Two-dimensional strain field analysis of reinforced concrete D-regions based on distributed optical fibre sensors

The introduction of Distributed Optical Fibre Sensing in experimental testing of reinforced concrete structures has enabled the acquisition of measurements with an unparalleled level of detail, providing an accurate and ubiquitous description of cracking and deflections throughout an element. However, most of the available research using this technology has focused on the study of beam specimens and high quality data for the calibration and development of models that can describe accurately the behaviour of D-regions in service is still lacking. For that reason, the application of distributed optical fibre sensing in D-regions remains a subject of interest. In this work a method for the deployment of fibre sensors in a multilayer configuration is presented for a wall element. An interpolation approach is then proposed, which combined with the distributed nature of the sensors enabled the description of detailed heat maps for the global and principal strain fields. The results indicated that shear strains can reveal the position of shear cracks well before they are formed whereas the maximum and minimum principal strains clearly show the crack pattern and crack development as well as the load transfer mechanisms including, for the first time, the experimental identification of a secondary strut-and-tie mechanisms

Crack monitoring in reinforced concrete beams by distributed optical fiber sensors

This paper investigates the use of distributed optical fiber sensors (DOFS) based on Optical Frequency Domain Reflectometry of Rayleigh backscattering for Structural Health Monitoring purposes in civil engineering structures. More specifically, the results of a series of laboratory experiments aimed at assessing the suitability and accuracy of DOFS for crack monitoring in reinforced concrete members subjected to external loading are reported. The experiments consisted on three-point bending tests of concrete beams, where a polyamide-coated optical fiber sensor was bonded directly onto the surface of an unaltered reinforcement bar and protected by a layer of silicone. The strain measurements obtained by the DOFS system exhibited an accuracy equivalent to that provided by traditional electrical foil gauges. Moreover, the analysis of the high spatial resolution strain profiles provided by the DOFS enabled the effective detection of crack formation. Furthermore, the comparison of the reinforcement strain profiles with measurements from a digital image correlation system revealed that determining the location of cracks and tracking the evolution of the crack width over time were both feasible, with most errors being below +/- 3 cm and +/- 20 mu m, for the crack location and crack width, respectively

Long-term performance of distributed optical fiber sensors embedded in reinforced concrete beams under sustained deflection and cyclic loading

This paper explores the performance of distributed optical fiber sensors based on Rayleigh backscattering for the monitoring of strains in reinforced concrete elements subjected to different types of long-term external loading. In particular, the reliability and accuracy of robust fiber optic cables with an inner steel tube and an external protective polymeric cladding were investigated through a series of laboratory experiments involving large-scale reinforced concrete beams subjected to either sustained deflection or cyclic loading for 96 days. The unmatched spatial resolution of the strain measurements provided by the sensors allows for a level of detail that leads to new insights in the understanding of the structural behavior of reinforced concrete specimens. Moreover, the accuracy and stability of the sensors enabled the monitoring of subtle strain variations, both in the short-term due to changes of the external load and in the long-term due to time-dependent effects such as creep. Moreover, a comparison with Digital Image Correlation measurements revealed that the strain measurements and the calculation of deflection and crack widths derived thereof remain accurate over time. Therefore, the study concluded that this type of fiber optic has great potential to be used in real long-term monitoring applications in reinforced concrete structures

The interplay between corrosion and cracks in reinforced concrete beams with non-uniform reinforcement corrosion

This paper investigates the interplay between corrosion of reinforcement and corrosion-induced cracking in reinforced concrete structures with non-uniform corrosion distribution based on the experimental results of a concrete beam simultaneously subjected to sustained deflection and accelerated corrosion through impressed current. Unlike previous studies, this work encompasses various refined techniques for the measurement of surface cracks, such as digital image correlation and distributed optical fiber sensors, as well as for the assessment of reinforcement corrosion, namely 3D laser scanning, to explore previously hidden aspects of the relationship between the two parameters. The applied techniques proved very effective in providing an unprecedented level of detail of both the crack development and corrosion distribution. More specifically, the formation and propagation of corrosion-induced cracks were accurately and constantly monitored over time and subsequently compared to the distribution of corrosion. The results revealed that determining the maximum corrosion level or even the location of the section with maximum corrosion based solely on visual inspection of the surface crack width may not be possible. However, the width of corrosion-induced cracks was found to increase linearly with the local corrosion level, implying that crack width monitoring can still be used to estimate the rate of corrosion degradation

Investigation on the influence of fibre reinforcement on chloride induced corrosion of RC structures

Corrosion of reinforcement is the main cause of deterioration of RC structures located in marine environments or subjected to de-icing salts. In order to delay the ingress of chlorides, such structures require the use of thick, dense concrete covers and strict crack width limitations. Given the crack limiting effect provided by fibres, it would be of interest to incorporate fibre reinforcement to conventionally reinforced concrete structures. Nevertheless, whereas fibre reinforcement might delay the penetration of detrimental agents into the concrete through crack control, there are other aspects which need to be addressed to determine whether fibres can improve the overall durability of RC structures. Several experiments were conducted within the present project, parts of them still ongoing, to investigate some of these aspects, namely: (i) the effect of fibres on the diffusivity of uncracked concrete; (ii) whether fibres may affect the corrosion onset of rebars for any crack width; (iii) whether steel fibres, due to their conductive nature, might influence the resistivity and consequently the corrosion rate of embedded rebars; and (iv) whether there is a risk of galvanic corrosion between steel fibres and rebars. Obtained results from chloride migration and bulk diffusion tests showed that fibres had no significant influence on the diffusion coefficient of concrete. Results from experiments in which RC beams were subjected to different loading conditions and thereafter naturally corroded through exposure to highly concentrated salt solution, showed a trend for earlier corrosion initiation with increasing crack width. Concrete mixes incorporating fibres exhibited similar or delayed corrosion onset compared to their plain concrete counterparts. While resistivity of FRC was consistently lower than resistivity of plain concrete measured under AC at 1 kHz, corrosion rate measurements based on the galvanostatic pulse technique showed no clear correlation between the presence of fibres and the corrosion rate

