Bond Behavior and Critical Anchorage Length Prediction of Novel Negative Poisson's Ratio Bars Embedded in Ultra-High-Performance Concrete

Abstract

Negative Poisson's ratio (NPR) reinforcement offers a novel solution to the usual trade-off between strength gains and ductility loss. Incorporating NPR into ultra-high-performance concrete (UHPC) effectively overcomes the ductility limitations of structural elements. However, the interfacial bonding between NPR reinforcement and UHPC is not sufficiently studied, especially its patterns and mechanisms, impeding the application of the materials. In this paper, the effects of nine design parameters (rebar type, prestrain, etc.) on the bond performance of NPR-UHPC through eccentric pull-out tests are investigated, and a quantitative discriminative indicator K_c for NPR-UHPC bond failure modes is established. The results showed that when K_c ≤ 4.3, 4.3 < K_c ≤ 5.64, and K_c ≥ 5.6, the NPR-UHPC specimens undergo splitting failure, splitting-pull-out failure, and pull-out failure, respectively. In terms of bonding with UHPC, the NPR bars outperform the HRB400 bars, and the HRB400 bars outperform the helical grooved (HG) bars. For the NPR bars, prestrain levels of 5.5%, 9.5%, and 22.0% decrease τ_u by 5.07%, 7.79%, and 17.01% and s_u by 7.00%, 15.88%, and 30.54%, respectively. Bond performance deteriorated with increasing rib spacing and decreasing rib height. Based on the test results, an artificial neural network (ANN) model is developed to accurately predict the critical embedded length l_cd and ultimate embedded length l_ud between NPR bars and UHPC. Moreover, the MAPE of the ANN model is only 53.9% of that of the regression model, while the RMSE is just 62.0%.

Keywords: bond behavior; bond failure mode; critical anchorage length; eccentric pull-out tests; negative Poisson’s ratio rebar; ultra-high-performance concrete.

Figure 1

Metallographic structure of the NPR bars and HRB400 bars.

Figure 2

Bonding mechanism and failure modes. (a) Bonding mechanism [3,28,29]; (b) Cracking [27]; (c) Typical failure modes and τ-s curves [28,29].

Figure 3

Experimental program.

Figure 4

Rebar testing [31].

Figure 5

Compressive and tensile results [31,34].

Figure 6

Specimen design (units: mm) [31].

Figure 7

Test process and test setup details. (A) Test process [31]; (B) Test setup.

Figure 8

Failure modes.

Figure 9

Contact interface of the rebar and matrix. (a) Interface of UHPC-NPR; (b) Interface of ECC-HS [37].

Figure 10

The meaning of ρ_ss and the variation in failure patterns with K_c [15,20].

Figure 11

Measured curves.

Figure 12

Ideal schematic of the bond response [28].

Figure 13

Effect of rebar type: (a,b) NPR and HRB400 and (c,d) HRB400 and HG.

Figure 14

Mechanism of the effect of pretension strain.

Figure 15

Effect of pretension strain.

Figure 16

Effect of rib height.

Figure 17

Effect of rib spacing.

Figure 18

Effect of l_d: (a,b) under V_f = 2.2% and (c,d) under V_f = 0.5%.

Figure 19

Effect of s_s: (a,b) under V_f = 2.2% and (c,d) under V_f = 0.5%.

Figure 20

Effect of curing time.

Figure 21

Effect of V_f.

Figure 22

Effect of c: (a,b) under V_f = 2.2%, and (c,d) under V_f = 0.5%.

Figure 23

Variation in τ_cr, τ_u, and s_u with K_s [15,20].

Figure 24

Comparison of the results.

Figure 25

Database.

Figure 26

Heatmap of R².

Figure 27

ANN modeling [31].

Figure 28

Valuation indicators.

Figure 29

Relative importance. (a) Operational principle [44,61]; (b) Calculation results.