3.4. Air versus Fresh Concrete Temperatures
The concrete temperature was plotted against air temperature during pour both, with respect to and irrespective of the design strength. As presented in
Figure 8 and
Figure 9, the same trend seemed to be followed. Indeed, when the air temperature exceeded a certain temperature, the concrete temperature was lower than the air temperature, and vice versa. Since the trend was the same regardless of the design strength, a graph was realized using all of the data, regardless of the design strength.
Figure 9 shows the correlation between concrete temperature and air temperature. One can note that the relationships between concrete temperature and air temperature with respect to and irrespective of the design strength, the data could be approximated by a linear function. Indeed, when the air temperature exceeds a certain temperature, the concrete temperature becomes lower than the air temperature, and vice versa. When the air temperature is about this temperature, the concrete and air temperatures are similar. To determine this temperature, which is the intersection between the function y = x and the function of concrete temperature, the data can be approximated by a linear function in Excel. Afterward, knowing the linear equation, the intersection between the two functions can be determined. Both with respect to the two design strengths of 3000 psi and 4500 psi, and irrespective of the design strength, linear regressions were made. In addition, it can be noted that this point may also be considered as an inflection point of a nonlinear function, approximating the function of concrete temperature.
On the one hand, considering the design strength of 3000 psi, using the equation of linear regression, the intersection point between the two functions is at x = 73.2 °F. Similarly, considering the design strength of 4500 psi, using the equation of linear regression, the intersection point between the two functions is at x = 74.2 °F. On the other hand, regardless of the design strength, using the equation of linear regression, the intersection point between the two functions is at x = 74.1 °F. The coefficients of determination in the two regressions for design strength of 4500 psi was about 0.5. In regards to the design strength of 3000 psi, the coefficient of determination was about 0.3. This low coefficient of determination, relative to the larger coefficient obtained for the 4500 psi regression line, can be explained by the fact that there was less data for this design strength. As a result, when either considering or ignoring the design strength, the intersection point was about 70 °F ± 5 °F. In other words, when the air temperature equals 70 °F ± 5 °F, the concrete and air temperatures were approximately the same. When the air temperature was below 70 °F ± 5 °F, the concrete temperature was higher than the air temperature. When the air temperature was above 70 °F ± 5 °F, the concrete temperature was lower than the air temperature.
Another methodology would involve assuming a range for air temperatures through a study of the results. One can note that when the air temperature was between 60 °F and 80 °F, the points were close to the line defined by the equation y = x, both with respect to and irrespective of the design strength. Outside of this range, the points are above or below this line. Hence, it can be deduced that for an air temperature of 70 °F ± 10 °F, the concrete and air temperatures are similar. It can also be noted that even if the concrete temperature varies against the air temperature, the strength f’c and slump remain constant with the varying air and concrete temperatures. Hence, it illustrates the good quality concrete of the concrete suppliers. In conclusion, regardless of the design strength, it can be deduced that when the air temperature is at 70 °F ± 10 °F, the fresh concrete and air temperatures are similar. During the hot season, this temperature can be reached in the early morning or late at night. Therefore, concrete should be poured when the air temperature is in this range to avoid a significant variation of fresh concrete temperature, so that using additional water or admixtures to regulate water content and workability is not necessary.