CEE 395- Materials for Constructed Facilities

Civil Engineering Materials

Admixtures

Week 6, Lecture 11

What are Concrete Admixtures ?

·  Ingredients other than PC, water, and aggregates

·  Impart a specific quality to either plastic (fresh) mix or the hardened concrete (ASTM C496)

·  Classified by chemical and functional physical characteristics

Why use 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

1. Air entrainers

2. Water reducers

3. High-range water reducers-superplasticizers

4. Retarders

5. Accelerators

6. Fine minerals

7. Specialty admixtures

I. Air entrainers

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.

Causes of Internal Stresses in Concrete due to water:

1. Freezing above critical saturation

Upon freezes, water expands in volume by 9%. If percent saturation exceeds 91.7%, the volume increase generates stress

2. Hydraulic pressure

Freezing water draws unfrozen water to it

3. Osmotic pressure

Water moves from the gel to capillaries to satisfy thermodynamic equilibrium and to equalize alkali concentrations

·  The movement of the unfrozen water generates stress depending on length of flow path, rate of freezing, permeability, and concentration of salt in pores

Admixtures

Week 6, Lecture 11 (Con’t)

·  The stresses reduce the durability of hardened concrete, especially with repeated cycles of freeze and thaw

·  Entrained air voids diameter = 0.01 mm to 1 mm (majority < 0.1 mm), total volume 1% to 7.5% of concrete volume

·  Frost resistance improves, and strength reduces less, with decreasing void size

·  What size is required? Measured by specific surface index = total surface area of voids in a unit volume of paste (should exceed 23,600 m2/m3 for frost resistance)

Benefits

·  Increases workability of fresh concrete

·  Increased durability; Better resistance to freeze thaw cycles, de-icers, salts, sulfates, and alkali-silica reactivity

Difficulties

·  Decreases strength (Figure 6.7)

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.

II. Water Reducers

·  Water reduces 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.

Admixtures

Week 6, Lecture 11 (Con’t)

III. Superplasticizers

·  They 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 superplasticizers 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

IV. Retarders

Used to delay the initial set of concrete

Why do we use them?

1. 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

** The use of retarders must be evaluated experimentally before incorporation in mix design **

V. Accelerators

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

Admixtures

Week 6, Lecture 11 (Con’t)

Calcium Chloride is the most widely used accelerator. Initial and final set times reduced

CaCl2, by weight
/ Initial Set Time, in hours
0% / 6
1% / 3
2% / 2

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 applications of concrete

Member Type
/
Chloride Ion Limit
Prestressed concrete / 0.06%
Reinforced concrete subjected to chloride in service / 0.15%
Reinforced concrete protected from moisture / 1.00%
Other reinforce / 0.30%
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

V. Fine Minerals

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

V.1. Cementitious

·  Have hydraulic cementing properties

·  Example:

blast furnace slag, natural cement and hydraulic hydrated lime

Admixtures

Week 6, Lecture 11 (Con’t)

V.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 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

Specialty admixtures

Other admixtures are available to improve concrete quality in a number of ways, such as workability agents, bonding agents, etc.

Table 6.9. Effect of Mineral Admixtures on Fresh Concrete

Quality 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 increases workability. Silica fume reduces workability; compensate by using super-plasticizers.
Hydration / Fly ash reduces 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 accelerator.

Admixtures

Week 6, Lecture 11 (Con’t)

Table 6.10. Effect of Mineral Admixtures on Hardened Concrete

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.