Rajan Sen

Durability of Fiberglass Pretensioned Beams

The durability of identical steel and S-2 glass/epoxy pretensioned beams subjected to wet/dry cycles simulating tidal effects was expermentally investigated. Eight identical steel and fiberglass pretesioned beams, half of them precracked at the critical midspan section, were placed in two tanks containing 15% saltwater solution. The ultimate capacity of the beams was determined at periodic intervals and compared against that of unexposed control specimens in air. The tests indicate a complete loss in the effectiveness of the fiberglass strands exposed to wet/dry cycles after an average of 6 months for the precracked beams and 15 months for the uncracked beams. There was no comparable loss of capacity in the identical steel pretensioned specimens.

Durability of E-Glass/Vinylester Reinforcement in Alkaline Solution

This paper presents results from a 9-month experimental study to evaluate the durability of E-glass/vinylester reinforcement used by the U.S. Navy in the construction of the MRI Pier in San Diego, California. A total of 36 specimens were tested. These were split into 4 groups of 9 specimens each exposed to simulated pore solution with a pH ranging from 13.3 to 13.5 for periods of 1, 3, 6, and 9 months. Of the 9 specimens in each group, 33% were unstressed, 33% were stressed to 10%, and the remaining 33% were stressed to either 15 or 25% of their ultimate short-term tensile strength. At the end of the exposure periods, surviving specimens were tested to failure to determine their residual tensile strength. Results showed that the E-glass/vinylester bars tested had very limited durability in this environment, especially at stress levels of 15% or higher. Overall, findings indicate that first-generation glass fiber-reinforced polymer bars are unsuitable as structural reinforcement in concrete members.

Silver-copper alloy nanoparticles for metal enhanced luminescence

Large metal enhanced luminescence was realized at the vicinity of easily fabricated Ag–Cu alloy nanoparticles upon tuning of their surface plasmon resonance spectra by changing only one experimental variable—the annealing temperature, for maximum spectral overlap with the emission and excitation spectra of the luminophores. We observed strong emission enhancement of luminophores (141.48±19.20 times for Alexa Fluor 488 and 23.91±12.37 times for Alexa Fluor 594) at the vicinity of these Ag–Cu nanoparticles, which is significantly larger than for pure Ag nanoparticles. We present theoretical calculations to provide insights into these experimental findings.

Durability of Aramid Pretensioned Elements in a Marine Environment

This paper presents results from a 3-year exposure study to evaluate the durability of aramid fiber reinforced plastic (AFRP) pretensioned elements used as piles driven in a marine environment. The aramid/epoxy composite ARAPREE was used in this investigation. Ten AFRP pretensioned beams designed to fail by rupture of the pretensioning rods were precracked and then exposed to wet/dry cycles in saltwater to simulate tide change. Exposed beams were periodically removed and tested to failure to determine their service and ultimate response. Changes in response relative to that of unexposed control specimens were used to evaluate durability. The results showed that exposure had a relatively minor effect on the ultimate capacity of the AFRP pretensioned beams. However, there was evidence of bond deterioration in the failure mode of specimens exposed for 2 years or more. More importantly, characteristic bond splitting cracks developed in one beam that had been exposed for 3 years. This suggests that while AFRP may have adequate chemical resistance to concretes alkalinity, its bond with concrete is vulnerable under wet/dry cycles. Thus, AFRP is not the ideal candidate for replacing steel in pretensioned piles driven in tidal waters.

Effectiveness of Fiber-Reinforced Polymer in Reducing Corrosion in Marine Environment

In this paper, results are presented from a long-term exposure study to assess the role of fiber-reinforced polymer (FRP) in mitigating corrosion in a marine environment. Twenty-two 1/3-scale models of prestressed piles cast with built-in chloride were exposed to simulated tidal cycles under outdoor ambient conditions for nearly 3 years. These included 8 carbon FRP (CFRP)-wrapped specimens, 8 glass FRP (GFRP)-wrapped specimens, and 6 controls. Embedded titanium reference electrodes and thermocouples were used to monitor the corrosion performance inside the wrapped region throughout the exposure period. The performance of the FRP was evaluated on the basis of bond and gravimetric tests conducted at the end of the exposure period. The results showed that the FRP-concrete bond was largely unaffected by exposure and both CFRP- and GFRP-repaired specimens significantly outperformed the controls. The underlying trend in corrosion rate measurements showed increases for the controls and reductions for the wrapped specimens. This was reflected by much lower metal losses in wrapped specimens compared with controls. These findings indicate that FRP is effective in reducing the corrosion rate in heavily chloride-contaminated prestressed concrete elements such as those exposed to a marine environment.

