Interfacial Shear Behavior of Composite Concrete Substrate to High-Performance Concrete Overly After Exposure to Elevated Temperature

Abstract

Basically, the interface shear strength between two concrete layers of varying ages must be sufficient to withstand the applied actions on the structure, specifically fire attack, which may cause the complete collapse of the composite structure. Thus, interfacial shear behavior was investigated and analyzed in this paper under the influence of a set of parameters, including temperature (25, 200, 400, and 600 °C), time exposure (30, 60, 90, 120, and 180 min), concrete type, and fibers type (polypropylene fiber (PPF), steel fiber (SF), and hybrid fiber) by employing a Z-shape push-off test. The test consists of two parts with different ages: normal strength concrete (NCS) and high-performance concrete (HPC). HPC includes high-strength concrete (HSC) and fly ash concrete (FAC). Initially, twenty-five Z-shaped push-off tests were made, four of which were cast as one unit (NSC/or concrete with hybrid (FSP)), and the rest were composite specimens. Furthermore, a 3D finite element model of a composite push-off specimen was developed to simulate and analyze the impact of various time and temperature exposures on the interfacial shear strength of composite specimen N-FSP. The results indicated that temperature degree and exposure time adversely affected the interfacial shear strength. Also, interfacial shear strength is significantly influenced by fiber types. Including combined fiber (SF + PPF) improved the interfacial shear strength by 114% compared to the composite specimen NSC-NSC after exposure to a temperature of 600 °C. In contrast, using PPF negatively affected the interfacial shear strength, recording only 84% of the composite specimen NSC-NSC. In addition, the inclusion of supplementary cementitious material enhanced the interfacial shear strength by 60.5% in the NSC-FAC composite specimen with 30% FA, compared to the NSC-NSC specimen. Finally, a finite element (FE) model was proposed with a satisfactory level of accuracy (0.95 to 1.03) in predicting the maximum shear strength. Additionally, the difference between the FE and experimental stiffness was between 0.92 and 1.07.

1 Introduction

Over the last several years, researchers have focused heavily on developing innovative repair techniques and materials for reinforced concrete (RC) structures. One of the traditional repair techniques is rehabilitation, which involves placing a new concrete layer on the existing one (Santos & Julio, 2007). Usually, the concrete-to-concrete interface between two concrete layers cast at different ages is the weakest area of the concrete structure. To ensure the reliable functioning of these connections, a quantifiable amount of shear strength at the interface must be mobilized (Julio et al., 2004). The impact of monotonic loading on the strength of concrete-to-concrete interface bonds has been the focus of a number of investigations (Bentz et al., 2018; Beushausen, 2010; Feng et al., 2020; Mansour & Fayed, 2021; Park & Kim, 2021; Santos et al., 2012; Tayeh et al., 2013; Valikhani et al., 2021). As a result of these studies, the primary mechanisms for shear transfer between concrete layers cast at varying ages may be identified: adhesion, friction, and dowel action. Moreover, the shear transfer stress depends on many variables including concrete substrate surface preparation (Feng et al., 2020; Tayeh et al., 2013), overlay concrete strength (Mansour & Fayed, 2021; Santos et al., 2012), bond agent (Park & Kim, 2021), shear connectors between the two concrete layers (Valikhani et al., 2021), moisture condition of the existing concrete layer at the time of casting (Bentz et al., 2018; Beushausen, 2010), fiber reinforcement type of the concrete overlay (Zanotti et al., 2018).

In the case of fire, horizontal cracks have been formed at the interface between the concrete layers under fire load in some studies (Ghazy et al., 2023). In contrast, others observed no crack propagating at the interface Baloch et al. (Baloch et al., 2023). According to Ghazy et al. (2023), the concrete type, strengthening side, connection type, and fire direction significantly affected the load–deflection curves of the composite slabs after fire exposure. Thus, studying the shear transfer stress between two concrete layers after exposure to elevated temperatures is essential for understanding composite structures' overall behavior under elevated temperatures.

Through the employment of slant shear, push-off, and splitting tensile tests, several researchers have experimentally examined the concrete-to-concrete interface bond at elevated temperatures (Abo Sabah et al., 2019; Gao et al., 2019; Haido et al., 2021; Sun et al., 2022). The findings show that the interface bond strength considerably decreased as the temperature increased. This is due to the deterioration of the cement paste under elevated temperature, the difference in thermal expansion between cement paste and aggregate, micro-cracks were developed, consequently, deterioration in the interface bond strength. In slant shear tests, positive correlations were observed between the maximum shear strength and the shear angle; larger shear angles corresponded to greater shear strengths. Sun et al. (Sun et al., 2022) concluded that the maximum shear strength after exposure to a temperature of 700 was 40%, 50%, and 57% of that at 100 °C for shear angles 30 ^◦C, 38 ^◦C, and 45 ^◦C, respectively.

