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Concrete constructions must be designed to last over time while maintaining their mechanical integrity and conditions of service during the service life they have been projected. In this regard, the evaluation of concrete and its constituents in laboratory and field tests is therefore essential to ensure the service conditions required to the concrete. The test for determining the resonance frequencies has traditionally been used to investigate the mechanical integrity of concrete cores, to assess the conformity of concrete constituents in different accelerated durability tests, and to ascertain constitutive properties such as the elastic modulus and the damping factor. The test conditions for determining the resonance frequencies in concrete samples are currently standardized by organizations such as ASTM —American Society for Testing Materials— or recommended by RILEM —International union of laboratories and experts in construction materials, systems and structures—. Its use is referred in standards for assessing the durability of concrete against freeze-thaw cycles and sulfate attack. However, this nondestructive technique has not only been applied under these standards, but it has been quite appealed for evaluation of mechanical properties in all kinds of durability tests. The damage evolution is commonly assessed from the reduction of dynamic modulus which is produced as a result of any cracking process. The mechanical behavior of concrete is intrinsically nonlinear and hysteretic. As a result of a hysteretic stress-strain behavior, the elastic modulus is a function of the strain. In dynamic tests, the nonlinearity of the material is manifested by a decrease of the resonance frequencies, which is inversely proportional to the excitation amplitude. This phenomenon is commonly referred as fast dynamic effect. Once the dynamic excitation ceases, the material undergoes a relaxation process whereby the elastic modulus is restored to that at rest. This phenomenon is termed as slow dynamics. These phenomena —fast and slow dynamics— find their origin in the internal friction of the material. Therefore, in cement-based materials, the presence of microcracks and interfaces between its constituents plays an important role in the material nonlinearity. In the context of the assessment of concrete durability, the damage evolution is based on the increase of hysteresis, as a result of any cracking process. In this thesis three different nondestructive techniques are investigated, which use impacts for exciting the resonant frequencies. The first technique consists in determining the resonance frequencies over a range of impact forces. The technique is termed Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). It consists in ascertaining the downward resonant frequency shift that the material undergoes upon increasing excitation amplitude. The relationship between the resonance frequency and amplitude of excitation is used to evaluate the nonlinear behavior. In this thesis, the NIRAS technique is used in Portland cement mortars 40x40x160mm3 produced with different water-cement and aggregate-cement ratios. The mortar samples are subjected to various degradation processes. In particular, the experimental program includes the evaluation of the dynamic properties of the mortars when subjected to increasing number of freeze-thaw cycles, and increasing levels of compressive load. The results show that the parameters derived from the nonlinear mechanical behavior exhibit larger variations than those obtained for the standard parameters (linear) such as resonance frequency or attenuation. Also, the experimental research program includes the effect of internal humidity and drying shrinkage cracking. The results show that drying can induce microcracks. This is especially true in low water-cement ratio and less aggregate content. The microcracking damage generation is detected by NIRAS technique. Moreover, the results indicate that the internal moisture is able to alleviate the mechanisms that lead to nonlinear behavior. Moreover, the NIRAS configuration adopted in this study allows observation of the fundamental flexural and longitudinal modes of vibration simultaneously. It is observed that different vibration mode families exhibit different nonlinear behavior. The precise causes of this behavior are still unknown, but it is plausible to attribute the differences to different hysteretic behavior under compression and shear motions. The NIRAS technique is also used in this thesis to assess the durability of fiber reinforced concrete. The incorporation of fibers to concrete improves mechanical properties, such as toughness and tensile strength, and helps to control cracking processes. The fibers contribute to increase the material nonlinearity which in turn depends on the characteristics of the interface between fibers and the cementitious matrix. However, in many cases the durability of the fiber is compromised in the cementitious matrix, which causes an impairment of the mechanical properties. In this sense, the concrete reinforced with glass fibers is a case of particular interest. The degradation process of the glass fibers is a special case of corrosion under stress, whereby the fiber chemically combines with the hydration products of the matrix, which in turn weakens the fiber and hardens the interface. Therefore, as the glass fibers deteriorate the material reduces its nonlinear behavior. The results show that the NIRAS technique is sensitive to the microstructural changes occurring during the degradation of the glass fibers in concrete. These results can be extrapolated to evaluate the durability of other fiber reinforced materials. The second technique consists in investigating the nonlinear behavior by analyzing the signal corresponding to a single impact. This is, to determine the instantaneous frequency, amplitude and attenuation variations corresponding to a single impact event. This technique is termed as Nonlinear Resonant Acoustic Single Impact Spectroscopy (NSIRAS). Two techniques are proposed to extract the nonlinear behavior by analyzing the instantaneous frequency variations and attenuation over the signal ring down. The first technique consists in discretizing the frequency variation with time through a Short-Time Fourier Transform (STFT) based analysis. The second technique consists of a least-squares fit of the vibration signals to a model that considers the frequency and attenuation variations over time. The technique of least-squares fit provides more accurate estimates of the signal amplitude, than those obtained through STFT based approach. However, the STFT based approach may be preferred in those signals containing multiple modes of vibration, wherein the least-squares minimization may result costly. Furthermore, NSIRAS and NIRAS techniques are compared. The dependence between frequency and amplitude differ between NIRAS and NSIRAS tests. The dependence between resonance frequency and amplitude exhibits a higher order dependence (nonlinear) when it is investigated in a single impact, and linear—frequency is inversely proportional to the excitation amplitude— when it is investigated from multiple impact events. This effect is attributed to the slow dynamic effect. It is then proposed to decompose the resonance frequency variation as, one strictly proportional to the amplitude of the signal, and one that directs away from proportionality the relationship between frequency and amplitude. After decomposing both contributions, it is observed that NIRAS and NSIRAS ascertain similar values of the proportionality constant. This procedure allows quantification of the nonlinear behavior from a single impact event. The application of NSIRAS techniques is demonstrated in mortar and concrete samples subjected to freeze-thaw cycles, thermal shock and carbonation. NSIRAS NIRAS techniques are particularly suitable for durability tests under laboratory conditions, or in any case, in applications where the standard resonance test may be employed. However, its use is excluded for the assessment of concrete structures. In fact, there are still few studies that have managed to extrapolate the possibilities of nonlinear evaluation to on-site measurements of concrete structures. The third technique used in this thesis can be used for onsite evaluation of structures. The technique is based on the Dynamic AcoustoElastic Test (DAET). The variations of elastic modulus as derived through NIRAS and NSIRAS techniques provide an "average" behavior and do not allow derivation of the elastic modulus variations over one vibration cycle. Currently, DAET technique is the only one capable to investigate the entire range of nonlinear phenomena in the material. Moreover, unlike other DAET approaches, this study uses a continuous wave source as “probe”. The use of a continuous wave allows investigation of the relative variations of the elastic modulus, as produced by an impact. Moreover, the experimental configuration allows one-sided inspection. This is especially important for on-site measurements.

Studies on nonlinear mechanical wave behavior to characterize cement based materials and its durability