1Texas Use of Precast ConcreteStay-in-Place Forms for Bridge

Decks

Brian D. Merrill, P.E.Texas Department of Transportation

Transverse Section

Precast Concrete Panels (PCPs)

4 CIP Concrete 4 PCP

2PCP Usage

Developed in 1963 Full scale use started in 1983 when a pay

item change went into effect and standard drawings were offered

Approximately 85% of all deck on stringers are built using this system

PCPs: How they work

Plans give details for full-depth CIP deck If PCP is authorized, PCP standard sheets (3

sheets inserted into plans) give all details for panel fabrication, panel placement, and reinforcing adjustments to CIP portion

3Typical Plan Detail for Full-Depth CIP Slab

PCP Standard Detail

41 1/21/4 Min

Panel Support Requirements

Advantages of Using PCPs

Speed Cost savings Safety Design

5PCP Advantage: Speed

Deck grading and forming can be completed in a few days

Placement of rebar takes half the time compared to full CIP deck

Reduced form removal (overhangs) time Can pour greater deck area in the same time

(PCPs take up half the deck volume)

PCP Advantage: Cost

Time = $$$ Form Setting Placement of Reinforcing Steel Less Time Working over Traffic Reduced Form Removal Time (overhangs) Lower Equipment Costs for Concrete

Placement Insurance

6PCP Advantage: Safety

Very stable in high winds (3000 lbs) Cannot fall between girders Only overhang forms need to be removed

PCP Advantage: Design

Prestressed Steel in Slab M+ Areas PCPs use 5000 psi concrete compared to

4000 psi for CIP concrete Excellent quality control on PCP fabrication

7Typical Plan Detail for Full-Depth CIP Slab

Design for Full-Depth CIP Slab

4000 psi Concrete, Grade 60 Steel 8 Thick Transverse Reinf: #5s @ 6 T&B Beam in Flexure Design: fs

8Design for PCPs

Standard details apply over the same span limits as the full-depth CIP slab (8.7)

M+ resisted by PCP/CIP composite section M- resisted by #5s @ 6 5000 psi concrete 3/8 Gr 270 Strands @ 6, mid-depth of 4

panel; stressed to 16.1 kips

Design for PCPs - Cont.

Panel tensile stress (ft) limited to 6f 'c Ultimate Flexural Capacity of PCP/CIP

Composite Slab (fsu) = (Panel Length)/2D + 2fse/3

#4 bars can be used for panels < 5 long Panels < 3.5 must use #4 bars Standards cover PCP placement on skews,

phased construction, & flared girders

9Note #3 bars

Limitations on PCPs

Curved Steel Plate Girders Widenings: not allowed in the bay adjacent

to existing structure Phased Construction: not allowed in the bay

adjacent to previous phase* Steel girders with narrow (

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Standard Detail for Phased Construction

No Panel here

Stage I Stage II

Header

Stage I Stage II

Modified Detail to Allow PCPs for Phased Construction

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PCPs: Keys to Success

Panel Fabrication Grading and Setting of Panels Concrete Placement

PCP Fabrication

3 Plants in Texas* Casting beds are 8 wide, 350 to 500 long Strands extend 3 on each side of panel 5000 psi concrete required but usually get

4000 psi in 14 hours using Type III cement Largest fabricator can produce up 300

panels a day

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Grading and Setting PCPs

Critical operation: must ensure adequate depth of CIP and rebar cover

Must ensure that panel support conditions are met mortar

Must account for girder deflection differences between interior/exterior girders

Often requires regrading of deck profile(up to 2 overpour allowed)

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1 1/21/4 Min

Panel Support Requirements

Differential Girder Deflections

50% 50%50% 25%25%

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Concrete Placement

Panels must be wetted (SSD) prior to concrete placement

Vibration along girder lines is crucial

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PCP Research

Test of Precast Prestressed Concrete Bridge Deck Panels 1963 In-house study

Study of In-Service Bridges Constructed with Prestressed Panel Sub-Decks, Project 145-1, 1970, Texas Transportation Institute (Texas A&M)

Development Length of Strands in Prestressed Panel Sub-Decks, Project 145-2, 1970, TTI

PCP Research - Cont.

Evaluation of a Prestressed Panel, Cast-In-Place Concrete Bridge, Project 145-3, 1972, TTI

Cyclic Load Tests of Composite Prestressed-Reinforced Concrete Panels, Project 145-4F, 1972, TTI

The Effect of Transfer Strand Extensions on the Behavior of Precast Panel Bridges, Project 303-1F, 1982, Center for Transportation Research (Univ. of Texas)

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Research Findings

Used to develop current standard details Load transfer of PCP deck compared favorably to

full CIP deck PCP/CIP deck composite action with prestressed

girders was verified Panel support conditions were verified the precast panel deck was stronger, stiffer,

and more crack-resistant than a CIP deck.

Problems

Longitudinal Cracking Transverse Cracking Deck Thickness/Rebar Cover Grading & Yield

Texas has had relatively few problems using PCPs

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Longitudinal Cracking Can reduce deck stiffness Cracks could expose M- steel to corrosive

elements Insufficient panel support (mortar) has

contributed to all major longitudinal cracking

Minor longitudinal cracking has been attributed to drying shrinkage of CIP concrete

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2 1/2 CIP Slab(3 1/4 per plans)

1 +/-

Major LongitudinalCracking

Original Panel Bedding Material:Preformed Bituminous Fiber Material

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New Panel Bedding Material:

High-Density ExtrudedPolystyrene Foam

ASTM C-578, Type VI40 psi*

Minor Longitudinal Cracking

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Transverse Cracking

Attributed to drying shrinkage of CIP concrete

Not considered to be a significant problem

PCP PCP

CIP Conc.

Transverse Cracks

Roughened Surface

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Grading and Yield

Panels can be difficult to grade if girder cambers vary widely - the high girder controls the grade

Yield (volume of concrete/square foot) can be affected if regrading is necessary

Conclusions

The PCP system is very popular with Texas contractors

Speed is the primary advantage realized by using the PCP system

The benefits of the PCP system far outweigh its limitations and the few negative consequences