Composites in Construction 2005 – Third International Conference Lyon, France, July 11 – 13, 2005
CONFINEMENT OF RC HOLLOW COLUMNS USING CFRP LAMINATES G.P. Lignola, A. Prota, G. Manfredi and E. Cosenza Dept. of Structural Analysis and Design, University of Naples Federico II Via Claudio 21, Naples, P.O.Box I-80125, Italy
[email protected] [email protected] [email protected] [email protected] ABSTRACT: Column jacketing with Fiber Reinforced Polymer (FRP) composites materials has been extensively studied in the last decade to address the issue of seismic upgrade and retrofit of existing Reinforced Concrete (RC) columns. Researchers have mainly focused their attention on solid columns. Very little has been done about hollow cross sections strengthened with FRP. In order to study the behaviour of non-circular hollow cross sections subjected to combined axial load and bending, and provide a contribution toward the comprehension of the resistant mechanisms in presence of FRP confinement, a total of six specimens, three as-built and three retrofitted with two plies of unidirectional Carbon FRP (CFRP) wraps, has been tested. The present work is a first step of a wider activity that will aim at evaluating the benefits that could be generated by an FRP wrapping, computing P-M interaction diagrams for hollow column confined with FRP, and defining practical design criteria for the strengthening of similar elements using FRP. The theoretical analyses will also assess under which conditions the approaches generally adopted for solid cross sections could be extended to the case of those hollow. 1.
INTRODUCTION
Many spectacular RC bridges have been constructed in Europe as well as in the United States and Japan. Where high seismic actions and natural boundaries require high elevation infrastructures, hollow bridge piers can be adopted to maximize structural efficiency of the strength-mass and stiffness-mass ratios and to reduce the mass contribution of the column to seismic response and high carrying demand on foundations. A large number of existing bridges have hollow piers, and this obviously results in larger flexural strength and stiffness than solid piers. It may be economical to use hollow RC members to minimize the weight of concrete members in some situations or the cost of concrete in some locations. Even modern codes of practice oriented to new design do not cover specific problems related to hollow sections. Hollow RC members may be required to dissipate energy by forming ductile plastic hinges when they are subjected to seismic or other lateral forces and the question of their available ductility may need to be studied. Bridge piers designed in accordance with old design codes may suffer severe damage during seismic events caused by insufficient shear or flexural strength, or low ductility. Due to its brittle nature, failure in shear of RC bridge piers has in any case to be avoided. This means that the member behavior is dominated by flexure and therefore it is important to investigate the flexural behavior of hollow cross sections. However, the research on the hollow bridge piers is lacking and the seismic performance of hollow bridge piers such as flexural and shear behavior, retrofit techniques, and its numerical analysis should be investigated. Most of the existing bridges, even if designed after modern codes had been introduced, can represent a source of risk in regions under high seismic risk. Inadequate seismic detailing contributed to premature column failures as soon as concrete spalling occurs during the seismic event. Retrofit systems consisting of concrete or steel jackets have been developed and thousands of installations based on this technology have been deployed. Different types of advanced composite column jacketing systems have been investigated to increase the speed of installation, to reduce maintenance and to improve durability. FRP materials retrofit provides successful
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solutions to strengthen, repair, and add ductility with no traffic disruption, rapid execution, long term durability and consequently lower overall cost of the application. [1] Compared to conventional materials such as steel and concrete, FRP materials have advanced properties in terms of strength-to-weight, stiffness-to-weight, durability (inertness to chemical and environmental factors) and fatigue and for this reason have been used worldwide to retrofit existing concrete bridges and buildings. This paper focuses on the upgrade and retrofit of existing square hollow columns using FRP composite materials to enhance flexural strength and ductility. 2.
