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CON 122 Session 1 - Introduction
 

CON 122 Session 1 - Introduction

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    CON 122 Session 1 - Introduction CON 122 Session 1 - Introduction Presentation Transcript

    • CON 122Concrete AdmixturesSession 1Introduction
    • What is an Admixture?“A material other thanwater, aggregates, hydraulic cement, andfiber reinforcement, used as an ingredientof a cementitious mixture to modify itsfreshly mixed, setting, or hardenedproperties and that is added to the batchbefore or during its mixing.” -ACI 116
    • General History of Admixtures Milk and Lard – Romans Eggs – Middle Ages in Europe Rice paste, lacquer, tung oil, molasses and bananas – China Cactus juice and rubber latex – Peru Bark extracts - Mayans
    • Chemical Admixture Purpose – ACI 212.3R“Chemical admixtures are used toenhance the properties of concreteand mortar in the plastic andhardened state.”
    • Concrete Chemical Admixtures Added to concrete mixture before or during mixing Primarily water soluble Ingredients other than  Cement  Water  Aggregates  Fibers
    • Benefits of Admixtures in Fresh Concrete decrease water content increase workability retard or accelerate time of setting entrain air / reduce unit weight reduce segregation reduce rate of slump loss improve pumpability, placeability, finishability
    • Benefits of Admixtures in Hardened Concrete modify rate and/or amount of bleeding increase strength increase durability improve aesthetics
    • Concrete Prerequisites Workable Finishable Strong Durable Watertight Wear Resistant
    • Concrete Chemical Admixtures Key Points Admixtures are materials added to concrete mixtures to modify concrete properties such as air content, water requirement, and setting time. Admixtures should complement, not substitute, for good concrete proportioning and practice.
    • Concrete Chemical Admixtures Key Points Admixtures may have unintended side effects. Therefore, run trial batches with job materials and under job conditions. Generally, for every 1% entrained air, concrete loses about 5% of its compressive strength
    • Concrete Chemical Admixtures Key Points Air-entraining admixtures are specified by ASTM C 260. Water-reducing and set-modifying admixtures are specified by ASTM C494 / AASHTO M 194.
    • Main Classes of Chemical AdmixturesWater Reducers Other  Normal, mid-range,  Viscosity modifiers & high-range Rheology controlSet Control  High Performance  Accelerators, retarders admixtures  Coloring admixturesDurability Enhancing  Other  Air-entrainers  Corrosion inhibitors  Shrinkage reducers  ASR inhibitors
    • Admixture Cautions Change of type, source, or amount of cement Change of aggregate type or grading Change of mixture proportions Change of job placement conditions Reinforcement of concrete Concrete appearance
    • Properties Influenced by Chemical Admixtures Plastic Concrete  Mix  Transport  Placing  Finishing Frost Resistance Strength  Early (1-7days)  Late (28 days)
    • Effects of Chemical Admixtures
    • Effects of Chemical Admixtures Air entrainers: Air-entraining admixtures work by stabilizing small air bubbles in the paste. Several different chemical forms of air entrainers are available, all with differing stability and differing effects on bubble size. In general, the greater the slump, the easier it is for air to be entrained. Retarders: Set-retarding admixtures may increase air content.
    • Chemical Admixture Testing and Sampling Plant production samples Admixtures tested to comply with specifications Quality control procedures Uniformity
    • Water-Reducing Admixtures PURPOSE: Water reducers are used to reduce the quantity of mixing water required to produce concrete of a given consistency.
    • Accelerating Admixtures PURPOSE: Accelerating admixtures are added to concrete to shorten time of set and accelerate early-age strength development. Main Types…  Chloride  Non-Chloride  Sub-Freezing
    • Accelerating Admixtures Reduce time of set Increase early-age strength / fast-track Reduce heating/protection time in cold weather Permit earlier stripping/reuse of forms Facilitate earlier use of structure
    • Retarding Admixtures PURPOSE: Retarding admixtures are added to concrete to offset acceleration and unwanted effects of elevated temperatures to keep concrete workable during placement and consolidation. Benefits include…  Long hauls  (extended-set control types)
    • Air-Entraining Admixtures PURPOSE: Air entrainers are primarily used to stabilize tiny bubbles in concrete during mixing to provide protection against cyclic freezing and thawing. Also used to improve workability
    • Air-Voids in Hardened Concrete
    • Corrosion-Inhibiting Admixtures PURPOSE: Corrosion Inhibitors are added to concrete to mitigate chloride- induced corrosion of embedded reinforcing steel. Main Types…  Calcium Nitrite (inorganic)  Amine & Esters (organic)
    • Corrosion-Inhibiting Admixtures MECHANISMS  Function by forming a protective oxide layer at the steel surface, as a result of a chemical reaction with ferrous ions.  Example: Calcium Nitrite  Function by forming a protective layer at the steel surface AND by reducing chloride ion ingress.  Example: Amine/Ester-Based
    • Shrinkage-Reducing Admixtures (SRAs) PURPOSE: Shrinkage reducers are used to reduce the drying shrinkage of concrete. Benefits include…  Reduced drying shrinkage cracking  Reduced curling in slabs  Improved aesthetics & water-tightness
    • Cost Benefits Job Specified  Construction Methods Concrete  High Performance  Cementitious Materials Concrete  Concrete Proportions  High Volume  Aggregates Placement  Job placement/curing  Finishing and Forming Adjustment of Yield
    • Chemical Admixture Considerations Admixture has not been used before Special cementitious materials specified Dosage of multiple admixtures Specific mixing and placement requirements
    • Method of Preparation and Batching Chemical Admixtures Affect  Properties  Performance  Uniformity Chemical Admixture Introduction  Concrete Plant  Truck-Mounted Tanks for Job-site
    • Batching and Discharging Systems Pumps Meters Timers Calibration Tubes Valves Required Tolerances
    • Chemical Admixture Recommendations Ready-to-use  Dosage Rate  Standard Solutions (% Solids)  Degree of Agitation or Recirculation  Storage and Dispensing Systems  Temperature Storage Solid  Premeasured Containers  Batched by mass
    • Dispensing Chemical Admixtures Timing of Batching Sequence Addition of dosage and mixing Addition of multi-admixtures  Compatibility  Affect of water demand  Flocculation Accurate calibration and visual observation Plant operator training
    • Concrete AdmixturesPlease return to Blackboard and watch thefollowing video: Video 1: Chemical Admixture Testing and Sampling
    • AdmixturesPlease return to Blackboard and watch thefollowing video: Video 1: Admixture Dispensers