Jonah Notes

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Formwork:

The term given to the whole setup of temporary moulds into which concrete is poured. Along

withformwork, a system is provided to support the moulds and the concrete till the concrete is setand hasgained enough strength this is known as the Staging

It is also the term given to either temporary orpermanent moulds into which concrete or similar materials are poured. The formwork constitutes 30% of the project cost and 60% of the project

construction time. A temporary framework is used to support thewalkway platform, the access

ways and that supports the labours who are laying the skin and placing thereinforcement is knowas Scaffolding.

The material which is used to form the mould into which theconcrete is pored is called as Skin.

The skin can be made up of ply, steel or aluminium. The process of placing the skin is known as

the Shuttering

Requirements of a good formwork system:

The essential requirements of formwork or shuttering are:

•The material for form should be cheap and it should be suitable for reuse for several times

•It should be practically waterproof so that it does not absorb water from concrete.

•Also itsshrinkage and swelling should be minimum.

•It should be as light as possible.

•It should be strong enough to take the dead and live loads during the construction

•The joints in the formwork should be rigid so that the bulging, twisting or sagging due todeadand live load is as small as possible. Excessive deformation may disfigure the

surface of concrete

•The construction lines in the formwork should be true and the surface plane so that the cost

of thefinishing the surface of concrete on removal of shuttering is least.

•A formwork should be such that it can be easily erected.

•A formwork should be easily removable without any damages to itself so that it can berepeatedlyused.

Factors to be considered for selection of system:

•The number of repeat uses

•The desired finish



•The amount of labour to install each use

•Availability of experienced personnels to install it

•Handling equipment available to move large sections around

•The amount of wet concrete deliverable per cast

•The amount of capital available

•The speed of project

•Versatility

•Pour to strip times

•The acceptability of cold joints

Materials

The various materials used as forms are:

Timber : It is the most common material for shuttering and is almost available everywhere. It

iscomparatively cheaper and could be easily shaped. Timber deteriorates under the stress of heatand contact with water. It is also difficult to evaluate the proper strength of timber. Thus,

ascientific approach of design becomes difficult.

Plywood: It has got a better surface and gives a smoother finish. These are useful andeconomicalfor larger panels with repetitive uses. Plywood is available is different varieties such

as resin ,plastic coated plywood, texturised surfaces for exposed concrete surface.

Hardwood : they are manufactured from wood fibre under controlled combination of

pressure,heat and moisture. These are tempered with the impregnation of drying oils which are

stabilizedby heating. They have improved strength, low water absorption, and better abrasionresistanceand are used as formwork lining.

Fibre forms: They are used as lost forms for concrete. They are left in place on the exposed

faceof the concrete where it gives architectural look and also improves acoustical andinsulationproperties

Gypsum boards: They are generally used to provide artistic design or ornamental pattern for theexposed concrete face. It is reinforced with an organic fibre or coir to make it tougher.

Concretequality may improve due to absorption of extra water present in concrete by these boards.

Plastic forms: PVC, neoprene and polyester strengthened with glass fibre are some of the

plasticforms in use. They are manufactured as per required shapes, do not rust and are easy toclean.

Lost forms: Forms, which are not removed after casting, form a part and parcel of the



structurethat behaves as composites. These are known as lost forms or permanent formwork.

Steel form: Steel forms consists of angles, tubes, joists, flat plates, are very much in use asshores,bracings, runners, slabs, shutters etc. Steel forms are also in use in combination with

timber.These are very strong and can be used repetitively without much damage done to these

forms.The design calculations can be easily done since the characteristics of steel are known.Theadvantages and disadvantages of the various forms are listed below.

Untreated wood

Advantages

Low initial cost

Low experience cost

Low weight

Versatility usually contains sugars that acts as a retardant and prevents sticking

Disadvantages

Poor finish, patching almost demanded where the finished concrete will be seen

Low per unit use (4-7)

High labour as each set must be rebuilt

Must wait until concrete is well set (24 hours) to strip

Leaves a dusty finish as there are sugars in the sap of soft wood

Untreated Plywood

Advantages

Same as above but gives a smooth finish

Disadvantages

Same as above but greater cost and less availability in some countries

Coated PlywoodDensified shuttering:It is manufactured from selected veneers peeled from

hardwood plantation and treatedwith specially manufactured phenolic resin for longer durability

and better performance. Thesurface of shuttering ply is overlaid with 10GSM phenolimpregnated film. The sides areresealed by acrylic paint to preserve moisture absorption and

swelling

Advantages

Will increase durability

Many plywood systems are available:



continually wet for 7 days, preferably 14 days.Membrane curing isthe most practicalwith today'sconstruction

schedules.The curing of the concrete is dependent on the quality of the film, the thickness, andthe uniformity of the film applied to the concrete. The curing membrane used is the leastexpensive component in the completion of quality concrete construction, and the mostover looked.

