DURABILITY AND LONG-TERM PERFORMANCE … Papers... · DURABILITY AND LONG-TERM PERFORMANCE MODELING OF FRP-CONCRETE ... durability of infrastructure under different environmental

DURABILITY AND LONG-TERM PERFORMANCE MODELING OF FRP-CONCRETE SYSTEMS

Oral Büyüköztürk*

Professor

Department of Civil and Environmental Engineering, Massachusetts Institute of Technology

77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A.

* Corresponding author: [email protected]

Denvid Lau

Ph.D. Candidate

Department of Civil and Environmental Engineering, Massachusetts Institute of Technology

77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A.

[email protected]

Chakrapan Tuakta

Lecturer, Ph.D.

Department of Civil Engineering, Kasetsart University

50 Boonsom Suvashirat Building, Ngamwongwan Rd. Lardyaw,JatuJak, Bangkok. Postcode 10900, Thailand

[email protected]

Abstract

Combining several materials to create a structural system has led to new kinds of innovative infrastructure applications. One of the major examples found in civil infrastructures is fiber-reinforced polymers (FRP)-concrete systems. Such systems consist of multiple materials including concrete, epoxy and FRP, constituting a cohesive unit to perform an intended function. The existence of the interfaces between the constituent materials introduces a challenging problem in determining durability of bond systems, in which moisture and variable temperatures are the two major environmental factors affecting the durability. In this paper, an evolutionary investigation on durability of bonded concrete-epoxy as a multi-layer material system subjected to moisture diffusion at different temperature levels is described. Two major fracture test specimens for characterizing fracture toughness were adopted, namely peel and shear fracture specimens, and sandwiched beam specimens, at the meso-scale level. The information from the fracture tests forms the basis of a long-term performance model proposed for FRP-bonded concrete systems. As this predictive model is mainly phenomenological in nature, an atomistic-scale investigation using molecular dynamics simulation is performed in order to understand the fundamentals of the durability in FRP-bonded concrete systems. The molecular dynamics simulation results show that the adhesion energy of the interface decreases by approximately 15% from dry to wet condition at the molecular level. Molecular dynamics is a potential fundamental approach as a basis for developing a new analysis and prediction paradigm. Research issues in such a development are discussed.

Keywords: concrete, debonding, durability, fracture, FRP, moisture, molecular dynamics

1. Introduction

Civil infrastructures are generally built to last for at least five decades. However, several external factors and unexpected events, such as increased load requirement, severe environmental exposure, or natural disasters, can affect their service. These may degrade the structures to the point that they can no longer perform their functions efficiently with respect to their strength and serviceability, ultimately shortening the intended service lives of the systems. In some cases, gradual degradation over time can eventually lead to catastrophic failure of infrastructure. Durability may be defined, for both material and structural levels, as

Proceedings of CICE 2012 6th International Conference on FRP Composites in Civil Engineering Rome, Italy, 13-15 June 2012 © International Institute for FRP in Construction (IIFC)

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the ability to maintain the intended functions for a certain period of time, during which degradation can occur. Different indicators are used to determine the durability to reflect different functions unique to that particular structural component. For example, resistance to surface abrasion may be used to determine long-term performance of concrete in a bridge slab, while water permeability is used for the case of concrete in a tunnel wall. Clearly, more than one indicator is used in determining durability of an infrastructure because it generally consists of many structural components, each performing more than one function.

Why is durability so important? Most civil infrastructures must be designed such that their service life is long enough in order to meet financial requirements. In addition, as environmental awareness is growing, the construction industry has to adapt to the notion of sustainability agreed upon by all parties directly and indirectly involved in construction projects. Replacing degrading components adds more impact to the environment because of additional resource usage and required waste disposal. Therefore, in engineering sense, sustainability may be defined as how well a structure is able to withstand different types of deterioration process and continue to perform its intended functions, or in other words, durability of structure. Thus, building an infrastructure has become a multidisciplinary task that combines knowledge and experience in engineering, environmental management, and economics. The life-cycle cost analysis and durability consideration of infrastructures are inevitably interrelated and have to be taken into account during planning, design, and building stages. In addition, an appropriate maintenance program has to be strategically implemented throughout the life of the structure to ensure durability after completion. As such, knowledge of durability and degradation mechanisms of construction materials will play a crucial role. Methods based on this information can then be developed to predict durability of infrastructure under different environmental and loading conditions at both material and structural levels.

