T2-6: Alkali Silica Reaction

Question 1

What percentage of concrete does aggregate account for?

Answer 1

50-80%

Question 2

What is ASR a common form of?

Answer 2

An alkali-aggregate reaction (AAR)
- these are deleterious reactions that can occur between the alkaline pore solution from the cement paste, and certain types of aggregate

Question 3

Aggregates contain reactive forms of silica. What is a type of this?

Answer 3

Quartz, if sufficiently strained or microcrystalline (chalcedony, cherts, flints)

Question 4

Describe the reaction of reactive silica in aggregate
- what is present?

Answer 4

The reaction of reactive silica in aggregate with hydroxyl ions (OH-) present (in the cement paste) leads to expansion and cracking
.

Question 5

What type of cracking does ASR produce?

What may the cracks be covered with if the ASR is very active?

Answer 5

‘Map’ or ‘pattern’ cracking

Cracks may be covered with carbonated gel

Question 6

Give an example of how the ASR crack pattern might be different if the concrete is restrained

Answer 6

e.g. cracks tend to run parallel to reinforcing bars

Question 7

When in a structure’s life do ASR cracks appear?

Compare to shrinkage cracks

Answer 7

Appear much later in a structure’s life than shrinkage cracks (which appear early)

Question 8

What happens in the first stage of the ASR reaction?

Answer 8

OH- (hydroxyl ions) present in alkaline pore solution attach to silicon atoms

Consequent breaking of silicon/oxygen bonds forms silanol (Si-OH) groups

Question 9

What happens in the second stage of the ASR reaction (x2)?

Answer 9

The Si-OH (silanol) groups react with further OH- ions, leaving negatively charged oxygen atoms

Negative charge on oxygen atoms is balanced by Na+/K+ ions that simultaneously diffuse into structure
- loosens structure, increases its capacity to absorb alkali cations and H20

Question 10

Describe the third* stage of ASR reaction
- what is formed?

Answer 10

The deformable, damaged framework can inhibit more water molecules and expands; highly mobile

The alkali-silica gel is highly unstable in the presence of Ca2+ (calcium) ions; reacts with them to form C-S-H

Question 11

What is the difference between the pozzolanic reactions and ASR

Answer 11

Pozzolanic reaction:
- alkali-silica gel is formed in a Ca2+ -rich environment and quickly converted to CSH

ASR:
- formed in a Ca2+ -poor environment
- cement paste cannot supply Ca2+ fast enough to prevent the gel from persisting for long expansion
- massive outflows of gel can result

Question 12

To summarise:
- when is alkali-silica gel formed, with respect to the supply of calcium ions?
- what does it quickly transform into
- what are the inner/outer products in the hydrated cement paste?

Answer 12

• Formed where there is a plentiful supply of Ca2+ ions from the alite reaction (ie. impure C3S)
• Alkali-silica gel transforms into CSH

Inner = PFA particle
Outer = CSH in a hydrated cement paste

Question 13

What are three necessary conditions for ASR?

Answer 13

• A reactive constituent in the aggregate
• High content of alkali oxides in the cement
• Water supply

Question 14

What are three avoidance conditions for ASR, and two reasons why?

Answer 14

To prevent ASR:
- Avoid aggregate containing reactive constituent (ie. it needs reactive constituent)
- Use low alkali cement
- Partially replace Portland cement with PFA of slag

Why:
- Alkali binding power of CSH is enhanced
- Hindered alkali migration due to reduced permeability

Question 15

Name a method of diagnosing ASR

What provides complimentary info?

Answer 15

Microscopial examination by transmitted light microscopy

Complimentary info from electron microscopy

Question 16

What may confirm the ‘ASR damage’?

ie. what does the crack contain

Answer 16

A crack in the particle of aggregate containing a potassium-rich gel
.