Evaluation of seismic shear capacity of prestressed concrete containment vessels with fiber reinforcement

Accepted Manuscript Evaluation of seismic shear capacity of prestressed concrete containment vessels with fiber reinforcement Young-Sun Choun, Junhee Park PII:

S1738-5733(15)00157-6

DOI:

10.1016/j.net.2015.06.006

Reference:

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To appear in:

Nuclear Engineering and Technology

Received Date: 24 March 2015 Revised Date:

2 June 2015

Accepted Date: 3 June 2015

Please cite this article as: Y.-S. Choun, J. Park, Evaluation of seismic shear capacity of prestressed concrete containment vessels with fiber reinforcement, Nuclear Engineering and Technology (2015), doi: 10.1016/j.net.2015.06.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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EVALUATION OF SEISMIC SHEAR CAPACITY OF PRESTRESSED CONCRETE CONTAINMENT VESSELS WITH FIBER REINFORCEMENT YOUNG-SUN CHOUN*and JUNHEE PARK

Integrated Safety Assessment Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea

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Number of Pages: 30 * Number of Tables: 8 * Number of Figures: 12

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* Corresponding author: E-mail address: [email protected] Tel.: +82 42 868 2036, Fax: +82 42 868 8256.

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ABSTRACT

Background: Fibers have been used in cement mixture to improve its toughness, ductility,

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and tensile strength, and to enhance the cracking and deformation characteristics of concrete structural members. The addition of fibers into conventional reinforced concrete can enhance

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the structural and functional performance of safety-related concrete structures in nuclear power plants.

Methods: The effects of steel and polyamide fibers on the shear resisting capacity of a prestressed concrete containment vessel (PCCV) were investigated in this study. For a

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comparative evaluation between the shear performance of structural walls constructed with (i) conventional concrete; (ii) steel-fiber reinforced concrete (SFRC); and (iii) polyamidefiber reinforced concrete (PFRC), cyclic tests for the wall specimens were conducted and

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hysteretic models were derived.

Results: The shear resisting capacity of a PCCV constructed with FRC can be improved

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considerably. When SFRC contains hooked steel fibers in a volume fraction of 1.0%, the maximum lateral displacement of a PCCV can be improved by more than 50%, in comparison with that of a conventional PCCV. When PFRC contains polyamide fibers in a volume fraction of 1.5%, the maximum lateral displacement of a PCCV can be enhanced by approximately 40%. In particular, the energy dissipation capacity in a fiber reinforced PCCV can be enhanced by more than 200%. Conclusions: The addition of fibers into conventional concrete increases the ductility and energy dissipation of wall structures significantly. Fibers can be effectively used to improve 2

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the structural performance of a PCCV subjected to strong ground motions. Steel fibers are more effective in enhancing the shear performance of a PCCV than are polyamide fibers.

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Keywords: Concrete shear wall Fiber reinforced concrete

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Hysteretic model Polyamide fibers

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Prestressed concrete containment vessel Seismic shear capacity

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Steel fibers

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1. Introduction

Large inertia forces caused by strong ground motions creates membrane shear forces in the cylinder wall of concrete containment vessels. The shear resistance of the vessels can be

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developed by interface shear transfer across the cracks, which is influenced by various parameters such as the initial crack width, amount of reinforcing restraint, and aggregate size [1]. The shear capacity increases as the initial crack width decreases, and the slip decreases as

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the transverse reinforcement increases [2]. The use of fibers is effective in resisting shear forces in concrete structures [3-6]. Fibers can provide equal resistance to stresses in all

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directions because they are randomly distributed throughout the concrete volume at a relatively small spacing. In addition, fibers increase the resistance to crack formation and propagation, and thus reduce the crack width and length.

The addition of fibers into plain concrete enhances the shear strength and ductility of

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reinforced concrete (RC) members, leading to a change in failure mode from a brittle shear failure to a ductile flexural failure [7]. RC members generally show a rapid deterioration in shear resisting mechanisms under a reversed cyclic load [8]. However, the use of high-

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performance fiber-reinforced cement composites (HPFRCC) provides excellent damage tolerance under large displacement reversals compared with regular concrete [9]. Steel fiber

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reinforced concrete (SFRC) is mostly applied in the field of industrial floor and tunnel constructions, but nowadays is suggested as a partial stirrup or punching reinforcement in RC beams and plates, respectively, or even as a complete substitution of conventional steel reinforcements in flat slab construction [10]. The shear behavior of structural walls using fiber reinforced concrete (FRC) was investigated in a limited number of studies. Parra-Montesinos and Kim [11] evaluated the use of HPFRCC in two low-rise structural walls under displacement reversals. In their 4

