admixtures

Question 1

____ 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).

Answer 1

admixtures

Question 2

____ admixtures are charged into the mix as solutions.

Answer 2

some

Question 3

Admixtures are classified by the following chemical and functional physical characteristics: ____

Answer 3

1. air entrainers
2. water reducers
3. retarders
4. hydration controller admixtures
5. accelerators
6. specialty admixtures

Question 4

PCA stands for: ____

Answer 4

portland cement association

Question 5

The Portland Cement Association (PCA) identifies four major reasons for using admixtures: ____

Answer 5

1. to reduce the cost of concrete construction
2. to achieve certain properties in concrete more effectively than by other means
3. to ensure quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions
4. to overcome certain emergencies during concrete operations

Question 6

_____ produce tiny air bubbles in the hardened concrete to provide space for water to expand upon freezing.

Answer 6

air entrainers

Question 7

As moisture within the concrete pore structure freezes, three mechanisms contribute to the development of internal stresses in the concrete: ____

Answer 7

critical saturation, hydraulic pressure, osmotic pressure

Question 8

Upon freezing, water expands in volume by 9%.

Answer 8

critical saturation

Question 9

Freezing water draws unfrozen water to it.

Answer 9

hydraulic pressure

Question 10

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

Answer 10

osmotic pressure

Question 11

_____ 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.

Answer 11

osmotic pressure

Question 12

_____ reduce the durability of hardened concrete, especially when cycles of freeze and thaw are repeated many times.

Answer 12

internal stresses

Question 13

_____ is recommended for all concrete exposed to freezing.

Answer 13

air entrainment

Question 14

Entrained air voids have diameters that range from ____, with the majority being less than 0.1 mm.

Answer 14

0.01 to 1 mm

Question 15

____ improves with decreasing void size, and small voids reduce strength less than large ones.

Answer 15

frost resistance

Question 16

The fineness of air voids is measured by the ______, equal to the total surface area of voids in a unit volume of paste.

Answer 16

specific surface index

Question 17

_____ improves concrete’s resistance to several destructive factor, including freeze-thaw cycles, deicers and salts, sulfates, and alkali-silica reactivity.

Answer 17

air entrainment

Question 18

____ also increases the workability of fresh concrete.

Answer 18

air entrainment

Question 19

High strength if ___ to attain with air-entrained concrete.

Answer 19

difficult

Question 20

Air entrainers are usually ____ and should meet the specification of ASTM C260.

Answer 20

liquid

Question 21

The agents enhance air entrainment by ____ the surface tension of the mixing water.

Answer 21

lowering

Question 22

____ of fresh or plastic concrete requires more water than is needed for hydration.

Answer 22

workability

Question 23

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.

Answer 23

water-reducing admixtures

Question 24

____ increase the mobility of the cement particles in the plastic miz, allowing workability to be achieved at lower water contents.

Answer 24

water reducers

Question 25

Water reducers are produced with different levels of effectiveness: ____

Answer 25

conventional, mid-range, high-range

Question 26

The high-range water reducer is typically called ______

Answer 26

plasticizer or superplasticizer

Question 27

____ can be used to indirectly to gain strength.

Answer 27

water reducing admixtures

Question 28

Hewlett (1988) demonstrated that water reducers can actually be used to accomplish three different objectives: ___

Answer 28

1. adding a water reducer without altering the other quantities in the mix increases the slump
2. 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
3. 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

Question 29

A ____ allows the use of a lower amount of mixing water while maintaining the same workability level.

Answer 29

water reducer

Question 30

______, 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.

Answer 30

superplasticizers (plasticizers)

Question 31

Superplasticizers can be used when:

Answer 31

1. a low water-cement ratio is beneficial
2. placing thin sections
3. placing concrete around tightly spaced reinforcing steel
4. placing cement underwater
5. placing concrete by pumping
6. consolidating the concrete is difficult

Question 32

When superplasticizers are used, the fresh concrete stays workable for a short time, _____ minutes, and is followed by rapid loss in workability.

Answer 32

30-60

Question 33

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.

Answer 33

retarders

Question 34

Retarders are used for several reasons, such as the following: ___

Answer 34

1. offsetting the effect of hot weather
2. allowing for unusual placement or long haul distances
3. providing time for special finishes

Question 35

_____ can reduce the strength of concrete at early ages (1 to 3 days).

Answer 35

retarders

Question 36

These admixtures have the ability to stop and reactivate the hydration process of concrete.

Answer 36

hydration-control admixtures

Question 37

Hydration-control admixtures consists of two parts: ____

Answer 37

stabilizer, activator

Question 38

Adding the ____ completely stops the hydration of the cementing materials for up to 72 hours.

Answer 38

stabilzer

Question 39

Adding the ______ to the stabilizes concrete reestablishes normal hydration and setting.

Answer 39

activator

Question 40

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.

Answer 40

hydration-control admixtures

Question 41

____ are used to develop early strength of concrete at a faster rate than that developed in normal concrete.

Answer 41

accelerators

Question 42

Accelerators are used to:

Answer 42

1. increases rate of strength gain
2. reduce the amount of time before finishing operations begin
3. reduce curing time
4. plug leaks under hydraulic pressure efficiently

Question 43

_____ is the most widely used accelerator (ASTM D98).

Answer 43

calcium chloride, CaCl2

Question 44

Concrete with CaCl2, develops ______ compared with plain concrete cured at the same temperature.

Answer 44

higher early strength

Question 45

The PCA recommends against using calcium chloride under the following conditions: ____

Answer 45

1. concrete is prestressed
2. concrete contains embedded aluminum such as conduits, especially if the aluminum is in contact with steel
3. concrete is subjected to alkali-aggreagate reaction
4. concrete is in contact with water or soils containing sulfates
5. concrete is placed during hot weather
6. mass application of concrete

Question 46

Several alternatives to the use of calcium chloride are available: _____

Answer 46

1. using high early strength (Type III) cement
2. increasing cement content
3. curing at higher temperatures
4. using non-calcium chloride accelerators such as triethanolamine, sodium thiocyanate, calcium formate, or calcium nitrate

Question 47

What are examples of specialty admixtures?

Answer 47

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

Question 48

Supplementary cementitious materials include: _____

Answer 48

fly ash, ground granulated blast furnace slag, silica fume, and natural pozzolans

Question 49

_____ is the most commonly used pozzolan in civil engineering structures.

Answer 49

fly ash

Question 50

____ is a by-product of the coal-fired electricity production.

Answer 50

fly ash

Question 51

Fly ash is classified (ASTM C618) as follows: ____

Answer 51

class N, class F, class C

Question 52

___ raw or calcined natural pozzolans, including diatomaceous earths, opaline cherts and shales, ruffs and volcanic ashes or pumicites, and some calcined clays and shales.

Answer 52

class N

Question 53

____ fly ash with pozzolan properties.

Answer 53

class F

Question 54

____ fly ash with pozzolan and cementitious properties.

Answer 54

class C

Question 55

Fly ash ____ the workability of the fresh concrete.

Answer 55

increases

Question 56

_____ 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.

Answer 56

slag cement

Question 57

____ is a by-product of the production of silicon metal or ferrosilicon alloys.

Answer 57

silica fume

Question 58

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).

Answer 58

natural pozzolans