archaeology

The load from the supporting structure was transferred to 32 numbers of brackets on the IC wall and 10 numbers of intermediate supports resting on the internal walls. The fabrication and erection work was completed in 90 days, thus achieving another milestone. The formwork supporting structure was first assembled on the ground to avoid any mismatch and the erection inside the Reactor Building was done using the Leibherr 650 T capacity departmental crane. The complete steel structure was erected in eight segments.

IC Dome Formwork

    
The formwork consisted of 12mm plywood with 100 x 150 mm (4"x6") backing wooden members, resting and the steel supporting structure through adjustable spindles.The complete IC dome formwork covering an area of 2000 sq.m. was divided into a number of panels and each panel was fabricated to curvature in all the three directions at the carpentry shop. Each panel was numbered, inspected and then dispatched to site to be erected and fitted in their respective locations.

Reinforcement

Cold twisted deformed bars are used as reinforcement rods in any concrete structure. The standard length of any reinforcement rods of various dia. is 10 m. When the length of the reinforcement rod is required to be more than 10 m., the usual practice is to join 2 or 3 rods to form one single rod by lapping the rods at their ends. In heavily reinforced structures, such lapped joints create congestion to the flow of concrete. To overcome this problem, lapping is replaced by various methods of joining such as butt welding, mechanical splicing, threaded coupling etc., which are generally done in a fabrication shop. In the case of Kaiga-2 IC Dome, which is also heavily reinforced, butt welding and mechanical splicing methods of joining rods were used.
The total quantity of reinforcement used in the IC dome was 220 MT with 1360 numbers of welded joints and 1350 numbers of mechanical splices. Welding/splicing of reinforcement bars was itself a difficult task owing to the large length of the rods to be handled. The average length of each rod after joining several rods was on an 40 m. with four welded/spliced joints per rod. The welder output per day was 17 joints and that of the splitter was 30 joints per day. Only pre-qualified welders were employed. Non-destructive and destructive tests such as dye penetration test; radiography and mechanical tests were conducted on the joints.     

Mechanical splicing was used where two rods of equal diameter were to be joined whereas welding was used at joints of unequal diameter rods. The complete job of welding/splicing took 80 days. The lifting and placement of rods were done, using a 35 m. long space frame truss, owing to the large length of the reinforcement rod. This method of placement of steel reduced the time considerably.

Pre-Stressing Works

As mentioned earlier, the containment structure is designed to contain the contaminated air during a postulated event of LOCA. To enable the above function, the containment (particularly the inner containment) should not show even minute cracks when the building pressure mounts during a postulated event of LOCA. For, such cracks would result in increased leakage of contaminated air to the outside atmosphere.

    
A plain reinforced concrete structure requires a very large quantity of reinforcement and thick concrete to prevent micro cracks. To avoid this and prevent the occurrence of micro cracks in concrete, high tensile steel strands/wires are used in the inner containment. These are later pulled and locked from both ends by bearing on the concrete. This removes the tensile stress, which is the main reason for cracking. The procedure of pulling these strands/wires (stressed) before loading on the containment is termed as pre-stressing of the cables and such a structure is termed as ‘Pre-stressed Concrete structure’.

Low relaxation stress which relieved high tensile strands for pre-stressing works, was used for Kaiga-2 IC dome. Lead coated dross bach flexible MS sheathing of 80 mm inner diameter were used to house 19 strands to form one cable.

The erection procedure of pre-stressing cables was an entirely new concept to minimise the time. A brief outline of this procedure is given below :

Those cables which were to be anchored in the ring beam at both ends (D-cables) were threaded into the sheathing on the ground and later lifted with the same space frame truss which was used for placement of reinforcement. Pre-fabricated templates to support the cables in position were used to keep the cables in their desired location in the IC dome. These cables which were to be anchored with one end in the ring beam and the other end in the stressing gallery (housed in the base raft) called J-cables were lifted without the sheathing, using a de-coiler.
All the 19 strands forming a cable, were bunched together (90 m. long). They were wrapped around a 6 m. diameter circular framework called the decoiler, made out of 80 NB pipe. At the ground level, an electrically operated winch with a steel rope was used to rotate the decoiler, which had a braking arrangement. It was used to rotate the decoiler to wrap the cable onto it at the ground. With the brakes in the applied position, the decoiler with the cable, was lifted using the tower crane to the IC dome top. A pilot manila rope with a steel hanger at the free end, tied to the end of cable, was inserted into the vertical sheathing in the IC wall until it reached the stressing gallery.     

