Experimental research on bonding mechanical performance of the interface between cementitious layers

: Interfaces of cementitious layers have widely existed in construction projects, and they are the weakest part of the whole building. In this article, laser scanning and ultrasonic pulse, splitting tensile, and semi-disc fracture tests were carried out to study the bonding performance of cementitious layers. Di ﬀ erent performance metrics, such as splitting tensile bond strength, ultrasonic pulse velocity


Introduction
Interfaces between cementitious layers occur widely in hydraulic, civil, and repair engineering, such as the layers of roller-compacted concrete (RCC) dams [1] and rollercompacted concrete pavement (RCCP) [2,3], the interface between old and new concrete [4], and the interface of printed concrete [5,6].The interfaces of the concrete layer poured at different times were the weakest parts of the overall structure.Therefore, it is important to investigate the bonding performance at the weakest interfaces.
The morphologies of the interfaces were studied at different levels to observe the interface's multiscale characteristics [7], which can unveil the structures of the interfaces.Li et al. [8] used scanning electron microscopy (SEM), energy dispersive spectroscopy, and splitting tensile tests to study the major properties of new-to-old concrete and found that the interface was porous and rich in large crystals.Xie et al. [4] divided the bonding interface into penetrating, strongly affected, and weakly affected layers and found that the interface was porous and abundant in Ca (OH) 2 .Qin and Xu [9] investigated concentrated initial defects in layer interfaces using a computer tomography method.
The mechanical performance of the interfaces was studied by some researchers.Daneshvar et al. [10] summarized the parameters affecting the bond strength and analyzed the influence of the different overlays on the interfacial bond strength.Ling et al. [11] studied the relationship between the compressive, splitting tensile, and shear strengths of RCC specimens with different layers.Qian and Xu [12] studied different layer interfaces with different time intervals by performing macro strength, permeability, and microstructure tests and found that longer interval times lead to weaker bond strength.Feng et al. [13] found that there was a linear relationship between the area of the substrate adhered to the ultra-high-performance concrete overlay and the substrate roughness had a significant influence on the bond strength.In contrast, the steel fiber type had only a slight influence on bond strength.
Micro-cracks and cavities reduce the load-bearing capacity and lead to failures of the concrete structure [14].There are more micro-cracks and cavities in the concrete layers [8,9], and hence, the shear-and tension-resistance ability of the interface between the concrete layers should be analyzed.Maier and Lees [15] summarized the parameters affecting the interlayer fracture behavior.Currently, there are some tests for the shear strength of a layer, such as the direct shear test [16][17][18], Z-type specimen test [19], semicircular bend (SCB) specimen test [20], and double-edge notched plate test [21].The tension fracture properties can be investigated using a three-point bending test [22], wedge splitting test [15,23], Brazilian disc test [24], and SCB specimen test [20].Among these methods, the SCB specimen type could be adapted to research the shear and tension fracture properties of interfaces with less materials usage and a simpler manufacturing process.
In this study, the mesoscopic and microscopic structures of interfaces between concrete and mortar layers were studied by scanning topography observation and SEM.The bonding characteristics of the interfaces between the cementitious layers were tested using splitting tensile and ultrasonic pulse tests.The shear and tension bonding properties of the interfaces were comprehensively investigated using an SCB specimen test.Subsequently, the relationships between bonding parameters are discussed.Therefore, the result suggests the bonding performance study of the interfaces between the cementitious layers.

Materials
Ordinary Portland cement manufactured by Jinyu was used in the experiment; its composition is shown in Table 1.Fly ash, sand, and aggregates were obtained from the Baihetan Dam Project and carried downstream of the Jinshajiang River.The mix proportions of the mortars are shown in Table 2.The mortar setting times were obtained from the penetration resistance test according to ASTM C403 [25], and a plastic sheet was paved on the surface of the mold during the test to prevent moisture loss.The initial setting time was 5 h 54 min, whereas the final setting time was 8 h 16 min.
The mix proportions of the concrete are listed in Table 3.The aggregate was sieved with a 16 mm square hole sieve to adapt the dimensions of the SCB specimens.Type-I fly ash and JM-II water reducers were used to prepare the concrete.The setting times of the concrete were tested such that the initial setting time was 12 h, while the final setting time was 16 h.

