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Concreting of the Bottom Plate of the Box-Shaped Foundation of the Lakhta Center Tower

magazine "High Buildings", VLADIMIR TRAVUSH, Chief Designer of GORPROJECT, CJSC; ALEXEI SHAKHVOROSTOV, CEO of “Inforsproekt” engineering bureau

01.04.2015

Vladimir TRAVUSH

Chief Designer
of GORPROJECT, CJSC


ALEXEI SHAKHVOROSTOV

CEO of “Inforsproekt” engineering bureau

The Tower building of the Lakhta Center multifunctional complex is 462 m hight and comprises 86 aboveground and 3 underground floors. The underground floors have a shape of an equilateral pentagon in plan, and each side is 57.25 m long. The construction of the underground floors of the building forms the box-shaped foundation, comprising the 3.6 m thick bottom slab, the 2.0 m thick top plate, the central structural core 28.5 m in diameter and 10 vertical stiffening diaphragms with the total height of 16.6 m. The peculiar feature of the structural layout of the Tower building is the circular central structural core, which receives most of the vertical load (about 70% of all vertical loads on the building). As a result, a large proportion of the building weight is transferred onto a small section of the foundation within the limits of the central core. The box-shaped foundation leans through a concrete bed upon the pile foundation of 264 two-meter piles 55 and 65 m long and performs the function of even load distribution from the core of the tower onto the pile foundation.

The carried out calculations showed, that the bottom plate of the box-shaped foundation experiences a great stretching stress: axial tension 2300  t/m, and bending moment 2150 tm/m. The total volume of concrete in the box-shaped foundation is about 46 thousand m3 .

Proceeding from technological considerations, according to its construction sequence the whole structure is nominally divided into three parts:

  • phase one – the bottom cast-insitu reinforced concrete raft of about 20.3 thousand m3 ;
  • phase two – the central part of the foundation of about 15.5 thousand m3 , comprising the cast-in-situ structures of reinforced concrete walls made from compressive strength class B80 concrete and reinforced concrete floor slab thickness of 0.40 m made from compressive strength class B60 concrete;
  • phase three – the top cast-in-situ reinforced concrete slab thikness of 2.0  m and volume about 10.5 thousand  m3 made from compressive strength class B80 concrete.

The bottom cast-in-situ pentagonal reinforced concrete slab of the boxshaped foundation rests on a reinforced concrete bed at the elevation of –21.250 m, the top of the slab – at the level of –17.650 m. The scheme of the slab is shown in Figure 2. The slab is designed to be made from compressive strength class B60 concrete; water tightness grade W8 and frost resistance grade F150.

The bottom foundation slab is reinforced with the help of class A500C working reinforcement 32 mm in diameter. The reinforcement cage consists of 15 horizontal grids with 150 mm spacing of the rods, which are evenly distributed along the height of the slab. The distance between the horizontal grids heightwise ranges from 200 to 300 mm. The average consumption of reinforcement is 452 kg/m3 . The protective concrete cover – 68 mm. An anti-shrinkage C-1 BP-1 grid with a 100 × 100 mm mesh is set in the protective concrete cover at a 25 mm distance from the slab surface. The scheme of the plate fragment reinforcement is shown in Fig. 3.

A special procedure was developed for the slab concreting, according to which the foundation slab casting must be carried out continuously to the full structure height with even placing of the concrete mix over the entire area from the base of the slab to the top with the front of concrete placement moving vertically.

In order to reduce the heat evolution of class B60 concrete it was specified that the concrete mix must have low energy potential and the rate of portland cement consumption not exceeding 360  kg/m3 equivalent to clinker containing no more than 8% of tricalcium aluminate, the temperature of the concrete mix delivered to the construction site must fall within the range from +5 to + 15°C. Concreting of the densely reinforced construction of the bottom foundation slab is carried out using self-compacting concrete mix with workability within the range of 60 to 65 cm. Particular attention was paid to the temperature regime during the concrete mix maturing. The concrete must have minimum heat

evolution and slower early hardening kinetics in normal temperature and humidity conditions. In addition the required compressive strength of concrete aged at least 1 day – 0 MPa, when at least three days – 15 MPa, aged at least 7 days  – 40  MPa, when at least 28 days  – 65 MPa. Concrete curing in the construction was carried out in the conditions preventing thermal shrinkage and ensuring that the concrete cooling rate in the core of the plate is no more than 2,0–3,0  °C per day and the temperature drop between zones sharing a border along the height of the slab is no more than 20°C. The required concrete strength at 90 days of normal curing must be at least 68.4 MPa.

