Shear Bond Research Articles

Structural behavior of Lightweight and High strength Layered Hollow Core Slabs

A new technique of Layered Hollow Core Slab (LHCS) has been used to obtain a slab with an optimum weight-to-strength ratio. Specimens with a 90 mm top layer of High Strength Concrete (HSC) and a 90 mm bottom layer of Lightweight Aggregate Concrete (LWAC) were examined. Nine full-scale slabs with dimensions of 1600* 450* 180 mm were tested under a 4-point loading test. The %core, a/d, RFT ratio, and connection method were the different studied parameters. A push-out test was conducted on triplet specimens to study the bond strength at the interface between HSC jacket and LWAC cubes using bond agent material or shear dowels, or without treatment, to determine which method of them is suitable for connecting the two layers of the tested slabs. Load, deflection, ductility, strain, crack pattern, and mode of failure were studied. The results indicate that ultimate strength is enhanced with decreasing a/d and %core and with an increasing RFT ratio of the LHCS specimens. Using shear dowels ensures an efficient bond between the two layers of the tested slabs. ANSYS program used for modelling the slab. The numerical study accepted the experimental data with a variation of less than 10% for all slabs.

Effect of hygrothermal acid rain environment on the shear bonding performance of CFRP-concrete interface

In this study, forty-five carbon fiber-reinforced polymer (CFRP)-concrete single shear specimens were designed to investigate the effect of hygrothermal acid rain environment. At first, the hygrothermal acid rain environment was simulated through a mechanical high-temperature dry-wet cycle and by artificially preparing acid rain solution with a pH value of 1.5. Subsequently, the effects of concrete strength and corrosion times on the interface failure mode, bond shear capacity, ultimate displacement, load–displacement behavior, and strain distribution in bonding interval were studied through the single shear test of CFRP-concrete. Then, the interface constitutive relation model was built based on the influence coefficient of hygrothermal acid rain. Finally, a reference method was proposed to divide the corrosion degree in hygrothermal acid rain environment, and the interface failure mechanism was expounded using a scanning electron microscope (SEM) for detection. The results indicated that the interface bonding performance was enhanced with the increase of the concrete strength, and the interface peeling position tended to change to the adhesive layer. The interfacial bonding performance increased first and then decreased with the increase of the corrosion times. When the corrosion number reaches ten cycles, the interfacial bond performance gets the best, the interfacial peel load and ultimate displacement of the three types of strength concrete(C30\\C40\\C50) were examined as 3.04 %, 3.50 %, 5.78 %, and 0.50 %, 0.49 %, 0.95 % higher than those of the uncorroded specimens, respectively. The SO42- ions in the acid rain invaded the surface of the concrete to synthesize an expansive substance CaSO4·2H2O, thus temporarily enhancing the interfacial bonding performance at the early corrosion stage. The load–displacement curve comprised four stages, including rising, oscillating, strengthening, and falling. In this study, an interface constitutive model was proposed based on the influence coefficient of hygrothermal acid rain, which is well consistent with the existing experimental data and exhibits high accuracy and safety. The relevant research results can provide theoretical support and design guidance for CFRP reinforcement projects in high-humidity and high-heat acid rain areas.

Agro-waste fiber reinforced mortar for augmenting the performance of masonry structures

The cumbersome nature of unreinforced masonry structures and the minimal tensile strength of the constituent materials make the masonry structure very brittle. As a result, unreinforced masonry structures perform inadequately when subjected to seismic loading. For this reason, the unreinforced masonry structures need to be strengthened so that they can withstand seismic forces. Hence, this study was completed to investigate the effect of coconut fiber on the bond behavior, strength, and ductility properties of masonry wallets. Coconut (coir) fiber was chosen because it has a good tensile strength and it is able to control cracks in the mortar. In the current study, mechanical properties of reinforcing materials were found. Microstructure studies using field emission scanning electron microscopy (FESEM) and X-ray diffraction analysis (XRD) on the control and reinforced mortar were also completed. A total of 116 groups of samples were prepared and tested to evaluate the compressive strength, shear bond strength, tension bond strength, and shear and flexural strengths of masonry. The length of coconut fiber was 15 mm, and three different percentages of fiber content (0.1 %, 0.2 %, and 0.3 % by weight) were considered to determine the optimum percentage or dose of coconut fiber. The test results showed that the incorporation of coconut fiber in the mortar caused a slight decrease in the compressive strength of mortar mix. However, the compressive strength, shear bond strength, tension bond strength, shear capacity, flexural strength, and ductility of masonry increased significantly. The enhanced performance of coir fiber reinforced masonry explores the suitability for the use of coir fiber reinforced mortar in masonry for improved behavior.

