distress, evaluation, and repair of historic concrete jarkko simonen, p.e. wiss, janney, elstner...
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Distress, Evaluation, and Repair of Historic Concrete
Jarkko Simonen, P.E.Wiss, Janney, Elstner Associates, Inc.
Introduction
• History• Common Distress Mechanisms– Corrosion– Freeze Thaw– Material problems
• Analysis or Evaluation Methods• Repair
Introduction
• Cement has a long history– Certain types of cement have been
used by the Babylonians, Egyptians, and Romans
– Portland cement 1824– Reinforced concrete 1867
• In the northwest one of the early examples of concrete construction is Fort Casey - 1890
• Generally the use of concrete became common after about 1900
Concrete Vintage Generalizations
Older Concrete (1900 to 1945)– Multiple layers– Placement in lifts– Higher w/c (lower f ’c)– Early reinforcing
systems– Carbonation– No air entrainment
Modern Concrete (1945 to present)– Homogenous pour– Improved placement– Lower w/c (higher f ’c)– Modern reinforcing– Carbonation– Air entrainment?– Admixtures
Introduction
Environment• Wet• Cold• CoastalDeterioration due to the environment• Corrosion• Freeze thaw
Corrosion
• Common in environments that contain salt and moisture• Distress manifests as
staining, cracking, and spalling of the concrete
Concrete provides a great environment for steel against corrosion
Concrete
Problem with rust• Corrosion of the rebar causes rust to form• Rust is 6 to 10 time less dense than steel • Increased volume causes concrete to crack
• Destroys natural passive oxide layer provided by portland cement paste
• Complicated chemical interactions• Hygroscopic
Effects of Chloride Contamination
Chloride Contamination
• Environment (soils, sea water)• Applied (deicing chemicals)• Integral (admixtures, aggregate, water)
CarbonationCarbonation
CO2 + H2O + Ca(OH)2 CaCO3 + H2Oatmospheric rain cement paste limestone water
CO2
NON-CARBONATEDCONCRETE
/ H O2
DEPTH OFCARBONATION
Carbonation
• Advances about 1 mm per year in normal concrete• Once carbonation
reaches steel, the steel is unprotected • Corrosion can affect
large areas
Freeze-thaw weathering regions (ASTM C33)
Freeze-thaw
• Damages the near surface region of concrete• Surface flakes off typically in horizontal layers• More aggressive if surface is exposed to chlorides
Air entrainment can be effective in mitigating freeze-thaw
ASR - Reaction between silica and hydroxyls (OH-) in the pore solution, forming silica gel. As the gel forms, it absorbs water and expands.
OH -
OH -
OH -
OH -
OH -
OH -
OH -
ALKALIS SILICA GEL (REACTION RIM)
SURROUNDINGPASTE
COARSEAGGREGATE
FINEAGGREGATE
ALKALIS SILICA GEL (REACTION RIM)
EXPANSIONCRACK
GEL ENTERINGCRACK
SURROUNDINGPASTE
COARSEAGGREGATE
FINEAGGREGATE
RADIALCRACK
Alkali Silica Reaction• ASR must have all three
components present to cause a problem‐ Reactive aggregates‐ Abundance of alkalis‐ Water
• In the northwest we have two out of three‐ Reactive Aggregates‐ Water‐ Generally cement has low
alkalinity• ASR aggravates F/T and corrosion
Assessment Methods
Field• Visual survey• Mechanical sounding survey• Corrosion assessment
‐ Half-cell potential measurements ‐ Linear Polarization Resistance Method
• Other non-destructive methods (rebound hammer, impact-echo, pulse velocity, etc.)
Assessment Methods
Laboratory• Chloride content• Depth of Carbonation (Phenolphthalein indicator)• Petrographic examination
Condition AssessmentsReferences• ACI 201.1R Guide to Making a Condition Survey of Concrete in Service• ACI 224.1R Causes, Evaluation and Repair of Cracks in Concrete
Structures• ACI 364.1R Guide for Evaluation of Concrete Structures Prior to
Rehabilitation• ACI 437.1R Strength Evaluation of Existing Concrete Buildings• ACI 546R Concrete Repair Guide• Technical Guidelines by International Concrete Repair Institute (ICRI)• Guide to Nondestructive Testing of Concrete• Others
Visual Survey• Identify distress mechanisms• Repair quantities• Repair locations/typesMechanical Survey• Identify hidden distress• Dislodge dangerous fragments• Gives you a better feel about the
concrete quality
Corrosion SurveysHalf cell surveys
– Identify potential areas of corrosion
– Repair quantities– Repair locations/types
Linear Polarization
– Corrosion rate/aggressiveness
NDT Investigative Techniques• Impact echo• Pulse velocity• GPR• Magnetic rebar locators
Laboratory Analysis
• Initial opinion of deterioration and conditions• Type of concrete exposure• Discuss testing with petrographer/chemist• Repair type being considered• Type of testing and expected results
Evaluation for Chlorides
Two methods• Cores• Drill/powder samples
Testing• ASTM C 1152: Acid-
Soluble Chloride in Mortar and Concrete
• ASTM C 1218: Water-Soluble Chloride in Mortar and Concrete
Evaluation for Carbonation
Petrography • ASTM C856: Standard Practice for
Petrographic Examination of Hardened Concrete – Freeze thaw– ASR– Finishing Problems– Identify substrate materials– Etc.
Repair
Concrete repair• Protection/Mitigation• Patch repairs• Reconstruction
Protection/Mitigation
• Coatings– Prevent moisture
• Electrochemical treatments– Cathodic protection– Chloride extraction– Re-alkalinization
• Other – Sealers
• Silanes• Siloxanes
– Migratory corrosion inhibitors
Patch Repairs
Considerations• Compatibility– Strength– Wear– Thermal
• Appearance– Color– Texture– Finish– Profile
Repairs should blend in
Blending Repairs
• Lift lines• Form board lines• Color• Texture
• Surface Preparation– Saw cuts– Rectangular
• Installation – Dry as possible
• Finishing• Curing
Samples and Mock-ups
• Cleaning• Coating removal• Color• Finish• Texture• Surface preparation• Design mix• All installation and finishing
procedures and• Techniques
Repair
Special Considerations with Historic/Architectural Concrete• Tasks are similar of work with other concrete• Options may be more limited• Rules of good concrete repair practice apply• Original design may need to be improved• Emphasis is needed on investigation, laboratory
analysis, samples, mock-ups, and trial repairs
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