CON 122 Session 5 - High Performance Concrete Admixtures
CON 122Concrete AdmixturesSession 5High Performance Concrete Admixtures
High-Performance Concrete―Concrete having desired propertiesand uniformity which cannot beobtained routinely using onlytraditional constituents and normalmixing, placing, and curingpractices.‖ - (NIST/ACI Workshop; May, 1990)
High-Performance Concrete in Simple Terms Concrete with performance characteristics over and above what is typically (“the norm”) that meets the project requirements.
High-Performance Concretes Developed to meet construction needs Often require combination of admixtures Typically, economically viable options
Properties of High-Performance Concretes Ease of Placement and Compaction without Segregation, Low Bleeding, Good Finishability, & Low Plastic Shrinkage High Early-Age Strength Volume Stability
Properties of High-Performance Concretes Increased Ductility & Energy Absorption (Toughness) Enhanced Long-Term Mechanical Properties Long Life in Severe Environments
Properties of High-Performance Concretes Can Be Grouped Into Three General Categories… Enhanced Fresh / Plastic Properties Enhanced Mechanical Properties Enhanced Durability Properties
What’s Next? The overview will be limited to only calcium nitrite and the amine-ester organic corrosion inhibitor, the two most widely used corrosion inhibitors in the world. Following the Overview, the bulk of the discussion will focus on the amine-ester inhibitor, which is now being introduced in the Middle East. Calcium nitrite will extend the setting characteristics of concrete.
Overview Corrosion – How Big a Problem in Bridges? The Corrosion Process Options for Corrosion Protection Corrosion-Inhibiting Admixture Calcium Nitrite Inhibitors
Corrosion of Bridge Structures Fulton Bridge in Cleveland, Ohio
Corrosion: How Big a Problem? ―The average bridge deck located in a snow-belt State with reinforcing steel and 40 mm (1.5 in.) of concrete cover has shown spalling in about 7 to 10 years after construction and has required rehabilitation in about 20 years after construction.‖Repair / Replacement Cost: ~ $ 20 billion & increasing
Corrosion Inhibitors Control Corrosion of Steel Reinforcement Dosage dependent on anticipated chloride level
The damage to this concrete parkingstructure resulted from chloride-inducedcorrosion of steel reinforcement.
Corrosion-Inhibiting System Definition: “An admixture (or system) that will significantly delay the onset and/or rate of corrosion and, thus, extend the useful service life of reinforced and pre-stressed concrete structures.” Though there are classical definitions for corrosion inhibitors in the literature, this simple definition corrosion inhibitor is the most relevant from an Owner’s perspective. Basically, an Owner is only interested in a corrosion inhibitor that will effectively delay corrosion and help achieve the intended service life of the structure.
Most Commonly Used Inhibitor The most commonly used corrosion inhibitor in the world is calcium nitrite. Calcium Nitrite is inorganic and comes in a 30% solution.
Corrosion Sequence Formation: Ferrous Oxide and Ferric Oxide Ferrous oxide reaction with chlorides to form rust Chloride ions continue attack until passivating oxide layer destroyed Volume of rust is greater, thus concrete cracks.
Corrosion of Steel in Concrete Electrochemical process that requires: Moisture & Oxygen Breakdown of Protective Oxide Layer (the Passive Layer)
Consequences of Corrosion in Concrete Delamination
Consequences of Corrosion in Concrete Cracking Spalling
Corrosion of Steel in Concrete: Net Effect Corrosion by-product (rust) induces tensile stresses within matrix…..
Calcium Nitrite Inhibitor: Advantages Historical data Effective with admixed chlorides Can double as an accelerator in cold weather applications Early concrete strengths are equal or better than reference mixes
Calcium Nitrite Inhibitor: Disadvantages Accelerating Effect Meets ASTM C 494 Requirements for Type C, Accelerating, Admixture
NOTE: ASTM Specification for Corrosion Inhibiting Admixtures ASTM C 1582/ C 1582M: Standard Specification for Admixtures to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete
Rule #1 for Corrosion Protection of Steel in Concrete Good Concreting Practices Good quality concrete Low water-cementitious materials ratio High-range water-reducing admixture Proper placement & consolidation Good Curing !!!
ACI 318 Classes for Corrosion Exposure Category Category Severity Class Condition Concrete dry or protected Not Applicable C0 from moisture Concrete exposed to moisture C Moderate C1 but not to external sources of chlorides Corrosion Protection of Concrete exposed to moistureReinforcement and an external source of chlorides from deicing Severe C2 chemicals, salt, brackish water, seawater, or spray from these sources
ACI 318 Requirements for Concrete for Corrosion Exposure Category Min.fExposure Max. ’c Additional Minimum Requirements Class w/cm (psi) Max Water-Soluble Chloride Ion (Cl-) Content in Concrete (percent by weight of cement) Related Provisions Reinforced Prestressed Concrete Concrete C0 n/a 2,500 1.00 0.06 None C1 n/a 2,500 0.30 0.06 C2 0.40 5,000 0.15 0.06 7.7.6, 18.16
Sources of Chloride• De-icing Salts for Snow & Ice Removal• Groundwater• Brackish Water• Seawater & Airborne• Mixture Ingredients
How to Reduce Concrete Permeability Lower Water-Binder Use Pozzolans & Slag Ratio & Use High- Cement Range Water Reducer Fly Ash & Natural Pozzolans Silica Fume Metakoalin
Effect of w/cm on Permeability Coefficient of Permeability Water-Cement Ratio
ASR Inhibitors—Lithium Carbonate Expansion of specimens made with lithium carbonate admixture.
External Sulfate Attack Source of sulfate ions in solution Access to cement paste
External Sulfates Natural sulfates of calcium, sodium magnesium, potassium Soils Ground water Ponds or rivers Seawater Sanitary, Industrial, and Agricultural waste
Sulfate Attack Mechanism Sulfate ions (SO4-2) react with hydration products (calcium hydroxide and aluminate hydrates) Reaction products result in swelling (mechanism is uncertain)
Sulfate Attack Mechanism Swelling pressures destroy cement matrix Affected by: Cement type Sulfate ion concentration in water or soil Permeability of concrete Presence water
External Sulfate Attack External to internalprogression of deterioration 50
Mitigation of Sulfate Attack Use low w/c Use sulfate resistant cement (Type V) Use supplementary cementitious materials Source: PCA
Effect of w/c Type V Cement Type V Cement w/c = 0.65 w/c = 0.39Visual Rating = 5 @ 12 Visual Rating = 2 @ 16 years years Source: PCA
Table for Sulfate Attack Class Water-soluble Sulfate (SO4) inClass Desc. sulfate (SO4) in soil, water, ppm % by weightS0 N/A < 0.10 < 150S1 Moderate 0.10 to 0.20 150 to 1,500S2 Severe 0.20 to 2.00 1,500 to 10,000S3 Very Severe > 2.00 > 10,000
Shrinkage Volume Reduction due to loss of moisture from a concrete matrix as it hardens and dries. Plastic Shrinkage Thermal Contraction Drying Shrinkage Autogenous Shrinkage Carbonation Shrinkage
Drying Shrinkage: Mechanism Loss of moisture Meniscus forms at air-water interface due to surface tension
Effect of SRAs on Plastic Properties of Concrete SRAs may increase bleed time and bleed ratio (10% higher). SRAs may also delay final set by 1-2 hours. Precautions needed to minimize impact on air-void system.
Effect of SRAs on Hardened Properties of Concrete May experience some loss in strength.