Experimental Investigation of the Bond Performance at the Interface between Engineered Geopolymer Composites and Existing Concrete

Abstract

Engineered geopolymer composite (EGC) exhibits ultra-high toughness, excellent crack control capability, and superior durability, making it highly promising for applications in bridge connecting slabs, wet joints of prefabricated components, and concrete structure reinforcement. However, the bond performance and failure mechanisms at the interface between EGC and existing concrete remain unclear. To elucidate the bond performance of EGC to existing concrete, direct shear tests were conducted on 15 sets of EGC–existing concrete bond specimens. This study explored the effects of existing concrete strength, interface roughness, and EGC strength on the bond performance and mechanisms. Additionally, a direct shear bond mechanical model was established to predict the interface bond strength. The results indicate that, with comparable compressive strength, the preparation of EGC can reduce the total carbon emissions by up to 127% compared to ECC. The failure mode of EGC-existing concrete bond specimens was mainly adhesive failure (except for specimen C30-III-G95), which can be categorized into serrated interfacial failure and alternating crack paths. The change in interface roughness was the primary factor leading to the transition between failure paths. The changes in interface roughness and EGC strength significantly influenced the bond performance. Under their combined effect, the interface bond strength of specimen C50-III-G95 increased by 345% compared to C50-I-G45. In contrast, the improvement in existing concrete strength had a relatively smaller effect on the increase in interface bond strength. Based on the experimental results and the bonding mechanism under direct shear stress, a direct shear bond mechanical model correlating existing concrete strength, interface roughness, and EGC strength was established. The model predictions showed good consistency with the experimental results. This study provides theoretical support and experimental data for the engineering application of EGC.

1. Introduction

Reinforced concrete structures are widely used in hydraulic and civil engineering. However, during their service life, the structures may experience creep and shrinkage [1,2,3], erosion from substances such as sulfates and chlorides [4,5,6], and excessive loads and fatigue [7,8], which can induce concrete cracking, rebar corrosion, and spalling of the concrete cover, ultimately leading to a decline in the structural safety and reliability. Nevertheless, demolishing buildings with safety hazards to reconstruct them must consider local economic development and environmental pollution issues, balancing the contradictions among structural safety, economic benefits, and environmental protection [9,10]. Based on this, reinforcing existing damaged structures, while ensuring structural safety and minimizing construction volume, is an effective way to promote the harmonious development of cities and nature.

Engineered cementitious composites (ECC), designed through micro-mechanics, exhibit significant tensile strain hardening and multiple cracking behaviors after the incorporation of a suitable amount of high-performance fibers into the cement-based mortar [11,12,13,14,15,16]. Additionally, the tensile cracks in ECC usually remain below 100 μm, effectively preventing the intrusion of harmful substances into the concrete [17]. Using ECC, a high-performance concrete, to reinforce aging structures not only maximally extends the lifespan of buildings but also significantly reduces the likelihood of needing secondary reinforcement [18,19,20], aligning well with the sustainable development goals of the construction industry. However, the production of ECC typically requires 2 to 3 times more cement than traditional concrete [21]. Given that cement manufacturing is a process with high energy consumption and high carbon dioxide emissions [22], using non-clinker cements (such as geopolymers) to produce EGC represents a promising alternative [23,24,25,26,27,28]. Studies show that compared to the production of ECC, the production of fly ash-based EGC reduces energy consumption by about 60% and carbon dioxide emissions by about 80% [29,30]. Furthermore, EGC also exhibits good chemical resistance [31,32], rapid hardening, and high early strength [33,34], making it highly potential in the reinforcement field.

The interface region between EGC and concrete is often considered a weak point, determining the failure mode of the entire component. This is due to discontinuities in material, high porosity accumulation, and concentration of micro-cracks at the interface between new and old concrete [35]. Therefore, improving interfacial bond performance can enhance the overall performance of the component. The strength and elastic modulus of the reinforcement material, the strength of the existing concrete, and the roughness of the interface are significant factors affecting interfacial bond performance [36,37,38]. Generally, increasing the strength of the reinforcement concrete can enhance the interfacial bond strength and shift the failure from the interface to the base material [39,40]. Research indicates that UHPC, as a reinforcement material, can reduce the amount of free water in the transition zone at the interface, decrease the porosity formed by air bubbles accumulating during vibration, and ultimately manifest as an increase in interfacial bond strength [41]. According to Kumar et al. [42], the elastic modulus of EGC is directly related to the interfacial bond strength between EGC and existing concrete. Moreover, EGC with a higher cracking strength exhibits stronger interfacial bonding with the existing concrete. However, when there is a significant difference in the elastic modulus between the reinforcement layer and the existing concrete layer, it can lead to localized stress concentration at the edge of the interface, thereby promoting the development of shear stress and early failure of the bonded interface [43]. Additionally, Semendary et al. [44] through direct tensile tests studying the effect of UHPC reinforcement on NSC and HSC at different curing ages, found that the interfacial bond strength growth rate of UHPC-HSC was higher than that of UHPC-NSC in the early stages (3–7 days) and exhibited higher interfacial bond strength at 28 days (UHPC-NSC at 3.15 MPa and UHPC-HSC at 3.45 MPa). Moreover, Courard et al. [38] state that stronger existing concrete shows superior interfacial bond performance when aggressive interface treatment methods (such as pneumatic bush hammering) are used.

