Download pdf - Presentation- Ground Slabs-Post-Tensioned Prestressed Concrete

5/20/2018 Presentation- Ground Slabs-Post-Tensioned Prestressed Concrete

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Post-Tensioned

Prestressed Concrete

Post-Tensioned

Slab-on-Ground

Construction


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Reinforced vs Plain Concrete

It is important to recognize the difference between

reinforced and plain concrete when referring to post-

tensioned or conventionally reinforced concrete slabs-

on-ground. The American Concrete Institute (ACI)

defines reinforcedconcrete as concrete designed to

satisfy the minimum requirements of the code. The

category of plain concrete was developed by ACI to

refer to concrete used in ground supported construction

where loads are light, stresses are low, life safety

concerns are minimal or non-existent and the minimumrequirements for reinforced concrete are not necessary.


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Occupancy Loads

Tension Tension

ft= Mc /I or M/Sb

How Prestressing Works

The Basics

Edge Lift Edge Lift


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ft= M/Sb (P/A)

CompressionForce Force

How Prestressing Works

For Slabs-on-Ground


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Advantages of Post-Tensioning

Inherent compressive strength of concrete

Less structural depth to achieve same design

strength

Economical use of building materials

Speed of construction


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Single Family Residential

Multi-Family Residential

Commercial / Industrial

Sports Courts

Applications of Post-TensioningSlab-on-Ground Foundations


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Post-Tension Steel

Tonnage Comparison

0

10000

20000

30000

4000050000

60000

70000

80000

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Buildings SOG Bridges R&S Anchors


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Foundations TypesClassified by BRAB Report 33

In 1962, The Building Research Advisory

Board (BRAB) issued Report 33 which

classified residential foundations into fourdesign categories based upon the degree

of severity of the supporting soil:

Type I: Unreinforced

Type II: Lightly Reinforced

Type III: Reinforced and Stiffened

Type IV: Structural


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The PTI design

procedure forexpansive soil is

based on a ribbed

slab layout.

PTI Design Procedure

Slab-on-Ground Foundations


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The key-word

is Slab-on-GROUNDGROUND

Slabs-on-Ground are designed to meet aspecific set of soil parameters.

The performance of ANY foundation isdependent up on obtaining accurate soilinformation about the site.

Expansive Soils

Plasticity Index (PI) Greater than 15 Expansion Index (EI) Greater than 20

Em, Ym, Bearing Capacity


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Site Inspection

Inspect site to look for unusual conditions

Drainage ditches or low areas that hold water

Trees that can influence soil moisture

Anythingthat appears out of the ordinary

Contact the geotechnical and structural engineerfor recommendations


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Site Preparation

Strip Site of Organics & Trash

The site should be

initially stripped of all

surface vegetation and

other deleterious

material.


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Site Preparation

Recompact Scarified Surface Material

The exposed

subgrade shouldbe scarified and

recompacted.


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Site Preparation

Identify Voids & Recompact

Remove trees,

including theroot system

Proof roll the

site to identify

any loose soil


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Site Preparation

Drainage

Grade the lot for

positive drainageaway from the

foundation during

and after

construction.


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Site Preparation

Compaction tests should be performed on all fill material used

during the site development phase. The quality, as well as the

compaction, of all fill material should be documented.

Fill should exhibit low expansion properties, be free of organicsand other deleterious material, and be compatible with the

existing soil characteristics.

READthe General Notes sheet prepared by the structural

engineer. This sheet may contain special instructions about

specific site preparation requirements.

Contact the structural engineer should anything be unclear or in

question.


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Components of an

Unbonded PT System

Unbonded Strand

Anchorages

Accessories


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Unbonded P-T Strand

Corrosion Protection "Grease"

High Density

PolyethyleneSheathing

P-T Coating

7 wire Steel

Strand

Unbonded strand consists of

three main components:

