M. Maalej

Introduction of Strain-Hardening Engineered Cementitious Composites in Design of Reinforced Concrete Flexural Members for Improved Durability

This paper proposes a new design for reinforced concrete flexural members to improve durability. The design makes use of the unique properties of a strain-hardening cementitious composite to limit crack width. The composite is used as a replacement for the concrete material that surrounds the main reinforcement in a regular reinforced concrete member. With this design, it was shown that crack widths under service load conditions can be limited to values never before achieved using conventional steel reinforcement and concrete. Under these conditions, it was concluded that it would be possible to prevent migration of aggressive substances into the concrete or reinforcement. Furthermore, accelerated corrosion due to longitudinal cracking or spalling can be reduced, if not eliminated, and spalling and delamination problems common to many of todays reinforced concrete structures can be prevented

EXTERNALLY BONDED FIBER-REINFORCED POLYMER FOR REHABILITATION OF CORROSION DAMAGED CONCRETE BEAMS

This paper reports the results of an experimental program designed to provide a realistic assessment of the potential of using fiber-reinforced polymer (FRP) materials in the repair and strengthening of reinforced concrete (RC) flexural members. The experimental program included seven RC flexural beams, 270-by-400 mm in cross section and 4,350 mm in length. Four of the seven RC beams were reinforced externally with one or two layers of carbon FRP (CFRP) composite. Variables considered included state of damage (damaged versus undamaged) and loading condition during bonding (loaded versus unloaded). Damage was introduced in four of the seven RC beams using an accelerated corrosion technique developed at the University of Toronto. Test results revealed that it is necessary to consider the effects of corrosion- and load-induced damage as well as sustained load on the load-carrying and deflection capacities of externally reinforced flexural members. Furthermore, it is concluded that it is possible to achieve adequate corrosion repair with externally bonded CFRP and minimal intervention. In particular, results showed that it is important to optimize CFRP layout to balance strength recovery with control of faulting and splitting, which could lead to premature member failure.

Toughening in cement based composites. Part I: Cement, mortar, and concrete

Abstract This paper reviews the mechanisms of toughening in unreinforced cementitious materials, including cement paste, mortar, and concrete. The paper emphasizes the microstructural aspects of the different fracture processes that can potentially take place in these materials, and point out any possible interactions between these processes. Reference is made to three types of fracture process — frontal, crack tip and wake processes, and estimates of contributions to composite toughness of the individual mechanisms are included. It is shown that the fracture mode of a cementitious material closely relates to the types of fracture process that occur in that material. Based on the understanding of the conditions under which certain toughening mechanisms can take place in a given material, it may be possible to control the material fracture mode by tailoring the material microstructure.

Toughening in cement based composites. Part II: Fiber reinforced cementitious composites

Abstract This paper reviews the mechanisms of toughening in fiber reinforced cement based composites. Reference is made to frontal, crack tip and wake processes, and estimates of contributions to composite toughness of the individual mechanisms are included. It is emphasized that the wake processes, which dominate the inelastic energy absorption during fracture development in these materials, can be well characterized by tensile stress vs crack opening relationships. The fiber/ matrix interface debonding energy, not usually important in fiber reinforced concrete, is shown to play an important role in new straining-hardening engineered cementitious composites as an additional frontal process with significant energy absorption capacity, thus giving rise to a cement based material with extremely high damage tolerance.

Modelling of rectangular RC columns strengthened with FRP

Abstract Existing analytical models for predicting the stress–strain or load–displacement response of fibre-reinforced polymer (FRP)-confined concrete are mostly derived for cylindrical plain concrete columns. In practice, however, typical concrete columns come in various shapes including circular, square, or rectangular and incorporate longitudinal and transverse steel reinforcements. Furthermore, strengthening or repairing is typically done while the column is under service loading. In this paper, an analytical model is proposed to predict the load–displacement response of wall-like (i.e. high aspect ratio) reinforced concrete columns strengthened with FRP wraps with and without sustained loading. The model assumes that the general load–displacement response of the strengthened column consists of two distinct branches: a parabolic ascending branch and a linear descending branch. The ascending branch is influenced by the lateral confining pressure from the transverse reinforcement as well as the FRP wraps, while the descending branch is influenced by the buckling of the longitudinal reinforcement and the failure of the core concrete. Comparisons between model results and experimental results indicate close agreement between the two.

