Methodology For Chimney Construction

 

METHODOLOGY FOR CONSTRUCTION OF 220 M HIGH RCC SINGLE FLUE BRICK LINED CHIMNEY AT DPPS SITE, DPL, DURGAPUR.

 

INTRODUCTION: Slip Slip formin forming g, cont continuous inuous pour poured ed,, conti continuousl nuously y form formed ed,, or  or slip slip for form m constr construct uction ion is a construction method in which concrete is poured into a continuously moving form. Slip forming is used for tall struc structure tures s (su (such ch as bridges, bridges, tower towers, s, buil buildin dings, gs, and dams), dams), as well well as horizon horizontal tal structures, structu res, such as roadwa roadways. ys. Slip forming enables continuous, non-interr non-interrupted, upted, cast-in-pla cast-in-place ce "flawless" (i.e. no joints) concrete structures which have superior performance characteristics to piecewi piec ewise se con constru structio ction n usin using g discre discrete te form elem element ents. s. Slip Slip formi forming ng relies relies on the quick-s quick-sett etting ing properties of concrete, and requires a balance between quick-setting capacity and workability. Concrete needs to be workable enough to be placed into the form and packed, yet quick-setting enough to emerge from the form with strength. This strength is needed because the freshly set concrete must not only permit the form to "slip" upwards but also support the freshly poured concrete above it. Slip forming is of two types: 1) Ve Vert rtica icall Slip Slip form forming ing.. 2) Ho Hori rizon zontal tal Slip Slip for formi ming ng.. In vertical slip forming the concrete form may be surrounded by a platform on which workers stand, placing steel reinforcing rods into the concrete and ensuring a smooth pour. Together, the concrete form and working platform are raised by means of hydraulic jacks. Generally, the slip form rises at a rate which permits the concrete to harden by the time it emerges from the bottom of the form. In hor horizont izontal al slip forming forming for pav paveme ement nt and traffi traffic c separat separation ion walls walls concre concrete te is laid down, vibrated, worked, and settled in place while the form itself slowly moves ahead. This method was initially dev initially devised ised and uti utilize lized d in Interst Interstate ate High Highway way con constru structio ction n ini initiat tiated ed by the Eisenho Eisenhower  wer  administration during the 1950s. In our job we are following the Vertical Slip Forming Concept.

Components of Slip form: Truss framework: Keeping the diameter of chimney in view, a truss framework is to be specially designed. Broadly, it has a central triangular frame and ring from which the radials emanate to all directions. The radials are equi-spaced. After every few radials, main trusses are provided at equal intervals, numbering six to twelve. The trusses are built up of joists, channels and angles while in between radials, also known as spider beams, are made of back-to-back channels. All the outer  ends of radials and trusses are connected by distance bars. Form panels: These are made of 4 mm thick MS plates, normally having a size of 1 m depth and Form  2.5 m length along the curvature of shell. Plates of this size are easy to handle. As the diameter of  chimney reduces with the increase in height, these plates overlap one another at the ends. Care needs to be taken that the overlapping of form panels is vertical, not inclined. Inclined overlapping

 

