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American Society for Testing and Materials

Structural Health Monitoring (SHM) can be understood as the integration of sensing and intelligence to enable the structure loading and damage-provoking conditions to be recorded, analyzed, localized, and predicted in such a way that nondestructive testing becomes an integral part of them. In addition, SHM systems can include actuation devices to take proper reaction or correction actions. SHM sensing requirements are very well suited for the application of optical fiber sensors (OFS), in particular, to provide integrated, quasi-distributed or fully distributed technologies. In this tutorial, after a brief introduction of the basic SHM concepts, the main fiber optic techniques available for this application are reviewed, emphasizing the four most successful ones. Then, several examples of the use of OFS in real structures are also addressed, including those from the renewable energy, transportation, civil engineering and the oil and gas industry sectors. Finally, the most relevant current technical challenges and the key sector markets are identified. This paper provides a tutorial introduction, a comprehensive background on this subject and also a forecast of the future of OFS for SHM. In addition, some of the challenges to be faced in the near future are addressed.

Fiber Optic Sensors in Structural Health Monitoring

The application of structural health monitoring (SHM) systems to civil engineering structures has been a developing studied and practiced topic, that has allowed for a better understanding of structures’ conditions and increasingly lead to a more cost-effective management of those infrastructures In this field, the use of fiber optic sensors has been studied, discussed and practiced with encouraging results The possibility of understanding and monitor the distributed behavior of extensive stretches of critical structures it’s an enormous advantage that distributed fiber optic sensing provides to SHM systems In the past decade, several R & D studies have been performed with the goal of improving the knowledge and developing new techniques associated with the application of distributed optical fiber sensors (DOFS) in order to widen the range of applications of these sensors and also to obtain more correct and reliable data This paper presents, after a brief introduction to the theoretical background of DOFS, the latest developments related with the improvement of these products by presenting a wide range of laboratory experiments as well as an extended review of their diverse applications in civil engineering structures

A review of distributed optical fiber sensors for civil engineering applications

The paper presents the experience of a working group within the RILEM Technical Committee 223-MSC ‘Masonry Strengthening with Composite materials’, aimed at developing a standardized, reliable procedure for characterizing the bonding mechanism of masonry elements strengthened with composite materials under shear actions. Twelve laboratories from European universities and research centers were involved. Two different set-ups were compared, for single-lap and double-lap shear tests (the latter in two versions). Four kinds of fiber fabrics, i.e., glass, carbon, basalt and steel, were applied with epoxy resins (wet lay-up system) to clay brick units, for a total of 280 monotonic tests. The results provided information regarding the response of externally bonded-to-brick composites in terms of observed failure mechanisms, load capacity, effective transfer length, and bond shear stress–slip behavior. The test results of the 12 laboratories constitute a set of statistically representative data which may conveniently be used for setting appropriate design provisions and guidelines.

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Round Robin Test for composite-to-brick shear bond characterization

Fibre Reinforced Cementitious Matrix (FRCM) composites are now widely used for repair and retrofitting existing structures. Guidelines for product qualification are needed to characterize the strengthening systems before they are made available in the market and installed. The paper proposes a procedure that combines the results of direct tensile and shear bond tests to provide engineering design parameters for externally bonded FRCM reinforcements. Due to the possible occurrence of different failure modes, the procedure provides results on the base of the weakest mechanism that takes place. Thanks to its simplicity, the proposed method is suitable for standard product qualification and material assurance purposes. In order to investigate its feasibility, the qualification procedure is applied to different reinforcements comprising basalt, carbon, glass, and steel textiles, bonded with either cement or lime mortars to clay bricks and tuff units.

A qualification method for externally bonded Fibre Reinforced Cementitious Matrix (FRCM) strengthening systems

The objective of this study is to investigate the efficiency of an innovative technique for strengthening masonry arches, based on the use of high strength steel cords embedded in either an epoxy (steel reinforced polymer) or mortar matrix (steel reinforced grout). Ten prototypes of brickwork arches strengthened by composite laminates were tested under a monotonic vertical load applied at the quarter-span. Load tests were performed to compare the behavior up to collapse of strengthened masonry arches; the influence of the types of reinforcement (steel and carbon fibers) and matrices (epoxy and cementitious), as well as location of the strengthening layer (intrados, extrados, and both) and the presence of anchorage systems has been investigated. The experimental results highlight the enhanced strength of the arches reinforced with steel cords, as well as the role of the mechanical anchoring with regard to the resulting final strength.

Strengthening of Brick Masonry Arches with Externally Bonded Steel Reinforced Composites

Fiber optic sensing technologies are emerging as valid alternatives for the health monitoring of civil structures. Distributed sensors based on Brillouin scattering add the unique capability of measuring strain and temperature profiles along optical fibers. Measurement is performed by establishing the correlation between fiber strain and temperature, and the frequency shift of the Brillouin backscattered light induced by a monochromatic light pulse. The technology holds potential for use on large structures and integrated transportation infrastructure. Its effectiveness has been assessed through scaled laboratory experiments, whereas field validation is limited to very few demonstration projects conducted to date. This paper presents a pilot application of Brillouin optical time domain reflectometry to measure strain profiles along the steel girders of a continuous slab-on-girder bridge subjected to diagnostic load testing. One of the exterior continuous girders required heat-straightening after falling during construction due to wind. The significance of applying a distributed measurement technique lies in the potential to assess the global girder response, which would be impractical and uneconomical using discrete measurement techniques. A 1.16 km long sensing circuit was installed onto the web of four girders. The circuit comprises bare optical fiber sensors, and a novel adhesively bonded fiberglass tape with embedded sensing fibers for strain measurement and thermal compensation. The strain profiles were first converted into deflection profiles and validated against discrete deflection measurements performed with a high-precision total station system. Structural assessment based on comparison of the strain profiles with the results of three-dimensional finite-element analysis of the bridge superstructure, and with specification mandated criteria, indicated that the response of the girder under investigation was within the design limits, and did not pose serviceability concerns. Factors that may affect measurement accuracy are finally discussed on the basis of the experimental and numerical results.

