FORM WORK

INTRODUCTION

When the concrete mixture is mixed with water it is at the liquid composition. At that time it is not a dense solid, before initials setting of cement. There for it is necessary to provide supports to held and gain the required shape. These temporary constructions, which are provided to keep the required shape of concrete, are called formwork.

When constructing formwork following things must be considered,

1.     The formwork should be strong enough to bear the load of reinforcement and wet concrete.

2.     It should not be curved because of the movement of working people and equipment’s.

3.     It should be erected correctly to get the required shape safely.

4.     The joints should be welled fixed avoiding grout leakages.

5.     Formwork should be constructed as to be easily dismantled and removed in sections in the desired sequence from the cast concrete, without shock, disturbance or damage.

Timber is the most widely used material for formwork. But steel formwork is also used and the finish is pleasing.

 MATERIAL USED FOR FORMWORK.

·        TIMBER

This is the most common material used for formwork construction. It is easy to work with and is also cheap. The moisture content of timber should be between 15%- 20%. So if the timber is dry, it will absorb moisture from the wet concrete that could weaken the resultant concrete member.

·        PLYWOOD

Strong thin board made by gloving layers with the direction of grain alternative. Plywood is widely used to construct formwork because it is strong; light weighted and supplied in easy handling sizes. The plywood sheet sizes available at site are 900mm x 2400mm and 1200mm x 2400mm. The thicknesses of plywood sheets are 12mm,10mm or 18mm.

·        STEEL

Steel also widely used material for constructing formwork nowadays. The smooth surface of steel is helpful to remove formwork easily and the concrete surface is pleasing and finished smooth. Steel formwork is strong enough to bare the load of the concrete & it is no doubt about the strength. Although the strength and finishing surface is much more it is somewhat difficult to handle the steel shutters. This is because of the weight of the large steel formwork. In our site we use steel plates to construct coloumns. They are called Docka plates and the procedure of that system is called Docka system.

DOCKA SYSTEM

It is a modern formwork system extreamly durable and robust made with hot dip galvanization. It is very strong and takes heavy loads. Docka specially easy and manufactured by experienced specialists. There are variable hights of  Docka plates. There is a Docka factory in Europe. Tabelforms load towers and even stairs towers can be constructed with it and for upper construction and supporting structure for every  height.There are 0.9, 1.2, 1.8m, heights. Concrete walls can be constructed easily.

REINFORCEMENT

Reinforced concrete is a composite material use where one single material cannot achieve the design requirements. Concrete is strong in compression but weak in tension. Hence, so that concrete to be good in construction, their should have a strengthening in the concrete. This strengthening (reinforcing) material is known as fibers. 

CHARACTERISTICS  OF  REINFORCING   MATERIAL  IN  CONCRETE

1.     It must have tensile strength.

2.     It must be able to achieve this tensile strength without undue strains.

3.     the material must be able to easily bent into any required shape

4.     Its surface must be capable of developing an adequate bond between the concrete and the reinforcement to ensure that the required design tensile strength is obtain.

5.     It must be available at reasonable cost.

The material which meets all the above requirements is steel in the form of bars, and is supplied in two basic types.

1.     mild steel

2.     high tensile steel

STEEL BARS USE IN REINFORCEMENT

the characteristic strength of mild steel is 250 N/mm2 and 460 N/mm. the surface of mild steel provides adequate bond but the bond of high- yield bars, being more critical with the higher stresses developed ,is generally by rolling on to the surface of the bar longitudinal or transverse ribs.

The reinforcement in concrete may be simple bar or series of bars, bend to a given schedule which known as bar schedule and tied according to the reinforcement drawings with stirrups. The normal diameters of bars used at this site are 6, 10,12,16,20 and 25.

Steel bars are available in various brands in the market some are locally manufactured and some are imported brands. Whatever brand is used by the contractor he should submit test reports for tensile strength, elongation.   