STATE OF ART
Studies on effectiveness of FRP wrapping for the seismic upgrade of RC non-circular hollow bridge piers are at an early stage [2÷4]. With respect to circular sections it has been underlined that the position of the neutral axis has an important role on section behavior [5]. Low axial load, moderate longitudinal steel percentage or reasonably thick wall could result in a neutral axis close to the inside face of the tube wall and the column may be expected to be ductile. Otherwise, if the neutral axis passes through the hollow portion at some distance from the inside face of the tube, the column can be expected to fail in a brittle manner as a result of rapid disintegration of the concrete in the compression zone. Tests [6] on non-circular columns showed the need to increase the amount of confinement of the longitudinal bars in columns with high levels of axial load to avoid premature failure caused by buckling of longitudinal reinforcement. Other studies [7÷9] are reported on the behaviour of thin walled concrete box piers and an experimental research on ten 1:4 scaled prototypes designed according to non-seismic standards [10,11], previously tested using shaking table technique. As found in circular hollow columns, local wall buckling reduces the capacity of the hollow column and must be avoided by appropriate detailing. Column jacketing with FRP composites materials has been extensively studied in the last decade [12, 13], to address the issue of seismic upgrade and retrofit of existing RC solid columns. Simple calculation procedures for the assessment of strength and deformation capacity of existing bridges have been proposed in recent years, but their reliability has been checked only on solid columns, while their extension to hollow members, especially rectangular, is much less investigated. Test [3] results showed that the ductility was larger as the number of FRP sheets and the shear strength of the hollow cross section increased, being able to eliminate all shear cracks and changing the failure mode of the specimen from shear to flexure. The theoretical load carrying capacity and general interaction relationships for hollow rectangular columns has been developed mainly for steel structures and approximately for box concrete cross sections as-build [14]. 3.
RESEARCH OBJECTIVES
The goal of this research is to study the behavior of square hollow piers subjected to combined high level of axial load and flexure and strengthened with FRP jacketing. A monotonic axial load applied eccentrically allows investigating the effect of the neutral axis position with respect to the ultimate failure mode experienced by the cross section and studying the effect of external wrapping by using FRP composite materials analyzing whether the retrofit solution adopted may change the ultimate failure mode by improving the overall section behaviour. When the neutral axis migrates into the cross section the failure mode is modified as the state of stress changes from pure compression into combined compression and flexure, and the flexural stresses become predominant. The present research has the aim of evaluating the opportunities of using FRP externally bonded reinforcement in confining hollow non-circular cross sections. The results and analysis of the experimental program confirm the validity of this strengthening methodology and will be used to develop design and construction guidelines on the seismic retrofitting and upgrade of RC non-circular hollow bridge piers. 2
4.
EXPERIMENTAL PROGRAM
4.1
Test Matrix
The experimental program has been planned on hollow columns in reduced scale. Test specimens reproduce in scale 1:5 real bridge piers having section area of 180x180 cm2, thickness of 30 cm and height equal to 650 cm. The selection of the scale factor has been conditioned from two considerations: the attempt to study specimens whose dimensions were sufficiently large to reproduce the behavior of real piers and the need to respect laboratory constraints. Studies [9] showed that 1:3 scaled prototypes had greater ductility than models under a flexure dominant loading condition. Tested specimens have hollow section dimensions of 36x36 cm2 and walls thickness 6 cm. The longitudinal reinforcement is 16 10 with 25mm concrete cover and stirrups 4@80mm (see Fig. 1). 360 240
e
60
Solid End
77,5 77,5 77,5 77,5
860
25
60
Ties Ø4@80
Rounded corners R=20mm
3020 1300
Hollow portion
Solid End
25
360
16Ø10
Figure 1 – Rectangular Hollow Cross Section.
The hollow portion of the column has a height of 130 cm, whereas the heads had solid section in order to distribute the load and avoid local failures. The concrete had cylindrical compressive strength of 28 MPa and the steel yielding at 506 MPa, ultimate strength of 600 MPa and strain of about 13%. The test matrix was designed in order to assess the FRP wrapping effectiveness in correspondence with three P/M ratios in terms of flexural strength increases (interpreting the modifications to the P-M diagram) and of ductility (analyzing load-deflection and moment-curvature curves). Accordingly three specimens are tested un-strengthened (Series U= un-strengthened), while the second three are strengthened with CFRP laminates (Series S=strengthened). The matrix of tests done to date is reported in Table 1, where e is the constant load eccentricity. The theoretical interaction diagram of the reinforced square hollow section is plotted in figure 2 based upon conventional RC theory (not taking into account premature failure modes as reinforcement buckling) and 3 points are highlighted in order to validate the numerical model and study the effect of external wrapping. The effect of neutral axis depth is analyzed and the solid marks are the theoretical predictions.
