Columns and shear walls means vertical elements form work we cane remove after24 hours but slab form work depend on span same like this(L*2+3) L=Maximum length of span

exp: We have max span is9 m

9*2+3=21

Remove form work also depend on type of cement which one we use ...Best of luck

http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/drying-concrete-vs-curing-concrete

The terms curing and drying are frequently used interchangeably with regard to the moisture condition of new concrete slabs. The following definitions clarify these terms.

Curing

Curing of concrete is defined as providing adequate moisture, temperature, and time to allow the concrete to achieve the desired properties for its intended use. This would mean maintaining a relative humidity in the concrete of greater than 80 percent, a temperature greater than 50 degrees Fahrenheit, and for a time typically ranging from three to 14 days depending on the specific application. When these recommendations are properly specified and performed in the field, the final properties of the concrete mixture will be achieved.

Drying

Drying of concrete is defined as providing the proper conditions to allow the concrete to achieve a moisture condition appropriate for its intended use. The moisture condition of a concrete slab is of significant importance for the application of moisture sensitive floor finishes such as vinyl composition tile, linoleum, wood flooring, and non-breathable coating like epoxy. The moisture condition is specified as a maximum relative humidity by percent or a vapor transmission rate in lb/1000 ft2/24 hr. A typical value specified for relative humidity may be less than 75 to 80 percent to assure the successful application of the flooring materials, while a commonly specified value for vapor transmission rate may be 3 lb/1000 ft2/24 hr.

ReferencesKosmatka, Steven H., Kerkhoff, Beatrix, and Panarese, William C., Design and Control of Concrete Mixtures, 15th Edition, EB001.15, 2002, 372 pg.

Kanare, Howard M., Concrete Floors and Moisture, EB119, 2008, 172 pg.

TYPES OF FORMWORK (SHUTTERING)

Formwork is an ancillary construction, used as a mould for a structure. Into this mould, fresh concrete is placed only to harden subsequently. The construction of formwork takes time and involves expenditure upto 20 to 25% of the cost of the structure or even more. Design of these temporary structures are made to economic expenditure. The operation of removing the formwork is known as stripping. Stripped formwork can be reused. Reusable forms are known as panel forms and non-usable are called stationary forms.

Timber is the most common material used for formwork. The disadvantage with timber formwork is that it will warp, swell and shrink. Application of water impermeable cost to the surface of wood mitigates these defects.

A good formwork should satisfy the following requirements:

1. It should be strong enough to withstand all types of dead and live loads.2. It should be rigidly constructed and efficiently propped and braced both horizontally and

vertically, so as to retain its shape.

3. The joints in the formwork should be tight against leakage of cement grout.

4. Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.

5. The material of the formwork should be cheap, easily available and should be suitable for reuse.

6. The formwork should be set accurately to the desired line and levels should have plane surface.

7. It should be as light as possible.

8. The material of the formwork should not warp or get distorted when exposed to the elements.

9. It should rest on firm base.

Economy in Formwork

The following points are to be kept in view to effect economy in the cost of formwork:

1. The plan of the building should imply minimum number of variations in the size of rooms, floor area etc. so as to permit reuse of the formwork repeatedly.

2. Design should be perfect to use slender sections only in a most economical way.

3. Minimum sawing and cutting of wooden pieces should be made to enable reuse of the material a number of times. The quantity of surface finish depends on the quality of the formwork.

Formwork can be made out of timber, plywood, steel, precast concrete or fibre glass used separately or in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary. For small works, timber formwork proves useful. Fibre glass made of pre-cast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.

Timber Formwork:

Timber for formwork should satisfy the following requirement:

It should be

1. well seasoned2. light in weight

3. easily workable with nails without splitting

4. free from loose knots

Timber used for shuttering for exposed concrete work should have smooth and even surface on all faces which come in contact with concrete.

Normal sizes of members for timber formwork:Sheeting for slabs, beam, column side and beam bottom

25 mm to 40mm thick

Joints, ledges 50 x 70 mm to 50 x 150 mmPosts 75 x 100mm to 100 x 100 mm

Plywood Formwork

Resin bonded plywood sheets are attached to timber frames to make up panels of required sizes. The cost of plywood formwork compares favourably with that of timber shuttering and it may even prove cheaper in certain cases in view of the following considerations:

1. It is possible to have smooth finish in which case on cost in surface finishing is there.2. By use of large size panels it is possible to effect saving in the labour cost of fixing and

dismantling.

3. Number of reuses are more as compared with timber shuttering. For estimation purpose, number of reuses can be taken as 20 to 25.