Specifically, durability of concrete structures has been an important technical, safety as well as economic issue attracting attention from researchers and practicing engineers. Many techniques have been developed to improve the durability of concrete. Corrosion of reinforcement may be prevented by replacing the steel reinforcement with alternative materials, such as stainless steel and fiber reinforced polymer (FRP) rods, or epoxy-coated steel. Implementing cathodic protection may also help prevent corrosion of steel reinforcement by stopping the release of ferrous ions from the steel. Controlling water-cement ratio (w/c ratio) during concrete mixing is also crucial. However, deterioration of concrete cannot be avoided and strengthening and retrofitting existing concrete structures generally becomes necessary.

In the retrofit application, composites and concrete are bonded together with an adhesive layer between them. FRP is one of the widely used composite materials; the use of FRP with concrete represents a multi-material system that can be readily found in civil infrastructure applications. In these applications, substrates are bonded together by an adhesive material, which is usually organic polymer-based. Existence of the interfaces between the constituent materials in the bonded system introduces a challenging problem in determining the durability of the bonded system. Such a system consists of multiple materials including concrete, epoxy and FRP, constituting a cohesive unit to perform an intended function. Combining several materials to create a structural system has led to new kinds of applications. The schematic diagram of the FRP-bonded concrete system shown in Figure 1(a) shows various layers, where the concrete-epoxy interface is our focus area in this paper. Various premature failures can be observed in an FRP retrofitted reinforced concrete beam involving interface failures (Figure 1b). For example, the failure mode of an FRP-bonded concrete beam failure can be initiated through the plate end debonding, or the flexural-dominated debonding initiated at the mid-span of the beam or sometimes through the flexure-shear debonding along the beam profile. The type of debonding mechanism that may happen is correlated to the environmental effects on the structural element. In many applications, durability of individual materials is well-documented. However, the properties of the interface can significantly deviate from those of the constituents, and special attention is required. This is due to the property variations in the interface resulting from chemical reaction between the substrates affecting adhesion forces [1].


Degradation mechanism of the interface may significantly differ from those of constituent materials. Even though the durability of FRP may be good because of its excellent corrosion resistance, when it is used with concrete to form the FRP-concrete system, the durability of the bonded system becomes a complex issue. In fact, the interface between epoxy and concrete may represent the critical element of the bonded system. Therefore, in this paper, durability of the FRP-concrete system with emphasis on the concrete-epoxy interface will be addressed as the key issue.

Figure 1. (a) A schedule diagram of the definition of interfaces in the FRP-bonded concrete system, (b) FRP

debonding from concrete can occur through (1) plate end debonding, (2) flexure-dominated debonding, and (3)

flexure-shear debonding.

2. Studies on Durability of FRP-Concrete Systems

Studies to understand durability and failure of fundamental mechanisms of bonded systems, such as FRP/concrete and FRP/steel systems, have generally used two types of approaches—strength-based and fracture-based approaches.