Prestressing Steel

PT Coating

Sheathing


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Anchorage System

Anchor

2-pc Wedge


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Standard System

Fixed End Anchorage


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Standard System

Stress End Anchorage

Anchor Pocket Former


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Standard System Assembly


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Stressing Anchorage

Incorrect Correct


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Stressing Anchorage

Replace Sheathing

Tape

1 inch Rule

Tendon Sheathing Repair


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Installation

Follow approved engineers drawings

Anchors are placed 6 inches from edge

Anchors are placed at T/2 or 3 below top of slab

Anchors are securelyattached to edge forms


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Installation

Follow approved engineers drawings

Anchors are placed 6 inches from edge

Anchors are placed at T/2 or 3 below top of slab

Anchors are securelyattached to edge forms


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Installation

Horizontal transitions

are smooth, clear all

openings by 3 inches

and are straight past

the opening


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Chairs are placed

and tied at each

tendon intersection atless than 3-6

centers

Installation

Chair Placement


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Installation

Chair Placement

IncorrectCorrect


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Installation

Draped tendons are used in some designs to

provide compression in the bottom of beams

resisting tensile stresses caused from edge lift.


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Installation

Drops in Slabs


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Plumbing Blockouts

Deflect tendons toavoid blockoutsthrough the slab.

Maintain minimum of3 concrete coverbetween tendon andblockout


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Plumbing Blockouts

Corner blockouts

can cause

concreting

difficulties. Rebar

solutions can

alleviate the

congestion and

prevent cornerfailures


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Anchorage Zone Penetrations

Penetrations through the anchorage zone

should be sleeved with schedule 40 pipe.


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Reentrant Corners

Initial curing tensile

stresses build-up at

reentrant (inside)corners causing

cracking to occur.

Rebar is typically

installed at these

locations.


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Stressing

Initial Concrete Strength (2,000 psi MIN)

Initial Marking (Elongations)

Calibration of Jack/Gauge

Stressing Force vs. Gauge Pressure

Final Elongation Measurement


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Cutting Stressing Tails


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Finishing Stressing Recess

Clean the pocket former recess of any

dirt, grit, oil or other material that will

prevent the grout from bonding to the

concrete

Grout pocket former recess with non-

shrink cement, sand and water mix that

will reach the minimum compressive

strength of the concrete slab.


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Inspections

Inspections should be conducted to insure

the quality of the construction.

Pre-pour: Installation of P-T and rebar

Pour: Placement of Concrete

Stressing: Tensioning of P-T Tendons


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Pre-Pour Inspection

Check the tendon and bar count

Check the placement of the fixed-end anchors for

the required cover over the end of the steel

Check to be sure that the stressing ends are

securely nailed to the forms (2 nails) and that the

tendon is perpendicular to the anchor

Check to be sure that you have adequate

concrete cover over all of the steel


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Pour Inspection

Do not add excessive water at the site

Place the concrete in one continuous

operation - NO COLD JOINTS

Consolidate (vibrate) the concrete around the

P-T anchors, especially in the corners where

multiple anchors are located.


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Pour Inspection

DO NOTdisplace or walk on any of the reinforcement

- P-T or rebar


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Post-Concrete Placement

Notify the P-T stressing company the day after

the concrete is placed to schedule stressing

Remove the forms - ALL OF THEM. Do thiswithin 3 days after the concrete is placed

Do NOT damage stressing tails in rough grade

Do NOT cover tendon tails with dirt or lumber


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Trouble-Shooting

Honeycombs

Low strength and poor quality concrete

Plumbing mistakes


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Trouble-Shooting

Correct Incorrect


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Foundation Maintenance

Do not alter the drainage pattern of the site

Provide a minimum of 3%-5% of slope away fromthe foundation with the first 5 feet

Roof drains should not dischargewater at the perimeter of thefoundation

Do not plant trees within the trees drip line

Do not cut off natural moisture around thefoundation by constructing decks or pools


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The long-term performance of any

slab-on-ground foundation is dependentupon good drainage and a moisture

maintenance program by the property

owner.

Performance


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PTI Certification Program

Consistency of material quality

Quality of extrusion and fabrication process

Traceability of components

Stressing equipment calibration

Company commitment to the long termperformance of the system


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Construction & Maintenance

More information concerning theconstruction & maintenance of post-tensioned slabs-on-ground, can be found ina manual published by the Post-TensioningInstitute.

The 3rdEditions is currently being reviewedby committee and will be available in thenext few months.

602-870-7540

www.post-tensioning.org


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Design Procedures & Code

602-870-7540

www.post-tensioning.org

New in 2005