Exam 2 Concept questions

Question 1

Ground granulated blast furnace slag (GGBFS) is blended with portland cement because the alkalinity
of the portland cement is needed to activate the GGBFS

Answer 1

True

Question 2

The highest temperatures in the cement kiln cause clinkering and are required to produce the tricalcium aluminate

Answer 2

The statement above is FALSE, because the highest temperatures produce tricalcium silicate

Question 3

Hydration becomes diffusion controlled at the beginning this stage of portland cement hydration

Answer 3

Stage 4

Question 4

Concrete setting and hardening first occur during this stage of portland cement hydration

Answer 4

Stage 3

Question 5

This stage of portland cement hydration is used for transporting and placing concrete

Answer 5

Stage 2

Question 6

Which of the following are reasons to use concrete?

Answer 6

Can be cast into any shape, Can be produced from locally available materials, it’s recyclable

Question 7

What affect does a water-reducing admixture have on concrete?

Answer 7

1. Gives the cement particles a negative charge so they repel each other
2. Release the water that was trapped between cement particles
3. Reduce the amount of water needed for a given slump
4. Increase the slump for a given amount of mix water

Question 8

How can alkali-silica reaction be controlled in concrete?

Answer 8

Add lithium and barium salts to the concrete

Question 9

How can sulfate attack be controlled in concrete?

Answer 9

Use types of portland cement with low tricalcium aluminate content

Question 10

How can corrosion of the reinforcement be controlled in concrete?

Answer 10

Use coatings on the reinforcing steel placed in the concrete

Question 11

Concrete containing pozzolans require additional time to cure

Answer 11

True

Question 12

Which of the following are important chemicals in portland cement?

Answer 12

Tricalcium aluminate, dicalcium silicate, tricalcium silicate, gypsum

Question 13

Which of the following are reaction products of portland cement and water?

Answer 13

Ettringite, calcium-silicate-hydrate, calcium hydroxide

Question 14

Which of the following chemical admixtures is used to increase concrete’s freeze-thaw resistance?

Answer 14

Air entrainer

Question 15

What causes the start of reinforcement corrosion in concrete?

Answer 15

Chloride ions destroy the passive layer

Question 16

What is the mechanism of sulfate attack in concrete?

Answer 16

Calcium hydroxide combines with sulfate which causes volume expansion and cracking

Question 17

What is the mechanism of alkali-silica reaction in concrete?

Answer 17

Aggregates react with alkali hydroxide to form an expansive gel that swells to crack the concrete

Question 18

Which of the following statements are true about pozzolans:
A. Pozzolans react with tricalcium aluminate to form more of the “glue”
B.Pozzolans can reduce the porosity of the concrete
C.
Volcanic ash and diatomaceous earth are examples of waste product pozzolans
D. Fly ash and silica fume are examples of natural pozzolans

Answer 18

Pozzolans can reduce the porosity of the concrete

Question 19

What are three factors that reduce concrete bleeding?

Answer 19

Increase the cement fineness, increasing the rate of hydration, using an air-entraining admixture

Question 20

What are three factors that contribute to concrete segregation?

Answer 20

Large amount of water in the mix, a large proportion of coarse aggregate, angular aggregate

Question 21

Which of the following techniques are examples of proper concrete placement for a slab on the ground with a slight incline?

Answer 21

Place concrete at the bottom of the slope, with additional concrete dumped onto the face of previously placed concrete

Question 22

Concrete is being delivered to a job site on a hot sunny day. Which of the following actions are likely to produce concrete that will remain workable in this situation?

Answer 22

Replace some of the portland cement in the mix with an equal weight of pozzolan, AND Add a set-retarding admixture

Question 23

How does the amount of mix water affect permeability?

Answer 23

More mix water causes larger capillary pores which allow water to easily flow into concrete

Question 24

What chemical is added to concrete to produce microscopic air bubbles and what is the purpose of the air bubbles?

Answer 24

An air entraining admixture which provides protects against freezing water.

Question 25

What chemical is added to concrete to increase the slump and how does it do this?

Answer 25

A water reducing admixture which stops the cement particles from clumping together.

Question 26

A concrete mix has a water-to-cement ratio of 0.45 and produces concrete with a compressive strength of 5500 psi. If air-entraining admixture is added to the mix, how should the amount of cement be changed to maintain a strength of 5500 psi?

Answer 26

Decrease the w/c

Question 27

Concrete with portland cement that is cast under water will never harden.

Answer 27

The statement above is FALSE, because portland cement is hydraulic.

Question 28

Which type of cement is used for structures in contact with soil containing high concentrations of sulfur?

Answer 28

Type V

Question 29

How is the portland cement that is resistant to sulfur different than Type I cement?

Answer 29

It has a reduced amount of C3A

Question 30

What will be the effect of replacing 25% of the portland cement in a batch of concrete with slag cement (GGBFS)?

Answer 30

The concrete will require a longer curing time

Question 31

The highest temperatures in the cement kiln produce what chemical in portland cement?

Answer 31

C3S

Question 32

The moderate temperatures in the cement kiln produce what chemical in portland cement?

Answer 32

C2S

Question 33

Ettringite has a needle-like shape, effects concrete setting, and makes a significant contribution to concrete strength

Answer 33

The statement above is FALSE, because ettringite does not make a significant contribution to concrete strength

Question 34

What causes flash set and how can it be remedied at the job site?

Answer 34

Flash set is caused by the rapid reaction of C3A and cannot be remedied at the job site

Question 35

The beginning of Stage 4 of cement hydration is marked by what event?

Answer 35

Hydration rate diffusion-controlled

Question 36

What stage of cement hydration is used for transporting and placing the concrete?

Answer 36

Stage 2

Question 37

What stage of cement hydration is hydration rate at its peak?

Answer 37

Stage 3

Question 38

What causes corrosion of concrete reinforcement and how is it prevented?

Answer 38

Chloride or carbon dioxide in the atmosphere destroys the passive layer; prevented by reducing the water-to-cement ratio

Question 39

What causes alkali-silica reaction and how is it prevented?

Answer 39

Silica in the aggregate reacts with the cement paste to form an expansive gel; prevented by the addition of silica fume

Question 40

How can concrete bleeding be reduced?

Answer 40

Replace 10% of the cement with silica fume

Question 41

A concrete mix with a 3000 psi compressive strength is developed. In the field, the contract asks to add additional water to the mix. What are the likely consequences of adding additional mix water?

Answer 41

1. Increased flowability as measured by a higher slump test value 2. Decreased compressive strength 3. Increased possibility of bleeding 4. Increased possibility of segregation

Question 42

How does the property of thixotropy affect concrete?

Answer 42

Increase workability when vibrated