Experimental Study on Concrete under Combined FRP-Steel Confinement

Abstract

The confinement of reinforced concrete (RC) compression members by fiber-reinforced polymers (FRPs) is an effective measure for the strengthening and retrofitting of existing structures. Thus far, extensive research on the stress-strain behavior and ultimate limit state design of FRP-confined concrete has been conducted, leading to various design models. However, these models are significantly different when compared to one another. In particular, the use of certain empirical efficiency and reduction factors results in various predictions of load-bearing behavior. Furthermore, most experimental programs solely focus on plain concrete specimens or demonstrate insufficient variation in the material properties. Therefore, this paper presents a comprehensive experimental study on plain and reinforced FRP-confined concrete, limited to circular cross sections. The program included 63 carbon FRP (CFRP)-confined plain and 60 CFRP-confined RC specimens with a variation in the geometries and in the applied materials. The analysis showed a significant influence of the compressive strength of the confined concrete on the confinement efficiency in the design methodology, as well as the importance of the proper determination of individual reduction values for different FRP composites. Finally, applicable experimental test results from the literature were included, enabling the development of a modified stress-strain and ultimate condition design model.

Keywords: CFRP; columns; confinement; load-bearing capacity; reinforced concrete; strengthening.

Figure 1

Confining action of a fiber-reinforced polymer (FRP) jacket.

Figure 2

Stress–strain model for FRP-confined concrete according to Lam and Teng [6].

Figure 3

Theoretical material behavior of carbon FRP (CFRP)-confined normal strength (a) and high strength (b) concrete columns according to different models and proposals collected from the literature [6,11,13,19,32,34].

Figure 4

Number of specimens as a function of the initial concrete strength, f_c0, used for the derivation of empirical design models for FRP-confined concrete by the authors of [6,11,13,32,33].

Figure 5

Arrangement of the fibers of the used carbon fiber (CF) sheets.

Figure 6

Preparation of the test specimens and application of the CFRP jacket.

Figure 7

Preparation of the test specimens and application of the CFRP jacket.

Figure 8

Test set-up.

Figure 9

Stress–strain curves of series D15-P-M1-1L-1, D15-P-M1-2L-1, and D15-P-M1-3L-1.

Figure 10

Typical failure of CFRP-confined plain concrete cylinders.

Figure 11

Typical failure of CFRP-confined RC cylinders.

Figure 12

Typical axial–transverse stress (a) and axial–transverse strain responses (b).

Figure 13

Dependence of Δf_cc (a) and ε_ccu (b) on the unconfined concrete strength, f_c0.

Figure 14

Strength enhancement Δf_cc (a) and ultimate strain ε_ccu (b) as functions of the relationship between confinement pressure and unconfined concrete strength.

Figure 15

Second modulus E_2,t (a) and second Poisson’s ratio ν₂ (b) as functions of the relationship between the confinement modulus and the unconfined concrete strength.

Figure 16

Values for k_ε determined from tests with different CFRP materials and calculated characteristic values k_εk (according to EN 1990:2002 [42]).

Figure 17

Strength enhancement (a), Δf_cc, and ultimate strain (b), ε_ccu, as functions of the ratio between f_l(j+w) and f_c0

Figure 18

Comparison between a confined concrete specimen (D30-P-M1-2L-1) and an RC specimen (D30-SR-M1-2L-2).

Figure 19

Comparison between a confined spiral-reinforced specimen (D25-SR-M1-2L-3) and a tie RC specimen (D25-TR-M1-2L-2).

Figure 20

Typical axial–transverse strain (a) and stress (b) responses of external CFRP confinement and internal transverse reinforcement (specimen D30-SR-M1-2L-2)

Figure 21

Proposal of Pellegrino and Modena [8] concerning k_ε (a) and a comparison between confined RC specimens with different numbers of longitudinal bars (b).

Figure 22

E_2,t (a) and ν₂ (b) as functions of the ratio between the confinement modulus and the unconfined concrete strength including the databases in [4,13,46,48].

Figure 23

Relationship between factor k₁ and the ratio between the confinement pressure and the unconfined concrete strength. Comparison of design models in [6,8,13,31,32,33] with experimental databases including those in [4,46,48].

Figure 24

f_cc (a) and ε_ccu (b) as functions of the ratio between the confinement pressure and the unconfined concrete strength including the databases in [4,13,46,48].

Figure 25

Strength enhancement Δf_cc (a) and maximum strain ε_ccu (b) as functions of the ratio between f_l(j+w) and f_c0 including the databases of [4,46,47,50].

Figure 26

Strength enhancement (a), Δf_cc, and maximum strain (b), ε_ccu, as functions of the ratio between f_l(j+w) and f_c0 including the databases in [4,13,46,47,48,49,50] (cf. Table 14 and Table 15).