Mechanics of Composite Materials, Vol. 48, No. 5, November, 2012 (Russian Original Vol. 48, No. 5, September-October, 2012)
MECHANICAL BEHAVIOR OF CONCRETE COLUMNS CONFINED BY BASALT FRP WINDINGS
I. Ciniņa, E. Zīle,* and O. Zīle
Keywords: confined concrete, basalt fibers, pretension The results of an experimental investigation of round concrete columns confined by a wound basalt filament yarn are presented. Basalt is an attractive material for strengthening purposes due to its low cost coupled with a good mechanical performance, especially at high temperatures. It is shown that the basalt FRP confinement provides a considerable strengthening effect. The winding equipment employed in this study has the ability to set a desired pretension force of the yarn and thereby to produce a prestressed confinement. It is found that the prestressed confinement notably delays the onset of intense internal cracking of concrete.
Introduction During the last decade, more and more research efforts have been concentrated on the use of FRPs (fiber-reinforced plastics) as a reinforcement of structural concrete elements. The deterioration of old structures, the need for strengthened structural members because of increased loads, and the rehabilitation of existing structures (especially in highly seismic regions), are some of the applications where the FRP reinforcement can be effectively utilized. FRPs can be used to confine concrete columns subjected to axial compressive loadings. Confined concrete possesses greatly enhanced strength and ultimate strain [1]. A FRP is linearly elastic up to failure, and when used as a confinement, it creates an ever-increasing lateral confining pressure on the concrete. As a result, a triaxial compressive stress state is created in the concrete. Basalt fiber is a novel material having appeared in recent years. Basalt is an inert, naturally occurring volcanic rock. Basalt filaments are made by melting basalt rock with no other additives used, and therefore they have the advantage in terms of cost. Less energy is needed for their production owing to the simplicity of the manufacturing process. Basalt-based materials Institute of Polymer Mechanics, University of Latvia, Aizkraukles St. 23, LV-1006, Riga, Latvia * Corresponding author; e-mail:
[email protected]
Translated from Mekhanika Kompozitnykh Materialov, Vol. 48, No. 5, pp. 783-792 , September-October, 2012. Original article submitted March 19, 2012. 0191-5665/12/4805-0539 © 2012 Springer Science+Business Media New York
539
5
6
4
2 1 3
Fig. 1. Schematic diagram of the yarn winding equipment: 1 — rotating specimen fixture, 2 — specimen, 3 — horizontal platform moving along the longitudinal axis of the specimen, 4 — pretension device attached to the platform, 5 — dynamometer, and 6 — yarn bobbin. are environmentally friendly and unhazardous and offer a high chemical stability [2, 3]. They are nontoxic, noncombustible, and resistant to high temperatures [4]. Moreover, the mechanical properties basalt FRPs are comparable to or better than those of E-glass FRP [5]. Sincee basalt fibers are also much cheaper than carbon ones, they might be a good alternative for strengthening purposes. As shown in [6], basalt fibers can be a good alternative among other FRP strengthening systems. In [7], the effectiveness of a confinement based on basalt fibers preimpregnated with an epoxy resin or latex and then bonded with a cement-based mortar was investigated. It was found that the confining system provided a considerable gain both in the compressive strength and ductility of concrete columns, inducing a less brittle failure mode than that achieved in glass FRP-wrapped members. In [8], the contribution of a glass-basalt hybrid system to the confinement of concrete was assessed. The objective of this study was to investigate the effect of a basalt FRP confinement on the behavior of round concrete columns under compressive loading. The confinement was created by winding a basalt yarn onto a rotating concrete specimen. This method [1, 9] allowed us to make a prestressed confinement by winding an initially pretensioned basalt yarn. The influence of prestress level on the confinement was analyzed. It is shown in [9] that, in terms of confinement quality, the yarn winding technique is superior to the conventional confining method, i.e., the manual wrapping of a FRP tape around a column. 1. Experimental 1.1 Concrete properties and confined specimens The compressive behavior of cylindrical concrete columns of length 300 mm and diameter 150 mm with two different compressive strengths was investigated. The concrete was prepared in a laboratory and allowed to harden in forms for 28 days. Then the columns were confined by winding a basalt yarn (collection of parallel continuous basalt filaments), impregnated with an epoxy resin, onto rotating concrete specimens. A schematic diagram of the yarn winding process is shown in Fig. 1. We used the winding equipment reported in [9]. A continuous flow of the yarn was supplied from a bobbin to a pretension device composed of four rubber-coated wheels. Each wheel was adjustable separately to obtain a pretension force desired. The placement of yarns was controlled by traversing the pretension device along the longitudinal axis of the specimen at a step of t = 2.25mm per rotation. The resin was applied to the concrete surface and yarns before winding. The winding process was conducted carefully in order not to damage the yarns in the course of winding. The confinement consisted of four