Assessment of an extrinsic polymer-based optical fibre sensor for structural health monitoring

Plastic optical fibre sensors offer remarkable ease of handling, and recent research has shown their potential as a low-cost sensor for damage detection and structural health monitoring applications. This paper presents details of a novel extrinsic polymer-based optical fibre sensor and the results of a series of mechanical tests conducted to assess its potential for structural health monitoring. The intensity-based optical fibre sensor proposed in this study relies on the modulation of light intensity as a function of a physical parameter (typically strain) as a means to monitor the response of the host structure to an applied load. Initially, the paper will reveal the design of the sensor and provide an outline of the sensor fabrication procedure followed by a brief description of its basic measurement principle. Two types of sensor design (fluid type and air type) will be evaluated in terms of their strain sensitivity, linearity and signal repeatability. Results from a series of quasi-static tensile tests conducted on an aluminium specimen with four surface-attached optical fibre sensors showed that these sensors offer excellent linear strain response over the range of the applied load. A comparison of the strain response of these sensors highlights the significant improvement in strain sensitivity of the liquid-filled-type sensor over the air-filled-type sensor. The specimens were also loaded repeatedly over a number of cycles and the findings exhibited a high degree of repeatability in all the sensors. Free vibration tests based on a cantilever beam configuration (where the optical fibre sensor was surface bonded) were also conducted to assess the dynamic response of the sensor. The results demonstrate excellent agreement with electrical strain gauge readings. An impulse-type loading test was also performed to assess the ability of the POF sensor to detect the various modes of vibration. The results of the sensor were compared and validated by a collocated piezofilm sensor highlighting the potential of the POF sensor in detecting the various eigen-frequencies of the vibration. Finally, preliminary results of a loading–unloading test of the same sensor design encased within a metal tube will be presented. The results obtained were encouraging offering the possibilities of employing the proposed device as an embedded sensor for damage detection in concrete beams.

Cover Cracking of Reinforced Concrete Beams Due to Corrosion of Steel

A finite element (FE) model is proposed to simulate the corrosion-induced cracking of reinforced concrete (RC) beams. The smeared cracking approach is used to model the cracking of ordinary concrete, ductile fiber-reinforced cementitious composites (DFRCC), and engineered cementitious composites (ECC). The model simulates the cracking of ordinary concrete beams and RC beams containing ECC and DFRCC materials. The strains obtained from the FE models are compared with that measured by the fiber-optic strain sensor (FOSS) gauge, which is placed between longitudinal steel bars at midspan ofRC beams during the accelerated corrosion test. The model could predict the corrosion-induced damage tolerance of ECC and DFRCC materials and found that it is several times higher than that of ordinary concrete. The model predicted the uniform damage in the ECC and the DFRCC materials due to corrosion compared with localized damage in ordinary concrete. The model also predicted that the delamination of the cover of the RC beams containing ECC/DFRCC materials will occur at a higher level of steel loss compared with that of an ordinary concrete beam. The better performance exhibited by the RC beam containing ECC/DFRCC materials is due to their higher tensile strain capacity, strain hardening, and multiple cracking behavior.

Fiber Optic Sensing for Monitoring Corrosion-Induced Damage:

This paper reports the feasibility of using embedded Fabry–Pé rot fiber optic sensors to detect and monitor the propagation of cracks and delamination within concrete beams induced by corrosion of the reinforcing bars. In this research, four series of reinforced concrete beams were subjected to varying degrees of corrosion-induced damage by modifying the composition of the concrete mix and subjecting all specimens to the same accelerated corrosion environment. The concept employed in this study involves embedding the Fabry–Pé rot sensor between two reinforcing bars to measure the transverse tensile strains associated with the longitudinal crack along the reinforcing bars (and in severe cases, delamination of the concrete beam) resulting from the radial expansion of the corroding rebars. Excellent correlation was obtained between the Fabry–Pé rot strain data and the amount of steel loss resulting from accelerated corrosion. In addition, the optical sensor strain readings and the reductions in the load-carrying and deflection capacities were also observed to exhibit strong positive correlation highlighting the potential of the optical sensor to monitor the progression of the rebar damage and the loss of structural integrity of the beams resulting from the extensive corrosion. The technique used in this study demonstrates the possibility of detecting corrosion-induced damage in reinforced concrete structures, particularly those where visual inspection is not possible.

Damage Quantification of Flexurally Loaded RC Slab Using Frequency Response Data

A one-way reinforced concrete (RC) slab was subjected to short-duration concentrated impact load and its dynamic characteristic for the virgin and damaged conditions were studied using two signal processing techniques. The recorded strain and acceleration signals were analyzed using the Fast Fourier Transform (FFT) and the Hilbert Huang Transform (HHT). From these analyses, the percentage reductions in the modal frequency for varying degrees of damage (or magnitude of applied load) were obtained. Based on the eigen-solution of a 3-element partitioned beam model, the frequency–damage relationship was also estimated using the observed initial flexural stiffness of the half-cycle hysteresis path associated with each stage of applied load. Both semi-empirical and experimental results showed close agreement and a 30% frequency reduction was observed between the virgin state and yield. Three quarters of the total frequency reduction from virgin-to-yield occurred within an applied load range of 30% of the yield load.

Structural health monitoring of smart structures

An intelligent monitoring procedure has been applied to static and dynamic field data collected from two innovative structural systems that have recently been constructed in the provinces of Manitoba and Nova Scotia, Canada. These structures incorporate innovative materials and systems and have been extensively instrumented with both fiber optic and electrical strain gauge sensors. Sensor data are remotely accessed and analysed, and performance indices appraising the structural health and performance of the structures are computed.