Introduction
The bonding properties of Fiber Reinforced Plastics (FRP) -concrete composites is directly related to the load carrying capacity and durability of FRP reinforced concrete structures. In addition, the study of bond properties can help us better understand how FRP bars behave in concrete and how they react under specific environmental conditions (e.g., high temperature, humidity, corrosion, etc.). This is valuable for predicting and improving the long-term performance of concrete structures. However, in offshore engineering, structures are exposed to strong corrosive environments all year round. Under the erosion of internal FRP bars by corrosive ions and the highly alkaline environment of the concrete pore solution, the matrix on the surface of FRP bars firstly suffers from deterioration phenomena such as dissolution, plasticization, and microcrack formation. Consequently, the bonding properties of the FRP-concrete interface are seriously affected, leading to the deterioration or failure of the concrete structure. Therefore, improving the corrosion resistance of FRP reinforcement is an important measure to ensure the long-term service of FRP-reinforced concrete structures. In this paper, the resin matrix modification of GFRP bars was realized by self-synthesized organosilicones and surface hydrophobic modification of nanofillers to obtain highly corrosion-resistant GFRP bars. To investigate the degradation of the interfacial adhesion between highly corrosion-resistant GFRP reinforcement and seawater-seasand concrete (SSC) in simulated pore solution of SSC at 60 ℃, and to reveal the mechanism of GFRP reinforcement-concrete adhesion enhancement through interfacial microstructure analysis.
Methods
Organosilicon synthesis: Hydrogen-containing silicone oil (PMHS) was mixed with γ-(2,3-epoxypropyloxy) propyltrimethoxysilane (KH560) in a mass ratio of 1:1, with ethanol as the solvent, and reacted under the catalytic effect of NaOH for 8 h at ambient temperature using a magnetic stirrer (200 r/min), and then kept for spare.CNT surface modification: 0.03% (mass fraction) of CNTs was stirred and dispersed into an ethanol and KOH organic solvent, and the CNT modifier was subsequently obtained by adding hydrogen-containing silicone oil (PMHS) and ethyl silicate (TEOS) after ultrasonic dispersion.Surface modification of EG: EG was dripped onto the surface of expanded graphite in the mass ratio of TEOS:PMHS=3:1 in a vacuum environment; subsequently, the samples were placed in a microwave oven at 800 W for a rapid expansion for 10 s. The modified expanded graphite was placed in the organic solvents of KOH and anhydrous ethanol with heating and stirring until the mixture was homogeneous.Preparation of modified epoxy resin hybrids: A bisphenol A-type epoxy resin (EGEBA), methyl tetrahydrophthalic anhydride curing agent (MeTHPA), and an accelerator (DMP-30) were mixed in a mass ratio of 100:85:5. Subsequently, 5% of the self-synthesized organosilicon was mixed with CNT and EG surface-modified nanofillers with a total solid content of 0.03% into the epoxy emulsion, and the mixed solution was stirred homogeneously using ultrasonication combined with triple-roller shear dispersion to obtain the modified epoxy resin mixture.Preparation and testing of pull-out specimens: modified GFRP bars were embedded in the upper surface of concrete to a depth of 5 mm, and the size of the concrete was set to 100 mm × 100 mm × 300 mm. Hydraulic jacks were used to carry out single-end pull-out tests on the bonded specimens, and a digital CCD camera was used to capture the displacement and deformation of the interface throughout the entire process of de-bonding.Microstructural characterization: the scaled-down specimens were prepared, and the microstructural changes at the FRP-concrete interface of the scaled-down specimens after corrosion were observed using scanning electron microscopy and X-ray computed tomography.
Results and discussion
From the pull-out damage pattern of the GFRP-concrete interface, it can be seen that after 120 d of corrosion, large areas of resin degradation on the surface of the N-GFRP reinforcement resulted in fiber exposure and carbon fiber rib detachment. Compared with N-GFRP, the corrosion degree of M-GFRP bar is less severe, and its surface only has speckled local yellowing and surface roughness phenomenon. Compared with the unmodified bars, the initial interfacial adhesion between corrosion-resistant GFRP bars and concrete was improved by 7.87%, and the interfacial adhesion was improved by about 15% in the 120 d-corrosion experiment. In addition, the maximum strain values at the interfaces of N-GFRP and M-GFRP with concrete decreased to 22.8×10–3、28.7×10–3, respectively in the 120 d-corrosion experiment. After the modification of the resin matrix of the FRP bars, the corrosion resistance of the bars is improved, which can better absorb and disperse the stress brought by the external load, making the stress distribution at the FRP-concrete interface more uniform and the deformation increased, thus slowing down the rate of loss of adhesive force.Corrosion to 120 d, the interfacial spacing between N/M-GFRP and concrete reached 158.11 μm and 67.91 μm, respectively, which increased by about 100% and 60% compared to the uncorroded condition. The Vr value at the M-GFRP-concrete bonded interface (23.35%) was higher than that of N-GFRP (18.52%). The surface substrates at the N-GFRP reinforcement-concrete interface all were detached and a new layer was exposed. In contrast, the localized matrix spalling at the M-GFRP-concrete interface was less corrosive than that of the N-GFRP bars. Therefore, the debonding of GFRP-concrete mainly depends on the degradation degree of the matrix at the interface. The affinity between the resin matrix and the concrete was improved by matrix modification, and the introduction of hydrophobic network led to the improvement of the waterproofing performance of the bars, which effectively mitigated the degradation of the resin matrix of the bars, and then improved their bonding performance.
Conclusions
The main conclusions of this paper are summarized as following. Compared with the unmodified bars, the initial interfacial adhesion between corrosion-resistant GFRP bars and concrete was improved by 7.87%, and the interfacial adhesion was improved by about 15% in the 120 d-corrosion experiment. In addition, the maximum strain values at the interfaces of N-GFRP and M-GFRP with concrete decreased to 22.8×10–3、28.7×10–3, respectively in the 120 d-corrosion experiment. Compared with the two, the maximum strain at the interface increased by about 25.87% after modification. The damage modes of the debonding process were dominated by concrete cracking and GFRP bar pullout. The degradation rate of the interfacial matrix was mitigated by resin matrix modification, which improved the adhesive properties of the GFRP-concrete interface.