Methods
Mortar and concrete specimens with a layer interface were prepared, as shown in Figure 1.In this process, the dimensions of the mold were 100 mm cubes.First, the mixture was poured into half the height of the mold, and then, the surface of the mixture was covered with a plastic sheet to prevent moisture loss.After specific time intervals, the plastic sheet was removed, and the upper half height of the mold was poured.Mortar specimens were prepared at 0, 5, 10, and 15 h at four different time intervals, while concrete specimens were prepared at 0, 10, 20, and 35 h at four different time intervals.When pouring the upper layer, 0.5% by weight of dye was added to the raw materials before mixing to distinguish the upper and lower layers for structural observation.The specimens with a certain strength were demolded after 1 day set in molds and then cured under the standard condition of 20 ± 2°C temperature and relative humidity not less than 95% until 28 days for tests.

Specimen preparation
The specimen for the scanner observation was drilled a hole with a 30 mm diameter along the layer interface of a 100 mm cube using a diamond drill of 38 mm diameter and then cut to approximately 15 mm in thickness.The observed surface was then ground and polished for scanning.The sample preparation process is illustrated in Figure 2.Then, the specimen from the scanner observation was cut to dimensions of approximately 10 mm × 5 mm × 2 mm for the SEM test.
The dimensions of the specimens for the splitting tensile and ultrasonic pulse tests were 100 mm × 100 mm × 100 mm, and three specimens of every interval group were prepared.The specimen for the SCB fracture test was drilled along the layer interface, and a cylinder with a diameter of 75 mm was obtained.Then, the cylinder was cut into thin circular plates with a thickness of 30 mm, and the circular plate was cut into a semi-circular plate.Finally, a crack with a length of 15 mm was fabricated.The preparation process for the SCB specimens is shown in Figure 3. Three specimens of each interval group and mode type were prepared for the SCB test.

Scanner topography observation
An HP LaserJet Professional M1136 scanner was used to observe the surface of the specimen.The maximum scanning resolution was 1,200 dpi.A picture of the layer interface was obtained, and the interface was drawn by a line.Bonding mechanical performance of the interface between cementitious layers  3 Because of the layered pouring, the layer interface is not a straight line.The solid line in Figure 4 shows the schematic of the layer interface.Pcurve is the length of the interface curve, and Acurve is the area surrounded by the continuous line of the interface and the dotted line through the highest point.h is the altitude difference from the peak to the valley on the solid line.Pcurve, Acurve, and h can be used to demonstrate the bond properties of the interface between layers.

SEM test
Before conducting the SEM test, the tiny sample was dried with a 40°C dryer for 1 day, and then the surface was cleaned with an ear wash ball and plated with platinum before being observed with SEM.After several preparation procedures, the specimens were placed under an emission SEM to obtain SEM images of the surface around the interfaces.

Ultrasonic pulse test
Ultrasonic pulse tests can use ultrasonic waves to analyze the received ultrasonic waves to represent the bond properties of the layer [9,12].Schematic and physical pictures of the ultrasonic pulse test are shown in Figure 5.A transmitting wave (250 kHz, 2 Vpp) was generated from the wave generator and amplified using a power amplifier.Two probes were placed on opposite surfaces parallel to the interface of the specimen using an ultrasonic couplant.The wave transmitted from the transmitting probe was weakened through the specimen and then captured by the receiving probe.The receiving and transmitting waves can be shown on the screen of the oscilloscope, as shown in Figure 6.From the two waveforms, the initial vibration point and first wave amplitudes of the transmitting and receiving waves were obtained and analyzed.The wave velocity and relative amplitude of the first wave were acquired to demonstrate the bond properties of the layer.

Splitting tensile test
A 100 mm cube was set between the two steel plates for the splitting tensile test, and two steel square strips of 5 mm ×  5 mm were placed between the steel plates along the interface, as shown in Figure 7.The yellow line indicates the location of the interface layer.The formula for splitting tensile strength is described in equation ( 1): where f ts is the splitting tensile strength (MPa), P is the maximum failure load (N), and A is the cross-sectional area of the specimen (mm 2 ).

