Chemical Admixtures - PowerPoint Presentation

Chemical Admixtures forConcrete Pipe Ver 1.1 Ed Mansky Grace Construction Products Febuary 11, 2014

Concrete Admixtures Learning Objectives Provide an introduction of chemical admixtures for use in concreteDiscuss definitions, uses, benefits, and cautionary measures

Introduction DefinitionA material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of a cementitious mixture to modify its freshly mixed, setting, or hardened properties and that is added to the batch before or during its mixing

Introduction A modern wetcast concrete is more than a mixture of cement, aggregate and water ADMIXTURES and MINERAL COMPONENTS are becoming as ESSENTIAL as PORTLAND CEMENT when making modern concrete Today Concrete without mineral components or admixtures is found in museums

Introduction Major reasons for using admixtures are:To reduce the cost of concrete constructionTo achieve certain properties in concrete more effectively than by other means To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse and varying weather conditions To overcome certain emergencies during concreting operations Keep in mind that no admixture of any type or amount can be considered a substitute for good concrete practice

Introduction Admixtures can be used to modify various fresh and hardened concrete properties: Fresh state Hardened state decrease water content increase workability reduce segregation reduce the rate of slump loss improve pumpability improve placeability & finishability modify the rate of bleeding retard or accelerate setting time improve freeze / thaw resistance improve impact & abrasion resistance inhibit expansion due to ASR inhibit corrosion reduce shrinkage cracking reduce permeability produce colored concrete produce cellular concrete

Chemical Admixture Types Water-reducing admixtures (plasticizers) Retarding admixturesAcceleratorsCorrosion inhibitorsAir-entraining admixtures Lubricants or surfactants

Water Reducers Ver 1.1

Water Reducers and High Range Water Reducers (HRWR / Superplasticizers)

Water Reducing Admixtures OverviewCan be used to reduce water content, improve slump or bothThree groups: low-, medium- and high-rangeHow it worksReducing flocculation and improves water efficiency Effects on Concrete Increased strength Increased slump and workability

Water Reducer and High Range Water Reducer Definition (ACI 116)Admixtures that either increase slump of freshly-mixed mortar or concrete without increasing water content OR maintain slump with a reduced amount of water, the effect being due to factors other than air entrainment

Water Reducer and High Range Water Reducer How they work Without Admixture With Admixture

Water Reducer and High Range Water Reducer How they work Without Admixture With Admixture

How Conventional Type A Water Reducers Work Cement grains naturally cluster together to form flocs, which trap water inside them + + + + + + + Water Cement + +

Floc Busting Action of Water Reducers 1. Water Reducer coats the cement grains 2. Water Reducer imparts negative charge to cement grains 3. Like charges repel cement grains away from each other Freed Water Cement - - - -

The Result: Increased Water Cut (Lower W/C Ratio) : Lower water/cement ratio means improved product strength and quality, “insurance policy” Improved Workability : Concrete that is easier to place and finish, with no sacrifice in quality + + + + + + + + + Cement flocs are broken up and water is evenly dispersed + admix = - - -

No Water Reducer (Materials are clinging together) Water Reducer (disperses materials for better flowability) Effect of Water Reducer

Steric (physical) Repulsion Force + + + + + Cement Cement COMB POLYMERS Cement Dispersion by Steric Repulsion Hooking units, Comb Backbone Comb Teeth Comb polymer Water + + + + + Cement Cement

How Superplasticizers Work- video

Why Water Reduction? Provides dispersion of cement particles Strengthreduces water demand while maintaining slump (workability) thus increases concrete strength Benefit lower concrete permeability can reduce concrete costs with less cementitious material if used correctly

Normal water reducer TYPE A (decreases the water requirements by about 5 – 10%)Ca or Na salts of lignosulfonic acidsalts of hydroxycarboxylic acidsCarbohydrate Gluconates Mid-range water-reducing (decreases the water requirements by about 6 – 12%) …. no ASTM Complex aqueous solution of lignosulfonates with accelerating admixtures Polycarboxylate ether with set and strength enhancing ingredients Superplasticizer TYPE F & G (decreases the water requirements by about 12 - 30%) Sulfonated melamine formaldehyde Sulfonated naphtalene formaldehyde Polycarboxylates Classification

The use of water reducer / superplasticizer: Increases slumpImproves flowImproves placeabilityImproves pumpability Improves finishability Improves formed surfaces Can have an effect on air content and setting Effects on Properties of Fresh Concrete

Slump Flow Test - SCC www.concrete-pipe.org

Effects on Properties of Hardened Concrete Due to Lower w/cm Ratio When using water reducer / HRWRCompressive strength increases Permeability decreases Chloride resistance increases Frost resistance improves Increases sulfate resistance Increases resistance to abrasion

Retarding & Set Stabilizing Ver 1.1

Retarding Admixtures and/or Set Stabilizers How it worksDecreases the rate of cement hydration (C3S) Admixture absorbs into calcium hydroxide atoms and temporarily inhibit their growth into larger crystals Effects on Concrete Delays Initial set Extends workability time

