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Low Rise Construction > Week 3 > Flashcards

Week 3 Flashcards

1
Q

What is standard cement made of?

A

Water, fine sand, crushed stone & admixtures

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2
Q

What is the purpose of hardened concrete?

A
  • Resists loads
  • Provides strength
  • Provides durability
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3
Q

What processes does fresh concrete allow for?

A
  • Transport
  • Placement
  • Compaction
  • Finishing
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4
Q

What are the 5 steps of concrete construction process?

A
  1. Mix design
  2. Transporting
  3. Placing
  4. Curing
  5. Stripping
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5
Q

During the ‘plastic’ stage, how does concrete take form?

A

Needs formwork

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6
Q

What are the types of concrete?

A
  1. Plain (non-structural)
  2. Reinforced (structural)
  3. Pre-stressed
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7
Q

What is the purpose for ‘General Purpose Portland Cement’?

A

General use

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8
Q

What is the purpose for ‘Rapid Hardening Cement’?

A

To gain early strength

(e.g. when early removal of formwork required)

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9
Q

What is the purpose for ‘Quick Setting Cement’?

A

To complete construction in short time

(e.g. in water)

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10
Q

What is the purpose for ‘Low Heat Cement’?

A

To avoid thermal stress in concrete

(e.g. mass concrete)

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11
Q

What is the purpose for ‘Sulphate Resisting Cement’?

A

To fight against sulphate

e.g. ground floor on sulphate bearing soil

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12
Q

What is the purpose for ‘White Cement’?

A

Architectural purposes

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13
Q

What is the purpose for ‘Coloured Cement’?

A

Architectural purposes

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14
Q

What is the purpose for ‘Air Entraining Cement’?

A

To improve workability

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15
Q

What is the purpose for ‘Hydrographic Cement’?

A

High workability & strength

(due to water repelling chemicals)

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16
Q

What is the purpose for ‘High Alumina Cement’?

A
  • High early strength
  • Resistance to high temperatures
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17
Q

What is the purpose for ‘Masonry Cement’?

A
  • For masonry construction only
  • Not structural
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18
Q

What is Type GP cement?

A

General Purpose Cement

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19
Q

What is Type GB cement?

A

General Purpose Blended Cement

  • Enhanced strength
  • Improve workability/pumpability
  • Reduce bleeding grouts
  • Lower drying shrinkage and creep
  • Increase resistance to sulphate/chloride attack
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20
Q

What is Type HE cement?

A

High Early Strength Cement

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21
Q

What is Type LH cement?

A

Low Heat Cement

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22
Q

What is Type SR cement?

A

Sulphate Resisting Cement

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23
Q

What is Type SL cement?

A

Shrinkage Limited Cement

24
Q

Water characteristics for cement:

A

Water must be clean and have set limits of:

  • Chlorides
  • Sulphates
  • Alkalis
  • Solids
25
Q

What are the 2 groups of aggregates?

A
  1. Course: crushed rock, gravel or screenings
  2. Fine: fine and course sands and crusher fines (particle size less than 5mm)
26
Q

What are the two types of sand?

A
  1. Concreting sand (used for concreting)
  2. Brickies/plasterers sand (NOT used for concreting)
27
Q

How do aggregates become ‘well graded’?

A

Range of sizes that fit together well in concrete

28
Q

How are aggregates measured?

A

In a sieve

29
Q

What are the benefits of rounded aggregates?

A

More workable than angular aggregates

30
Q

What are the benefits of angular aggregates?

A

Make concrete stronger

31
Q

What are the considerations of aggregate selection?

A
  • Strength
    (crumbly or flaky rock types (for example, sandstone) must not be used because they will result in a weaker concrete)
  • Resistance
    (to wear and tear and weathering)
  • Chemically inactive
    (with cement)
  • Free of dirt and clay
    (otherwise, the bond between the cement paste (i.e. the mix of cement and water) and aggregates will be weak)
32
Q

What are the types of admixtures?

A
  • Retarders
  • Accelerator
  • Air-entraining agent
  • Water-reducing admixtures
  • Superplasticizers
  • Corrosion-inhibiting admixtures
33
Q

What do retarder admixtures do?

A

Delay setting time

34
Q

What do accelerator admixtures do?

A

Shorten the setting time

(concrete setting time takes longer in colder climates)

35
Q

What do air-entraining admixtures do?

