of Concrete Chapter 9 Chapter Topics 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 ID: 151515
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Slide1
Curing and Protection of Concrete
Chapter 9Slide2
Chapter Topics
Importance of curingWhen to start curingCuring methodsEffects of high temperature on concrete
Causes and prevention of plastic shrinkage cracking
Effects of low temperatures on concreteSlide3
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.Slide4
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
TimeSlide5
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 evaporationSlide6
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.Slide7
Curing Methods
PondingSprinkling or fog spraying
Wet burlap or mats
Reinforced paper of plastic films
Curing compoundsSlide8Slide9Slide10
Curing Compounds
Forms a film to seal in moisture150 to 200 ft
2
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.Slide11Slide12
Effects of High Temperature
Hot weather precautionsUse 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 timesSlide13
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.Slide14Slide15
Plastic Shrinkage Cracking
To determine evaporation rateConcrete 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.Slide16Slide17
Minimize Plastic Shrinkage Cracking
Dampen subgrade and formsProtect against dryingFog spraying
Evaporation retarders
Plastic sheets or wet burlap
Erect windbreaks if possible
Place concrete in an enclosed building
Avoid placing in hot, windy weatherSlide18
Effects of Low Temperatures
Time to start floating2 ½ hours at 60F3 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 strengthSlide19
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.Slide20
Precautions Against Freezing
Protect concrete when temperatures are below 40FConcrete needs to be above 500 psi
Takes 1 day of curing at about 50F to reach 500 psi
Corners and edges most vulnerable to freezingSlide21
Protection Against Freezing
Use insulating blanketsPlace concrete in heated enclosuresUse accelerating admixtures
Calcium chloride if not steel
Non-chloride accelerator if steel
Use extra cement
Use Type III cementSlide22Slide23
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.Slide24
End of Chapter 9