Curing & placing

8/7/2019 Curing & placing

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PLACING, CURING, COMPACTION of CONCRETE


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Placing of ConcreteSegregationBondingTemperature controlMaximum drop

Compaction of ConcreteFinishing ConcreteCuring of Concrete

What is Curing?Is Curing Necessary?Why Curing Is So Important?Results of Proper CuringMethods of CuringWhat is Sealing?Why Seal?


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Placing of Concrete

*You cannot obtain the full value of well-designedconcrete without using proper placing

procedures.

*Good concrete placing and compacting techniques produce a tight bond between the paste andaggregate and fill the forms completely.

*Both of these factors ;contribute to the full strength and best appearance of concrete


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*Segregation

*Bonding

*Temperature control

*Maximum drop

The following are some of the principles ofconcrete placement:


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* Avoid segregation during alloperations, from the mixer to the point ofplacement, including final consolidation and

finishing.

* Thoroughly consolidate the concrete, workingsolidly around all embedded reinforcement and

filling all form angles and corners.

Segregation


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* Take appropriate steps

to control the temperature of fresh concrete frommixing through final placement.

* Protect the concrete from temperature extremesafter placement.

Bonding

* When placing fresh concrete against orupon hardened concrete, make surethat a good bond develops.

Temperature control


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Concrete should be placed, then screeded to the properelevation. The surface should be bullfloated before bleed water

accumulates on the surface. Finishing must be delayed untilwater sheen has disappeared from the surface.

The least amount of finishing should then be done: do not addwater to or overwork the surface. A broom finish is

recommended for exterior concrete, except when specialtextured finishes (like exposed aggregate or stamped orimpressed finishes) are desired.

In hot weather, fresh concrete should be protected from thesun and wind - even before it is finished. Confilm, a surface

evaporation retarder, is one way to protect concrete in hotweather.

Concrete can also be safely placed in cold weather whenproper precautions are taken


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1.Use of air entrainment.

2.Use of lower slump concrete.

3.Use of an approved accelerating

admixture.

4.Use of water reducers.

5.Increased cement content.6.Protection of aggregates against

frost.

7.Use of hot water or heated

aggregates.

8.Avoidance of placing concrete on a

frozen subgrade.9.Protecting the surface of the freshly

placed concrete from frost until the

strength of at least 3.5 mPa has been

achieved - this is usually a period of

not less than two days.

*Precautions that maybe taken in unfavourablecold conditions referredto above, incude therightside:

Cold Weather Concreting


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1.Elapsed time mixing and placing

should be minimised.

2.Exposure of mixers and agitators

to the hot sun, while waiting to be

unloaded, should be ;minimised.3.Use suitable retaring admixtures.

4.Avoid excessive mixing.

5.Wet forms, reinforcing, subgrade

and surrounding areas by spraying

with water shortly before placing.

6.Speed up placing.7.Use fog sprays shortly after

placement.

8.Give prompt curing and protect

exposed surfaces from drying out

*High temperatures result inmore rapid hydration ofcement, and hence earlystiffening of the concrete,

greater mixing waterdemands, increasedevaporation of mixing water,reduced strengths, and largevolume changes; and greater

chances of plastic cracking.

Hot Weather Concreting


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Concrete Placing Equipment


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Compaction ensures the optimum density of the mix isachieved, increasing compressive strength. Where

reinforcing is used, the bond between concrete andredistribution bar is improved. Permeability of theconcrete decreases, decreasing cold joints,honeycombing entrapped air and segregation.

Compaction


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Vibrators consolidate concrete by pushing the coarseaggregate downward, away from the point of vibration.

Vibrators allow placement of mixtures that

are too stiff to place any other way, such as thosehaving a 1- or 2-inch slump.

Stiff mixtures are moreeconomical because they require less cement and

present fewer segregation or bleeding problems.However, do not use a mix so stiff that it requires too muchlabor to place it

VIBRATION


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The best compacting tool is a mechanical vibrator.The best vibrators available inengineering construction battalions are called internalvibrators because the vibrating element is inserted intothe concrete.

Mechanical Vibrators


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When using an internal vibrator,insert it at approximately 46cm intervals into air-entrained concrete for 5 to 10 seconds and intononair-entrained concrete for 10 to 15 seconds. Theexact period of time that you should leave a vibratorin the concrete depends on its slump.


