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

7/16/2019 Presentation- Ground Slabs-Post-Tensioned Prestressed Concrete

http://slidepdf.com/reader/full/presentation-ground-slabs-post-tensioned-prestressed-concrete-56338564a8149 1/50

Post-Tensioned

Prestressed Concrete

Post-Tensioned

Slab-on-Ground

Construction



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 reinforced concrete 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.



Occupancy Loads

T e n s i o n T e n s i o n

f t = Mc /I or M/Sb

How Prestressing Works

The Basics

Edge Lift Edge Lift



f t = M/Sb – (P/A)

CompressionForce Force

How Prestressing Works

For Slabs-on-Ground



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



• Single Family Residential

• Multi-Family Residential

• Commercial / Industrial

• Sports Courts

Applications of Post-TensioningSlab-on-Ground Foundations



Post-Tension Steel

Tonnage Comparison

0

10000

20000

30000

4000050000

60000

70000

80000

1 9 9 4

1 9 9 5

1 9 9 6

1 9 9 7

1 9 9 8

1 9 9 9

2 0 0 0

2 0 0 1

2 0 0 2

2 0 0 3

Buildings SOG Bridges R&S Anchors



Foundations TypesClassified by BRAB Report 33

In 1962, The Building Research Advisory

Board (BRAB) issued Report 33 which

classified residential foundations into four design 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



The PTI design

procedure for expansive soil is

based on a ribbed

slab layout.

PTI Design Procedure

Slab-on-Ground Foundations



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



Site Inspection

Inspect site to look for unusual conditions

• Drainage ditches or low areas that hold water

• Trees that can influence soil moisture

• Anything that appears “out of the ordinary”

• Contact the geotechnical and structural engineer for recommendations



Site Preparation

Strip Site of Organics & Trash

The site should be

initially stripped of all

surface vegetation and

other deleterious

material.



Site Preparation

Recompact Scarified Surface Material

The exposed

subgrade shouldbe scarified and

recompacted.



Site Preparation

Identify Voids & Recompact

Remove trees,

including theroot system

Proof roll the

site to identify

any loose soil



Site Preparation

Drainage

Grade the lot for

positive drainageaway from the

foundation during

and after

construction.



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.

READ the 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.



Components of an

Unbonded PT System

• Unbonded Strand

• Anchorages

• Accessories



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



Anchorage System

Anchor

2-pc Wedge



Standard System

Fixed End Anchorage



Standard System

Stress End Anchorage

Anchor Pocket Former



Standard System Assembly



Stressing Anchorage

Incorrect Correct



Stressing Anchorage

• Replace Sheathing

• Tape

• 1 inch Rule

Tendon Sheathing Repair



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 securely attached to edge forms



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 securely attached to edge forms



Installation

Horizontal transitions

are smooth, clear all

openings by 3 inches

and are straight past

the opening



Chairs are placed

and tied at each

tendon intersection atless than 3’-6”

centers

Installation

Chair Placement



Installation

Chair Placement

IncorrectCorrect



Installation

Draped tendons are used in some designs to

provide compression in the bottom of beams

resisting tensile stresses caused from edge lift.



Installation

Drops in Slabs



Plumbing Blockouts

• Deflect tendons toavoid blockoutsthrough the slab.

• Maintain minimum of 3” concrete cover between tendon andblockout



Plumbing Blockouts

Corner blockouts

can cause

concreting

difficulties. Rebar

solutions can

alleviate the

congestion and

prevent corner failures



Anchorage Zone Penetrations

Penetrations through the anchorage zone

should be sleeved with schedule 40 pipe.



Reentrant Corners

Initial curing tensile

stresses build-up at

reentrant (inside)corners causing

cracking to occur.

Rebar is typically

installed at these

locations.



Stressing

• Initial Concrete Strength (2,000 psi MIN)

• Initial Marking (Elongations)

• Calibration of Jack/Gauge

• Stressing Force vs. Gauge Pressure

• Final Elongation Measurement



Cutting Stressing Tails



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.



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



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



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.



Pour Inspection

DO NOT displace or walk on any of the reinforcement

- P-T or rebar



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



Trouble-Shooting

• Honeycombs

• Low strength and poor quality concrete

• Plumbing mistakes



Trouble-Shooting

Correct Incorrect



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 tree’s drip line

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



The long-term performance of any

slab-on-ground foundation is dependent upon good drainage and a moisture

maintenance program by the property

owner.

Performance



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



Construction & Maintenance

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

The 3rd Editions is currently being reviewed by committee and will be available in thenext few months.

602-870-7540

www.post-tensioning.org



Design Procedures & Code

602-870-7540

www.post-tensioning.org

New in 2005