SCB specimen test
Concrete can fracture in modes I (tension), II (shear), and III (antiplane shear) [26,27].The fracture problems of modes I and II are the most frequent occurrence and damage [28].A four-point bend beam is often used for mode I and II fractures in concrete [28,29], which requires complex specimen preparation and massive material usage.SCB specimens have also been tested for both mode I and mode II fractures of rock [30,31], asphalt mixtures [32], and concrete [33].SCB specimens with less material usage are easier to form and test than four-point bending beams.The specimen can be loaded from mode I to mode II by changing the crack angle or loading distance.To simplify specimen preparation, the same specimen was used to test the mode I and mode II fracture toughness of the interface by changing the loading distance of the loading rollers, as shown in Figure 8.A symmetric loading method, as shown in Figure 8(a), was used for the mode I fracture, whereas an asymmetric loading method was used for the mode II fracture, as shown in Figure 8(b).
The stress intensity factor could be calculated by equation ( 2

Laboratory test
Before loading, the loading positions of the specimens are marked in red short lines according to S1 and S2 of mode I and II fracture tests.Mode I and II fracture tests were conducted, as shown in Figure 11, and two steel rollers were placed on the lower loading plate to control the loading span.The indoor SCB specimen test was carried out using a WDW-20 kN electronic universal testing machine for displacement control with a loading speed of 0.1 mm/ min.In the semi-disc test, the test piece was small, and the critical failure load P c was substituted into equation ( 2) to calculate the fracture toughness, i.e., the critical stress intensity factor [34], of the corresponding cementitious layers.
6 Experimental results 6.1 Results of the layered mortar

Micromorphology
The surface morphology of the mortar layer structures at different intervals could be obtained using a scanner.The images are shown in Figure 12.The interfaces between the upper and lower layers were evident because of the dyed-up layer.The schematics of the interfaces are shown in Figure 4, and the parameters of the interfaces within the 18.15 mm length range are given in Table 4.The larger the interval time, the smaller the parameters of the interface curve, and the flatter the interface.Therefore, shortening the interval time can increase the interpenetration and mixability of the upper and lower layers of the mortar.Monolithic mortar, rough interface, and flatter interface were chosen to analyze the microstructures of the different interfaces.Micro-topographic images of the layered mortar at 0, 5, and 15 h intervals are shown in Figure 13.The left images in every row are topographic images with a magnification factor of 50.The middle images in every row are magnified images of the red rectangles in the left images, and the right images in every row are magnified images of the red rectangles in the middle images.Two strip marks were engraved to sign the positions of the interfaces on the right side of Figure 14(d) and (g).The three right images, Figure 14(c), (f) and (i), show the images with the longer time interval, larger the pore area, and larger the pore diameter.

Ultrasonic pulse test
To compare the significance of the ultrasonic test parameters changing with the interval time, the normalized  parameter values were used for a comparative analysis according to the parameters of the monolithic mortar.The average normalized ultrasonic velocity and average normalized first wave amplitude ratio (received wave to transmitted wave) of the mortar interface are shown in Figure 14.The ultrasonic wave velocity passing through the layered mortar was insensitive to the interval time.
The first wave amplitude ratio was very sensitive to the interval time between the layers, which was testified in the literature [12].The amplitude of the first wave was very sensitive to the presence of the interface.Before the initial setting time, the first wave amplitude ratio decreased significantly with increasing interval time.After the final setting time, the first wave amplitude ratio decreased slowly with increasing interval time.

Splitting tensile test
The splitting tensile strength decreased almost linearly with an increase in the interval time between 0 and 10 h, as shown in Figure 15.However, the tensile strength decreased less between 10 and 15 h after the final setting time of 8 h 16 min.As the interlayer interval time increased, the tensile strength between concrete layers gradually decreased.From the split surface condition, the larger the interval time, the rougher the split surface.

SCB specimen test
The fracture test results for the layered mortar semi-discs are shown in Figure 16.K Ic decreased almost linearly with an increase in the interval between 0 and 10 h.K Ic decreased slightly during the 10 and 15 h intervals after the final setting time.K IIc decreased significantly during the 0 and 5 h intervals, while the change between the 5 and 10 h intervals was smaller.
The interlayer failure surface of the layered mortar in the SCB tests is shown in Figure 17.The failure surfaces of the monolithic mortar were uneven under both mode I and mode II loading.When the monolithic mortar underwent mode II loading, the crack in the specimen was deflected toward the S1 loading side, and the crack was finally deflected toward the uploading point.When the interface of the mortar underwent mode I loading, the cracks of the specimens extended vertically upward along the crack tip at the interface without any deflection.When the interface of the mortar underwent mode II loading after the final setting time, the cracks of the specimens also extended vertically upward along the crack tip at the interface without any deflection.The failure surface of the interface with a 5 h interval was slightly uneven under mode II loading.Irrespective of the type of loading mode, the larger the interval time, the larger the pores in the failure surfaces of the interface.

