# Long-Term Effects in Structures: Background and Recent Developments
## 3 Long-Term Effects in Concrete
### Concrete Creep  
Creep is a time-dependent deformation of concrete under constant load, which can be several times greater than the initial deformation [1; 2]. Concrete creep can be divided into two components:  
1. Basic creep occurs in conditions without movement of moisture;
2. Creep by drying: the part of creep that occurs when there is movement of moisture from or into the environment. For the purpose of practical design, it is not necessary to distinguish between basic creep and creep by drying [3]. Creep by drying, in addition to opening cracks, has the most significant impact on the nonlinear behaviour of structures, while the impact of basic creep on nonlinearity is almost negligible [45]. According to [2], there are three main groups of factors affect the occurrence of creep:  
1. The composition of the concrete mix;
2. The impact of the environment in which the observed structure is located;
3. The design of the system. Although the mechanism of creep is still not fully known, it can be said that creep depends on the properties and amount of cement paste and on the properties of the aggregate within the concrete mix. Generally, a concrete mix with a larger volume of aggregate (and a corresponding reduction in the content of cement paste) will lead to less creep deformation in contrast to a mix with a larger volume of cement paste. Also, the \"stiffer\" unit will try to \"restrain\" more time-dependent deformation behaviour, and thus reduce the creep effect. If aggregate is (partly) made from recycled concrete, its increasing in the concrete mix will lead to an increase in creep and shrinkage [46; 47]. It has also been  
Figure 9: Shrinkage strain components for normal concrete [34]. observed that the creep deformation is inversely proportional to the degree of cement hardening [48]. It is also known that an increase in water content [48] and air in the concrete mix causes an increase in creep deformation, which is approximately proportional to the square of the water-cement factor [48; 49]. Fresh concrete with better properties ensures a lower degree of creep and shrinkage development. A study in the literature [49] deals with this issue and proposes a new numerical approach to predict time-dependent flow of fresh concrete based on the discrete element method. The next group of factors influencing creep behaviour are the environmental conditions in which the concrete is prepared, installed and loaded. The creep size is particularly sensitive to the relative humidity conditions of the concrete sample. An increase in relative humidity causes a slower creep development and thus decline in the final amount of creep. The development of concrete creep over time also depends on the duration and type of curing of the concrete. Thus, increasing the curing period of wetting concrete is directly related to lower creep development, while curing concrete reduces the size of the final creep deformation up to 30% [2]. Creep is also very sensitive to temperature conditions; an increase in the degree of creep development is related to higher temperatures. An important factor influencing the development of creep deformation is the adopted design solution. The magnitude of the stress in the concrete element is linearly related to the creep deformation for stress values up to approximately 40 to 60% of the compressive strength [2]. The creep deformation is inversely proportional to the strength of the concrete at the moment of loading [48]. The size and the shape of the concrete element is another key factor. Thinner concrete elements will more easily allow the loss of moisture to the environment, and will thus experience greater creep deformation than thicker concrete elements [48]. Also, load time is very important. Concrete loaded in the later period is exposed to less total creep deformation. For instance, the same concrete loaded on the first day has an 18% higher creep deformation than concrete loaded on the third day [50].