# Long-Term Effects in Structures: Background and Recent Developments
## 1 Introduction  
Material parameters are generally time-dependent; that is, they change their properties over period of time. In some cases, this change is positive (for example, concrete increases its strength over time), but in some cases, change can induce the weakening of material and overall structure. That kind of negative change can lead to structural collapse.  
When discussing long-term effects on masonry and concrete structures, considerations include creep, shrinkage, aging and corrosion. The goal of this paper is to provide insight into these processes, accompanied by examples that underscore their significance and the potential consequences of neglecting them. Concrete is a mixture of cement, aggregate and water with additives. Cement is a mixture of calcium silicates and aluminates. After adding water in dry mixture of cement and aggregate, a number of exothermic chemical reactions start simultaneously. This process is commonly denoted with the term hydration. Hydration begins immediately after the cement comes in contact with water, but lasts for a very long period of time. During prolonged hydration, concrete increases its strength and modulus of elasticity and this process is called aging.  
Over time, concrete can be affected by reactive liquids and gases which can cause chemical, physical-chemical or colloidal deterioration and disintegration of concrete components. This process is known as concrete corrosion. Also, there is another type of corrosion--corrosion of reinforcement--which will not be discussed in this paper. Creep is the process of structure deformation under long-term (sustained) load.  
It is well established that body deforms under the influence of force. When a force is applied, the change is immediate. In addition, if the force is applied over an extended period, the material attempts to adapt, changing its shape and volume to redistribute stress.  
Generally, creep in concrete is defined as a time-dependent deformation due to long-term actions [1; 2]. These definitions imply that creep will lead to an increase in initialdeformation over time, while in the absence of creep, the initial deformation will decrease as a function of aging, shrinkage of concrete and relaxation of steel [3]. Structure stability can be seriously threatened if progression of shrinkage and creep deformation is neglected. Thus, the obligation to take creep into account when designing and dimensioning reinforced and prestressed structures is even more important. Therefore, considering creep is crucial when designing and dimensioning reinforced and prestressed structures, as it can cause stress redistribution in structures, which leads to a reduction in the load-bearing capacity.  
Concrete shrinkage can be described by defining a decreasing function of volume concerning moisture change in the concrete mixture. In the case of restrained deformations of concrete, shrinkage will initiate further stresses in the concrete, which will be accounted for by reinforcement. Therefore, reinforcement accounts for the reduction in shrinkage deformation [4; 5]. Displacement decreases over time [3] if shrinkage is present. Also, the influence of creep diminishes with time.  
The change in the strain in concrete over time is illustrated below (Figure 1). Shrinkage starts immediately after concrete mixture is prepared. At time t\({}_{0}\), the structure is loaded, causing immediate strain (\(\epsilon_{\text{m}}\)). Subsequently, creep develops, a process that gradually diminishes over time (Figure 1). Upon unloading, the structure immediately recovers a part of the strain, while another part recovers over a more extended period. The irreversible part of the strain is impossible to recover.  
As mentioned earlier, concrete aging contributes to the improvement of strength as well as other mechanical properties which are considered positive. On the other side, concrete corrosion occurs only in direct contact with water.