4REBAR DETAILS

SECTION SIZE	UNIT WEIGHT	CROSS SECTION AREA	WEIGHT OF 12M BAR	NO OF BARS FOR A TON
R6	0.222	0.283	2.664	375
R10	0.617	0.785	7.404	135
T10	0.617	0.785	7.404	135
T13	1.04	1.327	12.48	80
T16	1.58	2.011	18.96	53
T20	2.47	3.142	29.64	34
T25	3.85	4.909	46.2	22
T32	6.31	8.042	75.72	13
T40	9.88	12.56	118.56	8

R-Mild steel                        T-High tensile steel

 

BAR BENDING

Steel bars should cut and bend to the required shape so that they are fabricate  bind and place them within the form work .Bar bending is done by skilled person and he should be given the required bar schedule prepared  according to the reinforcement drawings.

BAR SHEDULE PREPARATION

Bar schedule is a special scheduling of steel bars which includes whole the details of each bar of a particular member required for bending the bar. The general bar schedule includes following details.

·        Member

·        Bar mark

·        Type and size

·        No. of member

·        No. bars in each

·        Total no.

·        Length of each bar (mm)

·        Total length (mm)

·        Shape of bending demension

Using the above method , we find out the quantity of  necessary bars in the site.

When we order bars, at first we should make the bar schedule.

LAPPING TWO STEEL BARS

Steel bars manufactures are of 6m lengths. But in practice we would need bars longer than 6m. In such cases we have to lap two bars as no failure occurs and as it produce its original strength. in this site, the recommended lap lengthy is 50 times the bar diameter if both bars are of same diameter. When lapping two bars of different diameters, the lap length is considered as 50 times the smallest diameter.

CRANCK

Crank is a slight bending in bars at the lap so that maintains the clear cover even at the lap position.

Crank length = ( d1+d2+5)*10mm

d1 =diameter of smaller bar.

d2 = diameter of largest bar

  COVERING FOR REINFORCEMENT

The purpose of providing cover is to protect reinforcement from harmful conditions such as whether; chemical attacks, heat and electricity that may reinforcement could be corroded. Hence before concreting, we should be careful weather sufficient numbers of covers are

provided so that ensure the protection of reinforcement. Usually clear cover can be provided by using cover blocks, stool and Donuts. Donut is made of plastic.

 

Cover blocks are made using cement and fine sand motor mixture at site premises. It depends on the grade of the necessary concrete. As example, if we need a cover blocks for the grade 15 concrete, the proportion among cement sand and aggregate of grade 15 is 1:3:6, therefore we make cover blocks by using cement and sand according to 1:3.

Stools are generally used in slab reinforcement to produce required covering and stop both the top layer net and bottom layer net touching together (top bottom net separation). The stool height and type is designed according to the slab thickness. There are two types of stools

In our site, cover blocks are used as follow.

·        For columns- 40mm

·        For walls      - 30mm

·        For beams    - 35mm

·        For slabs       - 25mm

SETTING OUT

Setting out is a major and key task that should pay more attention and care in any construction work. The accuracy of setting out in structural events is very important so that avoid many later problems.

INSTRUMENTS USE IN SETTING OUT

1)   Measuring tape

2)   Builders square

3)   Spirit level

4)   Profile board

5)   Dumpy level

6)   Theodolite

7)   Plumb bob

8)   Pegs

9)   Marking board 

ACCURACY IN SETTING OUT

1.   Steel tapes should not be sag while measuring

2.   Steel tape should be straight and should be parallel or align to the line along which measuring require.

3.   Each measuring should be taken from fixed datum in setting out.

4.   Survey instruments should be properly leveled and centered.

5.   Any errors in instruments should be taken in to consideration.

6.   Cross checking is important to ensure the accuracy.

LEVELS IN BUILDING (Vertical measuring)

In multi storied buildings construction, it is necessary to construct floors according to the levels given in the elevation drawings. Normally levels are given with respect to some datum. This datum is known as temporary bench mark (TBM). In this site, TBM setup at site denotes G.L + 0.3m. All the vertical distances of the buildings are measured with respect to the given datum. We can use engineering level and steel tape to transfer levels vertically.