| dc.description.abstract | Alkali-silica reaction (ASR) is a deterioration process in concrete that was discovered in the mid-1930s, and which still affects concrete structures all over the world. ASR is considered a multi-scale phenomenon; it is a chemical reaction that results in material deterioration and structural consequences. The product of the chemical reaction is a hydrophilic gel that swells by water absorption. On the aggregate scale (meso scale), the internal swelling leads to closely spaced microcracks that on the concrete scale cause a change in mechanical behaviour such as stiffness and strength. In addition, the swelling results in an overall expansion of the material. On the structural scale, the visible signs of ASR are often surface cracks and displacements, but there are also some invisible effects.
ASR results in expansion, and hence, introduces imposed deformation at the structural level. In contrast to thermal expansion, ASR will only cause the expansion of the concrete. Reinforcement bars will therefore have a restraining effect on the expanding concrete. In addition to the local incompatibility of strain between concrete and steel, differences in ASR expansions/strains at the structural level might occur, e.g. due to a moisture gradient. When imposed deformations are analysed, it is important that the material model can represent a realistic strain/deformation for a given history of stress. This means that concrete cracking and creep deformation might be of importance.
The concrete expansion due to ASR depends on the three-dimensional stress state, where the expansion is reduced with compression. Furthermore, the expansion, the micro-cracking and the change in the mechanical behaviour are interrelated. Consequently, the material both expands and deteriorates anisotropically when exposed to an anisotropic stress state.
The background for this PhD study was structural concerns due to ongoing ASR in Norwegian bridges, where the structural effects of ASR have become more evident in recent years. ASR-induced displacements of the bridge superstructures have led to inclined columns, and a reduction, or even closure, of expansion joints. Moreover, differences in expansions within a beam bridge in Norway (Elgeseter Bridge) have caused large vertical cracks.
An unanswered question regarding ASR is whether it leads to the loss of structural integrity of affected reinforced concrete structures. To answer this question, one needs to answer two other related questions: 1) how to calculate the ASR-induced stresses or load effects, and 2), what is the residual capacity when accounting for the induced stresses and material deterioration? This study aimed to answer the first question. A state-of-the-art 3D constitutive model of ASRaffected concrete and a suitable structural analysis method were developed to attain this aim. The material model and the structural analysis method were also applied to a real case, Elgeseter Bridge in Norway.
The concrete material model includes the following material effects: stress dependent ASR expansion, ASR-induced stiffness reduction, tensile strength reduction, cracking, creep, and compressive damage. A concrete expansion-based approach was chosen, where the ASR expansion relies on the input of only one predefined scalar field variable—the free ASR expansion —which is the expansion that would have occurred without stress. As this field is considered unknown, a structural analysis method was developed to identify it for an existing structure.
The structural analysis method is stated as an inverse problem; the unknown free ASR expansion field is back-calculated from a set of in situ measured displacements and cracks. For practical calculations, an inverse analysis is a reasonable approach to compensate for flaws in the model. The free ASR expansion is a suitable starting point for the modelling of the ASR expansion, especially when used in conjunction with the proposed structural analysis method.
From the perspective of the practising engineer, linear structural analysis is the preferred approach for designing and assessing existing structures. This study emphasises that the linear approach (engineering approach) may give too high load effects. To avoid unnecessary disapproval of continuous beam bridges suffering from ASR, the linear analysis should be conducted with great care.
Furthermore, the influence of stress on the imposed ASR expansion was found to be of great importance for accurate predictions of the ASR-induced stresses. The greater the sensitivity of the ASR expansion to stress, the smaller the load effect; the stress dependency of the ASR expansion adapts the imposed ASR strain field, reducing the load effects.
The results in this study were obtained based on an ASR expansion model that treats each principal stress direction independently. Further studies on the importance of including directional coupling (in the expansion model) when analysing similar structures are recommended. Other anisotropic expansion behaviours may lead to different results.
Keywords: Alkali-silica reaction, experimental investigation, constitutive modelling, load effects, non-linear finite element analysis, reinforced concrete structures | en_US |
