Protection of fresh concrete from rain, sunlight and other weather factors is very important. Bad results will occur in case of negligence. Similarly, protect concrete from any direct hit during other construction activities. Concrete gain strength with time. With the passage of time bond between cement, sand and aggregates strengthen up. Following measures should be taken into consideration for fresh laying concrete.
When it comes to protection of fresh concrete, Curing is the most important factor that must be considered in construction. In order to protect concrete from evaporation, cure the fresh surface after two hours of laying. Use water soaked gunny bags for wetting of the surface. Surfaces exposed to sunlight must be properly covered with water proof sheets, wet blanket or wet sharp sand to avoid excessive evaporation. Excessive evaporation may lead to undue cracks or reduction in anticipated strength of concrete. After 24 hours make small embankments for water storage of about 1inch depth on the surface of concrete. Protect concrete from direct sunlight for 14 days.
In case of foundation, masonry work can be started after 48 hours of concreting. But, even then curing of concrete must be continuing for 14 days.
Molds (that are used for concrete casting according to the desired shape) are part of form work. Similarly, temporary construction that is constructed for the support of molds is also a part of form work. Two things must be considered while designing form work. These are;
- Propping and Centring
Generally, material used for form works are plywood, steel and wood. If wood is used, then it must be of such specifications that it not get bent or bulk during exposed to rain or sunlight. Secondly nail can easily be fixed in it. Form work must be according to design and size.
Material should have enough strength that it carries self-load, concrete load and also absorb vibrations during works without any deformation. If any support gets settled during concrete works then use hard wood wedges for it support. Form work should be designed in such away that it can easily be removed in parts without disturbing other parts of building. Inner surface of form work should be plain so that plain surface of final concrete obtained. This saves mortar used for plastering.
Propping and Centering
Props are the vertical members used to support horizontal platform from sagging. It can be of steel pipes or wood. Props should have enough strength that it can easily carry the load of concrete easily. If wood bellies are used, then its diameter should be of 4inches at the center and 3inches on edges. Place two to three wedges under each prop. It will help in lifting props during tightening of form work and during removing of form work.
If height of props goes above 3.50meters or distance between two props is more than 4.5meters than brace the props with the addition of diagonal members. Before concreting, make sure engineer incharge has properly checked the form work and given orders for further proceeding.
Sometimes, it happens that few props get settled with the moving loads or vibration. To overcome this, continuously check each member and tighten up if required by adding more wedges under it.
Surface treatment of material (either steel or wood) used for form work is very important. It saves concrete bond with form work. Treat the surface of form work with raw linseed oil or soap solution before concreting.
Horizontal members of form work slightly deflect from center during concreting. To overcome this, provide camber (slightly lift up horizontal members from center). Generally, camber ratio for beam and slab is 1:250. While in case of cantilever, camber ratio is 1:50
Removal of Form Work
Form work should be removed with full precautions. Save concrete from any kind of shocks, because it affects the properties of concrete very badly. In case of beam and slab, first remove the props from sides and then proceed towards the underneath props. Remove the props of beam at the end.
The minimum time required before removing form work is as following;
- Sides of foundation, column, beam and slab require 2 days.
- Beneath slab supports of length up to four meters require 7 days
- Slab of more than 4 meters require 14 days
- Beam of up to 6 meters require 14 days
- Beam more than 6 meters require 21 days.