Materials Conservation – Concrete and Plaster
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Materials Conservation – Concrete and Plaster Carolyn L. Searls Senior Principal, Simpson Gumpertz & Heger Matthew Bronski Associate Principal, Simpson Gumpertz & Heger California Preservation Foundation Webinar 16 September 2014 www.sgh.com www.sgh.com 1 Learning Objectives • Understand the composition and properties of cementitious materials. • Recognize signs of concrete and plaster deterioration and their causes. • Learn reactive and proactive repair techniques for concrete and plaster. • Learn procedures for repairing uncoated, integrally colored plaster and concrete. 2 The Materials: Plaster, Concrete and Cast Stone • All are made up of binder + aggregates + water • Concrete = Cement (the binder), fine aggregate, coarse aggregate, admixtures and water • Cast Stone = Cement, fine aggregate, crushed rock, admixtures and water, precast in units • Plaster = Mortar applied to a substrate (masonry, concrete, or lath over sheathing or building paper) as a wall finish. Usually cement and/or lime (binder), fine aggregate, admixtures and water 3 Evaluation and Repair of Concrete and Plaster • Diagnose the problem – field investigation, lab testing, analysis and report. The treatment must be appropriate for the illness. • Design the repair – the right team • Select the correct materials • Construct field mock-ups • Select the right contractor • Provide quality control during repairs 4 Concrete • Roman concrete structures – lime putty + pozzolan (volcanic tuff rock at Pozzuoli) + aggregates = hydraulic concrete • Natural cement concrete – Erie Canal (1825), Brooklyn Bridge • Portland cement concrete – use in military fortifications in 1880’s in US • Reinforced concrete – 1900 – variety of bar types and placement strategies 5 Overview of Concrete as a Material - Properties A few key concrete properties: • Concrete is basically man-made stone (sedimentary) – Strong in compression, weak in tension (roughly 10:1) – Reinforcement provides tensile strength • As w/c ratio decreases (less water, “stiffer” mix) – Strength increases – Density increases – Absorption decreases – Durability increases – But… – “Workability” (by one def.) decreases • Romans understood this, as do engineers today Cast Stone • Precast concrete (cement, sand, crushed rock) with fine finish coat • Made to replicate stone or other materials • Used for copings, lintels, sills, ornaments on facades • Can be reinforced or unreinforced • Ability to be mass produced 7 Causes of Deterioration of Concrete • Environmental effects – water, freeze/thaw, salts (marine and deicing salts), biological growth – Most common problem: corrosion of embedded steel • Inferior materials and poor workmanship – alkali- aggregate reaction (ASR), poor consolidation, poor original quality, cold joints, lack of adequate concrete cover, sulfate attack from soil • Structural design defects – absence of expansion/contraction joints, structural failure • Improper maintenance – lack of maintenance allows water access, poor choice of repair materials, ie, non- breathable waterproof coating 8 Common Signs of Deterioration - Concrete • Cracks (not shrinkage) • Spalls and incipient spalls • Rust stains and signs of corrosion • Other stains • Deflection • Erosion ACI 364.1 “Guide for Evaluation of Concrete Structures before Rehabilitation” 9 Corrosion of Reinforcing Steel in Concrete (and Cast Stone) • Reinforcing steel initially protected by passivating layer • Corrosion occurs due to: – Carbonation – Chlorides • Expansion of rust product causes cracking and spalling 10 Methods to Detect Corroding Embedded Reinforcing • Visual observation and sounding for delaminated areas • Locate reinforcing steel with metal detectors and/or GPR • Half-cell potential (copper/copper sulfate) testing • Linear polarization corrosion rate testing • Test concrete for chlorides and carbonation to determine why corrosion is occurring – this will affect your repair GPR Copper-copper sulfate half cell testing 11 “Reactive” Repairs for Corrosion of Steel in Concrete • Patching • Epoxy injection ICRI “Guide for Selecting and Specifying Materials for Repair of Concrete Surfaces” 12 “Proactive” Repairs for Corrosion of Steel in Concrete • Inhibit moisture ingress – Flash sky-facing surfaces – Cut drip edges – Clear water repellants – Film-forming coatings (elastomeric) 13 “Proactive” Repairs for Corrosion of Steel in Concrete • Chemical treatments to alter corrosion process – Realkalization – Electrochemical chloride extraction – Surface applied corrosion inhibitors Re-alkalization treatment in process 14 “Proactive” Repairs for Corrosion of Steel in Concrete • Electrical treatments to alter corrosion process – Impressed current (active) cathodic protection – Sacrificial anodes to provide passive galvanic protection of localized repairs Sacrificial galvanic anode cross section (left) and installed (right). Images courtesy Vector Corrosion Technologies. 