1. Introduction
The scale of highway bridges in China is huge [
1], and many old bridges have a large number of structural and utilization problems, such as the corrosion of steel bars, serious aging of concrete, concrete spalling, excessive crack width, insufficient bearing capacity, and high prestressing losses [
2,
3,
4,
5,
6]. More than 30% of bridges that are currently in use require strengthening and repair, not just in our nation but also in developed nations like the US and Canada [
7]. When it comes to renovating both new and old bridges, ultra-high-performance concrete (UHPC) is a novel kind of strengthening material that offers several benefits [
8,
9,
10,
11,
12,
13]. It is constructed based on the principle of maximum packing density [
14] and exhibits excellent compressive, tensile, shear, and durability properties [
15,
16,
17,
18,
19,
20,
21]. In order to enhance the flexural performance of reinforced concrete (RC) beams or slabs, UHPC is frequently placed as a thin layer in the tensile zone of the beams or slabs due to its exceptional mechanical qualities and high durability [
22]. Numerous academics have looked into the flexural properties of UHPC-strengthened RC beams [
23,
24,
25], as well as the interfacial shear properties between UHPC and plain concrete [
26,
27,
28]. The shear properties between UHPC and RC determine the flexural properties of UHPC-strengthened beams.
N.K. Banjara et al. [
29] comparatively analyzed the effect of the reinforcement rate of UHPC strips and whether the UHPC ends are strengthened or not on damaged RC beams, and the test showed that utilizing interfacial adhesive gluing of UHPC strips leads to end interfacial bond damage, and the use of U-type CFRP ring wrapping of the UHPC ends effectively prevents the stripping damage of the strengthened beams and improves the ultimate bearing capacity by 30%. Al-Osta et al. [
30] examined the flexural behavior of RC beams strengthened with glued or cast-in-place UHPC strips in single-, double-, and triple-sided configurations. They discovered that the flexural behavior of the strengthened beams was not significantly impacted by the different interfacial bonding techniques (two types of epoxy adhesive versus sandblasting technique). P. Mário [
31] conducted 16 UHPC (50 mm thick)-strengthened RC beam flexural (shear) tests, which showed that when the interface was treated with air hammer chiseling, the overall working performance between the UHPC reinforcement layer and the RC beams was better, verifying the potential and effectiveness of UHPC for strengthening RC structures. Liu et al. [
32] conducted an experimental study of the shear and tensile behavior of UHPC-RC interface samples and used the interface model for finite element simulations of UHPC-strengthened concrete T-beams, which were more accurate using the CF interface model than using the perfect bond (PB) model. In subsequent simulations, it was found that the stiffness and shear capacity of the strengthened T-beams were greatly improved when the thickness of the UHPC layer was increased to 80 mm and the interfacial anchorage spacing was less than 300 mm. Liu et al. [
33] created ten concrete T-beams with varying steel bar configurations, UHPC layer thicknesses, and anchors at the repair interfaces in order to study the shear performance of UHPC-strengthened cast-in-place concrete T-beams. They suggested using U-shaped jacket configurations when a significant increase in beam stiffness was needed, and discovered that a 50 mm transverse layer and a 25 mm U-shaped jacket produced more ductile failure modes. Sun et al. [
34] investigated the interfacial shear performance of UHPC-strengthened RC structures using post-installed rebar connections. They designed three UHPC-based reinforcement forms: two-sided, U-form, and casing. They carried out 34 push-out tests. In comparison to the two-sided and U-form-strengthened specimens, they discovered that the specimens with casing reinforcement exhibited a notable clamping effect. A significant amount of frictional resistance is still produced by the clamping effect even in cases where the post-installed reinforcing bars’ embedment depth is insufficient. Gao et al. [
35] investigated the effect of different post-installed reinforcing bar embedment lengths on the interfacial shear strength and found that a length-to-diameter ratio greater than or equal to 2.5 is sufficient to give full play to the anchorage capacity of strengthened UHPC. Zhu Y [
36] and others used the method of “unit node localization tracking”, i.e., setting the “localization unit” with material properties much lower than those of the model components, to adjust the activation position of the UHPC reinforcement layer so as to make it coincide with the contact of the unloaded RC interface, and used the damage plasticity model of ABAQUS to simulate the intrinsic model.
It has been shown that the performance of UHPC-strengthened beams depends on whether good interfacial shear properties can be ensured between UHPC and plain concrete and that the post-installed rebar bonding technique is a better solution to the UHPC-RC bonding problem. However, few researchers have investigated the structural performance of UHPC-strengthened T-beams with the post-installed rebar bonding technique. Moreover, there have been many studies related to UHPC-strengthened beams, while research on UHPC-strengthened T-beams is still very scarce. In practical engineering, reinforced concrete T-beams are widely used in bridge structures due to their resistance to high bending and shear stresses [
37], and UHPC-strengthened T-beams may exhibit higher delamination possibilities [
33], so the problems related to the interfacial adhesion and flexural resistance of UHPC-strengthened T-beams require in-depth study.
