In this article we will discuss about:- 1. Meaning and Requirements of Form Work 2. Planning and Organising for Form Work 3. Materials Used 4. Design Load 5. Form Work of Columns and Walls 6. Points to be Kept in Mind before Placing Concrete in Form Work.
Meaning and Requirements of Form Work:
The temporary construction used as a mould for the structure in which concrete is placed and allowed to harden in the desired shape is known as form work or shuttering. The average cost of the form work is about 33 to 40% of the cost of the whole job. Hence the careful and economic design of the form work is essential. More over the appearance of the finished surface of concrete and progress of work are largely dependent on the use of properly constructed form work.
The main object of the form work is to give the necessary shape to the concrete and to support it while it is setting and hardening.
In order to obtain these objectives, from work should meet the following requirements:
1. It must be True to Grade and Alignment:
The surface of the shuttering in contact with the concrete must be true to grade and alignment, so that the desired type of surface finish may be obtained.
2. It must be Strong Enough:
The mould or shuttering has to bear the weight of concrete, and any other loading of man and machinery etc. Thus it should be strong enough to resist all these loads without undue distortion or deflection.
3. It must be Sufficiently Durable:
During the process of hardening of concrete in the mould, it may absorb some moisture and may alter its shape. If so happens, the resulting surface will be unsightly. Hence the mould must be sufficiently durable.
4. It should have Tight Joints:
During compaction, specially while compaction is provided by vibration, the cement mortar may leak out through joints, if they are not sufficiently tight. Hence to prevent the leakage of mortar, the joints should be tight enough, otherwise honeycombing will be developed near the leaking joints.
5. It must be Easy to Handle:
It must be made up of sections which can be handled readily by the available means and which can be erected and assembled quickly.
6. It must be Easy to Stripe:
It must be constructed in such a way that it can be stripped easily in the correct sequence without damage to the concrete.
7. No Nails etc. should protrude from it:
In case no plastering is to be done on the concrete, the shuttering should not be allowed to protrude any nail or wire etc., from it, as they will produce unpleasing surface.
Planning and Organising for Form Work:
When the use of shuttering is decided, the economics of different types of form work should be considered in relation to the basic requirements as noted above. The economics will include the initial cost of material, speed of assembling and striking and the amount of repeated use which can be made of the form work.
Materials Used for Form Work:
For the construction of form work, following materials may be used:
1. Timber used for Form Work:
Timber has been used for shuttering or form work ever since the reinforced concrete is used as a construction material.
Timber used for shuttering should possess the following properties:
i. It should be soft, as they are easy to plane and work with.
ii. Coarse grained knotty timber should not be used, as nailing is difficult in the region of knots. Also coarse grained timber will show its grains on the finished surface of the concrete.
iii. It should not be hard, as it is costly and difficult to work. They develop cracks on nailing.
iv. As shuttering undergoes a great deal of wetting and drying under rough conditions, hence timber used for shuttering should be partially seasoned. Kiln seasoned or fully dry timber is usually too dry and is liable to swell by absorbing moisture from concrete when used for form work. On the other hand, too green a timber will shrink excessively, particularly in hot weather. Good quality timber is preferable. It must retain its shape under very rough conditions of drying and wetting. Timber used for form work should be planed smooth on all four surfaces, but for props, joists, however can be rough sawn.
Sheeting or Sheathing:
It is that part of form work which comes into contact with the concrete face. It usually consists of 12.5 cm to 22.5 cm wide boards. The thickness of planks may vary from 2.5 cm to 5.0 cm depending on the load it is required to carry. Tongued and grooved boarding is very suitable for sheeting as it provides a smooth surface with good leak proof joints when new. However, this may give trouble after being used for several times.
In certain cases thick plywood is very suitable as sheathing material, provided it is bonded with water proof glue. Plywood can be obtained with a hard plastic face which provides a very smooth and durable surface. Though this form of sheeting is expensive than other forms of boarding, but it can be used for many more repetitions and also reduces the number of joints in the form work.
Absorptive Form Lining:
Absorptive form lining is used as a means of removing excess water from the concrete surfaces for eliminating sand streaks and surface voids and at the same time for producing an attractive surface finish. The absorptive material generally is 12.5 mm thick and is fastened inside the regular forms. Special care should be exercised to keep the lining dry and undamaged before and during placement and compaction of concrete. As the lining tends to adhere to the concrete, hence it should be stripped within 36 hours after placing the concrete. This lining has been found to increase the relative durability of concrete.