Accelerators Ver 1.1

Accelerators How they workIncreases rate of cement hydration (C3S)Why accelerate concrete?Shorten the setting time Quicker early strength Reduce bleeding Earlier finishing Improved initial protection against freezing Earlier use of structure / piece Reduction of protection time to achieve a given quality

Accelerating Admixtures 2 Classes of Accelerating Admixtures:Set AcceleratorStrength Accelerator (Early Age) Calcium Chloride is a well known accelerator, but BEWARE, it should not be used in reinforced concrete Several non-chloride, non-corrosive accelerators are available, but generally most are not as effective as calcium chloride

Calcium Chloride Very effective & economical acceleratorCautions when using chloride-bearing admixtureDO NOT use in ANY reinforced concretehigh potential to cause corrosion calcium chloride should not exceed 2% in non reinforced concrete calcium chloride should not exceed 1% when concrete contains uncoated aluminum conduit can cause discoloration issues (dark and light gray spots, especially on hard trowled finishes)

Time of Set Control Retarding vs Accelerating Always a compromise

Corrosion Inhibitors Ver 1.1

Corrosion Inhibitors How it worksPassive film enhances the protection of reinforcing steel from corrosion in the concreteGenerally, corrosion inhibitors are not needed to protect steel reinforcing, due to the passivating effect of the high pH in the concrete. Effects on Concrete May accelerate initial set May improve early age strength

Corrosion Inhibitors Control Corrosion of Steel ReinforcementDosage dependent on anticipated chloride level

Air Entraining Ver 1.1

Air Entraining Admixtures ASTM C260 OverviewCreate stable system of microscopic air bubblesHow it worksMicroscopic air bubbles gives water a place to expand during freezing conditions Effects on Concrete Increased resistance to freeze-thaw damage Improved workability of fresh concrete Every 1% air entraining potentially reduces the concrete strength by up to 5-10%

Air-Entraining Admixtures DefinitionAre used to produce concrete that is resistant to the effects of freezing and thawing and to improve workability V Polished section of air-entrained concrete as seen through a microscopic

Air-Entraining Admixtures Significance of freezing and thawingThe most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of de-icing chemicals used for snow and ice removal Contrary to fresh concrete which can be protected, we can’t avoid the exposure of mature concrete to alternating freezing and thawing Due to freezing and thawing, hardened concrete can suffer both internal as well as surface damage. Internal damage can lead to loss of strength and structural integrity.

Mechanism of Frost Damage Hydraulic pressuresCaused by the 9% expansion of water upon freezingGrowing ice crystals displace unfrozen waterIf a capillary is above critical saturation (91.7% filled with water) hydraulic pressures results as freezing progresses At low water contents, no hydraulic pressure should exist Critical saturation begins at 91.7% Water expands 9% on freezing Water is forced ahead of the advancing freezing front Internal hydrostatic pressures can disrupt the concrete

Effect of Entrained Air 2% air 4% air 6% air Surface Damage ASTM C 672: Samples are monitored visually and for loss of mass over a period of 50 or more cycles of freezing and thawing in presence of salts

Mechanism of Air Entrainment Air Hydrophobic end is attracted to air within bubbles Surface tension reduced – stable bubbles Charges around bubbles lead to repulsive forces between bubbles (prevent coalescence) Bubbles adhere to cement & aggregate particles – cohesion of mix improved + - + - + - Cement + - + - + - Cement + - + - + - + - + - + - Aggregate + - + - + - Aggregate AEA’s do not add air to the concrete…they make the air that is already there…STABLE

Un-wanted air in Concrete (Wetcast) How much air should be in concrete (no admixtures)? Less than 3% How much air should be in concrete with no AEA admixture, only WR or other admixture? Less than 3% If you have MORE than 3% AIR in concrete with NO admixtures… it’s a cement issue If you have MORE than 3% AIR in concrete with just WR (no AEA) … it’s an admixture issue or a cement issue The above assumes that the quality of the water is OK, bad water can cause air issues (too much-too little)

Entraining Air Many factors effect the development of air entrainment. Mixing time Materials Sequencing Temperature Mixing Action Equipment Dosage Rates Admixtures *Almost everything! www.concrete-pipe.org

Lubricants and Surfactants Ver 1.1

Lubricants & Surfactants (Drycast) How it worksDecreases surface friction and therefore aids in stripping formsIncreased reaction to vibration Improved moisture retention Effects on Concrete Can improve appearance Improved water tolerance Increase in surface paste

Dry Cast admixesProductivity improvement Reduction in crackingImproved surface swipeSharper joints / reduced repair

ASTM Specs Ver 1.1

ASTM C 494 ASTM C 260 ASTM D 98 ASTM C 869 ASTM C 1141 ASTM C 1017 ASTM C 937 ASTM C 979 Chemical Admixtures Air-entraining Admixtures Calcium Chloride Foaming Agents Admixture for Shotcrete Flowing Concrete Grout Fluidifier Pigments Current Admixture Standards

QUESTIONS?

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