2.1 Strength-based Approach

For the strength-based approach, most studies involve characterization of strength parameters such as ultimate strength, stiffness, and deformation. Various specimen configurations have been used to identify changes in these parameters with respect to several severe environmental conditions. These include prolonged exposure to different chemicals, wet-dry cycles, and freeze-thaw cycles Limited studies have shown that structural integrity of FRP retrofitted systems may be impeded due to the introduction of various environmental effects, and that debonding in the FRP/concrete interface region may play a major role in such premature failures. Under identical conditioning duration, the degree of stiffness reduction depends largely on the type of FRP being used. Testing of FRP retrofitted concrete beams under wet/dry cycles has for most cases registered reduction in flexural stiffness and ultimate strength after conditioning. Tommaso et al. [2] conducted three-point bending experiments on temperature conditioned FRP-plated beams to study the influence of temperature on the bonded system. It was found that failure modes changed from ductile to brittle as the conditioned temperature was decreased from 40 ºC to -100 ºC. Experimental works by Karbhari and Engineer [3] and Grace [4] investigated the influence of several environmental conditions such as saltwater, moisture, heat, and alkalinity on the structural performance of FRP-strengthened concrete beam elements. In particular, moisture effect was identified as an important environmental deterioration mechanism promoting premature system failures. Debonding was observed as one of the main failure modes [4]. Findings from these investigations regarding the detrimental effect of moisture were in agreement with other limited number of independent studies conducted on FRP-plated concrete specimens subjected to accelerated wet/dry cycles. Reductions in ultimate strength and stiffness due to such cycling were generally observed. Chajes et al. [5] reported decrements of 20% to 30% in ultimate strength for GFRP and CFRP retrofitted specimens that were subjected to 100 wet/dry cycles. In one study [6], it was experimentally determined that glass FRP bonded specimens lost up to 85% of its flexural stiffness after 20 wet/dry cycles, compared to 55% in the case of carbon FRP bonded specimens. Aramid FRP bonded specimens in the same test series appeared somewhere in between. Ultimate failure load also depreciates at various


degrees, depending on the epoxy systems being used. In one study [7], strength reductions ranging up to 33% were observed in FRP-plated concrete beams that were subjected to 300 wet/dry cycles in salt water. Failure was reported as a debonding mode that generally took place in the vicinity of the adhesive. In a study where glass FRP double lap shear specimens were being used [8], wet/dry conditioning resulted in increased shear slip, although it was concluded that the quantification of average shear stress could not effectively demonstrate the damage induced by the exposure.

2.2 Fracture-based Approach

With the fracture-based approach, generally the degradation parameter is the fracture toughness or the critical energy release rate corresponding to different levels of environmental exposure. In this approach, the interface fracture toughness is considered as a measure of resistance of a multi-layer material system against debonding. Fracture characterization of glass and carbon FRP bonded concrete subjected to continuous fresh water and synthetic seawater (or salt water) has been performed by means of a peel test [9]. It was concluded from the tests that very little reduction in mode I fracture toughness while significant reduction in mode II fracture toughness was observed from the short-term exposure of the interface to water. Tests on modified double cantilever beams after freeze-thaw and wet-dry cycles also showed marked reduction in fracture toughness of FRP/concrete bond [10]. Moisture does not only affect the adhesive bond in service, but also during the application of FRP on concrete surface. Tests on CFRP bonded to concrete with initially damp surface using a modified cantilever beam indicated reduction in bond strength for both dry and moisture-conditioned specimens [11]. Since the failure under effect of moisture generally occurs by either concrete delamination or concrete-epoxy interface separation, some studies have been devoted to determining the extent of degradation of bi-material systems. Results from fracture tests on bi-material specimens have shown that the interface between concrete and epoxy become weakened due to moisture exposure [12]. In fact, concrete-epoxy interface is the most critical region affecting the entire structural integrity of the bonded system when subjected to moisture.

In what follows, we will describe an evolutionary investigation on durability of bonded concrete-epoxy as a multi-layer material system subjected to moisture diffusion at different temperature levels [13-17]. Knowledge on durability of concrete/epoxy bonded systems is becoming essential as the use of these systems in applications such as FRP strengthening and retrofitting of concrete structures is becoming increasingly popular. Two major types of specimens for characterizing fracture toughness were adopted, namely peel and shear fracture specimens, and sandwiched beam specimens, representing meso-scale structures followed by a more fundamental study of the interface durability using molecular dynamics approach.