Advantages of Timber Shuttering:
Following are the advantages of timber shuttering:
i. Timber is very suitable material for form work due to its adaptability and ease in manufacture.
ii. It is a good heat insulator and helps to protect the concrete from hot sun and cold wind.
iii. Its initial cost is less.
i. Its life is short. It cannot be reused for a large number of times.
ii. It requires constant maintenance. Hence its maintenance cost is more.
2. Steel Form Work:
Steel form work has the following advantages and disadvantages.
i. It is more suitable for standard size units as for precast units where large numbers of uses are required.
ii. Its life is much longer than timber form work.
iii. It can be assembled easily within a short period.
iv. It gives a better smooth surface of concrete.
v. It can be stripped easily within no time.
i. Its initial cost is high.
ii. It does not provide the concrete with the same degree of protection from frost or hot sun shine as timber form work. Hence in extreme hot climate, extra protection against frost or sun shine is required.
Design Load for Form Work:
In the construction of floors and roofs, the form work has to bear the weight of wet concrete, live load due to labour and impact due to pouring of concrete besides its own weight. For the design of planks and joists in bending and shear, a live load including the impact may be taken as 370 kg/m2.
The surfaces of the form work should, be given a camber such that after deflection due to weight of concrete and reinforcement the surface of form work should remain horizontal. The sheathing with a full live load of 370 kg/m2 should not deflect more than 0.25 cms and the joists with 200 kg/m2 of live load should not deflect more than 0.25 cms.
Form Work of Columns and Walls:
In the design of form work for columns or walls, the hydrostatic pressure of concrete should also be taken into account.
This pressure depends on the following factors:
The greatest load to be carried by a form work is generally due to the weight of concrete. Thus the form work under the soffit of beams and slabs will carry a direct weight of the concrete. The weight of dense concrete may be assumed as 2300 kg/m3. Hence the pressure per unit area (1 m2) on the soffit will be 2300 kg per metre height of concrete.
The vertical sides of shuttering specially of deep beams or columns or walls are also acted upon by the pressure due to freshly placed concrete.
This pressure is affected by the following factors:
1. Rate of Placing of Concrete:
It has been observed that for each rate of placing, the pressure reaches a maximum at some depth and then decreases at even greater depths. The decrease is probably due to the initial setting and stiffening of concrete. It is also influenced by compaction of concrete, friction against the form and arching action within the concrete. Column forms are filed so rapidly that the concrete behaves as a fluid during the entire placement period.
2. Method of Consolidation:
The use of vibration for compaction causes greater fluidity of the concrete than the hand compaction. Thus vibration compaction develops greater pressure on the forms. This is because, as the fluidity is retained while vibration is in progress, the full hydrostatic head of the concrete under vibration is effective in producing lateral pressure. This pressure can be calculated by considering the height of concrete of a fluid having the same weight per cubic meter as that of concrete.
3. Temperature of Concrete:
The temperature of the concrete influences the maximum intensity of lateral pressure. It is due to the fact that at low temperatures the stiffening of the concrete is delayed and the concrete remains fluid for longer periods and for greater depths of placement. It has been observed that concrete placed at 50°F develops maximum pressure about 30% greater than concrete at 70°F.
The evaluation of effect of other factors is difficult hence their influence is neglected being small.
The value of maximum pressure can be computed in M.K.S. units as follows:
Pm = 2880 R1/3 for hand placed concrete
Pm = 3900 R1/3 for internally vibrated concrete
Hm = 1.09 R1/3 Height of max pressure from the top of concrete
Pm = Maximum pressure in kg/m2.
R = Rate of placing concrete in metre per hour.
These results are based for concrete having a slump of 15 cms at a temperature of 70°F. If external vibrators are used, the concrete may be vibrated at any depth. In that case the full hydrostatic pressure is taken.
then Pm = WH (M = weight of concrete. Here it is taken as 2400 kg/m3.)
ACI Formula for Pressure:
For wall forms with R not exceeding 2 m per hour.
