Jian-Guo Dai

Experimental Investigation of the Influence of Moisture on the Bond Behavior of FRP to Concrete Interfaces

The effects of moisture on the initial and long-term bonding behavior of fiber reinforced polymer (FRP) sheets to concrete interfaces have been investigated by means of a two-year experimental exposure program. The research is focused on the effects of (1) moisture at the time of FRP installation, in this paper termed “construction moisture,” consisting of concrete substratum surface moisture and external air moisture; and (2) moisture, in this paper termed “service moisture,” which normally varies throughout the service life of concrete. Concrete beams with FRP bonded to their soffits were prepared. Before bonding, concrete substrates were preconditioned with different moisture contents and treated with different primers. The FRP bonded concrete beams were then cured under different humidity conditions before being subjected to combined wet/dry (WD) and thermal cycling regimes to accelerate the exposure effects. Adhesives with different elastic moduli were used to investigate the long-term durability of ...

Bond-Slip Model for FRP Laminates Externally Bonded to Concrete at Elevated Temperature

AbstractThis paper presents a nonlinear local bond-slip model for fiber reinforced polymer (FRP) laminates externally bonded to concrete at elevated temperature for future use in the theoretical modeling of fire resistance of FRP-strengthened concrete structures. The model is an extension of an existing two-parameter bond-slip model for FRP-to-concrete interfaces at ambient temperature. The two key parameters employed in the proposed bond-slip model, the interfacial fracture energy, Gf, and the interfacial brittleness index, B, were determined using existing shear test data of FRP-to-concrete bonded joints at elevated temperature. In the interpretation of test data, the influences of temperature-induced thermal stress and temperature-induced bond degradation are properly accounted for. As may be expected, the interfacial fracture energy, Gf, is found to be almost constant initially and then starts to decrease when the temperature approaches the glass transition temperature of the bonding adhesive; the int...

Effect of Temperature Variation on the Full-Range Behavior of FRP-to-Concrete Bonded Joints

AbstractService temperature variations (thermal loadings) may significantly affect the behavior of the bond between externally bonded fiber reinforced polymer (FRP) and concrete. This paper presents an analytical solution for the full-range deformation process of FRP-to-concrete bonded joints under combined thermal and mechanical loadings. The solution is based on a bilinear bond-slip model and leads to closed-form expressions. The validity of the solution is demonstrated through comparisons with both experimental results and finite-element predictions. Numerical results from the solution are presented to illustrate the effect of thermal loading on the interfacial shear stress and slip distributions in addition to the global load-displacement response. Provided the material properties are not affected by temperature variations, a temperature rise is shown to increase the ultimate load, whereas a temperature reduction decreases the ultimate load; the latter can have serious implications for the safety of t...

Prestress Losses and Flexural Behavior of Reinforced Concrete Beams Strengthened with Posttensioned CFRP Sheets

An experimental study was conducted to investigate the flexural behavior and long-term prestress losses of reinforced concrete (RC) beams strengthened with posttensioned carbon fiber-reinforced polymer (CFRP) sheets. The experimental program consisted of tensile tests of flat CFRP coupons under sustained loads and flexural tests of a total of eight RC beams: six strengthened with posttensioned CFRP sheets, one strengthened with nonprestressed CFRP sheets, and one control beam. The main objective of the tests was to gain a better understanding of the long-term prestress losses of CFRP sheets in the posttensioned system under different prestress levels and strengthening ratios. It is shown that the prestress losses of CFRP sheets in the posttension system are mainly attributable to anchorage set (approximately 12.6 to 18.2% of the initial prestress), whereas the time-dependent losses caused by creep and shrinkage of concrete and relaxation of CFRP sheets are relatively small (approximately 2.3 to 3.9% of the initial prestress). DOI: 10.1061/(ASCE)CC .1943-5614.0000255.

Probabilistic Approach for Durability Design of Concrete Structures in Marine Environments

AbstractCorrosion of steel reinforcement due to chloride penetration is a major deterioration mechanism of reinforced concrete (RC) structures exposed to marine environments. In the present paper, a probabilistic model is presented to predict the chloride penetration profile in concrete and the risk of steel corrosion considering the time dependence of both the diffusion coefficient and the surface chloride concentration of concrete. Abundant field data from two projects in China and Japan are respectively collected. The consistency between these field data and the model prediction verified the reliability of this probabilistic model, based on which further parametric analyses were carried out to investigate the effects of different design variables on the probability of steel corrosion.

