CONCRETE: HEAT DEVELOPMENT UDC 691.32

Key words: Concrete, heat development, test method

1 SCOPE AND FIELD OF APPLICATION

The method can be applied to determine the amount of heatdeveloped in concrete during the hardening process. The heatdevelopment is described in terms of the produced energy asa function of the maturity of the concrete.

Thus, the entire heat generated by the hardening process isused to raise the temperature in the concrete sample. Thetemperature rise is registered as a function of time and usedas a direct measure of the generated amount of heat.

The information obtained may be used to predict thetemperature development and distribution in a structurewhere concrete hardening is taking place under varyingconditions.

2 REFERENCES

NT BUILD 191 Concrete, fresh: Sampling

3 DEFINITIONS

Adiabatic calorimetry, determination of energy generatedduring the hardening process in a concrete specimen, whenheat exchange between the concrete and the surroundings isprevented.

4 SAMPLING

The sampling procedure should be as stated in NT BUILD 191.

5 METHOD OF TEST

5.1 Principle

The energy production during hardening of a concrete sampleis calculated on the basis of measured temperatures in thesample and its surroundings depending on the method:

Adiabatic calorimetry

The concrete sample is kept in a container where thetemperature around the concrete is changed continuously tofollow the temperature in the concrete.

Quasi- adiabatic calorimetry

The concrete sample is kept in an insulated container. Thegenerated heat is partly used to raise the temperature in theconcrete and partly lost by convective heat transfer from theconcrete to the surroundings (into and through the insulation).The amount of heat generated is determined as the sum of theheat accumulated in the concrete and the heat lost to thesurroundings. The temperature rise in the concrete is used asa measure of the accumulated heat, and a temperaturetransducer built into the insulation wall of the container is usedto yield a measure of the heat loss.

Insulated block calorimetry

A well insulated large scale concrete block is used. The centralpart of the concrete will attain approximately adiabaticconditions during the first part of the hardening process. Thetemperature rise in the centre of the block is used as ameasure of the heat development. The method introduces aone-sided error. Since heat loss is neglected the magnitude ofthe error is increased with the length of the measuring time.

In a modified version of this method the concrete block isreheated when the heat development from hydration hasbecome negligible. Measurements of temperature during thesubsequent cooling process can be used to calibrate theindividual block and thus eliminate the one-sided error due tothe heat loss.

7GQ0”‘C:E Published by NORDTEST Tekniikantie 12, FIN-02150 Espoo, FINLAND Tel + 358 9 455 4600 Fax +358 9 455 4272G ISSN 0283-7153 Proj. 821-89 www.nordtest.org

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5.2 Apparatus

- Concrete mixer.

The passive stability of the box shall be better than± 0.5 °C/day.

- Thin-walled metal container.b) Haybox calorimeter.

- Temperature conditioning box, which can be either a), b) orc) depending on the method used:

a) Adiabatic calorimeter.

A box as shown in figure 1. The box shall be equippedwith a heating device, a fan, devices to measuretemperature in the concrete and in the air space insidethe box and an automatic system to operate the heatingsystem, so that the air temperature inside the box isalways the same as the temperature of the concrete.

A suitable insulated box equipped with devices tomeasure the temperature in the concrete and a propertythat can be correlated to the heat loss from the concretethrough a calibration factor. The principle is exemplifiedin figure 2.

The calibration factor for the box determined as afunction of measuring time (see section 6.1) shall beconstant within ± 2 % for measuring periods between1 and 3 days.

Figure 1 The adiabatic measuring box.

Concrete temperature

Heat t ransmit tance

Figure 2 Haybox calorimeter

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Concrete temperature

Insulat ion

Figure 3 Insulated mould

c) Insulated mould. where:

A cubical muold of effective side length not less than mw1 m, insulated on all six faces so that the coefficient of

= weight of water sample

transmittance is lower than 5 kJ/m2h°C, and equipped Cw = 4.182 kJ/kg°C (specific heat of water)

with a device to measure the temperature in the centre θ 0 = water temperature at start (t = 0)

of the mould. The principle is exemplified in figure 3. θt = water temperature at time t = t

l Data collecting equipment adapted to the devices built into fw,n and fw,n-1 = output from boxwall transducer at beginningthe temperature conditioning box. and end of a time interval, ∆tn.

