*2.1. Materials*

In the study, the RAC with full replacement ratio means that all of the natural coarse aggregates in the concrete have been replaced by recycled coarse aggregates. The specimens were obtained from PC32.5R Portland cement, recycled coarse aggregate, natural fine aggregate, natural fine sand, and urban tap water. The particle size of the recycled coarse aggregate was ranged 5–31.5 mm. The physical properties of the recycled coarse aggregate [28,29] were measured, and the results are shown in Table 1. The obtained results satisfied the requirements of "Recycled Coarse Aggregate for Concrete" GB/T 25177-2010.



Double 10 channel steel plates and double 6mm thick steel plates were bonded with epoxy resin in the experiment. The longitudinally stressed steel provided B16 reinforcement, the stirrup provided A6 reinforcement, and their mechanical performances are shown in Table 2.


**Table 2.** Mechanical performances of steel and stirrup.

Note: *E*1 is the elastic modulus of the elastic phase; *E*2 is the slope of the hardening section of the steel; <sup>ε</sup>*y* is the yield strain of the steel corresponding to *fy*; ε*s* is the strain of the steel; ε*u* is the peak strain of the steel corresponding to *fu*; *fy* is the yield strength of the steel; *fu* is the ultimate strength of the steel.

#### *2.2. Design of Specimens*

Nine push-out specimens were designed in the test to study the bond behavior and the bond slip between section steel and RAC. The e ffects of the concrete strength, the embedded length, the cover thickness, and the lateral stirrup ratio on the bond behavior and the bond slip between section steel and RAC were investigated in detail. The parameters of push-out specimens are shown in Table 3. All strain gauges were arranged at a certain interval on the flange steel plate and the web channel steel. This arrangemen<sup>t</sup> did not a ffect the bonding e ffect in the interface between section steel and RAC, and ensured the safety and the accuracy of the strain gauge. The section design of specimens and section steel are shown in Figures 1 and 2, respectively.

**Table 3.** Parameters of push-out specimens.


**Figure 1.** Section design of specimens.

**Figure 2.** Section design of section steel.

Pre-absorption treatment was conducted on recycled coarse aggregate before preparing RAC [30,31], and the mix design of RAC is shown in Table 4. The reason is that the recycled coarse aggregate is largely porous, which reduces the actual water/cement ratio in the cement slurry and the concrete mix ratio at the same concrete strength.

**Table 4.** Mix design of recycled aggregate concrete (RAC).


Pre-absorption treatment was conducted on recycled coarse aggregate before preparing RAC [30,31]. The reason is that the recycled coarse aggregate with large porosity absorbs a lot of water, and decreases the actual water-cement ratio in the cement slurry. The mix design of RAC is shown in Table 4.

The specimens were made in the seismic engineering laboratory of Xi'an University of Architecture and Technology. The 150 mm × 150 mm × 150 mm cube test blocks were also produced from the same RAC in the test. After the pouring was completed, the push-out specimens were cured in indoor standard conditions (with felt-covered watering and curing). The compressive strength (fcu) of specimens is shown in Table 3.

#### *2.3. Test Method*

The strain gauges were applied from dense to sparse distribution along the loading end to the free end, and were bonded to the steel plate by epoxy resin to measure the strain at the flange and the web. Four electronic slip sensors were uniformly arranged on one side of the flange and the web, which were developed by the research team [32], and the Slip-strain (S–ε) relationship of each electronic slip sensor was measured in advance.

The push-out test was carried out on the 2000t compression testing machine in the State Key Laboratory for civil engineering at Xi'an University of Architecture and Technology. Figure 3a,b show photos of the test setup and push-out specimen. The upper end of the specimen was fixed and the lower end was free. A mild steel plate with an "H" hole connected the bottom of the specimen with the loading platform. The topside of the section steel was attached to the compression testing machine with a complete steel plate. A foam pad was laid between the steel support and RAC to ensure flatness, and the loading rate was 0.3 mm/min. Slip occurred initially in the lower part and gradually in the upper part. Therefore, from the perspective of the section steel, the loading end of the specimen was defined at the lower end and the free end was in the upper portion. Double displacement meters were set at the loading end and the free end, respectively, which are shown in Figure 3c.

**Figure 3.** Photos and sketch of the test: (**a**) photo of test setup; (**b**) photo of push-out specimen; (**c**) sketch of push-out specimen.