2.3 Sandwiched Beam Specimens

As mentioned in the previous section, premature failures of the FRP-concrete system may occur in the vicinity of the concrete-epoxy interface. Fracture toughness of concrete/epoxy interfaces as affected by combinations of various degrees of moisture ingress and temperature levels has been quantified [15]. Normal strength concrete of Grade 40 (95% of the tested concrete samples have strength above 40 N/mm2) was used in this study. The maximum aggregate size was 10 mm. A commercial two-component 100% solids nonsag epoxy was employed; this epoxy is commonly adopted in current construction industry as a concrete bonding adhesive. To quantify the interfacial fracture properties, sandwiched specimens composed of an epoxy layer embedded in a concrete block were used. A notch at the concrete-epoxy interface was introduced so that the crack would be initiated at the tip of the notch. Sandwiched beam specimens containing concrete-epoxy interfaces as shown in Figure 2 were continuously moisture-conditioned in a temperature controlled water bath. We considered six durations of moisture-conditioning: 0 (dry), 2, 4, 6, 8 and 10 weeks; and two water bath temperatures: 23 °C and 50 °C. The high temperature conditioning represents a realistic upper bound of service temperature that could be reached in the soffit of reinforced


concrete beam. These sandwiched beam fracture tests can be used for the determination of the mode I and mixed-mode fracture toughness at the interface of a bi-material system by incorporating the relationship between the interface fracture toughness and the phase angle [18-20].

Considering the four-point bending specimen with a sandwiched epoxy layer as shown in Figure 2(a), proper techniques are required to sandwich an epoxy layer between two concrete blocks and create a perfect pre-crack at the interface. The stress intensity factor KI can be obtained by [21]

afK rI 1 (1)

where σr = 6M/bd2 in which M is the applied moment, a is the crack length, b is the width, d

is the height of the specimen and f1 is a correction factor for four-point pure bending. The corresponding mode I fracture energy release rate can be calculated as

1

22

1

1

2

E

af

E

KG rI

. (2)

where 1E is the Young’s modulus of the concrete substrate under plane strain condition.

Considering the four-point shear specimen shown in Figure 2(b), the stress intensity factors related to the loads and specimen geometry are given by

d

af

bd

MK bI 23

(3)

d

af

bd

QK sII 21

(4)

where fb and fs are correction factors depending on the ratio a/d [22]. M and Q and the applied moment and shear force at the crack location respectively. The corresponding energy release rate can be calculated as

1

22

E

KKG III . (5)

Typical failure modes of the sandwiched bending beam specimens with and without moisture conditioning are shown in Figure 3. Figure 3(a) shows the failure mode of the dry sandwiched bending specimen. Figure 3(b) shows the failure mode of the wet sandwiched bending specimen with 4-week moisture duration.

Figure 2. (a) Sandwiched four-point bending specimen, (b) sandwiched four-point shear specimen [15]


Figure 3 (a) Concrete delamination of bending beam specimen under dry condition, (b) Interface separation of

bending beam specimen after 4-week moisture conditioning [15]

For the sandwiched four-point bending specimens as shown in Figure 2(a), dry specimens exhibited failure in concrete itself, rendering a cohesive type of failure. The pre-crack, which was placed at the concrete/epoxy interface, kinked into the concrete upon reaching the peak load. The specimen failed by concrete delamination. This phenomenon was observed for both temperature groups. Failure surface was felt powdery to the touch and small grains of sand were clearly seen at the failure surface. Wet sandwiched bending beam specimens, on the other hand, exhibited a distinctive concrete-epoxy interface separation. It is observed that the shift of fracture failure mode from concrete delamination to interface separation is accompanied by a substantial decrease of the mode I interface fracture toughness. Unlike the pure bending specimen, the failure mechanism in the sandwiched four-point shear specimens (mixed mode case) did not change from dry to wet condition based on the experimental observation. It is observed that there is a decreasing trend in the mixed mode fracture toughness with increasing moisture duration, however, with a slower rate when compared with the variation of mode I interface fracture toughness. The variations of mode I and mixed mode fracture toughness under different moisture durations are shown in Figure 4.

