## 4 Experimental Results and Discussion
### Analysis of Linear Creep  
As mentioned above, TLBs' midspan deflections were measured and recorded under constant loading and climate-controlled conditions. The temperature in the climate room was 20 \({}^{\circ}\)C and the humidity--65%. Figure 4 presents the maximum values of midspan deflections in the tested TLBs due to linear concrete creep during the first 90 days according to the loading cases that were described in Section 3.3. As it follows from the graphs in the figure, in general the creep deflections' development is linear.  
Figure 3: A typical pattern of cracks in the TLBs: (**a**) general view; (**b**) fragment of the beam’s cracked zone, 1—border between the SFHSC and NSC layers.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & \multicolumn{5}{c}{**Cracks’ Opening, mm**} \\ \cline{3-6}
**Load Case** & **Specimen** & \multicolumn{2}{c}{**Crack 1**} & \multicolumn{2}{c}{**Crack 2**} & \multicolumn{2}{c}{**Crack 3**} \\ \cline{3-6}  & & **Depth** & **Width** & **Depth** & **Width** & **Depth** & **Width** \\ \hline \multirow{3}{*}{1} & Top & - & - & - & - & - & - \\  & Bottom & 80 & 0.1 & - & - & - & - \\ \cline{1-1}  & Top & 70 & 0.1 & 70 & 0.1 & 70 & 0.1 \\ \cline{1-1}  & Bottom & 100 & 0.1 & 70 & 0.1 & - & - \\ \cline{1-1}  & Top & 80 & 0.1 & 80 & 0.1 & 60 & 0.1 \\ \cline{1-1}  & Bottom & 100 & 0.1 & 100 & 0.1 & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 3: Cracks’ width in the TLBs immediately after loading.  
As expected, the dominant case from the viewpoint of creep deflections is Case 3 (loading, corresponding to 85% of the ultimate load). Compared to Case 1 (loading, corresponding to 70% of the ultimate load), the deflections in Case 3 at 90 days are 2.33 times higher, which indicates high non-linear deformations in the tested beams. For pre-cracked TLBs subjected to 70% of the ultimate load (Case 2), the creep deflection growth is similar to Case 1, but the deflections' values are about two times higher, which corresponds to the structural phenomenon [28].  
Following the design codes [7, 8], the deflection of beams corresponds to about 1/250 of their span. For the investigated TLB specimens, the span is 650 mm (see Figure 1), i.e., the deflections are limited by 2.6 mm. The maximum value of the midspan deflection that was obtained in the tested beams in Case 3 is 0.42 mm; therefore, at linear creep, the TLBs are safe. Moreover, considering the fact that for the measured deflections' values, the compressed concrete deformations are \(\epsilon_{c}\leq 0.5\%\)[15], therefore, it is evident that the tested TLBs are within the elastic stage. This fact is very important for the beams' design considering linear creep.  
At the same time, the lifetime of real beams is much longer than 90 days. It can be assumed that at a certain stage, non-linear creep will appear in TLBs. Therefore, it was decided to continue the tests over 90 days to obtain data on non-linear creep. It will allow comparing these data on TLBs with the available ones from the design codes to investigate the behavior of TLBs considering non-linear creep. This will be the main aim of our further research.