A

Increase workability while reducing segregation

36
Q

What do water-reducing admixtures do?

A

Used to increase workability without adding water

(by adding them, the water content can be reduced up to 10%)

37
Q

What do superplasticisers admixtures do?

A
  • Enhances workability without adding water
  • Water content may be reduced (by 12-30% by adding them)
38
Q

What do corrosion-inhibiting admixtures do?

A

Slow down corrosion in reinforcements

39
Q

What are the reasons for adding reinforcement steel in concrete?

A
  1. To withstand tensile stresses: concrete is weak in tension
    (reinforcement steel is used to withstand tensile stresses)
  2. To withstand shear stresses: concrete is weak in shear.
    (reinforcement steel is used to withstand shear stresses)
  3. Control cracks: concrete cracks can occur.
    (reinforcement steel is used to reduce shrinkage cracks to an acceptable level)
40
Q

How does cement paste + the nature of aggregates affect the workability of cement?

A
  • Amount of cement paste: a higher amount of cement paste (i.e. water plus cement) in a mix represents a more workable concrete
  • Shape of aggregates: rounded aggregates are much more workable than angular aggregates
  • Grading of aggregates: the aggregates of a range of sizes are more workable than the aggregates of the same or a few sizes
41
Q

How is cement workability measured?

A

Using a slump test

42
Q

How is the slump test measured?

A
  • Dry slump (20-50mm)
    Reduced water: low shrinkage but low workability
  • Normal slump (60-100mm)
    Suitable for general applications
  • Medium slump (120-160mm)
    Increased pumpability for high rise buildings above 130m
  • Flowing concrete (180mm+)
    Enhances placement, saving on labour,
    & required for off-form finishes
43
Q

Typical methods to increase slump of a concrete mix include:

A
  • Add more cement paste
  • Use approved admixture (plasticisers)

(DO NOT ADD WATER - leads to porous concrete)

44
Q

What does the ‘cohesiveness’ of concrete mean?

A

The degree that the ingredients of a concrete holds together.

45
Q

What are the 2 setting times?

A
  • Initial setting time:
    The elapsed time between water and cement are mixed and the paste starts losing its plasticity
  • Final setting time:
    The elapsed time between water and cement are mixed and the paste has completely lost its plasticity
46
Q

What are the characteristics of initial setting time?

A
  • Initial setting time 30-45min
  • Initial setting time can be delayed by adding retarders (for batching & transport)
  • After initial setting time, concrete is not to be disturbed (otherwise the bonds will break)
47
Q

What are the characteristics of final setting time?

A
  • Final setting time approx. 10hrs
  • Structural members (specifically horizontal) are inadequate to support self-weight
  • Concrete must remain supported until stripping time
48
Q

What do structural concrete beams need?

A

Reinforcement steal (rebar)

e.g. to resist tensile and compressive strength

49
Q

What are the thermal properties of concrete?

A
  • Low thermal expansion
  • Horizontal members are supported on rollers (allowing for expansion)
50
Q

What are the 4 steps of concrete preparation?

A
  1. Coarse and fine aggregates are mixed together
  2. A measured amount of cement is added to the aggregate mixture and mixed in
  3. Add a measured amount of water
  4. Mix all the materials until a consistent concrete (an even mix) is obtained
51
Q

What are the parameters of standard cement of housing?

A
  • Strength: 20 MPa or 25 MPa
  • Class: normal class concrete
  • Maximum aggregate size: 20mm
  • Slump: 100mm
52
Q

What are 2 types of cement tests?

A
  1. Slump test
  2. Compression test
53
Q

What are the 3 types of slump in a slump test?

A
  1. True slump:
    the concrete subsides while keeping more or less to the shape of the cone
  2. Shear slump:
    the top portion of the concrete shears off and slips sideways
  3. Collapse slump:
    the total collapse of concrete
54
Q

How are air bubbles removed from concrete?

A
  • Using vibrators
  • Vibrators should extend 150mm into the lowest layer
55
Q

What is the minimum curing period of concrete?

A

3 days (can range from 3-7 days)

56
Q

What is formwork stripping?

A

Formwork stripping done after the structural element is strong enough to withstand its ‘self-weight’ and ‘construction loads’

57
Q

What factors affect stripping time?

A
  • Type of formwork
  • Type of building element
  • Mix design
  • Weather

Low Rise Construction (5 decks)

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