STANDARD PROCEDURES FOR TEMPERATURE CONTROLLED CONCRETING

STANDARD PROCEDURES FOR TEMPERATURE CONTROLLED CONCRETING

1.0      PURPOSE:

This procedure covers the required methods for mass concreting in controlled temperature to prevent cracking caused by excessive tensile strains that result from temperature differential within the concrete mass. Cracking can be controlled by methods that limit the peak temperature to a safe level. These method statements have been prepared to provide a general guide, special considerations to be given in relation to the heat of hydration for concrete pours.

2.0      SCOPE:

These method statements includes e.g. pre-cooling procedure before & during concrete mix, supply & delivery of concrete, inspection of concrete at the workface and post cooling by a proper cooling upon a completion of concrete placement.

3.0      REFERENCE STANDARD:

2.1       IS: 456, IS: 10262, IS: 7861 (Part-1)

5.0      PROCEDURE:

5.1       The entire procedure can be organized into 3 steps

5.1.1    First step is a pre-cooling procedure with will be applied at the concrete batching plant to achieve a minimum concrete temperature supply.

5.1.2    Second step is to minimize time delivery of concrete since being dispatched from batching plant to the workface and to minimize time spending before concrete placement at the workface.

5.1.3    Third step is a post-cooling procedure with will be applied during concrete curing upon a completion of concrete placement.

5.2       A temperature differential between the center of the concrete mass and any about the outside faces will be kept minimum. Heat on the surface of the concrete is easily released toward the ambient whereas the center remains its high temperatures; this follows that the temperature differential will induce the internal restraint and subsequently introduces thermal contraction cracks.

5.3       The method statement also includes details of temperature differential control procedure during concrete placement with related to pouring sequence. Special care and planning is required as the volume of pour is large in relation to those normally being place on the workface.

6.0      CONTROLLING TEMPERATURE IN MASS CONCRETE:

6.1       Controlling temperature in mass concrete:

6.1.1    6.1 Concrete Mixing & Supply: 

            With a large concrete pour of this magnitude it is essential that proper pre-construction planning methods and proper checks be in place prior to execution of the works. A mass pour is considered to be demanded a special attention to be given to logistical and technical considerations such as concrete supply, casting sequence and temperature differential control by thermal quilts curing method etc. Finally, a report of temperature monitoring will be conducted.

6.1.1.    6.2 Concrete Mix Control (Pre-Cooling Procedure): 

It is a      It is a preparation against the possible effect to the concrete properties and construction operations. When these factors are explained to the relevant persons engaged to these activities therefore measures will be taken to eliminate or minimize undesirable effects that possibly introduced. This method defines lists of possible unfavorable effects and prevents practices intended to minimize them. Among these practices are such important measures as a pre-cooling ingredient, consideration of concrete temperature as placed, transportation time and curing technique. The method of concrete temperature control will be proposed as follows:

Ø  Use of ice as part of mixing water: This application is effective in reducing concrete temperature, because on melting alone, to absorb heat. The following measures will be applied:

a.       Spray cool water continually approximately at every 30 minutes interval to the course aggregate stock. This will be done at least 2 hours in advance prior to mixing of concrete.

b.      Add ice as a part of mixing water for the concrete mix to reduce the water temperature. The ice will be added until the water temperature is reduced to reach 8-10 Celsius Degree. The amount of ice added to the water is dependent on the air temperature at the certain mixing day.

c.       Spray cool water to the trucks mixers to reduce the temperature of the trucks mixers.

d.      At the batching, the first batch of the concrete shall be checked its temperature. The water temperature will be continued to reduce until the concrete temperature is lower than 30 Celsius Degree prior to the delivering to the workface.

e.       The above process will be regularly done during the entire period of supply to ensure that the same degree of temperature always be maintained.

f.        At the workface upon a delivery of the concrete, the fresh concrete will be re-measured whether the temperature is still within the specified tolerance.

Ø  Using Low Heat Concrete: mix design for low heat concrete mix will be submitted for approval prior to use. In addition, trail mix will be done prior to use.

6.1.1.    6.1.2 Since the temperature rise can be increased with a passage of time thus the period between mixing and delivery will be kept to an absolute minimum.  Attention will be given to coordinating and dispatching of the mixing trucks with the rate of placement to avoid delays in delivery. Details are as follows:

Ø  Planning: A pre-concreting meeting will be held internally and with concrete supplier to finalize agreement of the proposed concreting procedure.

Ø  Access: Designated site access routes for concrete supply will be prepared and traffic control measures implemented. The proper access must be organized to enable a continuous transportation of the concrete during the entire period of pouring so that the required temperature of concrete pour will be maintained.

Ø  Supply: Concrete batching plant capacity and numbers of delivery trucks will be planned to maintain a continuous supply of concrete so that an introducing to cold joint can be prevented. In addition to minimize temperature differentials occurred within the concrete mass during pour.

Ø  Pouring: Concrete will be placed in a predetermined sequence to avoid any possibility of an excessive temperature differential and cold joint formation within the foundation.

Ø  Quality Control: Sufficient quality control measures will be set up in place for checking concrete supply and deliveries throughout the duration of the pour.

6.1.2    Concrete Placement:

Due to one-go casting of each raft foundations / structures the concrete pouring should be executed during the night time in order that the workface temperature can be minimized during the entire placement period. Sufficient concrete pumps will be engaged for concrete placement in order that a flow pouring can be achieved. As such parking time of the concrete trucks waiting for pouring can be kept minimized thus the required placing temperature will be maintained.

6.1.3    Curing Procedure:

The set of foams and plastic sheets will be used for curing purposes. This will be done immediately once the concrete is reached its stiffening time by fully cover the whole  concrete surface and spread the sand (approximately 10 mm. thick.) onto the whole area of plastic sheets to prevent concrete temperature loss too quick and result high temperature differential between the surface and at the middle. Care must be taken during the entire curing to ensure by reading its temperature through a set of thermocouples that the temperature at the middle of the concrete and any outer surface to be kept minimum.

6.1.4         Test and Monitoring of the Concrete Mass after Poured:

6.1.4.1       6.1.4.1 As soon as the concrete has reached its stiffening time approximately 1 hour after pouring a set of plastic sheet will be covered fully the concrete surface. Sand will be filled fully onto the whole area where the plastics sheet cover with approximately 10 mm thick. The plastics cover and the side forms will be maintained to ensure that the temperature differential between the middle of concrete mass and any outer surface can be kept minimized.

6.1.4.2      6.1.4.2 To ensure that the temperature differential will be controlled a set of measuring apparatus will be set as shown by the above pictures. The measurement purposes are to finding the temperature development within the concrete mass. The results will show peak temperature and the temperature differential for such measuring points.

6.1.4.3        6.1.4.3 As previously explained, sets of thermocouples will be installed and embedded to the concrete at the bottom, middle and. Top and bottom sets will be approximately 30 cm of measuring from the edges. The heat generated will be converted to the circuit and sent to the digital thermometer which will record and show information of temperature measure.

    6.1.4.4  In general case the maximum temperature of mass concrete will peak at 48 hours after pour and will be constant for 7 days and will decrease to the air temperature after 14 days. 

7.0    INSPECTION

7.1       Inspection shall be carried out as per approved FQP.

8.0        DOCUMANTAION:

8.1       Reports and other documentation shall be generated and maintained as per   approved FQP and in the approved Format No. XXX.