Bond–Slip Performance between Steel Tube and Self-Compacting Fly Ash Concrete

Abstract

To reduce the amount of ordinary concrete and then reduce carbon dioxide emission, improving the engineering application range of self-compacting fly ash concrete (FASCC), this study explored the bond–slip traits between FASCC and a steel tube. Six samples were created, and bond–slip push-out tests were performed with varying concrete strength grades and steel tube internal setups. Digital image correlation (DIC) technology was applied to track the surface strain of four samples throughout the experiment. The results show that the outer surface of the steel tube stays mostly undistorted after the concrete is pushed out. Prior to reaching peak load, the load–slip curves of each specimen exhibit a primarily linear load–displacement relationship. Post-peak, the curves diverge into two distinct patterns, namely a sudden drop and a gradual decline. As the strength grade of the inner concrete increases, the interfacial bond between the steel tube and FASCC improves. Additionally, under the same conditions, the internal structure of the steel tube significantly enhances bonding strength. The FA40-Z specimen shows a maximum load that is 25.6% and 53.7% higher than the FA40-G and FA40-C specimens, respectively. The strain evolution patterns of steel tubes within FASCC and regular self-compacting concrete demonstrate similar characteristics. These observations provide valuable insights for the application of FASCC in engineering projects.

1. Introduction

Self-compacting concrete (SCC) stands out as a specific type of concrete recognized for its outstanding flowability and stability, allowing it to easily adapt to intricate construction settings [1,2]. Fly ash (FA) can be a beneficial substitute for some cement, acting as a cementitious element that improves SCC’s ease of use, longevity, strength, and density while reducing hydration heat and manufacturing expenses. Consequently, FASCC emerges as an eco-friendly, sustainable, and low-carbon construction material, showcasing exceptional efficiency in various aspects. The utilization of SCC in steel tubes enhances the structural components’ deformation resistance, bearing capacity, and ductility by combining the unique strengths of both materials while addressing their individual limitations [3,4,5,6]. Red mud contains abundant iron resources and potential pozzolanic activity minerals. The industrial utilization of red mud has a positive impact on low-carbon and sustainable development in the steel and cement industries [7]. The growing prominence of FASCC underscores these benefits. When applying SCC in steel tubes under different loading scenarios, it is crucial to meticulously assess the bond–slip behavior between the steel tube and SCC [8].

Extensive investigation into the impact of different factors on FASCC has been completed, including concrete strength [9,10,11], concrete curing age [12,13], the steel tube cross-section shape [14,15], the steel tube length–diameter ratio [10,16], the diameter–thickness ratio [17], and inner structural measures of the steel tube [18,19], on bond strength under various loading modes. These research studies have examined the distribution of bond stress and the relationship between bond stress and slip at the interface of conventional concrete-filled steel tubes (CFST). However, the process of vibrating concrete in intricate structures can pose challenges, potentially leading to delays and escalated costs. In order to tackle these challenges, self-compacting concrete (SCC) has been implemented in certain complex structures for its outstanding performance [20,21]. Studies indicate that the construction efficiency of self-compacting concrete-filled steel tube (SCCFST) columns surpasses that of ordinary concrete. SCC effectively resolves technical issues concerning the adhesion of concrete to the steel tube during construction [22,23]. Lu et al. [24] studied the bond performance of SCCFST columns that integrated steel fibers and proposed a method to predict bond strength. Huang et al. [25] performed a push-out test on a SCCFST square column, demonstrating that bond strength increases with the strength of the sandwich concrete and the thickness of the outer steel tube. Additionally, Ke et al. [26] examined the push-out behavior of SCCFST with rubber concrete. Qu et al. [27] carried out push-out tests on 17 low-expansion SCCFST columns, finding that bond strength was affected by factors such as the manufacturing method, the condition of the steel tube interface, concrete compressive strength, the diameter-to-thickness ratio, and the quantity of expansive agents. A 3D fractional elastoplastic constitutive model for concrete material [28] and a 3D non-orthogonal plastic damage model for concrete [29] were proposed. To study the bond performance between fiber-reinforced polymer bars and sea sand coral concrete (SSCC), 72 specimens of direct pull-out tests were designed, and relevant tests were carried out to explore the effects of fiber types, bar diameters, bond lengths, and SSCC strength grades [30].

