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Design and Control of Concrete Mixtures

Chemical Admixtures

Design and Control of Concrete MixturesCHAPTER 9


Overview

• Air-entraining admixtures• Water-reducing admixtures• Set retarding admixtures• Set accelerating admixtures• Specialty admixtures• Admixture compatibility• Storage and dispensing admixtures


Chemical Admixtures


Chemical AdmixturesThe major reasons for using chemical admixtures in concrete

mixtures are:1. To achieve specific properties in fresh and hardened concrete 2. To maintain the consistency 3. To overcome certain emergencies during concreting operations4. To make the mixture more economical5. To enable use of a wider selection of concrete materials

(including marginal materials)


Chemical AdmixturesConcrete Admixtures by Classification


Air-Entraining Admixtures


Mechanism of Air Entrainment

Thomas and Wilson 2002


Air-Entrained


Control of Air ContentThe amount of air entrained in concrete for a given dose of admixture will depend on:

•Concrete materials•Mixture proportions•Methods of transport•Placing and finishing methods•Curing•Aggregate proportions and gradation•Mixing time and intensity•Temperature•Slump


Impact of Air on Concrete Properties

Gilkey 1958


Impact of Air on Concrete Properties


Water-Reducing Admixtures

Adapted from Neville 1995


Water-Reducing Admixtures

• Normal – 5%-10% water reduction• Mid-range – 6%-12% water reduction• High-range – 12%-40% water reduction– Superplasticizers


High-Range Water Reducers


Flowing Concrete


Mechanisms of Water Reducers



Polycarboxylate Technology



Impact of Water Reducers on Properties of Concrete

Whiting and Dziedzic 1992


Impact of Water Reducers on Properties of Concrete


Set-Retarding AdmixturesSet Retarders are used to: (1) offset the accelerating effect of hot weather on the setting of concrete; (2) delay the initial set of concrete or grout when difficult or unusual conditions of placement occur(3) delay the set for special finishing techniques


Types of Set-Retarding Admixtures

•Lignosulfonates•Hydroxycarboxylic acids•Polysaccharides (corn syrup)•Organophospates•Phosphonates•Sugars and their derivatives•Selected inorganic salts


Set Accelerating Admixtures

• Accelerates hydration and strength development

• Calcium chloride – most common– Up to 2% by mass of cementing material

• Triethanolamine (TEA), inorganic salts – alternatives


Set Accelerating AdmixturesStrength development of concrete can also be accelerated by other methods:1. Using Type III or Type HE high-early strength cement2. Lowering the water-cement ratio by adding 60 kg/m3 to 120

kg/m3 (100 lb/yd3 to 200 lb/yd3) of additional cement to concrete3. Using a water reducer4. Curing at higher temperatures


Set Accelerating Admixtures

• Calcium chloride effects on concrete– Increase corrosion– Discoloration– Increased shrinkage– Long-term strength reduction– Reduced durability


Hydration-Control Admixtures

• Two-part system:– Stabilizer/retarder that stops hydration– Activator that restarts hydration

• Allows for suspending overnight or for long hauls


Workability-Retaining Admixtures

• Provide workability retention without affecting set times or early-age strength

• Can be used with water-reducing admixtures• Reduces the need for slump adjustments


Corrosion Inhibitors


Shrinkage-Reducing Admixtures (SRAs)


Crack-Reducing admixtures (CRAs)

• Based on a specialty alcohol alkoxylate• Perform the same as SRAs− Provide smaller crack width in the event of

cracking•Provides internal stress relief compared to SRAs


Permeability Reducing Admixtures

• Two water penetration mechanisms:– Capillary absorption (wicking)– Direct ingress under pressure

• Two types of PRA:– Non-hydrostatic (PRAN)– Hydrostatic (PRAH)


Permeability Reducing Admixtures-Non-Hydrostatic (PRAN)• Referred to as “damp-proofers”• Most PRANs are hydrophobic in nature• PRANs provide water repellency pand reduced absorption

(wicking)• Sometimes used to reduce the transmission of moisture

through concrete in contact with water or damp soil• Hydrophobic admixtures are usually not effective when

concrete is in contact with water under pressure


Permeability Reducing Admixtures-Hydrostatic (PRAH)

• Referred to as “waterproofers”• Shown to be effective in reducing permeability under

pressure• Also been shown to reduce concrete corrosion in

chemically aggressive environments• Consist of products that produce pore blocking deposits• Shown to increase autogeneous healing of leaking,

hairline cracks


Alkali-Aggregate Reactivity Inhibitors

Stark 1992


Coloring Admixtures (Pigments)


Air Detrainers

• Reduce the air content in concrete• Used when the air content cannot be reduced• Rarely used


Miscellaneous Admixtures• Pumping aids• Bonding admixtures and agents• Grouting admixtures• Gas-forming admixtures


Miscellaneous Admixtures• Fungicidal, Germicidal, and Insecticidal Admixtures• Viscosity Modifying Admixtures• Self-Healing Admixtures


Admixture Compatibility

• Cement-admixture or admixture-admixture incompatibilities are encountered

• Slump loss, air loss, early stiffening, and other fresh and hardened concrete properties may be affected

• Solved by:– Dosage rate or sequence– Altering cement or admixture composition


Less Than Expected Water Reduction

• Composition of cementitious materials• Presence of other set-control admixtures• Temperature• Clay minerals• Admixture dosage• Slump loss

– Can be attributed to temperature, reactivity of cement, dispersion and availability of admixture, expansive clays

– Typically avoided by delaying addition of water-reducer


Less Than Expected Retardation

• May be caused by increase in C3A• Too much retardation caused by:– Low C3A– Low cement reactivity– Excessive admixture– High levels of SCMs– Low temperatures


Storage and Dispensing Chemical Admixtures


Summary• Air-entraining admixtures• Water-reducing admixtures• Set retarding admixtures• Set accelerating admixtures• Hydration-control admixtures• Workability-retaining admixtures• Corrosion inhibitors• Shrinkage-reducing admixtures• Permeability reducing admixtures• Alkali-aggregate reactivity

inhibitors

• Coloring admixtures• Pumping Aids• Bonding admixtures and agents• Grouting admixtures• Gas-forming admixtures• Air-detrainers• Fungicidal, germicidal, and

insecticidal admixtures• Viscosity modifying admixtures• Admixture compatibility• Storing and dispensing chemical

admixtures


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