H. R. Hamilton

Small Beam Bond Test Method for CFRP Composites Applied to Concrete

This paper presents the development of a test method that can be used to test the bond capacity of carbon fiber-reinforced polymer (CFRP) composites bonded to concrete. The rationale for the selection of the test method is described along with the results of the experimental work used to refine the test configuration and procedures. The research objectives were to develop a test method that (1) can be used to evaluate the durability of the FRP-concrete bond (adhesion failure mode); (2) facilitate multiple replicate for statistical validation; (3) is simple to conduct; and (4) provides comparative results that are easy to interpret. The method utilizes a small concrete beam modeled after the modulus of rupture test, which is typically used to measure concrete tensile strength. A number of small beam sizes and loading configurations were considered during the investigation. The final recommended specimen configuration is 4×4×14 in. ( 100 mm×100 mm×356 mm ) beam with a half-depth saw cut at midspan. A 1 in. ...

Durability of Concrete Beams Externally Reinforced with CFRP Composites Exposed to Various Environments

In this paper, the effects of various environmental exposure conditions on concrete beams externally reinforced with carbon-fiber-reinforced polymer (CFRP) composites were studied. To evaluate the overall environmental effects on the durability of the interfacial bonding between concrete and CFRP, different commercial composite systems were investigated under various exposure conditions by using a new test method suggested previously. Results showed that the flexural strength of the beam specimens was reduced with environmental exposure. The commercial systems showed significant differences in the relative strength loss, although they were all intended for the same application, suggesting that durability is highly dependent on the epoxy system used in the composite. Under exposure to water and other fluids with different temperatures, CFRP systems employing wet lay-up composites showed high durability with the exception that one system revealed a considerable loss in strength with elevated temperature and time. A system using a precured laminate exhibited a large reduction in strength with exposure. Exposure to brackish water caused an increase in the flexural strength by virtue of buildup of barnacles, which protected the specimens from deterioration and increased the strength by acting as an additional reinforcement.

Bond Durability Factor for Externally Bonded CFRP Systems in Concrete Structures

Lack of understanding and confidence in the long-term performance of externally bonded fiber-reinforced polymer (FRP) composites in concrete structures still inhibits their application in repair of aged structures. While synergistic effects of multiple exposure conditions can be severe, it is generally agreed that the single most significant issue with externally bonded FRP composites is their susceptibility to degradation when exposed to moisture. This research utilized small-beam three-point bending test specimens to study FRP-concrete bond performance when subjected to accelerated conditioning environments (immersion in water and exposure to high humidity at elevated temperatures). Bond strength retention (Rb) was determined by dividing the conditioned strength by the average control strength. Test results from the present research were combined with other test data to form a database of over 700 test results. By utilizing an apparent analogy of FRP-concrete bonded systems to adhesive anchors, a bond durability factor (BDF) that quantifies loss in bond capacity due to accelerated conditioning is determined equivalently as a characteristic test value for adhesive anchors. Given their resistance to the environment, only carbon-fiber-reinforced polymer (CFRP) composites were examined. For the purpose of the analyses, and based on available data, it was determined that all CFRP systems may be split into three categories: wet-layup without putty, wet-layup with putty, and precured laminate. BDF corresponding to wet-layup without putty was determined to be 0.60. BDF for wet-layup with putty was not established due to the observed sensitivity to increasing conditioning temperature. CFRP laminate specimens failed prematurely by composite rupture or at the adhesive-composite interface; BDF was not determined as long term durability data corresponding to FRP-concrete bond failure mode was not available.

Heating Methods and Detection Limits for Infrared Thermography Inspection of Fiber-Reinforced Polymer Composites

The use of fiber-reinforced polymer (FRP) composites to strengthen existing civil infrastructure is expanding rapidly. Many FRP systems used to strengthen reinforced concrete are applied using a wet lay-up method in which dry fibers are saturated on site and then applied to the surface. This research investigated using infrared thermography (IRT) as a nondestructive evaluation (NDE) tool for detecting air voids and epoxy-filled holes in FRP systems bonded to a concrete substrate. Four small-scale specimens with FRP thicknesses ranging from 1 to 4 mm (0.04 to 0.16 in.) containing fabricated defects were constructed and inspected in a laboratory setting. Three heating methods (flash, scan, and long pulse) were employed and a quantitative analysis of resulting IRT data was used to establish detection limits for each method. Scan heating was shown to be most effective for basic defect detection. Air-filled defects at the FRP/concrete interface as small as 2. 9 cm 2 (0.45 in. 2 ) were detected in a 4 mm (0.16 in.) thick FRP system. Defects as small as 0.3 cm 2 (0.05 in. 2 ) were detected in a 1 mm (0.04 in.) thick FRP system.