Currently, there is a significant lack of experimental research exploring the bond–slip characteristics between steel tubes and FASCC. This study aims to fill this gap by performing push-out tests on FASCC and steel tubes, employing DIC technology. The tests are carried out across various concrete strengths and internal configurations of steel tubes, allowing for the examination of bond–slip curves, strain distribution patterns, and interface damage evolution at different stages. This holistic approach facilitates an investigation into the changing bonding behavior between FASCC and steel tubes, as well as an analysis of the failure modes of the specimens under push-out loads. These findings offer valuable insights for the advancement of FASCC.

2. Test Overview

2.1. Test Materials and Mix Design

Six CFST column specimens were arranged for push-out testing, each with different structural configurations, concrete types, and strength categories as the main variables.

Table 1. Test specimens’ design.

Specimens	Concrete Grades	Concrete	L/mm	d/mm	t/mm	Inner Structure
FA40-C	C40	FASCC	350	219	4	—
FA40-Z	C40	FASCC	350	219	4	Longitudinal rib
FA40-G	C40	FASCC	350	219	4	Stud
SCC40-C	C40	Ordinary SCC	350	219	4	—
FA60-C	C60	FASCC	350	219	4	—
FA60-Z	C60	FASCC	350	219	4	Longitudinal rib

Note: L is the specimen length, d is the steel tube outer diameter, and t is the steel tube thickness.

presents the key design specifications of these samples. All steel tubes and longitudinal ribs utilized were constructed from Q235 B grade material. The yield strength was 296 MPa. Its ultimate strength was 371 MPa and the elongation rate was 32.8%. The internal composition of the steel tube is illustrated in Figure 1. The ribs were welded with a steel tube with a 4 mm effective throat thickness.

Test specimens are illustrated in Figure 2. The longitudinal ribs had dimensions of 275 × 60 × 4 mm, while the short steel bars without heads measured φ8 × 60 mm. Ordinary SCC comprises coarse and fine aggregate, cement, water, and a water-reducing agent. The FASCC is developed using fly ash as a partial replacement for cement in a specific ratio. Below are the detailed specifications of the raw materials. The coarse aggregate consisted of continuously graded gravel with particle sizes ranging from 5 to 20 mm. The fine aggregate was natural river sand with a fineness modulus of 2.3. The cement was of ordinary Portland P.O 42.5 grade. The fly ash was grade II. The water reducer exhibited a water reduction efficiency of 30%. These specifications are delineated in

Table 2. Mix ratio of SCC (kg·m⁻³).

Test Specimen	Water Cement Ratio	Cement	Fly Ash	Coarse Aggregate	Fine Aggregate	Water	Water Reducing Admixture
SCC40	0.37	486.5	—	950	809.6	180	4.87
FA40	0.37	291.9	194.6	950	809.6	180	4.87
FA60	0.31	309.6	206.4	909	774	160	7.74

.

2.2. Test Method and Loading System

Bond–slip tests were carried out between the SCC and steel tubes with real-time measurement of displacement fields on one side of the specimen using DIC equipment which was manufactured by Xintuo Sanwei company in Shenzhen of China. A matte white coating was applied to one side of the samples and ink dots were used to generate speckles. The displacement gauge was mounted on the predetermined steel bar at the free end of the SCCFST columns to enable the precise measurement of displacement values. Furthermore, push-out tests on the SCCFST columns were conducted as illustrated in Figure 3.

The testing loading process consisted of two clear steps, namely preloading and formal loading. Initially, in the preload phase, a 10 kN load was applied and maintained for 1 min. It was crucial to verify data acquisition normality during this period. Following this, the load was gradually decreased to zero. Moving to the formal loading step, displacement control loading was used. The loading rate was set to 0.4 mm/min. The loading halted when the longitudinal displacement reached 40 mm or if no significant changes in load capacity were observed.

