The assessment of concrete structures affected by alkali-silica reaction (ASR) is a complex problem due to the multiscale nature of this long-term phenomenon. The reaction starts within the concrete constituents with the formation of an expansive alkali-silica gel at reaction products level. Being the expansive gel confined within the concrete micro-structure, an internal pressure is built up that induces damage at aggregate level. This micro-cracking affects the mechanical characteristics of the material at concrete level. At structural level, the performance of members and of structures itself can thus be compromised by the reaction. Since the material characterization is one of the main points of attention within a structural assessment, this thesis work aims to study the deteriorating impact of ASR on concrete in terms of both expansion and degradation of the mechanical properties. Both experimental and modelling approaches are followed. The experimental investigation, which includes laboratory tests supplemented with literature data, shows a statistically relevant relationship between the concrete expansion and the degradation of mechanical properties of ASR-affected concrete specimens stored under free-expansion conditions. Rather than the compressive strength, the elastic modulus results the best indicator of ASR signs in concrete by showing the fastest degradation rate, leading to the lowest residual value. By comparing the behaviour of unaffected and affected concretes in terms of strength-stiffness correlations a substantial difference is observed. Considering that unaffected and affected concrete experimentally appear as substantially different materials at concrete level, a multiscale modelling approach, ranging between aggregate and concrete level, is adopted to explore the deteriorating impact induced by ASR. An analytically solved micro-poro-fracture-mechanical model, which is based on a limited number of input parameters, is adopted. The approach considers the micro-cracking phenomenon as the common damage mechanism associated to the internal swelling and the external mechanical loading. The model is able to approximate the behaviour of unaffected concrete under uniaxial loading as well as the relation between stiffness and strength of unconstrained ASR-affected concrete. However, the lack of permanent deformation in its formulation results as a limitation. In conclusion, this thesis work highlights the importance of a multiscale analysis to explore the ASR phenomenon in concrete and concrete structures. The deteriorating impact of ASR on concrete can be correlated to the microcracking phenomenon at aggregate level and should be considered both in terms of expansion and mechanical degradation. The proposed multiscale material modelling approach results as a method for the material characterization, which can be extended to both the reaction products and the structural level.