Regarding shear strength at the interface during a fire, surface preparation of the concrete is crucial. According to Chen et al. (Chen et al., 2023), after being exposed to an elevated temperature of 200 °C, the smooth surface only maintained 20–35% of its shear strength at room temperature, while the rough surface retained approximately 65–87.5% of its shear strength at room temperature for the same condition. In contrast, Shang et al. (2021) revealed that the surface roughness adversely affected the shear strength when the surface roughness was higher than 3.5 mm.

Regarding existing fiber in the overlay concrete, Goa et al. (2019) experimentally studied the bond strength of the concrete-to-concrete interface after exposure to elevated temperatures through conducted slant shear tests. The parameters include overlay concrete types, NSC or engineering cementitious composite (ECC), and scenario of fire effect; the first scenario, the concrete substrate subjected to temperature up to 800 °C, then repaired with overlay concrete, while the second scenario, the concrete substrate repaired with concrete overlay then exposed to elevated temperature up to 800 °C. Their results indicated that the utilized composite NSC/ECC observed higher bond strength than NSC/NSC. Furthermore, the bond strength improved with increasing temperature up to 200 °C; the bond strength gradually decreased with temperature increase. Shang et al. (2021) conducted a push-off test on a concrete substrate heated up to 800 °C and strengthened with engineered cementitious composite reinforced with polyethylene (PE-ECC) or polyvinyl alcohol (PVA-ECC) fibers. The results showed that the PVA-ECC specimen recorded a shear strength of 2.27 MPa with an increasing ratio of 17.6% as compared to the PVA-ECC specimen after exposure to 600 °C. In addition, concrete type plays an essential role in the interface shear strength. Chen et al. (2023) indicated that the inclusion of high-strength concrete can improve the interface shear strength and depend on the concrete substrate strength, where existing HSC in the two interface sides led to an increase in the shear strength by 119% as compared to use normal strength concrete in the two-interface side. Moreover, the bond agent had an evident effect on the bond strength at ambient and elevated temperatures. Chen et al. (2023) determined that using cement mortar at the interface between two NSC layers can improve the shear strength by 40%, 32%, and 119% at 20 °C, 200 °C, and 600 °C, respectively, as comparison without a bond agent.

High-performance concrete (HPC) is one of the traditional repair materials for RC structures due to its high mechanical properties and durability (Khaliq & Kodur, 2011). From previous studies (Babalola et al., 2021; Ghazy et al., 2022; Khaliq & Kodur, 2011; Poon et al., 2001), existing 30% fly ash (FA) can improve the mechanical properties of concrete after exposure to elevated temperature. Moreover, adding steel fiber (SF) and polypropylene fiber (PPF) has a significant effect in enhancing the mechanical properties at high temperatures (Algourdin et al., 2020; Ghazy et al., 2022; Khaliq & Kodur, 2011; Li & Liu, 2016). Furthermore, the use of HPC can improve the post-fire behavior of RC members (Baloch et al., 2023; Ghazy et al., 2023; Khaliq & Kodur, 2013).

However, limited papers focus on the Z shape push-off test of different concrete cast at different ages after exposure to elevated temperature (Chen et al., 2023; Shang et al., 2021). Thus, this investigation aims to study the interface shear strength at ambient temperature or after exposure to elevated temperature through the Z-shape push-off test. Firstly, the authors examined the effect of different temperatures 25, 200, 400, and 600 on the interface shear strength for Z specimen cast with NSC and fly ash concrete containing hybrid fiber (0.5% SF + 0.5% PPF) (N-FSP). Also, studying the interfacial shear behavior of N-FSP specimens heated to temperatures of 600 °C for 30, 60, 90, 120, and 180 min. Secondly, from the first stage, the temperature of 600 °C and 90 min exposure time were selected to investigate the effect of concrete types and fiber types on the interfacial shear behavior at ambient and after exposure to elevated temperatures. The results were compared to monolithic NSC and FSP specimens. Accordingly, the results of this study will assist engineers in comprehending the behavior of RC members enhanced with HPC in a high-temperature environment. In addition, the data obtained from this study can be used to validate finite element models of composite RC members under fire exposure conditions.