3. Experimental Results and Analysis

3.1. Failure Pattern

In the initial loading stage, no slippage is detected between the steel tube and the SCC. At a displacement ranging from 2.4 to 2.7 mm, a slight slippage begins at the loading end. With further displacement, a subtle tinkling noise can be perceived within the steel tube, followed by a louder bang. Upon reaching peak load, the concrete at the loading end separates from the inner wall, resulting in a sharp decrease in load and significant slippage. It is noteworthy that specimens FA40-Z and FA60-Z, which feature longitudinal ribs, exhibit comparatively subdued noises.

The test results indicate that the outer surface of the steel tube has maintained its integrity without any significant deformation. The concrete inside also remains undamaged, with only minor debris falling out at the loading end. Furthermore, there are similarities observed in the bond failure modes among different specimens at the loading end.

3.2. Load–Slip Curves

The load–slip curves from the tests are displayed in Figure 4, where P represents the load and S represents the displacement at the loading end.

Figure 4 illustrates the following:

(1)

The P–S curves of the tests display clear peak load points and exhibit similar patterns until reaching the peak load. The relationship between load and displacement remains mainly linear. Upon reaching the peak load, the specimens experience a sudden drop, with the exception of FA40-Z and FA60-Z. It is important to note the significant slip at the loading end. The load–slip curves of FA40-C and SCC40-C contrast noticeably with those of FA40-Z and FA40-G. Once FA40-C and SCC40-C reach the peak load, there is a sharp decrease in load, followed by a rapid decline. This decrease is due to the evident reduction in load after the chemical bonding force between the FASCC and steel tube is disrupted. Consequently, the mechanical bite force and friction force become active despite their relatively minor magnitudes. Due to the restraint effect of the steel tube on the inner concrete, the FA40-C, FA60-C, and SCC40-C specimens without structural measures have similar properties.

(2)

Specimens featuring longitudinal rib structures show enhanced bonding performance. Samples with longitudinal ribs offer clear benefits, displaying substantially higher maximum loads in comparison to other specimens. The FA40-Z specimen shows a maximum load that is 25.6% and 53.7% higher than the FA40-G and FA40-C specimens, respectively. Moreover, there is an increase in strength retention rate during the later loading stages.

(3)

The P–S curves display unique features. Initially, the chemical cementation force acts on the contact surface between the FASCC and steel tube, preventing any slippage. With increasing displacement, the chemical bond force decreases until the initial slip occurs at the loading end, marking the transition to the second stage. At the slip position, the chemical bond force no longer exists, while it persists at the non-slip position. Meanwhile, the mechanical resistance force between the FASCC and the steel tube remains in effect. During this stage, the load–slip curves show a linear increase. As the free end slips, the curves enter the third stage. At this juncture, the interface chemical bond force vanishes, leaving the bond force solely composed of the mechanical resistance force and interface friction resistance.

3.3. Analysis of DIC Strain Nephrogram

In order to explore the progression pattern of strain in the steel tubes during the push-out tests, the DIC system is utilized. Upon reaching maximum strain, strain cloud charts for individual samples are recorded as shown in Figure 5. In these charts, positive values indicate tensile strain, whereas negative values represent compressive strain.

Based on the strain value acquired from the DIC testing system, Figure 6 demonstrates the strain profiles of the external surface of the steel tube. The x-axis shows different load levels, while the y-axis presents the strain values at each measuring point for different load levels. Specifically, strain measuring point 1 is positioned 25 mm away from the open end, strain measuring point 3 is placed 25 mm from the loaded end, and strain measuring point 2 is at the center of the specimen. Moreover, P_u represents the maximum load of the specimens.

From Figure 5 and Figure 6, the following can be observed:

(1)

The DIC strain cloud diagram offers a straightforward depiction of the variations in surface strain on steel tubes throughout various loading phases. Both the FA SCCFST and the traditional SCCFST show comparable patterns of strain evolution for the steel tube. Initially, the longitudinal strain values at different measuring points on the steel tube are relatively minimal, and this strain progressively increases in a linear fashion with the escalation of the load level.