Pm = 0.73 + [144R/(1.8T + 32)]
Max. allowable P = 10t/m2 or 2.4 H. whichever is less
(ii) When R > 2 m.
Pm = 0.73 + [212/(1.8T + 32)] + [45R/(1.8T + 32)]
Max. allowable pressure = 10t/m2 or 2.4 H. whichever is less
(iii) For column form,
Pm = 0.73 + 144R/(1.8T + 32)
Max. allowable pressure = 15t/m2 or 2.4 H, whichever is less.
Pm = Max. lateral pressure t/m2. (tonnes/m2)
R = Rate of concrete placement in meter per hour.
T = Temp, of concrete.
H = Max. height of fresh concrete in the form.
t = Weight in tonnes
Pm = 1100H
H = height of layer in metre not to exceed 4V.
V = Rate of pouring in meres (pour/hour)
This formula is derived from the theory of cohesionless material as P = γ H tan2 (45° – ϕ/2) putting g = 2500 kg/m3.
φ = 23° for concrete
P = 2500 H tan2 [45° – (23°/2)] = 1100 H
The effect of temperature on the maximum concrete pressure on form work is shown in Fig. 25.2.
4. Tolerances in Form Work:
The form work should be constructed as per design to the shape lines and dimensions as shown on the drawings. However tolerances can be allowed as given below as per I.S. 456-1978.
According to Dr. Jaikishan and Dr. O.R Jain a pressure equivalent caused by a liquid having density 2300 kg/m3 for heights of concrete from 1.5 m to 6 m may be taken as 1200 kg/m3. For intermediate heights of concrete poured with 1/2 to 3/4 metre an hour, the corresponding weight for the whole mass may be interpolated between 2300 kg/m3 and 1200 kg/m3 assuming the variation as a straight line law. As a guide line following values may be assumed as shown in table 25.2.
5. Cleaning and Treatment of Forms:
Before placing the concrete in the form, all rubbish, particularly chippings, and saw dust etc. should be removed carefully and the surface of the form work in contact with concrete should be cleaned and wetted or treated with mould oil etc., but care should be taken that oil does not come in contact with reinforcement nor it should stain the surface of the concrete nor it should have any adverse effect on its hardening. Steel surfaces require heavier oil than that used for timber.
If wetting of timber is not done before placing concrete, the boards will soak water from concrete and swell, resulting in uneven and bulging surface of concrete. Secondly the boards may not be so firmly attached to the concrete that at the time of stripping form work, the surface of green concrete will be pulled off resulting in unsightly surface of the concrete work.
6. Time of Stripping of Form Work:
As per I.S. 456-2000, forms should not be struck until the concrete has attained strength at least twice the stresses to which the concrete may be subjected at the time of removal of the form work. The strength referred to shall be that of the concrete using the same cement and aggregates, with the same proportions and cured under the conditions of temperature and moisture similar to those existing at work.
In special circumstances the strength development of concrete can be assessed by placing companion cubes near the structures and curing under the same conditions as that of the structure. Where ever possible, form work should be left longer as it will help the curing of the concrete. In normal circumstances and where ordinary port-land cement is used, and temperature does not fall below 15°C, forms should generally be removed after the expiry shown in table 25.3.
For other cements the stripping time recommended for ordinary port-land cement may be suitably modified.
Thus at the end it can be said that form work must be sufficiently strong to resist all vertical and horizontal loads acting on it and have water tight joints to prevent leakage of cement mortar. It should retain its proper shape, if once it deforms during placement of concrete, it will be impossible to pull it back to its original position.
Points to be Kept in Mind before Placing Concrete in Form Work:
Following points should be kept in mind while placing concrete in the form work:
1. The geometric dimensions should be checked carefully with steel rule or steel tape. The accuracy of horizontal surfaces may be checked by spirit level and verticality by plumb bob.
2. In order to control the seepage of cement mortar through joints all openings more than 2 mm should be filled up properly.
3. The form work should be free from dust or any kind of rubbish.
4. In order to check the absorption of water by form work from green concrete, the form work either should be well wetted or oiled properly. It will give smooth surface of the concrete work.
5. The nails etc. should not protrude from the form work. It should be checked and any nails or timber piece found protruding should be removed and mould should be repaired suitably.
6. The alignment of the form work should be checked carefully and it should not be disturbed during concreting operations.