2 Experimental Work

2.1 Material

A Z-shape push-off test consists of two parts: normal strength concrete (NCS) and high-performance concrete (HPC). Concrete mixtures have been prepared using ordinary Portland cement (CEM 42.5), crushed limestone with a maximum nominal size of 10 mm, river sand, water, and polycarboxylate superplasticizer. HPC includes high-strength concrete (HSC), fly ash concrete (FAC) by replacing 30% of cement with FA type F, FAC with 0.5% polypropylene fiber (PPF), 0.5% steel fiber (SF), and hybrid fiber (0.5% PPF + 0.5% SF). Fig. 1 presents the fibers employed in this study. Table 1 displays aggregate characteristics, whereas Tables 2 and 3 highlight fiber characteristics. The components of concrete mixtures are given in Table 4.

Fig. 1

Types of the used fibers

Table 1 Physical properties of the aggregates utilized

Table 2 The chemical, physical, and mechanical properties of the employed cement and FA (according to the manufacturers)

Table 3 Properties of employed fibers (SF, and PPF) and steel rebars (according to the manufacturers)

Table 4 Components of various concrete mixtures

Where HRWR refer to high-range water reducers, FA: fly ash SF: steel fiber, and PPF: polypropylene fibers.

2.2 Specimen Perpetration

The interfacial shear strength of HPC and NSC cast at various ages at ambient temperature and after exposure to elevated temperature was investigated by making and testing a total of 25 Z-shaped push-off tests. The Z-shape specimen dimensions were 100 × 200 × 400 mm, where the interface area was 100 × 160 mm². To prevent edge failures brought on by potential flexural loads, the reinforcement rebar with a 10 mm diameter is included. This study used no shear connectors across the interface to demonstrate the pure shear between the two concrete parts. Fig. 2 depicts the specimen dimension and reinforcement details.

Fig. 2

Specimen dimensions and reinforcement details (all dimensions in mm)

The first part of the Z-shape was cast with NSC (N) as the concrete substrate; for studying the effect of concrete type, the second part was cast with NSC specimen (N–N) as reference one and compared with specimen cast with HSC (specimen N–H), FAC (specimen N-F). Specimens that have FAC incorporating 0.5% steel fiber (specimen N-FS), 0.5% polypropylene fiber (specimen N-FP), and hybrid fibers (specimen N-FSP) were considered for studying the influence of fiber types. The following procedures were used to create the composite specimens. (1) Half of the Z-shape was fabricated, reinforcement was placed inside, and then the NSC was cast. (2) After 24 h, the concrete part was demoulded and cured in water for 28 days. (3) According to Fib, the NSC surface was roughened by 1.5 mm to increase the bond strength at the interface (CEB-FIP & Model Code., 2010). Moreover, previous research (Jiang et al., 2020, 2021; Yang et al., 2022; Zhang et al., 2020a, 2020b) had demonstrated that grooved treatment with dimensions varied from 10 to 20 mm significantly enhanced the shear strength, while increasing the space between grooves led to a decrease in the interfacial shear strength. In this study, two grooves with 1.5 cm width and 1 cm depth were fabricated in the NSC surface, as shown in Fig. 3. The surface was cleaned from dust and wetted by spring water. (4) The NSC part was placed into the mold, and HPC was cast; after 24 h, the Z-shape specimen was demoulded and cured in water for 28 days, then the specimens were placed at room temperature of 25 °C for 30 days until the time of heating, to prevent the concrete from becoming too damp, which could cause explosive spalling as mentioned by Ahmad et al. (Ahmad et al., 2020), Ghazy et al. (2023). Fig. 4 shows the steps of casting composite specimens. In addition, four Z-shape specimens were cast as one unit with NSC/or FSP for comparison with the composite specimen at ambient temperature and after exposure to a temperature of 600 °C. Table 5 presents the summary of specimens.

Fig. 3

Grooves arrangement and dimensions (all dimensions in mm)

Fig. 4

Images from specimen perpetration

Table 5 Details of the investigated specimen

For studying the mechanical properties of concrete mixes at room temperature and after exposure to elevated temperatures, six cubes with dimensions of × 1010 × 10 cm³ were cast from each concrete mix for compression test (BS En 12390–3, 2019), while six cylinders have 10 cm diameter and 20 cm height were taken for tensile strength (BS En 12390–6, 2009).