Durability of Carbon Fiber-Reinforced Polymer/Epoxy/Concrete Bond in Marine Environment

This paper presents results from a 2-year exposure study to evaluate the durability of the epoxy bond formed with concrete and carbon fiber-reinforced polymers (CFRP) in a marine environment. The objective of the study was to assess the likely long-term performance of epoxy systems that had actually been used to repair highway bridge elements in the state of Florida. An equally important objective was to quatify bond deterioration in terms of residual strength so as to provide a measure of the degradation caused by exposure. Twenty-four unstressed slab specimens were used to investigate five different epoxy systems exposed to four different environments. Five of the 24 specimens--one for each epoxy system--were controls kept in an air-conditioned laboratory. The remaining 19 slabs were exposed to one of three environments: wet/dry cycles in salt water, combined wet/dry and thermal cycles in salt water, and outdoor. Long-term performance was evaluated both qualitatively (visual inspection) and quantitatively from destructive shear and tension tests conducted at the end of the exposure period. The results showed there was some deterioration in bond, particularly in specimens exposed to wet/dry cycles. This could be more readily detected by destructive testing rather than visual inspection. Overall, the results are promising and suggest that several competing epoxy systems are likely to be durable in Floridas harsh marine environment.

Effective Repair for Corrosion Control Using FRP Wraps

This paper presents results from a multiyear study to evaluate the role of prewrap substrate preparation on corrosion mitigation in a marine environment. Seventeen one-third scale prestressed piles were corroded to 20% metal loss to simulate severe corrosion. Subsequently, two types of prewrap substrate preparation were carried out: (1) full repair in which the delaminated concrete was removed and the section reformed and (2) epoxy injection repair in which the cracks were sealed and the surface cleaned. Specimens were then wrapped using carbon fiber-reinforced polymer (CFRP) and exposed to simulated tidal cycles at 60°C for 28 months. The postexposure wrap performance was evaluated from gravimetric testing in which the metal loss in all specimens was measured. Results showed that the performance of the full repair and the epoxy injection were comparable with relatively minor increased steel loss despite the severity of the exposure. In contrast, the steel in unwrapped controls exposed to the same environ...

Measurement of Oxygen Permeability of Epoxy Polymers

This paper describes an experimental technique that refines and extends available methods to measure the oxygen permeability of polymers such as epoxies that cannot easily be prepared as thin films. A simple Fick’s law-based quasi-steady-state diffusion model was developed to extract the permeation coefficient from the experimental data. The validity of this technique was established by comparing results with published values. Tests were subsequently conducted to determine the oxygen permeation rates for five representative commercially available epoxy polymers. The results showed that the oxygen permeation rates for the different epoxies were comparable, making them all suitable for corrosion repair. This suggests that repair costs may be optimized by selecting the most cost-effective epoxy system. The applicability of the findings is discussed and an illustrative numerical example is presented.

DURABILITY OF ARAMID FIBER REINFORCED PLASTIC PRETENSIONED ELEMENTS UNDER TIDAL/THERMAL CYCLES

This paper presents results from a 33-month exposure study to assess the likely effect of diurnal/seasonal temperature change on the durability of aramid fiber reinforced plastic (AFRP) pretensioned piles driven in tidal waters. The aramid fiber composite ARAPREE was used in the investigation. Twelve precracked AFRP pretensioned beams, designed to fail by rupture of the prestressing rods, were placed outdoors in two saltwater tanks. The beams were simultaneously exposed to wet/dry cycles (simulating tides) and hot/cold cycles (simulating temperature variation). The effect of exposure was evaluated from bending tests conducted periodically over the nearly 3-year exposure period. The results showed that exposure led to visible cracking and a rapid deterioration in bond. Accompanying strength reductions ranged from 43% to 55% in specimens exposed for more than 21 months (15,500 hr). This provides persuasive evidence of the unsuitability of AFRP as a pretensioning element for piles driven in tidal waters. Two-dimensional finite element analysis indicated that moisture absorption by the AFRP rods had been more important than temperature change for the bond degradation observed in the study.

Oxygen Permeability of FRP-Concrete Repair Systems

Fiber reinforced polymers (FRP) are increasingly used for repairing corrosion-damaged concrete structures. The performance of the repairs is critically dependent on the oxygen permeability of the FRP-concrete system. This paper presents the results of an experimental study in which the oxygen permeation of concrete and FRP-concrete systems were determined. In the study, concrete specimens with three different water-cementitious ratios were initially tested. Subsequently, uni directional one and two-layer carbon and fiberglass material were bonded to the concrete surface, and the oxygen permeation of the FRP-concrete systems were determined. The results showed a significant reduction in the oxygen permeation of concrete after FRP had been bonded. The best performance was obtained for FRP bonded to concrete with the highest water-cementitious ratio. An expression for the equivalent thickness of FRP-concrete systems was derived by using Fick’s law. This facilitates the evaluation of the effectiveness of alternate FRP-concrete corrosion repair schemes.