HYDRATION HEAT

If you have ever touched a newly disarmed concrete structure, you know very well that the concrete is warm, that's because the cement hydration reactions, as known, are exothermic.

In any case, the trend of the hydration development curve over time can always be divided into distinct phases of acceleration and deceleration.

According to the diagram below it is possible to see that the maximum acceleration phases occur after the first hours after casting.


There are numerous factors which influence the acceleration and deceleration of the heat development processes.

Simplifying, we can say that acceleration occurs in the presence of:


  • High ambient temperatures

  • High clinker fineness


Again, simplifying we can state that deceleration, on the other hand, is favored by:


  • Mineral additions in partial replacement of clinker

  • Water reducing admixtures that also allow a reduction in cement

  • Retardant admixtures that slow down the hydration processes.


MINERAL ADDITIONS


Silica fumes are a by-product of the silicon industry and iron-silicon alloys which have micropozzolanic qualities.

They appear as spheres with dimensions between 0.01 and 1 mm, that is 50-100 times smaller than those of cement. These react with calcium hydroxide as well as pozzolan, ash, blast furnace slag. The small size allows the positioning of the particles in the capillary pores and the pozzolanic qualities allow the formation of bonds in addition to those of cement hydration.

This activity facilitates the saturation of part of the capillary pores, decreases the porosity of the concrete and therefore its permeability, as well as improving its mechanical performance.


OTHER INVOLVED FACTORS


The concrete temperature will be the result of the cement hydration temperature and the heat exchange that occurs with the outside environment since it is almost impossible to work in adiabatic conditions.

Some factors must therefore be considered:


  • The thickness of the structure to be casted

  • The air temperature and ventilation

  • The insulation of the formwork

  • The thermal conductivity of concrete



The Thermal Δ between the concrete external surface and its core, in extreme cases, can be many tens of degrees and in the case of massive castings it can persist for long periods, even years. When the ΔT is considerable, a certain degree of differential thermal expansion can occur.


This fact can lead to the development of cracks which can compromise the stability, permeability and durability of the structure. Il nucleo della struttura, molto più caldo, va incontro ad una certa dilatazione termica, capace di indurre significativi sforzi di trazione sulle zone corticali, che restano invece immuni da fenomeni espansivi.

According to some predictive models, an acceptable Δ T value between the core and the cortical area is set between 20 and 25 ° C.


Some measures that can be taken to keep the ΔT value within the acceptability limits are:


  • Minimize the cement content keeping the performance level

  • Give preference to the use of mixed cements (type II, III, IV or V, preferably "B")

  • Search for the maximum possible ratio between aggregate volume and cement volume

  • Use aggregates with a relatively large maximum diameter

  • Use mineral additions to partially replace cement

  • Use retardant admixtures

  • Choose the best moment during the day to perform the pouring

  • In cold weather remove formwork as late as possible

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