FORMWORK - Government Engineering College, Ajmer

ormwork

Column Formwork

Column Formwork Column formwork is made usually with eithertimber or metal panels. The principle is to create an enclosed box withframes at the exact size of the column and fix ittightly on the kicker left from base or at the laststage of column concreting. The box is held in position by steel columnclamps or bolted yokes and supported bytimber studs or props

Beam Formwork

Beam Formwork Beam formwork consists of open throughsection and because it is not closed at thetop requires more supporting framework torestrain the sides. The supports need to be maintained to thesoffit and also provide lateral support to thesides. In timber this is done by the use of aheadtree across the top of a vertical member. Metal panels are used with corner pieces,but timber headtrees are needed for verticalsupport.

Slab Floor Formwork

Slab Formwork Floors require a large area of formwork to beprovided usually fronı beam to beam. Timber floor formwork consists of timberboards or plyvvooıİ sheets supported on afraınework and resting on a series of timberjoists. Again timber and metal props can be used forvertical supports. Metal panels can be used and bolted or clippedtogetherand held in place by a system of metalbeams or a tabular scaffold system. Adjustable props need for levelling purposes

Wall Formwork

Wall Formwork Wall formwork is a simpler than for otherconcrete units as the actual forces against itare less, most of the load being carriedvertically downwards. The panels at both sides are held in position byties. Ties are also used as spacer, arranging wallthickness. WaII support systems are usually slopingprops at satisfactory intervals.

FORMWORK STRIKING TIMESThe time to be allowed before formwork can beremoved naturally depends on many factors,such as:1.The type of concrete mix used (type of cement)Rapid hardening cement mix requires less time,whereas high water cement ratio needs longertime for striking the formwork.2.The type of structural member being castSoffit of beams and slabs or sides of beams orcolumns require different time.3.TemperatureHigh temperature can cause rapid curing ofconcrete and formwork can be struck in shortertime than low temperature weather.

British Standards Formwork Striking TimesMinimum Striking TimesStructural MemberformworkFormwork to columns,beam sides and wallsFormwork to slabs(props left in position)Surface Temperature ofConcrete16 C7 C2 C9 hours 12 hours1 day4 days7 days11 daysFormwork to beam soffit 8 days(props left in position)Props to slabs11 days14 days21 days14 days21 days21 days30 daysProps to beam soffits15 days

Turkish Standard Formwork Striking rtlandCementSides ofbeams,walls andcolumnsSlabsProps of beamsand largeopening slabsforms3 days8 days21 days2 days4 days8 days

FORMWORK TIES When concrete is placed in wall formwork, thepressure exerted by the fresh concrete tends toforce the opposite sides of the formwork apart. Normal economic solution to this problem is tocollect the force exerted by the concrete firstlyonto the face material, generally plywood, andthen to the walling to distribute the force intosoldiers on either side of the formwork. The soldiers are prevented from moving apartby use of steel rods called tie rods passingthrough the concrete to connect the twosoldiers together. Tie systems are in two categories:non-recoverable ties and recoverable ties.

Non-recoverable ties1. Snap ties The principal of snap ties is that it is cast intothe wall and has normal wedge connection ateach and for fixing to the formwork. Once the wall is concreted and the formworkis ready for removal the snap tie is thenphysically over-stressed and the ends "snap"generally inside the concrete. The snap tie arrangement also acts as aspacer to the formwork so that order

1. Snap ties The principal of snap ties is that it is cast into the wall and has normal wedge connection at each and for fixing to the formwork. Once the wall is concreted and the formwork is ready for removal the snap tie is then physically over-stressed and the ends "snap