## 3 Results and Discussion  
### Effect of SAP Internal Curing on the Cracking and Corrosion Initiation  
The first cracks were visualized on the reference walls WR1 and WR2 five and eleven days after casting, respectively. The cracking pattern in WR1 after 28 days is shown in Figure 5. For the SAP-containing walls, no cracks were noticed during the monitoring period of 24 months. Embedded optical fiber sensors were used for monitoring the shrinkage strain inside the walls. Furthermore, the ASTM C1581-04:2010 [24] test method was employed to measure the restrained shrinkage using steel rings. This test was conducted for concrete samples taken from one concrete truck from each wall. The shrinkage levels of the walls WR1 and WSC as measured by the optical fiber sensors (see Figure 6) showcase the effects of the internal curing promoted by the SAPs, especially at early ages, as a consequence of a proper water release by the SAPs (here proper water release is defined as a controlled and continuous release of water at around the moment when self-desiccation starts to take place in the hardening matrix [25; 26]). Compared to the reference wall WR1, the SAP-containing wall WSC presented a reduction of 55% in the strain levels at 7 days. At the age of 28 days, the difference in shrinkage strain between both walls was 18%. At an early age, the tensile stresses in the surface layer may exceed the tensile strength of the material, which results in early-age cracking [27]. In this case, the effects of the internal curing promoted by SAPs are clearly  
Figure 5: Observed cracking in wall WR1, four weeks after the casting, showing the measured height [in mm] and average width [μmm]. Figure 6: Strain/deformation in the WR1 and WSC walls, measured with the embedded fiber optic (SOFO) sensors. observed: the strain level found in WR1 when the first cracks were detected (around \(-150\)\(\upmu\)m/m, 5 days after casting) is only attained in WSC 14 days after casting. The absence of cracks in the SAP walls indicates that the reduction in strain level due to internal curing during the initial stages, when the concrete is still gaining strength, functioned as a limiting factor for crack formation in the SAP walls. This demonstrates the advantage of employing SAPs for internal curing and prevention of cracking. In terms of the time of cracking, the results from the restrained shrinkage with the steel rings (see Figure 7) also confirm the positive effects of the internal curing, as the sample prepared with concrete from WR1 presents an indication of cracking (marked as the discontinuity in the strain curve) around 15 days after casting, while the specimens prepared with concrete from WSC do not indicate any cracking potential during the whole monitoring period of 21 days. [PERSON] et al. [28] showed that the time until cracking could be prolonged for mixtures with an increasing volume of internal curing agents. As stated by the authors, the extension in cracking time was primarily attributed to reduced shrinkage, but other factors such as a decreased elastic modulus, increased relaxation, and enhanced fracture toughness of mixtures containing internal curing agents could also contribute to the observed increase in cracking time. In Figures 8 and 9, the corrosion potential measured by some of the electrodes embedded in walls WR1 and WSC is presented. To ensure the proper functioning of the system, initially, eight electrodes located near the access box were connected to the monitoring unit. The potential measurements were conducted over a duration of approximately one month. Subsequently, two months after the walls were cast, other electrodes were monitored following the appearance of cracks. Over a 4-month monitoring period, the measurements remained stable at all the sites monitored. However, after 6 months, a significant decrease in the potential value was observed for the sensors located near one of the cracks in the reference wall WR1 (at points 37 and 38 in particular).