On deformation and fracture of PBX simulant employing modified three-body potential peridynamics model with deformation-based failure criteria

The modified three-body potential peridynamics (PD) model is used to study the macroscopic mechanical responses of polymer bonded explosive (PBX) mock materials. Combined with PD functions, under small deformation conditions, bond shear form of the PD model is derived from the micro-potential incorporating modified three-body interaction potential, giving some attention to the applicable condition for its two expression forms, i.e., general form and bond shear form. Subsequently, an improved mode Ⅱ failure criterion considering collective shear deformation of family members is proposed to enhance the deformation-based failure criteria adopted in the present study. A series of PBX simulants associated with elastic deformation and brittle fracture behaviors are carried out by the proposed numerical method. And the numerical results are verified by experimental and other numerical ones published previously, which show that the proposed numerical method is suitable for modeling PBX mock materials.

Dynamic simulation of aircraft electro-impulse de-icing using bond-based peridynamics

Electro-impulse de-icing is a lightweight type of mechanical de-icing system that consumes little energy, provides high reliability, and offers wide application prospects. In this study, the bond-based peridynamics theory was used to simulate the dynamic damage process and evolution law of aircraft electro-impulse de-icing. The ice layer was simplified as a brittle material that satisfies the linear elastic constitutive relation. A numerical analysis model of the ice layer, aircraft skin substrate, and their interface was established to simulate the dynamic responses under a high strain rate. Considering the anisotropy of ice adhesion on the substrate, the interfacial bonds were described as shear bonds and tensile bonds, and the critical elongations of the two were derived. The tensile and shear capacities of the ice on the substrate were then simulated using these critical elongations, and the peeling rates of single ice particles and interfacial ice layers were taken as indexes describing the efficacy of electro-impulse de-icing. The process of electro-impulse de-icing was then analyzed for an aluminum substrate with two adjacent clamped edges. Finally, the results of the peridynamic simulation were compared with those of existing experiments and finite element models to verify the effectiveness of the peridynamic approach.

Mechanical behavior of rotary friction welding joints composed of laminated veneer bamboo substrate and bamboo dowel

Rotary friction welding is a new connecting technique to produce joints by melting and re-solidification processes of substances, which has been pioneered in wooden furniture and structures due to the high interface strength. Bamboo has similar chemical components to wood and is a practical supplement to timber for applications. In this regard, the bamboo dowel was explored in this work to connect laminated veneer bamboo substrates using the rotary friction welding technique. The bond strength and shear strength of the proposed joints were experimentally evaluated by pull-out tests and single-shear tests based on the orthogonal array testing methodology. The effects of pre-drilled hole diameter, rotation rate, welding angle and substrate thickness on the mechanical behavior were investigated. The failure modes and load-carrying capacity were analyzed and discussed. Two failure modes including the pull-out failure and dowel fractured failure in pull-out tests and three failure modes including the pull-out failure, shear failure and hybrid failure of dowels in single-shear tests, were observed. It was found that the bond strength and shear strength of joints were primarily influenced by the pre-drilled hole diameter and the welding angle, respectively. The maximum bond strength of 4.40 MPa and maximum shear strength of 4.24 kN were achieved with the pre-drilled hole diameter of 8 mm and the welding angle of 45°, respectively. Furthermore, the results of scanning electron microscope observation and Fourier transform infrared spectroscopy analysis indicated that the bamboo fibers wound on the dowel surface and the relative contents of lignin increased. The research would provide the theoretical and technical basis for the application of bamboo rotary friction welding.

Hot-Melt and Pressure-Sensitive Adhesives Based on Styrene-Isoprene-Styrene Triblock Copolymer, Asphaltene/Resin Blend and Naphthenic Oil.

