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ABSTRACT Heuristically known at least since the first half of XIX century, the self-healing capacity of cement-based materials has been receiving keen attention from the civil engineering community worldwide in [...]

Abstract

The aim of this work is to develop a procedure to follow the evolution of micro and macro-crack in unreinforced and reinforced masonry walls by means of Acoustic Emission signal analysis. In particular the main goal of this experimental work want to be the definition of a characteristic signal parameter able to represents the evolution of the fracture dimension in the masonry walls during the fracture process. To this end laboratory tests were carried out at the Non-Destructive Testing Laboratory of the Politecnico di Torino.

Experimental researches conducted in recent years at the Non Destructive Testing Laboratory of Politecnico di Torino have revealed an unexpected thermal response of concrete subject to short duration cyclic compressive tests: a cooling phenomenon has been observed on medium-quality concrete cores and this contradicts Kelvin’s thermodynamic laws. Such a decrease in temperature is not to be ascribed to the evaporation of absorbed water, while it depends on the applied load intensity, the application frequency and the total number of cycles performed. Moreover, the cooling phenomenon is accompanied by such mechanical effects as performance enhancement and increasing brittleness, observed through static tests performed at the end of cyclic compression: an increase in the failure strength and the elastic modulus of the material as well as a reduction in max. displacement at failure are seen to occur. The present research takes its starting point in the above observations and aims to investigate the relations between thermal and mechanical effects in concrete, in order to check the possibility to use the variation of temperature at the end of a short duration cyclic compressive test as an indicator for the evaluation of damage, the final aim being to develop in the future a methodology for diagnosis of existing structures.

The aim of this work is to analyse the mechanical response of the masonry specimens under long-term action by means of cyclic tests. To this end laboratory tests were carried out at the Non-Destructive Testing Laboratory of the Politecnico di Torino. The Acoustic Emission technique was employed to assess the damage evolution, and the mechanical properties decay in order to evaluate the extent and the evolution of micro and macro-cracking due to cyclic action until structural collapse in masonry blocks and mortar layers.

The paper presents the results of an experimental investigation conducted on bricks. Prism-shaped specimens were produced. Laboratory tests were carried out at the Non-Destructive Testing Laboratory of the Politecnico di Torino. They were subjected to static compressive tests for the determination of their mechanical properties and cyclic compressive tests for the analysis of their fatigue behaviour. The aim of this work is to apply a failure criterion based on the observation of the static and cyclic curves to estimate the fatigue life and the long-term behaviour of the brick. Subsequently, it will be possible to give a relation between the static-displacement and cyclic-displacement of the material tested.

An experimental analysis on a set of strengthened masonry walls has been carried out by means of cyclic loading tests in order to simulate the creep effects. Laboratory tests were carried out at the Non-Destructive Testing Laboratory of the Politecnico di Torino: ad hoc brickwork specimens were manufactured and reinforced by FRP and strengthening mortar. The main goal of this experimental work is to demonstrate that in the choice of the masonry reinforcement is important to value not only the increment of the ultimate strength of the material reinforced, but also its response and durability in term of long-term action, called as creep phenomenon.

The results of an experimental study concerning the behavior of concretes made with recycled aggregates under repeated mechanical and thermal actions are presented. A comparison with the performances achieved by similar concretes made with traditional aggregates in the same working conditions eventually made it possible to estimate the actual quality and durability of the recycled concretes examined and identify possible limitations in their use. The laboratory protocol here adopted is suggested as a practical experimental procedure to test hardened concretes and pre-qualify their performances with respect to compressive actions and thermal solicitations.

The evolution of concrete behavior in the proximity of a joint under the effect of varying external pressures is studied by means of a novel nonlinear ultrasonic technique denoted as Scaling Subtraction Method. The results obtained show that the proposed method is effective in describing the occurrence of micro-structural changes near the joint and detect potential conditions for crack opening and damage initiation.

The presence of damage or cracks modifies the elastic properties of a sample, introducing nonlinear components in the elastic response to an ultrasonic excitation. Among other effects, we discuss here conditioning and memory, i.e. the transition of the sample to a new (non equilibrium) elastic state when perturbed by a strain, even at a relatively low amplitude. A quantification of these phenomena can be the basis for a novel Nondestructive Method to evaluate the integrity of a structural or mechanical component.

One of the signatures of the presence of cracks in a sample is the nonlinearity in its elastic response to an impingent ultrasonic wave. The Fourier analysis is often inadequate to monitor the evolution of nonlinearity, since the signal-to-noise ratio of higher order harmonics is very low. In order to overcome this drawback, we suggest an alternative procedure to extract nonlinearity indicators from a recorded ultrasonic signal, based on the amplitude dependence of the response of the system. The procedure is first described and then used to analyse the evolution of the nonlinearity due to cracks induced by a quasi-static loading in mortar samples. Our approach allows to distinguish the compaction phase from the micro-damage progression and the pre-rupture phases.

The results of an experimental research on plain concrete are presented. The non-linear behavior of both virgin and damaged samples is investigated by means of ultrasonic tests: recent theoretical models, indeed, have pointed out that mono-frequency ultrasonic excitations bring to light such phenomena as harmonic generation and sidebands production, which are essentially due to the material classical or hysteretic non-linearity. The estimation of the harmonic components parameters (amplitudes and phases) is achieved through a signal processing technique based on MUltiple SIgnal Classification (MUSIC) system, which reveals to be optimal for the specific signal model here considered. The experiments described in this paper show that the material non-linear features increase with increasing level of internal micro-cracking, thus suggesting the possibility to use the ultrasonic signal analysis in the frequency domain as a valuable tool for damage assessment.