Bmc Concreting

7/28/2019 Bmc Concreting

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Cold-Weather Concreting

Under- Water Concreting

AND


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Introduction

Without curing, concrete is just like brittle mixture of various construction ingredientsthat can break down under excessive loads or even under normal loading conditions.

Concrete under water curing, curing concrete below freezing and extremely hottemperatures needs special attention.

Curing means hardening of concrete mix under controlled conditions. Temperature andmoisture content of concrete are two important aspects of curing and depending upon

these two factors, concrete achieve its strength. Efficient curing ensures that concrete isdurable, resistive to corrosion and is resistant to wind and heat. Period of curing is

different for different types of work and temperature conditions. In wet and freezing

conditions conventional methods do not work.


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Concrete can be successfully placed, finishedand cured in winter so long as you understandwhat affect low temperatures have on the freshand early age concrete.

The setting of concrete involves a series of

complex chemical reactions that are verysensitive to temperature. This reaction, undernormal conditions, gives off a considerableamount of heat. If it is so cold that the reactionslows down, the reaction and heat evolution isdelayed - consequently, the strength gain will

be drastically reduced.

The use of admixtures can over-ride some ofthe negative affects of low temperatures andenable you to continue to place concrete in lowtemperatures


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Cold Weather


Unfavourable conditions?The unfavourable conditions includetemperatures below 5 degrees on a fallingthermometer, or 2 degrees on a risingthermometer, or where it becomesimpractical to work and finish the concreteadequately.The precautions listed in the Standardinclude the use of air entrainers, using low

slump concrete, using admixtures(accelerators and water reducers,)increasing cement contents, using someform of frost protection, and avoidingfrozen ground.


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Practical Site measures


1. Do not place concrete on frozenground. If snow is forecast cover thesub-base to protect it or delay the pour.

2. Check the predicted temperatures forthe day, and decide whether or not toproceed if temperatures are fallingbelow 5 degrees, or if a frost ispredicted overnight.

3. If a decision is made to proceed get thecovers necessary to protect theconcrete from freezing.


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Placement


1. Place the concrete as early as practicable in themorning to give plenty of time to finish thesurface in daylight.

2. Do not wet up the concrete. Ensure the slumpis not above 80 mm.

3. Do not attempt to finish the concrete until all

bleed water has evaporated. This can takesome time on cold windless days.

4. It may be necessary to remove the bleed wateron cold windless days. Whatever system youuse take great care not to mix the water intothe top of the concrete. This will lead to a soft

dusty surface.5. Do not spread cement onto the surface to dry

up the top. This will lead to dusting andexcessive crazing of the surface

6. Do not overwork slabs that have had a delayedset due to low temperature.


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Curing conditions


If icy or frosty conditions are expected immediately after finishingoperations have been completed, do not use water to cure the topsurface as this will freeze and disrupt the top surface before it hashad a chance to develop any strength.Cover the slab with straw and plastic to ensure the top of the slabremains above freezing point.If more severe weather is expected replace the insulation for atleast the duration of the cold spell for good measure (or four days asa reasonable rule of thumb).Do not seal freshly placed concrete unless the sealer is specifically

designed for application to new concrete at low or very lowtemperatures.If the concrete is warm after the use of the insulation, carefullyremove the insulation in a way that will not drop the temperature ofthe concrete more than 20 degrees over a 24hour period. This willavoid the risk of thermal shock cracking developing


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Stick to the design code, maintain proper water-cement ratio, and ensure a safe gapbetween the reinforcement bars. The water to cement ratio should not be more than 0.40 inwet and freezing conditions.In extremely cold regions, a propane heater and a polyethylene enclosure can be used tokeep the temperature above the freezing point.Wet and freezing conditions mean moisture, which means corrosion when related to steeland concrete. Use Portland Type III cement as it helps in easy setting without degrading the

quality of concrete.Use additives like fly ash, silica fume, and furnace slag as they help in controlling formationof chloride ions, which is one of the main reasons for steel corrosion in concrete.Concrete under water curing for flatwork applications becomes easy with previous concrete.Previous Concrete is all coarse aggregates and it contains a negligible percentage of fineaggregates, especially sand. Additives are mixed into it that do not allow water to penetrateinside the concrete surface.Use concrete sealants so that water does not seep inside the concrete. Sealants not onlyincrease the life of concrete but also help in preventing concrete curing failure. In extremelycold regions, only a breathable concrete sealant must be used, as it will allow theevaporation of water and moisture, helping in fast setting of the concrete.