Asphaltene/resin blend (ARB) extracted from heavy crude oil was used to modify poly(styrene-block-isoprene-block-styrene) (SIS) to make it an adhesive. There were prepared double and triple mixtures containing 10–60% SIS, 10–40% ARB, and 10–50% naphthenic oil used as an additional plasticizer. The viscoelasticity of the mixtures at 25 °C and 120 °C was studied, their flow curves were obtained, and the temperature dependences of the loss tangent and the components of the complex modulus were measured. In addition, the mixtures were used as hot-melt adhesives (HMAs) and pressure-sensitive adhesives (PSAs) in the shear, peel, and pull-off tests of the adhesive bonds that they formed with steel. Both naphthenic oil and ARB act as plasticizers for SIS and make it sticky. However, only the combined use of ARB and the oil allows for achieving the best set of adhesive properties of the SIS-based mixture. High-quality HMA requires low oil content (optimal SIS/ARB/oil ratio is 50/40/10, pull-off adhesion strength (τt) of 1990 kPa), whereas a lot of the oil is needed to give SIS characteristics of a PSA (SIS/ARB/oil is 20/40/40, τt of 100 kPa). At the same time, the resulting PSA can be used as a hot-melt pressure-sensitive adhesive (HMPSA) that has many times lower viscosity than HMA (13.9 Pa·s versus 2640 Pa·s at 120 °C and 1 s−1) but provides a less strong adhesive bond (τt of 960 kPa).

Flexural Behavior of RC Beams Strengthened with GFRP Laminate and Retrofitting with Novelty of Adhesive Material

Two unique approaches were proposed to strengthen the bond between the glass fiber reinforcement polymer (GFRP) and the RC concrete surfaces. The two bonding materials are epoxy (EP) and geopolymer (GPP) with different ratios of short glass fibers (SGF). The experimental program includes seven reinforced concrete (RC) beams that have the same cross-section (150 mm × 200 mm) and are 1500 mm in length. The first beam is the control beam (B0-Control). The next three beams B1-0-GPP, B2-0.6-GPP, and B3-1.2-GPP have GPP with SGF ratios of 0%, 0.6%, and 1.2%, respectively. The last three beams B4-0-EP, B5-0.6-EP, and B6-1.2-EP have EP with SGF ratios of 0%, 0.6%, and 1.2%, respectively. The results show that the failure loads of beams B1-0-GPP, B2-0.60-GPP, and B3-1.2-GPP are greater than the control beam B0-Control by approximately 20.80%, 25.60%, and 31.40%, respectively, whereas the failure loads of beams B4-0-EP, B5-0.6-EP, and B6-1.2-EP are greater than the B0-Control by approximately 16.90%, 26.90%, and 26.10%, respectively; it is also noted that debonding occurs. In addition to the adhesive material, GPP has a great effect on increasing the beam’s failure load capacity due to the enhanced interfacial bond shear strength. Additionally, a finite-element program ABAQUS is performed to verify the experimental results.

Understanding meso interfacial structures between z-pins and laminates in z-pinned laminates

Z-pin pull-out tests from the laminates were conducted to explain the components of the meso interface structure of z-pin/laminate and the debonding position. Three types of z-pins with different molding qualities were investigated. The results revealed that the z-pin/laminate interface was composed of two meso interface components connected in series, including the interface between the outer layer resin of z-pin and the resin of laminate (Interface Ⅰ) and the interface between z-pin's own fiber and its own matrix resin (Interface Ⅱ). Debonding would occur at the weaker one. The z-pin with the best molding quality possessed the highest interfacial bond shear stress, τd, and the debonding mainly occurred at Interface Ⅰ. Regarding the z-pin with the worst molding quality, its τd was the lower one and the debonding primarily occurred at Interface Ⅱ. While this kind of z-pin exhibited the higher resistance at the final frictional pull-out stage.

Behaviour of EBRIG CFRP sheet-concrete joint: Comparative assessment with EBR and EBROG methods

