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?
1. Reduce cost of concrete construction
2. Achieve specific concrete properties more effectively
3. Ensure quality of concrete during mixing, transporting, placing, and curing in adverse weather condition
4. Overcome emergencies during operations
Classification of concrete admixtures:
1. Air entrainers
2. Water reducers
3. High-range water reducers-superplasticizers
6. Fine minerals
7. 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 freeze thaw cycles, de-icers, salts, sulfates, and alkali-silica reactivity
- Decreases strength
- Effect can be reduced in moderate strength concrete by lowering water cement ratio and increasing cement factor
Composition of Air Entrainers:
- 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
Usually liquid meets ASTM C260 specifications.
Water Reducing concrete admixture:
Water reducers can result in 3 things:
1. Increased slump at constant w/c
2. Increased strength, by lowering the water content
3. Reduced cost of the cement
How do they do this?
Water reducers increase the mobility of the cement particles in the plastic mix, allowing same workability to be achieved at lower water contents.
Superplasticizers are “high-range” water reducers. Superplasticizers are used when placing:
1. Thin sections or around tightly spaced reinforcing steel
2. concrete underwater
3. concrete by pumping
4. consolidating the concrete is difficult
Note: When super plasticizers are used, the fresh concrete stays workable for only a short period of time (30 min to 60 min), which is why they are usually added at the site
Retarding concrete admixture:
Used to delay the initial set of concrete. Why do we use them?
1. To offset the effect of hot weather
2. Allow for unusual placement or long haul distances
3. Provide time for special finishes
Possible adverse effects of retarders
1. Reduce early age strength
2. Reduction of time between initial and final set
Possible advantages of retarders
1. Air entrainment
2. Increased workability
3. Reduction of time between initial and final set
Note: The use of retarders must be evaluated experimentally before incorporation in mix design
Accelerating concrete admixture:
Used to reduce the time required to develop final strength characteristics in concrete
Possible reasons for using accelerators:
1. Reduce the amount of time before finishing operations begin
2. Reduce curing time
3. Increase rate of strength gain
4. Plug leaks under hydraulic pressure efficiently
5. Offset effect of cold weather
Calcium Chloride is the most widely used accelerator. Initial and final set times reduced
CaCl2 by weight
Initial Set Time in Hrs.
The PCA (Portland Cement Association) recommends against using calcium chloride when:
1. concrete is prestressed
2. concrete contains embedded aluminum such as conduits
3. concrete is subjected to alkali-aggregate reaction
4. concrete is in contact with water or soils containing sulfates
5. concrete is placed during hot weather
6. Mass application of concrete
Alternatives to CaCl2
1. high early strength cement (type III)
2. increase cement content
3. cure at higher temperature (if feasible)
4. 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 properties
Have hydraulic cementing properties Example: blast furnace slag, natural cement and hydraulic hydrated lime
- 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 calcinated natural pozzolans, e.g. diatomaceous earths, 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
Fly ash is the most commonly used pozzolan in civil engineering structures
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 Concrete
Fly ash reduces water requirements
Silica fume increases water requirements
Fly ash and silica fume reduce air content
It can be avoided by increasing air entrainer.
Fly ash, ground slag, and inert materials generally increases workability. Silica fume reduces workability; compensate by using super-plasticizers.
Fly ash reduces heat of hydration. Silica fume may not affect, but superplasticizers used with silica fume can increase heat.
Fly ash, natural pozzolans, and blast furnace slag increase set time; Can compensate by using accelerator.
Table: Effect of Mineral Admixtures on Hardened Concrete
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.
Generally reduced reactivity, extend of improvement depends on type of admixture.
Improved due to reduced permeability.