results in a horizontal drag component, further resulting in torsional effects on the slip form assembly. Wallers: Wallers are those components of slip form equipment which stiffen the form panels and transfer the loads to yokes. These must be made strong enough to bear the lateral pressure of  concrete laid inside the form panels. Wallers are made of steel and are braced suitably.  Yokes:: A yoke consists of yoke legs and yoke beams; they are primarily used to transfer the load  Yokes of the forms to the hydraulic jacks. These also support working decks and prevent form panels from bulging. Yokes are the most important components of slip form equipment. These are made of steel sections. All the loads on slip form and truss assembly get transferred to the hydraulic jacks through the yokes. A yoke assembly has one outer yoke leg, one inner yoke leg, one lower yoke beam on which the hydraulic jack rests and one upper yoke beam. Working decks: This part of slip form equipment enables the performance of all operations of slip form equipment. equipment. For chimn chimney ey construction, construction, two such decks are creat created. ed. The upper deck receives all the mater materials ials including concrete to be poured, reinforcemen reinforcementt steel, insert inserts, s, block- outs and any other materials. The lower deck allows the movement of engineers and workers for checking the  jacks, providi providing ng force to them to lift the system system,, checkin checking g the param parameters eters of chimn chimneys, eys, tying reinforcement bars and multiple other works. Hanging Decks: These are the lower decks provided along the inner and outer periphery of the chimney shell, and are also called mason’s platforms. These These consist of angular frames suspended from the yokes and are connec connected ted by laying timb timber er planks bridging the frames. These are used to apply final finish to the chimney shell. Lifting equipment: The lifting equipment consists of a number of jacks, one jack installed on each yoke beam. Hydraulically operated jacks have a central hole in them through which the jack rod passes. These jacks work on the ball grip principle. When the hydraulic pressure is applied to the  jack, the piston inside the jack gets pressed down, pulling along with it the spring provided inside the jack. The spring under pressure makes the balls of the top catcher squeeze making making them grip the jack rod. The lower ball catcher gets released as soon as the upper ball catcher grips the rod. On release of hydraulic pressure, the spring gets released upwards, taking along with it the top sleeve provided inside inside the jack. The lower ball catcher again gets loaded, clamps the jack rod and the process continues. Therefore, one of the two ball catchers is always gripping the jack rod. Jack rods: Solid jack rods are used to transfer the loads from the hydraulic jacks to the structure below. These jack rods either get embedded in concrete and are left there or completion of slip form equipment’s work. Pumping station: All the hydraulic jacks of slip form equipment are connected to a centrally placed pumping station. This pumping station is electrically run and applies oil pressure to the jacks for  their upward movement, thereby lifting the whole assembly. The upward movement increments to be given to the jacks are pre-calculated. This pumping station itself provides oil pressure to the perimeter jacks also which rotate the spindles to adjust the perimeter of slip form assembly to match the theoretical requirements. A manifold outlet system is attached to the pumping station to release oil under pressure to various hydraulic circuits which include lifting jacks, inner perimeter   jacks and and outer outer perimeter perimeter jacks jacks Extraction jack equipment: These jacks help in extraction of jack rods on completion of slip form work. These are hydraulically operated and applied on the jack rods, one by one, in sequence.

 

Whenever the extraction jacks are to be used, sleeves must be provided around the jack rods to avoid their bonding with concrete and facilitate their easy extraction. Miscellaneous equipment: Bracin Bracings, gs, for form panels (Waller Post, Intermediate Intermediate Form Support & Spindle), Turn Buckle Jack, adjustment screws for wall thickness, radius adjustment and yoke inclination, safety nets to catch the slipping men and materials and grouting pumps to grout the holes created in the shell on extraction of jack rods, are other miscellaneous equipment that form part of slip form equipment.

Concrete equipment: During the construction of the chimney shell using slip form equipment, concreting work can be done for a height of about 70 m to 110 m with the help of Uplift Concrete Pump & for the balance height, winch or high capacity uplift concrete pump can either be used. The daily requirement of  concrete for the shell is worked out. The required quantity of concrete keeps reducing as more heigh hei ghtt is ga gain ined ed.. Conc Concre rete te trans transpor porta tatio tion n arra arrang ngem ement ents s ar are e simila similarr to those those pr prov ovid ided ed fo for  r  construction of the shell using conventional formwork. For compaction of concrete, needle vibrators are to be provided on the working deck at different locations to cover the full periphery of the shell. Internal Corbels of chimneys: Single flue chimneys have internal RCC corbels provided at regular  height intervals as an essential component. It is not possible for a chimney to serve its purpose unless unle ss thes these e plat platfor forms ms are prov provided ided.. The These se cor corbels bels are requ required ired to sup support port the int intern ernal al flue flues s carrying flue gases. The internal flues are built-in, fire-resistant refractory brick masonry. It is not possible to run this masonry from the ground up to the full height of chimney as it is unable to support itself. Therefore, the provision of internal corbels is essential. Time constraint: Internal corbels of a chimney require a significant amount of time for construction. Generally, these are provided at height intervals of about 10 m. A 220 m high chimney therefore, has around 21 such corbels. In the lower parts of the chimney, the diameter is quite large therefore, these corbels contain RCC beams. There are a number of such beams, inserts and openings to be provided in these corbels. Heavy quantity of concrete is required for each such platform. As the height of the chimney increases, its diameter decreases, and the size of beams and corbels also decreases. Comparatively, less quantity of concrete is required for higher corbels. However, the height involved acts as a constraint here. While the bulk quantity of concrete and supporting arrangements pose a challenge in lower platforms, the height does so for higher level platforms.  Another unique feature of these corbels is that these are one above the other, within the interna internall diameter of the chimney. Therefore, no two platforms can be taken up at a time unless some innovative steps are devised. Space constraints, too, add to the problem. Building so many corbels in a sequence, from bottom to top, thus becomes a difficult activity. Innovative steps: The innovation demands two platforms taken up at a time in form of five-fold arrangements of scaffoldings, shuttering, equipment and manpower. Also, extreme level safety measures are required as the work is going overhead for the workers deployed on the lower  platforms. Any small object falling from the upper platforms gains high acceleration since such a height is involved and hits the workers at lower levels like a bullet. Electrical Winch: Four electrical winches are required to run the concrete and material buckets to the platforms. Generally, two such hoists are put on job to transport concrete carrying buckets and one or two additional winches are put on carrying steel reinforcement and other materials to the platforms if ducts or openings to run the same are available. The winches are often 2.5 - 5 ton capacity, single drum type, equipped with magnetic brakes and using 16 mm & 19 mm  mm  diameter 

