Durability of Concrete Flashcards
What is an alkali-aggregate reaction?
When the aggregate contains reactive components, it can be attacked by the pore solution (Portland cement contains small amounts of Na and K that stay in the pore solution)
What are the main Alkali guidelines put in place in the UK?
-Limits om alklai content of concrete based on aggregate reactivity classification
-Most cements sold in the UK are low-alkali
-Blending with slag or pozzolans helps protect from damage
Describe the accelerated testing method (ASTM).
Mortar bars are places in 40 g/L NaOH at 80 degrees, and the expansion is measured after 16 days
What are some advantages and disadvantages of the accelerated testing method?
-Adv: Fast and widely used
-Disadv: Known to give false results (because its an extremely aggressive test)
What are the issues with carbonation?
-Atmospheric CO2 can act as an acid, which reduces the pH of cement which can induce corrosion of the steel reinforcements
-Happens fastest at intermediate humidity (~65%) or under wet-dry cycles
-The main limitation on concrete service life in warm climate areas without marine exposure
How do you measure carbonation?
Measure the depth of CO2 ingress after exposure to CO2 (often at a higher concentration)
Why could carbonation be a positive thing?
It is a possible route to global CO2 reduction, reversing some of the emissions from cement production
What is freeze-thaw damage?
Degradation based on the ingress of water, when it freezes water expands ~9% and even more when higher salt levels are present
What happens during a freeze-thaw test?
-Samples (usually cylinders or slabs) experience freeze-thaw cycles repeatedly
-Tens to hundreds of cycles used
-Measure changes in elastic modulus, dimensions, mass
-Sometimes just give a visual rating of damage
How can freeze-thaw damage be prevented?
-Put appropriately sized bubbles into concrete
-Gives the water space to expand into as it freezes