Figure 4 (a) Mode I and (b) mixed-mode interface fracture toughness variation under different moisture contents

[15]


2.4 Peel and Shear Fracture Specimens

The effect of continuous moisture ingress was investigated quantitatively and qualitatively by means of peel and shear fracture tests [13, 17]. Figure 5 shows a schematic diagram of the peel and shear specimen. The CFRP plate in both specimens is 1.28 mm thick and made of unidirectional carbon fibers. Special consideration is given to shear fracture to make sure that the bond line satisfies the requirement for development length, i.e. stress in the FRP plate varies from maximum value at pulled end to zero at the other end. A pneumatic needle scaler was used to roughen the concrete surface to obtain optimal bonding property through mechanical interlocking. Possible dust and debris resulted from surface roughening were removed by means of compressed air. Before applying the CFRP plates and the epoxy, the surfaces of the CFRP and the concrete were cleaned with methyl ethyl ketone to ensure proper adhesive bonding surface. The bond thickness was kept to be 1 mm uniformly over the bond region.

For both the peel and shear fracture tests, the load was applied at the end of the CFRP plate as shown in Figure 5. Loads applied on CFRP and displacements were recorded from the load cell and machine crosshead, respectively. The LVDT was fixed on the loading frame serving as a reference point for measuring the displacement of the CFRP plate. As the specimen was pulled upward at 1 mm/min along the bond line direction, loads were applied on the CFRP and the displacements were recorded by the load cell and the LVDT, respectively. The effects of continuous moisture uptake or residual debonding resistance were quantified by the tri-layer fracture toughness [14]. Specimens were conditioned in a moisture environment at 23 °C using water tanks, and at 50 °C using an environmental chamber before testing. It was determined from a 3D diffusion simulation that in order to have distinguishable levels of moisture in the bond line, peel and shear fracture specimens should be conditioned for 2, 4 and 8 weeks. The load–displacement behavior and debonding modes were observed and correlated with the moisture level obtained from the finite element diffusion simulation.

It has been found that elevated temperature causes more bond degradation [Figure 6] in both mode I and mode II fracture [13]. A shift in failure mode has been observed for carbon FRP bonded concrete systems before and after moisture conditioning. Control specimens usually exhibited a conventional concrete fracture type of failure, while moisture-conditioned specimens failed mostly at the epoxy/FRP interface or epoxy/concrete interface [Figure 7]. This indicates that an interface weakening mechanism was activated by the presence of water molecules in the bond.

Figure 5. Schematic diagram of the peel and shear fracture specimens


Figure 6. Effect of moisture on bond fracture toughness of CFRP/concrete interface obtained from fracture

specimens loaded in mode I and mode II [13]

Figure 7. Shift in failure surface (a) from concrete delamination in dry specimen to (b) interface separation in

wet specimen [13]

2.5 Discussion

The peel and shear specimens of FRP-concrete system are in fact originated from the concept of modified double cantilever beams (MDCB) [9, 23-26]. With these specimens, loads can be applied easily at the end of FRP strips. However, the interface fracture energy estimated by these tests, when compared to the sandwiched beam specimens, was often higher. In fact, the normal peel test involving primarily bending of the FRP strip may have significant effect in quantifying the interface fracture energy of the bonded system because of various factors such as plasticity in the vicinity of the crack tip due to bending. The four-point bending tests on sandwiched beam specimens investigate the concrete-epoxy interface directly, and, thus, are more applicable in quantifying the interfacial fracture toughness between concrete and epoxy.

