Concrete Durability Flashcards
Explain how durability of concrete is affected by porosity and permeability
The more permeable a concrete is, the easier it is for aggressive agents to penetrate and degrade the concrete. Larger and a higher quantity of pores means they are more interconnected, causing higher permeability.
Explain how the w/c ratio affects porosity
A lower w/c ratio means cement particles are closer together, reducing capillary porosity, causing total porosity to be lower
Explain how the type of cement affects porosity
Blended cements (PFA, BFS etc) have lower capillary porosity as the additives react with the clinker to form more C-S-H. This decreases the capillary porosity as more gell pores are produced
Describe how the curing process affects permeability of concrete
As cement hydrates, the hydration products fill channels, reducing permeability
How do sulphates attack concrete
Sulphates present in solids are picked up by groundwater and transported into the concrete.
How does sulphate attack damage concrete
The sulphates react with mono-sulfates to form ettringite. The volume of one mole of ettringite is greater than one mole of sulphate, so expansion occurs. Delayed formation of ettringite causes cracking as there is no room for expansion if the cement has set.
List 3 ways of avoiding sulphate attack
Use cements with low porosity and permeability.
Use of SCM’s means lower portlandite (mono sulphates) content.
Use Sulphate resisting Portland cement
Explain how carbonation leads to the corrosion of steel reinforcement in reinforced concrete
Atmospheric CO2 dissolves in water to form carbonic acid. The acid neutralises alkaline pore water and reacts with portlandite in the cement paste. This decreases the pH of concrete pore solution to 8. Manganite is no longer the anodic product at this pH so the steel is passivated and corrosion can occur.
Explain how the rate of Carbonation is affected by relative humidity
At low RH not enough water present so no carbonic acid is produced-carbonation doesn’t take place.
At high RH, pores are saturated, CO2 travels much slower through water. No transport will occur over 50-70% humidity.
State a simple model of carbonation
Carbonation begins at the surface and proceeds onwards. As carbonic acid reacts with portlandite, calcium carbonate is produced. CO2 must then diffuse through this new layer of calcium carbonate.
Explain how chlorides cause the pitting of steel and how this can lead to large scale failure.
Chlorides cause the weakening of the oxide film, creating a pit. This pit becomes an anode as it is depleted in oxygen. The difference in electrolyte concentration causes the flow of electrons from the anode to the cathode. This causes the dissolution of iron from the anode meaning the pit increases in size. This leads to a greater chance of being depleted in oxygen.
