## 4 Discussion
### Load-Induced Strain  
The interpretation of the creep deformation of concrete based only on the creep coefficient \(\Phi(t,t_{0})\) can be misleading, especially when these coefficients are used for the creep analysis of concrete structures [57]. It should be noted that the creep coefficient is a normalised quantity. It depends not only on the actual creep strain \(\varepsilon_{\rm p}(t,t_{0})\), but also on the elastic deformation \(\varepsilon_{\rm el}(t_{0})\), which in turn depends on the magnitude of the applied stress and the Young's modulus of the concrete at the age of loading \(E(t_{0})\). The creep coefficients in Figure 6 are calculated assuming that the initial strain measured immediately after loading the test cylinders to the prescribed stress level \(\sigma(t_{0})\) corresponds to the elastic deformation \(\varepsilon_{\rm el}(t_{0})\). The Young's moduli at the age of 2 days (Table 5) are calculated as \(E(t_{0})=\sigma(t_{0})/\varepsilon_{\rm el}(t_{0})\). For a complete description of concrete material under load, at least one more parameter must be known in addition to the creep coefficient. Another way of expressing the response of a viscoelastic material to a long-term load is the compliance function \(/(t,t_{0})\). Using the compliance function, the time-dependent strain  
Figure 8: The shrinkage strain of concrete mixes at an age of 90 days (the error bars represent the standard deviation). of samples under constant compressive load is expressed as \(\varepsilon(t)=J(t,t_{0})\)\(\sigma(t_{0})\), where \(\sigma(t_{0})\) is the compressive load applied at age \(t_{0}\). \(\varepsilon(t)\) consists of the instantaneous (or elastic) strain at the moment of loading and the time-dependent strain, i.e., \(\varepsilon(t)=\varepsilon_{\text{el}}(t_{0})+\varepsilon_{\text{p}}(t,t_{0})\). It follows that \(J(t,t_{0})=[1+\Phi(t,t_{0})]/E(t_{0})\). The compliance function, or simply compliance, is the sum of the elastic compliance, which is constant and equal to \(1/E(t_{0})\), and the creep compliance, which is time-dependent and is expressed as \(\Phi(t,t_{0})/E(t_{0})\). In contrast to \(\Phi(t,t_{0})\), compliance function is a material property that provides a complete description of the concrete under the applied load, at least for linear creep analysis [57]. Figure 9 shows the compliance function for the concrete mixes analysed in this work. Error bars in Figure 9 represent the combined measurement uncertainty of the compliance function calculated from the standard measurement uncertainty of the creep coefficient Equation (4) and the standard deviation of the 2-day Young's modulus (Table 5). The physical interpretation of the compliance function is that it represents the strain per unit stress and is expressed in units 1/unit stress. Therefore, at the same stress level, greater compliance corresponds to greater strain. The initial value of compliance measured during the experiment appears at about 15 min (0.01 days), which corresponds to the time required to apply a load and measure the initial change in length. Since the measured compliance spans several orders of magnitude of time, these values are plotted on a logarithmic scale (Figure 9). For SCC mixes with a \(w/c\) ratio of 0.5, the lowest compliance is measured for mixture SCC0 (Figure 9b). The initial compliance depends on the concrete stiffness, so it is expected that the mix with the highest Young's modulus will have the lowest initial compliance. Recycled aggregate increases compliance, especially when both coarse aggregate fractions are replaced. The effect of replacing both coarse fractions with recycled aggregate became evident after the 10 th day under load, when the slope of the compliance curve started to deviate from the compliance measured for the SCC0 and SCC10 mixes (Figure 9b). This shows that the rate of creep strain after an initial period of 10 days is higher in the mix SCC20 than in the mix SCC10.