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ACD > T1: Pre- and Post-Cracking Behaviour of RC > Flashcards

T1: Pre- and Post-Cracking Behaviour of RC Flashcards

(60 cards)

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1
Q

What is the difference between a direct and indirect load?

A

Direct - loads applied directly to a structure, like dead weight or concentrated forces

Indirect - act on a structure indirectly, potentially due to factors like stress changes, temperature variations, or settlement

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2
Q

What is the difference between SLS and ULS?

A

SLS - ensures a structure remains functional and comfortable under normal use; will perform to a satisfactory standard under in-service conditions

ULS - ensures the structure doesn’t collapse under extreme loads

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3
Q

What is the difference between one-way and two-way slabs?

A

One-way slabs - span and transfer loads in one direction, typically the longer span

Two-way slabs - span and transfer loads in both directions, generally supported on all four sides

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4
Q

[NAQ] difference in load transfer between one-way and two-way slabs

A
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5
Q

What does grade 35/45 concrete have?

Which value is used in design?

A
  • A 28-day cylinder strength of 35 MPa (used in design)
  • A 28-day cube strength of 45 MPa
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6
Q

What is the commonly used grade of steel reinforcement, and what is its characteristic yield strength?

A

Type B

F_yk = 500 MPa

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7
Q

Describe thermal contraction

What happens when concrete is restrained on one side?

A

When you cast concrete, it shrinks (thermal contraction)

  • Being restrained on one side means it can’t shrink
  • Get vertical cracks originating from side that is restrained, which is under tension
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8
Q

Name a mechanism of concrete shrinkage

A

Loss of moisture (drying shrinkage)

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9
Q

Describe concrete

A

A series of coarse/fine aggregates stuck together with an adhesive or binder

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10
Q

What is concrete adhesive (or binder) made of?

How is it formed?

A

Hardened cement paste

Formed by hydration between cement and water

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11
Q

What can cause both:
- cracking as concrete cools during the first 2 or 3 days
- a volume reduction (shrinkage), which can also cause cracking after pouring

A

The exothermic hydration reaction

ie. temperature generated without restriction can cause cracking

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12
Q

What does concrete behaviour do under constant load?

A

It creeps - continues to move slightly

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13
Q

How does concrete behaviour react to changes in the:
a) ambient temperature
b) humidity

A

Temperature - expanding or contracting

Humidity - by swelling or shrinking

(NB. very slightly)

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14
Q

Why is concrete usually reinforced or prestressed?

A

Because it has low tensile strength

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15
Q

Why is steel compatible with concrete (x2)?

A
  • Similar thermal expansion characteristics
  • Low Poisson’s ratio
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16
Q

How is steel reinforcement usually protected against corrosion?

A

Protected by a low permeability layer of alkaline cement paste (concrete cover)

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17
Q

Why does the steel reinforcement have a passive layer?

A

Because iron oxides form on its surface, due to the high alkalinity of concrete

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18
Q

Name the dominant mechanism of tensile creep, and one of the four mechanisms of compressive creep?

A

Microcracking

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19
Q

What does cement paste contain?
- What is it used for?
- What property does it have?
- What does it contain?

A

It is a mixture of cement and water, often used as a binder in concrete

  • It is porous
  • Contains microcracks
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20
Q

What can produce cracks?

A

Excessive tensile stress

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21
Q

What is the initial and final setting time of concrete?

A

Initial - 2 hours
Final - 10 hours

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22
Q

a) at what time is the typical peak hydration temperature?

b) what can peak hydration temperature typically be up to?

A

10 hours

48 hours

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23
Q

What part of the concrete has the highest temp, and why?

Where do cracks form and what can get rid of them?

A

The middle of the concrete

  • Middle gets hotter and wants to expand, being held back by the surface; area near formwork goes into tension and can start to create small cracks in surface
  • Re-vibrating can get rid of these cracks
  • Eventually, cracks in middle can link with outer cracks (becoming through cracks)
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24
Q

What are three different types of concrete

A

Portland cement
- CEM 1 (most common, but being used less)

Aluminuous cements
- e.g. high alumina cement (HAC)

Blended cements
- e.g. portland cement + partial cement replacement
- improved durability

