Admixtures for Concrete Pipe Concrete of Today A modern wetcast concrete is more than a mixture of cement aggregate and water ADMIXTURES and Supplementary Cementitious Materials SCM are becoming as ESSENTIAL as PORTLAND CEMENT when making modern concrete ID: 768522
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Admixtures for Concrete Pipe
Concrete of Today A modern wetcast concrete is more than a mixture of cement, aggregate and water ADMIXTURES and Supplementary Cementitious Materials (SCM) are becoming as ESSENTIAL as PORTLAND CEMENT when making modern concrete
Admixtures for Concrete Pipe Used to modify the performance or a property of fresh or hardened concrete. High Performance (engineered) concrete routinely possible
Introduction Major reasons for using admixtures are: To improve concrete durability To 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 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
Admixtures What Are They? ACI 116.R-2 Material other than water, aggregates, hydraulic cement and fiber reinforcement used as an ingredient of concrete or mortar and added to the batch immediately before or during its mixing.
Concrete Admixtures ASTM C 260: Air Entraining Agents ASTM C 494: Chemical Admixtures Type A - Water Reducing Type B - Retarding Type C - Accelerating Type D - Water Reducing & Retarding Type E - Water Reducing & Accelerating Type F - High Range Water Reducing Type G - HRWR & Retarding Type S – Specific Performance Based ASTM C 1017 (Flowing Concrete) Type I Plasticizing Type II Plasticizing & Retarding
Air Entrainment Plastic Concrete Added Workability Reduced Segregation and Bleeding Improved Finishing Hardened Concrete Increased Freeze-Thaw Durability Reduced Permeability Resistance to Alkali and Sulfate Attack
Good entrained air void system: mostly entrained air Poor entrained air void system: mostly entrapped air
Freeze/Thaw Durability Parameters Air Content Average Chord Length Specific Surface Spacing Factor
Un-wanted Air in Concrete (Wetcast) How much air should be in concrete (no admixtures)? Depends on concrete ingredients How much air should be in concrete with no AEA admixture, only WR or other admixture? Depends on concrete ingredients
Un-wanted Air in Concrete ( Wetcast ) If you have MORE than 3% AIR in concrete with NO admixturesVery likely that cement, SCM, or aggregate issue The above assumes that the quality of the water is OK, water can cause air issues as well
Un-wanted Air in Concrete ( Wetcast ) If you have MORE than 3% AIR in concrete with just WR (no AEA) Could be cement, SCM, aggregate, or admixture The above assumes that the quality of the water is OK, water can cause air issues as well
Water Reducers and High Range Water Reducers ( HRWR = Super Plasticizers)
Water Reducing Admixtures . . . What Are They? ACI 116.R-2 Admixtures that either increase the 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 Reducing Admixtures Overview Can be used to reduce water content, improve slump or both Three groups: low-, medium- and high-range How they workReduces flocculation and improves hydration efficiency Effects on concrete Increased strength Increased slump and workability
Why Water Reduction? Provides dispersion of cement Strength Reduces water content while maintaining slump (workability) thus improves concrete strength BenefitLower concrete permeability Lower shrinkage (less cracking) Improved durability More economical $ admix < $ cement for equivalent strength
Estimated Strength Gain with Water Reducers Product Type Class Water Reduction Typical Dosage Strength Gain Water Reducer Low 4-7% 2-4 oz /cwt ~ 10% Mid-Range Mid 5-12% 6-12 oz /cwt ~ 15%+ HRWR (NSFC) High 12-25% 10-18 oz /cwt ~ 20%+ HRWR (PC) High 12-35% 3-8 oz/cwt ~ 20%+
Visual Effects of WRs
Water Reducer and High Range Water Reducer Without HRWR With HRWR
How Dispersants Work
How Dispersants Work Cement grains Dispersion obtained due to the presence of excess water Attractive forces between cement grains cause FLOCCULATION and LOSS OF FLUIDITY
How Dispersants Work Flocculation With Loss of Fluidity and Workability hydration products
How Dispersants Work Dispersant Molecules Cement Dispersion by Electrostatic Repulsion Surface Adsorption
How Dispersion Really Looks
High Range Water Reducer Use in Flowing Concrete Flowing concrete = 8” to 11” slump Self consolidating concrete = 22” to 32” flow
Effects of HRWR on Properties of Fresh Concrete Increases slump Improves flow Improves placeabilityImproves pumpability Improves finishability Improves formed surfaces Can have an effect on air content and setting
Effects of HRWR on Properties of Hardened Concrete Improved Strength Improved Durability Chloride resistance increases Frost resistance improves Increases sulfate resistance Increases resistance to abrasion
Set Retarding Admixtures Retarders / Hydration Stabilizers
Why Use Set Retarding Admixtures? Hot Weather: Increase water demand Increase set times Hasten evaporation rate Increase potential for plastic shrinkage cracking Accelerate slump loss Long Haul / Long Hold Use of retarders is not a substitute for adequate hot weather concreting procedures
Set Retarding and/or Hydration Stabilizing Admixtures How they work Decreases the rate of cement hydration (C3S) Admixture absorbs onto cement grains and temporarily inhibits crystal growth Effects on Concrete Delays Initial set Extends workability time Differences between Retarder and HS Range of control over the variety of hydration processes at the surface of the cement grain Retarders can slow, HS can stop
Set Accelerating Admixtures Accelerators
Accelerators Some common names: Calcium, CC, NCA, non chloride & accelerator Allows concrete to be “used” faster
Accelerators Speed up both initial and final time of set Speed up strength development General Categories: Chlorides and Non-Chloride
Accelerators How they work Increases rate of cement hydration (C3S) Why accelerate concrete? Shorten the setting timeQuicker early strength Reduce bleeding Earlier finishing Improved initial protection against freezing Earlier use of structure Reduction of protection time to achieve a given quality They are NOT anti-freeze agents
Calcium Chloride Very effective & economical accelerator Cautions when using chloride-bearing admixture DO NOT use in ANY reinforced concrete high 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) Although often the most effective, non-liquid CC (pellets/flake) is not recommended
Non-Chloride Accelerators (NCA) Non – corrosive Linear dosage NOT Anti-freeze
Corrosion Inhibitors
Corrosion Inhibitors How it works Passive film enhances the protection of reinforcing steel from corrosion in the concrete Generally, 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 Reinforcement Dosage dependent on anticipated chloride level
Lubricants and Surfactants
Lubricants & Surfactants (Drycast) How it works Decreases surface friction and therefore aids in stripping forms Increased reaction to vibration Improved moisture retention Improved production efficiency Effects on Concrete Can improve appearance Improved water tolerance Increase in surface paste
Dry Cast Admixtures Reduction in cracking Improved surface swipe Sharper joints / reduced repair
ASTM Specs
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
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 & finishabilitymodify the rate of bleedingretard or accelerate setting timeimprove freeze / thaw resistanceimprove impact & abrasion resistance inhibit expansion due to ASR inhibit corrosion reduce shrinkage cracking reduce permeability produce colored concrete produce cellular concrete
QUESTIONS?