∆tn = time interval = tn - tn-1

N = number of time intervals from t = 0 to t = t

6 TEST PROCEDURE

6.1 Calibration

Adiabatic calorimeter

An inert specimen (water, old concrete) with a certaintemperature is placed in the box. The temperature in thespecimen shall be recorded at least once per hour over aperiod of 2 days. The passive stability shall be better than

± 0,5 °C/day. The calibration shall be performed at minimum 2different levels representative for the range of temperaturesactually used. Typically 60°C and 30°C can be used.

Haybox calorimeter

A container filled with water with a temperature ofapproximately 40°C is placed in the box. The temperature inthe water and the output from the transducer in the box wallshall be recorded at least once per hour over a period of3 days.

The calibration factor shall be determined by:

The calibration factor function shall be constant within ± 2 %after 24 hours measuring time.

6.2 Testing of heat development

Method A and B

The net weight of the fresh concrete sample, placed andcompacted in the metal container, shall be determined within

1 % by weighing the container with and without concrete.±The container is placed in the box and the temperature of theconcrete and the output from the box wall transducer (onlymethod B) shall be recorded as a function of time relative tothe time when water was added to the concrete mix. Themeasurements shall normally be continued until at least 300hours of maturity have been reached. Measurements shall betaken at intervals of not more than 30 minutes.

Method C

mw . cw . (θ 0 - θt )at =

fw,n + fw,n-1 . ∆tnn=1 2

The fresh concrete is placed and compacted in the mould.The temperature in the centre of the concrete and the ambientair temperature shall be recorded at least once per hour overa period normally not less than 3 days.

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6.3 Expression of results

Heat development

The measured data are transformed by calculation so that thegenerated heat is presented as a function of the maturity of theconcrete. The maturity, M, is given by:

M = Σ H ( θi ) . ∆tii=1

where:

∆ti = legth of time interval i

θi = mean temperature in time interval i

H(θ) = velocity function

E(θ) 1[ ( 1= EXP . -

R )]293 273 + θ

R = gas constant = 8.314 J/mol K.

E(θ) = activation energy for the concrete mix

The value used for E (θ) shall be stated. If the correct value isunknown the following can be used:

E (θ) = 33.500 J/mol for θ > 20 °C

E (θ) = 33.500 + 1470 (20 - θ) J/mole θ < 20 °C.

The heat development, Q, is expressed as energy per kgcement (or cement + flyash + microsilica) and is determinedby:

∆Qn = ∆Qacc,n + ∆Qtrans,n

[ at . ρc fc,n + fc,n+l= (θn+1 - θn) . cc . ρc + . . ∆tn ]

c mc . c 2

N-l

Q = Σ ∆Q nn=1

where:

∆Q acc, n = The change of accumulated energy in theconcrete during the time interval ∆tn

∆Q trans, n = The change of transmitted energy from theconcrete to the surroundings during the time

interval ∆tn.

qn and qn+1 = Concrete temperature at beginning and endof a time interval ∆tn.

C = content of cement (or cement + flyash +microsilica) per m3 concrete, kg/m3

ρc = density of concrete, kg/m3

Cc = specific heat of concrete, KJ/kg°C

The second term applies only to method B.

m c = weight of concrete sample, kg

fc,n and fc,n+l = output from box wall transducer at beginningand end of a time interval, ∆t.

at = calibration factor

∆tn = time interval = tn+1 - tn

7 REPORT

The test report shall include the following information:

a)

b)

c)

d)

e)

f)

g)

h)

i)

j)

k)

l)

m)

Name and address of the testing laboratory.

Identification number of the test report.

Name of the organization or person who ordered thetest.

Purpose of the test.

Identification of the concrete mix.

Method of sampling and other circumstances.

Date of the test.

Test method.

Identification of the test equipment and instrumentsused.

Any deviations from the test method.

Test results.

Inaccuracy of the test result.

Date and signature.

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