Abstract
Reducing the risk of through-cracking in mass concrete during the hardening phase is mainly a question of reducing the hydration heat and, thus, reducing the temperature-increase in a hardening concrete member restrained against movement. The report deals with on-site curing box tests and gives some data on what to expect with regard to hydration heat and temperature increase when testing a concrete with a certain composition in future tests.
A series of FEM-analyses were run in DIANA, and the temperature developments for a various set of concretes, curing box designs as well as ambient conditions were evaluated. The main result for each analysis is given as the maximum temperature increase (Tmax) in the core of the curing box. The majority of analyses have been based on data for four concretes; ANL FA, ANL FA + 8FA, ANL FA + 16FA and Ready-mix 1 (50FA) with a fly ash content of 19 %, 27 %, 35 % and 50 %, respectively. It was found that when increasing the size of the curing box, ΔTmax increases, i.e. the temperature development sensitivity with regards to the surrounding conditions decreases. This effect is more pronounced the lower the ambient temperature. Likewise, the same effect is obtained by increasing the insulation thickness. The effect of wind speed was also investigated, and it was found that the effect of 0 and 5 m/s wind is insignificant regardless of the given curing box sizes and insulation thicknesses.
An extended series of temperature development analyses, with a variation of temperature conditions and curing boxes, were run for ANL FA + 16FA. For all studied curing box alternatives, there seems to be an approximately linear correlation between ΔTmax and ambient temperature, where the slope of the trend line represents the curing box’s sensitivity to the surrounding conditions. Among the studied curing box alternatives it is found, for instance, that the sensitivity of Tmax to the ambient temperature is about the same for a 0.2 m3 curing box with 200 mm insulation and for a 1.0 m3 curing box with 100 mm insulation. As expected, the largest 1.0 m3 curing box with 200 mm insulation has the lowest sensitivity due to the ambient conditions, but there is still a difference of 5 oC in ΔTmax between the two extreme ambient temperature cases T = -15 oC and T = 30 oC. Hence, even when using the curing box with the least temperature sensitivity, it is still necessary to consider the ambient temperature. It can be debated whether one curing box is more favourable than the other, as all seem to have an approximately linear correlation between ΔTmax and ambient temperature, and, thus, the ambient conditions can in principle be corrected for in all cases.
An extended series of analyses, with a variety of ambient temperatures and initial concrete temperatures, was conducted for the defined curing box reference case, Size = 1.0 m3, tinsulation = 100 mm, v = 0 m/s. The results show a rather linear correlation between ΔTmax and ambient temperature. One linear model for ΔTmax versus ambient temperature was established. The model gives good agreement with the analysis results for the concretes in question.
In addition, more as a curiosity, a series of analyses were performed with the aim to find out what wall thicknesses give a ΔTmax that is approximately equivalent to that of the various curing boxes with 100 mm insulation thicknesses. The results indicate that curing boxes with size 1.00 m3, 0.51 m3 and 0.22 m3 represent wall thicknesses of about 1500 mm, 1300 mm, and 1100 mm, respectively. These are approximate numbers, as it was also found that the relation to some extend was dependent on the hydration heat characteristics of the concrete.