G4 - LT #2 - Process of concrete curing Flashcards
What are two required conditions for proper concrete curing?
Moisture & favorable temp.
What is considered a favorable air temperature for concrete curing?
15 to 20 degrees and over cast
Explain the hydration process
Chemical reaction of water & cement
List 4 methods of curing concrete:
1) Water coating
2) Water covering
3) Curing compound
4) Mechanical barrier
How are chemical hardeners used?
Sprayed or sprinkled unto surface and floated in
Why are some curing compounds colored?
Showing coverage
If 2 coats of curing compound are used, how is the second one applied?
90 degrees to the first coat
What is the minimum curing time required to allow the concrete to gain 70% of its 28 day strength
3 days
What are two purposes of chemical sealants?
Durable water proof finish, improved looks & cleanable surface
How are powdered hardeners applied to concrete?
Telecast on & floated or troweled in
Why should additional water not be added to the concrete when placing concrete in hot weather?
Weakens concrete
How is the concrete mixture cooled in hot weather?
1) Refrigerate mix water
2) Spray aggregate w/ cold water
What causes plastic shrinkage cracking?
rapid water loss
What is the effect on the concrete of using fossil fuel-burning heaters in unvented enclosures?
Co2 weakens concrete & causes dusting
What is hydration?
Chemical reaction that takes place when the water reacts with the cement in the concrete mixture
What is the process of keeping concrete moist called?
Curing
Concrete not cured at all will attain % of strength ?
50 % of 28 day str.
Concrete cured 3 days will reach % of str?
70 %
Concrete cured for 7 days reaches % of str?
90 %
If concrete is moist continuously, it will continue to gain strength, T OR F?
T
What happens to a slab that freezes?
It spalls, needs redo
When is the damage reduced from freezing for concrete?
Once it reaches 3.5 MPA (500 PSI) damage reduced
What form is suspect. to freeze?
Narrow concrete walls 8” or less
What is done to warm a mixture when placing in cold temps?
1) Add hot water to the aggregate
2) Cement is added if aggregagte/water is below 38 degrees
Cement added to very hot water causes what?
A flash set
T or F - The heat from the hydration of the cement will keep massive concrete (more than 8 ft thick) warm enough to resist freezing
True
What does an accelerator do in the hydration process?
Speeds up hydration, adding more heat, warming the cocnrete. Increased hydration speeds up the strength gain, reaching 3.5 MPA faster.
Used for narrow and thin slabs
Adding water to make concrete workable does what?
Makes it unworkable and discardable
Excess exposure to carbon dioxide will cause the surface of the concrete to become:
Weak & dusty
When the job site temp is over 25 degrees, what happens?
Rapid evaporation of the water in the concrete mixture and the surrounding materials.
Water loss accelerates the settling & creates plastic shrinkage cracks
What does retarding admixture do?
Lengthens setting time without long term effects on the strength gain of concrete
What does super plasticizing admixture do?
Allow extreme slumps to be obtained without increasing the water/cement ratio or decreasing the strength
How can you extend the curing process of a concrete form after stripping?
Spray the wall with a curing compound
What is the difference between a sealer and a hardener?
Sealer aids in moisture retention and curing while, giving a durable water proof finish
Hardener hardens the surface for a durable finish to wear, physical damage and some chemicals
What can be added to a new or existing concrete surface to increase the hardness?
Sodium silicate reacts with the free lime in the concrete to form a hard, dustproof surface.