Even though the interfacial fracture toughness captured from these two types of specimens differ in magnitude, the substantial decreasing trend of the moisture affected interfacial fracture toughness can be observed in both specimens, together with the shift of failure. It can be concluded that this deterioration will occur regardless of the specimen type and should be considered early in the structural design stage in order to avoid any premature failure during the service life of the FRP-concrete system. Analytical and empirical formulae based on conventional interfacial fracture mechanics have been limited to mechanical debonding and are unable to predict the behavior when chemical effects are involved. In fact, crack propagation under moisture effect is complex and further explanation at a more fundamental scale is necessary.


3. Model for Long-Term Performance

Based on the meso-scale specimens (e.g. sandwiched beam specimens, and peel and shear specimens), an initial conceptual model to predict the long-term structural performance of FRP-bonded concrete systems is proposed [16]. For this purpose, we use the results from our previously discussed peel and shear fracture specimens and additional experimental information on the wet-dry cyclic behavior of the specimens. In order to determine the effect of conditioning duration and the number of wet-dry cycles on the residual strength of the adhesive bond, peel and shear fracture specimens were subjected to various wet-dry cycles, with three weeks being the longest continuous conditioning duration. It should be mentioned that the specimens were left to dry for four days in the laboratory at room temperature and approximately 60% relative humidity after each continuous moisture ingress. With this model, the reduction in the FRP-concrete bond strength under continuous moisture exposure is expressed in the form of an exponential decay with respect to interface moisture content:

( ) (5)

where the coefficients A and b are obtained from the continuous moisture condition tests (Figure 8) and the ratio of transient to threshold moisture contents (C/Cth). For simplicity, it is assumed here that the rate of this deterioration remains constant for any wet-dry cycle, while the rate is dependent only on the intermediate moisture content. The findings [16] show that the rate of deterioration is generally high during the first few wet-dry cycles, but will decrease to an almost constant value as the number of wet-dry cycles increases beyond three cycles as shown in Figure 8. Because civil structures are designed to withstand much more wet-dry cycles during service life, this assumption is deemed reasonable for this analysis. Therefore, owing to the cyclic nature of the problem, a general form of the moisture cyclic degradation model is proposed as follows:

( )

(6)

where Γ = interface fracture toughness obtained from Eq. (5); N = number of wet-dry cycles. The coefficients q and n are determined from the meso-scale fracture tests under several sets of moisture cyclic duration. Eq. (6) implies that as the moisture concentration approaches the threshold value, a fewer number of wet-dry cycles will be required for the fracture toughness to reach an asymptotic value.

Figure 8. (a) Effect of number of wet-dry cycles on the residual fracture toughness of peel fracture specimens; (b)

Effect of number of wet-dry cycles on the residual fracture toughness of shear fracture specimens [16].

Eq. (6) is integrated to determine the number of cycles, Nth, that cause the fracture toughness to reach the asymptotic value:

∫

∫

( )

(7)

where Γ0 and Γth = initial fracture toughness of the control specimens (dry) and the asymptotic fracture toughness (wet), respectively. The coefficients in Eq. (5) and (6) are specific to the certain type of adhesive used in this study. Nonetheless, the methodology presented here is generally applicable to other commercially available structural adhesives, and further studies are needed to establish the variation of the coefficients based on the


variety of epoxy types used in practice. The integration of Eq. (7) results in a function of C, which is the level of intermediate moisture content between each wet-dry cycle:

( ) (8)

Substituting various values of C/Cth into Eq. (8), it is noticed that the number of cycles reaches an asymptotic value when C/Cth approaches 1. It is predicted that higher intermediate moisture content and higher temperature will require slightly fewer wet-dry cycles for the fracture toughness to reach the asymptotic value.