Changes in interface roughness can significantly affect the interfacial bond strength, mainly due to increased interfacial shear friction and mechanical interlocking between different concrete layers [45]. Wang et al. [46] found that, when using EGC to reinforce existing concrete, increasing the ductility of EGC does not result in higher interfacial bond strength, whereas improving the interfacial roughness effectively enhances the interfacial bonding performance. Under shear conditions, sufficient interface roughness can change the location of failure in a component (from the bond interface to either the reinforcement layer or the existing concrete layer) [43]. However, the study by Costa et al. [47] suggests that the beneficial effects of increased interface roughness on shear and tensile bond strength have boundary effects, and beyond a certain level of interface roughness, the interfacial bond strength will stabilize. Using sand blasting to treat the bonding interface is an effective method to improve interface roughness. The treated bond interface surface texture is neither too rough nor too smooth, and it allows the aggregate to be exposed, avoiding the mechanical damage to existing concrete caused by chiseling [39]. Additionally, interface agents and the moisture content of the interface can also affect the bond performance to some extent [41,48]. Different interface agents have varying effects on the bonding interface. Jiang et al. [49] found that using cement paste interfacial agent and polymer modified interfacial agent on the bonding interface, through direct shear tests, the cement paste interfacial agent can enhance the chemical bonding between ECC and existing concrete, while the polymer modified interfacial agent has no effect. Some researchers have pointed out that applying interface agents might lead to a loss of interface roughness and a weakening of the fiber bridging effect [50]. Moreover, the mechanism of how the moisture content of the bonding interface affects the bonding strength when pouring the reinforcement layer is still unclear. One view suggests that the moisture content of the interface during the pouring of the reinforcement layer depends on the materials chosen for the substrate and the overlay [43]. Farzad et al. [51] indicated that when ordinary concrete is used as the overlay material, wetting the bonding interface can increase the bonding strength, while keeping the bonding interface dry is more beneficial when ultra-high-performance concrete is used as the overlay material. Summarizing the above literature, reports on the interfacial bonding performance of EGC-existing concrete are still relatively scarce. Additionally, variations in the strength of existing concrete, which can result in different reinforcement effects of EGC, have not been considered (as the strength of existing concrete cannot be consistent in actual engineering). EGC is an ideal reinforcement material that can enhance the durability of reinforced structures, and the fiber bridging effect produced by the organic fibers can improve the shear performance of the bonding interface. Therefore, researching the interfacial bonding performance of EGC- existing concrete is of significant importance.

This study prepares EGC using fly ash and granulated blast furnace slag as binder materials and controls the quality ratio of fly ash to slag (8:2, 7:3, 6:4, across three groups) to adjust the strength of EGC. Pneumatic bush hammers are used to roughen the interface of existing concrete. This study explores the effects of existing concrete strength, interface roughness, and EGC strength on the bond performance of EGC–existing concrete, analyzes the failure modes and the development patterns of interfacial bond strength, and establishes a direct shear bond mechanics model to predict interfacial bond strength, providing a reference for practical engineering applications. By elucidating these aspects, this study contributes to understanding the interfacial bonding mechanical behavior of EGC–existing concrete, enabling more rational application of EGC as a reinforcement material. However, the material curing conditions, interface treatment methods, and environmental exposure conditions considered in this study are relatively limited and require further in-depth research.

2. Experimental Program

2.1. Raw Materials and Mix Proportion

The materials used to prepare EGC primarily consist of binder materials, aggregates, alkaline activators, water, ultra-high-molecular-weight polyethylene fibers (PE fibers), and retarders. The binder materials used include Class F fly ash (FA) and S105-grade granulated blast furnace slag (GGBFS), both provided by Longze Water Purification Materials Co., Ltd. in Gongyi City, China. The chemical composition and loss on ignition are as shown in

Table 1. Chemical composition and loss on ignition of binder materials (wt%).

Binder Materials	CaO	SiO2	Al2O3	SO3	Fe2O3	MgO	TiO2	Others	Loss on Ignition
GGBFS	34.00	34.50	17.70	1.64	1.03	6.01	0	5.12	0.84
FA	4.01	53.97	31.15	2.20	4.16	1.01	1.13	2.37	4.60

. The aggregate is quartz powder (QP) sourced locally from Guangzhou, China. The particle size distribution of the binder materials and aggregates is shown in Figure 1. The alkaline activator is prepared by mixing a preconfigured solution of 10 mol/L sodium hydroxide and sodium silicate solution (modulus 2.25, mass fraction of m(SiO₂):m(Na₂O):m(H₂O) = 29.99 wt%:13.75 wt%:56.26 wt%) at a ratio of 1:2, which is added to a planetary mixer along with additional water during the mixing process to prepare EGC. The PE fibers used are produced by Beijing Quantum Terra New Materials Technology Co., Ltd, Beijing, China. The physical and mechanical parameters of these fibers are shown in

Table 2. Physical and mechanical properties of PE Fiber.

Length (mm)	Diameter (mm)	Tensile Strength (MPa)	Elastic Modulus (GPa)	Density (g/cm3)	Elongation (%)
18	24	2500	120	0.97	3.7

. Additionally, to ensure that the freshly mixed EGC remains in a pliable state for a sufficient duration to meet pouring requirements, BaCl₂ is used as a retarder.

The materials used to prepare the existing concrete include P.II 42.5R ordinary Portland cement, fine aggregates, coarse aggregates, super-plasticizer (SP), and water. The ordinary Portland cement is sourced from Yuebao Cement Co., Ltd. in Guangzhou City, China. The fine aggregate used is river sand with a maximum particle size of 2.36 mm, and the coarse aggregate used is granite crushed stone with a particle size range of 5–10 mm (both sourced locally in Guangzhou, China). The SP is produced by Jiangmen Qiangjian Building Materials Company, Jiangmen, China and mainly consists of a sodium naphthalene sulfonate formaldehyde condensate. The mixing water used in this experiment comes from laboratory tap water.