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TABLE 1. Compressive Properties of Plain Concrete Compressive strength fco, MPa
Ultimate axial strain εco, %
Ultimate lateral strain εlo, %
Young’s modulus Eb, GPa
Poisson ratio ν
12.3 49.2
0.45 0.30
0.43 0.20
15.6 28.6
0.18 0.22
TABLE 2. Split-Disk Test Results Number of layers
Thickness, mm
Strength, MPa
Young’s modulus, GPa
Ultimate hoop strain, %
4 8
0.79 1.58
729 798
34.7 45.0
2.10 1.75
and eight layers of yarns. The pretension force P applied to the yarn was equal to 0 or 200 N. The confined specimens were left to dry for 10 days at a temperature of 22°C. Eight unconfined plain concrete columns and 24 confined columns were tested. The specimens were subjected to a monotonic uniaxial compression loading up to failure. The loading rate was 10MPa/min, following the ASTM C 39/C39M–99 standards. The axial and lateral strains were measured by vertically and horizontally oriented strain gages glued on two opposite sides of the specimens at the column midheight. The axial compression load was measured by the loading cell of the testing machine. The averaged results of plain concrete tests are summarized in Table 1. The following labeling of specimen groups will be used throughout the text and in figures: the average strength of plain concrete (MPa) — the number of confinement layers — the pretension force of the yarn (N). 1.2. Basalt confinement properties A KVT1200Tex13E basalt yarn produced by Basaltex was used in this study. Its properties, according to manufacturer’s data, were as follows: tensile strength > 1755MPa, tensile modulus 87 GPa, density 2.67 g/dm3, linear density 1200 tex, and filament diameter 13μm. Split-disk tests according to the ASTM D 2290 standard were performed to determine the hoop properties of the basalt FRP jacket. Two types of composite rings were made — with four and eight layers. The diameter of the rings was 155 mm and width 22 mm. All the rings were tested at a displacement rate of 2 mm/min up to failure. The results are summarized in Table 2. 2. Effect of Initial Pretensioning When the initial loading path in the nondimensional space of normalized compressive axial σz /fco and lateral σl /fco stresses reaches the strength line, intense internal cracking of concrete begins. Because of the confinement, the concrete columns does not fail, but the loading path changes its direction and becomes roughly parallel to the strength line [10, 11]. The strength line can be described by the simple formula [11]
σz 1 −ν σ l = −1 + ⋅ , (1) f co f co ν
where fco is the plain concrete strength, and ν is the Poisson ratio of plain, undamaged concrete. The lateral stress is calculated as
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z fcc
^
*p
z
^
z* f co
^
^
l lo
fl
Fig. 2. Influence of the lateral stress on the knee point: loading paths of nonprestressed confined (• • •) and prestressed confined (▬▬) specimens and the strength line (– – –).
σl = −
Ejh R
ε j,
(2)
where Ej is the elastic modulus of the FRP jacket, h is the thickness of the jacket, R is the radius of the column, and εj is the hoop strain in the FRP jacket. The lateral strain of the confined concrete εl is equal to the hoop strain in the FRP jacket: εl = εj . Formulas (1) and (2) and lead to the simple formula for the compressive strength of confined concrete columns fcc
f cc 1 −ν f l = 1+ ⋅ . (3) f co f co ν In the formula, fl is the ultimate lateral pressure
fl =
Ejh R
ε ju ,
where εju is the ultimate hoop strain in the FRP jacket. The confinement thickness is calculated as
h=
nS0 , t
where S0 is the cross-sectional area of the yarn, t is the winding pitch of the yarn, and n is the number of yarn layers. The loading path of a confined specimen is schematically shown in Fig. 2. The intersection point of the initial loading path and the strength line is called the knee point or the limit of linearity. It is shown in [9] that the axial stress at the knee point can be estimated by the formula
Ejh σ *z = − 1 + (1 −ν ) f co , Eb R
(4)
where Eb is the initial elastic modulus of plain concrete. In most cases, σ *z ≈ f co . The lateral pretensioning of the FRP jacket increases the axial stress at the knee point. Due to the initial lateral stress σ lo produced by the pretensioned FRP jacket, the initial loading path will intersect the strength line at a higher axial stress (see Fig. 2). The initial lateral stress can be expressed as
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σ lo = −
nP . (5) Rt
80
z, MPa
a
120
z, MPa
b
100 60 80 40
60 40
20
l, % 0
120
cc, %
20 0
2 1
0
1
2
3
z, MPa
4
5
6
7
c
180
l, %
cc, %
2 1 0 1 2 3 4 5 6 7 8
z, MPa
d
100 140 80 100 60 40 20 0
60
l, % 2
1
cc, %
20 0
0
1
2
3
4
l, % 2
1
cc, % 0
1
2
3
4
5
Fig. 3. Stress–strain curves σ z −ε cc of confined concrete columns: a) 12–4–0 (––––) and 12–4–200 (▬▬); b) 12–8–0 (––––) and 12–8–200 (▬▬); c) 49–4–0 (––––) and 49–4–200 (▬▬); d) 49–8–0 (––––) and 49–8–200 (▬▬). The compressive strains and stresses are assumed to be positive.
