Deterioration Of Concrete

Question 1

Name the 2 types of concrete attack

Answer 1

Physical attack & Chemical attack:

Question 2

Physical Attacks include:

Answer 2

 Impact Damage. 
 Overloading. 
 Freeze/Thaw Cycles. 
 Corrosion of Reinforcement. 
 Abrasion.

Question 3

Chemical Attacks include:

Answer 3

 Alkali-aggregate Reaction (AAR). 
 Sulphate Attack. 
 Chloride Attack. 
 Carbonation. 
 Corrosion of Inset Steelwork. 
 Biological Attack.
Degradation of concrete by environmental action is termed, Weathering.

Question 4

Alkali-Aggregate Reaction (AAR)

Answer 4

 Aggregate containing silica will undergo alkali- silica reaction, this forms a gel & begins to absorb water.
 The gel swells & exerts a pressure within theconcrete.
 Gel may weep from the surface of the
concrete, termed Exudation, can lead to
spalling.

Question 5

Sulphate Attack

Answer 5

 Seawater contains sulphates, if allowed to penetrate into the concrete, will react with the hydrated cement to form a crystalline solid (Ettringite).
 This causes expansion, leading to cracking & spalling.

Question 6

Chloride Attack.

Answer 6

 If seawater seeps next to the reinforcement within concrete, Chlorides will destroy the passive Fe2O3 skin.
 This initialises the reinforcement corrosion process, resulting in cracking & spalling.

Question 7

Carbonation

Answer 7

 If atmospheric CO2 reaches the reinforcement within the concrete, passivation will be lost.
 This will result in reinforcement corrosion, cracking & spalling.

Question 8

Biological Attack.

Answer 8

 Under Anaerobic conditions, sulphate-reducing bacteria living in the small voids in the concrete, produce H2S.
 Some of the H2S may be captured by aerobic bacteria, that oxidise it to form Sulphuric Acid.
 This dissolves away the carbonates from the cement & weakens the material, leading to cracking & spalling.

Question 9

Corrosion of Inset Steelwork.

Answer 9

 More likely than reinforcement corrosion.
 Different types of steel components cast into the structure - riser clamps - steel skirts - towing eyes.
 Should any of this be in contact with the internal reinforcement, the exposed steelwork acts as an anode & the reinforcement becomes the cathode.

Question 10

Name the 3 categories of concrete deterioration

Answer 10

 Category A – Defects.
 Category B - Areas of Concern.
 Category C – Blemishes.

Question 11

Category A (Defects).

Answer 11

Usually require detailed investigation & are likely to generate an intervention to repair. 
 Crack. 
 Impact Damage. 
 Popout. 
 Delamination. 
 Variable Cover. 
 Exposed Reinforcement. 
 Tearing. 
 Poor Repair.

Question 12

Category B (Areas of Concern).

Answer 12

Usually monitored but may require an intervention to repair.
 Embedded objects.
 Recessed metal plates. 
 Cast-in sockets. 
 Abrasion. 
 Water-jet damage. 
 Honeycombing.

Question 13

Category C (Blemishes).

Answer 13

Surface imperfections that have no structural significance.
 Construction joint.
 Rubbing-down marks.
 Blowholes.
 Formwork misalignment. 
 Scabbling.
 Regular horizontal
 Good repair.
ridges.
 Resin mortar repair.
 Irregular horizontal
 Sealant run.
ridges. 
 Vertical drag marks. 
 Curing compound.

Question 14

There are a few additional imperfections that the report does not categorise:

Answer 14

 Missing or damaged sealant.
 Surface staining.
 Corrosion of inset steelwork.
 Biological attack.

Question 15

Surface Staining may be the result of:

Answer 15

 Exudation.
 Efflorescence.
 Lime Encrustation.
 Corrosion.
Corrosion is the only stain likely to be seen during an underwater survey - should be reported.
When photographing staining, include a colour bar, so the photo is presented in true colour.
Engineers may then be able to gauge the age of the defect by the colour of the stain.