 

ropes. 16 & 19 mm diameter wire ropes are additionally required as guide ropes for the concrete carrying buckets. D-shackles, U-clamps, two sheave pulleys are also required to complete the running arrangement. While the flue ducts can be used to run the concrete buckets, some more openings are to be norm normally ally left in the platforms for ventilation ventilation purposes. purposes. It is not possible to use a tower crane inside a chimney for construction of platforms as its erection and movement interferes with the corbels to be constructed. Therefore, winches prove to be the best equipment for the purpose. Passenger Cage: One passenger cage is required to transport the engineers and workers to the platforms and back to the ground level. The passenger hoist should be a double drum type with magnetic magnet ic brakes, overload alarm and limit switches as safety guards.19 guards. 19 mm diameter wire ropes should be used to run the passenger cage. Guide ropes should keep the cage in position to avoid its extra swing or accidents. Bucket trolleys: Simi Similar lar to the concreting of the shell, the best arrangement arrangement to carry the concrete filled buckets from the transit mixers shed to the hoist point is to carry the buckets over trolleys running along the top platform. This involves minimum effort on the part of workers who keep sliding the trolleys between the points of discharging of transit mixers and bucket lifting points. Concrete unloading arrangements: Space constraints on the platforms make it very difficult to unload the concrete there as the concrete is to be unloaded on a temporary platform built over the corbels to be concreted. Secondly, these platforms have to be near the hoisting point of concrete carrying buckets. In a way, there are no options in deciding the location of temporary platforms over  which concrete has to be unloaded. A swinging type chute should be installed near the concrete bucket position. When the bucket is to be emptied, the chute is swung and adjusted below it. After  the unloading of the bucket, the support below the chute is removed to hang it vertically and to give way to the bucket to travel down. Concrete vibrators: Concrete vibrators are required to be stationed at each platform to fully compact the concrete laid in the beams and corbels. Plenty of spare needles need to be kept available as the needles often need repair and replacement during construction. Plate or form vibrators are to be avoided as the work is carried out at very high levels and any chances of  accidents due to high vibrations caused by the form vibrators in the forms and support system need to be avoided.

Slip form Assembly: This "Sli This "Slip p form" form" assemb assembly ly is an ext extrem remely ely complicat complicated ed form. form. A lot of control controlled led functions functions are especially designed to adjust this form's diameter, taper and the poured shell thickness, as this fancy form rides the jack rod supports embedded in the pre-placed concrete voids in the shell of the chimney. Below is a summary of some key construction features: 1. The form rises non-stop (continuously) at a rate of approx 0.3 meter/hour. 2. Each Jack is mounted to a yoke, to which which is secured the inside and outside form panels and the working platforms. 4. Each Jack rod is removed upon completion of the shell. This leaves typically 32 mm & 48 mm diameter diamet er holes internal to the wall, extending extending for the fill height of the concrete shaf shaft. t. To protec protectt