2.2. Specimens and Preparation

According to ASTM-C469/C469M-22 [52] and JSCE [53] requirements, cylindrical axial compression specimens and dumbbell-shaped axial tensile specimens of EGC were prepared. A total of three types of EGC were designed, and three specimens of each type were prepared to test the compressive and tensile properties of EGC. The specific dimensions of the specimens are shown in Figure 2a,b.

The applied external load may be eccentric, causing bending moments at the bond interface, which can alter the magnitude and direction of the primary shear stresses. Therefore, this study uses cubic double-sided bonded specimens for testing. A total of 45 EGC–concrete bond specimens were prepared, with their specific designs shown in Figure 2c The mix proportions for EGC and existing concrete are provided in

Table 3. EGC mix design (kg/m³).

Groups	FA	GGBFS	QP	Activator	Water	BaCl2	PE Fiber
G45	956.4	239.1	239.1	478.2	71.7	12.0	19.4
G70	849.7	364.1	242.8	485.5	72.8	12.1	19.4
G95	736.3	490.9	245.4	490.9	73.6	12.3	19.4

Note: The “G” in “G45” stands for “Geopolymer”, and “45” indicates that the EGC strength grade is 45 MPa. Other groups have the same notation.

and

Table 4. Existing concrete mix design (kg/m³).

Groups	Cement	Coarse Aggregate	Fine Aggregate	Water	SP
C30	316	1126	750	208	-
C50	444	1112	622	222	-
C70	588	1028	578	164	3

Note: The “C” in “C30” stands for “Cement”, and “30” indicates that the existing concrete strength grade is 30 MPa. Other groups have the same notation.

, respectively.

Before pouring the EGC reinforcement layer, this experiment requires surface treatment of the existing concrete, which has been cured for 28 days. The surface of the existing concrete is roughened using a pneumatic bush hammer to achieve the pre-designed interface roughness targets, forming Type I, Type II, and Type III interfaces.

Based on the influencing factors set in the experiment (EGC strength grade, existing concrete strength, and interface roughness), the 45 EGC-existing concrete bond specimens are divided into 15 groups, with three specimens per group. The specific grouping is shown in

Table 5. EGC–existing concrete bond specimens of grouping design.

Groups	Existing Concrete Strength Grade	Interface Type	EGC Strength Grade
C30-III-G45	C30	III	G45
C30-III-G70	C30	III	G70
C30-III-G95	C30	III	G90
C50-I-G45	C50	I	G45
C50-I-G70	C50	I	G70
C50-I-G95	C50	I	G90
C50-II-G45	C50	II	G45
C50-II-G70	C50	II	G70
C50-II-G95	C50	II	G90
C50-III-G45	C50	III	G45
C50-III-G70	C50	III	G70
C50-III-G95	C50	III	G90
C70-III-G45	C70	III	G45
C70-III-G70	C70	III	G70
C70-III-G95	C70	III	G90
C70-III-G95	C70	III	G90

. The symbol definition for the groups is as follows: The first character (C30, C50, C70) indicates the strength grade of the existing concrete, the second character (I, II, III) represents the type of bond interface, and the third character (G45, G70, G95) represents the strength grade of EGC. For example, C30-I-G45 refers to a specimen where the existing concrete is bonded to EGC through a Type I interface (with the existing concrete strength grade as C30 and the EGC strength grade as G45).

2.2.1. Production of The Existing Concrete

According to JGJ 55-2011 [54], existing concrete is produced using a forced mixer to mix the concrete materials. Before casting, a 50 mm thick baffle is embedded in the cubic mold to facilitate subsequent bush hammering work. The mixing process is as follows: First, the cement and aggregate are added to the mixer and mixed for 2 min, then the water-reducing agent and water mixture are poured into the mixer and mixed for another 2 min. Finally, the freshly mixed concrete is poured into the mold and vibrated to form. The surface of the mold is covered with plastic wrap, demolded after curing in the laboratory indoor environment for 24 h, then placed in water for curing for 28 days before bush hammering. Additionally, extra cubes of all groups of existing concrete are prepared to test the compressive strength at 28 days, with three per group.

2.2.2. Interfacial Roughness Treatment

After the existing concrete had reached a curing age of 28 days, it was removed from the water tank and dried for the application of three different interface roughness levels through bush hammering. Different types of scabbler heads were used with a pneumatic bush hammer to roughen the surface of the existing concrete, resulting in varying depths of roughening. For Type I interfaces, a lychee-faced scabbler head was used to lightly roughen the surface without removing the of laitance of existing concrete. Type II interfaces were treated by using the lychee-faced scabbler head for deep roughening. Type III interfaces involved deep roughening with the lychee-faced scabbler head followed by further roughening with a pineapple-faced scabbler head. Both Type II and Type III interfaces removed the laitance from the surface of the existing concrete. Specimens with different interface types were strictly processed according to the specified roughening requirements, and the consistency of interface treatment effects was reflected by the stability of the measured interface roughness data. The roughening effects for Type I, Type II, and Type III interfaces are illustrated in Figure 3.

