by the ACI Committee 437 for Strength Evaluation of Existing. Concrete Structures. The strengthening of the parking garage, mainly based on FRP composites, ...
EDITORIAL
Research Needs and Unresolved Issues of Composites for Built Infrastructure Edoardo Cosenza Professor of Structural Engineering, Dept. of Structural Analysis and Design, Univ. of Naples Federico II, Naples 80125, Italy.
Gaetano Manfredi Professor of Structural Engineering, Dept. of Structural Analysis and Design, Univ. of Naples Federico II, Naples 80125, Italy.
This special issue of the Journal of Composites for Construction is part of the activities related to the international workshop Composites in Construction: A Reality held in Anacapri 共island of Capri, Italy兲 July 20 and 21, 2001. The meeting was a collaborative activity between the Department of Structural Analysis and Design of the University of Naples Federico II and the Center for Infrastructure Engineering Studies of the University of MissouriRolla; it was hosted in Villa Orlandi, International Center for Scientific Culture of the University of Naples. In the last few years, the great interest in fiber reinforced polymer 共FRP兲 composites for use in construction and the worldwide expectation that such advanced materials could significantly improve the performance and durability of new or deteriorated constructed facilities has induced many researchers to promote meetings and stimulate discussion on this topic. Along with the attention of the scientific community, significant efforts are continuously made by manufacturers and producers in order to develop innovative manufacturing technologies and propose more stable and stronger constituent materials. After the latest ‘‘Fiber Reinforced Plastics for Reinforced Concrete Structures 共FRPRCS兲’’ conference was held at the University of Cambridge on July 16 –18, 2001, the workshop organizers felt that a specialty two-day gathering, intended as a follow-up to the latest Cambridge event and opened to some invited participants, could be a unique opportunity to rethink and reflect on the state-of-the-art, look at an agenda for research in the new century, and possibly set the course for future initiatives. In this spirit, 30 leaders in FRP technology 共academics, contractors, engineers, and manufacturers兲 were invited to attend the workshop and were associated to a specific subtopic within a given matrix; each participant presented their contribution in a general session. All the papers were collected in the ASCE volume Composites in Construction: A Reality edited by E. Cosenza, G. Manfredi, and A. Nanni. Attendees were then split into break-up sessions before coming back into a general session where controversial aspects, unresolved issues, and needs of research were finally discussed and prioritized according to participants’ votes and opinions. Considerations were drawn on the present situation of composites for the built infrastructure, and a working plan was formulated for the international scientific community in terms of opportunities, barriers, and needed research, as well as action items that the participants themselves would commit to carry out or start in the near future. The main research needs summarizing the records of the different sessions are outlined below; for simplicity, they are re-
ported following the order of the break-up sessions and include elements of the general sessions. • Reinforced and Prestressed Concrete: standard test methods to characterize the properties of FRP reinforcement; confirmation of high-durability performance; development of specifications for different grades of FRP bars and defining clearly their bond properties. • Structural Shapes and Systems: integration of different processing methods; optimization of hybrid shapes; development of standards for production processes and test methods; unified reduction safety factors; design criteria accounting for imperfections and premature failures; and construction specifications. • Repair and Strengthening: standard tests to assess fiber performance and reliability; improvement of existing models; separation between mechanical models and practical algorithms; serviceability behavior; development and testing of mechanical anchorages; and standard quality-control procedure. • Materials, Durability, and Characterization: need to understand durability/performance mechanisms such as fatigue, creep, moisture, influence of additives, temperatures, fire, UV radiation, degradation, biological effects, and galvanic currents; influence of the interaction of FRP concrete on the durability of new or strengthened members; performance-based specifications; serviceability and ultimate performance; recyclability and poor performance due to fire or high temperature exposure; and tools for life-cycle cost assessment. • Analysis, Design, and Codes: shear behavior of FRP RC elements or strengthened concrete members; issues for externally bonded columns such as interaction steel FRP, cyclic behavior under seismic loads, combined bending, effectiveness of confinement for rectangular sections; serviceability aspects of FRP RC members; models to predict debonding failures; local and global behavior of masonry strengthened with FRP; address design/construction detailing as well as model calibration and safety factors; and harmonization of different guidelines. • Manufacturing, Construction, and Marketing: design-build approach to market new technology and demonstrate/capitalize on the benefits of the new technology. In addition to papers collected in the ASCE volume, organizers thought it could be interesting to ask some workshop attendees to develop further contributions aimed at providing a first response to some controversial issues, and unresolved aspects also emerged from the discussion within the final general session. Those papers are collected in the present special issue. The first proposed by Benmokrane et al. deals with the durability of glass FRP 共GFRP兲 bars. As underlined by the workshop discussion, the costs of GFRP bars make them a promising alternative to traditional steel bars for reinforced concrete 共RC兲 structures governed by durability issues. However, the disagreement of workshop participants on the future of glass GFRP bars evidenced that long-term performance should be clarified in order to better understand the JOURNAL OF COMPOSITES FOR CONSTRUCTION / AUGUST 2002 / 141
