Exam 2 Concept questions Flashcards
Ground granulated blast furnace slag (GGBFS) is blended with portland cement because the alkalinity
of the portland cement is needed to activate the GGBFS
True
The highest temperatures in the cement kiln cause clinkering and are required to produce the tricalcium aluminate
The statement above is FALSE, because the highest temperatures produce tricalcium silicate
Hydration becomes diffusion controlled at the beginning this stage of portland cement hydration
Stage 4
Concrete setting and hardening first occur during this stage of portland cement hydration
Stage 3
This stage of portland cement hydration is used for transporting and placing concrete
Stage 2
Which of the following are reasons to use concrete?
Can be cast into any shape, Can be produced from locally available materials, it’s recyclable
What affect does a water-reducing admixture have on concrete?
- Gives the cement particles a negative charge so they repel each other
- Release the water that was trapped between cement particles
- Reduce the amount of water needed for a given slump
- Increase the slump for a given amount of mix water
How can alkali-silica reaction be controlled in concrete?
Add lithium and barium salts to the concrete
How can sulfate attack be controlled in concrete?
Use types of portland cement with low tricalcium aluminate content
How can corrosion of the reinforcement be controlled in concrete?
Use coatings on the reinforcing steel placed in the concrete
Concrete containing pozzolans require additional time to cure
True
Which of the following are important chemicals in portland cement?
Tricalcium aluminate, dicalcium silicate, tricalcium silicate, gypsum
Which of the following are reaction products of portland cement and water?
Ettringite, calcium-silicate-hydrate, calcium hydroxide
Which of the following chemical admixtures is used to increase concrete’s freeze-thaw resistance?
Air entrainer
What causes the start of reinforcement corrosion in concrete?
Chloride ions destroy the passive layer
What is the mechanism of sulfate attack in concrete?
Calcium hydroxide combines with sulfate which causes volume expansion and cracking
What is the mechanism of alkali-silica reaction in concrete?
Aggregates react with alkali hydroxide to form an expansive gel that swells to crack the concrete
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
Pozzolans can reduce the porosity of the concrete
What are three factors that reduce concrete bleeding?
Increase the cement fineness, increasing the rate of hydration, using an air-entraining admixture
What are three factors that contribute to concrete segregation?
Large amount of water in the mix, a large proportion of coarse aggregate, angular aggregate
Which of the following techniques are examples of proper concrete placement for a slab on the ground with a slight incline?
Place concrete at the bottom of the slope, with additional concrete dumped onto the face of previously placed concrete
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?
Replace some of the portland cement in the mix with an equal weight of pozzolan, AND Add a set-retarding admixture
How does the amount of mix water affect permeability?
More mix water causes larger capillary pores which allow water to easily flow into concrete
What chemical is added to concrete to produce microscopic air bubbles and what is the purpose of the air bubbles?
An air entraining admixture which provides protects against freezing water.
What chemical is added to concrete to increase the slump and how does it do this?
A water reducing admixture which stops the cement particles from clumping together.
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?
Decrease the w/c
Concrete with portland cement that is cast under water will never harden.
The statement above is FALSE, because portland cement is hydraulic.
Which type of cement is used for structures in contact with soil containing high concentrations of sulfur?
Type V
How is the portland cement that is resistant to sulfur different than Type I cement?
It has a reduced amount of C3A
What will be the effect of replacing 25% of the portland cement in a batch of concrete with slag cement (GGBFS)?
The concrete will require a longer curing time
The highest temperatures in the cement kiln produce what chemical in portland cement?
C3S
The moderate temperatures in the cement kiln produce what chemical in portland cement?
C2S
Ettringite has a needle-like shape, effects concrete setting, and makes a significant contribution to concrete strength
The statement above is FALSE, because ettringite does not make a significant contribution to concrete strength
What causes flash set and how can it be remedied at the job site?
Flash set is caused by the rapid reaction of C3A and cannot be remedied at the job site
The beginning of Stage 4 of cement hydration is marked by what event?
Hydration rate diffusion-controlled
What stage of cement hydration is used for transporting and placing the concrete?
Stage 2
What stage of cement hydration is hydration rate at its peak?
Stage 3
What causes corrosion of concrete reinforcement and how is it prevented?
Chloride or carbon dioxide in the atmosphere destroys the passive layer; prevented by reducing the water-to-cement ratio
What causes alkali-silica reaction and how is it prevented?
Silica in the aggregate reacts with the cement paste to form an expansive gel; prevented by the addition of silica fume
How can concrete bleeding be reduced?
Replace 10% of the cement with silica fume
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?
- Increased flowability as measured by a higher slump test value
- Decreased compressive strength
- Increased possibility of bleeding
- Increased possibility of segregation
How does the property of thixotropy affect concrete?
Increase workability when vibrated