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Manual consolidation methods require spades,puddling sticks, or various types of tampers. Toconsolidate concrete by spading, insert the spadealong the inside surface of the forms ,

through the layer just placed, and several inches intothe layer underneath. Continue spading or puddlinguntil the coarse aggregate disappears into the concrete.

Hand Methods


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FINISHING CONCRETE

The finishing process provides the final concretesurface. There are many ways to finish concretesurfaces, depending on the effect required. Sometimes

you only need to correct surface defects,fill bolt holes, or clean the surface. Unformedsurfaces may require only screeding to proper contourand elevation, or a broomed, floated, or trowelled finishmay be specified.


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The top surface of a floor slab, sidewalk, orpavement is rarely placed at the exact specifiedelevation.

Screeding brings the surface to therequired elevation by striking off the excess concrete.

Two types of screeds are used in concrete finishingoperations: the hand screed and the mechanicalscreed

SCREEDING


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Hand screeding requires a tool called a screed. This is actually atemplet (usually a 2-by-4) having astraight lower edge to produce a flat surface (or acurved lower edge to produce a curved surface).

M

ove the screed back and forth across the concrete using a sawingmotion. With each sawing motion, move the screed forward an inchor so along the forms.

This forces the concrete built up against the screed face into the lowspots. If the screed tends to tear the surface, as it may on air-entrained concrete due to its sticky nature, eitherreduce the rate of forward movement or cover the lower edge ofthe screed with metal.

Hand Screed


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The mechanical screed is being used more andmore in construction for striking off concrete slabs onhighways, bridge decks, and deck slabs. This screedincorporates the use of vibration and permits the useof stronger, and more economical, low-slump concrete.

It can strike off this relatively dry materialsmoothly and quickly. The advantages of using avibrating screed are greater density and strongerconcrete.

Vibrating screeds give abetter finish, reducemaintenance, and save considerable time due to the speed atwhich they operate. Vibrating screeds are also much lessfatiguing to operate than hand screeds.

Mechanical Screed


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Mechanical screeding machine


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Proper curing greatly increases the durability ofconcrete. Concrete should be cured with a curingcompound sprayed on the surface immediately afterthe final finishing, or by approved moist curing

methods.

Concrete should cure 3 to 7 days before being putin service. (For driveways, car traffic may beginafter 3 days; trucks and other vehicles, 7 days.)

A cure and seal compound with an ultraviolet (UV)protectant is recommended to keep your concretefrom gaining a yellow tint.

Curing


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Curing is the maintaining of a satisfactorymoisture content and temperature in concrete,so that the concrete may develop the desiredstrength and hardness.

Correctly cured, concrete shrinks, cracks anddusts less, while gaining strength, durability and

wear resistance.

What is Curing?


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Without an adequate supply of moisture, the Portland

Cement in the concrete cannot chemically react toform a quality product.

Drying may remove the water needed for thischemical reaction called "hydration," and the concrete

will be weak.

Temperature is an important factor in proper curing,since the rate of hydration is temperature dependent.

For outdoor concrete, relative humidity and windconditions are also important; they contribute to therate of evaporation from the concrete.


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Tests show that improper curing can easily cut the strength ofeven the best concrete by 50%.

Concrete placed under high temperature conditions will gainearly strength quickly, but later strengths may be reduced.

Concrete placed in cold weather will take longer to gainstrength, delaying form removal and subsequent construction.

Properly cured concrete provides improved durability.

Well cured concrete has better surface hardness andtherefore is more watertight.

Is Curing Necessary?


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Improper curing can easily cut the strength of even the bestconcrete by 50%.Curing simply means keeping the water in theconcrete where it can Do its job of chemically combining withthe cement to change the cement into a tough "glue" that willhelp develop strong, durable concrete.

Good curing means keeping the concrete damp and at about70F (21C) until the concrete is strong enough to do its job.

Recommended practice calls for a minimum of seven days

curing for ambient temperatures about 40F (4C) or the timenecessary to attain 70% of the specified compressive or flexuralstrength, whichever period is less.

Why Curing Is So Important ?


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All concrete must be cured to attain maximumstrength. Correctly cured concrete is best from everystandpoint: It shrinks less, cracks less and dusts less.It is stronger, more durable and has a more wearresistant surface.

Start curing the concrete as soon as possible after ithas hardened. Early drying- especially in hot, windyweather- must be prevented or the concrete will notattain its full potential quality.


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Methods of Curing

Membrane Curing Compounds

Water Spray

Waterproof Paper

Damp Burlap

Accelerated Curing


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The most common method of keeping moisture in theconcrete. Membrane curing compounds are low in cost andcan be easily brushed or sprayed on immediately after the

concrete is laid without worrying about marring thesurface.