2.3 Heating Stage

After 60 days, the specimens were heated in an electric furnace with dimensions of 500 × 400 × 400 mm. The heating rate was 50 °C/min to simulate the actual fire situation. In the case of studying the temperature effect on the interface shear strength, specimens N-FSP and N–N were heated at 200 °C, 400 °C, and 600 °C, where the specimens were exposed to temperature from all sides, as shown in Fig. 5a. When the furnace reached the target temperature, it stayed for 90 min, after that, the furnace switched off. The specimens cooled down naturally to room temperature. To observe the effect of time exposure on the interface shear strength, specimen N-FSP was exposed to a temperature of 600 °C for different times of 30, 60, 90, 120, and 180 min.

Fig. 5

Specimen under different loads

2.4 Loading Test

A direct shear test was conducted using a compression machine with a loading rate of 0.01 kN/sec, as shown in Fig. 5b. The load was recorded automatically, while the displacement was recorded using a digital dial gauge (DG). The crack propagation and failure pattern have been observed during the test.

3 Results

3.1 Failure Pattern

After exposure to elevated temperatures, no spalling occurred during the fire test neither under different temperatures nor at different times, except specimen N-FSP which consists of NSC and fly ash concrete with hybrid fibers, showed explosive spalling and contributed to a total collapse of Z-shape push off specimen after exposure to a temperature of 600 °C for 180 min as shown in Fig. 6b. This is due to the fact that steel fibers bridge the cracks and prevent the mitigation of vapor water to the atmosphere as discussed by Algourdin et al. (2020).

Fig. 6

Effect of elevated temperature on a specimen

Under shear load, none of the specimens showed flexural failure because there was enough flexural (main) steel. Fig. 7 presents the failure pattern for a monolithic specimen cast with NSC/or FAC (SF + PPF). Specimen N showed a brittle failure; the aggregate interlock action caused the failure plane to form along the pastes around the aggregate particles, while SF delayed crack propagation in specimen N-FSP. Alimrani and Balazs (2020) displayed that this is because the SF acts as a bridge over the fractures and increases the interlocking activity at the shear interface. At 600 °C, the degradation in the concrete mechanical properties made the concrete more softening, where the cracks were more obvious, as shown in Fig. 7c, d.

Fig. 7

Crack patterns of monolithic specimen N and FSP

Fig. 8 shows three distinct failure patterns that may be observed in composite specimens depending on the crack propagation. Both Chen et al. (2023) and Shang et al. (2021) highlight that interfacial roughness plays a major role in enhancing the interfacial bond between the two concrete parts after fire exposure. Cohesive failure of the concrete substrate indicated that the full failure occurred in the NSC part, as shown in Fig. 8a. When compared to NSC's tensile strength, this indicates a great interfacial bond. This failure was observed in specimens N-H₂₅, N-H₆₀₀, N-PF₂₅, N-FS₂₅, N-FS₆₀₀, N-FSP₂₅, N-FSP₂₀₀, N-FSP₄₀₀, N-FSP₆₀₀, N-FSP_30min, N-FSP_60min, and N-FSP_90min. Mixed failure A: adhesive failure with a fracture in the post-cast concrete that is apparent in composite specimen N-FP₆₀₀ as depicted in Fig. 8b. This is due to the fact that the melting of PPF at a temperature of 160 °C and total evaporation at a temperature of 360 °C resulted in producing additional voids, consequently softening concrete (Ghazy et al., 2022). Mixed failure B means the fracture of both NSC and post-cast concrete adjacent to the interface (Fig. 8c). Table 6 presents the failure pattern for different composite push-off specimens.

Fig. 8

Failure pattern in composite push-off test

Table 6 Failure pattern for different composite push-off specimens

3.2 Interfacial Shear Properties

This section discusses the interfacial shear behavior for monolithic and composite push-off specimens at ambient temperature and after exposure to elevated temperature. The output of the test includes maximum interfacial shear strength. ${\tau }_{max}$ and calculated by Eq. 1,

$${{\varvec{\tau}}}_{{\varvec{m}}{\varvec{a}}{\varvec{x}}}=\frac{{{\varvec{P}}}_{{\varvec{m}}{\varvec{a}}{\varvec{x}}}}{{\varvec{A}}}$$

(1)

where ${P}_{max}$ is the maximum vertical load, and A is the interfacial shear area (160 × 100 mm²).

Fracture energy is the area under the interfacial shear strength–slip curve, which is calculated by Eq. 2,

$$G={\int }_{0}^{{\delta }_{t}^{f}}\tau d\delta$$

(2)

where ${\delta }_{t}^{f}$ is the failure slip.