With the decoiler hung in position, the pilot rope was pulled from the stressing gallery simultaneously regulating the decoiler brakes. Once the cable reached stressing gallery, the decoiler was moved in the air, simultaneously unwinding the coil. It was laid on the templates to hold the cable in position on the IC dome. The sheathings of 3 m. length each, were later threaded to the cables and the joints sealed with couplers and M-seal. With above method, an average of ten to twelve J-cables and twelve to fifteen D-cables were placed in one day. The complete operation of cutting, placement and alignment of cables took 60 days. The total number of cables in the IC dome was 170 numbers.

Top Formwork

As per the code of practice for concrete structures, the slope of the top profile of any concrete structure exceeding 15 deg. to the horizontal should be provided with shuttering, also called as top formwork, to maintain the top profile. With the maximum slope of the top profile of the IC Dome being 36 deg., top formwork was provided for almost the full area.

After various mock-ups, the top formwork scheme was evolved.It consisted of structural steel trusses laid in the radial direction providing 40 mm cover to the top reinforcement. 25 mm thick and 200 mm wide wooden planks were cut to the required size and shaped and numbered, so as to fit in their desired locations covering area of 1200 sq.m. These planks, were erected progressively along with the concreting of the IC dome and wedged adequately against uplift with wooden wedges to the truss.

Concrete and Concreting

 

The quantity of concrete to be placed in the IC dome including the ring beam was of the order of 1950 cu.m. It is very difficult to handle such a large quantity of concrete in a single pour. In such cases, pouring of concrete is done in segments by maintaining an inter-pour gap of few days between adjacent segments.

Before starting the actual concrete work, the workers were divided into various categories such as masons, carpenters, riggers, finishing gang etc. All the workmen were briefed about their work before the start of every pour. This method of educating the workmen and in turn obtaining a good quality job was unique in the field of civil engineering.

The IC dome along with the ring beam was divided into eight pours - three horizontal joints in the ring beam and five in the IC dome. The construction joints in the IC dome were in the north-south and east-west directions, running perpendicular to the direction of the pre-stressing cables. Concrete was produced in the batching plant situated outside the plant site and transported to site in transit mixers. The temperature of concrete was controlled to less than 23 deg. C. by replacing water with ice flakes to prevent cracking of fresh concrete due to temperature gradient occurring during release of heat of hydration of cement. Workability in terms of slump of concrete was maintained at not less than 175 mm, to achieve easy placement. The concrete mix design details are as follows:

1. Grade - M 60/20/175/23 deg. C.
2. Cement content - 475 Kg/Cu.m
3. Water binder ratio - 0.32
4. Micro silica (silica fumes) - 7.5% by weight of cement
5. Superplasticizer - 2% by weight of cement
6. Retarder - 0.1% by weight of cement

    
Concrete sampling for cubes and cylinders were done both at the ground level as well as at the placement position on the IC dome.Concrete was pumped to the IC dome top using two concrete pumps and placed in the required pours using two concrete placers which had swing in multi-direction. Except for the crown pour where quantity of concrete was small, for all the other pours, both the concrete placers were put to use simultaneously, to avoid cold joint and achieve faster progress.

As mentioned in the earlier paragraphs, the top formwork was placed progressively along with placement of concrete. These planks were removed within three hours of placement for finishing of the top surface. Once the finishing was over, the surface was sprayed with curing compound and covered with plastic sheets to prevent loss of water from the concrete. After eight hours of placement of concrete,
the plastic sheets were removed. The top surface was covered with wet hessian cloth and water curing was continued for at least ten days.