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investigation, two types of fiber were used: a 1.5% volume fraction of ultrahigh molecular weight polyethylene fibers and a 2.0% volume fraction of hooked steel fibers. The experimental results indicated that the use of HPFRCC materials in lightly reinforced low-

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rise walls represents a viable alternative for ensuring adequate behavior during seismic events. Polyethylene fibers in a 1.5% volume fraction were more effective than hooked steel fibers in a 2.0% volume fraction in terms of reducing the crack spacing and width. However,

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there was no significant difference in the overall hysteretic response of the wall specimens. Carrillo, et al. [12] showed that SFRC walls can exhibit a seismic performance comparable to

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that of conventionally reinforced walls in terms of strength and deformation capacities. More diagonal cracks of smaller width were observed as the fiber dosage was increased, and a better uniform distribution of web cracking was shown as the aspect ratio of the fiber increased.

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Previous experimental investigations indicate that the use of fibers in conventional concrete can enhance the shear resistance of prestressed concrete containment vessels (PCCVs) in nuclear power plants. This study evaluates the shear resisting capacity for a

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PCCV constructed using SFRC or polyamide fiber reinforced concrete (PFRC). For a comparative evaluation of the shear capacity of structural walls constructed using

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conventional concrete or FRC, cyclic tests for the wall specimens were conducted. The hysteretic models were derived from the test results and were used as part of a pushover analysis of the PCCVs.

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2. Experimental program for shear walls

Two types of fibers were used for cyclic shear tests: steel and polyamide fibers. Steel fibers have been most widely used in FRC applications because steel is highly compatible with

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cement composites. Polyamide fibers (often called nylon fibers) are known to have an excellent resistance to moisture, alkalis, and chemical environments. The experimental study investigated the shear response of structural walls constructed using conventional RC,

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reinforced-SFRC (R-SFRC), and reinforced-PFRC (R-PFRC) under reversed cyclic loads.

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2.1 Test specimens

In a PCCV model subjected to lateral force at the level of the spring line, a through-wall failure around the circumference of the cylinder wall occurs at about a quarter of the overall

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wall height from the basemat [13, 14]. Failure mode is defined as concrete cracking, rebar yielding, loss of bond between concrete and rebar, and through-thickness cracking of the cylinder wall. A major failure mode is the shear failure at the junction of the cylinder wall

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and basemat.

Fig. 1 shows hoop and vertical reinforcing bars in the section of the cylinder wall at a

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quarter of the wall height from the basemat. The rebar is placed in one layer in each direction on each face. Based on the wall reinforcements, the dimensions and reinforcement details of the test specimens were determined, as shown in Fig. 2. The specimen consists of three parts: 1. The loading beam, which transfers the loads into the wall. 2. The wall, which models the lower part of a cylinder wall of the PCCV. 3. The footing, which anchors the specimen to a strong floor.

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All of the reinforcing bars had a nominal yield strength of 400 MPa. Vertical and horizontal reinforcement ratios provided in the wall of the specimens were determined by

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considering the reinforcement details for the PCCV wall section.

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Fig. 1 - Wall reinforcement of PCCV and test specimen.

2.2 Concrete mix proportions

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Fig. 2 - Geometry and reinforcement details of test specimen (in mm).

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Concrete mixes with a compressive strength of 42 MPa are given in Table 1 for plain and fiber reinforced concretes. To evaluate the effect of fibers on the shear response, equivalent mix proportions were used for plain concrete and SFRC, except for the proportions of water-

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reducing agents and fibers. A 1.0% volume fraction of hooked-end steel fibers was added for SFRC, whereas a 1.5% volume fraction of straight polyamide fibers was used for polyamide

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fiber reinforced concrete (PFRC). Steel and polyamide fibers used for FRC are shown in Fig. 3 and their properties are listed in Table 2.

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Mix proportions

SFRC

PFRC

325.50

325.50

376.00

Water (kgf/m )

162.75

162.75

188.00

Coarse aggregate (kgf/m3)

938.77

938.77

722.00

3

748.89

748.89

883.00

19

19

20

81.38

81.38

94.00

Water-reducing agent (kgf/m )

2.60

3.66

-

Air-entraining agent (%)

0.15

0.15

0.2

Superplasticizer (%)

-

-

2.0

Viscosity agent (%)

-

-

0.15

40

40

3

Cement (kgf/m ) 3

Sand (kgf/m ) Coarse aggregate size (mm) 3

Fly ash (kgf/m ) 3

40

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Water/cement ratio (%)

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Plain concrete

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Table 1 - Mixture details of the concrete used in specimens.