The axial stress at the knee point for the prestressed concrete calculated by the formula [9] is
Ejh 1 −ν σ *z p = 1 + (1 −ν ) σ lo . − f co + E R ν b
(6)
In actuality, the transition from the first to the second part of the experimental loading path is gradual. Therefore, the experimental axial stress at the knee point is determined from the intersection of continuations of the linear sections of the first and second parts of loading paths. 3. Results and Discussion The stress–strain curves σ z −ε cc of basalt FRP-confined concrete columns are presented in Fig. 3 and evidence show an almost bilinear response, which is typical of FRP-confined concrete columns. Table 3 shows the averaged experimental characteristics of the confined concrete specimens. They all failed abruptly due to failure of the basalt FRP jacket in the hoop direction. It is seen that the basalt FRP confinement provides a considerable strengthening effect. Due to time-dependant effects in the prestressed confined specimens, the prestress level in the confinement after the drying period can be much lower
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TABLE 3. Experimental Results Specimen 12–4–0 12–4–200 12–8–0 12–8–200 49–4–0 49–4–200 49–8–0 49–8–200
σ loe
σ loa МPa
0 4.7 0 9.5 0 4.7 0 9.5
εlu
εcc %
0 1.5 0 3.9 0 4.4 0 4.9
5.81 5.16 7.55 6.68 3.37 3.24 5.13 4.46
1.85 1.46 1.79 1.39 2.16 1.73 1.94 1.64
f cc f co 5.59 5.07 8.80 8.18 2.21 2.19 3.34 3.12
σ *z
f co
1.01 1.55 1.12 2.06 1.07 1.37 1.10 1.47
Note: σ loe and σ loa are the expected and achieved initial lateral stresses; σ loe is calculated using formula (5); εcc and εlu are the average ultimate axial and lateral strains of confined concrete specimens. than the expected one [9]. The initial lateral stress σ loa can be determined from the loading paths of prestressed and nonprestressed confined concrete columns. The lateral stress produced by the prestressed confinement is
σl = −
Ejh R
ε j + σ lo .
(7)
If formula (7) is used to calculate the experimental lateral stress and σ lo is equal to the initial lateral stress σ loa achieved, then the second parts of experimental loading paths of prestressed and nonprestressed specimens will approximately coincide. Thereby the initial lateral stress can be determined. The experimental loading paths of all tested specimens are shown in Fig. 4. The values of the initial lateral stress achieved are given in Table 3. The actual values of the stress are much smaller than the expected ones. The expected initial lateral stress σ loe was achieved only for specimens 49–4–200. The relative increment of the axial stress at the knee point due to the initial prestress is calculated as
∆=
σ *z p − σ *z σ *z
⋅100%,
where σ z∗ and σ z∗ p are found from Eqs. (4) and (6), respectively. The experimental and estimated relative increments of the axial stress at the knee point are shown in Fig. 5. The initial lateral stress σ loa achieved was used to calculate the estimated value of Δ. It is seen from Fig. 5 that the values of Δ are in good agreement with experimental ones and pretensioning of the basalt FRP jacket considerably increases the axial stress at the knee point by delaying the start of intense internal cracking of concrete. Since the pretensioned FRP jacket contains prestretched yarns, the prestressed confined specimens have a lower ultimate lateral strain and, in turn, lower compressive strength and ultimate axial strain than the nonprestressed ones (see Table 3). However, this fact is of no importance if the load applied is smaller than the limit of linearity. It was found that the average ultimate hoop strain obtained from the split-disk tests (1.93%) was the same as the average ultimate lateral strain of nonprestressed confined specimens, demonstrating a high quality of the wound FRP jacket. The yarn winding technique drastically reduces the number of misalignments, which are responsible for the different stretching of fibers and, hence, the premature failure of the FRP jacket. Due to the high number of misalignments in FRP wrappings created by the hand lay-up technique, the ultimate lateral strain of columns confined with such wrappings is up to 40% lower than the ultimate hoop strain obtained from the split-disk tests [1, 11].