Corrosion initiation in cracked fibre reinforced concrete: Influence of crack width, fibre type and loading conditions

This paper reports results from an ongoing project aimed at investigating the influence of fibre reinforcement on corrosion of rebar in chloride environments. Material tests showed that the resistivity of concrete decreased with the addition of fibres, whereas the chloride migration coefficient remained unaffected. Fibres at low dosages (<1.0% vol.) did not significantly affect the compressive and flexural strength of concrete but greatly enhanced its toughness. The results from corrosion tests showed a tendency of an earlier initiation of corrosion with increasing crack widths, while a small improvement was observed by the addition of fibres in terms of delayed corrosion initiation

Performance requirements for Swedish transport infrastructure - A pre-study of challenges and possibilities

In recent years, significant worldwide research has been conducted regarding the performance assessment of bridges and the concept of performance indicator has been introduced However, there are still significant discrepancies in how these indicators are obtained and used. Simultaneously, it is desirable to achieve processes and methods that are direct, i.e. that measured values are directly compared with projected values over time. This project concerns methods for verification of technical performance requirements. The feasibility study brought together interdisciplinary researchers, consultants, and entrepreneurs to gather knowledge, anchor the research agenda, and implement performance requirements. The project concludes that there is a need for a “Holistic multi-parameter verification/validation system” that relies on the knowledge gained in structural health monitoring research

Monitoring of new and existing stainless-steel reinforced concrete structures by clad distributed optical fibre sensing

The implementation of structural health monitoring systems in existing civil engineering structures could contribute to a safer and more resilient infrastructure as well as important savings. Due to their light weight, small size, and high resistance to the environment, distributed optical fibre sensors (DOFS) stand out as a very promising technology for damage detection and quantification in reinforced concrete structures. This dataset includes information of DOFS featuring an external polymeric cladding with rough surface, deployed in a stainless-steel reinforced concrete beam subjected to four-point bending. Several sensor positions, both embedded in the concrete and attached to the surface, are included in a multilayer configuration. The data of the sensors includes two series of test, first cyclic loading under service loads and lastly cyclic loading to failure. Additionally, data from Digital Image Correlation and the actuator recordings are included for cross-validation purposes

CYBERBRIDGE: AN INTERACTIVE TOOL TO PROMOTE ACTIVE LEARNING IN STRUCTURAL ENGINEERING COURSES

Understanding basic concepts and assumptions is a crucial step in the learning process of any subject, yet today it is not always achieved. A change of the current teaching method is suggested in this paper through the implementation of new learning activities based on digital tools. The use of interactive tools has been shown to be an effective method in the field of medicine. Although less frequent, interactive tools could also be beneficial in the field of civil engineering, particularly in structural engineering, where real lab experiments are large, costly and time consuming. By building a small-scale interactive bridge model, the students can have access to a live experiment in the classroom, which enables them to put in practice the concepts learnt during the lectures. This tool seeks promoting a deeper understanding of the subject, thereby increasing the ability of the students to describe different concepts and explain the physical meaning behind the analytical equations taught in structural engineering courses

Incorporation of pre-existing cracks in finite element analyses of reinforced concrete beams without transverse reinforcement

Cracking in reinforced concrete (RC) bridges and other structures is common and not necessarily detrimental. However, some cracks may grow past specified limits and, aside from aesthetic and durability aspects, may influence the ductility and structural capacity of an RC member. This is not generally reflected in current assessment methods and, therefore, improved methods are needed. The aim of the current work was to develop a modelling methodology to incorporate pre-existing cracks into finite (FE) analysis for improved structural assessments. Two different approaches were investigated: (1) weakening the continuum elements at the position of a crack and (2) introducing discrete crack elements with weakened properties. In both approaches, a total-strain based model was used in the continuum elements. These modelling approaches were applied to analyses of experiments, in which concrete beams had been pre-cracked and tested in four-point bending. The pre-existing cracks led to differing failures limiting the deformation capacity, plus varying ultimate capacity and ductility. In the current study the weakened-elements approach captured the failure characteristics, ultimate capacity and ductility more accurately than a standard FE analysis without cracks included; the discrete-crack approach, on the other hand, did not. Furthermore, the bending stiffness differed between the experimental tests and the FE analyses. Damaged bond properties and closure of cracks in the compressive zone were identified as probable causes. Moreover, the choice of shear retention used for the weakened elements was shown to noticeably affect the predicted capacity and ductility. In conclusion, the weakened-elements approach was the most straightforward to implement. It was less time-consuming and led to better agreement with experimental results, compared to the discrete-crack approach. Based on this study, the weakened-elements approach is regarded as a promising approach for the structural assessments of tomorrow