The sand patch test was used to measure the roughness of the bond interface. The specific implementation process involved enclosing the surface of the specimen with plastic boards, ensuring that the top of the plastic board was level with the highest point of the bond interface. Then, a material with a known bulk density was poured onto the surface of the existing concrete, and the mass of the poured sand was measured to determine the average depth of sand pouring into the bond interface. According to references [17,55], after cleaning the residual bonding material on the interface of the existing concrete, the interface roughness was calculated using Equation (1). Here, m represents the mass of the sand used for pouring; A represents the area of the bond interface, which is 225 cm²; and ρ represents the bulk density of the pouring sand, measured in this experiment as 1.61 cm³. The calculated roughness values for Type I, II, and III interfaces are 0.15 mm, 1.15 mm, and 2.15 mm, respectively.

$$Roughness=\frac{m}{A\times \rho }$$

(1)

2.2.3. Pouring of the EGC

The existing concrete part of the EGC–existing concrete bond specimens, after interface treatment, is placed in a 150 mm cubic plastic mold, leaving a symmetric gap of 25 mm on each side, as shown in Figure 4. The preparation process for EGC is as follows: Mixing is carried out using a planetary mortar mixer with dual-speed settings, with a low speed of 75 r/min and a high speed of 135 r/min. First, all dry materials (FA, GGBFS, QP, and BaCl₂) are poured into the mixer and mixed at low speed for 2 min. After the dry materials are uniformly mixed, the alkaline activator solution and additional water are slowly added and mixing is continued at a low speed for 1 min, then at a high speed for 2 min. Finally, while maintaining low-speed mixing, PE fibers are evenly added within 3 min. After mixing, the freshly mixed EGC is poured into the molds for axial tensile tests and axial compression tests, as well as the molds containing the existing concrete cubes (using a tamper during the process to ensure even distribution of EGC over the bond interface). Then, the molds are placed onto a vibrating table to eliminate air bubbles. All specimens are covered with plastic wrap, demolded after curing in a laboratory indoor environment for 24 h, then placed in water to cure for 28 days before being dried, in preparation for testing.

3. Experimental Setup and Procedure

3.1. Axial Compressive Test for EGC

The setup and installation method for the axial compression test of EGC (Figure 5) are set according to ASTM-C469/C469M-22 [52]. Loading is performed using a universal testing machine with a displacement loading speed of 0.2 mm/min. To observe the compression deformation of EGC, two 50 mm long resistance strain gauges and linear displacement gauges are placed in the axial compression direction of the cylindrical specimens.

3.2. Axial Tensile Test for EGC

The setup and installation method for the axial tensile test of EGC (Figure 6) follow the guidelines of JSCE [53]. The test is conducted using a microcomputer-controlled electro-hydraulic servo universal testing machine at a displacement loading speed of 0.5 mm/min. Tensile deformation data from the central measuring area of the dumbbell-shaped specimens are collected using linear displacement gauges set on both sides of the specimens, with a collection frequency of 1 Hz.

3.3. Axial Compressive Test for Existing Concrete

The setup and installation method for the axial compression test of existing concrete (Figure 7) are established according to JGJ 55-2011 [54]. The test is conducted using a universal testing machine with a force loading mode at a loading speed of 0.5 MPa/s. The compressive strengths of the three types of existing concrete are shown in

Table 6. Compressive strength of existing concrete.

Groups	Compressive Strength (MPa)
C30	35.6 (1.27)
C50	54.9 (1.89)
C70	75.3 (2.53)

Note: the values are standard deviations in parentheses.

.

3.4. Direct Shear Test of EGC-Existing Concrete Interface Bond Performance

The experimental setup and installation method for the direct shear bond performance test of the EGC–existing concrete interface are shown in Figure 8. A steel shim measuring 150 mm in length, 100 mm in width, and 20 mm in thickness was placed flat on the top surface of the existing concrete portion of the specimen. Two steel shims measuring 150 mm in length, 25 mm in width, and 20 mm in thickness were placed at the bottom of the EGC portion of the specimen to support it. Load was then applied to the rigid shim on the top of the specimen to generate shear force at the interface between the EGC and the existing concrete. Loading was carried out using a universal testing machine. According to references [56,57], the displacement loading speed was set at 0.2 mm/min. Linear displacement gauges were positioned on both sides of the specimen to collect slip values at the bond interface. The slip value at the bonding interface can reflect the improvement effect of EGC on the shear brittleness failure of the specimen.

After the specimen fails, we recorded the ultimate load P_u and calculated the interfacial bond strength of the specimen using Equation (2). In Equation (2), τ represents the interfacial bond strength of the specimen and P_u is the ultimate load at the bonded interface of the specimen.

$$\tau =\frac{Pu}{2A}$$

(2)

4. Results and Discussion

4.1. Basic Mechanical Properties and Environmental Benefits of EGC

The basic mechanical parameters of EGC and the tensile stress–strain curve can be found in

Table 7. Mechanical properties of EGC.

Groups	Compressive Strength (MPa)	Elasticity Modulus (GPa)	Initial Cracking Strain (%)	Ultimate Tensile Strain (%)	Initial Cracking Stress (MPa)	Tensile Stress (MPa)
G45	49.6 (1.15)	8.88 (0.695)	0.0340 (0.0200)	6.10 (0.258)	1.63 (0.0360)	5.09 (0.167)
G70	72.2 (0.364)	14.1 (0.144)	0.136 (0.0380)	7.21 (0.283)	2.53 (0.167)	6.32 (0.171)
G95	96.6 (0.545)	18.8 (0.883)	0.198 (0.122)	7.93 (0.482)	3.20 (0.0930)	8.00 (0.336)

Note: the values have standard deviations in parentheses.

and Figure 9, respectively. From Figure 7, it is evident that as the proportion of slag in the binder materials increases, the basic mechanical properties of EGC are significantly enhanced. Studies indicate that incorporating slag to replace fly ash in EGC can enhance material strength due to the higher CaO content and reactivity of slag compared to fly ash [58]. Moreover, CaO can promote the production of C-S-H/C-A-S-H gel during the hydrolysis and polymerization reactions of the binder materials, further improving the mechanical properties and microstructure of the geopolymers [59].

