# Long-Term Effects in Structures: Background and Recent Developments
## 3 Long-Term Effects in Concrete
### Concrete Shrinkage  
Concrete shrinkage occurs during the process of concrete hardening due to hydration of cement, as well as the process of drying concrete due to loss of moisture to the environment. The volume of hardened concrete decreases. Shrinkage obtains its maximum on surfaces of the element exposed to drying and decreases towards the inside of the element. The main factors affecting shrinkage are as follows [28; 29; 30; 31]:  
1. Mechanical effects from temperature changes;
2. Thermal effects of cement hydration;
3. Hydrological effects related to hydration;
4. Hydrological effects related to climate;
5. Compressive strength of concrete;
6. Consistency of the fresh mix;
7. Type and content of cement;
8. Water-cement (w/c) factor;
9. Ratio of fine aggregate according to the total amount of aggregate;
10. Air content;
11. Effective element thickness.  
According to the literature [32; 33], shrinkage can be divided into four main types:  
* Autogenous shrinkage--reduction in volume of the mixture of water and cement during hydration;
* Plastic shrinkage--created by the vacuum which occurs when water evaporates from the pores of the capillaries;
* Drying shrinkage--due to the loss of moisture from the concrete, i.e., the release of water from the concrete into the environment; this occurs with hardened concrete;
* Carbonization shrinkage--a reduction in volume by carbonization as a consequence of chemical reactions in cement stone caused by carbon dioxide from the environment.  
Each type of shrinkage is schematically illustrated concerning concrete age (Figure 9), along with ratios between each type of shrinkage [34].  
A large number of factors affecting shrinkage makes it impossible to generalize one universal model. Researchers have adopted a phenomenological approach, collecting data over long period of time. A contribution in this area was made by numerous authors, some of whom are listed below [21].  
One of the earliest models describing concrete shrinkage originates from [PERSON] [5] (1913). It explains the phenomenon of shrinkage by physical-chemical processes at the beginning of the hardening of concrete. After World War II, research on concrete shrinkage intensified. Several models were proposed, based on the phenomenological parameters: [PERSON] (1958), [PERSON] (1977) and [PERSON] (1978) [21].  
From the mid-1990s, several significant studies were conducted. [PERSON], [PERSON] and [PERSON] (1995) proposed a macroscopic-scale analytical model of autogenous shrinkage and two years later improved this model. [PERSON] and [PERSON] (1997) used thermodynamic approach to determine autogenous shrinkage of hardening cement paste. [PERSON] et al. (1998) proposed a model which is based on micro-mechanical influence of water in pore structure of the concrete [21; 31].  
It can be shown that reduction in the shrinkage strain would substantially diminish initials cracks too [35; 36; 37; 38]. Achieving this could involve replacing Portland cement with Sulfoaluminate cement (SAC), which can be combined with shrinkage-reducing admixture. Assuming regular concrete strength, autogenous shrinkage as well as the early-age strength of concrete, will decrease with the addition of fly ash [39; 40]. Conversely, high-strength concrete will exhibit increased shrinkage under the same conditions [41].  
Current research on shrinkage primarily focuses on a deeper understanding of autogenous and drying shrinkage [31], particularly in construction stages [42], using new types of cement [43], or with different types of concrete mixtures with different admixtures: various types of aggregate, fibres, etc. [44].  
Despite numerous theories, shrinkage remains incompletely explored, and not all causes of this physical phenomenon have been explained. Over the last 5 years, Web of Science has registered over 80 papers dedicated specially to concrete shrinkage.