Initial stiffness is the relative maximum shear strength (${\tau }_{max}$) to the corresponding slip (${\delta }_{t}^{0} )$ as presented in Eq. 3, according to Sun et al. (2022).

$$K=\frac{{\tau }_{max}}{{\delta }_{t}^{0}}$$

(3)

The effects of different temperatures, different time exposure, concrete type, and fiber type on the interfacial shear characteristics of composite push-off specimens are presented in Table 7 and Figs. 9, 10, 11, 12, and 13.

Table 7 Interfacial shear properties of push-off specimens

Fig. 9

Interfacial shear strength–slip displacement curve

Fig. 10

Effect of elevated temperature on the residual interfacial shear strength

Fig. 11

Effect of time exposure on the interfacial shear properties of composite specimen N-FSP

Fig. 12

Maximum shear strength of different specimens after 600 °C

Fig. 13

The relative interfacial shear strength of roughness surface at elevated temperature

3.3 Interfacial Shear Load Slip Displacement Curve

Fig. 9 presents the interfacial shear strength versus slip displacement for all specimens at room temperature and after exposure to elevated temperature. For all specimens, the interfacial shear strength–slip curve can be divided into two main parts (Fig. 9). In the first part, the interfacial shear strength increased linearly with a higher rate at a small slip less than 0.05 mm, where the adhesive bond is the main contribution in the interfacial shear strength mentioned by FIB (CEB-FIP, Model Code., 2010). In the second stage, the load increases slowly and, in some cases, becomes constant with a rapid increase in the slip. This behavior can be discussed by: with increasing load, the adhesive bond deteriorates rapidly, and the shear friction introduced by mechanical interlocking becomes the main load transfer mechanism and resists the slip increase.

Moreover, the temperature degree and temperature time adversely affected the interfacial shear behavior. The maximum shear strength of the specimens decreased; at the same time, the ductility increased except for the specimen exposed to a temperature for 120 min, which recorded the least ductility, as shown in Fig. 9b. In contrast, the effect of concrete type was not obvious at room temperature; the use of fibers can improve the ductility of the specimens. In addition, its effect is more visible after exposure to elevated temperature, where the composite push-off specimens containing SF (N-FSP, N-SF) displayed better behavior than the specimen without SF, as shown in Fig. 9f. In contrast, the specimen included PPF showed brittle behavior after exposure to elevated temperature. It should be noticed that the interfacial shear strength–slip curve for monolithic specimens N and FSP was stiffer and recorded higher shear strength as compared to composite specimens, as shown in Fig. 9c–f. This is due to the adhesion between cement paste and aggregates, mechanical interlocking of aggregates, and SF at the interfacial shear zone, which delay cracks propagation.

3.4 Effect of Temperature

The interfacial shear strength of composite push-off specimens N–N and N-FSP at different temperature exposures up to 600 °C are presented in Fig. 10. The output results indicated that specimen N-FSP showed better performance than specimen N–N at ambient and after elevated temperature. The interfacial shear strength of specimen N–N decreased rapidly with increasing temperature up to 400 °C, where the remained residual shear strength at 400 was 54% of that at ambient temperature. After 400 °C, the rate of deterioration in the residual interfacial shear strength decreased to a record 44% after exposure to a temperature of 600 °C. This significant deterioration can be explained by the evaporation of free and bond water at 100 °C Babalola et al. (Babalola et al., 2021), dehydration of C-S-H at 560 °C, the mismatch in the thermal expansion of cement paste and aggregate, which create micro-cracks at interfacial transition zone Ma et al. (Ma et al., 2015). These cracks weaken the aggregate interlocking Schneider (Schneider, 1988). Whereas the interfacial shear strength of specimen N-FSP decreased gradually with increasing temperature, the shear strength was about 96.6%, 89.3%, and 81.3% of that at ambient temperature for 200, 400, and 600 °C. The results are relevant to Abo Sabah et al. (Abo Sabah et al., 2019). This improvement is due to the increase in the temperature up to 200 °C made as autoclave and accelerated the hydration process of the anhydrate cement and FA particles at the interfacial zone and produced additional silicate hydrate gel, which filled the void in this area and consequently increase adhesion as observed by Gao et al. (2019) and Ghazy et al. (2022). In general, the residual interfacial shear strength of specimen N-FSP was 9%, 25%, 87.5%, and 114% higher compared to specimen N–N at temperatures of 25, 200, 400, and 600 °C, respectively. This increase was because SF plays an evident role in bridging the crack Shang et al. (Shang et al., 2021) and increasing the mechanical interlocking Alimrani and Balazs (Alimrani & Balazs, 2020). Moreover, Fig. 10b indicates that specimen N-FSP confirmed a slight increase in the fracture energy with increasing temperature and recorded an increase of 15% after exposure to a temperature of 600 °C, while specimen N–N exhibits a significant decrease in the fracture energy. This behavior proved the significant role of using SF fiber in enhancing the ductility of the specimen after exposure to elevated temperature.