The concrete was prepared according to the standardized technological scheme with due consideration of GOST 7473-2010 requirements as to the accuracy of components proportioning and the peculiarity connected with loading and mixing of the main components of the mix and powder admixtures, the whole process divided into two stages: the stage of proportioning, loading and mixing of the components in the stationary mixer of a concrete plant; the stage of transit mixing in the process of transportation of the mix to the construction site.

In order to protect from rainfall and meet the specified requirements to the temperature regime of concrete curing and comfortable work organization, protective covers were erected over the entire area of the plate concreting (see fig. 4), which helped to maintain the required temperature regime of air heating.

To manage the temperature regime heat generators with variable power were used. Control of temperature regime of concrete curing in the bottom foundation plate is carried out with the help of automated systems developed on the basis of temperature sensors which are installed in different zones of the concreted slab: in the core and on the periphery of the structure at three elevation points, as well as in the upper part of the slab in the zones where the walls of the box-shaped foundation are located.

The general scheme and the number of zones in the bottom slab of the boxshaped foundation where the temperature sensors are installed are shown in fig. 5. At points 1, 4, 7, 10, 13, 14, 22, 31 control pipes are installed according to Node “A”. Control pipes, meant for periodic inspection of the sensors reading, are installed at a distance of 15–200 mm from the temperature sensors.

The day before the start of concreting of the structure, the bottom of the slab and reinforcement cages are heated to the temperature of + 3 ... + 5°C.

The temperature of the concrete mix, poured into the structure must not differ from the temperature of the reinforcement cage, the bottom and the walls of the fence around the perimeter of the slab by more than 12 °C and range from + 5 to + 15 °C.

The concrete mix was poured into the structure with the help of 18 concrete pumps (fig. 6, 7, 8). In order to prevent concrete disintegration tremie pipes with 125 mm inner diameter were used. The maximum height of the free drop while pouring the concrete mix in the bottom tier of the slab did not exceed 1.0 m, in the central and upper tiers – 1.3 m. The pipes were installed at the rate of three pieces per each concrete pump in accordance with the scheme in fig. 6.

After pumping the cement mortar, prior to feeding the concrete mix into the structure, a small portion (approximately 0.1 m3 ) of concrete mix, delivered to the construction site is pumped and then dropped into a special container or dump. Pumping of the mix started after each concrete pump received at least two or three mixer trucks. Intervals in the work of each concrete pump feeding the mix must not exceed 1.0 hour.

In case of a longer delay in the delivery of concrete the pumping rate was reduced to ensure that there is enough mix in the pump hopper and the concrete conveying pipe until the arrival of the next mixer truck. A laboratory for concrete quality control was set up right at the entrance to the construction site.

Mixer trucks could discharge the mix into the concrete pumps only after the laboratory had checked the quality of the concrete mix on samples taken from the mixer trucks and given their authorization.

The concrete mix was fed to the placement zones indicated in fig. 6 simultaneously by all concrete pumps with a view to even distribution of the mix over the entire area of the slab (fig. 9). Besides, to ensure the spread of the mix from the center to the periphery of the structure the pouring rate in the central zone of the slab (the core zone) was higher than in peripheral zones.

Portions of the mix were successively fed through a flexible link from each of the steel concrete conveying pipes into the tremie pipes (fig. 9). The volume of the mix portion continuously fed into each tremie pipe – 24 ... 32 m3. After each portion is placed the mix is fed into the next tremie pipe. The estimated diameter of the spread of the concrete mix with SF (slump flow)  = 60–65  cm from each tremie pipe – about 12–13 m, the approximate spread area of the mix – 130–133  m2 .

The approximate elevation rate of the self-compacting concrete layer – 7.0... 7.5 cm/hour. When self-compacting concrete mix was poured into the structure its compacting occurred due to gravity without any compulsory vibration impact.

The exposed surface of the slab within the space between protruding reinforcement for the walls is trowelled after which a vapour-moisture-proof coating is installed to prevent concrete shrinkage from dehydration: the surface of the structure was sprayed with aqua-dispersive film-forming emulsion, covered with heat-insulating blanket (geofabric, geotextile, burlap), and then with roll polythene. The said vapourmoisture-proof coating is installed within 1...4 hours after trowelling the exposed surface of the slab. Moreover, to prevent the water-retaining material from drying after 2–3 hours, water at +10 ... + 25°C was poured over the concreted areas.