 

from water entrainment after the chimney is completed, the jack rod voids are capped top and bottom with grout. 5.Radius wallers extend between the yoke frames on both the outside and the inside surfaces of the concrete wall, and additional form panels (which lap the panels fixed to the yokes) will be attached to these wallers. 6. The diameter of the slip form assembly is controlled by a radial adjustment fitted to each yoke frame, and the taper of the column or slope of the wall is set by an adjustment at the top of each yoke frame. 7. The thickness thickness of the concrete wall will be contro controlled lled by adjustmen adjustments ts that are secured to the inside form sheets and the yoke frames. 8. The construction construction of the shaft, that is the slipping of the form, is usually carried out continuously continuously on a 24-hour day, 5-day week basis, with a temporary shutdown each weekend. 9. It is the usual practice of the slip form construction construction team to require the customer customer to provide center cen terline line mar markin kings gs for sta startin rting g the chi chimne mney, y, suc such h as lin lines es to demarc demarcate ate the orienta orientation tion of  chimney centre point, flue, access, or other openings, as well as foundation slab elevation data. 10. Hoisting the work personnel to the top platform of the slip form is based on the utilization of a "man-cage" rigged with cable grabbers and two guide cables, so that in the event of a hoist line cable breakage, the grabbers would actuate and suspend the cage from the guide cables. This man-cage is typically operated by a safety hoist, equipped with limit switches and operated under  power in both directions.

Mechanism of Slip form System: This technology for chimneys construction was developed in the early sixties following the principle of the concrete extrusion method already used in the early century for silos construction. The first use of the slip forming system was adopted for simple cylindrical structures where the windshield diameter was constant through all the chimney height. As a result of this application the need to erectt and dism erec dismantl antle e exp expens ensive ive scaffo scaffoldin ldings gs was eliminat eliminated ed and the advant advantage age to provide provide a homogenous and seamless structure could finally be achieved. A further development of this basic techn tec hnolo ology gy was was prov provide ided d addin adding g the the poss possib ibili ility ty to er erec ectt conic conical al shape shapes s th thus us allow allowin ing g th the e construction of tall chimneys where the use of wall tapered shells was a basic design condition impose imp osed d from fro mwing the stru al ovem des design igners . The applicatio applic of ign slip pr form techno tec could also be pos possib sible le fo follo llowi ng structur the the ctural impr improv ement enters. of co conc ncre rete te ation mi mix xn des design prov ovid idin ing ghnology thelogy use usecou ofld ad addi ditiv tives es (plasticizers, (plastici zers, retarder retarders, s, etc.) to control settin setting g time and workability depend depending ing on different different site conditions. Description of the system The system consists essentially of a complex of metal forms (panels) that slide from below upwards providing to extrude the concrete chimney wall which emerges as a monolithic and seamless structure. structu re. The slip form applica application tion provides in princip principle le one or more working working platforms platforms connected connected by the mean of an hydraulic system to a series of jacks climbing on vertical jack rods which represent the statical supporting structure of the system. The conceptual idea of the slip forming is that the working platforms are capable to climb together with the form panels along the vertical jack rods by the mean of the hydraulic movement of climbing jacks, thus making possible the continuous extrusi extr usion on of the concre concrete te wal walll of the chi chimne mney. y. Thi This s con concept cept has been applied applied usi using ng dif differ ferent ent technologies which at first allowed the construction of very simple cylindrical shell chimneys and towers up to perform much complicated conical an multi-tapered structures.

 

It has to be highlighted that conical structures require the application of more sophisticated slip forming systems allowing the relative sliding movement of the panel forms towards the centre of the structure to be built thus reducing both shell diameter and wall thickness. This has been made possible by the introduction of sliding yokes connected through a main rigid structure by the mean of sliding supports which control both translation and rotation of the outer and inner shuttering towards the centre. The formwork circumference is automatically adjusted by radially moving the frame jokes by the mean of Turn Buckle Jacks and Spindle Rods thus reducing the formwork diameter on the top side of the shuttering while the bottom edge of the formwork is radially restrained against the already hardened concre restrained concrete. te. In this way fresh concrete concrete located on the upper  part of the shuttering is confined to be extruded according to the required design geometry. Statical scheme of the system The statical system for a conical slip forming is consisting in a reticular truss of radial beams directly connected to a central ring: in the simplest system the central ring consists of a double plate connecting the radial beams (Spider Beams). The spider beams are supported on yokes which are directly connected to the hydraulic jacks. The movements of the jacks are controlled by an operator  and the rise speed of the system is widely depending on many variables, such as concrete setting time, the quantity of steel rebars to be installed, installed, the presen presence ce of embedded embedded items, the provision of  block corbels or platforms to be built, etc.. The jack rods are initially bottom supported on top of the foundation and as soon as the shuttering will move upwards climbing on the steel rods they will be free to move inside a recess hole provided by the mean of a sliding jacket (Recovery Pipe) directly connected with the jacks. This allows the rod not to stick to the concrete as soon this will start to harden, thus after releasing the frictionofforces and givingofadditionally the possibility to recover climbing climbin g rods the completion a certain portion the slip formed structure. structure. The rods willthe be prevented to buckle inside the holes due to the horizontal reaction force of the already hardened concrete. The sliding operations will also depend on the very careful control of concrete setting time made by experienced slip forming operators to prevent the rods buckling as soon as the concrete emerges from the bottom side of the shuttering. Rods buckling need to be carefully prevented mostly in particular locations where the provision of openings (ducts entrances, ventilation louvers, temporary bottom doors, etc.) do not allow the same to be restricted inside the concrete wall: in this cases it will be necessary to install additional steel frames or to pour provisional concrete columns columns for the whole height of the opening. The main components of the slip forming equipment are: - the shuttering forms - the yokes - the hydraulic jacks and oil pump for lifting - the main working platforms - the hanging decks The shuttering forms The forms consist of the effective shuttering used to contain the concrete during casting operations. The forms are composed by mild steel panels. The height of each panel is normally 1.00 m. The forms are supported on the Form Box and provide a double screw system which allows to control the wall thickness through the rotation and translation of the panels relatively to the yokes

