## 4 Discussion
### Load-Induced Strain
The replacement of cement with fly ash in the mix SCC30 resulted primarily in an increase in initial compliance while the shape of the compliance curve is very similar to that of the mix SCC20. For normal concrete mixes, the effect of the amount of recycled aggregate is also visible after the initial 10-day period under load (Figure 9a). Even if only the creep compliance is analysed, the replacement of cement with fly ash is found to increase the creep strain. [PERSON] and [PERSON] tested concrete with 25% and 35% fly ash replacement and found that for samples loaded at 28 days, the creep strain measured after 120 days decreased with increasing fly ash content [72]. This phenomenon was attributed to the greater increase in strength due to pozzolanic reactions after loading. The contribution of fly ash to strength development depends on both the \(w/c\) ratio and the fly ash replacement ratio [73]. With a high proportion of fly ash (>45%) and a high \(w/c\) ratio, the compressive strength of concrete with fly ash cannot exceed the compressive strength of control concrete [73]. The comparison of the creep coefficients shown in Figure 6b shows that the creep coefficients are lowest for the mix SCC30, which could lead to the false conclusion that strain under load is lowest for this mixture. But as already mentioned, this is because the creep coefficient does not contain complete information about the behaviour of the concrete under load. Figure 9: Comparison of the compliance function of mixes subjected to constant compressive load at the age of 2 days with mix SCC0: (**a**) normal concrete mixes; (**b**) SCC mixes with \(w/c\) ratio 0.5; (**c**) SCC mixes with \(w/c\) ratio 0.4. Since creep originates from hydrated cement paste [62], mixes with a lower cement paste content and less water available for hydration exhibit a lower creep strain. This led to a lower load-induced strain of the SCC40 and SCC50 mixes compared to the SCC10 and SCC20 mixes. However, this cannot explain the lower strain of mix SCC50 compared to mix SCC40, as the test results of the other mixes clearly show that the incorporation of a higher amount of recycled aggregate increases the creep strain. A look at the results of the compressive strength test after 2 and 28 days (Table 5) shows that the mix SCC50 exhibited a greater increase in strength of 25 MPa compared to 18.7 MPa measured on samples from mix SCC40 in that period. In addition, the mix SCC50 has a higher aggregate/cement ratio compared to the mix SCC40 (Table 3). Both effects, the lower cement paste content and the greater increase in strength, counteracted the effect of the greater volume of recycled aggregate, so the measured load-induced strain of the mix SCC50 was lower overall than that of the mix SCC40. For the mix SCC60, the initial compliance is comparable to the stress-induced strain measured for the mix SCC30, which is due to the lower initial stiffness at 2 days of age due to the cement replacement by fly ash. At longer load duration mix SCC60 shows the highest rate of creep strain which can be attributed to the incorporation of fine recycled aggregate. Although very little data are available on the influence of fine RCA on creep, studies have shown that it increases creep strain [31]. [PERSON] et al. found that the lowest creep in SCC with RCA occurred in the mix with the highest compressive strength and the smallest pore size distribution [36]. Figure 10 shows the creep compliance after 28, 90 and 180 days under load together with the 28-day compressive strength. The initial compliance can be derived from Table 5 as the reciprocal value of the 2-day Young's modulus. The data in Figure 10 confirm that creep was lowest in the mix with the highest compressive strength. However, the mixes NC2 and SCC20 exhibited higher creep values than the NC1 and SCC10 mixes despite their higher compressive strength. Both mixes, NC2 and SCC20, had a lower cement paste content and consequently a higher aggregate/cement paste volume (Table 3), which should also contribute to the lower creep strain.