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25
What are three types of partial cement replacements?
**PFA** - **fly ash** - waste from **coal** fired power stations **SF** - **silica fume** - produces **high strength** concrete **GGBS** - ground granulated blast-furnace slag
26
For adequate mixing, what are two performance requirements of concrete?
- **Cohesive** mix that doesn't suffer from **segregation** and **bleeding** - Adequate **workability**
27
What are three performance requirements of concrete?
- Minimise **shrinkage** and **early thermal cracking** - Minimise **permeability** (and maximise durability) - Provide **required strength** (compressive and/or tensile) .
28
How is shrinkage and early thermal cracking minimised?
Having a **high aggregate:cement** ratio (aggregates are rigid and restrict contraction of cement paste)
29
How is permeability minimised (and durability maximised)?
Keeping a **low water:cement** ratio
30
What are the three main components of grey/green Portland Cement powder? Gypsum is one - what does it do?
**Calcium silicates** - C2S, C3S - quality and strength **Calcium aluminates and aluminoferrite** - C3A and C4AF **Gypsum** - prevents flash setting
31
What is cement hydration?
The reaction that occurs when **cement and water** are mixed
32
Describe hydration stage 1 What are the main compounds produced?
The **cement powder and water are mixed**: - takes 30 mins for reaction to start - main compounds produced are **Calcium Silicate & Aluminate hydrates** and **Calcium Hydroxide**
33
Describe hydration stage 2 What happens to the unhydrated particles?
**Hydration starts**: - initial set can occur within about **2 hours after mixing** - unhydrated particles become coated with a **gel of calcium silicate hydrates** (C-S-H) and **aluminate hydrates** (Aft and AFm) - **calcium hydroxide** (C-H) crystals start to form NB. initial set is the change of state from a liquid to a stiff paste (solid)
34
[NAQ] microstructure of cement paste 1
35
[NAQ] Ettringite (Aft) expansion during very early stage of hydration
36
What is much more chemically stable than the Ettringite
**AFm** (monosulfate) NB. conversion of AFm **back into Etrringite/Aft** after concrete has set and hardened can lead to **severe cracking of the cement paste** .
37
Describe hydration stage 3 What forms between the gel coated particles?
'**Final set**', strength gain: - **Strong chemical bonds** form between the gel coated particles - **Cement matrix** starts to gain in strength - **2 to 10 hours** after mixing - Temp of concrete **rises and then cools**
38
Describe hydration stage 4 What continues to form?
**Introduction of pore system**: - **more gel and calcium hydroxide forms**; more chemical bonding takes place - cement matrix gains strength - **hydration reaction slows down** - 'pockets' of mix water are isolated in pores known as **capillary pores**
39
What does the pore water (in capillary pores) causes the cement to have a pH of? Is this acidic or alkaline?
**pH 13** - highly alkaline
40
At what time does concrete typically gain about 2/3rds of its 28-day strength?
Between **3 and 7 days**
41
[NAQ] view of cement paste during very early stages of hydration
42
Name the main hydration products Why are more products developed during the hydration reaction, and what is the overall result?
- Calcium silicate hydrates (**C-S-H**) - Calcium hydroxide (**C-H**) - *Calcium aluminate hydrates* (**AFt** converted to **AFm**) More products develop during hydration reaction to **fill pore spaces** (particularly the C-S-H) - overall result is **shrinkage**
43
Describe hydration stage 5 What is hoped at this stage?
Hydration reaction '**ceases**': - unhydrated cement known as **anhydrous residue** - hoped at this stage that the **number of capillary pores is very small**
44
What is the number of capillary pores dependent on (x2)?
- **W/C** ratio - Degree of **curing**
45
For high-quality concrete, what must be done and why?
Must **reduce the permeability** to a minimum - because the hardened cement paste is porous and permeable
46
Summarise the hydration process
47
What bond has a big influence on the composite behaviour and ability of RC to perform? What term is used?
The **bond between cement paste and reinforcement** If bond remains in tact, the two materials exhibit '**strain compatibility**'
48
What helps create an effective cement/steel bond? What must there be to permit effective transfer of stress?
- The choice of **materials** (steel has a comparatively **low Poisson’s ratio** and that the **coefficients of thermal expansion** of the materials are very similar) - Must be sufficient concrete (**cover**) around reinforcement to permit effective transfer of stress
49
What must the cover be at least equal to, for the main reinforcement?
At least equal to the **bar diameter**
50
What material properties does concrete have with respect to: a) brittleness b) ductility c) linearity (in compression)
a) **rigid** brittle b) **no ductility** c) **non-linear in compression**, as the stress approaches the peak value
51
Define ductility Which of steel/concrete is ductile/brittle?
Ducility - ability to be **drawn or stretched into a wire** without breaking Steel = ductile Concrete = brittle
52
[NAQ] actual vs idealised steel reinforcement stress/strain curves
53
Describe a BM/curvature (short-term) relationship curve for an RC beam subjected to bending What are the three 'stages'?
1. Pre-cracked 2. Post-cracked 3. Plastic behaviour
54
Describe the pre-cracked condition of a RC beam What analysis method is valid?
- At start (linear), section is **uncracked** with **linear elastic behaviour** (low rebar stress, stress in concrete less than tensile strength) - **Point A** is the **serviceability limit state of cracking** **Linear elastic analysis**
55
Describe the post-cracked condition of a RC beam What analysis method is valid? What happens at B?
- Between A and B, **further cracking** occurs and **stress in steel increases** - Up to B, behaviour is **non-linear approximately elastic** - At B, steel has **started to yield** **Non-linear elastic analysis**
56
What can be said to describe what is happening between A and B?
Stabilised crack pattern
57
[NAQ] post-cracked condition load/strain graph, with number of cracks
58
Describe the plastic behaviour of a RC beam What is reached at C, and what happens to the steel after C? What does the concrete section **behave like**? What analysis method is valid?
- At C, **ultimate moment capacity** is reached - Beyond C, **steel deforms plastically** - **Large rotations experienced** - Concrete section **behaves like a hinge**, with other hinges established in other highly stressed parts of structure **Plastic methods of analysis** (e.g. yield line)
59
What is needed to cause the structure of a RC beam to collapse?
**Sufficient hinges established** to form a **mechanism**
60
What is needed for plastic methods of analysis to be valid (x2)?
- Need to demonstrate that there is **sufficient ductility for hanging action to occur**, - And **that the concrete does not crush** (e.g. as at D)

ACD (5 decks)

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