• This study was the first attempt to evaluate the behavior of EBRIG FRP-concrete joints in comparison with EBR and EBROG joints. • A novel setup was developed to prepare the EBRIG specimens and carry the lap-shear tests on them. • The results including bond strength, failure modes, and other bond properties, confirmed the significant superiority of EBRIG installation method over the EBR and EBROG ones. • PIV technique was adopted to achieve the deformations fields and other bond properties. Grooving methods (GM) have remarkably shown higher efficiency than Externally Bonded Reinforcement (EBR) in anchoring the Fiber Reinforced Polymers (FRP) used for flexural retrofitting concrete structures. They are categorized into two types: Externally Bonded Reinforcement on Grooves (EBROG) and Externally Bonded Reinforcement In Grooves (EBRIG). The behavior of the EBR and EBROG joints were evaluated in many studies, but the behavior of EBRIG ones has not been assessed yet, and the present study is the first attempt to achieve the above aim. Accordingly, 12 concrete prisms were strengthened using one or two-layer of Carbon FRP (CFRP) sheets bonded via the EBR, EBROG, and EBRIG methods and then tested by a single lap-shear setup. In addition to evaluating bond strengths and failure modes, the bond behavior, including load-slip and shear stress-slip curves, and the slip, strain, and shear stress longitudinal profiles were acquired by Particle Image Velocimetry (PIV) technique. The results indicated substantial enhancement in the bond properties of specimens strengthened using grooving methods over the EBR. Besides, the EBRIG joint revealed a higher performance compared to the EBROG joint. For instance, applying the EBRIG method instead of EBROG led to 39% and 120% bond strength enhancement for one and two-layer FRP reinforced specimens, respectively. Additionally, all the EBROG joints failed due to cohesive debonding, whereas FRP sheets either ruptured or deeply debonded in the EBRIG joints reinforced with one or two layers of FRP.

Properties of plybamboo manufactured from two Malaysian bamboo species—

Abstract The aim of the presented investigation was to explore the physical and mechanical properties of plybamboo manufactured from two Malaysian commercial bamboo species – Gigantochloa levis (beting) and Gigantochloa scortechinii (semantan). Three-layer-plybamboo with dimensions of 12 × 350 × 350 mm was fabricated using melamine urea formaldehyde (MUF) as a binder. The moisture content, thickness swelling, water absorption, linear expansion parallel and perpendicular to the grain were done for determination of the physical properties. Whereas, mechanical properties were evaluated via bonding shear strength, modulus of rupture (MOR) and modulus of elasticity (MOE) and compression strength parallel to the grain. Results indicated that semantan plybamboo exhibited excellent mechanical properties along with better bond shear strength than beting plybamboo. MOR and MOE value for semantan and beting plybamboo were 130.86 N/mm2, 19661.30 N/mm2, 115.92 N/mm2 and 25331.5 N/mm2, respectively. Moreover, the findings proved that the properties of both plybamboo satisfy the minimum standard requirement stipulated in the Malaysian Standard.

Bond strength between substrate and post-cast UHPC with innovative interface treatment

Ultra-high performance concrete (UHPC) has been utilized in construction joint by virtue of superior strength, crack resistance and durability. However, fiber bridging at the interface between substrate UHPC and post-cast UHPC is limited in current engineering practices, which may lead to the loss of various performances. The objective of the present research is to identify the bond performance of the UHPC-UHPC interface bridged by the steel fibers, using different surface treatment techniques including chipping and steel wire mesh (SWM) pretreatment. To this end, direct tensile tests, direct shear tests and slant shear tests were carried out on the specimens with no treatment, chipping and SWM treatment. The experimental results show that SWM treatment can greatly play the role of fiber bridging at the interface, thus increasing the tensile and shear bond strengths by 55.3% and 333.3%, respectively, compared with interfaces without treatment. The calculation model of the interfacial bond tensile strength and shear strength considering the effect of the fiber bridging was established. Furthermore, conversion of bond tests between tension and shear was discussed, and linear transformation relationship was found among direct tensile tests, direct shear tests and slant shear tests.

Strengthening of existing stone arch bridges using UHPC: Theoretical analysis and case study

Ultrahigh-performance concrete (UHPC) is widely used in retrofitting damaged structures, but few studies on the reinforcement of stone arch bridges have been reported. This paper presents a simplified analysis and its application for a damaged stone arch bridge. The finite strength of the masonry, the friction and rotation between voussoirs and the bond shear strength between UHPC and stone were considered. A double-sided shear test was carried out to determine the bond shear strength where stones were taken from the considered bridge. Four UHPC strengthening configurations, including intrados, extrados, U-hoop (wrapping for three sides) and □-hoop (wrapping for four sides) reinforcement configurations were discussed. The collapse mechanisms and load collapse multipliers were analyzed in their original and strengthened configurations. The analyses performed indicate that the interface strategies of scabbling and post-installed rebar can ensure ductility and interface bonding strength. The considered non-reinforced bridge shows a lack of load-carrying capacity of approximately 40% with respect to that required by the demand of local heavy traffic. To obtain load collapse multiplier values greater than one, the respective thicknesses of the UHPC reinforcement layer required by the four UHPC reinforcement configurations are 10 cm, 8.3 cm, 6.8 cm and 6.0 cm. The structural stiffness and carrying capacity have been significantly improved after reinforcement, which shows that UHPC is reliable for reinforcing a stone arch bridge and offers the value of popularization and application.