To further investigate the environmental benefits of EGC, an environmental assessment is conducted using the total CO₂ emissions per unit volume of concrete (CO₂-e). Additionally, for comparison, ECC with compressive strengths [60,61,62] similar to the three EGC types discussed in this study are selected to demonstrate the lower carbon footprint of EGC. The calculation of CO₂-e is based on Equation (3) [63,64]:

$$C{O}_2-e=Q\times EC\times GWP$$

(3)

In the equation, Q represents the quantity of fuel consumed during production activities (kg), EC represents the energy content of the fuel (J/kg), and GWP represents the global warming potential parameter of the fuel (kg CO₂-e/J). EC × GWP (kg CO₂-e/kg) represents the CO₂ emission index.

Table 8. CO₂-e emission of different materials.

Materials	Cement	GGBFS	FA	Silica Fume	Quartz Qowder	NaOH	Na2SiO3	SP	PE Fiber
Emission factor (kg CO2-e/kg)	0.830 [63]	0.019 [64]	0.009 [64]	0.014 [65]	0.046 [64]	1.915 [64]	1.514 [64]	0.720 [64]	1.960 [65]

lists the carbon emission factors for each of the main raw materials.

Table 9. CO₂-e emission of different mixtures.

Materials	Mix Groups
	G45	G70	G95	Reference (C45) [61]	Reference (C70) [62]	Reference (C95) [60]
Cement	-	-	-	492.190	746.095	581.000
GGBFS	4.543	6.918	9.327	-	-	14.250
FA	8.608	7.647	6.627	6.404	4.061	-
Silica Fume	-	-	-	-	1.274	3.220
Quartz Sand	10.999	11.169	11.288	21.822	20.746	23.000
NaOH	92.491	93.903	94.947	-	-	-
Na2SiO3	211.117	214.340	216.724	-	-	-
SP	-	-	-	2.880	14.400	32.400
PE Fiber	29.372	29.372	29.372	37.240	39.200	39.200
CO2-e (kg/m3)	357.129	363.348	368.285	560.537	825.776	693.070

displays the total carbon emissions for the three types of EGC discussed in this paper and their corresponding ECC counterparts for comparison. From the data in Table 9, it can be seen that the total carbon emissions of EGC primarily originate from the production of the alkali activators, whereas the carbon emissions of ECC mainly stem from the production of cement. Under conditions of comparable compressive strength, the total carbon emissions produced from manufacturing EGC are significantly lower than those from producing ECC, with a reduction of up to 127%. Therefore, compared to ECC, using EGC as a reinforcing material in engineering applications offers superior environmental benefits.

4.2. Failure Modes of Specimens in Interfacial Bond Direct Shear Test

The failure modes of the EGC–existing concrete bond specimens are shown in Figure 10. All specimens exhibit clear cracking when reaching the ultimate load P_u, followed by a sharp drop in load, displaying a quasi-brittle failure mode. However, due to the bridging action of fibers at the interface, the existing concrete and EGC do not immediately separate and peel off. Most specimens produce sounds of concrete cracking twice, which may be due to the heterogeneity of the concrete material causing different bond strengths at the interfaces on either side. Eventually, the bond interfaces on both sides of the specimen fail in succession.

Based on the location of the main cracks after specimen failure, the failure modes can be categorized into cohesive failure and adhesive failure. Within adhesive failure, there are two failure paths: serrated interfacial failure and alternating crack [66]. As shown in Figure 10, all specimens, except for C30-III-G95, exhibited adhesive failure. Jiang et al. [49] indicated that increasing the interface roughness enhanced the mechanical interlocking effect between the existing concrete and ECC. Additionally, the fiber bridging effect improved the shear performance of the specimens, and the excellent deformability of ECC prevented premature failure of the ECC section. Ultimately, the existing concrete section was sheared off, leading to the failure of the specimens. Specimen C30-III-G95 followed this pattern, with the lower strength of the existing concrete leading to cohesive failure. Although the other specimens all exhibited adhesive failure, changes in interface roughness affected their failure paths.

(1) With constant strengths of existing concrete and EGC, an increase in interface roughness changes the failure path. Specimens with type I interfaces exhibit serrated interfacial failure, while those with type II and III interfaces show alternating crack failure. This is because type I interface specimens undergo only slight roughening without removing the surface laitance of the existing concrete. The lower strength of the laitance layer causes the laitance and some concrete to be pulled out [35], forming serrated interfacial failure. In contrast, type II and III interface specimens had the surface laitance removed and a certain roughness created, leading to a better bonding interface between EGC and the existing concrete. This shifts the failure path to alternating crack, with both materials interlocking. Additionally, roughening introduces micro-cracks on the concrete surface, enhancing mechanical interlocking at the interface [38], resulting in the shearing of some coarse aggregate during failure. However, the transition from type II to type III interface does not significantly increase the amount of EGC remaining on the concrete part of the interface. (2) With constant existing concrete strength and interface roughness, increasing EGC strength results in more concrete being retained on the EGC part of the specimen after failure. (3) With constant interface roughness and EGC strength, increasing the strength of the existing concrete results in more EGC being retained on the concrete part of the specimen and less concrete on the EGC part.

During the experiment, the ultimate load P_u is recorded when the specimen fails, and the interfacial bond strength τ can be calculated using Equation (2).

Table 10. Summary of interfacial bond strength and failure modes of EGC–existing concrete bond specimens.

