Assessment and Restoration of

Post-tensioned Waffle Slabs in Parking

Ramp Structures

Gabriel A. Jimenez, Ph.D., P.E., S.E.

Principal/Managing Director

Walter P Moore

• Longer spans and/or heavier loads requires increasing the slab thickness in order to limit deflections

• Dead load becomes the major portion of the design load

• Economical reasons: rib the slab in both directions to provide greater stiffness with less weight

Motivation

• Rows of concrete ribs at right angles to each other with solid heads at the columns.

• Square pans or dome forms to create the void, or recess, between the ribs.

• Standard dome forms: 24-inch or 36-inch module with sloping sides forming a bevel of 1 ½-inch/ft from the vertical.

• Use: Office buildings, parking structures, warehouses

Waffle slabs

• Top of slab reinforcement: WWF or conventional reinforcement

• PT reinforcement started in the 1960s – Monostrand

Post-tensioned waffle slabs

Susceptible to corrosion

Courtesy of Adapt technical note

• Concrete deterioration generally signals the onset of accelerated distress

• Symptoms of concrete deterioration: – spalling, cracking, scaling, rust staining and abrasion

• Deterioration mechanisms primarily responsible: – Corrosion Related Distress – Cracking – Material Related Degradation – Others

Concrete deterioration

• Concrete is a naturally porous material

• Concrete pores have diameters ranging from 15 - 1,000 A (10 millionths of mm)

• Chloride ion diameter is less than 2 A

• All concrete is susceptible to chloride ion contamination by virtue of its natural porosity

Chloride contamination

MOISTURE AND

CHLORIDE ION

ROAD SALT

RUST BUILD-UP

CAUSING

PRESSURE

DELAMINATION

OR FRACTUREOPEN SPALL

SALT WATER

CONTINUOUS

CORROSION

• Corrosion - dynamic electro-chemical process & induces progressive deterioration

• Corrosion by-products (rust) - 5 to 8 times larger (volume) than the parent metal

Corrosion

MOISTURE AND

CHLORIDE ION

ROAD SALT

RUST BUILD-UP

CAUSING

PRESSURE

DELAMINATION

OR FRACTUREOPEN SPALL

SALT WATER

CONTINUOUS

CORROSION

Areas of vulnerability

Areas of vulnerability

Typical deterioration

Testing

Instrumental Testing Acoustic testing/hammer sounding Impact-echo Impulse response Covermeter Ground-penetrating radar Rebound hammer Corrosion potential testing Acoustic monitoring

Material Testing Compressive strength Petrographic examination Chloride content Carbonation Metallurgical and physical examination of the prestressing steel strand

Ground Penetrating Radar

Screwdriver penetration test

Exploratory openings

Exploratory openings

Core sampling and corrosion potential

Analysis

Repair strategies • No tendon repair

Insignificant strand deterioration no loss of prestressing force

• Selective abandonment Tendon is severely

deteriorated/damaged No repair needed based

on a design having greater design capacity than current code

• Repair Severe deterioration –

loss of prestressing force

• Tendon supplement/replacement

Existing tendons insufficient for current code/use

Requires installation new tendons

• Selective/Full structure replacement

Insufficient capacity or significant deterioration

• Stabilization/Strengthening

Restoration of structural capacity by external post-tensioning

Drawings and specifications

Repairs

Punching shear

Tendon splice repair

• Connects 2 sections

• Steel cylinder with spring-loaded wedges

Center stressing splice • Stressing/splicing from an intermediate location

• Sometimes in combination with splice coupler

• Cast metal with pre-molded wedges

Center stressing splice

Sheathing repair • All damaged sheathing must be repaired

• Sleeves/waterproofing tape/coatings suitable for PT applications

Drying and regreasing

Procedure developed for the push-through system

Dries annular space between tendon and sheathing

Drying and regreasing

Thank you for your attention

Gabriel A. Jimenez, Ph.D., P.E., S.E.

Walter P Moore

www.walterpmoore.com

713-630-7485 gjimenez@walterpmoore.com