Method Statement For Temperature Controlled Concrete

CONTENT

1.      Method Statement

2.      Appendix 1.0      -        Manpower organization chart

3.      Appendix 2.0      -        ISO 9001 certificate & SAC-SINGLAS approved letter

4.      Appendix 3.0      -        Mix design

5.      Appendix 4.0      -        PBFC test report (BS 4246)

6.      Appendix 5.0      -        CP 65 : Part 2 : 1996 (1999) – Code of Practice for

Structural Use of Concrete

7.      Appendix 6.0      -        Temperature Monitoring Recorder & Thermocouple Wires

Type T

8.        Appendix 7.0   -        Location of Thermocouple Points & Conditions of

Monitoring

Method Statement For

 Temperature Controlled Concrete

1.0    GENERAL

This method of statement covers the works of temperature controlled concrete.

2.0    ORGANISATION

The  organization  involves  key  quality  assurance,  production  &  management personnel, from the central laboratory and batching plant. (Appendix 1.0)

Our quality management system on ready-mixed concrete operation obtained ISO

9001 certificate on 13/03/2003 and renewed on 09/03/2009(Appendix 2.0).

3.0  BATCHING FACILITIES

Location	Capacity, (m3/hr)	Nos of trucks	Status
Plant 1	200	30	Main plant
Plant 2	100	15	Support plant

4.0  CONCRETE MIX DESIGN

Refer to Appendix 3.0 for the proposed mix design.

5.0    TEMPERATURE CONTROLLED CONCRETE

In thick section concrete pour, thermal cracking may bound to occur if no special considerations regarding the following items are made:

a.      Cement content & type b.      Aggregate type

c.      Internal & external restraints d.      Pour size, thickness

e.      Initial mix temperature

a.      The main problem associated with thick section concrete pouring is the heat of hydration.    The  hydration  of  cement  is  an  exothermic  reaction  and  the  heat developed during hydration can result in excessive temperature rise.   Associated with this temperature rise are thermal stresses generated by restraints to thermal movement which cause thermal cracking on the concrete.  The cement content shall be optimum in order to attain a low peak temperature & yet still attain its strength.

Cement type shall be Portland Blast Furnace Cement, PBFC type ‘B’ complying to BS 4246.  PBFC ‘B’ with 70% of slag has a lower heat of hydration and therefore ideal for use in temperature controlled concrete.

Other notable advantages in using PBFC concrete are as follow:

i.   Better chloride & sulphate resistance.

ii.   Greater pore filling capacity reducing water permeability. Refer to Appendix 4.0 for test report on high slag blast furnace cement.

b.      As the combined aggregate comprises of some 75% by weight of concrete, it is not surprising that the thermal expansion coefficient of concrete is dependent primarily on the aggregate type.  To minimize the likelihood of early thermal cracking, the choice of aggregate is also significant in relation to the tensile strain capacity or crack resistance of the concrete.  In particular for this project, we shall use 20mm crushed granite which will yield lower thermal expansion concrete.  Further, tensile strain capacity shall be relatively higher than gravel.

c.      Pour size/thickness influence the  rise  in  temperature curve.    As  the  minimum dimension increases, the rate of heat dissipation from the center is reduced and therefore the temperature rise increases.

Temperature related cracking control is based on the control of the temperature differential between interior and exterior of the mass concrete.

Attention should be given to the concrete just below the exposed surfaces and in the vicinity of edges and corners. The heat generated during the hydration process will dissipate into the surrounding air dependent upon the temperature differential.  The net temperature rise in the concrete adjacent to the surface is less than in the interior producing a gradual increasing temperature gradient from the surface to the interior. Where the heat can dissipate concurrently in two or more as in the case of edges and corners  of  concrete  elements,  the  temperature  drop  is  inevitably higher.    This condition if unchecked will lead to the development of tensile strains sooner than on the sides or tops of the element.

Temperature differential is a function of distance, hence the term temperature gradient. From our past experience thermo cracks shall not happen if the max temperature differential is not more than 20.0 oC (Refer to Appendix 5.0) for a distance ranged from 0.5m to 1.0m. The temperature gradient shall be controlled using the insulation method on the surface of the mass concrete in order to prevent rapid cooling of the concrete surface while the core remains warm. 50mm thick polyform boards with low thermal conductivity value shall be fixed on the top surface of the structure. The maximum peak temperature shall be less than 65 oC.

For further protection from external weather, polythene sheets or canvas shall cover the top insulating boards.

External water pounding are not allowed at the surface and the side of the cast structure.  Any water pounding shall be pumped or removed immediately so as not to accelerate the cooling of the concrete surface.

d.      Reducing the initial mix temperature causes a reduction in the rate of hydration.  In addition to reducing the rate of hydration, the peak temperature will be reduced; hence the subsequent temperature drop to ambient will also be reduced.

Since peak temperature usually occurs at the concrete core, we shall limit temperature at the time of placement of the centre portion to be less than 28°C.

This temperature can be achieved by the following methods:

i.     Control  the  temperature  of  aggregate  by  shading  of  the  aggregate stockpile & sprinkling water on the crushed aggregate.

ii.   Use of chilled water to batch chilled concrete.

iii.   Limit temperature of cement of 45°C at the time of mixing.

6.0    CONCRETE TEMPERATURE MONITORING

To monitor the concrete temperature, temperature sensing devices shall be installed within the concrete element.  As the objective of this exercise (field monitoring) is to obtain information on the temperature distributions within the element, hence the sensing device is to be distributed so as to give a representative temperature profile.

The sensing device to monitor the temperature within the concrete section shall be

Type T (Copper-constantan) thermocouple wires with temperature range between 0

– 360°C.

This sensing device shall be connected to a temperature monitoring recorder (Refer to Appendix 6.0) which records & plots the  temperature data of each sensing location. The thermal stresses due to temperature gradient in the transverse direction have always been found to be critical. Hence, thermocouples shall be spaced along the transverse direction (thickness), vertically for slab/pile cap and horizontally for wall. The  sensing location shall  be  at  the  core, next  at  the  edge/corner. Each location shall have thermocouples spaced evenly and shall be 100mm from the outer surface of the concrete.

Temperature readings shall be taken and monitored until the core temperature falls below 55˚C. Temperature readings shall be reported for every hour. The

temperature differential shall be calculated for nearby two thermocouples within the same location.