Groups	Failure Mode	Interfacial Bond Strength (MPa)	Standard Deviation (MPa)
C30-III-G45	alternating crack	1.18	0.0515
C30-III-G70	alternating crack	2.01	0.0793
C30-III-G95	cohesive failure	2.61	0.0831
C50-I-G45	serrated interfacial failure	0.674	0.0209
C50-I-G70	serrated interfacial failure	1.07	0.104
C50-I-G95	serrated interfacial failure	1.31	0.0498
C50-II-G45	alternating crack	1.11	0.0259
C50-II-G70	alternating crack	1.74	0.0243
C50-II-G95	alternating crack	2.40	0.0241
C50-III-G45	alternating crack	1.46	0.0400
C50-III-G70	alternating crack	2.18	0.0318
C50-III-G95	alternating crack	2.82	0.0356
C70-III-G45	alternating crack	1.74	0.0886
C70-III-G70	alternating crack	2.53	0.0636
C70-III-G95	alternating crack	3.29	0.0751

summarizes the interfacial bond strengths and failure modes of the specimens. Data from Table 10 indicate that increases in the existing concrete strength, interface roughness, and EGC strength all positively affect the interfacial bond performance. Additionally, the standard deviation values of the bonding interface strength for all specimens are relatively small. This indicates that the specimen preparation method used in this study has repeatability. Furthermore, the significance of the three experimental variables was tested using the Bonferroni correction to analyze the data and ensure the reliability of the conclusions.

Table 11. Results of tests for main effects among the three experimental variables.

Source of Variation	Type III Sum of Squares	Degrees of Freedom	Mean Square	F	p	Partial Eta Squared
Corrected Model	19.887 a	14	1.421	250.760	<0.001	0.992
Intercept	171.496	1	171.496	30,273.940	<0.001	0.999
Existing Concrete Strength	1.588	2	0.794	140.136	<0.001	0.903
Interface Roughness	2.674	2	1.337	236.018	<0.001	0.940
EGC Strength	12.710	2	6.355	1121.825	<0.001	0.987
Existing Concrete Strength * Interface Roughness	0.000	0	-	-	-	0.000
Existing Concrete Strength * EGC Strength	0.038	4	0.009	1.656	0.186	0.181
Interface Roughness * EGC Strength	0.058	4	0.015	2.581	0.057	0.256
Existing Concrete Strength * Interface Roughness * EGC Strength	0.000	0	-	-	-	0.000
Error	0.170	30	0.006	-	-	-
Total	233.214	45	-	-	-	-
Adjusted Total	20.057	44	-	-	-	-

Note: ^a indicates R² = 0.992 (Adjusted R² = 0.988). * is a symbol used to connect different experimental variables together to form an interaction term.

presents the results of the tests for main effects among the experimental variables. From Table 11, it is evident that all three experimental variables exhibit main effects. This indicates that different existing concrete strength, interface roughness, and EGC strength significantly affect the interface bond strength of the specimens. Therefore, the differences in the experimental results are due to the specific experimental treatments. However, the interaction terms of existing concrete strength * EGC strength and interface roughness * EGC strength have p > 0.05, indicating no statistical significance.

4.3. Load–Slip Curve of the Specimens

The load–slip curves of all specimen bond interfaces are shown in Figure 11. From Figure 11, it can be seen that before reaching the ultimate load P_u, the relationship between the load and interface slip is generally linear. After reaching the ultimate load P_u, the load drops sharply but does not drop to zero. Due to the bridging action of fibers at the interface, the load–slip curve exhibits a certain degree of ductility. Because of the inconsistency in bond strength on both sides of the specimens, some specimen load–slip curves show two peaks.

4.4. Analysis of Interfacial Bond Strength Parameters

4.4.1. Interface Roughness

The effect of interface roughness on the interface bond strength is shown in Figure 12. When the strength grade of the existing concrete is C50 and the strength grade of EGC is C45, the interface bond strengths of Type II and Type III interface specimens increase by 0.436 MPa and 0.786 MPa, respectively, compared to an interface bond strength of 0.674 MPa for Type I interface specimens. The increase in interface roughness has a significant impact on the development of interface bond strength. Additionally, as seen in Figure 12, when the strength of the existing concrete and EGC remains constant, the interface bond strength increases with increasing interface roughness. This is because increasing the interface roughness is equivalent to increasing the bonding surface area while also enhancing the mechanical interlocking at the bond interface [67].

4.4.2. Existing Concrete Strength Grade

The effect of existing concrete strength grade on interface bond strength is shown in Figure 13. When the strength grade of EGC is G45, and the interface type is Type III, compared to an interface bond strength of 1.18 MPa for specimens with existing concrete strength grade C30, the interface bond strengths for specimens with existing concrete strength grades C50 and C70 increase by 23.1% and 32.1%, respectively. According to data from Table 8, the bond strength at the interface increases with the increase in the strength of the existing concrete when the interface roughness and EGC strength remain constant. This is because, in high-strength concrete, the adhesion between cement paste and aggregates is higher than in lower-strength concrete, making it less likely for aggregates to be pulled out along with the EGC overlay, thereby enhancing the bond strength at the interface [44]. Additionally, high-strength concrete can form a more robust bond at the interface with microcracks created during pneumatic chiseling of the overlay [38]. However, combining data from Table 8 and the analysis of Figure 11, the increase in the strength of existing concrete does not significantly enhance the bond strength at the interface.

