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 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 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
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
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