durability

Question 1

What is the most significant form of deterioration in concrete

Answer 1

corrosion of reinforcement

Question 2

What is permeation, diffusion, absorption and wick action

Answer 2

flow via pressure gradient, flow via concentration gradient, flow by capillary action, flow where one side is drying and the other is exposed to water

Question 3

What are structural cracks

Answer 3

cracks that are intended to form on the tension side of concrete. Since cracks cannot be eliminated, structural cracks are added to control the cracking

Question 4

Despite the high porosity of concrete, why doesn’t the steel rebar corrode

Answer 4

The rebar forms a passive iron oxide layer and also concrete is highly alkaline so any water that enters the concrete becomes an alkaline solution. Metals only corrode in acidic environments, so the alkaline solution protects it

Question 5

Why is concrete solution so alkaline. give approx pH

Answer 5

The addition of potassium oxide and sodium oxide which are high in solubility. Also other constituents in concrete like CaO are also quite basic. results in pH of 13.5

Question 6

How can the steel passive layer be destroyed (2 ways)

Answer 6

Carbonation and chloride induced corrosion

Question 7

how does carbonation work

Answer 7

The carbon dioxide reacts with water in pore solution to form carbonic acid. This acid then reacts with hydration products to form calcium carbonate. This reduces the pH to about 8.5 and removes the steel passive layer

Question 8

Why is carbonation more prevalent at cracks

Answer 8

It is easier for CO2 to diffuse in at the crack

Question 9

Why does the rate of carbonation slow down

Answer 9

The carbonation of concrete produces CaCO3 which is denser and stronger than normal cement paste. This reduces porosity so it is harder for CO2 to diffuse deeper into the concrete

Question 10

Why is carbonation is slower in wet conditions

Answer 10

The cracks and pores become saturated with water, this makes the CO2 diffuse in slower

Question 11

How does chloride induced corrosion work

Answer 11

The chloride acts as a catalyst to destroy the steel passive layer

Question 12

What is pitting corrosion

Answer 12

fast but local corrosion.

Question 13

What are common sources of external chloride on concrete structures

Answer 13

Seawater salts and de-icing salts

Question 14

How does the steel corrode in chemical terms (transfer of electrons)

Answer 14

The iron in steel dissolves and forms Fe2+ and releases two electrons. The electrons are taken by H2O and O2 to form OH- ions. The OH- ions are then attracted to the Fe2+ ions

Question 15

why does the corrosion of steel cause cracking even when there are no loads applied

Answer 15

The corrosion of steel forms iron oxides which are much greater in volume than steel. This expansion within concrete causes it to crack

Question 16

What is the chloride content necessary for the catalytic depassivation to propagate

Answer 16

0.4% by weight in concrete

Question 17

What is the method used to estimate the time for the chloride corrosion to begin propagation

Answer 17

fick’s second law of diffusion. Plots a graph of chloride contents at different depths for different times since operation begins.

Question 18

What is frost action

Answer 18

Water expands 9% when freezing, so the ice pushes against the pore walls. This repeated expansion and contraction damages the cement paste

Question 19

How can frost action be mitigated

Answer 19

Use denser cement, use more aggregates, used air entraining agents

Question 20

What is sulphate attack and what is it caused by

Answer 20

Sulphate in groundwater from decay of organic matter penetrates into the concretes and reacts with the hydration products, particularly aluminate phases to form gypsum and ettringite which have larger volumes so cause cracking

Question 21

What are secondary deposits of ettringite

Answer 21

The ettringite crystals dissolve and recrystallise in the air voids

Question 22

What is thaumasite sulphate attack

Answer 22

Another form of sulphate attack which occurs when in presence of carbonate. Thaumasite replaces CSH and the concrete expands and becomes a soft mush. Thaumasite is comprised of CSH, sulphate and carbonate

Question 23

What promotes thaumasite formation

Answer 23

low temperatures

Question 24

what offers resistance to thaumasite formation

Answer 24

Cements with fewer aluminate phases

Question 25

what is alkali aggregate reaction

Answer 25

The reaction between alkali and reactive aggregates causes a hydrophilic gel to be formed which expands when exposed to water and causes cracking

Question 26

how to mitigate sulphate attack

Answer 26

Use low penetrability concrete, use less aluminate in cement add coating