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Admixtures for Concrete Pipe

Admixtures for Concrete Pipe

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Admixtures for Concrete Pipe

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  1. Admixtures forConcrete Pipe

  2. 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

  3. Admixtures for Concrete Pipe Used to modify the performance or a property of fresh or hardened concrete. High Performance (engineered) concrete routinely possible

  4. 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

  5. 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.

  6. 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

  7. 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

  8. Good entrained air void system:mostly entrained air Poor entrained air void system:mostly entrapped air

  9. Freeze/Thaw Durability • Parameters • Air Content • Average Chord Length • Specific Surface • Spacing Factor

  10. 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

  11. Un-wanted Air in Concrete (Wetcast) • If you have MORE than 3% AIR in concrete with NO admixtures • Very 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

  12. 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

  13. Water Reducers and High Range Water Reducers (HRWR = Super Plasticizers)

  14. 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.

  15. Water Reducing Admixtures • Overview • Can be used to reduce water content, improve slump or both • Three groups: low-, medium- and high-range • How they work • Reduces flocculation and improves hydration efficiency • Effects on concrete • Increased strength • Increased slump and workability

  16. Why Water Reduction? • Provides dispersion of cement • Strength • Reduces water content while maintaining slump (workability) thus improves concrete strength • Benefit • Lower concrete permeability • Lower shrinkage (less cracking) • Improved durability • More economical • $ admix < $ cement for equivalent strength

  17. Estimated Strength Gain with Water Reducers

  18. Visual Effects of WRs

  19. Water Reducer and High Range Water Reducer Without HRWR With HRWR

  20. How Dispersants Work

  21. How Dispersants Work Cement grains Attractive forces between cement grains cause FLOCCULATION andLOSS OF FLUIDITY Dispersion obtained due to the presence of excess water

  22. How Dispersants Work Flocculation hydration products With Loss of Fluidity and Workability

  23. Surface Adsorption Cement Dispersionby ElectrostaticRepulsion Dispersant Molecules How Dispersants Work

  24. How Dispersion Really Looks

  25. High Range Water Reducer Use in Flowing Concrete Flowing concrete = 8” to 11” slump Self consolidating concrete = 22” to 32” flow

  26. Effects of HRWR on Properties of Fresh Concrete Increases slump Improves flow Improves placeability Improves pumpability Improves finishability Improves formed surfaces Can have an effect on air content and setting

  27. 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

  28. Set Retarding Admixtures Retarders / Hydration Stabilizers

  29. 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

  30. 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

  31. Set AcceleratingAdmixtures Accelerators

  32. Accelerators • Some common names: • Calcium, CC, NCA, non chloride & accelerator • Allows concrete to be “used” faster

  33. Accelerators Speed up both initial and final time of set Speed up strength development General Categories: Chlorides and Non-Chloride

  34. Accelerators • How they work • Increases 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 • Reduction of protection time to achieve a given quality • They are NOT anti-freeze agents

  35. 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

  36. Non-Chloride Accelerators (NCA) • Non – corrosive • Linear dosage • NOT Anti-freeze

  37. Corrosion Inhibitors

  38. 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

  39. Corrosion Inhibitors Control Corrosion of Steel Reinforcement Dosage dependent on anticipated chloride level

  40. Lubricants and Surfactants

  41. 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

  42. Dry Cast AdmixturesReduction in crackingImproved surface swipeSharper joints / reduced repair

  43. ASTM Specs

  44. Current Admixture Standards

  45. Admixtures can be used to modify various fresh and hardened concrete properties:

  46. QUESTIONS?