-

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Fibers (%)

8

1.0

1.5

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Fig. 3 - Steel and polyamide fibers used for FRC specimens.

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Table 2 - Fibers used in FRC specimens. Type

Length (mm)

Diameter (mm)

Aspect ratio

Tensile strength (MPa)

Steel

30

0.5

60

1,100

30.28

2.31

13

650

End-hooked Straight

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Polyamide

Shape

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2.3. Concrete properties

Fig. 4 shows compression and tension test results for plain concrete, SFRC, and PFRC specimens. As indicated, both steel and polyamide fibers provide significant improvements in

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the toughness of plain concrete. The peak stress appears at a large strain in PFRC because polyamide fibers allow a deformation at the early stage. The mechanical properties of the hardened concrete were obtained using molded cylinder specimens. Table 3 summarizes the

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compressive strength and elastic modulus for the three types of concrete at the time of testing. Comparing the properties with plain concrete specimens, the SFRC specimens had 11% and

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10% higher values in compressive strength and elastic modulus, whereas the PFRC specimens had 11% and 4% lower values in compressive strength and elastic modulus,

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respectively.

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Fig. 4 - Compression and tension behaviors for plain concrete and FRC specimens.

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Table 3 - Measured mechanical properties of the concrete used in the specimens. Compressive strength (MPa)

Elastic modulus (MPa)

Plain concrete

40.2

20,134

SFRC

44.7

22,058

PFRC

35.8

19,227

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Type

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2.4. Test setup

It is assumed that a simple cantilever subjected to static-cyclic lateral loads can represent the behavior of a shear wall under earthquake conditions. Figure 5 shows the test setup for the

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cyclic tests of the specimens. The specimen was mounted on a thick reaction floor and strongly anchored by high tension anchor rods in the vertical direction to prevent an uplift. Steel blocks are placed and anchored on both sides of the footing to prevent horizontal sliding

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of the specimen during lateral loading. Lateral displacements were applied through a 3,000 kN hydraulic actuator connected to the loading beam of a specimen at one end and a strong

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reaction wall at the other end. To eliminate out of plane movements during the test, a specially designed steel jig was installed at the top of the loading beam. Positive loading was applied by extending the actuator against the test specimen, while negative loading was applied by pulling four steel bars of 64 mm in diameter. Three linear voltage differential

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transducers (LVDTs) were placed at the center of the loading beam and at the top and mid-

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heights of the specimen wall, for monitoring the in-plane displacements.

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Fig. 5 - Setup for reversed cyclic test.

2.5. Displacement history

The specimens were subjected to a displacement loading history of the reversed cycle tests, as shown in Figure 6. Two cycles were applied to each drift at up to 3.5% for investigating the degradation in strength and stiffness during the repeated cycles.

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Fig. 6 - Lateral displacement history for reversed cyclic tests.

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3. Experimental results

3.1. Load-drift response

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Fig. 7 shows the observed hysteresis responses for the RC, R-SFRC, and R-PFRC specimens. It was revealed that the addition of fibers in an RC wall can enhance its shear resisting capacity significantly. The increase in the shear force capacity was less than about 10%, but the increase

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in the drift capacity was significant. The drift capacity for the R-SFRC and R-PFRC specimens increased to 3.5% and 3.0% from 2.25%, as summarized in Table 4. In particular, the ductility

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and energy dissipation were significantly improved in R-SFRC and R-PFRC specimens. The ductility was increased by 59% in the R-SFRC specimen and 28% in the R-PFRC specimen. The energy dissipation capacity was increased by 188% in the R-SFRC specimen and 85% in

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the R-PFRC specimen.

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Fig. 7 - Lateral force versus drift response for different concrete walls.

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Table 4 - Lateral force and displacement response Property

Lateral force (kN)

Displacement (mm)

Drift Ductility Energy (%) dissipation (MN·mm)

Minimum Maximum Minimum Maximum Yield -1,287

1,254

-35.96

35.72

12.30

2.25

2.90

228

R-SFRC

-1,356

1,384

-52.58

52.07

11.29

3.50

4.61

656

R-PFRC

-1,223

1,258

-44.06

43.36

11.72

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3.00

16

3.70

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3.2. Crack patterns and failure modes

Fig. 8 shows the cracking patterns and failure modes for the three specimens after the cyclic tests. Both fiber-reinforced specimens exhibited a larger number of cracks of a small width.

