durability Flashcards
What is the most significant form of deterioration in concrete
corrosion of reinforcement
What is permeation, diffusion, absorption and wick action
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
What are structural cracks
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
Despite the high porosity of concrete, why doesn’t the steel rebar corrode
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
Why is concrete solution so alkaline. give approx pH
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
How can the steel passive layer be destroyed (2 ways)
Carbonation and chloride induced corrosion
how does carbonation work
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
Why is carbonation more prevalent at cracks
It is easier for CO2 to diffuse in at the crack
Why does the rate of carbonation slow down
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
Why is carbonation is slower in wet conditions
The cracks and pores become saturated with water, this makes the CO2 diffuse in slower
How does chloride induced corrosion work
The chloride acts as a catalyst to destroy the steel passive layer
What is pitting corrosion
fast but local corrosion.
What are common sources of external chloride on concrete structures
Seawater salts and de-icing salts
How does the steel corrode in chemical terms (transfer of electrons)
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
why does the corrosion of steel cause cracking even when there are no loads applied
The corrosion of steel forms iron oxides which are much greater in volume than steel. This expansion within concrete causes it to crack
What is the chloride content necessary for the catalytic depassivation to propagate
0.4% by weight in concrete
What is the method used to estimate the time for the chloride corrosion to begin propagation
fick’s second law of diffusion. Plots a graph of chloride contents at different depths for different times since operation begins.
What is frost action
Water expands 9% when freezing, so the ice pushes against the pore walls. This repeated expansion and contraction damages the cement paste
How can frost action be mitigated
Use denser cement, use more aggregates, used air entraining agents
What is sulphate attack and what is it caused by
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
What are secondary deposits of ettringite
The ettringite crystals dissolve and recrystallise in the air voids
What is thaumasite sulphate attack
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
What promotes thaumasite formation
low temperatures
what offers resistance to thaumasite formation
Cements with fewer aluminate phases
what is alkali aggregate reaction
The reaction between alkali and reactive aggregates causes a hydrophilic gel to be formed which expands when exposed to water and causes cracking
how to mitigate sulphate attack
Use low penetrability concrete, use less aluminate in cement add coating