The overall process of Concrete shall include
- Mixing & Batching
- Making of Formwork
- Transporting Concrete
- Placing Concrete in the Formwork
- Curing of concrete
- Stripping of Formwork
Process of Concrete:
1. Mixing & Batching:
Before mixing on concreting, the batching should be done.
Batching of Concrete:
The ingredient of concrete should be mixed by the weight as per design. But on field it can be measured by volume or by weighing. The measurement and batching is done depending on the production of required quantity and also depends on quality of concrete. The required quantity of Concrete (of given mixture proportions) can be calculated by the “Absolute Volume Method”.
a. Volume batching.
b. Weight batching.
Mixing of Concrete:
Concrete is mixed by any two methods, based on requirement as per quality and quantity of concrete required.
Mechanical Mixing: Normally for mass concrete, where good quality of concrete is required, mechanical mixer is used.
Hand Mixing: Mixing by hand is employed only in specific cases.
The mixing shall be continued until there is a uniform distribution of the materials and the mass is uniform in color and consistency. For the segregation after unloading from the mixer, the concrete should be remixed for the at least 2 minutes or as per manufacturer.
All the test of fresh concrete should be performed to check consistancy and workability after mixing.
2) Making of Formwork
- Formwork or shuttering is a temporary mould or structure to provide the support to fresh concrete to form a structural member till the concrete gets sufficient strength to stand it own.
- This helps the structural member to gain sufficient strength to carry its self-load by its own and load from other members.
- There are many types of structural formwork or shuttering based on their use, materials and the type of structural members. They can be named based on their function. However, main functioning of the formworks or shuttering remains the same.
- There are some guidelines for removal of formwork that need to follow for removal.
3. Transporting of concrete:
- Concrete needs to be transported to formwork as rapidly as possible, after mixing to prevent the segregation, losses of any ingredients, ingress of foreign matter and loss of water from concrete due to temperature rises in hot weather. To main workability and consistency, concrete must be transported rapidly.
- During hot or cold weather, concrete shall be transported in deep containers means to provided less exposer of surface area to climate to reduce the loss of water by evaporation in hot weather and heat loss in cold weather may also be adopted.
- In the case of Ready Mix concrete (RMC), the transportation duration should not be more. To maintain the workability and to reduce the losses during transportation in case of RMC, admixtures can be used.
4. Placing concrete in the formwork:
- The placing concrete in the formwork shall be placed properly avoiding the re-handling and re-transporting to its destination member.
- Concrete should be placed and compacted before it initial setting commences and should not be disturbed subsequently.
- Concrete should be placed in such way that, there is no segregation and bleeding after placing.
- Concrete should not be placed from more than 1.5m height as the maximum permissible free fall of concrete is 1.5m.
- Methods of placing should be such as to preclude segregation. Care should be taken to avoid displacement of reinforcement or movement of formwork. As a general guidance, the maximum permissible free fall of concrete may be taken as 1.5m.
5. Compaction of concrete:
- Concrete should be thoroughly compacted after it gets placed and fully worked around the reinforcement, around embedded fixtures and into corners of the formwork.
- There are two methods of compaction.
- Mechanical vibrators: Concrete shall be compacted using mechanical vibrators. Over vibration and under vibration of concrete are also harmful and must be avoided.
- Manual compaction: For the small placed like beam or shear wall rod tamping is preferred.
- Vibration to the very wet mixes should also be avoided.
- In the case of external vibrations, the design of formwork should be done accordingly.
6. Curing of concrete:
- The process of preventing the loss of moisture from the concrete while maintaining a specific temperature routine, is called as ‘Curing of concrete’.
- The curing should also prevent the development of high temperature gradients within the concrete.
- Purposes of Curing are as follows:
To boost the speed of Hydration of Concrete to achieve desired Strength
To Improve durability of concrete by reducing cracks
To Increase abrasion resistance
To Improve microstructure by developing better hydrate gels and solid mass.
Methods of Curing:
- In moist curing, exposed surfaces of concrete shall be kept continuously in a damp or wet condition by Spraying with Water or Ponding or by covering with a layer of Sacking, Canvas, Hessian or similar materials and kept constantly wet for at least 7 days from the date of concreting.
- The period of curing shall not be less than 10 days for concrete exposed to dry and hot weather condition.
b. Membrane Curing:
- In membrane curing, impermeable membranes such as polyethylene sheeting covering closely the concrete surface may also be used to provide effective barrier against evaporation or Curing Compounds may be applied to the surface after concrete has set.
7. Stripping Time of formwork:
Stripping time of forwork is the time for which the concrete is required to gain the sufficient strength after that the formwork can removed. Formwork may be removed after stripping time, when concrete has achieved a certain amount of strength that is generally half or 50% of what concrete should achieve at loading and I.S.456 recommends as follows
1. Walls, Columns, and Vertical Faces of Structural Members 16 – 24 Hours
2. Soffit Formwork of Slabs (Props to re-fixed immediately after removal) 3 days
3. Beams (Props to re-fixed immediately after removal) 7 days.
4. Removal of props.
- For Slabs Spanning up to 4.5m 7 days
- For Slabs Spanning over 5.5m 14 days
- For Beams Spanning 6m 14 days
- For Beams Spanning over 6m 21 days