15 Plaster • Mud plaster – used on interiors and exteriors of adobe • Lime plaster – often mixed w/sand and reinforced w/hair • Gypsum plaster – interior only • Exterior cement plaster (“stucco”) – Cement, lime and sand 16 Exterior Cement Plaster • Applied as a parge coat to finish masonry or concrete walls – In multiple coats, wide thickness variation – Barrier wall construction • Applied to wood lath or wire lath over a weather resistive barrier and sheathing or open frame construction – Modern: 3 coats, 7/8 in. thick – Expansion joints every 144 sf to reduce cracking – Modified drainage wall construction • Integral color or painted 17 Plaster – Common Signs of Deterioration • Cracking • Crazing or map cracking • Separation from backing or between coats • Spalling • Crumbling or powdering • Stained • Corrosion 18 Delaminating Plaster Methods to Detect • Sounding • Infrared thermography • Bond strength (pull off or shear) testing Falling Hazards 19 Plaster Repairs • Analyze existing plaster (remove sample) • Develop possible matches – Match sand • Check against building code = ASTM C926 proportions 20 Plaster Repairs From PCA, “Repair of Portland Cement Plaster (Stucco) Patches require moist curing 21 Memorial Auditorium • Bakewell and Brown, 1937 • Cement plaster over concrete • Campus Landmark • Building envelope assessment 22 Memorial Auditorium Existing Conditions 23 Memorial Auditorium • Condition survey – Samples and laboratory testing – Determined causes of deterioration • Close up survey – Tapping to identify and remove falling hazards – Quantify delaminated areas for pricing • Develop and discuss repair options (eventually used them all): – Clean and remove graffiti – Patch cracks and spalls – Fog coat or paint all or part of plaster – Replace all or part of plaster • Analyze samples, design mix • Mockups • Repair: client expectations 24 Mockups – Graffiti Removal, Patching, Cleaning Cleaning – Antimicrobial D/2 Graffiti removal – Citristrip (acrylic paint) and Wipe Out (aerosol paint) 25 Repair Gutters and Stains Near Roof 26 Clean and Patch Walls Before After Considerations • How perfectly do the patches have to match? (skim coat or fog coat vs. patching) • How much of the original fabric should remain? • Is the natural weathered appearance of unpainted plaster OK? • When one area is skim coated, adjacent weathered or patched areas appear imperfect. 27 Garden Walls Cleaned and patched Skim coated with plaster (need clean, unpainted surface) 28 Theater Entrance Mostly skim coated 29 West Side Cleaned and patched 30 The Southwest Museum • Built in 1914, Los Angeles’ oldest museum • Local Landmark & National Register • Cast in place reinforced concrete with cement plaster parge coat • Problems addressed – Damage from Northridge earthquake – structural evaluation and repairs – Water infiltration at roofs walls, and windows – Cracking, spalling and delamination of plaster 31 The Southwest Museum – Exterior Wall Investigation • Accessed walls using industrial rope access – Tapping and listening for a “hollow sound” to detect delaminated areas – Mapped cracks, delaminated areas, and other damage – Removed cores for materials testing 32 The Southwest Museum – Field Investigation Results • Plaster delaminated on all sides of tower; particularly near top • Up to five layers of plaster over concrete • Plaster appeared “harder” than “soft” concrete beneath • Plaster on rest of building in better condition than on tower 33 The Southwest Museum Laboratory Testing – Petrography & Mortar Analysis • 5 core samples removed - Plaster layers fell off concrete when cores removed even in “sound” areas • Petrographic examination (ASTM C856) and Mortar Analysis (ASTM C1324 – chemical + petrographic) – Composition of materials – Causes of deterioration – Appropriate repairs 34 The Southwest Museum – Laboratory Testing • 4 to 5 layers of plaster – Two inner layers from original construction (1 cement : 3.5 to 4.5 sand) – Two outer layers (1 cement : 1 lime : 2 sand) – Voids between layers; fractures within layers • High w/c ratio (low strength) concrete beneath Fractures parallel to surface due to Vertical fractures in outer layer volume changes (temperature & moisture) 35 Southwest Museum – Plaster Repair Considerations • Inherently problematic system – Outer layers of plaster stronger than inner plaster; stronger than concrete beneath – Failures due to defects from original construction + variation in moisture content over time – volume changes, shear stress, warping and delamination of plaster from concrete • Repair Considerations – How much to remove and replace? – Compatibility of new