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z /fco
9 8 7 6 5 4 3 2 1
Specimens 49 8 200 49 4 200 12 8 200 12 4 200
l /fco
0
0.5
1.0
1.5
2.0
, % 0
2.5
20
40
60
80
100 120 140
Fig. 4. Experimental loading paths of all confined specimens tested. Fig. 5. Experimental (□) and estimated (■) relative increments Δ of the axial stress at the knee point due to the initial prestress.
9
est cc ,%
12
( fcc/fco)est
10 7 8 5
6 4
3
exp cc , % 1
3
5
7
( fcc/fco)exp
2 1
9
2
4
6
8
10
12
Fig. 6. Experimental and estimated (by Eq. (8)) ultimate axial strains of basalt FRP-confined concrete specimens 12–4–0 (■), 12–4–200 (□), 12–8–0 (●), 12–8–200 (○), 49–4–0 (▲), 49–4–200 (Δ), 49–8–0 (♦) and 49–8–200 (◊). The solid line — perfect correspondence. Fig. 7. Experimental and estimated (by Eq. (3)) normalized compressive strengths of basalt FRPconfined concrete specimens. Designations as in Fig. 6.
For estimating the ultimate axial strain of carbon FRP-confined round concrete columns, the following formula was proposed in [11]:
Ejh ε cc = ε co + 0.17 ε ju − ε lo Rf co
(
)
0.65
.
(8)
A comparison between the results given by formula (8) and the experimental ultimate axial strain of basalt FRP-confined concrete columns is shown in Fig. 6. Although formula (8) underestimates the ultimate axial strain, a satisfactory correlation between the results can be observed. Also, a good correlation between experimental values of the compressive strength and those given by formula (3) can be observed in Fig. 7.
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Conclusions The experimental results found show that the confinement consisting of a moderate number (4-8) of wound basalt yarn layers provides a considerable gain both in the compressive strength and the ultimate axial strain. This makes basalt fibers a good alternative to the much more expensive carbon fibers for strengthening purposes. The yarn winding technique allows one to fully utilize the high deformational capacity of basalt yarns. It has been demonstrated that a prestressed confinement can increase the limit of linearity by up to 84%. The prestressed confinement in our in study was created by winding an initially pretensioned basalt yarn. Formulas (3) and (8) are recommended for estimating the compressive strength and ultimate axial strain of basaltFRP-confined concrete columns. Acknowledgments. This work was funded by ESF via project 2009/0209/1DP/1.1.1.2.0/09/APIA/VIAA/114. The authors express gratitude to R. Apinis, V.Skruls, and U. Vilks for assistance. REFERENCES 1. V. Tamuzs, et al., “Behavior of concrete cylinders confined by carbon-composite tapes and prestressed yarns. 1. Experimental data,” Mech. Compos. Mater., 42, No. 1, 13-32 (2006). 2. B. Wei, H. Cao, and S. Song, “Environmental resistance and mechanical performance of basalt and glass fibers,”. Materials Sci. and Eng., A, 527, 18-19, 4708-4715 (2010). 3. B. Wei, H. Cao, and S. Song, “Tensile behavior contrast of basalt and glass fibers after chemical treatment,” Materials & Design, 31, No. 9, 4244-4250 (2010). 4. M. Černý, et al., “Partially pyrolyzed composites with basalt fibres – Mechanical properties at laboratory and elevated temperatures,” Composites Pt A: Appl. Sci. and Manufacturing, 40, No. 10, 1650-1659 (2009). 5. V. Lopresto, C. Leone, and I. De Iorio, “Mechanical characterisation of basalt fibre reinforced plastic,” Composites Pt B: Engn., 42, No. 4, 717-723 (2011). 6. J. Sim, C. Park, and D. Y. Moon, “Characteristics of basalt fiber as a strengthening material for concrete structures,” Composites Pt. B: Engineering, 36, No. 6-7, 504-512 (2005). 7. M. Di Ludovico, A. Prota, and G. Manfredi, “Structural upgrade using basalt fibers for concrete confinement,” J. of Composites for Construction, 14, No.5, 541-552 (2010). 8. A. De Luca, et al., “Structural evaluation of full-scale FRP-confined reinforced concrete columns,” J. of Composites for Construction, 15, No. 1, 112-123 (2011). 9. E. Zīle, M. Daugevičius, and V. Tamuzs, “The effect of pretensioned FRP windings on the behavior of concrete columns in axial compression,” Mech. Compos. Mater., 45, No. 5, 457-466 (2009). 10. V. Tamuzs, R. Tepfers, and E. Sparnins, “Behavior of concrete cylinders confined by carbon composite. 2. Prediction of strength,” Mech. Compos. Mater., 42, No. 2, 109-118 (2006). 11. Tamuzs, V., et al., “Behavior of concrete cylinders confined by a carbon composite 3. Deformability and the ultimate axial strain,” Mech. Compos. Mater., 42, No. 4, 303-314 (2006).
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