Steps required to safeguard newly placed concrete curing time
Newly placed Concrete Curing Time in the preliminary phase of setting (up to 12-24 hrs.) should be safeguarded from frost, mist, rain and drying winds with proper ways.
Uncovered concrete should be retained drenched with proper methods for seven days when the concrete is placed or for three days when rapid hardening cement is utilized. A layer of gunny bags should be used to wrap concrete, canvas, hessea or identical absorbent materials and retained continuously drenched for seven days from the date of arranging concrete (More details IS-456-2000).
In the following phases of hardening (after 12-24 hrs.) concrete is cured by ponding and flooding with water to retain it continuously drenched or by applying some curing compound or water proofing coating (that don’t permit concrete water to vacate).
Rate of hardening of concrete can be raised i.e. concrete curing time can be minimized with following ways :-
• Hot water curing
• Steam curing
• Using accelerator admixture
By steam curing at 80 degree centigrade, 70% of the strength is attained in 4-5 hours. When Portland Pozzolana Cement(PPC) is utilized in place of OPC, curing period should be raised to 20 days for concrete work. PPC obtains required strength in 35 days as against 28 days in case of OPC.
Caution - Allowing vaporizing of water from concrete at the time of hardening can provide following negative effects:
a) Reduction of concrete strength (caused by incomplete hydration of concrete)
b) Shrinkage cracks (caused by concrete shrinks for removal of water)
c) Increase in voids
Ideal temperature for concrete pouring should reamin 21 degree Centigrade. In cold weather, concreting should be dropped under 4.5 degree centigrade since water can freeze and break up concrete.
Minimum Period before Striking Form Work:
1. Props to Slabs
• Spanning up to 4.5 M 7 Days
• Spanning over 4.5 M 14 Days/p>
2. Props to beams and arches
• Spanning up to 6 M 14 Days
• Spanning over 6 M 21 Days