Influence of short glass fibres and spatial features on the mechanical behaviour of weft-knitted textile reinforced concrete elements in bending

Weft-knitted textiles made from high-strength fibrous materials offer great potential for use as a flexible stay-in-place formwork and reinforcement system since they allow creating complex geometries (i.e. doubly curved or folded) and introducing spatial features such as ribs within the fabric. However, the closed surface of the textile and its placement at the bottom edge of the concrete element present major challenges regarding bond, which may lead to premature failure due to delamination of the reinforcement initiated by a substantial opening of a governing crack. This study investigates the influence of short integral glass fibres and spatial bond ribs on the mechanical behaviour of weft-knitted textile reinforced concrete elements subjected to bending and their potential to increase the shear resistance. To this end, an experimental campaign consisting of 14 four-point-bending tests was conducted, where the specimens were examined regarding their load-deformation behaviour, crack kinematics and failure modes. The contribution of the short glass fibres to the load-bearing mechanism was estimated with Pfyl’s fibre engagement model, based on the material characterisation results from prism tests on fibre reinforced concrete members. The Tension Chord Model was used to predict the stress-strain relationship of the reinforcement and the crack widths in the constant moment zone, which yielded a good correlation between the predictions and the experiments. The short fibres mostly contributed to the flexural response in the serviceability limit state, but only slightly increased the shear resistance due to their low fibre effectiveness. The introduction of bond shear connectors was shown to be essential to prevent the premature delamination of the reinforcement. The use of tall connectors allowed doubling the bearing capacity with respect to the flat reinforcement without any spatial features, effectively bridging the governing crack and reaching the tensile capacity of the longitudinal textile reinforcement.

Shear behaviour of interface between normal-strength concrete and UHPC: Experiment and predictive model

Ultra-high performance concrete (UHPC) has various excellent mechanical properties and durability which make it as a suitable repair material for bridge decks repair. However, the bond shear strength between UHPC and concrete substrate is still considered as knowledge gap in the literature. In this investigation, the bond shear properties of UHPC–NSC were systematically investigated using single side shear test, and the shear bond strength and bond failure mode were obtained. Different factors including surface roughness, steel fibre type, and substrate strength which affect the bond strength of UHPC–NSC were discussed. The results suggested that for the as cast substrate surface, bond failure occurred at the interface and the floating pulp layer adhered to UHPC. For specimens with a rough surface, the debonded interface showed that the coarse aggregate on the rough surface adhered to the UHPC. There was a linear relationship between the area of substrate adhered to the UHPC overlay and the bond shear strength. The substrate roughness has a significant influence on bond strength, while the steel fibre type has only a slight influence on bond strength. Bond strength mechanical model were built to predict the interfacial shear strength. The calculated results are in good agreement with the experimental results, indicating that the model can reliably predict the UHPC–NSC interface shear strength.

Experimental investigation on punching shear behavior of the embedded column base for CFSTs

In this paper, the punching shear behavior of the embedded column base for the concrete-filled steel tube is experimentally studied. Push-out tests were firstly conducted to obtain the bond strength between the steel tube and the foundation concrete. Punching tests were then performed on ten specimens to reveal the working mechanism of the embedded column base with shear studs. The test results indicated that shear studs changed the load path, enlarging the punching section of the foundation and improving the punching shear resistance of the column base. It was also found that the diameter of the shear studs and the distance between the shear studs could affect the punching shear resistance significantly. Finally, a method was proposed to determine the punching shear resistance of the embedded column base for the concrete-filled steel tube, by adding up the contributions from the different components of the system, including the bond force and shear resistance of studs above the punching section, and the resistance of foundation. The predicted results showed good agreement with the experimental results, with errors not greater than ± 15%.