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Effect of Low Temperatures on Strength

Hydration Ceases at14 F


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Effect of FreezingFresh Concrete Up to 50% reduction of

ultimate strength canoccur if frozen Within a few hours

Before reaching a

strength of 3.5 MPa (500psi)

Frozen only once at anearly age

With curing nearly allstrength can be restored

Less resistance toweathering

More permeable


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Type III or HE high-early-strength cement

Additional portland cement (60 to 120 kg/m3 or 100 to 200

lb/yd

3

) Chemical accelerators

Methods to accelerate strength gain:


Accelerate the rate of:

Hydration (setting)

Early-age strength gain

Calcium chloride accelerators: Increase drying shrinkage, potential

reinforcement corrosion, potential

scaling

Darken concrete

Accelerating Admixtures


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Preparations for PlacingConcrete in Cold Weather

-Snow, ice, and frost should be removed.-Temperatures of surfaces and metals incontact with concrete should be abovefreezing...may require insulating or heatingprior to placement.-Have materials and equipment in place toprotect concrete during and afterplacement.-Insulated blankets, tarps, straw covered with

plastic sheets-Heaters (make sure to vent to avoidcarbonation..thus dusting)-Enclosures and possible insulated forms


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BASIC COLD WEATHER CONCRETINGGUIDELINES

1) Use air-entained concrete when exposed to

freeze/thaw conditions.

2) Keep surfaces in contact with concrete free of ice& snow, and at a temperature above freezing.

3) Place and maintain concrete at recommended

temperature.

4) Place at lowest possible slump.

5) Protect "plastic" concrete from freezing ordrying.

6) Protect from early-age freeze/thaw cycles untiladequate strength is achieved.

7) Limit rapid temperature changes whenprotective measures are removed


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Underwater Concreting

-Underground construction always poses serious threats to curing of concrete. Forexample, underground curing of concrete in tunnels is always affected by seeping water.

The surface always remains wet and setting of concrete takes time, rendering it weakand unstable.

-Similarly working on a basement or foundation also has moisture issues. Working inwatery conditions requires waterproofing, which increases costs and secondly drying the

area also takes plenty of time.-Similarly, curing of concrete in wet conditions needs special care and treatment; you

cannot just place it and forget about it. Despite the fact that curing is not possiblewithout water, too much water will definitely render the concrete weak and result incracks on the surface. In regions with high rainfall, concrete curing in time becomes very

crucial.-Similarly in wet and waterlogged conditions, curing needs to be done carefully. If thesurface is too wet, the first thing to do is to dry the surface by heating. It may not bepossible to dry the surface completely and that is where the free water to cement ratio

becomes crucial.-This ratio must be set according to the moisture content of the surface because if thewater cement ratio exceeds the desired limits, it will result in capillary pores on the setconcrete, resulting in poor strength and durability. Additives can also be used to speed

up the process of concrete setting

.


