**8. Distribution of Concrete Compressive Strength for Selected Structural Elements of the Bridge**

As mentioned at the beginning of the paper, the in-place concrete temperature measurements were performed with a focus on estimation the real concrete compressive strength. The contractor planned to prestress every typical segment after reaching 60% of characteristic strength for class C 60/75, so it was exactly 45 MPa for cubic strength. Due to that fact, the maturity method was implemented [2,41]. The curve plotted after the third stage of the study is shown in Figure 33.

**Figure 33.** Maturity curve related to cubic strength after three stages of research.

Based on the numerically obtained maps of the temperature field and the maturity–strength relationship expressed in the equivalent age *te* domain, the distribution of early concrete compressive strength was presented. In each of the considered cases, the strength was calculated according to the concrete maturity curve for cubic samples, developed after the first, second and third stage of the specimen's validation tests performed during bridge operations. Figures 34–36 illustrates the compressive strength changes of young concrete for the bottom plate (stage I) and top slabs (stage II and III) versus the first three days of hardening. It can be seen that the age needed to achieve the required strength to prestress, and differed significantly in each monitored element. For plates with the same thickness, concrete reached 45 MPa in 15–48 h (Figure 35, Figure 36). In the case of the top plate monitored during third stage of research, the desirable concrete strength in the surface zone was achieved definitely later than for the bottom and middle part of the slab. The presented studies confirmed the impact of boundary conditions on early age compressive strength of cast-in-place concrete. Based on conducted investigations, the actual strength of in-place concrete and the possible dates of prestressing were determined.

**Figure 34.** Map of the early age compressive strength distribution of the bottom slab (stage I, 93 cm thick, *T*<sup>10</sup> *days env* = 22.1 oC).

**Figure 35.** Map of the early age compressive strength distribution of the top slab (stage II, 56 cm thick, *T*<sup>10</sup> *days env* = 20.1 oC).

**Figure 36.** Map of the early age compressive strength distribution of the top slab (stage III, 57,2 cm thick, *T*<sup>10</sup> *days env* = 4.4 oC).