In summary, 15 sets of push-out simulation tests were carried out using the control variable method with the horizontal spacing of shear-resistant steel bars, yield strength of shear-resistant steel bars, compressive strength of matrix concrete, and diameter of shear-resistant steel bars as parameters using ABAQUS software [
38]. The software then analyzed the impact of each rebar planting parameter on the specimen interface’s shear performance in order to shed light on the shear performance of the UHPC-RC reinforcement interface. The push-out simulation tests showed how to design the horizontal spacing of shear-resistant steel bars and the diameter of shear-resistant steel bars in the UHPC layer for the numerical simulation of damaged T-beams that have been strengthened with UHPC using this bonding method. A new theoretical formula for the shear strength of the UHPC-RC interface was derived based on the push-out test simulation results. Then, ABAQUS software was used to conduct finite element analysis of bending tests on T-beam specimens with different layout forms of the post-installed reinforcing bars, longitudinal spacing of the post-installed reinforcing bars, UHPC-strengthened position, and thickness of the reinforcement layer. The best base layer reinforcement configuration and placement were suggested in order to fully utilize the benefits of UHPC material and guarantee the overall longevity and safety of the strengthened structure.
6. Conclusions
This paper uses ABAQUS to investigate the flexural performance of damaged T-beams strengthened with UHPC using the post-installed rebar bonding technique. Using the post-installed rebar bonding technique, finite element simulations of push-out tests were first used to study the shear performance of the interface between UHPC and RC. The following primary conclusions were reached after the flexural performance of UHPC-strengthened damaged concrete T-beams was numerically simulated using the push-out test results from the finite element simulation:
- (1)
The finite element simulation methods used in this paper are all very accurate. The load–deflection curves, load–midspan displacement curves, peak loads, and failure modes that were found from the simulations match the test results very well.
- (2)
Through the push-out test finite element simulation, this paper proves that when the compressive strength of matrix concrete increases, the yield strength of shear-resistant steel bars continues to increase, and the anchorage capacity of shear-resistant steel bars is gradually enhanced. The horizontal spacing of shear-resistant steel bars is too small and easily causes local damage to the concrete substrate; the horizontal spacing of shear-resistant steel bars is too large and will cut the adhesive effect of the steel bar; the recommended horizontal spacing of shear-resistant steel bars of 8d~12d is more reasonable. If the diameter of shear-resistant steel bars is too small or too large, it will reduce the interface shear strength and ductility of UHPC-RC. It is recommended that the diameter of shear-resistant steel bars be 10~14 mm.
- (3)
Rebar shear friction and pinning force are the main sources of shear-bearing capacity at the UHPC-RC reinforcement interface. In this paper, the reinforcement shear friction force is taken into account as an influencing factor, and the shear capacity calculation formula of the old and new concrete interface in the Technical Specification for Assembled Concrete Structures is optimized. The proposed shear capacity calculation formula of the UHPC-RC reinforcement interface is more suitable for the actual situation.
- (4)
It was discovered that the damaged T-beam can achieve a more satisfactory reinforcement effect when the layout form of the post-installed reinforcing bars is a square-shaped form and the longitudinal spacing of the post-installed reinforcing bars is 300 mm. The parametric analysis’s findings demonstrated that as the longitudinal spacing of the post-installed reinforcing bars increased beyond 300 mm, the strengthened beams’ flexural performance dramatically declined. The cracking load and peak load of the strengthened beams were greatly increased by the increase in the thickness of the reinforcement layer. From the point of view of safety and economy, the UHPC-strengthened position is recommended to be the bottom reinforcement. However, the reinforcement effect of the three different reinforcement locations with the same amount of UHPC material is a question worth studying.
It should be pointed out that, although several key parameters concerning the shear strength of the UHPC-RC interface have been investigated in this paper, in order to more comprehensively explore the influencing factors affecting UHPC-strengthened T-beams, control variable studies should be carried out on these key parameters at a later stage in view of the differences in the steel bar adhesive, steel bar placement, and casting direction adopted in the actual reinforcement project. And it is not clear why the results of strengthening T-shaped beams are different from those of strengthening rectangular beams, and control variable studies should also be carried out on the basis of this problem at a later stage. Based on the U-shaped reinforcement structure using UHPC, a control variable study on the relationship between bottom thickness and side thickness can be carried out to specify a more reasonable configuration of the U-shaped reinforcement structure.