Table 1 – Test Matrix
Specimen Code
Loading Condition
e (mm)
U1 - S1 U2 - S2 U3 - S3
Combined Compression and Flexure Combined Compression and Flexure Combined Compression and Flexure
80 200 300
3
4.2
Instrumentation and Test Setup
All columns have been monitored with at least four strain gauges placed on the internal steel longitudinal reinforcement on opposite sides of the section. LVDTs have been used to monitor deflections and end rotations and then derive cross section deformations. LVDTs on the opposite faces in the bending plan gain a medium behaviour of the concrete in tension and compression that can be compared to strain gauges data. These data from LVDTs, strain gauges and load cell are collected continuously by a data acquisition system. In order to load columns with an eccentric normal load, a suitable steel device has been designed and realized. The correct distribution of the load is guaranteed also at growing spins of the columns ends. The test machinery does not permit to directly place the specimens. Therefore two open hinges have been arranged and a system of self-centering fixing in order to facilitate the alignment and the columns under the machine and, moreover, to bear high loads without warping, compromising the test or altering restraints assumptions (cylindrical hinges theoretically allows to transmit the column only axial load and bending but no shear). The load is applied in cycles through hydraulic actuators in force control at the first stage with 2kN/sec rate, then in displacement control until rupture at 0.002 mm/sec. 4.3
Strengthening Scheme
A total of 2 plies of CFRP wet lay-up unidirectional fabric (600 g/m2) have been applied in all S specimens for the entire specimen height. Tensile modulus of elasticity of the FRP material is 230 GPa and ultimate tensile strength is 3450 MPa. Plies are applied with no longitudinal overlapping, but are overlapped between each ply. Corners have been rounded with a radius of 20mm as prescribed in many codes. The number of plies is considered an upper limit of the economical and technical applicable amount, also accounting for the scale reduction. Nevertheless it has been observed that the influence of the number of layers of FRP on the specimen under eccentric loading is not so pronounced as that of the specimen under concentric loading [15].
5.
PRELIMINARY EXPERIMENTAL RESULTS
5.1
Strength
The executed tests evidenced a good agreement between theoretical predictions and test results (Fig. 2 and Tab. 2) of as-built columns and a small strength increment of strengthened ones with respect to the former ones (generally 15%), but a remarkable ductility increment (Fig. 3). It is underlined that series 1 specimens are not straight comparable due to different eccentricities. In Table 2, Mmax=P·e is the experimental ultimate flexural moment and Mth,U is the theoretical prediction of ultimate flexural capacity of un-strengthened column. Wrapping has moreover avoided, or delayed, buckling of steel longitudinal reinforcement, allowing the full development of the load capacity of the concrete.
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3000
2500 U1 S1
P [kN]
2000
e=80, x=327 1500
S2
1000
e=200, x=182
U2
U3
500
S3
e=300, x=110
0 0
20
40
60
80
100
120
140
160
180
200
M [kN-m]
Figure 2 - Theoretical P-M Diagram and experimental points (blank marks).