The heat insulating coating helps to achieve the cooling rate of concrete in the core of the structure of no more than 2...3°C a day; and the temperature difference between adjacent zones along the slab height of no more than 20°C.

The period of safe removal of the curing tent while retaining heat insulation at the outside temperature up to –10°C is at least 6 days after the pour. When the air temperature drops below –10°C, this period must be prolonged to 7–11 days. Before removing the tent it is necessary to provide thermal protection of cold protruding reinforcement bars by covering them from above with 3 layers of “Etafom”, or with one layer of “Etafom” and by putting on heatinsulating covers such as “Vilaterm.” After the tent is removed, scaffolding is installed to assemble the reinforcement cages of the walls, the scaffolding being turned into a sort of protective tent. The air inside the tent is heated until the temperature difference between the concrete surface and the air is no more than 17  °C. After that the covers are removed from the protruding reinforcement bars.

Concrete quality control to determine the required characteristics was performed at the plant after mixing the loaded components for at least 5 min. The sample, taken from the first mixer truck in each shift (12 hours), is used to determine the cone flow, the average density and the temperature and to revise the concrete composition on the basis of printouts of actually pre-dosed components. Samples, taken from the next four mixer trucks, are used to determine the cone flow, the average density and the temperature. When the specified parameters are stabilized at the given level, samples from each mixer truck are used further on only for workability control, whereas the temperature of the mix is controlled on samples taken from every tenth mixer truck. Workability retention and segregation resistance are assessed during the concrete mix optimization prior to its delivery.

Signs of disintegration and water gain are determined by a laboratory worker first visually on the sample of the mix taken for slump flow measurement. In case of clear signs of disintegration, the mix must be tested by the “water gain” parameter according to GOST 10181-2000.

On-site concrete quality control is carried out to determine whether the concrete mix delivered to the construction site meets the regulatory requirements.

For this purpose, the workability of the mix is determined by the slump flow according to GOST 10181-2000, visual assessment of disintegrability is performed, the actual density and temperature of the concrete mix are identified, and control samples are moulded for further testing. Samples taken from the first five mixer trucks in each batch (the volume of concrete mix poured continuously for 12 hours) from each of the production plants, are used to determine the workability, average density and temperature. When these parameters are stabilized at the given level (see table) further control of their workability is conducted on samples from concrete mixer, and of the temperature – from every tenth mixer truck.

The concrete mix must not show any signs of disintegration or water gain, in the context of the construction site are determined visually by a laboratory worker on the sample of the mix taken for workability measurement. In case of clear signs of disintegration the mix must not be accepted for pouring into the structure.

Dilution of received concrete mix with additives is carried out under strict supervision and according to the calculations of representatives of the plant laboratory. The above mentioned procedure is performed once on each truck mixers. If, after its implementation, the mobility of the concrete mix does not comply with the Regulations, the mixture can not be repaired on site, shall be rejected and returned to the supplying plant.

The procedure for recovery to normal mobility is the responsibility of the manufacturer of concrete mix and shall be performed by the services of the plant laboratory on site.

Sample-cubes with 100 mm edges to determine the strength at 7, 28 and 90 days are moulded in the amount of 12 pieces from the pour of the concrete mix, delivered by one production plant and poured into the structure during one working shift. Two series of control samples (6 pieces each) are moulded from the pour of the concrete mix discharged in the first and second half of the concrete batch.

Control samples must be kept in normal (relative humidity 95 ± 5%, temperature + 20 ± 2о С) conditions. The measurement of the concrete temperature during its maturing (for 30 days) in the construction of the foundation plate starts right after its placement in the following way: within the first 3 days – after 4 hours, within the next 7 days – after 8 hours, within the next 10 days – after 12 hours, from 20 days onward – after 24 hours.

The temperature measurement is performed up to the moment when the difference between the minimum daily outside temperature and the maximum temperature of the surface layers of the structure drops by less than 20°C.

Concrete compressive strength is determined at 7, 28 and 90 days in accordance with GOST 10180-2012 31 and GOST 914-2012. Tests are carried out on at least two series of control sample-cubes with 100 mm edges from each batch of concrete mix ready for transportation.

A batch is the volume of the concrete mix of constant composition, prepared from the same components during one working shift.

