Sunday, December 15, 2019

Materials for Formwork, Types of Formwork , Advantages of steel for over timber form-work

Materials for Formwork
formwork can be made out timber, plywood,steel precast concrete or fibre, glass, used separately or in combinations.
the type of material to be used for formwork depends upon the nature of construction as well as the availability and cost of material.
for small works involving less number of re-uses, timber formwork proves economical.
fibre-glass is used mainly for making moulds for repetitive castings of precast concrete products.
moulds made up of precast concrete, fibre glass and aluminium are used in cast-in-situ construction such as waffle slabs or members involving curved surfaces.

Types of Formwork
1.Timber Formwork
it is used for the formwork should satisfy the following requirements.
(i) it should be well seasoned.
(ii) it should be light in weight .
(iii) It should be easily workable with nails without splitting.
(iv) It should be free from knots.

  • the sizes of timber sections for different components of form-work depend upon the span of the slab or beam, floor to floor height and the centre spacing of centering supports.
  • The size of timber planks or joists that can be adopted for different components of formwork for shuttering of 4.5 m span and 3.5 m height.
  • for normal construction work where repetitive use of shuttering is possible , the quantity of timber shuttering can be worked out on the assumption that one set of shuttering can be used 10 to 12 times.

                         Table.1 Sizes of members for timber formwork
S.No.     Components of formwork              Size                                                                                                                (Varying according to the spacing of            centering props from 1 metre to 1.2 metres.)
(i.) Sheeting for slabs, beam and           25 mm to 40 mm--- thick
column side and beam bottom

(ii.)Joists, ledges                                50 x 75 mm to---cross section 50 x 150 mm

(iii.) Posts                                             75 x 100 mm. to 100 x 100 mm. .....

(iv.) Ballies                 Not less than 100 mm at mid length and 80 mm at thin end

2. Plywood Formwork
Use of plywood instead of timber planks is getting popular these days.
In this case resin bonded plywood sheets are attached to timber frames to make up panels of required sizes.
the panels thus formed can be easily assembled by bolting in the form of shuttering.
this types of shuttering ensures quality surface finish and is specially recommended in works where large exposed areas of concrete are to be constructed such as floor slabs faces of retaining walls etc.
it may prove to be cheaper in certain cases .
(i) By use of large size panel it is possible to effect saving in the labour cost of fixing and dismantling.
(ii) No of re-uses are more as compared with timber shuttering. for estimation purposes, number of re-uses can be assumed as 20 to 25 .
(iii) it is possible to have perfectly plain and smooth surface (without joint marks) by use of plywood shuttering. thus expenditure on surface finishing can be saved.
3.STEEL FORMWORK
this consists of panels fabricated out of thin steel plates stiffened along the edges by small steel angles.
  • The panel units can be held together by two or more clamps pr bolts provided along each edge and the shuttering can be assembled and kept in alignment by use of horizontal or vertical centring of timber or steel.
  • The panels can be fabricated in large number in any desired modular shape or size.
  • The usual size for wall or slab panel varies from 60 cm x 60 cm to 60 cm x 120 cm. 
  • this types of shuttering is considered most suitable for circular or curved shaped structures such as tanks, Columns , chimney etc.and for structures like large sewer, tunnels,and retaining walls.

Advantages of steel for over timer form-works 

 (i) Steel forms are stronger, more durable and have longer life as compared with timber forms. 
(ii) They can be put to sufficiently larger number of te-uses. For estimation purposes the number of reuses can be assumed to vary from 100 to 120
(ii) Steel forms can be installed and dismantled with greater ease and speed which results in saving in labour cost for this item of work. 
(iv) The quality of exposed concrete surface obtained by use of steel forms is excellent and it needs no further treatment. 
On the other hand construction carried out by use of timber formwork invariably requires plastering to obtain desired finish of the concrete surface. Thus there is saving in the cost of finishing the surface by use of steel forms.
 (v) There is no danger of the formwork absorbing water from the concrete and hence the chances of honeycombing are minimised.
 (6) They are not liable to shrink or distort and hence it is possible to achieve better workmanship and higher degree of accuracy by use of steel forms.   



Economy in Form-work, Economy of shuttering , Economy of Centering, Effect economy of in the cost of form-work

Economy in Form-work
it may be noted that the total cost of concrete construction includes the cost of form-work as well.
Construction of form-work involves considerable expenditure in terms of cost of material, cost of labour for fabrication, erection and removal of form-work time element.
in case of buildings, the cost of form-work may vary range between 30 to 40% of the cost of concrete work.
in case of special structures like bridges, tall chimney, dams etc. the cost of form-work may range between 50 to 100% of the cost concrete work, or even more.

