Curing is the ensurement of planned properties in concrete through the optimising of the hydration process.
The time dependent course of hydration is contingent upon the temperature of the concrete matrix. Low temperatures mean here a slowdown of hydration until standstill at about -10°C. At temperatures under 0°C structure damage occurs in the fresh concrete mix, which can influence the durability of the concrete. From a quality viewpoint there must therefore be a minimum temperature and from a production viewpoint a permitted maximum temperature achieved for the concrete matrix.
Primarily the heat of the concrete matrix is created by the release of energy during the chemical reaction. The hydration heat development is dependent on the type of cement and the degree of hydration (time elapsed).
1 kg Cement (e.g. CEM I 32,5 R) develops up to about 200kJ of "heat energy" in the first 24 hours of hydration. The heat divides itself in the concrete matrix between mixing water, supplemental materials, additives as well as the cement itself.
With a contemplation of the heat balance it is however fundamental, that the temperature progression of the concrete matrix does not take place linear, rather one function is sufficient, which is conditional on the heat development in the first hours of hardening.
The summarial heat development of the concrete matrix amounts to:
Q = CP · γ · ∆t · V (Fundamental Equation)
With a view to the physical properties of the material in the mixture (concrete) being considered the equilibration yields to:
Q∑ = QH – [QZ/A/S/L (f = CP · γ · ∆t · V )
∆tConcrete = CPConcrete· γConcrete · VConcrete
Q∑ Heat excess
QH Hydration heat development
QZ Heat intake capacity of the cement
QA Heat intake capacity of mixing water
QS Heat intake capacity of the sand/gravel
QL Heat intake capacity of the air space
QP Specific heat intake capacity (Cement/Water/Sand/Air)
γ Specific equilibrium (Cement/Water/Sand/Air/Concrete)
∆t Temperature difference of materials
When the output temperature of the material mix is designated with t1 , one obtains the simple relationship:
t1 + ∆t = t2
The interpretation of the simultaneous equation shows a familiar picture.
In winter activity (cold supplemental materials, cold mixing water, etc) and with the same amount of cement t2 reaches only a low value. In summer activity (warm supplemental materials) t2 is correspondingly larger. The consequences on the time dependent course of strength development are well known.