Steel Formwork

This consist of panels fabricated out of thin steel plates stiffened along the edges by small steel angles. The panel units can be held together through the use of suitable clamps or bolts and nuts. The panels can be fabricated in large number in any desired modular shape or size. Steel forms are largely used in large projects or in situation where large number reuses of the shuttering is possible. This type of shuttering is considered most suitable for circular or curved structures.

Steel forms compared with timber formwork:1. Steel forms are stronger, durable and have longer life than timber formwork and their reuses

are more in number.2. Steel forms can be installed and dismantled with greater ease and speed.

3. The quality of exposed concrete surface by using steel forms is good and such surfaces need no further treatment.

4. Steel formwork does not absorb moisture from concrete.

5. Steel formwork does not shrink or warp.

Construction of formwork:

This normally involves the following operations:

1. Propping and centring2. Shuttering

3. Provision of camber

4. Cleaning and surface treatment

Order and method of removing formwork:

The sequence of orders and method of removal of formwork are as follows:

1. Shuttering forming the vertical faces of walls, beams and column sides should be removed first as they bear no load but only retain the concrete.

2. Shuttering forming soffit of slabs should be removed next.

3. Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.

Rapid hardening cement, warm weather and light loading conditions allow early removal of formwork. The formwork should under no circumstances be allowed to be removed until all the concrete reaches strength of atleast twice the stresses to which the concrete may be subjected at the time of removal of formwork. All formworks should be eased gradually and carefully in order to prevent the load being suddenly transferred to concrete.

Figure 1 to 6 shows formwork for different types of members in civil engineering construction.

Figure 1(a): Details of timber formwork for RCC beam and slab floor

Figure 1(b): Details at section (A) shown in above figure

Figure 2(a): Elevation

Figure 2(b): Details of timber formwork for circular RCC column

Figure 3(a): 150 3D View

Figure 3(b): Details of timber formwork for square or rectangular RCC column

Figure 4: Sectional plan showing details of timber formwork for an octagonal column

Figure 5: Details of formwork for stair

Figure 6: Timber formwork for RCC wall

Table: Period of removal of formworkS. No. Description of structural member Period of time

1 Walls, columns and vertical sides of beams 1 to 2 days2 Slabs (props left under) 3 days3 Beam soffits (props left under) 7 days4 Removal of props to slabs

 

 (a) For slabs spanning upto 4.5 m 7 days

 (b) For slabs spanning over 4.5 m 14 days

5 Removal of props to beams and arches 

 (a) Spanning upto 6 m 14 days

 (b) spanning over 6 m 21 days

Formwork Safety Checklist during Design:1. Formwork should be properly designed for the structural element considered and its working drawing should be available at site.

2. Design of formwork should consider all the loads it will experience during casting of concrete structural members.

3. Strength of materials used for formwork should be adequate to support structural load as well as other loads imposed on it.

4. Formwork design should indicate the rate of concrete pour, height of concrete pour, temperature and sequence and schedule of concrete pours.

5. Working drawing of formwork should have detailed dimensions including pouring pocket size, compaction opening and cleanouts.

6. Formwork design should consider the safe bearing capacity of soil.

Formwork Safety Checklist during Construction:Following inspection should be carried out before starting the concreting of structural member:

1. Inspection of entire formwork system for details from bottom to top of formwork for proper load transfer in safe manner.

2. Inspection of working scaffolds, ladders, runways, ramps and crossings.

3. Maintenance of good housekeeping around working area and passage.

4. Guarding of peripheral edges and floor openings.

5. Adequate space for safe working.

6. Safety training of workmen involved in formwork and concreting works.

7. Use of all personal protective equipment (PPEs).

8. Formwork, rigging inserts and connections checked for correct installation and periodically checked for wear and correct position.

9. Removal of all unused and hanging forms, loose materials etc. stored on exposed floors.

10. Inspection of all props and shores for adequacy to handle all the loads.

11. Removal of defective props.

12. Alignment of props such as verticality, height and spacing between props should be inspected.

13. All props should be rested on bearing plates.

14. Props should be placed on hard bearing surface.

15. Safe nailing and firm locking of clamps on adjustable props.

16. Lateral stability of formwork and complete fixity at the joint between props when one prop is placed on the top of the other.

17. Proper bearing below the stringers and joists at points of supports.

18. De-shuttering and removal of props below concrete slabs and beams after development of adequate strength in concrete.

19. Construction loads not placed on freshly cast slab or beams while removal of formwork or before concrete attaining required strength.

There can be many more checklists for formwork which has not been written here. If you think any addition has to be made, please write those in comments.

