IntroductionIntroduction AdvantagesAdvantages Dis-advantagesDis-advantages Methods of Pre-stressingMethods of Pre-stressing Mix DesignMix Design CuringCuring Laboratory TestLaboratory Test Visit Report Visit Report

Pre cast concrete members are assembled and fastened together on the jobsite .

They may be unreinforced ,reinforced or prestressed .

Precasting is beneficial because higher quality control and higher strength concrete.

Prestressing is the deliberate creation of internal stress in a structure or system to

improve its performance.

Very fast Construction.Very fast Construction. Less Man Power.Less Man Power. High thermally insulatedHigh thermally insulated

Better shear resistanceBetter shear resistance High strength steel and concrete High strength steel and concrete

PC : 20-35 % reinforcement in RCPC : 20-35 % reinforcement in RC Lighter section , Longer spansLighter section , Longer spans

PC depth ~ 65-80% of RC depthPC depth ~ 65-80% of RC depth

Lighter foundations possibleLighter foundations possible Crack lessCrack less

Better corrosion resistance, better durabilityBetter corrosion resistance, better durability Deflection controlDeflection control

Higher material costs.Higher material costs. Pre-stressing is an added cost.Pre-stressing is an added cost. PC Form work is more complex than RC PC Form work is more complex than RC PC is Not as ductile as RC.PC is Not as ductile as RC.

Prestressing is applied to concrete by means of Prestressing is applied to concrete by means of high strength tendons (usually steel) in tension, high strength tendons (usually steel) in tension, passing through the concretepassing through the concrete..

This can be achieved in one of two ways, namely This can be achieved in one of two ways, namely pretensioning and post tensioning, the main pretensioning and post tensioning, the main distinction between the two methods being distinction between the two methods being weather the steel is tensioned before or after the weather the steel is tensioned before or after the concrete is cast, hence the terms “pre” and concrete is cast, hence the terms “pre” and

“post“post.”.”

Pre-tensioning :Pre-tensioning : Steel tensioned before casting of concrete.Steel tensioned before casting of concrete.

Strands are tensioned – concrete is cast around the Strands are tensioned – concrete is cast around the strands- strands are released when concrete attains strands- strands are released when concrete attains required strength required strength..

Summary :Summary :

1.1. Stress strands .Stress strands .

2.2. Cast concrete .Cast concrete .

3.3. Cut strandsCut strands .

4. Force transferred to concrete by bond .

Post-tensioning :Post-tensioning :

Steel tensioned after concrete is cast .Steel tensioned after concrete is cast .

Post-tensioning is primarily an in situ operation uese Post-tensioning is primarily an in situ operation uese in large span bridges.in large span bridges.

Prestress force is transferred to concrete via the Prestress force is transferred to concrete via the end anchorage .end anchorage .

Summary:Summary:

1.1. Provide tendon inside duct.Provide tendon inside duct.

2.2. Cast concrete.Cast concrete.

3.3. Tension tendon after concrete attains required Tension tendon after concrete attains required strength.strength.

4.4. Inject grout into duct.Inject grout into duct.

All precast elements prestressed and nonprestressed elements are cured by steaming.

The reason for curing by steaming is to accelerate the curing by heating without drying.

The process takes place by covering the elements with canvas and then leading steam from a boiler plant to the elements under the canvas. The steam curing cycle is given on the below :

(1) Pre-steaming period. Must never last less than 3 hours.Too early steaming of fresh concrete can adversely affect the final strength of the concrete.

(2) Temperature rise period. The length of this period is until max temperature is reached. With an idealized temperature rise rate of 20 deg C per hour it will typically last 2 - 3 hours.

(3) Period at max temperature. The length of this period is practically until the needed concrete strength for demoulding / detensioning is reached. Typically 6 hours for nonprestressed concrete and 12 hours for prestressed concrete.

)4 (Cooling period. The cooling period before uncovering of canvas should for good economy and to avoid the risk of cracks last at least one hour. However if needed for recast and if experience says that it can be done without risk of cracks then the period can be shortened down.

Procedure for steam curing

Compression test :Compression test :

Shear test :Shear test :

Aggregates properties test :Aggregates properties test :

Company system:Company system:

1.1. SalesSales

2.2. EstimationEstimation

3.3. ContractingContracting

4.4. DesignDesign

5.5. ApprovalApproval

6.6. FabricationFabrication

7.7. DeliveryDelivery

8.8. ErectionErection

Some of the company products:Some of the company products:

1.1. Hollowcore slab Hollowcore slab

2.2. Double tee(TT) slabDouble tee(TT) slab

3.3. ColumnsColumns

4.4. BeamsBeams

5.5. WallsWalls

HCS 150

HCS 185

HCS 200

HCS 265

HCS 380

HCS 320

HCS 300

HCS 400

LOAD BEARING CAPACITY GRAPHLOAD BEARING CAPACITY GRAPH

DEFLECTION

LOAD BEARING CAPACITY GRAPHLOAD BEARING CAPACITY GRAPH

DEFLECTION