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What are Concrete Admixtures?
Ingredients other than cement, water, and aggregates that impart a specific quality to either plastic (fresh) mix or the hardened concrete (ASTM C496) is called concrete admixture.Why use Concrete Admixtures?
- Reduce the cost of concrete construction
- Achieve specific concrete properties more effectively
- Ensure quality of concrete during mixing, transporting, placing, and curing in adverse weather condition
- Overcome emergencies during operations
Classification of Concrete Admixtures:
- Air entrainers
- Water reducers
- High-range water reducers-superplasticizers
- Retarders
- Accelerators
- Fine minerals
- Specialty admixtures
Air entraining concrete admixture:
Air entraining concrete admixtures produce tiny air bubbles in the hardened concrete to provide space for the water to expand upon freezing.How do they work?
They are anionic (water-hating) agents that form tough, elastic, air-filled bubbles. These bubbles reduce stresses caused by movement or freezing of water. They provide more volume for expansion and shorter flow path.Benefits of air-entraining admixture:
- Increases workability of fresh concrete
- Increased durability; Better resistance to freezing thaw cycles, de-icers, salts, sulfates, and alkali-silica reactivity
- Decreases strength
- The effect can be reduced in moderate strength concrete by lowering water cement ratio and increasing cement factor
- Salt of wood resins (Vinsol resin)
- Synthetic detergents
- Salts of sulfonated lignin (by-product of paper production)
- Salts of petroleum acids
- Salts of proteinaceous material
- Fatty and resinous acids
- Alkylbenzene sulfonates
- Salts of sulfonated hydrocarbons
Water Reducing concrete admixture:
Water reducers can result in 3 things:- Increased slump at constant w/c
- Increased strength, by lowering the water content
- Reduced cost of the cement
Superplasticizers
Superplasticizers are "high-range" water reducers. Superplasticizers are used when placing:- Thin sections or around tightly spaced reinforcing steel
- concrete underwater
- Concrete by pumping
- consolidating the concrete is difficult
Retarding concrete admixture:
Used to delay the initial set of concrete. Why do we use them?- To offset the effect of hot weather
- Allow for unusual placement or long haul distances
- Provide time for special finishes
- Air entrainment
- Increased workability
- Reduction of time between initial and final set
Accelerating concrete admixture:
Used to reduce the time required to develop final strength characteristics in concrete Possible reasons for using accelerators:- Reduce the amount of time before finishing operations begin
- Reduce curing time
- Increase rate of strength gain
- Plug leaks under hydraulic pressure efficiently
- Offset effect of cold weather
CaCl2 by weight | Initial Set Time in Hrs. |
0% | 6 |
1% | 3 |
2% | 2 |
- Concrete is prestressed
- Concrete contains embedded aluminum such as conduits
- Concrete is subjected to alkali-aggregate reaction
- Concrete is in contact with water or soils containing sulfates
- Concrete is placed during hot weather
- Mass application of concrete
- High early strength cement (type III)
- Increase cement content
- cure at higher temperature (if feasible)
- Triethanolamine, sodium thiocyanate, calcium formate, calcium nitrite or calcium nitrate
Fine Minerals as concrete admixtures:
Fine mineral admixtures added in large amounts (20% to 100% of cement weight) to improve the characteristics of plastic and hardened concrete. Classification based on chemical and physical properties1. Cementitious
- Have hydraulic cementing properties Example: blast furnace slag, natural cement and hydraulic hydrated lime
2. Pozzolanic
- Siliceous and aluminous material
- Little or no cementitious value
- In presence of moisture, will react with calcium hydroxide to form compounds with cementitious properties 15% of PC weight is hydrated lime. Addition of pozzolan could make the hydrated lime into a cementitious material
Classification of fly ash and natural pozzolans
- Class N: Raw or calcined natural pozzolans, e.g. diatomaceous earth, opaline cherts and shales, ruffs and volcanic ashes, & some calcined clays and shales
- Class F: Fly ash with pozzolan properties Class C: Fly ash with pozzolan and cementitious properties
Special concrete admixtures:
Other admixtures are available to improve concrete quality in a number of ways, such as workability agents, bonding agents, etc. Table: Effect of Mineral Admixtures on Fresh ConcreteQuality Measure | Effect |
Water Requirements | Fly ash reduces water requirements Silica fume increases water requirements |
Air Content | Fly ash and silica fume reduce air content It can be avoided by increasing air-entrainer. |
Workability | Fly ash, ground slag, and inert materials generally increase workability. Silica fume reduces workability; compensates by using super-plasticizers. |
Hydration | Fly ash reduces the heat of hydration. Silica fume may not affect, but superplasticizers used with silica fume can increase heat. |
Set Time | Fly ash, natural pozzolans, and blast furnace slag increase set time; Can compensate by using the accelerator. |
Quality Measure | Effect |
Strength | Fly ash increases the ultimate strength but reduces rate of strength gain. Silica fume has less effect on rate of strength gain than pozzolans. |
Drying shrinkage and creep | Low concentrations usually have single effect. High concentrations of ground slag or fly ash may increase shrinkage. Silica fume may reduce shrinkage. |
Permeability and absorption | Generally reduced permeability and absorption. Silica fume is especially effective. |
Alkali-aggregate reactivity | Generally reduced reactivity, extend of improvement depends on type of admixture. |
Sulfate resistance | Improved due to reduced permeability. |