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Performance Requirements for UnderwaterConcrete in Structural Applications

Flowability and Self-Compaction

Workability Retention within Work Window Cohesion Against Washout, Segregation, andLaitance Formation Low Bleeding Low Heat of Hydration

Controlled Set Time Compressive Strength Adequate Bond


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Principal Parameters in MixDesign

Particle Packing Characteristics - Sand

Content, Gradation, Size, and Shape The water-to-fine ratio - Enough Fine to MakeItFlowable and Cohesive (0.85-1.0 by volume) Cementittious Material Content High VolumeFly Ash plus Silica Fume

Dispersion characteristics - Proper Use ofChemical Admixtures HRWR and Set-retarder


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Specific Gravity: 2.85Absorption: 1.1%Maximum NominalSize: 3/4-inchAppearance: Cleanand round-shapedwith smooth surfacetexture

Coarse Aggregates

Specific Gravity: 2.72Fineness Modulus: 2.9Absorption: 3.0%

Natural River Sand

Fine Aggregates


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High Volume Fly Ash Concrete for

Underwater Repair

Reducting the heat ofhydration in mass

concrete Increasing concreteflowability withoutcompromising cohesion Facilitating concreteflowability retention and

extended set time


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Coarse Aggregate: Gravel of 3/4" (20mm) max. size.

Use 50-55 % of the total aggregate by weight.

Sand, 45-50% of the total aggregate by weight.

Cement: Type II ASTM (moderate heat of hydration),

600 lbs/yd3Pozzolans: ASTM 616 Type N or F, 100 lbs/ yd3

Water/Cement Ratio: 0.42 (0.45 Maximum).

Water-Reducing Admixture (preferably it is also

plasticizer): Do not use superplasticizers.

Air-Entrainment Admixtures: To give 6% total air.Retarding Admixture: To increase setting time to 4-24

hours, as required.

Slump: 6 1/2" 1"

This mix will develop compressive strength in the

range of 5,600 - 7,000 psi(pounds per square inch) at 28days.

For Structural concrete following must be

considered


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Tremie method

Method on how to place concrete underwater,

in offshore concreting, since cement looses itsstrength and fade away under water.Tremie Concrete is done by using aformwork/pipe which will have one end of theformwork/pipe above water and other bottomend immersed under the water and with the

help of gravity.

A tremie is a watertight pipe, generally 250mmin diameter, having a funnel shape hopper at itsupper end and a loose plug at the bottom ordischarge end.

The valve at the discharge end is used to de-water the tremie and control the distribution ofthe concrete.The tremie is supported on a working platformabove water level, and to facilitate the placing itis built up in 1 to 3.5m section.


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Tremie method

During the concreting, air and water must be exclude from the

tremie by keeping the pipe full of concrete all the time.In charging the tremie a plug formed of paper is first inserted intothe pipe as the hopper is filled the pressure of fresh concreteforces the plug down the pipe, and the water in the tremie Idisplaced by concrete.

For concreting, the tremie pipe is lowered into position and thedischarge end is kept as deeply submerged beneath the surface offreshly placed concrete as the placed concrete as the head ofconcrete in tremie permits.As concreting proceeds the pipe is raised slightly and the concreteflows outwards.Care should be taken to maintain continuity of concreting withoutbreaking the seal provided by the concrete cover over thedischarge end.. The tremie should never be moved laterally though freshlyplaced concrete.It should be lifted vertically above the surface of concrete andshifted to its new position.


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Tremie method

When large quantities of concrete are tobe placed continuously, it is preferable toplace concrete simultaneously anduniformly through a battery of tremies,rather than shift a single tremies from

point to point.It has been recommended that thespacing of tremies be between 3.5 and 5mand that the end tremies should be about2.5m from the formwork.The risk of segregation and non-uniform

stiffening can be minimized by maintainingthe surface of concrete in the forms aslevel as possible and by providing acontinuous and rapid flow of concrete.

3-5 Times Depth of Tremie Pours

P P k d


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Pre Packed concrete

Description: Placing coarse aggregate inthe form and filling the voids withgrout. It is used on large repair jobs, andusually grout is pumped through groutpipes from the bottom up

Uses: Prepacked concrete is used whereplacement of cast-in-place concrete isnot practical. It is also used underwaterwhere the tremie or pumped methods are

not practical due to inaccessibility. It issuitable for vertical surface repairs thathave a minimum thickness of 8 to 10 cm (3to 4 inches)


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Thank you.

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