Effects of moisture on debonding in FRP-retrofitted concrete systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 169-176).FRP (fiber reinforced polymer) retrofit systems for reinforced concrete (RC) structures have been widely used in the past 10 years, and numerous studies on its short-term debonding behavior have been conducted extensively. Nevertheless, long-term performance and durability issues regarding such debonding behavior still remain largely uncertain and unanswered. In this study, comprehensive experimental and analytical investigations of debonding in FRP bonded concrete systems under long-term environmental exposure, namely moisture, have been performed to develop mechanics-based predictive debonding failure models and related design tools for the prevention of debonding failures. The experimental approach involves an investigation of debonding under moisture ingress, moisture reversal, cyclic moisture conditioning, using the concept of fracture mechanics and meso-scale tri-layer fracture specimens. Prolonged exposure to moisture condition has been shown to result in significant degradation of the FRP/concrete bond strength, while an irreversible weakening in the bond strength has also been observed in the fracture specimens under moisture cyclic condition. A predictive model has been developed based on the experimental results of the fracture specimens under variable cyclic moisture conditions for service life prediction of the FRP-strengthened systems. To incorporate this quantification method and experimental data, finite element analysis and correlation study have been conducted for RC beams externally strengthened with FRP plate. The cohesive zone model for interfacial fracture has been implemented, for which the properties of the interface were obtained from the meso-scale fracture tests. Test on laboratory-scale FRP-strengthened RC beam specimens under continuous moisture conditions has demonstrated the applicability of the methodology. In addition, molecular dynamics simulation has been conducted to gain more fundamental understanding of interface fracture behavior in bi-layer material systems (i.e. crack initiation and propagation direction) and the effects of material and interface properties at atomistic scale. Finally, a recommendation has been made to the current design of RC beams strengthened with FRP to prevent premature failure due to long-term exposure to moisture.by Chakrapan Tuakta.Ph.D

Natural fiber-reinforced light-weight cement blocks prepared from waste for sustainable development

Low density of the light-weight cement blocks offers an advantage in terms of dead load reduction, which is advantageous in structural design-reduction of size and numbers of load-carrying structural components is possible. Production of cement blocks, however, generally requires Ordinary Portland Cement (OPC) which creates harmful environmental impacts. Utilization of waste as alternative raw materials for cement production is a route to alleviate the problem. This research aims at synthesizing eco-friendly cement-like material for production of light-weight cement blocks. The cement-like material were prepared from eggshells, cockleshells, and rice husk ash (RHA). With high content of calcium oxide and silica, eggshells and cockleshells are potential sources of calcium, whereas RHA is a good source of silicon. Additionally, a fuel-efficient solution combustion technique was employed in synthesis of the cement-like material. Phase identification analysis of the synthesized powder indicated that tri-calcium silicate (C3S), di-calcium silicate (C2S), tri-calcium aluminate (C3A),and tetra-calcium alumino ferrite (C4AF), which are main constituents of OPC, were obtained. To fabricate eco-friendly light-weight cement blocks, the synthesized cement-like material were mixed with cement, water, and additional RHA and cast into blocks. The optimal compressive strength and density of the cement blocks were in comparable range with the standard light-weight concrete defined by Thai Industrial Standards Institute (TISI) and American Concrete Institute (ACI 213,2001). With jute fiber reinforcement, enhanced compressive strength of 20% was achieved, while elimination of spalling after compression test was clearly evident, implying a more ductile failure

Water degrading effects on the bond behavior in FRP-strengthened masonry

Fiber reinforced polymers are being extensively used for external strengthening of masonry structures. However, durability of this strengthening technique under environmental conditions is still under inves- tigation. Previous studies indicate that moisture plays an important role in the durability of bond between FRP and substrate. Moisture can cause degradation in the bond behavior and also in the mechan- ical properties of the constituent materials. This paper presents and discusses the results of an experi- mental investigation on the effects of moisture on the bond behavior in FRP-strengthened masonry bricks. The degradation in the bond performance has been investigated by performing pull-off and pull-out tests on the conditioned specimens. The change in the mechanical properties of the materials has also been investigated. Comparative analysis has been performed and the results are presented and critically discussed.This work was partly funded by project FP7-ENV-2009-1-244123-NIKER of the 7th Framework Program of the European Commission, which is gratefully acknowledged. The first author also acknowledges the financial support of the Portuguese Science Foundation (Fundacao de Ciencia e Tecnologia, FCT), through grant SFRH/BD/80697/2011

Use of FRP in bridge structures

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2005.Includes bibliographical references (leaves 45-46).Fiber reinforced polymer composite (FRP) is a new construction material, gradually gaining acceptance from civil engineers. Bridge engineering is among the fields in civil engineering benefiting from the introduction of FRP composite. Its advantages over traditional construction materials are its high tensile strength to weight ratio, ability to be molded into various shapes, and potential resistance to environmental conditions, resulting in potentially low maintenance cost. These properties make FRP composite a good alternative for innovative construction. In the past 10 years, experiments have been conducted to investigate the applicability of FRP composite in bridge structures, including the applications of FRP composite girder and bridge deck, column and beam strengthening, etc. This document will first present the basic information of FRP composite, including its mechanical behaviors and manufacturing processes relevant to civil engineering applications. Then the application of FRP composite in bridge engineering will be investigated, through three case studies. Four main issues contributing to the slow acceptance of the material into construction industry as a whole, despite its success in aerospace and automobile industries, are discussed at the end.by Chakrapan Tuakta.M.Eng

Structural solution using molecular dynamics: Fundamentals and a case study of epoxy-silica interface

In this paper, the molecular dynamics (MD) simulation technique is described in the context of structural mechanics applications, providing a fundamental understanding of the atomistic approach, and demonstrating its applicability. Atomistic models provide a bottom-up description of material properties and processes, and MD simulation is capable of solving the dynamic evolution of equilibrium and non-equilibrium processes. The applicability of the technique to structural engineering problems is demonstrated through an interface debonding problem in a multi-layered material system usually encountered in composite structures. Interface debonding may lead to a possible premature failure of fiber reinforced polymer (FRP) bonded reinforced concrete (RC) structural elements subjected to moisture. Existing knowledge on meso-scale fracture mechanics may not fully explain the weakening of the interface between concrete and epoxy, when the interface is under moisture; there is a need to study the moisture affected debonding of the interface using a more fundamental approach that incorporates chemistry in the description of materials. The results of the atomistic modeling presented in this paper show that the adhesive strength (in terms of energy) between epoxy and silica is weakened in the presence of water through its interaction with epoxy. This is correlated with the existing meso-scale experimental data. This example demonstrates that MD simulation can be effectively used in studying the durability of the system through an understanding of how materials interact with the environment at the molecular level. In view of the limitation of MD simulation on both length- and time-scales, future research may focus on the development of a bridging technique between MD and finite element modeling (FEM) to be able to correlate the results from the nano- to the macro-scale.National Science Foundation (U.S.) (Grant CMS-0856325