Wire Breakage Detection Using Relative Strain Variation in Unbonded Posttensioning Anchors

Recent problems with grouted posttensioning tendons in U.S. bridges have led to interest in tendons that are not grouted, but rather filled with a pliable corrosion protection system such as grease or wax. The use of a flexible filler material in place of cementitious grout results in an unbonded tendon, which not only affects the structural behavior but also enables the application of new methods for improved monitoring and maintenance. This paper outlines a novel continuous monitoring approach that aims to detect wire breakage in unbonded tendons based on relative strain variation in the anchor head (or wedge plate). The feasibility of the method is investigated using a finite-element model calibrated with experimental data from a seven-strand anchor head. The model is subsequently extended to a 19-strand anchor head to assess the method’s performance on detection, localization, and quantification of tendon damage. The numerical results confirm the correlation between wire breakage and relative strain variation among the monitoring points and illustrate the method’s effectiveness in determining the location and severity of damage.

Experimental and Analytical Evaluations of Confinement Reinforcement in Pretensioned Concrete Beams

The AASHTO LRFD Bridge Design Specifications require that confinement reinforcement be placed around prestressing strands in the bottom bulb of pretensioned concrete beams. Although the AASHTO specifications contain prescriptive requirements for the quantity and placement of confinement reinforcement, the effect of such reinforcement on end region behavior is not well understood. To evaluate the function and effect of confinement reinforcement, 12 tests were conducted on precast pretensioned beams loaded in three-point bending at a shear span-to-depth ratio of 1.0. Variables in the test program included strand size, strand quantity, prestressing force, and the presence or lack of confinement reinforcement. In the tests, the confinement reinforcement neither prevented nor delayed strand slip but improved shear capacity and displacement ductility and prevented splitting failure. Experimental results also indicated transverse strain in the concrete above the bearing pad, which eventually led to splitting failure in test beams without confinement reinforcement. An elastic finite element analysis of the test beam was conducted to model the strain distributions in the test beams before cracking. Results from the analyses matched well with the experimental results. In particular, the magnitude and distribution of the transverse concrete strain measured at the end of the beam agreed well with the experimental results.

NDE of fiber-reinforced polymer composites bonded to concrete using IR thermography

Infrared thermography is a non-destructive evaluation technique that can be used to identify debonded areas in FRP strengthening systems applied to concrete. This research provides a summary of IR thermography experiments that were conducted on full-scale AASHTO girders strengthened with four different FRP composite systems. Significant findings were that the thickness of the FRP system as well as the material composition strongly influences the ability to detect defects at the FRP/concrete interface. Additional experiments were conducted on small-scale specimens with implanted defects. Results from these experiments indicate that IR thermography is capable of detecting defects under multi-layer FRP composite systems; however the defect signal strength and time to maximum signal vary significantly from single-layer systems.

Detection and Assessment of Structural Flaws in Concrete Bridges with NDT Methods

Researchers investigated the applicability of various nondestructive testing (NDT) methods on full-scale concrete bridge structures, including ultrasonic pulse velocity tomography and the impact-echo method. The combination of the two methods enabled researchers to three-dimensionally evaluate material consistency and locate flaws or regions of deterioration within structural members with a new level of efficiency. Although research involving NDT methods has been conducted for many years, the use of NDT methods for primary investigation of full-scale structural elements is not typically performed. In this study, the investigation of bridge structures for flaws and damage resulted in the discovery of a previously undetected yet recurring form of damage, which led investigators to suspect a design flaw. The results of the ultrasonic pulse velocity tomography enabled researchers to assess the actual cause of the damage, which allowed the formulation of recommendations for the repair and redesign of the defective structural elements.

Visual Rating and Strength Testing of 40-Year-Old Precast Prestressed Concrete Bridge Piling

The rating of bridge piles is primarily visual and requires judgment to quantify damage levels and, ultimately, to estimate the piles remaining structural capacity. A series of tests was run to evaluate the existing capacity of piles that have been in service in a harsh saltwater environment for more than 40 years. In 2004, the Bryant Patton Bridge over Apalachicola Bay in the Florida Panhandle was replaced with a new bridge. The original bridge, which was demolished as part of the construction project, was constructed in 1963. During demolition, 12 prestressed concrete piles with varying levels of corrosion damage were recovered. Two of the selected piles were equipped with cathodic protection at the time of their installation in 1994 as part of a repair project that involved most of the pilings supporting the bridge. Visual inspections were conducted by Florida Department of Transportation divers on the selected piles before they were removed. After recovery of the piles, they were cleaned and flexural...

Experimental and Numerical Evaluation of Unbonded Posttensioning Tendons Subjected to Wire Breaks

AbstractThe development of corrosion as a result of tendon exposure to moisture or corrosive media often leads to wire fractures. Such fractures in unbonded tendons are expected to induce global strand and anchor response through the progression of prestress loss from the break to the end anchors. Radial pressure, interwire friction, and lateral confinement, however, affect the magnitude of prestress loss carried to the anchors and have important implications on breakage detectability by tendon monitoring methods that rely on anchor response. This paper presents an experimental investigation of tendon behavior after successive wire cuts in confined and unconfined conditions and demonstrates the effects of confinement on stress recovery. In addition to analysis of strand prestress loss and axial strain along individual wires, the strand’s dynamic postbreakage response was examined. Moreover, the effects of deviators on stress recovery were tested with a multistrand tendon specimen. A finite-element model o...