In real-life applications, existing RC beams are designed such that the service load should not cause any structural failure during the service life. This design philosophy also applies to the case of an FRP-strengthened RC beam. Given a loading history during the design service life of an RC beam, the driving force that can cause premature debonding may be characterized by the energy release rate (ERR) of an existing interfacial crack at a given time. According to ACI (2008), the stress in steel reinforcement is limited to 0.8 times its yield strength to avoid any inelastic deformation. This limitation implies the maximum load a beam will carry during its service life. However, this design principle is not conservative, in that a premature failure by debonding may occur at a lower level of the load. The ERR corresponding to the maximum load can indicate potential premature debonding failure when compared to available fracture toughness of the interfaces and the bulk materials (i.e., concrete and epoxy). The two input parameters of the proposed predictive model are the ERR and the interface moisture content, both of which are calculated from FEM. Application of this conceptual model to a pre-cracked FRP-plated RC beam has been demonstrated for the prediction of debonding failure under moisture cycles [16].

4. Atomistic Modeling at the Interface

The above predictive model can be regarded as a phenomenological model, meaning that the basis of the model is experimental observation and analysis. In order to understand the debonding mechanism fundamentally, an atomistic model at the interface is required for studying the interaction among various atoms and molecules at the vicinity of the interface. When moisture is diffused into the adhesive material, it is in either one of two forms—free water (equilibrium water) or bound water (excess water) [27]. Free water occupies the free volume of the epoxy resin and does not cause swelling. On the other hand, bound water chemically reacts with the polymer chain through hydrogen bonding, disrupting the chain, and enhancing the chain mobility, causing swelling to occur. In other words, the effective cross-link density is reduced when adsorption takes place. This can alter the properties reversibly by plasticization, or irreversibly by cracking and hydrolysis [28]. It can also cause swelling stress and deterioration of strength in adhesive and at the adhered interface. Figure 9 schematically shows the destruction of secondary bonds between adhesive and substrate in moisture environment.


Figure 9. Attack by water molecules at the interface of adhesive and substrate. One of adhesion mechanisms is

known to be hydrogen bonding between adhesive and substrate. However, under moisture conditions these

bonds can be broken by hydrolysis, as can the bond between the polymer chains.

Figure 10. (a) Molecular dynamics simulation showing the water molecules interacting with silica in concrete;

(b) energy reduction due to water effect computed at the molecular scale; this reduction may translate into larger

amounts at the macro-scale.

Based on fundamental understanding of molecular structure from atomistic modeling involving principles of physics and chemistry, a characterization of the nano-mechanical properties at the concrete/epoxy interface under the effect of moisture has been conducted [29]. Silica, which is a commonly found material in nature in the form of sand or quartz and is the major component in concrete (~40% by mass), was used as a representative of concrete in the atomistic modeling process. Such atomistic model can be placed inside a water box for studying the moisture effect as shown in Figure 10(a). The atomistic approach is able to quantify the decrease of adhesive energy in the interface by examining the changes in the physical forces of attraction and repulsion between molecules in the two materials – changes that can lead to failure of the concrete-epoxy interface. It should be mentioned that the energy barrier (Eb) measured during the entire simulation process is equal to the energy required to separate the bonded epoxy-silica system and hence the energy barrier refers to the adhesion energy. When peel and shear forces were measured, the simulation results showed that in a “wet” scenario, adhesive energy decreased by approximately 15% compared to a “dry” scenario, as shown in Figure 10(b). This reduction at the molecular scale may translate into greater adhesive energy reductions in large-scale structures due to the interaction of local effects which may significantly reduce the structural integrity of the bonded system.