4.4.3. EGC Strength Grade

The influence of EGC strength grade on interface bond strength is illustrated in Figure 12 and Figure 13. As shown in Figure 10, for specimens with interface Type I and existing concrete strength grade C50, compared to an interface bond strength of 0.674 MPa for specimens with EGC strength grade G45, the interface bond strength increased by 0.396 MPa and 0.636 MPa for specimens with EGC strength grades G70 and G95, respectively. Additionally, Figure 12 illustrates that for specimens with existing concrete strength grade C30 and interface Type III, compared to an interface bond strength of 1.18 MPa for specimens with EGC strength grade G45, the interface bond strength increased by 70.0% and 120% for specimens with EGC strength grades G70 and G95, respectively. It can be observed that the interface bond strength increases with increasing EGC strength. Eduardo et al. [40] suggest that for a given shear stress, the use of higher-strength reinforcement materials generates higher normal stresses at the interface, thereby enhancing the interface bond strength.

Furthermore, with the existing concrete strength and EGC strength held constant, the increase in bond strength varies with different levels of interface roughness. As the interface roughness increases, the bond strength enhancement is more significant when using high-strength EGC as the reinforcement layer. For instance, the bond strength of specimen C50-III-G95 is 354% higher compared to that of specimen C50-I-G45. This is because increased interface roughness maximizes the mechanical interlocking and friction between the high-strength EGC and the existing concrete. Therefore, when the strength of the existing concrete remains constant, the simultaneous increase in EGC strength and interface roughness contributes more significantly to the enhancement of bond strength at the interface.

4.5. Modeling of Interfacial Bond Strength between EGC and Existing Concrete

Based on the results of bilateral direct shear bond tests and the mechanism of direct shear force bonding, this paper establishes a direct shear bond mechanical model by conducting elasto-plastic analysis and incorporating the Mohr–Coulomb theory. This model correlates interface roughness, existing concrete strength, and EGC strength to predict the interface bond strength.

According to a relevant reference [68], the interface bond strength τ can be predicted using the following Equation (4):

$$\tau =\frac{\sqrt{3}}{3}\left({\sigma }_{s1}+{\sigma }_{s2}\right)\frac{\beta }{1+\beta }+{\tau }_s^0\frac{1-\beta }{1+\beta }$$

(4)

Here, σ_s1 and σ_s2 represent the uniaxial compressive strengths of the existing concrete and EGC, respectively. ${\tau }_s^0$ is the shear stress on the velocity discontinuity surface, and β is a velocity reduction coefficient ranging from 0 to less than 1.

By combining the Mohr–Coulomb strength theory and simplifying the calculations, the Mohr stress circle envelope can be approximated as a straight line, resulting in the following equation:

$${\tau }_s^0=\frac{1}{2}\left({\sigma }_1-{\sigma }_2\right)\cos \phi$$

(5)

In the equation, σ₁ represents the tensile stress at the bond interface, σ₂ represents the compressive stress at the bond interface, and $\phi$ is the internal angle of friction.

The stress at the bond interface is determined by the weaker material. In this experiment, the strength of the existing concrete is close to or less than that of EGC, and the bush hammering of the interface induces microcracks in the concrete, reducing its strength. This corresponds to the experimental observation that most of the concrete at the bond interface is pulled out. Therefore, σ₁ can be represented by the tensile strength of concrete, and σ₂ can be represented by the compressive strength of concrete, with the relationship between concrete’s tensile strength σ₁ and compressive strength σ₂ as follows [69]:

$${\sigma }_1=0.3{\sigma }_2^{2/3}$$

(6)

The tangent of the friction angle $\phi$ is the coefficient of friction. Therefore, the friction angle $\phi$ can be regarded as a parameter reflecting the interface roughness. And cos$\phi$ can be expressed by the function f(H), where H is a variable. From Equations (5) and (6), it follows that:

$${\tau }_s^0=\frac{1}{2}\left(0.3{\sigma }_2^{2/3}-{\sigma }_2\right)f\left(H\right)$$

(7)

Combining Equations (4) and (7), the interface bond strength τ can be expressed as follows:

$$\tau =\frac{\sqrt{3}}{3}\left({\sigma }_{s1}+{\sigma }_{s2}\right)\frac{\beta }{1+\beta }+\frac{1}{2}\left(0.3{\sigma }_{s2}^{2/3}-{\sigma }_{s2}\right)f\left(H\right)\frac{1-\beta }{1+\beta }$$

(8)

Here, a quadratic function is used to model the impact of interface roughness H on interface bond strength. Letting $f\left(H\right)=c{H}^2+dH+e$, $a=\frac{\sqrt{3}\beta }{3\left(1+\beta \right)}$, $b=\frac{1-\beta }{1+\beta }$, and substituting into Equation (8), we obtain:

$$\tau =a\left({\sigma }_{s1}+{\sigma }_{s2}\right)+\frac{1}{2}b\left(0.3{\sigma }_{s2}^{2/3}-{\sigma }_{s2}\right)\left(c{H}^2+dH+e\right)$$

(9)

The fitting of the experimental data from this study results in the following:

$$\tau =0.02269\left({\sigma }_{s1}+{\sigma }_{s2}\right)+\frac{0.34716}{2}\left(0.3{\sigma }_{s2}^{2/3}-{\sigma }_{s2}\right)\left(0.01325H^2-0.10062H+0.22786\right),R^2=0.94$$

(10)

The comparison of predicted values from the direct shear bond mechanics model for EGC and existing concrete against experimental values is shown in Figure 14 and

Table 12. Comparison of experimental and predicted values of interface bond strength.