The yokes The form panels is fixed to the Form Box which is then screwed to the steel yokes placed radially at regular intervals (1.50 - 2.50m approx.) and connected to a spider beam supporting system. The

 

yokes are consisting of vertical steel frames perpendicular to the chimney wall, each supported by a spider beam system connected to a central ring. Each yoke provide a support connection for one hydraulic hydrauli c jack which are used to pull upwards the frame and consequently consequently the whole system. system. The yokes are also used to fix both the external and the internal hanging deck frames. The jacks and the hydraulic system Normally for each yoke one jack is used with lifting capacity of 6.0t & 12t. All jacks are connected Normally together by a communicating oil pipe system and by an electrical high pressure oil pump. Each jack acts as an actuator under the manual / automatic control of the oil pump which allows the jack to climb along the rod through double effect friction grip clutches pushing upwards the yokes and consequently consequ ently the reticular truss. The jacks are climbing on smooth jack rods located in the middle partt of the par the concr concret ete e wall. wall. Each Each rod is com compo posed sed by multi multipl ple e sectio sections ns wh which ich ar are e exten extended ded continuously by screw couplings. A steel pipe jacket (Recovery pipe) is connected to each jack, during climbing operations the jacket provides to protect the rod against any contact with fresh concr con cret ete e thus thus lea leavin ving g a perm perman anent ent ho hole le insid inside e the the wall wall an and d allow allowing ing the po poss ssib ibili ility ty of th the e subsequent recovering of each rod under a certain level. The working platforms The arrangement of the working decks is widely depending on each different patented equipment but normally comprises of the following: - the main deck -- the deck decks the top hanging - the additional decks The main deck The main deck is located below the level of the climbing jacks and is normally used for the installation of horizontal and vertical steel reinforcement, for concrete casting operations and for  maintenance of the hydraulic circuit and jacking system. For larger diameters the main deck is consisting in a circular gangway supported by hanging through tendons from the spider beams.  The top deck The top deck is provided as a closure to the main deck and is mainly used to store the steel reinforcing rebars and to distribute the concrete from the main bucket to the outer wall by the mean of a concrete distributor, pump or hand driven wheelbarrows. The closure of the floor is normally provided by the mean of wooden planks directly supported on the steel spider beams. On top of the deck there is usually located the supporting truss for the hoist pulley system used to lift both concrete and steel rebars on the platform and the additional hydraulic accessories.  The hanging decks Below the main deck two hanging decks are provided for the finishing and curing operations of both internal and external concrete surfaces. The decks are mainly consisting in circular walkways supported by the mean of vertical steel frames located below each yoke. Wooden planks duly fixed are provided spanning between each frame to allow for easy access and working operations. An additional safety net installed below the hanging decks protects the bottom site area against any possible fall down of materials from the upper working platforms.   The additional decks

 

 Additional decks can be provide  Additional provided d in specific working working areas where where the necessity to install install particular  items (local block corbels, ladders, stairs, etc.) or equipments (permanent lift, hoist, etc.) during slip forming is required. Additional equipment The slip form system system is normall normally y com complet pleted ed wit with h the followin following g equipm equipment ent dependin depending g on the geometries of the chimney to be erected: - top hexagonal beam including pulley device system for winch ropes connection - electrical system and provisional lightning protection - water distribution system through high pressure pumps - concrete distributor  - winch for material lifting