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Because the fibers are randomly distributed through the volume of the concrete at a much closer spacing, the crack spacing is much closer and the crack width is smaller in the fiber reinforced specimens. The RC specimen, which failed at a 2.25% drift, exhibited a typical

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diagonal failure mode with a crushing in the corners. The R-SFRC specimen failed after a breaking and yielding of vertical reinforcing bars at a 3.5% drift. The main failure mode of

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the R-SFRC specimen was a horizontal sliding failure owing to the shear. The R-PFRC specimen, which failed at a 3.0% drift, exhibited a combined failure mode of diagonal and flexural failures. The fibers limit the opening of the tension crack and lead to a sheardominated failure from the diagonal failure in the RC specimen. It was revealed that the use

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of fibers can change the failure modes of the structural walls.

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Fig. 8 - Failure cracking patterns for different concrete walls.

3.3. Lateral equivalent stiffness

Fig. 9 shows the lateral equivalent stiffness and its degradation for the three specimens with an increase in the wall drift. The equivalent stiffness is estimated as the slope between the peak positive and negative displacements for the first cycle. It can be observed that the initial equivalent stiffness at a 0.1% drift for the R-SFRC specimen is approximately 14% larger 17

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than that for the RC specimen. For all specimens, the lateral equivalent stiffness decreased by more than 50% of the initial equivalent stiffness of 0.5% drift. For both of the fiber reinforced specimens, shear capacity exists even though the lateral equivalent stiffness decreases by

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more than 90% of the initial equivalent stiffness.

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Fig. 9 – Degradation of lateral equivalent stiffness with drift increase.

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4. Shear resisting capacity

4.1. Hysteretic models

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Based on the hysteresis responses of the test specimens shown in Fig. 7, the hysteretic models for the RC, R-SFRC, and R-PFRC members were derived, as shown in Fig. 10. The strength

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and displacement properties of the hysteretic models are summarized in Table 5.

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Fig. 10 - Hysteretic models for RC, R-SFRC, and R-PFRC walls.

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RC

R-SFRC

R-PFRC

Crack strength (kN)

668

959 (1.44)

578 (0.87)

Crack displacement (mm)

3.54

4.14 (1.17)

2.52 (0.71)

Maximum shear strength (kN)

1,323

1,442 (1.09)

1,306 (0.99)

Displacement at maximum shear strength (mm)

19.08

18.04 (0.95)

16.26 (0.85)

Shear strength at maximum lateral displacement (kN)

1,209

829 (0.69)

795 (0.66)

Maximum lateral displacement (mm)

32.12

49.58 (1.54)

46.20 (1.44)

Ductility

2.57

3.77 (1.47)

4.35 (1.69)

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* ( ): ratios to RC values.

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Property

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Table 5 - Strength and displacement properties of hysteretic models.

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4.2. Pushover analysis of PCCVs

For a pushover analysis of the PCCV, the containment structure was represented by a lumped-mass stick model, shown in Fig. 11, which has a different eccentricity between the

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mass center and rigidity center at each level of lumped mass. The mass of the model includes all of the mass of the walls, slabs, and heavy equipment. As a material model, the hysteretic model of OpenSees [15] was used to simulate a degradation of the strength and stiffness. The

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pinch factors used for the hysteretic model were derived from the hysteretic response of the walls. For the element modeling, the nonlinearBeamColumn element was selected from the

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OpenSees elements library. Because the stress caused by seismic ground motions is concentrated on the lower part of the cylinder wall, the behavior of the wall was modeled by a nonlinear model and the behavior of the dome was modeled by a linear model. The properties of nonlinear models for a conventional PCCV (PCCV/RC) were

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determined using the horizontal load-displacement relationship for the PCCV specimens, presented in Reference 13. The assumed shear response at the wall of the PCCV/RC are summarized in Table 6. To determine the cracked stiffness, the flexural and shear rigidities

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were taken as 0.5 times those for an uncracked wall, as given in ASCE 43-05 [16]. The

Vu =

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ultimate shear capacity of the conventional concrete cylinder wall can be obtained from [17]:

v u πDt α

(1)

where vu is the ultimate shear stress capacity, (πDt/α) represents the effective shear area. D is the centerline diameter of the cylinder wall, t is the wall thickness, and α, a factor to convert

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cross-sectional area to effective shear area, is a function of the moment to shear times the cylindrical outside diameter ratio. The ultimate shear stress capacity, vu (psi), is given by:

v u = 0.8 f c′ + (ρσ y )AVER ≤ 21.1 f c′

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(2)

where f c′ is the compressive strength of concrete in psi units, and (ρσy) represents the average

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of the effective steel ratios, times a yield stress in the hoop and meridional directions. The strength and displacement properties of the nonlinear models for wall element #1 are

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summarized in Table 7.