Results of the layered concrete 9.1.1 Meso-morphology
Similar to the mortar interface, the scanner images of the layered concrete test piece are shown in Figure 18.The parameters of the interface curve within 18.15 mm are listed in Table 5.Before the initial setting time (12 h), at the 10 h interval, the upper and lower concrete layers had the greatest influence, and the aggregates of the upper layer interpenetrated within the lower concrete layer.After the final setting time (16 h), the interpenetration between the upper and lower layers decreased rapidly at the 20 h interval, especially at 35 h.The parameters decrease as the interlayer interval time increases.The area (Acurve) and height difference (h) of the layered concrete changed significantly with the interval time.

Ultrasonic pulse test
Like the interface of the mortar, the average normalized ultrasonic velocity and average normalized first wave amplitude ratio of the layered concrete are shown in Figure 19.The average normalized first wave amplitude ratio of the layered concrete was also more sensitive than the average normalized ultrasonic velocity to changes in the interval time.The first wave amplitude ratio decreased significantly before the initial setting time with the increase in the interval time.It decreased relatively  Bonding mechanical performance of the interface between cementitious layers  9 slowly with the increase in the interval time after the initial setting time.The first wave amplitude ratio increased slightly at 35 h intervals, which might be because more energy of the ultrasonic wave was needed to pass through the concrete specimen of the long interval interface, and it was difficult to obtain an accurate first wave amplitude of the receiver wave.Therefore, a larger-energy ultrasonic wave should be used to penetrate the layered concrete specimen.

Splitting tensile test
The splitting tensile strength of the concrete layer decreased with increasing interval time, as shown in Figure 20.After   the final setting time, the splitting tensile strength of the concrete layer changes relatively slowly.In addition, many aggregates broke on the failure surface of the splitting tensile test, and the area of the broken aggregate decreased as the interval time increased.The failure surface of the interface at 30 h interval was flat, without broken aggregates.

SCB specimen test
The fracture test results for the layered concrete semi-discs are shown in Figure 21.K Ic decreased almost linearly with an increase in the interval between 0 and 20 h.K Ic decreased relatively slowly between 20 and 30 h after the final setting time (16 h).The change in K IIc with the interval time was the same as that of K Ic , but K IIc of the layered concrete was more sensitive to the change in interval time.Compared with the results of the layered mortar, the presence of the aggregate affected the mutual penetration of the upper and lower layers and changed the sensitivity of the bonding properties.The rule of the failure surface to the interval time of the layered concretes was basically the same as that of layered mortars, but the failure surface of the layered concrete was rougher than that of layered mortars owing to the existence of aggregates.

Discussion
13.1 Correlation between fracture parameters and meso-morphology parameters Figure 22 shows that the interlaminar meso-morphology parameters had a good correlation with mode II fracture toughness for both the layered mortar and concrete.The correlation coefficient between K IIc and h was not less than 0.9846.However, the correlation between K Ic and h is not general, and the correlation coefficient of the layered concrete is 0.9997, whereas that of the layered mortar is 0.5778.As K IIc is related to the friction of the interface and the height difference h represents the roughness of the interface, K IIc and h are more closely related than K Ic and h.

Correlation between fracture parameters and amplitude ratios
Figure 23 shows that the correlations between K IIc and the amplitude ratio were higher for both the layered mortar and concrete, and the correlation coefficient was not less than 0.9590.The relationship between K Ic and the     amplitude ratio was relatively lower than that with K IIc ; however, the correlation coefficient was not less than 0.8709.Considering the layered concrete, the fracture toughness of modes I and II was highly correlated with the amplitude ratio, and the correlation coefficient was not less than 0.9832.

