18th Hellenic Conference on Concrete (page in Greek)| 29-31 March 2018
Eugenides foundation, Athens
Non Destructive Testing methods have contributed crucially to the in-situ and time effective evaluation of constructions structural integrity. Although their advantages, they suffer from the important drawback of physical contact requirement as in most cases their implementation demands the installation of sensors on monitoring structure surfaces. Present paper deals with the application of non-contact and remote scanning technique of Laser Scanning Vibrometry for the control of concrete structural elements. According to the specific method, vibration velocity of construction is acquired in frequency domain on several points of a monitoring grid and velocity distribution maps are designed by applying a two-dimensional polynomial regression. As velocity maps imprint the individual vibration patterns of each constructional member, in context of present study are examined the changes that velocity maps undergo when a structural damage occur. The above-mentioned methodology is applied for the monitoring of dynamic behavior changes a steel-fiber reinforced concrete beam exhibits, when failures under three-point bending.
Laser Scanning Vibrometry in Concrete and Masonry Non Destructive testing (LSV-CoMaND) is a research project which deals with the development of an analytical protocol and data post-processing system for the non-contact and non-destructive detection of structural flaws in concrete and masonry constructional elements, exploiting Laser Scanning Vibrometry (LSV). read more…
LSV-ComMaND project is funded from Technical University of Crete’s Special Account for Research as postdoctoral research (Oct. 2016 – Sep. 2017).
Applications in concrete structural health monitoring
Presentation in context of summer school Mechanics Meets Informatics, Technical University Campus, Chania, July 4-14, 2016.
Presentation file: (pdf)
Efficient inspection of engineering structures demands the frequently and reliable monitoring of several functional features. In case of mechanical behavior control it is necessary to be estimated the structural integrity level and health condition of a construction, both locally and globally . Several parameters such as strength of building materials and stiffness of constructional members have to be checked for sufficient assessment of integrity level.
Non Destructive Testing (NDT) has contributed essentially in improvement of health monitoring procedures. Providing the option of structural parameters assessment without the need of material specimens sampling , control of structural control can be achieved in-situ and in short time. A wide variety of NDT techniques based on different physical principles have been proposed and developed for commercial use, such as ultrasonic testing, rebound hammer, impulse-echo method, thermography, ground penetrating radar etc.
Present lecture deals with the family of NDT techniques which derive from observation of constructions vibrations characteristics. It is well known that when a construction has been excited from an external dynamic load, vibrates in an unique and specific pattern. The vibration modes are related with constructions dynamic features like resonant frequencies. Dynamic characteristics are physically correlated with the mechanical properties of building materials, the geometric properties of individual members and the boundary conditions (fundaments, structural members’ connection joints etc.) of construction. As implementation principle of vibration based NDT can be termed the monitoring of changes in dynamic behavior of a construction when a mechanical damage occurs. In context of lecture’s presentation are quoted some applications of Impact–Response methods and Laser Scanning Vibrometry in concrete NDT.
Fig 1. Impact-Response scanning of a cracked beam. Impact excitation in several points – Acceleration response in a steady point (Control Point) .
DAAD SUMMER SCHOOL | Chania 2016
with the support of the German Academic Exchange Agency DAAD
4 – 14.07.2016, Technical University of Crete, Chania, Greece
Cooperating groups from the Technical University of Crete and the Technical University of Braunschweig with the financial support of the German Academic Exchange Agency (DAAD) organize a summer school in Chania with the title ‘Mechanics meets informatics’. The summer school is addressed to graduate students of both Universities who are interested to work further in this direction and establish contacts with their colleagues and the lecturers of the school. Interested students should send an application to the organizer before 15th of May 2016. A committee composed of the lecturers from the Technical University of Crete will make the final selection. Participation to the school is free of charge. Participation to some of the social activities (dinners, excursions) may require a small contribution to cover the cost. Students who will attend the summer school to the end will obtain a Certificate of Attendance.
Further informations available from:
Prof. Georgios E. Stavroulakis
Dr. Magdalene Marinaki
Dr. Eleftheria Sergaki
Applications in Non Destructive Testing (NDT) of concrete, masonries and heritage structures
Lecture in context of post-graduate course Instrumental monitoring of monuments and heritage structures” of TUC’s School of Architectural Engineering (Code: ARCH105 | Spring semester). Course Instructor: Prof. Costas P. Providakis
Lecture presentation (pdf). Published under the terms of creative common license.
Monuments and historic buildings represent the constructed section of cultural heritage of a society, a nation or a country. Conservation of monumental constructions is aiming both to the reviving of their classical aesthetic, concerning their initial appearance, and to the restoration of their structural durability. In many cases arise a conflict between the demand of minimum intervention on monuments faces and the need of repairing or replacing parts of problematic constructional elements.