There are 6 pieces of control samples in each series, including:

  • 2 samples - for the test at 7 days;
  • 2 samples – for the test at 28 days;
  • 2 samples – for the test at 90 days.

Series of control samples are moulded in one-piece molds from the pour of the concrete mix, discharged in the first and second half of the concrete batch.

Control sample-cubes must be labeled with the number, date and time of sampling. Statistical analysis of test results is carried out in accordance with GOST  18105-2010 and GOST  31914- 012. The required concrete strength at 90 days of normal curingat (a temperature of 20 ± 2  °C and relative humidity of 95 ± 5%) must be no less than 68.4  MPa.

Control of concrete compressive strength in the construction of the foundation slab at the design age (90 days), is carried out on control samples according to GOST 10180 and GOST  31914-2012 and on samplecores taken from the construction in accordance with GOST  28570-90 and GOST 31914-2012. The number of cores taken from structures must be no fewer than 20  pieces. Places for future core sampling are selected according to the approximate lay-out of core-sampling points shown in Figure 11.

Samples made from cores must comply with GOST 31914-2012 (have a minimum 70 mm diameter and polished parallel end surfaces).

VLADIMIR TRAVUSH, Chief Designer of GORPROJECT, CJSC;
ALEXEI SHAKHVOROSTOV, CEO of “Inforsproekt” engineering bureau

 


A.A. Bobkov

IFC Lakhta Center
Executive Director

Why was decided to withdraw from the traditional technology of concreting the foundation plate with joints and decided to apply a single pouring of such a large amount of concrete? Are there any analogues of a single pouring of slabs of such proportions for public buildings?

This decision was made due to several reasons, but first and foremost due to the uneven distribution of the load from the Tower building on the bottom slab of the box-shaped foundation. The total weight of Tower will be more than 650 thousand tons, and in the end it will fall on the bottom slab of the foundation and the pile foundation under the Tower, it will be distributed by means of the core and the radial walls inside the foundation box. Occurence of the construction joints in the bottom slab could lead to future cracks, impossible to be repaired. Structural reliability and integrity are always the most important factors to any construction. Especially – in high-rise construction. Therefore, we decided to accept the solution proposed by the General Designer – GORPROEKT, CJSC and its Chief Designer – D.Sc. in Engineering, Academician, V. I. Travush.

In the world there surely are the analogues of comparable areas of single pour of foundation structures. Nevertheless in the Guinness Book of Records the registered record of single pour of a foundation slab refers to a 16 300 cubic-meter concrete structure (“Venice” hotel, Las Vegas, USA), which is less than our volume for 3324 cubic meters. Now we’ve set a new world record.

Important to point out that in our case the very process of pouring of the concrete structure was much more complex: the work was not carried out on the surface of the earth but in a pit that was 17.500 m deep and was limited by a trench under a closed canopy at the outdoor night temperature of down to –11 ° C. Frankly speaking, it was not easy. By the end of the pouring the temperature under the canopy reached already +22 degrees.

How did you manage to synchronize the operation of 13 plants and exercise concrete quality control over production sites?

This problem was successfully solved by the general contractor. A week before the start of pouring we developed and approved with the customer a detailed delivery schedule of the concrete mix and we indicated the expected volumes every employed plant had to provide per hour. 10 days prior to the pouring the approved nominal composition of the mixture was tested at all plants without an exception to fine tune the batching and to provide rigorous parameters for receipt control of concrete at the entrance to the site.

And they even had mixers that failed the receipt control in the labs on the construction site. Once the mixture was produced, the plant was taken a sample and measured the parameters of the mixture before sending the mixer. After that they were communicated along with the car plate to the logistics team so that these were passed on to the receipt control labs at the site. The contractor’s logistics manager control the entire route from the production to the location site of every mixer with the mixture and was well aware of the car tags that had left and where they were. Much work has been done.

How far from the construction site were the plants located and what was their maximum distance?

The concrete plants engaged in the delivery of the mixture to the structure of the bottom slab are located on the border of St. Petersburg adjacent to the Beltway. This allowed us to eliminate the factor of traffic jams within the city limits. Two plants are situated on the construction site. The most remote one – “Betomix-Sofiyskaya” – is located 91 km away from the construction site.

How you were succeeded to organize the steady delivery of concrete to the site during 49 hours, and did you ask the city authorities for help?