Following steps should be followed to effect economy in the cost of form-work .
(i) The Building should be planned in such a way that there are minimum number of variations in the size of rooms, floor area etc. so as to permit re-use of the form-work moulds repeatedly.
(ii) The Scheme of the form-work should be efficiently planned and suitably designed to determine the most economical but safe sizes of different components including the supports or props.
(iii) the Form-work should be constructed in such a way that timber (where used) is cut to the minimum and it can be struck off with ease an in such  works  re-use with least damage.




(iv) In case the form-work is made out of rough timber and is not constructed properly, the resultant concrete surface will be irregular and full of defects.     
  The expenditure involved in rectification of defects in such works is invariably more than the saving made in the cost of form-work.

Friday, December 13, 2019

Form work & A good requirements of form work, Satisfy Condition of Form work

Form work - Centering, Shuttering or form-work is a sort of temporary construction provided for laying cast-in-situ concrete to required shape.


A good form-work should satisfy the following requirements.

(i.) It should be strong enough to withstand all types of dead and live loads such as self weight,weight of reinforcement , weight of wet concrete,loads due to workmen, construction equipment, other incidental loads and forces caused by placement and consolidation of concrete, imposed upon it during and after casting of concrete. 
(ii)  It should be rigidly constructed and efficiently propped and braced(both horizontally and vertically) so as to retain its shape without undue deflection.
(iii) The joints in the form-work should be tight against leakage of cement grout.
(iv) The form-work should be constructed in such a manner that it may permit the removal of various parts is desired sequence without jarring of damaging the concrete.
(v) The material of the form-work should be cheap, easily available and should be suitable for re-use several times.
(vi) The form-work should be set accurately to the desired line and levels and should plain surfaces.

(vii) The form-work should be as light as possible.
(viii) The material of form-work should not wrap of get distorted when exposed to sun, rain or water during concreting.
(ix) The form-work should rest on firm base.








Thursday, December 12, 2019

Embedment length and Development of Length of Rienforcement

Embedment length
The length of embedded steel reireinforcement, 1 provided beyond a critical section.
The fibre length also defines the embedment length. This is especially important for thick, short hooked end steel fibre and low strength concrete.
A slippage of the reinforcement at the beam-column interface is observed because of the accumulation of the rebar strain along the embedment length in the beam-column connection.
Therefore, it is important to understand the proper embedment length of the longitudinal reinforcement considering the interface behavior between the infilled concrete and the steel casing in the Cast In Steel Shell pile.
Development Length
A development length can be defined as the amount of reinforcement(bar) length needed to be embedded or projected into the column to establish the desired bond strength between the concrete and steel (or any other two types of material)

Fig 1: Development length in Footing
Reason for providing Development length
  • To develop a safe bond between the bar surface & the concrete so that no failure due to slippage of bar occurs during the ultimate load conditions.
  • Also, the extra length of the bar provided as development length is responsible for transferring the stresses developed in any section to the adjoining sections (such as at column beam junction the extra length of bars provided from beam to column).
  • where less development length against the required is provided the structures will be prone to encounter failure due to slippage of joints, bonds, anchors & Laps, in such cases the bars will not yield first but the failure will happen at joints & laps prior to yielding of reinforcement bars.

Fig 2: Development length as per IS 1786
Calculation of Development Length

Where
Ø = nominal dia of reinforcement bar
s= Stress in bar at the section considered at design load
bd= Design bond stress
The above given formula is used to calculate the required development length in mm for any given dia of bar, same formula is used for limit state method as well as working stress method.
The only change in calculation in both methods is due to the different value of design bond stress; the values of design bond for Limit State & working stress are as follows;
Table No 1: Design Bond Stress in Limit State Method
Design Bond Stress in Limit State Method
Concrete Grade
M20- 1.2
M25-1.4
M30-1.5
M35-1.7
M40 and above-1.9
(For Plain Bars in Tension
Design Bond Stress )
bd,N/mm2)
M20- 1.92
M25-2.24
M30-2.4
M35-2.72
M40 and above-3.04
(For deformed bars in tension)
(Design Bond Stress bd,N/mm2)
Footnotes-
Development Length of Reinforcement Bars

Types of drawings in any construction project:

Types of drawings in any construction project: 1. IFC Drawing: Detailed drawings considered final, issued, and approved by the design team f...