The area of concrete in compression reduces as the load eccentricity grows (due to the reduction of the neutral axis depth). Greater transverse deformations are achieved in the columns loaded with smaller eccentricity (moreover, for the same flexural moment they have greater axial load applied and greater compressed areas for the aforesaid effect of the neutral axis). The neutral axis depth induces also a reduction of the axial load carrying capacity of the section (when eccentricity grows, flexural capacity increases even though axial load is reduced). Curvature increases with growing of eccentricity and that is the column deflection increase, consequently. Table 2 – Test Results (1)
5.2
Specimen Code
e (mm)
Mmax (kNm)
U1 U2 U3 S1 S2 S3
52 200 300 80 200 300
117.7 187.7 181.4 170.8 216.2 208.4
Increase Mmax vs. Mth,U (%) 15,4 3,5 7,5 28,0 19,2 23,4
Failure Modes
The behavior of the columns has been characterized by the buckling of the compressive reinforcement bars that have generally determined the failure mode. For bigger eccentricities the axial load is reduced and consequently also the buckling has been less noticeable. The rupture of the column has occurred at the basis of the hollow column, in correspondence with discontinuity due to the change from the solid section of the ends to the hollow section, or at midspan where the P-I effects increment flexural moment (Fig. 4a), even though P-I effects are not taken into account at this time. Specimen U1 rupture was brittle with sudden crushing and cover 5
220
expulsion of the concrete in the compressive part with concrete strains as small as 2,5‰. The longitudinal rebars buckling was observed (Fig. 4b). Also for column U2 and U3 the concrete reached strains lower than 2,5‰. In the U3 the brittle behavior after peak can be observed in Figure 3. Test S1 has been characterized by small eccentricity of the load, but cross section reduced and the behavior was less brittle in comparison to the as build one. The effect of the CFRP wrapping in S columns has avoided the longitudinal steel bars buckling that caused a brittle behavior in the reference U columns. For example, in S3 test (Fig. 3) the curvature, 80%max, on the softening branch at 80% of ultimate load is 3.78 times the peak curvature, max. For the same eccentricity the dissipated specific energy, E80%max, of S3 specimen computed on the softening branch at 80% of ultimate load is about 6.8 times the dissipated specific energy, Emax, of the brittle U3 specimen (see Table 3). Energy values are computed as the area under the M/ diagram at any given . The S3 test was deliberately stopped at about 70% of ultimate load. In the S3 test, wide cracks were observed in the tensile side (Fig. 4c). In Figure 4d the formation of a bulge of the FRP due to the remarkable shortening of the column is shown. 250
S3
Mmax 200
Flexural Moment [kNm]
M80%max
150
U3
100
50
0
max
0
0,00005
80%max
0,0001
Curvature
-1
0,00015
0,0002
[mm ]
Figure 3 – Flexural Moment vs. Curvature Diagram. Comparison U3 and S3.
It has been observed a gradually more ductile behavior when increasing the eccentricity. In fact, for small eccentricity the section is fully compressed. The behavior of the concrete affects the total behavior of the element. When increasing the eccentricity it is observed instead that the neutral axis moves towards the inside of the section. The total behavior then is determined also from the tensile property of the steel. Table 3 – Test Results (2)
Specimen Code
e (mm)
Mmax (kNm)
(106mm-1)
Emax (106N)
(106mm-1)
E80%max (106N)
U1 U2 U3 S1 S2 S3
52 200 300 80 200 300
117,7
5,611
398
5,611
398
1
187,7
12,726
1.462
12,726
1.462
1
181,4
23,503
3.375
23,503
3.375
1
170,8
10,515
1.183
34,194
4.797
3,25
216,2
15,747
2.134
45,18
7.907
2,87
208,4
34,635
5.615
130,816
23.113
3,78
max
6
80%max
/ (-)
80%max
max
a)
b)
c)
d) Figure 4 – Failure Modes of specimens: U2(a-b), S3(c-d).
6.
CONCLUSIONS
The two main objectives of the study can be summarized as follows: to evaluate the seismic performance of as-built hollow piers with respect to solid ones and assess the chance to adopt the same computation method; to evaluate the seismic performance of hollow retrofitted piers with respect to solid ones and quantify the benefits of column wrapping in terms of strength and ductility. First experimental results outlined herein reveal that composite wrapping can enhance the structural performance on concrete columns under eccentric loading to some extent in terms of strength improvement (about 15%), but significantly in terms of ductility (it has been observed that curvature is about three times greater and dissipated specific energy about six times in the wrapped configuration at 80% of ultimate load respect to the as-built column). It is underlined that the lateral confining pressure exerted by the wraps also provides additional restraint against buckling of longitudinal steel bars. The present work is a first step of a wider activity that aims to improve the knowledge and develop an effective design method for fast strengthening of hollow bridge columns so that bridge function can be quickly restored. The strengthening aims to upgrade seismic capacity in terms of strength and ductility. For this purpose it is required to complete the experimental program. Experimental results and the related database can be used for further validation of CFRP strengthening design methods addressing hollow cross section and calibrating the current available models to non-circular hollow bridge piers.
7.
ACKNOWLEDGEMENT
The authors would like to gratefully acknowledge the support of the Italian Government funding the Research Project M.I.TRAS. The FRP strengthening of the columns was supported by MAPEI Spa., Milan, Italy. The contribution of Messrs. Balsamo and Zaffaroni is acknowledged. 7
8.
REFERENCES
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