We managed to arrange the complex process that does not have any counterparts on our own and with the contractors’ help. We did not turn to the authorities for help. The logistics solutions were carefully estimated and simulated in advance. The maximum traffic of the mixers was switched to the weekends and night time. The plants to deliver were selected with account of the geography so that the route to the site covered the Beltway, outside the city. The clear traffic engineering at the entrance to the site and along the territory was achieved with the help of thirty traffic guides from the contractors. Thus, thanks to the smooth and efficient work of all the participants in the process we managed to carry out the complex operation without failure, traffic restrictions at the entrances and, most importantly – without any delays in the delivery of concrete.


E.V. Morozova

Construction Director of
the IFC Lakhta Center

How do you exercise the quality control of the dry components of concrete mix and the necessary additives at different production sites?

The receipt test of the quality and performance of inert materials (gravel, sand, cement, slag) was actually carried out in three stages: the receipt test of concrete plants (including the preparation of receipt test protocols during the delivery and check of materials at the plants), then the representatives of the general contractor inspected and run random sampling of material specimens and pass them on to the laboratory approved with the customer beforehand; after that the technician who had developed the nominal composition of the mixture checked all the 13 plants.

The last stage of the performance test of the mixture components was the technician that was in charge of developing the technology of concrete pouring as well as the technician from every manufacturer. They made the mixture by means of adjusting the water content depending on the humidity of the sand.

What was the composition of the concrete and how did you monitor that all the plants would adhere it?

The recipe of the concrete mixture contained a certain proportion of cement, sand, gravel that was 5–10- mm, liquid plasticizer, slag as a filler, water and a delay mechanism for the chemical reaction in the concrete. It is no secret that with such a mass of the concrete mixture there are two essential factors necessary to ensure that the structure is homogeneous and strong. They are the delay in heating up of the mixture core as well as the life expectancy of concrete and its retaining the characteristics of mobility in order to avoid the solidification of the surface of the mixture and the formation of cold joints. The batching control was automatically carried out by plant dispensers (the human factor was completely eliminated when the characteristics of the components were determined during the receipt test).

How do you monitor the concrete in the mixers before it’s poured into the structure?

Once the mixer entered the construction site it was directly sent to the receipt test lab center. There it submitted samples of the concrete mixture to have its input parameters analyzed (the temperature of the mixture, air entrainment, concrete flow or mobility, mixture density). When the parameters of the ready-mix concrete complied with the concrete production procedure, the mixer was directed to a special area where the traffic guide led it to the vacant concrete pump.

What was the working schedule of the concrete pumps?

The concrete pumps continuously worked throughout the entire pouring period as stopping the pump can lead to clogging of pipes with concrete remnants. That way one will have to have the entire supply system cleaned. We could not afford this during the single pour. In case unexpected emergencies happened, we kept 5 sets equipment to be replaced immediately at the site.

What is the capacity of concrete pumps and how can it be changed?

The question is set in a slightly wrong way as the main goal of a pump is to pump the mixture entered the bunker. And here the determining factor is build-up pressure. Our branded B60W12F150 model had the average speed of concrete pumping at pressure of 150 MPa equal to 27 cubic meters. per hour, but our contractor and we adjusted it within the range of 180–200  MPa so as to reach the speed of concrete pouring by averagely 450 cubic meters per hour. We did it for 18 pumps. The deliveries allowed us to do it. In general the pouring of the lower plate reached 446 cubic meters per hour. Very few people believed we would succeed.

What is the mobility of the concrete mixture and did you have to use vibrators?

B60 mixture is heavy itself and when we had to pour concrete we continuously used self-compacting concrete. This type of concrete is very sensitive to vibration and can start segregating. We used vibrators locally as an impulse   – during the inspection of the structure we determined that some areas were starting to consolidate, then we used vibrators to “revive” the concrete but it lasted no more than 10–20 seconds per vibration point. It was simply dangerous to keep it longer.

How did the supply of concrete to the foundation change with time and how long did the pouring take?

At the beginning of concrete pouring we always pump laitance through the concrete pumps and the latter are started in a sequence. Thus, within the first three hours of concrete pouring the pace grew from 250 cubic m. per hour up to 380 cubic m. per hour. The rate of 440 cubic m. per hour was reached during fourth hour of concrete pouring. At the end of pouring we have a natural slowdown as it is necessary to accurately check the volume of the mixture to be laid and to consider the requirements for the protective concrete layer above the cage of reinforcement and for the screeding process. Then surveyors come into the scene. Thus, we have periods of acceleration and stopping just like a car. My assessment was more pessimistic than the actual result. The entire structure was concreted within 49 hours from the beginning of the process and with the account of the screeding of the surface of the slab.