All of the manufacturers directions must be completely

followed for maximum protection. Compounds that have anultraviolet (UV) protectant are recommended as somecompounds give concrete a yellow tint.

Membrane Curing Compounds


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A good curing method, but only if the concrete is kept continuouslydamp.Allowing the concrete surface to dry between sprinkling can causecrazing of the surface and cracking of slabs.

Water Spray

Waterproof Paper

Holds moisture in the concrete by preventing evaporation. Thismethod is acceptable for slabs but not practical for wall,irregular surfaces or colored slabs.

Damp Burlap

Sometimes an effective method of curing. The burlap must bewashed and free of anything that might stain or harm theconcrete. It is not practical for large areas, and it must be keptdamp to prevent water lost through evaporation


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Sealing is a glossy film sprayed or rolled on concreteslabs to make them resistant to liquid and dirtimpregnation.

It provides good resistance to traffic, abrasion,alkalis and many low concentration chemicals.

Sealers are designed to keep moisture out of the

concrete, especially in the first year when theconcrete is curing and gaining strength. This providesfor scale-resistant concrete.

What is Sealing?


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In order to prevent concrete surface scaling duringthe winter, it is a good idea to consider the applicationof a quality concrete surface sealer for all concretesurfaces that will be exposed to wetting and freezing.

The presence of a surface sealer before winter willminimize the amount of water penetrating into theconcrete, which decreases the likelihood of freezing

damage.

A surface sealer will enhance the performance ofconcrete and result in greater concrete service life.

Why Seal?


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Application of heatApplication of heat

The development of strength of concrete isThe development of strength of concrete isa function of not only time but also that ofa function of not only time but also that of

temperature. When concrete is subjectedtemperature. When concrete is subjected

to higher temperature it accelerates theto higher temperature it accelerates the

hydration process resulting in fasterhydration process resulting in fasterdevelopment of strength. Concrete cannotdevelopment of strength. Concrete cannot

be subjected to dry heat to accelerate thebe subjected to dry heat to accelerate the

hydration process as the presence ofhydration process as the presence ofmoisture is also an essential requisite.moisture is also an essential requisite.

((steam curing )steam curing )


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Application of heatApplication of heat

The exposure of concrete to higherThe exposure of concrete to highertemperature is done in the followingtemperature is done in the followingmanner:manner:

Steam curing at ordinary pressure.Steam curing at ordinary pressure.

Steam curing at high pressureSteam curing at high pressure(Autoclaving).(Autoclaving).

Curing by InfraCuring by Infra--red radiation.red radiation.

Electrical curing.Electrical curing.


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Steam CuringSteam Curing


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Effect of Steam CuringEffect of Steam Curing


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Effect of steam curingEffect of steam curing


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Effect of TemperatureEffect of Temperature


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Steam Curing CycleSteam Curing Cycle

10

PresetRise

CuringMaximum Temperature

5 10 15 20 25


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Electrical CuringElectrical Curing

Concrete can be cured electrically by passing anConcrete can be cured electrically by passing an

alternating current (Electrolysis troublealternating current (Electrolysis trouble

will be encountered if direct current is used)will be encountered if direct current is used)

through the concrete itself between twothrough the concrete itself between two

electrodes either buried in or applied to theelectrodes either buried in or applied to the

surface of the concrete. Care must be taken tosurface of the concrete. Care must be taken to

prevent the moisture from going out leaving theprevent the moisture from going out leaving the

concrete completely dryconcrete completely dry


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Curing by InfraCuring by Infra--red Radiationred Radiation

Curing of concrete by InfraCuring of concrete by Infra--red Radiation has beenred Radiation has been

practiced in very cold climatic regions in Russia.practiced in very cold climatic regions in Russia.

It is claimed that much more rapid gain of strength canIt is claimed that much more rapid gain of strength can

be obtained than with steam curing and that rapid initialbe obtained than with steam curing and that rapid initial

temperature does not cause a decrease in the ultimatetemperature does not cause a decrease in the ultimate

strength as in the case of steam curing at ordinarystrength as in the case of steam curing at ordinary

pressure. The system is very often adopted for thepressure. The system is very often adopted for the

curing of hollow concrete products. The normal operativecuring of hollow concrete products. The normal operative

temperature is kept at about 90temperature is kept at about 90C.C.

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Curing & placing