Construction activities: The peculiarity of the slip forming system is that concrete operations are conduced continuously providing concrete pouring during both day and night shifts. The sequence of the construction works encompasses the following main activities - Vertical and horizontal rebars installation - Concrete pouring and vibration - Raising of slip form shuttering - Control of deviations and plumpness - Concrete finishing and curing - Equipment installation Vertical and horizontal rebars installation The steel rebars are normally cut to 6.0m length to allow for easy handling, lifting to the working deck and further installation. Bars are usually lifted by the mean of an auxiliary winch and are stored on the top platform or on the available space of the main working deck .Vertical rebars are subsequently moved to their final position and overlapped to the existing ones using tie wire connections. Horizontal reinforcement is normally fixed to the external side of external and internal vertical rebars. Concrete pouring and vibration Ready mix concrete is transported to the base of the chimney by transit mixers and moved to the top of the main deck by the use of concrete buckets lifted by winch. As soon as the bucket will reach the working deck concrete will be transferred to a distributor and subsequently poured inside the shuttering of the circumferential wall. In some cases and when the available space is critical to the workin working g activit activities ies concret concrete e dis distrib tributi ution on is mad made e manually manually by the mean mean of wheelba wheelbarro rrows ws transferring transfer ring small concre concrete te quantities to perman permanent ent hoppers directly directly discharging discharging the material material from the top deck inside the slip form shuttering. Concrete will be vibrated to remove the air content and to allow the material to flow inside the geometrical shapes of the form. Normally concrete need to be poured within final setting time of  concrete after its preparation in the batching plant to the time of distribution in order to avoid segregation of its components and excessive heating due to hydration. Pouring will proceed in layers. As soon as the slip form will move upwards the fresh concrete will replace the hardened concrete located in the central part of the shuttering while the formed concrete wall will emerge from theforming) bottom side of theinfluenced form. The by setting time process (andto consequently the raising speed of  the slip is widely concrete heating due cement hydration mostly in the

 

central part of the shuttering, shuttering, plasticizers and retarders are widely used to control the setting time particularly to allow for concrete workability in hot conditions. Raising of slip form shuttering The slip form is normally operated by well trained personnel who control the raising speed of the shuttering system as soon as concrete has been poured. The slip forming operators will control the raising of the shuttering acting on the hydraulic pump, the raising is normally performed at 2 to 3 cm steps providing to check concrete consistency inside the shuttering. As soon as the fresh concrete is sufficiently hardened the operator will proceed to raise the shuttering thus allowing new fresh concrete concret e to be poured in subseq subsequent uent layers. The raising speed is mostly depending from concrete setting time. Where the geometries of the chimney, the congestion of reinforcing rebars located in proximity of ducts openings or corbels or the particular tapering of the shell do not allow the raising process to go so fast, the operator will also adapt the slip forming speed to perform all construction activities. It is fundamental that concrete setting time will be checked and adjusted during the progressing of the works to each of the construction phases thus allowing the work to proceed in a full optimized way. Control of deviations and plumpness The control of deviations during slip forming is normally made by the slip forming operator using optical or laser plumbs. The deviations from vertical axis are monitored through a set of three or  more tags installed on top of the base foundation prior to the start of slip forming operations, the operator provides to fill in a data report every 0.5m of slip forming including the deviations from the vertical axis adjusting during theofprogressing of the Thefrom experience of the operator  is fundamental to allowthe forsame the correction the deviations andworks. rotations the vertical axis. Concrete finishing and curing Concrete finishing activities are performed from the hanging deck and comprise the provision of  small repairs to fresh concrete and the application of a membrane curing compound which is necessa nece ssary ry to avoid avoid the rele release ase of moi moistu sture re con content tent from the exposed exposed concrete concrete surfac surface e thus preventing shrinkage effects which are detrimental to structure durability. In most cases it is a common com mon practice practice to cur cure e con concre crete te by spr sprayin aying g wat water er distrib distribute uted d from from an annular annular ring pipe pipe suspended to the bottom side of the main deck and adding a protection by the application of  hessian cloths to the outer surface, thus preventing concrete to be subject to the direct effect of sun and wind exposure.