Thermal Diffusivity
The average thermal diffusivity number for the mixes was determined to be 0.80
ft2/day which is about 35% less than the amount suggested in ACI 207.1R-96 for
concrete mixes with lime stone aggregate (ACI suggested number is 1.22 ft2/day). ACI
207.1R-96 does not specify the maximum aggregate size of the samples that were used to
determine the concrete thermal diffusivity. Since the major application of mass concrete
is in dams, the numbers reported by ACI 207.1R-96 may be originated from the samples
with very large aggregate size. However, in FDOT approved mass concrete mix designs
coarse aggregate occupies about 45% of the mix volume. The rest is filled with cement,
fine aggregate, and water. The mixture of cement and sand has a thermal diffusivity of
about 0.40 ft2/day (Xu and Cheng, 2000). The thermal diffusivity of water is 0.13 ft2/day.
The thermal diffusivity of limestone has been reported between 1.00 to 1.40 ft2/day in
different sources with an average 1.20 ft2/day. Based on these diffusivity numbers it is
reasonable for a concrete containing about 45% limestone and the rest containing
materials with much lower thermal diffusivity to have a thermal diffusivity lower than
that of the limestone. Another observation of theses tests (as it was shown in Chapter 4)
was that concrete thermal diffusivity reduces with the replacement of higher percentage
of pozzolans.
The thermal diffusivity number affects the results of calculations to determine the
maximum temperature and temperature difference (thermal gradient) in a mass concrete
element. In Figure 5-17 the effect of thermal diffusivity on the maximum temperature rise
for mass concrete elements with thickness of 5.5, 6.5 and 7.5 feet is shown. The
maximum temperature rise was calculated using Schmidt method. Two different curing
conditions were assumed. At first it was assumed that no insulation is used during the