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 • Coastal Deterioration 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 Effects of Chloride Contamination • Destroys natural passive oxide layer provided by portland cement paste • Complicated chemical interactions • Hygroscopic Chloride Contamination • Environment (soils, sea water) • Applied (deicing chemicals) • Integral (admixtures, aggregate, water) Carbonation C O2 / H 2O DEPTH OF C A R B O N A T IO N N O N -C A R B O N A T E D CONCRETE CO2 + H2O + Ca(OH)2 atmospheric rain cement paste CaCO3 + H2O limestone water 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 OH - ASR - Reaction between silica and hydroxyls (OH-) in the pore solution, forming silica gel. As the gel forms, it absorbs water and expands. COARSE AGGREGATE OH - OH - A L K A L IS S IL IC A G E L (R E A C T IO N R IM ) OH - OH - OH - OH - S U R R O U N D IN G PASTE F IN E AGGREGATE R A D IA L CRACK COARSE AGGREGATE E X P A N S IO N CRACK G E L E N T E R IN G CRACK A L K A L IS S IL IC A G E L (R E A C T IO N R IM ) S U R R O U N D IN G PASTE F IN E AGGREGATE 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 Assessments References • 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/types Mechanical Survey • Identify hidden distress • Dislodge dangerous fragments • Gives you a better feel about the concrete quality Corrosion Surveys Half 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: AcidSoluble Chloride in Mortar and Concrete • ASTM C 1218: WaterSoluble 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 ?