Interpretation formulas for in situ characterization of mortar strength

Abstract The in situ characterization of mortar mechanical properties is a mandatory activity in every structural assessment of existing monumental buildings. Fine and complex numerical models are meaningless if the material parameters are hypothetical or based on reference values since there exist thousands of mortar compositions and brick types and aging processes do change values in a not easily predictable way. Therefore direct investigation of the mortar courses is a key ingredient, and this requires specific tools able to deal with thin layers of materials and requirements of conservation. There are however several ways able to investigate mortars without extracting consistent specimens, and most of them can be interpreted within the common framework of a Mohr-Coulomb plasticity constitutive behavior. Several of these techniques are even capable to discriminate superposed repointing layers of sufficient depth. In the paper, some versatile and effective mortar investigation techniques are reviewed and the formulas for the strength class definition are presented. Then, the predictive ability of the proposed formulas is checked against large experimental campaigns. Finally, an interesting investigation on the Ponte Taro bridge in Parma is used in comparing the results of different experimental tools. The data show a general good agreement among the mechanical properties extracted with the proposed procedures

Caratterizzazione dinamica di ponti in calcestruzzo: analisi del ponte Bacchelli a Bologna

Il presente lavoro di tesi viene svolto all’interno del “Progetto SHAPE – Predicting Strength Changes in Bridges from Frequency Data Safety, Hazard, and Poly-harmonic Evaluation”, progetto che fa parte del programma “ERA-NET Plus Infravation 2014 Call” finanziato dalla Comunità Europea e che si occupa della valutazione dello stato di sicurezza dei ponti esistenti attraverso la definizione di un sistema di acquisizione e monitoraggio dei parametri modali al fine di percepirne le variazioni. L’elaborato si compone principalmente di due parti, la prima introduce il monitoraggio, le principali tecniche di identificazione del danno e riprende alcune delle principali nozioni inerenti all’analisi del segnale e alla dinamica delle strutture; la seconda parte, invece, si focalizza principalmente sull’applicazione dell’analisi nel dominio delle frequenze di segnali acquisiti sperimentalmente mediante la tecnica di analisi modale operazionale di tipo Output-Only. La caratterizzazione dinamica viene condotta su due ponti modello, realizzati in scala 1:4 presso il laboratorio LISG della Scuola di Ingegneria e Architettura di Bologna, e per un ponte esistente, il ponte R. Bacchelli, situato sempre a Bologna. L’acquisizione dei dati accelerometrici si è operata a seguito di un’eccitazione impulsiva nel caso dei modelli in scala, e di un’eccitazione random, quale quella del traffico, per quanto riguarda il ponte esistente in condizione di esercizio, l’elaborazione dei dati è stata effettuata mediante tecniche di analisi del segnale in ambiente Matlab, i cui risultati sono stati poi confrontati con quelli ottenuti da modelli FEM realizzati con il software Straus7 ed infine si è proceduto ad un’analisi critica dei risultati

Damage Identification of Structures through Vibration-Based Structural Monitoring Systems

The thesis has been carried out within the “SHAPE Project - Predicting Strength Changes in Bridges from Frequency Data Safety, Hazard, and Poly-harmonic Evaluation” (ERA-NET Plus Infravation Call 2014) which dealt with the structural assessment of existing bridges and laboratory structural reproductions through the use of vibration-based monitoring systems, for detecting changes in their natural frequencies and correlating them with the occurrence of damage. The main purpose of this PhD dissertation has been the detection of the variation of the main natural frequencies as a consequence of a previous-established damage configuration provided on a structure. Firstly, the effect of local damage on the modal feature has been discussed mainly concerning a steel frame and a composite steel-concrete bridge. Concerning the variation of the fundamental frequency of the small bridge, the increasing severity of two local damages has been investigated. Moreover, the comparison with a 3D FE model is even presented establishing a link between the dynamic properties and the damage features. Then, moving towards a diffused damage pattern, four concrete beams and a small concrete deck were loaded achieving the yielding of the steel reinforcement. The stiffness deterioration in terms of frequency shifts has been reconsidered by collecting a large set of dynamic experiments on simply supported R.C. beams discussed in the literature. The comparison of the load-frequency curves suggested a significant agreement among all the experiments. Thus, in the framework of damage mechanics, the “breathing cracks” phenomenon has been discussed leading to an analytical formula able to explain the frequency decay observed experimentally. Lastly, some dynamic investigations of two existing bridges and the corresponding FE Models are presented in Chapter 4. Moreover, concerning the bridge in Bologna, two prototypes of a network of accelerometers were installed and the data of a few months of monitoring have been discussed

Damage identification of cracked reinforced concrete beams through frequency shift

The safety evaluation of reinforced concrete (RC) bridges is of the outmost importance, both for the early warning of critical states below a given safety margin and owing to plan maintenance cycles of the infrastructural network. Structural health monitoring based on dynamic testing has become widespread in the last 20\ua0years, leading to very effective operational algorithms able to extract valuable structural features from the recorded signals. However, although in principle it is possible to identify position and severity of the damage by using a finite element model, still some identification issues are unresolved due to the non-linear nature of the oscillations of a cracked beam. In fact, the available experimental data show, for a given damage pattern, a significant underestimation of the natural frequencies given by cracked beam numerical models. This paper presents an approximate solution for the problem of a vibrating damaged RC beam with opening\u2013closing (breathing) cracks. The solution is based on the static equivalence of the kinetic energy and allows incorporating most of the features of a beam loaded above the cracking limit and oscillating under the self-weight with breathing cracks. The comparison with a wide data set collected in the literature points out the predictive capability of the developed analytical formulas. An independent test confirms the theoretical results

Static and Dynamic Investigation of the Taro Masonry Bridge in Parma, Italy

The Taro bridge has a very long history since was built by Antonio Cocconcelli in 1820. The bridge is composed of 20 arch spans and is arranged in the shape of three center Perronet arches with flood holes in the piers. The road over the bridge has two lanes and is set for the highest truck loading category. After the second World War has been used intensively for road transportation and due to some intrinsic features of the structure, in recent times many brick and stone detachments occurred. Recently, in order to assess the health of the bridge, static and dynamic testing was carried out, by using a set of 8 trucks filled with sand. A Finite Element Model was worked out for the data interpretation, and the parameters of the model were identified on the basis of the vibration frequencies. In order to test the effectiveness of the model, a comparison is made with the static load testing, finding a very good agreement. The main identified parameter are the compliance of the foundation blocks, and the elastic modulus of the fill. The analysis shows that models with different number of spans are all capable to identify the main frequencies of the bridge, if the correct value of the main parameters is introduced