Correlation between fracture parameters and splitting tensile strength
Figure 24 shows that there was a strong correlation between the splitting tensile strength and K Ic of the interface between the layered mortar and concrete, with a correlation coefficient of not less than 0.9744.K Ic could be predicted by tensile strength [35], and splitting tensile strength is the ability to resist tensile failure.The correlation between the splitting tensile strength and K IIc was relatively weaker than that with K Ic of the layered mortar.The correlation coefficient between the splitting tensile strength and K IIc of the layered mortar was only 0.8098.

Conclusions
In this study, the interfaces of layered cementitious materials with different interval times were investigated using mesoscopic structures, ultrasonic pulse tests, splitting tensile tests, and SCB specimen tests.The bonding properties of the interfaces were then determined and evaluated.1.As the time interval increased, the total area of the pores and the range of weak substances at the interface of the layered mortar increased.The interpenetration of the upper and lower layers of the layered mortar and concrete weakened with an increase in the interval time.
Owing to the different coagulation properties of the layered mortar and concrete and the presence of coarse aggregates in the layered concrete, the interpenetration between the layered concrete was greater at the same intervals.2. The ultrasonic velocity was less sensitive to the interval of the interface than the first wave amplitude ratio of the received and transmitted waves in the ultrasonic pulse test.Therefore, the first wave amplitude ratio can be used to study the characteristics of the interface between the layered cementitious materials.3. K IIc and K Ic of the SCB specimen test decreased with an increase in the interval of the interface between cementitious layers.K IIc was more sensitive to the interval than K Ic of the layered concrete, whereas K Ic was more sensitive to the interval before the initial setting time than K Ic of the layered mortar.4. The results of the ultrasonic test and the splitting strength are correlated well with the fracture toughness parameters of the layered cementitious material.K IIc of the layered cementitious material had a strong correlation with the meso-morphology parameters, which were related to the toughness of the interface.K Ic did not have a clear correlation with the meso-morphology parameters.Therefore, meso-morphology tests, ultrasonic pulse tests, and splitting tensile tests can be conducted to determine the fracture toughness of the interface between cementitious layers.

Figure 1 :
Figure 1: Pouring process of the specimen.

Figure 2 :
Figure 2: Process of specimen preparation for scanner topography observation: (a) drill along the layer interface, (b) cut to about 15 mm in thickness, (c) the thin plate, and (d) grinding and polishing.

Figure 3 :Figure 4 :
Figure 3: Preparation process of SCB specimen: (a) drill along the layer interface, (b) thin circular plate, (c) prefabricated crack, and (d) semicircular specimen with load point mark.
where K I and K II are the stress intensity factors for mode I and mode II (MPa m 1/2 ), respectively; P is the load (N); R is the radius of the specimen (mm); t is the thickness of the specimen (mm); a is the crack length (m); Y I and Y II which are functions of the specimen geometry and loading condition, are the geometry factors for modes I and II, respectively; and S1 and S2 are the loading distances of modes I and II.In the mode II fracture toughness test, S2 should be determined via numerical simulation to ensure mode II failure.

Figure 8 :
Figure 8: Semi-circular specimens: (a) loading method of mode I fracture and (b) loading method of mode II fracture.

Figure 6 :
Figure 6: Waveform of the transmitting and receiving wave.

Figure 10 :
Figure 10: Variation of the geometric factor of fracture toughness: (a) Y I -S2/R and (b) Y II -S2/R.

Figure 11 :
Figure 11: SCB test loading diagram: (a) mode II fracture loading test and (b) mode I fracture loading test.

Figure 14 :
Figure 14: Result of ultrasonic pulse test of the layered mortar with different interval times.

Figure 15 :
Figure 15: Splitting tensile strength of the layered mortar.

Figure 16 :
Figure 16: Result of SCB test of the layered mortar.

Figure 17 :
Figure 17: Failure surface of SCB specimens of the layered mortar.

Figure 19 :
Figure 19: Result of the ultrasonic pulse test of the layered concrete with different interval times.

Figure 20 :
Figure 20: Splitting tensile strength of the layered concrete.

Figure 21 :
Figure 21: Result of the SCB test of the layered concrete.

Table 1 :
Chemical composition of cement

Table 2 :
Mix proportion of mortar

Table 3 :
Mix proportion of concrete

Table 4 :
Topography parameters of mortar interfaces

Table 5 :
Topography parameters of concrete layer