Static and Fatigue Bond Characteristics of Interfaces between CFRP Sheets and Frost Damage Experienced Concrete

Synopsis: Both short and long-term performances of repaired or strengthened concrete structures using external FRP bonding are greatly affected by states of bonding substrates, which are covercrete and may have experienced various damages. One of them is frost damage in cold regions. This paper intends to investigate how the initial frost damages in concrete influence the static and fatigue bond performances of CFRP/concrete interfaces. Concrete specimens were exposed to freeze and thaw cycles before being bonded with CFRP sheets. The initial frost damage of concrete was controlled approximately at three different levels in terms of its relative dynamic modulus of elasticity, which was 100% (non frost damage), 85% and 70%, respectively. Test results showed that failure modes of CFRP/concrete bonded joints with initial frost damage in concrete were the delamination of covercrete. By contrast the joints without initial frost damage failed in a thin concrete layer as usual. Moreover, CFRP/concrete joints with and without initial frost damage showed different manners in their interface bonding strength and stiffness. If the initial frost damage existed in concrete substrate the effective bond length of CFRP/concrete joints was increased due to the decrease of the bonding stiffness and interfacial fracture energy. Fatigue testing results indicated that the linear slopes of S-N curves of CFRP/concrete bonded joints were not influenced by the initial frost damage. The initial frost damage did not shorten the fatigue life of CFRP/concrete joints if a same relative tensile stress level was kept in the FRP sheets, where the relative tensile stress level was defined as a ratio of the applied tensile force in FRP sheets for the fatigue tests to the maximum static pullout one achieved in each test series.

Long-Term Behavior of Prestressed Old-New Concrete Composite Beams

This paper presents both theoretical and experimental studies of the long-term behavior of prestressed old-new concrete composite beams under sustained loads. General differential equations governing the relationship between the incremental deflection and incremental internal forces of the composite beams were deduced in the theoretical study. Closed-form solutions for simply supported composite beams were obtained and validated using test results reported in previous literature on steel-concrete composite beams. The experimental investigation consisted of static long-term load tests carried out on four prestressed old-new concrete composite beams. The behavior of the old-to-new concrete interface, time-dependent deflections, concrete strains, and prestress losses was carefully observed over 260 days. The long-term test program showed that the midspan deflections and concrete strains increased with time because of creep and shrinkage of the new prestressed concrete. The slip strains at the old-to-new concrete interface were found to be relatively small, indicating that the interface bond was sound enough to prevent slip and that the prestressing loads were effectively transferred to the old concrete. The proposed theoretical models predicted the long-term behavior of the prestressed old-new concrete composite beams with an acceptable degree of accuracy. DOI: 10.1061/(ASCE)BE.1943-5592.0000152.

Simple method for predicting temperatures in insulated, FRP-strengthened RC members exposed to a standard fire

AbstractFire safety is a significant concern for fiber-reinforced-polymer (FRP)–strengthened RC structures, particularly for indoor applications. To satisfy fire resistance requirements, fire insulation layers may be provided to protect FRP-strengthened RC members. This paper presents a simple, design-oriented method for predicting temperatures in insulated FRP-strengthened RC members under standard fire exposure. The proposed method consists of two sets of formulas: one set for predicting temperatures in unprotected FRP-strengthened RC members exposed to a standard fire; and another set to convert a fire insulation layer into an equivalent concrete layer. As a result, an insulated FRP-strengthened RC member can be analyzed as an unprotected RC member with an enlarged section for which a similar simple method has previously been established by these authors. In the present study, a finite element (FE) approach for the temperature analysis of insulated FRP-strengthened RC members was first developed and th...

Intermediate crack-induced debonding in RC beams externally strengthened with prestressed FRP laminates

Intermediate crack-induced debonding is often a dominant failure mode in fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams in flexure. It has been extensively studied for RC beams externally strengthened with unstressed FRP laminates. However, very little work has been done on FRP debonding for RC beams strengthened with prestressed FRP. This article presents a sectional analysis model for predicting the flexural capacity of RC beams strengthened with prestressed FRP laminates with due consideration of different failure modes. The focus is placed on the effective strain in the prestressed FRP at the ultimate states of intermediate crack-induced debonding or rupture of debonded FRP. Through back-calculation analysis of 51 RC beams strengthened with post-tensioned FRP laminates, a model for predicting the effective strain of prestressed FRP for ultimate strength prediction based upon sectional analyses was developed and validated through comparisons with test results.

Finite element modeling of insulated FRP-strengthened RC beams exposed to fire

This paper presents a finite element (FE) model for the thermo-mechanical analysis of insulated FRP-strengthened reinforced concrete (RC) beams exposed to fire. In the model, the effects of loading, thermal expansion of materials, and degradations in both the mechanical properties of materials and the bond behavior at FRP-to-concrete and steel-to-concrete interfaces due to elevated temperatures are all considered. The validity of the FE model is demonstrated through comparisons of FE predictions with results from existing standard fire tests on insulated FRP-strengthened RC beams.