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Fundamental technology
Basic Technology

Curing is the ensurement of planned properties in concrete through the optimising of the hydration process.

Physical Restrictions

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 )

                                             Q                            
∆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 
      Volumes

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.

Temperature effect
Temperature effect

The completeness of the hydration is dependent on the availability of the stoichiometricly required water. A withdrawal of water from mixture in areas close to the outer surfaces of the concrete product is possible through evaporation on the outer surface, if the humidity of the ambient air does not show saturation.
The air humidity shows the gaseous proportion of the water contained in the air. The capability of air to take up water rises with increasing temperature.
Should air saturated with water vapour be cooled, liquid water seperates from the air by means of condensation.
A common declaration for the amount of water vapour in air is the relative air humidity in %, which is calculated with the following formular:

φ =          s        * 100 [%]
               S

s - specific air humitity in g/kg
S - maximal specific air humidity in g/kg
The numerical values can be taken from the Mollier chart.

The effects of air humidity can be made clear with the example of a curing room having the dimensions 10 mtr. wide, 25 mtr. long and 3 mtr. high with a product volume of 100 mtr.³. With an exit temperature of 20°C and relative air humidity of 70%, a heating of the space to ca. 30°C would require a water vapour capacity of 9100 gr to achieve saturation of the air. Without supplementary water from an external source this water volume would be extracted from the concrete mixture.
Mollier-chart
Mollier-chart

Simplified these dependances establish the requirements:

  1. Protection of the concrete upper surface against premature drying through the minimisation of the vapour pressure fall between the product outer surface and the ambient air, in that the water absorption capacity of the air, dependant on the temperature level, is readjusted.

  2. Protection against a sub-optimal hardening process in the concrete mixture via an influence on the chemical reaction of the cement by means of the controlled transfer of heat energy over the product outer surfaces. 
 
Idealized course of a curing curve
Idealized course of a curing curve
Constructive Commentary on the ifs systembau Curing System

The technical realisation of the technological requirements is achieved by:

    - infinitely variable temperature increase of the ambient air around the concrete form up to 60°C
    - infinitely variable increase of the relative humidity of the ambient air up to > 95%

The regulation of heat transfer is made over a flat capillary-tube heat exchanger with an effective surface of 1:1,3 to the installation surface. The heat transfer to the concrete product ensues as 70% heat radiation and 30% convection. The capillary-tube heat exchangers are made of synthetic material and, with a weight of 1,0 kg/m² and a wall thickness of 2,0 cm, are placeable without problem in existing curing areas.

Capillary heat exchanger
Capillary heat exchanger

The system works with a closed water circulation, on the feed side with a temperature of < 90°C and a working pressure of < 2.0 bar. Occupational safety requirements are thus not accured. The curing space is at all times accessible during operation.

On the primary side the heat requirement is generated from a standard heat source (Oil, gas or electrical operation). A required low temperature demand can also be covered from alternative energie sources - heat pump, process residual energy etc. The system is just as suitable for cooling the concrete outer surface when, for example, an excessive temperature in the concrete mixture has to be avoided. In this case the heat source would be replaced or expanded with a cooling unit.

Primary heating water loop
Primary heating water loop
Secondary heating water loop
Secondary heating water loop

The regulation of the air humidity takes place over a binary-jet system (compressed air/water). With freely selectable guidelines for spray times and spray volumes, a permanent balancing between set value and actual values regulates the individually set target value interactively, beginning with the start value for humidity level. A supersaturation of the air (forming condensation) is thus extensively avoided.

Through the temperature difference between the colder product surface and (warmer) ambient air the air is cooled off (heat transfer) in the outer surface vicinity of the concrete and thereby a relative moisture level of > 95% up to the condensation of air humidity on the concrete outer surface is achieved.

Valves for controling the relative humidity
Valves for controling the relative humidity
Nozzle
Nozzle
Sensor temperature/relative humidity
Sensor temperature/relative humidity

In every curing zone a combination sensor is installed for the gathering of air parameters (temperature and relative humidity). Additionally and for integration into the controls as a reference signal and/or for gathering environmental data a combination sensor is installed in both the production hall and the external area.

For the calibration of the facility management manual sample measurements are taken with an infra-red thermometer/hygrometer of the actual outer surface temperature/humidity on the product outer surface. The comparison of the measured air temperature/humidity with the concrete outer surface temperature/humidity shows as a rule constant factors for the individual zones.
To determine the concrete parameters with sufficient accuracy the measured values must be weighted. The input of these factors as correlation values is made into the control programme if the PC.
For every zone of the curing independent set values can be freely given for both parameters. Every set value can be additionally allocated a time interval, so that the temperature and humidity profiles are programmable over the curing period. By this means various curing sequences can be drawn up, e.g. for the formular, product type or the season. 
Input layout for programming a curing process
Input layout for programming a curing process

To the end of the curing period the control orders, over programme option, to additionally give a freely selectable constant (Δt) as a set value to the measured values of the hall sensors or the exterior sensors. This leads to an automatic decay ramp for the temperature (or the humidity) to the end of the curing period.

A system control after the maturity development (e.g. Saul) is possible. The securing of quality standards such as for example temperature increase and temperature decrease ramps is assured through the free selection of Δt [K] Values/Hour.

Digital actual indication
Digital actual indication
Course of the temperature/humidity-curve
Course of the temperature/humidity-curve
All process data is continuously gathered by a measuring system and stored in a freely selected rhythmn. The data will be shown up to date as digital values or as a graph. The expansion of the stored data with company or customer details etc., for the creation of production protocols / quality protocols, is at all times possible. The system has the required interface for an online connection Intranet/Internet. The programming and administration of the data is therefore independent of the location of the facility. The curing system is supplied and regulated over a central control station. All components are internationally available products and possess the respective national certifications.
Curing control center
Curing control center

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