Distributed Strain Measurement in Steel Bridge with Fiber Optic Sensors: Validation through Diagnostic Load Test

In the fall of 2002, a two-storey parking garage in Bloomington, Indiana, built with precast prestrestressed concrete (PC) double-T beams, was decommissioned due to a need for increased parking-space. This led to the opportunity of investigating the flexural performance of the PC double-T beams, upgraded in the positive moment region with steel reinforced polymer (SRP) composite materials, representing the first case study where this material has been applied in the field. SRP makes use of high-strength steel cords embedded in an epoxy resin. This paper reports on the test results to failure of three beams: a control specimen, a beam strengthened with one ply of SRP and a third beam strengthened with two plies of SRP anchored at both ends with SRP U-wraps. Results showed that SRP can significantly improve both flexural capacity and enhance pseudo-ductility. Preliminary analytical work shows that the same approach used for externally bonded fiber reinforced polymer (FRP) can be satisfactorly used for SRP.

Performance of Double-T Prestressed Concrete Beams Strengthened with Steel Reinforcement Polymer

2 Disclaimer The contents of this report reflect the views of the author(s), who are responsible for the facts and the accuracy of information presented herein. This document is disseminated under the sponsorship of the Center for Infrastructure Engineering Studies (CIES), University of Missouri-Rolla, in the interest of information exchange. CIES assumes no liability for the contents or use thereof. 3 The mission of CIES is to provide leadership in research and education for solving society's problems affecting the nation's infrastructure systems. CIES is the primary conduit for communication among those on the UMR campus interested in infrastructure studies and provides coordination for collaborative efforts. CIES activities include interdisciplinary research and development with projects tailored to address needs of federal agencies, state agencies, and private industry as well as technology transfer and continuing/distance education to the engineering community and industry. One-way RC slabs are very common as a floor system in US structures. It often happens that structures need to be renovated due to many factors, such as placing new staircases, elevators, additional skylights or (more often) wiring and ducts through the existing floor slabs. In these instances, openings usually need to be created, weakening the existing floor system. Fiber-reinforced polymer (FRP) matrix composites can provide an efficient method for strengthening slabs after installation of cutouts , but it is important to define guidelines for the designer, particularly when the openings are created in the negative moment region. This paper presents the results of an experimental program that investigated the behavior of RC one-way continuous flat slabs with openings in both the positive and negative moment regions, strengthened with externally-bonded CFRP laminates following two different schemes of strengthening. The significance of this work is that instead of testing specimens manufactured in the laboratory, the authors had the possibility of using as a research test bed, an old parking garage scheduled for demolition. Experimental results revealed that externally-bonded CFRP sheets can significantly increase the flexural capacity of the system for those specimens containing an opening in the positive moment region. For specimens with an opening in the negative moment region and strengthened using top-surface CFRP laminates, shear failure occurred at a lower load capacity than the original unstrengthened specimen. This has considerable implications for the design of such negative-moment strengthening schemes, and reasons for this behavior are discussed.

Experimental results of one-way slabs with openings strengthened with CFRP laminates

This paper reports on the results obtained on the applicability of the diagnostic cyclic load test method in comparison with the existing 24 h test procedure adopted in ACI-318. A parking garage, owned by St. Louis County, St. Louis, Missouri, scheduled for demolition during the summer of 2002 was used as a research test bed before demolition. This structure, a two-story steel and reinforced concrete (RC) frame with one-way RC slabs built in 1970's, was ideal, in terms of size and construction system, for performing comparative field experimentation on load testing. Investigation and validity of acceptance criteria of existing and proposed testing methods were performed. Two identical RC slabs were tested, according to both the standard procedure (ACI-318-02) and the proposed diagnostic load testing. In both instances, the applied total test load was such that the slab did not pass the load test. This allowed characterizing the critical test parameters that govern acceptability and draw conclusions on their values. After load testing, both slabs were loaded until partial collapse was reached. This allowed making observations on the margin of safety with respect to collapse, a determination that is not generally possible in a proof test. The analysis of the experimental data provides professionals with evidence on the validity of in situ assessment for the adequacy of structural members.

Paolo Casadei

Papers

Strengthening of Brick Masonry Arches with Externally Bonded Steel Reinforced Composites

Distributed Strain Measurement in Steel Bridge with Fiber Optic Sensors: Validation through Diagnostic Load Test

Performance of Double-T Prestressed Concrete Beams Strengthened with Steel Reinforcement Polymer

Experimental results of one-way slabs with openings strengthened with CFRP laminates

In Situ Load Testing of Parking Garage Reinforced Concrete Slabs: Comparison between 24 h and Cyclic Load Testing