Concrete strength withmoisture present for curingaccording to the Portland Cement Association

Age Days5000 psi4000 psi3000 psi2000 psi1000 psi

CONCRETE STRENGTHConcrete strength increases with age as long as moisture and a favorabletemperature are present for hydration of cement. In the figure shown below concrete

that isIn air entire time is only 55% of the strength of Moist-cured entire time concrete at 28days.In air after 3 days is 80%, and In air after 7 days is 90%. A Quality curing and sealingcompound will allow the concrete to continue in strength gain beyond 28 days as showninthe chart for moist-cured.Strength gain in colder temperatures slows down. 40o F. concrete will be 35% of its designstrength in seven days as compared to 75% for 73o F. concrete.Concrete poured in lower temperatures needs to be covered to maintain higher concretetemperatures to get sufficient strengths to withstand freeze-thaw cycles.

CURING COMPOUNDS and CURING AND SEALING COMPOUNDS

Difference of CURING compounds and CURING AND SEALING compounds is theresin used. Curing compounds use a resin that breaks down in months, and curing andsealing compounds use a resin that will resist sun light, abrasion, most chemicals andwillbe on the surface of the concrete for a while.CURING compounds are meant for curing the concrete only. No other sealer or adhesive for a floor covering can be put down unless the curing compound is strippedoff.



c. Open the building to fresh air at the time of the pour to ventilate exhaust fumesfrom trucks, troweling machines, etc.d. The concrete is most susceptible in the first 24 hours. Use a membrane curingand sealing compound as soon as possible, to block the carbon dioxide gas fromthe concrete. A uniform and heavy film will be necessary. It was proven thatburlap and poly for curing do not offer adequate protection.

References1. Haque, M.N., "Some Concretes Need 7 Days Initial Curing", Concrete International, February 1990, pp.42-45.

CRACKS IN CONCRETE SURFACESTwo basic causes of cracks in concrete are (1) stress due to applied loads and (2)stress due to drying shrinkage or temperature changes.Stress Cracks can be eliminated by having a proper subbase and proper concretedesign for the load.Shrinkage CracksDrying shrinkage is an inherent, unavoidable property of concrete. Shrinkage of plain concrete drying is .72 inches per 100 feet from its plastic state to a dried state with50% relative humidity, this shrinkage will take place when the moisture leaves the

concrete.To minimize these cracks we need proper placement of saw cuts, and proper curing toholdthe moisture in the concrete long enough to get adequate tensile strength gain in theconcrete before the moisture leaves and the concrete shrinks. With adequate strengththeconcrete should pull together and crack in the saw cuts and not in between them.Plastic Shrinkage CracksPlastic Shrinkage Cracks is a type of cracking that appears on the surface of freshlyplaced concrete during finishing operations, or soon after. Most often, this form of cracking

is caused by the rate of evaporation water being greater than the rate at which it isbeingreplaced by bleed water. The surface shrinks while underlying concrete maintains aconstant volume.Contributing factors:a. increased wind velocity.b. ambient temperature increase

c. a decrease in relative humidity.To minimize plastic cracking:a. dampen the subgrade.b. don't work in the sun or wind, erect a roof over the slab and a wind barrier.c. schedule work for the early morning or late afternoon.

d. have sufficient man power and equipment.e. cover the concrete with poly in between finishing operations.f. spray on a evaporation retarder, Tri-Film, to form a liquid monomolecular barrier on thesurface of the concrete after each finishing operation.

Craze CrackingCrazing is the occurrence of numerous fine hair cracks in the surface of a newlyhardened slab due to surface shrinkage. These cracks form an overall hexagonalpattern.



Causes:a. Rapid surface drying usually caused by either high air temperatures, hot sun, or drying winds, or a combination of these.b. not using an evaporation retarder during placement in hot or windy conditions.c. excessively high slump, over troweling the surface, floating the surface when thereis an excess amount of moisture at the surface.

d. over use of a jitterbug, vibrating screed, darby or bull float may contribute by working anexcess of mortar to the surface.

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