admixtures Flashcards
____ are ingredients other than portland cement, water, and aggregates that may be added to concrete to impart a specific quality to either the plastic (fresh) mix or the hardened concrete (ASTM C494).
admixtures
____ admixtures are charged into the mix as solutions.
some
Admixtures are classified by the following chemical and functional physical characteristics: ____
- air entrainers
- water reducers
- retarders
- hydration controller admixtures
- accelerators
- specialty admixtures
PCA stands for: ____
portland cement association
The Portland Cement Association (PCA) identifies four major reasons for using admixtures: ____
- to reduce the cost of concrete construction
- to achieve certain properties in concrete more effectively than by other means
- to ensure quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions
- to overcome certain emergencies during concrete operations
_____ produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing.
air entrainers
As moisture within the concrete pore structure freezes, three mechanisms contribute to the development of internal stresses in the concrete: ____
critical saturation, hydraulic pressure, osmotic pressure
Upon freezing, water expands in volume by 9%.
critical saturation
Freezing water draws unfrozen water to it.
hydraulic pressure
Water moves from the gel to capillaries to satisfy thermodynamic equilibrium and equalize alkali concentrations.
osmotic pressure
_____ permit water to flow from the interlayer hydration space and capillaries into the air voids, where it has room to freeze without damaging the parts.
osmotic pressure
_____ reduce the durability of hardened concrete, especially when cycles of freeze and thaw are repeated many times.
internal stresses
_____ is recommended for all concrete exposed to freezing.
air entrainment
Entrained air voids have diameters that range from ____, with the majority being less than 0.1 mm.
0.01 to 1 mm
____ improves with decreasing void size, and small voids reduce strength less than large ones.
frost resistance
The fineness of air voids is measured by the ______, equal to the total surface area of voids in a unit volume of paste.
specific surface index
_____ improves concrete’s resistance to several destructive factor, including freeze-thaw cycles, deicers and salts, sulfates, and alkali-silica reactivity.
air entrainment
____ also increases the workability of fresh concrete.
air entrainment
High strength if ___ to attain with air-entrained concrete.
difficult
Air entrainers are usually ____ and should meet the specification of ASTM C260.
liquid
The agents enhance air entrainment by ____ the surface tension of the mixing water.
lowering
____ of fresh or plastic concrete requires more water than is needed for hydration.
workability
The excess water, beyond the hydration requirement, is detrimental to all desirable properties of hardened concrete. Therefore, ______ have been developed to gain workability and, at the same time, maintain quality.
water-reducing admixtures
____ increase the mobility of the cement particles in the plastic miz, allowing workability to be achieved at lower water contents.
water reducers
Water reducers are produced with different levels of effectiveness: ____
conventional, mid-range, high-range
The high-range water reducer is typically called ______
plasticizer or superplasticizer
____ can be used to indirectly to gain strength.
water reducing admixtures
Hewlett (1988) demonstrated that water reducers can actually be used to accomplish three different objectives: ___
- adding a water reducer without altering the other quantities in the mix increases the slump
- the strength of the mix can be increased by using the water reducer by lowering the quantity of water and keeping the cement content constant
- the cost of the mix, which is primarily determined by the amount of cement, can be reduced. the amount of cement is then reduced to keep the water-cement ration equal to the original mix
A ____ allows the use of a lower amount of mixing water while maintaining the same workability level.
water reducer
______, or high-range water reducers, can either greatly increase the flow of the fresh concrete or reduce the amount of water required for a given consistency.
superplasticizers (plasticizers)
Superplasticizers can be used when:
- a low water-cement ratio is beneficial
- placing thin sections
- placing concrete around tightly spaced reinforcing steel
- placing cement underwater
- placing concrete by pumping
- consolidating the concrete is difficult
When superplasticizers are used, the fresh concrete stays workable for a short time, _____ minutes, and is followed by rapid loss in workability.
30-60
Some construction conditions require that the time between mixing and placing or finishing the concrete be increased. In such cases, ____ can be used to delay the initial set of concrete.
retarders
Retarders are used for several reasons, such as the following: ___
- offsetting the effect of hot weather
- allowing for unusual placement or long haul distances
- providing time for special finishes
_____ can reduce the strength of concrete at early ages (1 to 3 days).
retarders
These admixtures have the ability to stop and reactivate the hydration process of concrete.
hydration-control admixtures
Hydration-control admixtures consists of two parts: ____
stabilizer, activator
Adding the ____ completely stops the hydration of the cementing materials for up to 72 hours.
stabilzer
Adding the ______ to the stabilizes concrete reestablishes normal hydration and setting.
activator
These admixtures are very useful in extending the use of ready-mixed concrete when the work at the job site is stopped for various reasons.
hydration-control admixtures
____ are used to develop early strength of concrete at a faster rate than that developed in normal concrete.
accelerators
Accelerators are used to:
- increases rate of strength gain
- reduce the amount of time before finishing operations begin
- reduce curing time
- plug leaks under hydraulic pressure efficiently
_____ is the most widely used accelerator (ASTM D98).
calcium chloride, CaCl2
Concrete with CaCl2, develops ______ compared with plain concrete cured at the same temperature.
higher early strength
The PCA recommends against using calcium chloride under the following conditions: ____
- concrete is prestressed
- concrete contains embedded aluminum such as conduits, especially if the aluminum is in contact with steel
- concrete is subjected to alkali-aggreagate reaction
- concrete is in contact with water or soils containing sulfates
- concrete is placed during hot weather
- mass application of concrete
Several alternatives to the use of calcium chloride are available: _____
- using high early strength (Type III) cement
- increasing cement content
- curing at higher temperatures
- using non-calcium chloride accelerators such as triethanolamine, sodium thiocyanate, calcium formate, or calcium nitrate
What are examples of specialty admixtures?
workability retaining
corrosion inhibitors
damp-proofing agents
permeability-reducing agents
fungicidal, germicidal, and insecticidal admixtures
pumping aids
bonding aids
grouting agents
gas-forming agents
coloring agents
shrinkage reducing
Supplementary cementitious materials include: _____
fly ash, ground granulated blast furnace slag, silica fume, and natural pozzolans
_____ is the most commonly used pozzolan in civil engineering structures.
fly ash
____ is a by-product of the coal-fired electricity production.
fly ash
Fly ash is classified (ASTM C618) as follows: ____
class N, class F, class C
___ raw or calcined natural pozzolans, including diatomaceous earths, opaline cherts and shales, ruffs and volcanic ashes or pumicites, and some calcined clays and shales.
class N
____ fly ash with pozzolan properties.
class F
____ fly ash with pozzolan and cementitious properties.
class C
Fly ash ____ the workability of the fresh concrete.
increases
_____ is made from iron blast furnace slag. It nonmetallic hydraulic cement consisting basically of silicates and aluminosilicates of calcium, which is developed in a molten condition simultaneously with iron in a blast furnace.
slag cement
____ is a by-product of the production of silicon metal or ferrosilicon alloys.
silica fume
A ____ is a siliceous and aluminous material which, in itself, possesses little or no cementitious value but will, in finely divided form and in the presence moisture, react chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties (ASTM C595).
natural pozzolans
