**1. Introduction**

Substructural components, such as piers and piles, are critical to the safety of bridge structures. Due to the low requirements of design codes decades ago, material degradation, and environmental erosion, the existing components generally suffer from delamination of the concrete covers, exposed ribs, and riverbed cutting. These defects are not easy to be detected because the substructure components are almost underwater throughout their life cycle, which limits the function of such bridges, even leading to collapse. Therefore, many conventional strengthening methods and techniques, such as the bonded steel plate method [1], enlarging section method [2], planting bar method [2], fiber-reinforced polymer (FRP) method [3,4], and prestressed strengthening technique [5], have been proposed and applied in practical engineering. Although these techniques have perfect design theories,

**Citation:** Wu, S.; Ge, Y.; Jiang, S.; Shen, S.; Zhang, H. Experimental Study on the Axial Compression Performance of an Underwater Concrete Pier Strengthened by Self-Stressed Anti-Washout Concrete and Segments. *Materials* **2021**, *14*, 6567. https://doi.org/10.3390/ ma14216567

Academic Editor: Krzysztof Schabowicz

Received: 31 August 2021 Accepted: 25 October 2021 Published: 1 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

mature construction techniques, and good strengthening effects, they are time-consuming, expensive, and traffic-disrupting because of the necessary construction of cofferdams, which are used for drainage before strengthening works. Consequently, increasing attention has been paid to undrained strengthening technology recently.

A number of undrained strengthening techniques have been developed and applied to practical engineering in the past decade, such as the jacket strengthening method [6], FRP underwater strengthening method [7,8], and precast concrete segment assembly method (PCSAM) [9]. These methods are economical, fast, and traffic-friendly compared with conventional methods, but also have some fatal deficiencies. For instance, the jacket strengthening method can greatly improve the durability of components but piers cannot be wrapped in fiber sleeves in deep water, which leads to strengthening failure. For the FRP underwater strengthening method, although the underwater strengthening effect is good, the empty gap generally occurs in the interface between the FRP and components because of the ineliminable water. As a result, this method tends to fail when the piers are submerged in water for a long time after strengthening. Additionally, the diving operation involves high costs, safety risks, and slow progress in deep water and under certain water pressure and flow rates. On the contrary, the PCSAM first uses precast concrete segments to wrap the components, then the segments are connected into the sleeve using wire ropes, and finally the interspace between the sleeve and the components is filled with the filling material to achieve undrained strengthening. The PCSAM can be used to strengthen the components in deep water. This approach has many advantages, including the simple construction method, reliable construction quality, short construction period, and low cost. However, there are still some problems with the PCSAM, such as the large prestress loss and poor durability of the wire ropes, poor accuracy and connection performance of the sleeves, large strength loss of the filling material, and uncertain bonding properties between the filled concrete and sleeves. As a consequence, there is an urgent need to solve the problems existing in the PCSAM and to develop a new strengthening method that can better serve in the reinforcement of damaged components.

Based on the PCSAM, this paper proposes a new undrained strengthening method named the improved precast concrete segments assembly method (IPCSAM), which takes advantage of three theoretical reinforcement techniques, namely the increasing section method, outer sleeve method, and prestressing method. The IPCSAM proposed in this paper not only uses self-stressed anti-washout underwater concrete (SSAWC) as the filling material, but also improves the connection of the segment sleeve, which is referred to as the shield lining segment. The SSAWC, as the filled concrete, can not only cut down the strength loss of the underwater concrete [10–12], but can also increase the bonding strength between the filled concrete and the sleeves [13]. On the one hand, due to the additional appropriate expansion agent added to the SSAWC compared with the AWC, the concrete becomes denser and the self-stress makes the bond between the sleeve and the filled concrete stronger. On the other hand, the shield lining segments have high precision in prefabrication [14], good connection effects [15], and good durability [16]. The lining concrete segment sleeve (LCSS) is developed according to the characteristics and theory of shield lining segments under the consideration of existing problems, while the prefabrication precision, durability, and joint connection strength of the segment are improved.

The axial compression performance of the strengthened columns is so significant that numerous studies have investigated this topic. Fakharifar [17] found that the bearing capacity and ductility of columns strengthened with steel sleeves could be increased by 20%. Wang [18] used a CFRP sleeve to strengthen a column, the bearing capacity of which was greatly improved. In [9], the peak load and displacement of the column strengthened by the PCSAM were increased by 28% and 20%, respectively. Seible [19] proposed a design method for strengthening columns with FRP sleeves, while Tang and Wu and Sun [20] proposed a simplified moment–curvature calculation model for the cross-section of the column strengthened by the PCSAM.

This paper attempts to investigate the strengthening effect of the IPCSAM and the axial compression performance of the strengthened columns. Firstly, the implementation strategy for the IPCSAM is introduced. Then, 6 specimens are prepared by taking account of influential factors, such as self-stress, the thickness of filled concrete, and the concrete strength of the LCSS, while the mechanical properties of the specimens are also studied under axial compression loading. Next, the extended parameter analyses are carried out via numerical simulations. Finally, based on the experimental results and extended parameter analyses, the calculation method for the bearing capacity is established for the components strengthened with the IPCSAM.
