*7.1. Bottom Slab (Stage I)*

The slab was concreted in June, when the average ambient temperature of 10 days was 22.1 ◦C. It was assumed that throughout the analyzed period, on the bottom surface of the slab the formwork (plywood and girders) was mounted. The top surface of the plate was exposed to the weather conditions, but between 23 and 94 h it was covered with a styrofoam layer, 5 cm thick. The average wind speed in this period was approximately 16 km/h = 4.44 m/s [39], thus according to the proposition presented in Figure 20b, coefficient <sup>α</sup>*f c* <sup>=</sup> 12.6 W/(m<sup>2</sup> · <sup>K</sup>). The initial temperature of the mixture was 26.7 ◦C.

Using the finite difference method, the space domain was subdivided in *ms* = 94 nodes whose distance Δ*x* was 1 cm (Figure 21). Moreover, for the explicit numerical integration a time increment *dt* met stability criterion. The calculations were carried out for a constant (Figure 22a) and a daily time variation of ambient temperature measured over and above the concrete slab, interpolated to the adopted step *dt* (Figure 22b). In Figure 22, as well as in the forthcoming ones, dashed lines indicate the results from the numerical simulations, whereas solid lines correspond to the concrete temperature measured at the appropriate depths. Thermo-physical parameters were adopted according to Tables 2 and 5. Figure 23 shows the temperature distribution, over time and plate thickness, in two variants. In Figure 23b the influence of daily temperature fluctuations can be seen, especially on the upper surface of the slab. The numerical results of temperature evolution show good agreement with the experimental data for both cases. According to Figure 22b, the relative error between measured and numerical value of maximum concrete temperature was equal to 1.6%, 0.9% and 4.5% for points p4, p5 and p6. The bigger differences concerned the time occurrence of the peak because the relative error reached 12.7%, 20.5% and 29.6%, respectively.

The maximum concrete temperature of the bottom plate was noted after 25 h of hardening, reaching a temperature of 67.8 ◦C at point p5 (Figure 22). The temperature of 70 ◦C was not exceeded, which the standard [40] gives for the limit value. In the core of the monitored slab, the conditions close to adiabatic existed. The temperature of self-heating of concrete caused by hydration reaction was 41.1 ◦C.

**Figure 21.** Space discretization.

**Table 5.** Thermophysical parameters—bottom slab, 93 cm thick, stage I.


**Figure 22.** The concrete temperature of bottom slab, 93 cm thick (stage I): (**a**) Constant ambient temperature and (**b**) variable, measured ambient temperature.

**Figure 23.** The temperature distribution of the concrete bottom slab (stage I): (**a**) Constant ambient temperature and (**b**) variable, measured ambient temperature.