5. Further Research

5.1 Meso-scale coupled stress-diffusion

In a service condition, strengthened concrete structures are subjected to mechanical and environmental stresses simultaneously. The studies on the durability of FRP-bonded concrete system reported in this paper have not addressed the problem of stress-coupled deterioration. However, a preliminary work reported in [16] was conducted in our group. In this preliminary investigation, fracture tests on peel specimens under coupled moisture diffusion and stress were conducted [16]. In all concerned levels of sustained load, there was a brief jump in deflection during the first few hours, which could be attributed to instantaneous microcracking in the epoxy. The deflection then increased in small magnitudes owing to the creep effect in the bond line. This effect is significantly pronounced in specimen under high intensity of sustained load at high temperature. On the other hand, the effect of coupled stress-diffusion on bond strength was not obvious under low stress intensity. Such effect of sustained stress coupled with moisture diffusion can further degrade the strength of FRP-concrete interface leading to premature failure at load levels much lower than prediction. The effect of sustained load on interface fracture toughness in a specimen under moisture


effects needs to be further studied for the implementation in long term performance model to take into account the effect of stress-diffusion coupling.

5.2 Atomistic model describing concrete

The development of the reactive force field (ReaxFF) core model of concrete, which is capable in describing the possible reaction inside concrete, is essential for a more accurate atomic model. The development process will require the knowledge in quantum mechanics (QM). Observables to be included in the force field training set are bond order, valence angle, torsion angle, heat of formation, and Mulliken charge distribution. Successive optimization should be conducted in order to find optimal ReaxFF parameters that fit the training set.

5.3 Finite element modeling coupled with molecular dynamics

In practice, the length scale of structural models is much larger than that of the atomistic models. In order to implement the fundamental knowledge of local interface behavior in the entire model of the structural system, molecular dynamics (MD) modeling should be coupled with the conventional finite element modeling (FEM). Due to the limitation of MD modeling on both length and time scales, bridging between MD and FEM needs to be established. Several techniques have become available to bridge FEM and MD [30-32]. The main objective of these techniques is to avoid wave reflection at MD and FE interface. A concurrent coupling technique based on the idea of quasicontinuum method is found to be suitable in bridging the atomistic and continuum paradigms. This technique reduces the degree of freedoms in the atomistic region by employing the Cauchy-Born (CB) rule, which postulates that a uniform deformation gradient at the macro-scale can be mapped to the same uniform deformation at atomistic scale [33, 34]. It is noted that the FEM internal force can be obtained using the CB hypothesis in conjunction with the atomistic forces utilizing the same interatomic potential. The connection between the FEM equations of motion can be achieved if a CB continuum model is used and the FEM mesh is meshed down to the atomic spacing. A correction can be made to the CB rule to account for non-uniform deformation in some materials [35, 36] and free surfaces when extracting information at continuum scale (i.e. force and energy) from representative atoms in the atomistic region and subsequently performing energy minimization [37, 38]. This will allow seamless information transfer between FEM and MD regions because, unlike other techniques, no boundary will be present between them.

6. Conclusion

Understanding durability is important for the design and life cycle analysis of civil infrastructures. Combining multiple materials leads to new type of applications, such as FRP strengthening system for concrete structure, though such a system also introduces new challenges in determining the durability of multi-material systems. Possibility of interfacial degradation has to be taken into account in the design and maintenance planning for such a structural system. The shift in failure at the concrete/epoxy interface under the effect of moisture has been captured in various types of fracture specimens. Based on the experimental findings from the meso-scale fracture tests, a long term performance prediction model for FRP-bonded concrete system has been developed. As this predictive model is mainly based on the phenomenological observation from the meso-scale experiment, a fundamental approach using molecular dynamics simulation is necessary for a complete understanding on the shift in failure mechanism under the effect of moisture. This new approach is able to quantify the decrease in adhesion energy in the interface by examining the changes in the physical forces of attraction and repulsion between molecules in the two materials. In the molecular scale, the adhesive energy in a “wet” scenario decreased by approximately 15% compared to a “dry” scenario. In order to develop a more complete framework as a basis for service life prediction, areas of further research should include (1) the stress-moisture coupled effects on FRP-concrete systems, (2) atomistic modeling of


concrete and (3) the coupling between molecular dynamics simulation and finite element modeling.

7. Acknowledgement

This research was supported by the National Science Foundation (NSF) through the grants CMS Grant No. 0510797 and Grant No. 0856325.

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