PSI - Issue 78

Sabatino Di Benedetto et al. / Procedia Structural Integrity 78 (2026) 1697–1704

1699

Bryson and Mathey (1962) explored the influence of steel surface conditions. They found that sandblasted or rusted steel surfaces developed higher bond stresses than those with mill scale. After slip, however, post-slip bond stresses were similar regardless of surface condition. The flange surfaces were identified as the primary bonding areas. Hawkins (1973) investigated casting orientation and reinforcement size. Vertically cast specimens exhibited greater bond capacity than horizontally cast ones, due to better aggregate distribution and fewer voids. While tie reinforcement did not significantly affect pre-slip bond strength, it enhanced post-slip resistance. Steel section size did not influence bond capacity provided the embedment length-to-depth ratio remained constant. Roeder (1984) analysed bond stress distribution using strain gauges along specimen length. Under service loads, bond stress followed an exponential distribution, becoming more uniform near ultimate capacity. Observed slip was linked to crack formation along the steel – concrete interface. Roeder suggested that cracking causes irreversible damage, and loading beyond the slip threshold should be avoided in cyclic conditions. While concrete strength appeared to improve bond capacity, subsequent studies (Hamdan and Hunaiti 1991) challenged this claim. In this framework, the aim of the present study is to examine the bond behaviour between concrete-filled steel tubes and reinforced concrete plinths. Several relevant studies can be cited in this context. Huang et al. (2021), Cao et al. (2023) and Xie et al. (2023) investigated the bonding performance of steel and ultra-high-performance concrete (UHPC), including UHPC confined in steel tubes, through push-out testing. Their results indicate that replacing conventional concrete with UHPC improves interface bonding. However, limited research has addressed the interaction between steel tubes and surrounding concrete layers. A few experimental studies by Li et al. (2020), Yongwang et al. (2020), and Weixiao et al. (2017) focused on the bond at the interface between steel tubes and the concrete core in double- layer steel tube systems. These studies identified the inner steel tube’s diameter -to-thickness ratio as the dominant factor affecting bonding strength. Han et al. (2016) further explored how the steel tube diameter and external concrete strength influence interface bonding, concluding that stronger concrete or smaller tube diameters improve performance. Qian et al. (2015) conducted launch tests on 10 specimens to measure shear resistance at the steel – concrete interface, reporting a bonding strength of 1.0 MPa and recommending a design value of 0.7 MPa. These findings deviate significantly from conventional design codes, such as Eurocode 2 (EN 1992-1-1, 2004), Eurocode 4 (EN 1994-1-1, 2004) and the Model Code 1990 (Telford, 1993), which primarily address bond behaviour of reinforcing bars rather than steel tubes. However, the current study aims to demonstrate that the requirements given for smooth bars in the Model Code 1990 can also provide accurate predictions for the evaluation of the bond behaviour of concrete-filled steel tubes embedded in concrete plinths. The paper is structured as follows: Section 2 reviews formulations on bond stress capacity found in the literature; Section 3 presents results from a pull-out test on a concrete-filled steel micropile embedded in a concrete plinth; Section 4 describes the implementation of a numerical modelling approach in OpenSees (McKenna, 2011) and validates the model by comparing it with the experimental test results; Section 5 presents the results of a parametric study conducted on different geometric and mechanical configurations of composite micropiles embedded in concrete plinths, and introduces a regression-based formulation for estimating the bond resistance. 2. Formulations available in the literature for the bond stress capacity The Italian building code (NTC2018) and Eurocode 2 (EN 1992-1-1, 2004) provide specific guidelines for evaluating the bond stress between steel and concrete. According to the Italian code, with reference to ribbed reinforcement bars in reinforced concrete, the design bond stress is related to the characteristic tensile strength of concrete and can be calculated using Eq. (1): =2.25 1 2 / (1) In Eq. (1) 1 is a coefficient accounting for the quality of bond, 2 is a coefficient related to the bar diameter; is the partial safety factor for concrete. These coefficients are defined based on the bond quality and the bar diameter. Eurocode 2 provides a similar formulation, but including additional, as shown in Eq. (2): =2.25 1 2 3 4 5 1 2 (2) In Eq. (2): 1 considers the effect of the form of the bars; 2 takes into account the effect of concrete minimum cover; 3 refers to the effect of confinement by transverse reinforcement; 4 is for the influence of one or more welded bars along the design anchorage length; 5 considers the effect of the pressure tranverse to the plane of