFRP Repair TechniqueTwo types of GFRP materials were used in the present study to repair corrosion-damaged specimens. Woven Roving and Unidirectional Cloth Glass Fibre Reinforced Polymer was used to rehabilitate the corrosion-damaged columns. The Woven Roving has interwoven fibres oriented at 90º to the longitudinal axis of the fabric. The Unidirectional Cloth has fibres oriented in single direction. The GFRP wrap materials were tested according to the ASTM D 638. The tensile strength, elastic modulus of GFRP wrap material are summarized in Table 3.
|Table 3 – Mechanical Properties of GFRP|
|Wrap Material||Thickness (mm)||Elasticity Modulus (MPa)||Tensile Strength (MPa)|
The column surface was made rough and cleaned with an air blower to remove any dust and loose particles. A high-pressure air jet was then used to clean the surface from dust or any foreign particles after the grinding process. The wet lay-up technique was used to wrap the FRP layers. This technique was started with applying an iso-phthalic polyester resin to the surface of the concrete column. Next, the dry fibrous reinforcement material was wrapped to fully cover the resin. A roller was then used to remove the entrapped air bubbles and press the resin to penetrate into the fabric. The roller was continuously used until the resin was reflected on the surface of the fabric, an indication of fully wetting. On the top of the fabric, another layer of resin was applied. This completed one FRP repair layer. The procedure was repeated to apply the subsequent layers. The wrapped specimens were then left at room temperature to allow curing of the bonded GFRP laminates.
Test ArrangementsAll the column specimens were tested in a loading frame of capacity 2000 kN. The column specimens were loaded under a monotonic uni-axial compression up to failure. 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 expansion of the column. Fig. 2 shows the test up and instrumentation provided for the test specimens.
|Table 4 – Summary of Test Results|
|Specimen||Ultimate Load (kN)||Ultimate Axial Deformation (mm)||Ultimate Lateral Deformation (mm)|
|CD 10 CON||1000||3.30||0.36|
|CD 10 WR 3||1075||3.87||0.46|
|CD 10 WR 5||1150||4.24||0.50|
|CD 10 UDC 3||1220||4.26||0.54|
|CD 10 UDC 5||1300||4.53||0.61|
|CD 25 CON||900||3.21||0.31|
|CD 25 WR 3||1040||3.74||0.43|
|CD 25 WR 5||1090||4.00||0.46|
|CD 25 UDC 3||1190||4.13||0.51|
|CD 25 UDC 5||1250||4.36||0.59|
Results and Discussions
The experimental values obtained from tests were summarized in Table 4. From this table, it is seen 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 ultimate axial compression capacity of corrosion-damaged columns was enhanced by GFRP wrapping. The axial compression capacity of corrosion-damaged columns was found to restore the ultimate strength by GFRP wrapping. On overall observations the corroded and then repaired columns were about 40% stronger than the non-corroded un-strengthened columns. It is also to be noted that the UDCGFRP wrapped corrosion-damaged columns resulted in higher ultimate load, this is because fibers in the hoop direction result in the highest hoop tensile strength. The concrete columns wrapped with 3 mm thick WRGFRP and UDCGFRP showed an increase in ultimate axial load by 8% and 22% respectively and those with 5 mm thick WRGFRP and UDCGFRP wrap column exhibited an increase of 15% and 30% respectively for 10% level of corrosion damage. For columns subjected to 25% level of corrosion damage, WRGFRP and UDCGFRP of 3 mm thickness increased the strength level by 18% and 32% respectively and 5 mm thick WRGFRP increased the load capacity by 22% and for 5 mm thick UDCGFRP exhibited an increase of 39%. These results clearly indicate that the effect of GFRP on corrosion damage remarkably improved with increasing wrap thickness.
Failure ModesThe failure of unwrapped corroded columns occurred due to the cracking and reduction in cross sectional area of steel reinforcement. The failure of wrapped specimens was typically marked by rupture of the fibre at or near the mid-height of the specimen. Although it was sudden, the failure could be predicted by the appearance of white patches at the top of the column as the result of the fibre stretching.
Stress-Strain ResponseTypical stress-strain curves for corrosion damaged specimens wrapped with WRGFRP and UDCGFRP are shown in Fig.3.
The curves to the right represent the plots of axial compressive stress versus axial strains whereas the curves to the left show the plots of axial compressive stress versus lateral strain. The curve indicates a significant enhancement in strength and ductility of corrosion damaged concrete. Recent studies by Toutanji (1999), Chaallal et al (2000), Bae et al (2009) have shown a similar bilinear trend for FRP wrapped columns with no descending branch. The response in the first region of the curves was similar to that of control 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 was fully activated and provides an effective confinement to the cracked concrete and thus increased its axial compressive strength. The response in the plastic region was dependent on the stiffness of FRP sheet wrapping. 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. The curves clearly show that confinement with GFRP enhanced the performance of concrete, both its strength and ductility, under axial load. It can also be inferred that the wrapped columns have more energy absorption capacity before failure.
|Table 5 – Ductility Indices|
|Specimen||Deflection Ductility||Energy Ductility|
|CD 10 CON||1.90||3.01|
|CD 10 WR 3||2.73||4.03|
|CD 10 WR 5||2.88||4.08|
|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 WR 3||2.67||4.13|
|CD 25 WR 5||2.88||4.21|
|CD 25 UDC 3||3.72||5.43|
|CD 25 UDC 5||3.89||5.61|
ConclusionIn this study, the effect of GFRP sheet wrapping on rehabilitation of HSC columns from corrosion of steel reinforcement was investigated and the following conclusions were made.
- GFRP wrapped corrosion- damaged concrete columns enhanced its load carrying capacity of the columns.
- GFRP wrapped corrosion-damaged concrete columns exhibited higher axial and lateral strains at the ultimate stage.
- The corrosion–damaged-strengthened columns possess enhanced ductility in comparison to corroded-unwrapped columns.
- UDCGFRP wrapped corrosion-damaged concrete column exhibit a better performance when compared to WRGFRP.
- The maximum increase in ductility was found by 65% and 123% for respectively for WRGFRP and UDCFRP wrapped corrosion-damaged concrete columns.
- The failure mode in all the wrapped columns consisted of rupture of fibres.
AcknowledgmentThe authors acknowledge the financial assistances of the University Grants Commission, New Delhi through its Grant scheme, which enabled for conducting this research. The experimental work was carried out in the structures laboratories of the Annamalai University, Chidambaram.
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