#### **3. Results and Analysis**

#### *3.1. Failure Procedure and Mode*

The failure modes of the specimens can be divided into two types: splitting failure mode and bursting failure mode. The failure procedure was roughly as follows: at the initial stage of the specimen loading, there was no obvious change on the surface of each specimen. When the specimen was loaded to 40%–75% of the ultimate load, the initial cracks appeared on the surface of the specimen. With the increase of loading, the initial cracks were mostly concentrated near the loading end at the web, and a small part appeared in the middle. In this time, the initial cracks propagated rapidly, the initial cracks at the loading end extended toward the free end, and the initial cracks in the middle expanded toward both sides as the load increased. When loading to 80%–90% of the ultimate load, the sliding increment of the loading end was obvious and the load increased gently. The initial crack gradually developed into a through crack, and the maximum crack width reached 2–3 mm. As the load continued increasing to the ultimate load, the load sharply dropped to 50%–70% of it, therefore the specimen was considered to be broken by the through crack. If the load continued to increase, the changes were minimal and stabilized with the increasing drifts. It was considered that the load was a residual load. In the process, multiple cracks were generated, in which the original cracks developed secondary cracks, and the damage of RAC increased. The loading ended when the section steel was pushed out 4–6 mm.

The failure mode for SRRC-1, SRRC-4, and SRRC-8 was bursting failure, and the rest specimens showed splitting failure. Splitting failure is the typical failure mode, and here the initial cracks appeared at the loading end of the web sides. With increased load, the cracks extended to the free end and some fine cracks appeared on the specimen gradually. When the load reached the peak load, the initial cracks extended to the upper part of the specimen. Then the load fell rapidly and tended to be gentle gradually. There was a penetrating crack on the flange side and at web sites at this stage, as shown in Figure 4 (taking SRRC-5 as an example).

**Figure 4.** Splitting failure of SRRC-5: (**a**) web; (**b**) flange.

With bursting failure, the initial cracks occurred in the middle of the flange or the web. As the load increased, the initial cracks gradually expanded toward the loading and free ends, and some new fine cracks occurred. When the load reached about 80%–90% of the ultimate load, the initial cracks continued expanding and widening, and there were many obvious secondary cracks. As shown in Figure 5 (taking SRRC-1 as an example), through cracks were present on both the flange and the web sides after failure. This is one of the main features of bursting failure that makes it different from the splitting failure.

**Figure 5.** Bursting failure of SRRC-1: (**a**) web; (**b**) flange.

It can be seen from SRRC-4 and SRRC-8 specimens that a high lateral stirrup ratio and high cover thickness make the specimen more prone to bursting failure. The reason for this phenomenon is that a high lateral stirrup ratio and high cover thickness are effective in preventing the deformation of concrete and further improving the cracking load of cracks.

#### *3.2. Characteristic of P–S Curves*

The loading end slip curve (P-S curve) can be simplified to the model shown in Figure 6. Here, The load is divided into two categories, each of them showing basically the same changes, which are divided into three parts: rising, sag, and gentle loads. Type (I) is characterized by a large initial load (65%–75% of the peak load), with a residual load that is slightly lower than the initial load. Type (II) is characterized by a small initial load (40%–65% of the peak load), with a residual load that is slightly higher than the initial load. The P–S curves of the specimens are shown in Figure 7.

The following definitions of the characteristic points in Figure 6 are given:


**Figure 6.** P–S curve models of the loading end: (**a**) Type (I); (**b**) Type (II).

**Figure 7.** P–S curves of each specimen: (**a**) SRRC-1; (**b**) SRRC-2; (**c**) SRRC-3; (**d**) SRRC-4; (**e**) SRRC-5; (**f**) SRRC-6; (**g**) SRRC-7; (**h**) SRRC-8; (**i**) SRRC-9.

In this paper, the P–S curves of the loading end are divided into four stages: nonslip, slip-crack, descending, and residual.


#### *3.3. Influence Analysis of Various Factors*

The bond strength between the section steel and RAC can be considered to be evenly distributed along the length of the section steel under the push-out test conditions. The average bond strength can be expressed by Equation (1).

$$
\overline{\pi} = \frac{P}{L\_{\text{t}} \cdot \text{C}} \tag{1}
$$

where τ is the average bond stress in MPa; *P* is the load in N; *Le* is the embedded length of section steel in mm; and *C* is the perimeter of section steel in mm.