Groups	Existing Concrete Strength (MPa)	Interface Roughness (mm)	EGC Strength (MPa)	Experimented Interface Bond Strength (MPa)	Predicted Interface Bond Strength (MPa)	Error (%)
C30-III-G45	35.6	2.15	49.6	1.18	1.52	−28.8
C30-III-G70	35.6	2.15	72.2	2.01	2.04	−1.49
C30-III-G95	35.6	2.15	96.6	2.61	2.57	1.53
C50-I-G45	54.9	0.15	49.6	0.674	0.499	26.0
C50-I-G70	54.9	0.15	72.2	1.07	1.01	5.61
C50-I-G95	54.9	0.15	96.6	1.31	1.55	−18.3
C50-II-G45	54.9	1.15	49.6	1.11	1.23	−10.8
C50-II-G70	54.9	1.15	72.2	1.74	1.75	−0.575
C50-II-G95	54.9	1.15	96.6	2.40	2.28	5.00
C50-III-G45	54.9	2.15	49.6	1.46	1.73	−18.5
C50-III-G70	54.9	2.15	72.2	2.18	2.25	−3.21
C50-III-G95	54.9	2.15	96.6	2.82	2.78	1.42
C70-III-G45	75.3	2.15	49.6	1.74	1.95	−12.1
C70-III-G70	75.3	2.15	72.2	2.53	2.46	2.77
C70-III-G95	75.3	2.15	96.6	3.29	3.00	8.82

. From Table 12, it can be seen that when the EGC strength is lower, there is a larger discrepancy between the predicted values from the bond mechanics model and the experimental values. This is because the derivation of the mechanical model assumes that the strength of the existing concrete is less than or equal to the strength of EGC. However, the data error for most groups is within 20%, indicating that the bond mechanical model derived in this study shows good consistency between the predicted and experimental values.

Additionally, the applicability of the proposed bond mechanics model needs to be verified using experimental data from existing relevant literature. The validation results of the bond mechanics model’s applicability are shown in Figure 15 and

Table 13. Comparison of experimental and predicted values of interface bond strength from relevant reference.

Number	Existing Concrete Strength (MPa)	Interface Roughness (mm)	Post-cast Material Strength (MPa)	Experimented Interface Bond Strength (MPa)	Predicted Interface Bond Strength (MPa)	Error (%)	Ref.
1	66.3	0.324	32.9	0.500	0.300	66.6	[17]
2	66.3	0.779	32.9	1.04	0.722	44.1
3	66.3	1.21	32.9	1.59	1.07	48.3
4	45.3	2.23	93.9	2.07	2.69	−23.1	[70]
5	45.3	4.46	94.1	2.23	2.98	−25.3
6	45.3	5.67	113.4	4.96	3.21	54.6
7	45.3	5.74	90.3	3.64	2.66	36.7
8	45.3	2.39	97.9	3.24	2.83	14.4
9	45.3	4.53	83.7	3.08	2.74	12.3
10	45.3	4.43	89.8	2.58	2.89	−10.7
11	45.3	6.01	93.7	3.43	2.65	29.2
12	45.3	2.11	99.9	1.75	2.79	−37.2

. As can be seen in Figure 15, some validation data exhibit a significant error (>30%) compared to the predicted values of the bond mechanics model. By analyzing the reasons for the large prediction errors combined with the post-cast material parameters provided in Table 13, it can be observed that: on the one hand, the strength of the post-cast material is lower than the strength of the existing concrete, violating the assumption premise of the bond mechanics model; on the other hand, the interface form of the existing concrete in this study is a chiseled interface, whereas the interface forms of samples numbered 6, 7, and 11 are grooved interfaces, which significantly increase the bond area and thereby the bond strength. In summary, when the strength of the post-cast material is greater than the strength of the existing concrete and the interface form of the existing concrete is a chiseled interface, the bond mechanics model’s prediction accuracy is high. Therefore, the bond mechanics model proposed in this paper can be used to predict the bond strength of the interface between existing concrete and post-cast material under direct shear conditions and can serve as a reference for engineering design.

5. Conclusions

This study conducted direct shear tests on EGC–existing concrete bond specimens, altering the strength of existing concrete, interface roughness, and EGC strength. It examined how these variables affect failure modes and interface bond strength, resulting in a shear bond mechanics model with minimal prediction errors. The study also analyzed how varying fly ash and slag ratios of EGC impact its mechanical properties. Based on the findings, the following conclusions and future outlooks are presented:

(1)

Increasing the slag proportion in EGC enhances its microstructure, improving the compressive and tensile properties. Additionally, EGC production results in up to 127% lower carbon emissions compared to ECC.

(2)

Adhesive failure was the primary mode observed. The fiber bridging effect at the interface prevented immediate separation of the existing concrete and EGC, reducing brittleness. Higher interface roughness altered the failure path and increased the bond surface area, enhancing mechanical interlocking.

(3)

Within certain limits, interface bond strength increased with greater interface roughness, existing concrete strength, and EGC strength. Interface roughness and EGC strength had significant impacts, while existing concrete strength had a lesser, but still positive, effect.

(4)

A direct shear bond mechanical model was developed, correlating existing concrete strength, interface roughness, and EGC strength. Validation showed the model is applicable when the post-cast material is stronger than the existing concrete and when the concrete interface is chiseled. Prediction errors were generally within 30%. This study used fly ash and slag, activated with sodium hydroxide and sodium silicate, to make EGC. Future research should test other precursors and activation systems for EGC and concrete interfaces.

(5)

This study focused on interface roughness, existing concrete strength, and EGC strength, without considering interface moisture content or bonding agents. Other factors like curing conditions, interface treatment, and environmental exposure also affect bond performance. Adverse conditions like wet–dry cycles, seawater erosion, and freeze–thaw cycles also deserve to be explored for their potential impact.