To obtain the model properties for a PCCV constructed with SFRC (PCCV/R-SFRC) and a PCCV constructed with PFRC (PCCV/R-PFRC), the property ratios derived from the hysteretic models for the test specimens, shown in Table 5, were applied to the model

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cyclic.

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properties for the PCCV/RC. Two types of pushover analysis were conducted: monotonic and

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Fig. 11 - Containment model for pushover analysis.

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Table 6 –Assumed shear response at the wall of the PCCV/RC for horizontal loading. Event milestones

Shear strength

Shear strain (rad)

Diagonal cracking

0.6 Vu

0.0025

Vu

0.008

0.94 Vu

0.010

Vertical rebar yielding

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Structural failure

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Table 7 - Properties of nonlinear models for wall element # 1 in PCCV stick model.

Crack strength (kN) Crack displacement (mm) Ultimate shear strength (kN) Displacement at ultimate shear strength (mm) Shear strength at maximum lateral displacement (kN)

PCCV/R-SFRC

PCCV/R-PFRC

663,331

955,196

577,098

8.4

9.8

6.0

1,105,551

1,205,051

1,094,496

26.8

25.5

22.8

1,039,218

717,061

685,884

33.5

51.6

48.3

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Maximum lateral displacement (mm)

PCCV/RC

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4.3. Shear resisting capacity

The monotonic and cyclic pushover analyses resulted in the capacity curves and cyclic loops for PCCVs constructed with different types of concrete, as shown in Fig. 12. The maximum

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shear strength, maximum lateral displacement, and energy dissipation capacities for PCCV/RC, PCCV/R-PFRC, and PCCV/R-SFRC are shown in Table 8.

The maximum shear strength and lateral displacement for a PCCV/R-SFRC were

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approximately 9% and 52% greater than those for a PCCV/RC, respectively, and the maximum lateral displacement for a PCCV/R-PFRC was approximately 42% greater than

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that for a PCCV/RC. The energy dissipation capacities were approximately 390% and 207% larger in a PCCV/R-SFRC and PCCV/R-PFRC, respectively. The addition of fibers into a conventional RC does not increase the maximum shear strength of a PCCV greatly, but significantly increases the maximum lateral displacement of a PCCV. In particular, the

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energy dissipation capacity in a fiber-reinforced PCCV is enhanced remarkably. Steel fibers are more effective than polyamide fibers at improving the seismic resisting capacity of a

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PCCV.

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Fig. 12 - Pushover responses for PCCVs constructed with different concretes.

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Table 8 - Maximum resisting capacity of PCCVs for a lateral force

PCCV/RC

Maximum shear strength (MN) 1,105

Maximum lateral displacement at the top (cm) 30.8

1,205

46.8

PCCV/R-PFRC

1,094

43.8

214,617

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PCCV/R-SFRC

Energy dissipation capacity (MN·cm) 43,840

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5. Conclusions

The effects of steel and polyamide fibers on the shear resisting capacity of a PCCV were investigated. For a comparative evaluation of the shear performance of structural walls

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constructed with conventional concrete, SFRC and PFRC, cyclic tests for the wall specimens were conducted and the hysteretic models were derived. It was shown that an addition of fibers into a conventional concrete significantly increases the ductility and energy dissipation

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of wall structures and can change the failure modes.

The shear resisting capacity of a PCCV constructed using FRC can be improved

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considerably. Steel fibers are more effective at enhancing the shear performance of a PCCV than are polyamide fibers. When SFRC contains hooked steel fibers in a volume fraction of 1.0%, the maximum lateral displacement of the PCCV can be improved by more than 50% in comparison with that of conventional PCCV. When PFRC contains polyamide fibers in a

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volume fraction of 1.5%, the maximum lateral displacement of a PCCV can be enhanced by approximately 40%. In particular, the energy dissipation capacity in a fiber reinforced PCCV can be enhanced by more than 200%.

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Fibers can be effectively used to improve the structural performance of a PCCV subjected to strong ground motions. Currently, however, fiber reinforced concrete is not used

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for the main structural members. Further studies are needed to apply fibers to safety-related concrete structures in nuclear power plants.

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Conflicts of interest

Acknowledgments

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All contributing authors declare no conflicts of interest.

This work was supported by the National Research Foundation of Korea (NRF) grant funded

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by the Korea government (MSIP) (No. 2012M2A8A4025985).

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Systems, and Components in Nuclear Facilities, ASCE/SEI 43-05, VA., 2005.

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