3.5 Effect of Time Exposure

The composite push-off specimen N-FSP was used to study the effect of different time exposures of 30, 60, 90, 120, and 180 min at a temperature of 600 °C on the residual interfacial shear properties, as depicted in Fig. 11. From the results, it can be noticed that by increasing time of exposure the specimen N-FSP exhibits degradation in its interfacial shear strength with slow rate up to 90 min, the interfacial shear strength was 95%, 91%, 81.3% of the initial shear strength at room temperature for 30, 60 and 90 min. The results agree with Mathews et al. (2021). After 90 min, the shear strength decreased sharply to nearly 50% of that at room temperature. Because increasing exposure time results in microcracks, the concrete becomes softer and loses its mechanical properties. The same trend was observed in the initial stiffness, which recorded 82%, 77.7%, 59.7%, and 39.5% of the initial stiffness at room temperature for 30, 60, 90, and 120 min, respectively. This proved that the concrete mechanical properties at the interface influence the interfacial shear performance. On the other hand, the fracture energy observed marginally increases with a time increase up to 90 min to reach 15% higher than that of the initial temperature. At 120 min, the specimen was softer and brittle, significantly deteriorating the shear strength behavior. After exposure to a temperature of 600 for 180 min, the specimen showed explosive spalling. This is attributed to the SF bridging the cracks and preventing escaping water evaporation, as motioned before Algourdin et al. (2020).

3.6 Effect of Concrete Type

Specimens N–N, N–H, and N-F were used to observe the effect of concrete type on the interfacial shear strength after exposure to an elevated temperature of 600 °C for 90 min; the interfacial shear strength was plotted in Fig. 12. From the results, it can be observed that the concrete type has a significant effect on the interfacial shear strength at ambient temperature and after exposure to elevated temperature, where the enhancement ranged between 15 and 20% higher than that of specimen N-N₂₅ at room temperature. Similar findings were identified by Júlio et al. (2006); Momayez et al., 2004), whereas increased compressive strength of post-cast concrete was attributed to improved interfacial properties and increased bond strength. The specimen N-F₂₅ showed the highest shear strength due to the presence of FA, which consists of SiO₂ and Al₂O₃ that react with Ca(OH)₂ and result in additional C-S–H and fill the pores at the interface and improve the interface of the existing concrete, as discussed by Gao et al. (Ghazy et al., 2022). After 600 °C, the composite specimen N-F₆₀₀ displayed a shear strength of 1.83 MPa, which was 63.5% of its initial shear strength with an increasing ratio of 60.5% than that of N-N₆₀₀. The thermal stability of FAC at elevated temperatures increases the thermal resistance of the transition zone to high temperatures, which improves the shear strength of specimen N-F than N–N.

On the other hand, specimen N-H₆₀₀ showed more retreating in shear strength after exposure to a temperature of 600 °C, which was 49% of its initial shear strength but stayed higher than that of N-N₆₀₀ by approximately 19%. The results agree with Chen et al. (2023). The difference in the thermal expansion between NSC and HSC contributes to developing microcracks at the interfacial zone, consequently reducing the bond between two concrete parts, as observed by Ghazy et al. (2023). Furthermore, compared with monolithic specimen N₆₀₀, the shear strength was 1.5 lower and 32.6% higher for specimens N-H₆₀₀ and N-F₆₀₀, respectively.