Construction Joint Preparation

To obtain a good bond between concrete of adjacent pours, the usual practice is to hack the surface with chisel and hammer. However, in the case of IC dome, the surface to be roughened is not completely accessible for hacking. To overcome this problem, a liquid called surface retarder was used.
A surface retarder, retards the setting of concrete to a depth of 5 mm from the surface. This concept was used for the first time in NPC and the same was studied in various mock-ups before putting to use. In case of ring beam, the surface retarder was sprayed on the exposed construction joint and in case of the IC dome, the same was applied using sponge on the stopper formwork (bulkhead) at the construction joint before the placement of concrete. After the placement of concrete (the stopper formwork in the case of IC dome was removed) the exposed construction joint was green cut using air-water jet at 7 kg/sq.cm to expose the aggregates. This provided a rough surface between the present concrete surface and the concrete to be poured in the adjacent pour. A special green cutting nozzle was devised at the NPC plant site mechanical workshop.     

Long Term Structural Monitoring of Inner Containment

For long term structural monitoring of the inner containment and to study the loss of prestress with time, Vibrating Wire Strain Gauges (VWSG) were embedded at two diametrically opposite locations in the ring beam and at thirteen locations in the IC dome. The wires connecting all the VWSGs to the data-logger were routed along the pre-stressing cable sheathing and brought out of the IC dome at the springing level. These wires were further routed in a 100 NB GI pipe to the control room situated at the ground level, for data logging. Strain and temperature measurements were taken every 30 minutes after placement of concrete in a particular pour.

Conclusion

 

With an inter-pour gap of seven days between adjacent pours, the concreting of the IC dome, including the ring beam was completed in 70 days. As a design requirement, concreting of pour-1 in the ring beam was started after placing almost complete reinforcement and complete placement of pre-stressing cables.

In March 1998, the construction of Kaiga-2 IC dome was achieved, thus, creating a record in the process. It was accomplished by the Civil Engineering community of NPCIL and Kaiga Project in particular.

Discussions

1. Inter Pour Gap

The inter-pour gap has always generated discussions during concreting of important structures. The inter-pour gap determines the speed with which we achieve the progress and complete the construction activity. Inter-pour gap depends not only on the design criteria but also on the constraints at site. By meticulous planning and engineering of the construction activity, the inter-pour gap can always be reduced to the minimum possible.

2. Simultaneous Placement of Concrete

As can be seen from sketch No. 2, Pour Nos. 5 and 6 are sub-divided into 5A, 5B and 6A, 6B. As per stipulations, Pour Nos. 5A and 5B were to be cast simultaneously and so also, 6A and 6B. The quantity of concrete to be handled in pour 5, was 300 cu.m and that in pour 6, was 200 cu.m. To achieve the stipulation of simultaneous pouring of concrete, plant and machinery as listed below were used:

1. Batching Plant - 30 Cu.m/hr capacity 1 + 1 standby
2. Transit Mixers - 6 Nos.
3. Concrete pumps - 30 Cu.m/hr. capacity 2 + 2 stand by with standby pipeline to carry concrete.
4. Concrete Placer - 2 Nos. No standby.
5. Tower Crane - 1 No. for placement of concrete during breakdown of concrete placer.

With the kind of plant and machinery as indicated above, the process of simultaneous placement of concrete became very smooth.

3. Cleaning of Pour No. 2

After casting Pour No. 2 of the ring beam, there is a considerable time gap before the IC Dome is connected to the ring beam. During this period, a lot of debris gets accumulated in Pour No. 2 mainly due to green cutting of construction joints of various IC Dome Pours. Though windows of size 250mm X 100mm were provided in the inner formwork at the level of Pour No. 2 to facilitate removal of debris, a better alternative would have been to fabricate the inner shutter in such a way so as to have a horizontal joint of the shutter panels at a height of 100 to 150mm above Pour No. 2. After casting Pour No. 2, the shuttering to the above mentioned joint can be removed, thus creating a continuous window of 100 to 150 mm high at the level of Pour No. 2. High pressure air-water jet at 7 kg/sq.cm pressure can be used to clean the pour. Later, the shuttering for a height of 100-150 mm can be provided.

Mr. H.T.Jagadish is Scientific Officer (Civil), NPCIL, Kaiga