Considering, for example, a partially damaged masonry wall of a historical building; the damages could be appeared either as fractured blocks or as corroded parts of mortar. A possible restoration process it could be containing the replacement of cracked blocks or the refilling of interfaces between blocks with a properly fabricated restoration mortar. In both above cases a new portion of material will be placed on initial structure’s body. The sufficient embedding of new materials in masonry’s body depends from the relativity between their mechanical properties and the mechanical properties of the existed building materials. Furthermore, in order to be restricted the area of intervention it must be localized accurately the damaged zone. Damaged zones usually are characterized from the decrease of structural members mechanical strength and generally, from the alteration of constructional materials mechanical properties. Taking into consideration the above facts it can be stated that the reliable estimation of structure’s mechanical properties and integrity level, is crucial for the design of the repairing method.
Nondestructive testing (NDT) techniques provide the options of in-situ a) mapping of possible damage zone, b) estimation of monuments structural integrity before the restoration and c) evaluation of repairing techniques effectiveness, after the finalization of restoration procedure. Moreover NDT methods are unchained from the need of specimen sampling, a procedure that induces further artificial damages in a historical structure. Several nondestructive methods have been developed for the fulfillment of a wide range of monitoring purposes, based on different theoretical principles.
Some of the most widespread NDT methods regarding the monitoring of structural integrity rely on the observation of constructions response to dynamic loads excitation (Doebling et al, 1996). The excitation of structures can be either guided, generated from dynamic motion actuators or arbitrary, originated from ambient wave source (wind, electromechanical installations noise, traffic, earthquakes etc). The vibrating modes of a structural system are governed by its dynamic features which are time-invariant and independent form excitation type. The most vital dynamic features are the resonant frequencies and the respective spectral amplitudes of the construction. Resonant frequencies are strongly related with the mechanical parameters of structure (stiffness, mass and damping) and tend to be altered in cases where these parameters change as result of a structural damage.
Laser Doppler Vibrometry (LDV) is a non-contact technique for the remote acquiring of vibration velocity time histories on specific control points of a construction. Vibration velocity is approximated by measuring, via an interferometer, the Doppler effect-caused phase shift of a laser ray which is interacting optically with a vibrating surface. Recent developments in LDV instrumentation have expanded the single point survey to automatic scanning of vibration velocity in a multi-point monitoring grid. This technique has been termed as Laser Scanning Vibrometry (LSV) and provides time-efficient control of large areas on vibrating surfaces (Fig 1.).
Fig 1. Scanning of a masonry structure’s vibrating face under ambient excitation (natural vibration). Color Map: Snapshot of displacement Fourier amplitudes distribution in 119 Hz.
The present lecture deals with the basic aspects of LDV and LSV, and the application of them in NDT of concrete and masonry structures. All the experiments have been executed utilizing the facilities and equipment (Polytec Inc, PSV) of Applied MEchanics Laboratory (AMEL) of TUC.
Damage detection in concrete structures using smart piezoelectric sensors as concrete’s aggregates
Extended Abstract | E.V. Liarakos, Jan2015.
The present PhD thesis deals with the application of piezoelectric materials in non-destructive testing of concrete, and in particular, with their applicability in damage detection of structural elements. Moreover, the application of piezoelectric materials in monitoring of early age concrete mechanical behavior is investigated. Due to the need for continuous gathering of information regarding the condition of concrete structural members from the first stages of their construction, one of the main aims of the present research is the design and development of monitoring devices based on piezoelectric materials, which are embedded to concrete mass and characterized as “smart” piezoelectric aggregates.
The main technique that has been employed to monitor the structural integrity of the concrete is the method of Electro-Mechanical Impedance (ΕΜΙ). This method is based on a comparative analysis between a reference response spectrum that corresponds to the state of the “healthy” structure (undamaged structure) and a spectrum which is associated with a state where the structure suffers from damages and/or degraded mechanical properties. In the context of this thesis a novel method for performing the comparative analysis has been developed. This method is based on the statistical control of changes that occur between the reference spectrum and the response spectrum of damaged structure. Statistical control of the changes is implemented by identifying specific confidence limits regarding to the differences that occur among the recorded spectra of undamaged structure.
In the filed of instrumental monitoring of concrete structural systems using “smart” piezoelectric aggregates, a cost-effective prototype integrated system has been developed for the automatic recording of Electro-Mechanical Impedance at regular time intervals (Fig. 1). This system includes a low-cost electronic board, referred as AD5933 EB (Evaluation Board), and a prototype Teflon based “smart” piezoelectric aggregate. Moreover, this system utilizes wireless technology to transfer data from the AD5933 EB to a client computer and enables the assembly of the impedance measurements in a suitably designed database. The main contribution of the proposed system is that it enables every user that has access rights in the database, to be connected remotely via internet and to extract, via properly scripted queries, the desired Electro-Mechanical Impedance measurements for real-time structural integrity assessment and post-processing.
The method of the Electro-Mechanical Impedance in conjunction with the statistical analysis performed on the recorded response spectra using the integrated prototype recording system, have been implemented in a number of characteristic tests of structural concrete elements. More specifically, the applications that have been examined within the present doctoral study are the following: a) monitoring of stiffness and strength development of the early age concrete, from the 3 first hours after casting until the age of 28 days, b) detection of damages in a cubic concrete specimen that is gradually led to failure via a axial loading, and c) detection of damages in a concrete beam specimen that is gradually led to failure through a three-point bending loading.