How many workers were involved in the concrete pouring of the bottom slab?

How many people worked per shift and how many shifts did you have? In total there were 18 pumps to supply concrete participating in the pouring. 750 people attended to them in 3 shifts of 8 hours. The shift schedule was intentionally reduced from 12 hours to 8 so that we could avoid the fatigue factor of the workers and the lack of attention towards the mixture to be poured. The attending team of the supply pump consisted of 8 people, 6 of them working in the structure, 1 of them controlling the pump on the surface of the pit and 1 of them leading the team. In addition, we had a logistics team (54 people, also divided into 3  shifts) and receipt test lab centers – a total of 8  centers (42 people more).


S. V. Nikiforov

Chief Engineer of the
IFC Lakhta Center

How did you arrange the quality control over the supplied reinforcement and sockets to connect the joints?

The quality control of the reinforcement and sockets is run during the receipt test. To do this we took the samples from each batch. According to the technical specifications of the manufacturers of sockets, during the first test was run the sampling out of 50 joints, and during the others – the sampling out of 500 joints.

To determine the mechanical properties of the reinforcement and to ensure that the sockets complied with the specifications indicated in the certificates we conducted tensile tests according to GOST. Upon the successful completion of the test the sockets were taken into service.

What is the number of wells installed to control the temperature of the consolidating concrete?

The temperature control of the concrete consolidating process was performed by means of an automated system with the help of temperature sensors. The total number of 38 temperature sensors was set to control the temperature in different zones of the plate to be concreted: in the core and on the periphery of the structures at 3 elevation points as well as in the upper area of the slab. The 8 overflow pipes (wells) designed to run periodic checks of the sensors are installed at the distance of 150-200 mm from the temperature sensors.

What was the number of specimens to test the concrete grade selected during the concreting?

The control of concrete strength under pressure in the mixture batches delivered to the construction site was carried out in accordance with the GOST on the basis of control specimens produced out of the samples of the concrete mixture taken during the concreting of the structure.

Within 1 batch we received the volume of concrete supplied by the one manufacturer and poured into the structural slab during one shift (12 hours). We produced no fewer than 2 sets of control samples out of every batch.

The control of concrete strength under pressure in the foundation slab during its project life (90 days) is carried out by means of control samples according to GOST 10180-2012 and GOST 31914–2012 and by means of core samples taken from the structure in accordance with GOST  28570-90 and GOST 31914-2012. Their number was no less than 20 pieces.

How do you control the process of consolidation of concrete?

The control over concrete consolidation is carried out by means of testing control samples taken during the concreting of the structure at the age of 3, 7, 28 and 90 days. On top of that to clarify the reference data on the strength we’ve run a test of the concreted structure by means of a non-destructive method of shear test according to GOST 22690-88.

How many samples were taken to control the strength of the concrete?

The total number of the samples taken out of the concrete mixture from all the 13 plants and delivered to the construction site was 936 pieces.

What did you do to take care of the poured concrete?

Care is an important factor to maintain the integrity and strength of a massive concrete structure. When the concreting was finalized we started taking care of the concrete of the foundation slab in order to ensure gain in strength and to prevent temperature and shrinkage cracks.

The principles of care in terms of a massive reinforced concrete structure are justified by estimating the state of the structure under thermal stress and are set in the covering with heat insulating roll materials when the concreting is over and the moisture-impermeable coating is set.

According to the calculations carried out it was necessary to ensure crack resistance of the concrete during its hardening by applying temporary shelters (heated enclosures) and blanket insulation. The insulation was installed once the upper layer acquired the required sufficient hardness.

The thermal coating consists of four layers of thermal insulating roll materials such as “Etafom”. It is applied to a steam- and moisture-impermeable film to gradually equalize the temperature of the hardening concrete along the structural cross section as well as the surface layers of the structure and the outside air. The purpose of the above mentioned coating is to ensure that:

– the cooling rate of the concrete in the core of the structure is no more than 2...3 °C per day;
– The temperature difference between the adjacent areas along the height of the slabs is no more than 20 °C;
– The temperature difference of the surface layers of the concrete slab and the outside air is no more than 20 °C.

According to the monitoring the maximum temperature in the core of the structure was achieved on the 5th- 6th days and reached 70 °C. The decline rate of the temperature after the maximum temperature is about 1°C per day

 




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