3.7 Effect of Fiber Type

For monolithic FSP, the interfacial shear strength was 44.8% and 164.5% higher than that of monolithic N at room temperature and after exposure to a temperature of 600 °C for 90 min. The results agree with Alimrani and Balazs (2020). This is due to the fiber inclusion often made as shear connectors across the interface, resulting in delayed crack propagation and improved resistance to fracture growth (Zanotti et al., 2014). The interfacial shear strength of composite specimens N-FS₆₀₀, N-FP₆₀₀, and N-FSP₆₀₀ after exposure to an elevated temperature of 600 °C for 90 min are presented in Fig. 12. The results demonstrated that fiber plays an important role in the interfacial shear strength after exposure to elevated temperature. The composite specimen N-FSP displayed the highest interfacial shear strength with an increasing ratio of 114% compared to specimen N-N₆₀₀, while the improvement for N-FS₆₀₀ reached 68.8%. The improvement in the interfacial shear strength is due to the use of fibers, which often improve the ability of the concrete to resist fracture growth, reduce overlay shrinkage, and reduce porosity close to the interface, as discussed by Daneshvar et al. (Daneshvar et al., 2022). On the contrary, the incorporation of PP fiber had adverse effects on the shear strength after exposure to elevated temperatures, where specimen N-FP₆₀₀ displayed the least shear strength; it was 15.6% lower than that of N-N₆₀₀. The same behavior was observed by Alimrani and Balazs (Alimrani & Balazs, 2020). The deterioration in the interfacial shear strength of specimen N-FP₆₀₀ is due to the melting of PPF at elevated temperatures and the creation of more voids. This resulted in the weakening of the concrete microstructure and the degradation of the adhesive bond properties at the interface.

4 Discussion

For more observations, the interfacial shear strength in this study and previous studies conducted by (Abo Sabah et al. (2019); Haido et al., 2021; Gao et al., 2019; Chen et al., 2023) were presented in Fig. 13 as relative interfacial shear strength, which equals the interfacial shear strength at elevated temperatures related to that at room temperature. The interfacial shear strength was obtained for rough surface conditions in this comparison. From the outcome of the figure, it can be noticed that the concrete compressive strength has a relevant effect on interfacial shear strength, where composite specimens with HSC showed a significant deterioration as compared to those with NSC. This is due to the dense microstructure of HSC, which prevents escaping the vapor water, resulting in more cracks and deteriorating the interfacial bond strength, as observed by Chen et al. (2023). Also, it can be found that the interfacial shear strength improves with including fibers. The fiber bridges the crack and delays crack propagation, leading to increased interfacial shear strength. Moreover, using supplementary cementitious materials can enhance the interfacial shear strength at elevated temperatures.

5 Finite Element Analysis

To simulate and analyze the impact of various time and temperature exposures on the interfacial shear strength of composite specimen N-FSP, a 3D finite element model of a composite push-off specimen was developed. Fig. 14 displays the dimensions of the push-off specimen used to mimic the experimental specimen. Two models were used to predict the impact of temperature on the interfacial shear strength. The first model was the heat transfer model, which was used to determine the temperatures at each node. The second model is the structural model for calculating the interfacial behavior after heating, and it takes the nodal temperatures calculated by the heat transfer model as an input.

Fig. 14

FE model of composite push-off specimen; a concrete parts, b reinforcement model, c boundary conditions

The FE of the composite push-off specimen and the boundary condition are depicted in Fig. 14. The heat transfer model for the NSC and FSP parts employs a DC3D8 eight-node linear heat transfer brick. The structural model employs C3D8R bricks with eight nodes, decreased integration, and an hourglass-shaped control element. The reinforcement was simulated using a heat transfer model comprised of a 2-node heat transfer link (DC1D2) and a structural model comprised of a 2-node linear 3D truss (T3D2). NSC and FSP parts were modeled using a 10 mm mesh size. An embedded region model was utilized to simulate the bond between reinforcement and concrete, where concrete was chosen as the host region and reinforcement as the embedded region.

5.1 Interfacial Properties

According to experimental results, the failure occurred by crushing NSC; thus, the traction separation model produced by ABAQUS (ABAQUS Manual, 2013) was used to simulate the vertical shear plan. The traction separation model is characterized by the initial stiffness K (elastic linear part), the point of maximum shear ${\tau }_{max}$ and corresponding slip ${\delta }_{t}^{0}$ (point of damage initiation), fracture energy G and failure slip ${\delta }_{t}^{f}$ (damage evaluation) as shown in Fig. 15. These parameters were taken from experimental work results (see Table 6). According to Farouk et al. (2022), softening degradation was evaluated using the linear softening law. Furthermore, it is expected that only the horizontal contact surface will experience friction; thus, a penalty tangential model with a friction coefficient of 0.3 was used, according to Zhang et al. (2020c). Thus, in accordance with Farouk et al., the FSP part surface was chosen as the master surface, and the NSC part surface as the salve surface (Farouk et al., 2022).