Bond-slip behaviors of composite steel-reinforced high strength lightweight aggregate concrete columns with innovative X-shaped steel sections

This paper provides an insight into the bond-slip performance of the new type of steel-reinforced high strength lightweight aggregate concrete (SRLWAC) column with innovative X-shaped steel sections through experimental and theoretical studies. Three SRLWAC specimens were tested under push-out force; the variables are the confinement by stirrups (ρsv) and shear connectors (SCs). Force-slip curves, force axial and horizontal deformations, failure modes, and crack patterns of the specimens were investigated; the force transfer, bond-stress, bond shear transmission length, and strains were studied. Comparisons between the bond strength of the studied specimens and ordinary types column specimens from 13 previous studies showed a superior bond capacity for the new type specimens. Five related models of bond capacity were discussed, and the utility of the models was put forward. A simplified load-slip model was proposed and verified. The most feasible model and the simplified model were used in the parametric study. The results showed that the material strengths and geometric configurations parameters affect the bond strength capacity. The confinement and SCs have effects on the bond and failure mode. The natural bond-stress combined with SCs produced smaller bond strength than that from both natural bonds and using higher ρsv. Recommendations for future works are developed.

Non-ordinary state-based peridynamic fatigue modelling of composite laminates with arbitrary fibre orientation

For the safe application of carbon fibre-reinforced polymer (CFRP) materials in engineering structures, fatigue life prediction and failure modelling are essential. In this paper, an application of a nonordinary state-based peridynamics (NOSB-PD) approach for the fatigue failure prediction of multilayer CFRP was presented. To characterize the structure of composite laminates, matrix and fibre bonds were introduced within the plies, and normal and shear bonds were introduced between the plies. The fatigue simulation model of composite materials was established by combining the fatigue theory of peridynamics and Paris-Erdogan law, and a method to obtain model parameters was proposed. To demonstrate the accuracy of the model, tension and fatigue experiments were conducted. By comparing the results of tensile and fatigue experiments, it was shown that the model developed in this paper can accurately predict the failure process of multidirectional CFRP laminate under the action of tensile or fatigue loading. The crack initiation and propagation for different laminae during the tensile and fatigue processes can be obtained from the proposed model.

Effect of temperature on the properties of polymer cement composite material

AbstractIn the actual working environment for the polymer cement composite, the effect of temperature on its properties cannot be ignored. In order to investigate the properties of polymer cement composite in extreme temperature environment, this study conducted bond test, tensile test, and shear test of composites at different temperatures (−30, −15, 25, 40, and 60°C). Through measuring the elastic recovery rate of specimens, as well as relevant strength indexes, deformation indexes and energy consumption indexes under tensile and shear loads, the effect of temperature on the bond property, tensile property, and shear property of the composite were investigated. The results show that the polymer cement composite has a good bond property, and no damage occurs at any temperature or after cold drawing and hot pressing cycle. Temperature has a great influence on the tensile property and shear property of polymer cement composite. At a low temperature, the tensile property and shear property improve, the strength and energy consumption indexes increase noticeably, and the elongation at break increases while the peak strain reduces in terms of deformation indexes. At a high temperature, the tensile property and shear property deteriorate, and the strength, deformation, and energy consumption indexes all fall. As a result, the polymer cement composite is more suitable for cold areas.

Experimental Study and Degradation Analysis on Interfacial Shear Modulus of Steel and Steel Fiber-Reinforced Concrete Composite Structure

To address the problems of mutual interference between shaped steel and reinforcement bars and the difficulty of concrete pouring in the construction of steel-reinforced concrete (SRC) composite structure, the steel and steel fiber-reinforced concrete (SSFRC) composite structure without rebars was proposed. Bond behavior between the shaped steel and steel fiber reinforced concrete is the basis to ensure two kinds of materials work together. 20 square specimens were designed and tested by push-out test to study the bond performance and shear modulus of the interface between the shaped steel and steel fiber reinforced concrete. The effects of steel fiber ratio (ρsf), embedded length (Le), and concrete cover thickness (Css) on the interfacial shear modulus (G) were analyzed quantitatively. The degradation law of G was studied by defining the degradation variable of interfacial shear modulus (Da). The results show that the increase of ρsf contributes to the ascent in G, and the loading end displacement (D) at the end of degradation related to G also increase. With the rise of Le, the value of G decreases gradually and the D becomes larger when G starts to deteriorate. In addition, there is a positive correlation between G and Css. Besides, the higher Css leads to a slower degradation process of G.