Curing and protection of concrete

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Curing and Protection
of Concrete
Chapter 9
Chapter Topics
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•
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•
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Importance of curing
When to start curing
Curing methods
Effects of high temperature on concrete
Causes and prevention of plastic shrinkage
cracking
• Effects of low temperatures on concrete
Curing
• Curing refers to the methods used to keep
concrete moist and at the temperature needed to
develop the required strength, wear resistance,
and durability.
• The main concern is preventing moisture loss
from the slab surface, especially at early ages.
• Controlling this loss may require actions any time
after placement, including before and after final
finishing.
Importance of Curing
• Hydration is the hardening of concrete due depends on a
chemical reaction between the cement and water.
• The speed of hydration depends on temperature, but
hydration also calls for moisture to be present.
• Right conditions for curing involves three factors:
– Moisture
– Temperature
– Time
Moisture for Curing
• Hydration of cement slows as concrete dries.
• That is why slab surfaces must be kept from drying in
the first several days after placement.
• If the water is kept in the concrete, hydration will
continue, but if all the free water evaporates, hydration
practically stops.
• Protect the concrete surface either by applying water
or preventing evaporation
When to Start Curing
• In hot weather, it’s important to start curing very quickly.
• Curing may have to start after placing and before
finishing is completed by using fog sprays or evaporation
reducers.
• To minimize plastic shrinkage cracks, take steps to reduce
evaporation during finishing operations and for several
days thereafter.
• Planning so fogging nozzles, evaporation reducers, or
both are available and ready for use before the concrete
arrives.
Curing Methods
• Ponding
• Sprinkling or fog spraying
• Wet burlap or mats
• Reinforced paper of plastic films
• Curing compounds
Curing Compounds
• Forms a film to seal in moisture
• 150 to 200 ft2 per gallon
• White or gray colors added to curing compounds to reflect
sunlight and show that the compound is being uniformly
applied.
• Don’t use on surfaces that will be covered with concrete,
mortar, tile, many floor coverings, or paint, unless tests
show that the compound can be adequately removed or
the manufacturer states in writing that it will not weaken
the bond between the slab and the covering.
Effects of High Temperature
• Hot weather precautions
– Use concrete mix that works for hot weather
– Select a concrete slump for rapid placement and
consolidation
– Place and finish concrete as fast as possible
– Schedule night pours
– Protect against moisture loss at all times
Plastic Shrinkage Cracking
• Form while the concrete is soft or plastic, usually
soon after concrete is placed.
• Occurs when the slab surface is drying faster than
the bleed water is rising to the surface.
• Plastic shrinkage cracks are most likely to appear
when the concrete is warm, and the weather is
dry (low relative humidity), windy, and/or hot.
Plastic Shrinkage Cracking
• To determine evaporation rate
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Concrete temperature
Air temperature
Wind speed
Relative humidity
• When the rate of evaporation exceeds about 0.2 lb of
water/ft2/hour, plastic shrinkage cracking may occur.
• With concrete that doesn’t bleed much, plastic
shrinkage cracking can occur at evaporation rates lower
than 0.2 lb of water/ft2/hour.
Minimize Plastic Shrinkage Cracking
• Dampen subgrade and forms
• Protect against drying
– Fog spraying
– Evaporation retarders
– Plastic sheets or wet burlap
• Erect windbreaks if possible
• Place concrete in an enclosed building
• Avoid placing in hot, windy weather
Effects of Low Temperatures
• Time to start floating
– 2 ½ hours at 60F
– 3 ½ hours at 40F
– 5 hours at 35F
• Early strength gain is very slow at temperatures
below 50 °F
• Curing period must be extended to attain a
certain strength
Effects of Low Temperatures
• Placing warm concrete on a cold subgrade can result in some problems that
are very difficult to solve.
• Concrete in contact with the subgrade will cool rapidly, remain plastic, and
might still be bleeding while concrete at the surface will set and beginning to
harden.
• A “crust” forms at the surface just as the slab is ready to be floated and
troweled.
• When the workers walk on the slab, the concrete feels rubbery, and it’s
difficult to obtain a flat and level surface during finishing.
• Bleed water can also collect beneath the crust and form a plane of weakness
that may cause the surface to delaminate under traffic.
• It’s best to place warm concrete on a subgrade that has a temperature no
more than 30°F less than the concrete temperature.
Precautions Against Freezing
• Protect concrete when temperatures are below
40F
• Concrete needs to be above 500 psi
• Takes 1 day of curing at about 50F to reach 500
psi
• Corners and edges most vulnerable to freezing
Protection Against Freezing
• Use insulating blankets
• Place concrete in heated enclosures
• Use accelerating admixtures
– Calcium chloride if not steel
– Non-chloride accelerator if steel
• Use extra cement
• Use Type III cement
Air-Drying of Slabs after Curing
• It’s good practice to permit outdoor slabs in cold
climates to dry for as long as possible, preferably
30 days after curing.
• Drying reduces moisture content and thus the
danger of damage caused by freezing saturated
concrete.
• Avoid using deicing salts until the concrete has
dried and has been through one winter.
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