(2)

With the increase in load, the longitudinal strain at all measurement points gradually increases. It is noteworthy that the rate of concrete stress growth at the free end is considerably higher than that at the loading end. This suggests that the interface at the loading end experiences slippage before the free end. Throughout the loading process, the strain distribution of the steel tube stays consistent, indicating that the bond between the steel tube and SCC remains unbroken, albeit somewhat compromised.

4. Analysis of Bond Strength and Influence Factors

4.1. Calculation of Bond Strength

The mechanical behavior of concrete is vital in determining the bond performance between FASCC and steel tubes. Due to differences in mechanical properties between SCC and traditional concrete, along with the substantial influence of steel tube internal structures on bond strength, a thorough examination of the FASCC strength grade and steel tube inner structures is conducted. The bonding performance between FASCC and steel tubes is intricately linked to bonding strength. To simplify the analysis, tests presume a consistent distribution of bond strength along the length of the steel tube and FASCC. The bond strength under peak load is denoted as τ_u, while P_u represents the peak load. Additionally, A denotes the effective contact area, considering the surface area of the ribs in the FA40-Z and FA60-Z models.

$${\tau }_{\mathrm{u}}={\displaystyle \frac{P_{\mathrm{u}}}{A}}$$

(1)

The peak load and the average bond strength of the specimens are shown in Figure 7.

From Figure 7, the following is evident:

(1)

The adhesive strength of the FASCC increases while the internal structures remain unchanged, leading to a notable enhancement in the adhesive strength of the FA SCCFST columns. This improvement is mainly due to the reduced shrinkage of FASCC with higher strength grades, consequently reinforcing the mechanical grip and chemical adhesion between FASCC and steel tubes.

(2)

While the P–S curves of specimens FA40-C and FA60-C show similarities, there is a significant difference in bond strength of 0.59 MPa. On the other hand, the bond strength variance between FA40-Z and FA60-Z specimens is 0.44 MPa, indicating a noteworthy increase of 21.7%. However, when compared to the non-structural specimens, the bond strength decreases by 5.4% and 10.5%, respectively, due to the inclusion of the longitudinal rib area in the calculation. This illustrates that the longitudinal rib structure enhances the overall stability of the specimen, making buckling failure less likely under heavy loads. The internal structures effectively minimize the relative sliding between FASCC and the steel tube.

(3)

The contact area between FASCC and the steel tube expands due to the inner structures, thereby enhancing the mechanical bite force and friction resistance. These enhancements are closely linked with the mechanical properties and surface friction resistance of FASCC.

4.2. Gap between the Test and Engineering Application

This study compares the bond strength values of all specimens with the standards of various countries. The bond strength of all specimens ranges between 2.03 MPa and 2.73 MPa, exceeding the recommended bond strength values of 0.225 MPa in the Chinese standard [31], 0.4 MPa in the American standard [32], British standard [33], and Australian standard [34], as well as 0.55 MPa in the European standard [35]. The bond strength value of the tests is much higher than the value applied in engineering.

5. Conclusions

(1)

The load–slip curves obtained from tests on the steel tube and FASCC of varying strength grades display consistent patterns. Prior to reaching the peak load, there is a linear relationship between load and displacement. Post-peak load, the curves diverge into two categories, namely sudden drop and gradual drop.

(2)

The strain behavior of FA SCCFST and conventional SCCFST follows similar trends. The DIC strain cloud image effectively captures variations in surface strain of the steel tube at different loading phases. Strain on the steel tube at the measuring point incrementally rises with increasing load. The strain growth rate at the free end outpaces that at the loaded end.

(3)

The bonding performance at the interface of FA SCCFST improves as the strength of the inner concrete increases. In comparison to FA40-G and FA40-C, the peak load of FA40-Z rose by 26.4% and 55.7%, respectively, while the bond strength decreased by 29.5% and 5.4%, respectively. The longitudinal ribs significantly enhance the overall stability and diminish the risk of buckling failure, demonstrating that internal structures effectively prevent sliding between the steel tube and FASCC. These research findings can serve as guidance for designing engineering structures in this field.