potential and marketability of GFRP. A better understanding of GFRP’s long-term performance could also allow the modification of the reduction factors presently adopted by design guidelines that may be unjustifiably conservative. The paper by Benmokrane et al. attempts to satisfy that need. GFRP bars produced by three different manufacturers have been examined within the experimental campaign. The influence of type of fiber, type of matrix, manufacturing details, bar size, shape, and coating on the durability performance of GFRP bars is assessed. In addition to an extended set of experimental data, the issue of accelerated tests under load is raised in the paper. Two possible test setups are proposed for subjecting the bar sample to a sustained stress level; conclusions are drawn on the advantages of each. In order to study moisture and ion diffusion, three different exposures 共alkaline solution, pore water solution, and embedding in concrete兲 have been tested during the conducted experiments. Microstructural analyses are also used to identify the most relevant stress corrosion mechanisms. The paper provides insights about the role of resin to limit microcrack propagation, the need for stress-level limitations to prevent stress rupture, and the influence of voids, cracks, and fiber-matrix interface properties on the migration of ions through the bar. After the first paper focused on durability and material properties, the contributions by Pilakoutas et al. and Monti and Santini cover issues related to the design of members internally or externally reinforced with FRP, respectively. Pilakoutas et al. focus on the flexural short-term behavior of over-reinforced FRP RC beams with reference to the European design guidelines. The writers discuss the results obtained from theoretical simulations on many configurations of rectangular beams internally reinforced with either CFRP or GFRP. Based on those outcomes, they analyze the role of the material, the partial safety factor of FRP reinforcement, and other parameters. Some design recommendations are outlined, an expression for the minimum amount of FRP reinforcement is proposed, and the unresolved issues that need further research are mentioned. Monti and Santini discuss a reliability-based procedure allowing for the calibration of the partial safety factor of FRP that should be adopted in designing the flexural strength of RC beams. Because calibration concerns the partial factor of FRP, the analysis by Monti and Santini considers only the case of a strengthened beam whose failure is due to FRP. By following an incremental approach from a given to a target situation, the writers aim at developing a procedure providing the amount of FRP, the corresponding compressive concrete strain, and the design value for FRP yielding to a target reliability. Following the same approach and using the Monte Carlo method for the simulations, the authors expect to calibrate in the near future the partial safety factors for FRP used in other applications such as shear strengthening of beams or confinement of columns. The fourth paper, by Ueda et al., presents the results of an experimental campaign aimed at studying the behavior in tension of RC members strengthened with FRP. As underlined during the workshop discussion, the lack of data on this aspect requires that model calibration should be based on many experimental data. The writers use the recorded lab results as background for the future development and calibration of a constitutive model in pure tension of concrete elements internally reinforced with steel bars and externally bonded with FRP laminates. The presented study covers crucial aspects of serviceability behavior, details on the influence of FRP reinforcement on crack width and crack spacing, steel bond stresses, and stress-strain relationships of both steel and concrete. The paper provides a valuable contribution toward a better understanding of concrete tension stiffening, crack spacing, 142 / JOURNAL OF COMPOSITES FOR CONSTRUCTION / AUGUST 2002
and basic material 共concrete and steel兲 relationships in FRP reinforced members; attention is also devoted to the deterioration of FRP bond properties close to cracks. The paper by Tan covers the confinement of RC columns. In particular, the contribution deals with the experimental behavior of rectangular members externally confined with composites. As emerged from the workshop discussion, this topic represents one of the most debated within the scientific community, along with the interaction between internal steel stirrups and FRP confinement, cyclic behavior under seismic loads, damage tolerance properties 共e.g., residual state of stress兲, and combined bending. The paper presents the results of an extensive experimental work conducted on rectangular columns; strain gauges on internal steel reinforcement and on the composite jacket allowed them to assess the deformation characteristics of columns reinforced with either glass or carbon fibers. The contribution of transverse and longitudinal fibers to the axial capacity is assessed, and the role of anchors in improving the effectiveness of transverse fibers and preventing the buckling of longitudinal fibers is recognized. Based on experimental outcomes, strain values for the longitudinal and transverse fibers are suggested toward a reliable prediction of the axial capacity of the FRP-confined member. The main message that emerged during the workshop from the break-up session on manufacturing, economics, construction, and marketing was that a design-build approach may promote alliances of manufacturers, designers, contractors, and users, to demonstrate and capitalize on the benefits of FRP technology. The paper by Kelley et al. confirms the validity of a similar approach describing a field application on a deficient parking garage. Along with explaining the design philosophy for strengthening the concrete decks of such a posttensioned RC structure, the paper reports a typical experience that an engineering firm can face when dealing with similar jobs. Prior to CFRP strengthening acceptance by the client, detailed and advanced in-situ, nondestructive tests were carried out in order to assess the performance of unstrengthened and strengthened members. The case study is pioneering in recognizing within the American context the validity and opportunity of load tests, demonstrated also by the latest actions taken by the ACI Committee 437 for Strength Evaluation of Existing Concrete Structures. The strengthening of the parking garage, mainly based on FRP composites, was also characterized by the adoption of steel framing supports that performed better than FRP in some locations. The break-up session on structural shapes and systems underlined the fact that an effort should be made for the development of optimized FRP products. The example of hybrid glass/carbon pultruded FRP shapes was discussed. They have been manufactured in order to lower cost and maximize performance; however, tests on hybrid beams have shown that the carbon fiber-matrix interface was a weak link, and this weakness may lead to a delamination problem. Such a case study confirms that new opportunities should be explored, but the effectiveness of new profiles and systems needs to be demonstrated and improved by experimental characterization. In the paper by Lopez-Anido and Xu, an hybrid FRP composite-glulam panel system for bridge decks is presented; the idea of coupling a traditional material such as wood and FRP in order to maximize the performance of the final system appears very interesting. Along with technological aspects, the results of laboratory tests on the new system are discussed, and the influence of two different fiber orientations on its structural performance is evaluated. Based on recorded data and observed failure mode, a formulation based on lamination theory is proposed for predicting deformability performance of the panel.