Fig. 15

Cohesive element parameters

5.2 Concrete Constitutive Model

As presented in Table 8, the thermal parameters for NSC and FSP needed by the heat transfer model, such as thermal conductivity, specific heat, and density, are obtained from Zalhaf (2022). For the purpose of simulating the nonlinear behavior of NSC and FSP upon exposure to elevated temperature, the concrete damage plasticity model (CDP) was included in the structural model. For FSP, the relationships presented by Li and Kodur (1996) and Lok and Xiao (1998) are used to represent the uniaxial compressive stress–strain curve and the tensile stress–strain curve after exposure to various temperatures, as shown in Fig. 16b, d. Whereas EN1992-1–2 (Eurocode2., 2004) and Massicotte et al. (1990) display the stress–strain curves for NSC under compression and tension, respectively, as in Fig. 16a, c. The modulus of elasticity for NSC and FSP were calculated using correlation $E=4700\surd {f}_{c}$, where ${f}_{c}$ is the concrete compressive strength according to ACI 318–19 (Committee, 2019). The coefficient of deterioration in the modulus of elasticity mentioned by EN 1992–1-2 (Eurocode2., 2004) and Khaliq and Kodur (2011) were used for calculating the deterioration in the modulus of elasticity of NSC and FSP, respectively. Reinforcement rebar temperature impacts on thermal and structural properties were simulated employing EN 1993-1-2 (2005). The steel yield stress and its modulus of elasticity are taken as 360 MPa and 200 GPa, respectively. The Poisson’s ratio is taken as 0.2 and 0.3 for concrete and steel materials, respectively, according to EN 1992–1-2 (Eurocode2. 2004). Depending on the experimental observation, no spalling occurred for composite specimen N-FSP at different temperatures and different times up to 120 min, so the spalling is ignored in the FE model.

Table 8 Thermal properties of concrete mixes

Fig. 16

Stress–strain curves for NSC and FSP after exposure to temperature

The shear strength–slip curves obtained from the FE model under different temperatures and different times of exposure are plotted in Fig. 17 as a comparison with experimental curves. As indicated in the figure, the FE model gave results consistent with experimental results. It demonstrates that using the cohesive model to simulate interfacial properties after high temperatures can improve FE modeling. Table 9 displays the FE model-derived interfacial shear characteristics. Maximum shear strength predictions have an accuracy of 0.95–1.03. There was also a discrepancy of 0.92–1.07 between the FE stiffness (${K}_{FE}$ and the experimental stiffness ${K}_{Exp}$.

Fig. 17

Predicted shear strength–slip curve versus the experimental results. a Different temperatures. b Different times of exposure

Table 9 Predicted interfacial shear properties

Fig. 18 shows the failure pattern from the FE model and experimental test. From the Tension damage, it can be observed that the failure occurred by NSC crushing. Furthermore, Fig. 18b demonstrates that the stress concentration is at the shear failure plane.

Fig. 18

Finite element results

6 Conclusion

The main aim of this paper was to investigate and analyze the interfacial shear behavior between two layers under the influence of a set of parameters, including temperature, time exposure, concrete type, and fiber type by employing a Z-shape push-off test. The test consists of two parts with different ages: normal strength concrete (NCS) and high-performance concrete (HPC). Finally, a finite element (FE) model was proposed to depict and validate the experimental work. Following is a summary of this study's key findings:

• 1) Temperature degree and exposure time had adverse effects on the interfacial shear strength.

• 2) Specimen N-FSP performed better than specimen N–N at different temperature exposures. The interfacial shear strength increased by 24%, 27.7%, 87.5%, and 114% at 25, 200, 400, and 600 °C, respectively.

• 3) Despite the fracture energy of composite specimen N-FSP increased with increasing temperature, it decreased for specimen N–N. This proves the importance of using SF fiber in improving the ductility of specimens at high temperatures.

• 4) Fiber types significantly influence interfacial shear strength. Including combined fiber (SF + PPF) improved the interfacial shear strength by 114% compared to composite specimen N–N after exposure to a temperature of 600 °C. In other words, using PPF negatively affected the interfacial shear strength, which recorded only 84% of composite specimen N–N.

• 5) Enhancement of interfacial shear strength at elevated temperature achieved using supplementary cementitious material, where composite specimen N-F incorporating 30% FA showed improvement by 60.5% higher than N–N

• 6) While HSC improves the interfacial shear strength of push-off specimen at room temperature, it showed a significant deterioration after exposure to elevated temperature, which retains 49% of its interfacial strength at room temperature.

• 7) In general, the monolithic specimens N and FSP had the maximum shear strength compared to the composite specimens.

• 8) The finite element model of the interfacial shear strength of composite specimen at elevated temperature using cohesive elements gave a good accuracy with test results.