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Construction Materials > Concrete part 1 > Flashcards

Concrete part 1 Flashcards

1
Q

What are the 3 main ingredients of concrete?

A

1) Cement powder (PC)
2) Water
3) Aggregates (fine and coarse)

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2
Q

How does concrete form?

A

1) Cement powder is mixed with water to form cement paste.
2) Water causes the dissolution of the cement powder and the ions that are released react during the hydration process to form new phases called hydration products (gel) which binds the paste.

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3
Q

What are the cement powder grains made from?

A

Calcium silicate the most abundant being tricalcium silicate (up to 75% of the mass).

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4
Q

What is the composition of the hydration product?

A

Calcium and silicon are the most abundant elements in the hydration product. C-S-H is non-stoichiometric and can form with the different relative amount of Ca and Si.

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5
Q

What is the typical ratio x/y called?

A

The calcium to silicon ration or C/S ratio, typically between 1 and 2.

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6
Q

What is the cement gel or HP made from (largely)?

A

C-S-H

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7
Q

How wide are the gels mesopores?

A

2-50nm wide.

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8
Q

What is there not enough of in PC?

A

Si to convert all Ca into C-S-H. Excess Ca reacts with water forming calcium hydroxide also called Portlandite (CH).

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9
Q

What is Portlandite or CH?

A

A crystal, less complex and less important than C-S-H for mechanical properties but it’s soluble and keeps the concentration constant, fixing pH to 11.

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10
Q

What does fixing the pH to 11 prevent?

A

Prevents steel rebars from corrosion.

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11
Q

What can aluminium replace in the hydration product?

A

It can replace Si in C-S-H, forming C-A-S-H.

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12
Q

What is the issue with using Al during the hydration product?

A

C3A dissolves very fast, causing fast production of C-A-S-H and hydrogarnet which links the cement grains in a process called “flash setting”.

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13
Q

What does flash setting do?

A

Makes the concrete unworkable.

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14
Q

How can we prevent flash setting?

A

Add gypsum

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15
Q

What does gypsum do?

A

Dissolves releasing sulphur in the solution, which forms two classes of materials; AFt and Afm.

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16
Q

What is the most common AFt?

A

Ettringite

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17
Q

What is the role of ettringite?

A

Forms nano-crystals that coat the C3A stopping its dissolution and stopping flash setting.

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18
Q

What is primary ettringite?

A

Its when with time (hours and days) the ettringite reorganises into larger needle crystals.

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19
Q

What is the most common AFm?

A

Monosulfoaluminate

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20
Q

What happens when extra sulphur is added?

A

The Monosulfoaluminate binds it and converts it to secondary ettringite, very expansive reaction (volume increases).

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21
Q

What can happen when secondary ettringite forms?

A

Hard mature concrete can cause cracks due to said expansion.

22
Q

Why do we not use seawater to make concrete?

A

Chloride causes corrosion of the steel reinforcement but it can be equally as strong if not stronger.

23
Q

What are the 3 microscopic properties of aggregates?

A

1) Chemical composition
2) Surface texture
3) Porosity

24
Q

When is concrete strong?

A

In compression but weak in tension.

25
Q

How do we add tensile strength to concrete?

A

We usually add reinforcement bars or a pair of concrete slabs with steel scaffolds or beams.

26
Q

What is the width of the capillary pores?

A

Greater than 50nm.

27
Q

If there is more water in the original mix what happens to the number of capillary pores left and whats effect does this have?

A

There will be more. The more capillary pores the more interconnected they get, providing pathways for external chemicals to permeate into the concrete.

28
Q

What are the sizes of fine and coarse-grained aggregates?

A

Fine- less than 2mm

Coarse- greater than 2mm

29
Q

What are the 3 key features of aggregates?

A

1) Quantity
2) Good grading
3) Shape- impacting strength and workability

30
Q

Are fibres always included in concrete construction?

A

No, they impede micro-cracks from various degradation process.

31
Q

What are fibres made out of?

A

Steel or plastic.

32
Q

Why are glass fibres dangerous?

A

Amorphous silica causing ASR reaction.

33
Q

What is the size of gel pores?

A

Smaller than capillary pores (<50nm).

34
Q

What do gel pores do?

A

React strongly with water causing volume changes upon drying and wetting concrete.

35
Q

Where are gel pore intrinsic to?

A

C-S-H and C-A-S-H phase.

36
Q

What are the two rocks that create PC?

A

1) Shale- rich in Al and Si

2) Limestone- rich in Ca

37
Q

What is the process to create PC?

A

Shale and limestone ground together obtaining a granular material that goes to the kiln.

38
Q

What is a kiln?

A

A huge cylinder is more than 10m long. Its pre-heated to 600 degrees C and the granular material is progressively heated up to more than 1450 degrees C

39
Q

What is the role of the kiln?

A

Transfer limestone and shale into the main phases of cement:

1) C2S- dicalcium silicate- 900 degrees C
2) C3A- Tricalcium aluminate- 1200 degrees C
3) C3S- Tricalcium silicate- 1400 degree C

40
Q

Why must aggregates be washed?

A

To preempt chemical reactions when added in the concrete mix.

41
Q

What does workability depend on?

A

Water content, additives and types of aggregates.

42
Q

What is the test to assess workability?

A

The slump test.

43
Q

What happens when there has been insufficient workability?

A

Gaps are left between the rebar and therefore its too fluid causing segregation of aggregates (i.e. aggregates all at the bottom of the mix).

44
Q

How can we prevent and remove air bubbles?

A

Hand compaction or mechanical vibration.

45
Q

How long doe it take for ordinary cement to set?

A

Few hours

46
Q

What does setting depend on? (4)

A

1) Chemical composition
2) Additives
3) Temperature
4) Not so much on water content

47
Q

When can frameworks be removed?

A

After 1-2 days.

48
Q

How long must you wait for the concrete to cure (gain strength) before loading it?

A

28 days

49
Q

What are the best conditions for curing?

A

High humidity and temperature between 0-20 degrees C.

50
Q

Why humidity good for curing?

A

Supplies water to sustain hydration.

51
Q

What effect does temperature have on curing?

A

If the temperature is too low, setting and curing take too long and if the temperature is too high, the C-S-H forms with more gel pores.

Construction Materials (9 decks)

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