You can also download this document for formwork safe practices

In normal circumstances (generally where temperature are above 200C), and where ordinary cement is use, forms may be struck after expiry of following periods:

Walls, columns and vertical sides of beams 24 to 48 hours as may be decided by the engineer in charge

Slabs (props left under) 3 daysBeam soffits (props left under) 7 daysRemoval pr props to slabs(a)  Spanning upto 4.5 m(b)  Spanning over 4.5 m

7 days14 days

Removal of props to beams and arches(a)  Spanning upto 6 m(b)  Spanning over 6 m

14 days21 days

 

Concrete Formwork

For rapid hardening cement 3/7 of the above period will be sufficient in all cases except vertical sides of slabs, beams and columns which should be retained for 24 hours.

To be specific, the code states that where ordinary Portland cement (OPC) is used the forms of the following types of RCC members can be removed after the indicated time: -

(a) Walls, columns and vertical faces of all structural members: 24 to 48 hours (b) Slabs (props left under): 3 days (c) Beam bottoms (props left under): 7 days (d) Removal of props under slabs: Spanning up to 4.5 m - 7 days; Spanning up to 4.5 m - 14 days (e) Removal of props under beams and arches: Spanning up to 6 m - 14 days; Spanning up to 6 m - 21 days For other types of cements, the stripping time recommended for ordinary Portland cement should be suitably modified.

I am constructing my house and the foundation work is in progress. I need your advice on how to provide DPC at the plinth level in the correct manner so that dampness does not develop in the building. Please guide me regarding the cement concrete mix proportion, chemical to be used etc.

Smrity Khaneja, Dehradun

The foundation of a building is always in touch with moisture present in the soil and due to capillary action in the brickwork the moisture has the tendency to rise up in the building. To prevent this rise of water above the plinth level, DPC (Damp Proof Course) is provided. I will briefly explain how DPC is to be provided in the correct manner.

First of all, the foundation work of the entire building should be constructed up to the plinth level and it should be ensured that the top level is same for entire work. DPC is made up of cement concrete of mix 1:2:4 (1-cement: 2-coarse sand: 4-12.5mm stone aggregate) mixed with a good quality waterproofing compound.

The waterproofing compound should be mixed as per the manufacturer's instructions. The DPC should be of 40mm thickness and should be of uniform thickness. It should not be provided at the locations of the door openings.

The top level of DPC should match with the planned finished floor level. As DPC is cement concrete it should be cured with water for 7 days and after it has dried, a uniform layer of 85/25-grade bitumen should be applied over the surface.

This completes the providing of DPC. If the DPC is properly provide it will give lifelong protection against rising dampness in walls from the soil. Please note that even small crack in the DPC could lead to rise of dampness.

Revibration

Revibration of concrete is the intentional systematic vibration of concrete which has been compacted some time earlier. It should not be confused with the double vibration which sometimes occurs with the haphazard use of immersion vibrators or multiple passes of a vibrating-beam screed.Whilst it is generally agreed that revibration of concrete can be beneficial to its strength, its bond to reinforcement and its surface finish, the practice is not widely used, partly due to the difficulty of knowing just how late it can be applied. A good rule of thumb is that revibration may be used as long as the vibrator is capable of liquefying the concrete and sinking into it under its own weight.

Situations in which revibration may be beneficial include:

1. To bond layers of concrete into those preceding them. In elements such as walls, deep beams and columns, which are being filled in successive layers, the vibrator should just penetrate the previous layer.

2. To close plastic shrinkage and settlement cracks.These may form within the first few hours of concrete being placed and may be able to be closed by vibration, providing it is done early and prior to the hardening of the concrete. However, a reasonable level of energy input is required since mere reworking of the surface, may simply close the cracks superficially. They may then reopen as the concrete dries out.

3. To improve the surface finish at the tops of columns and walls by expelling the air which tends to congregate there as the concrete settles in the formwork.

4. To improve the wear resistance of floors. Revibration, coupled with a trowelling action, helps to create a burnished wear-resistant surface layer.

Like   CommentRevibration is delayed application of vibration to concrete which has placed and compacted already. Now question is why we should make delay in compaction effort. This condition arises where concrete is placing in layers for a relatively deep member. Where first layer is vibrated and may have hardened partially and application of vibration is transmitted to the underlying layer. In some cases this is done intentionally. 

Now question arises what are the consequences of disturbing partially hardened concrete or why revibration is provided intentionally. Yes friends there have some beneficial effects of revibration of concrete. The aggregate particles are get rearranged and voids from either by air or water is removed, thus providing more compacted concrete mass to form a solid concrete of high strength properties. But, when and how should we use this repeating vibration? Dear reader we will find the answer of these questions in the next posts.