Chemical Attack on Concrete Flashcards
How can Chloride corrosion be prevented in cold regions?
Salts are applied to concrete for de-icing
What is the main ways to prevent Chloride attack?
-Reduce permeability
-Denser binder
-Pozzolanic reactions help in the long term (by producing more C-S-H from Portlandite)
What effect does chloride have on steel?
Makes it rust
Describe the ponding test for Chloride permeability.
-Concrete cylinder or slab, with a pool of chloride solution on top, and wait several months
-Measure how far chloride has travelled into the material, to calculate the diffusion coefficient
What are the advantages and disadvantages of the ponding chloride permeability test?
-Simple and accurate
-Very slow and labour intensive
Describe the rapid chloride permeability test.
Apply voltage and correlate the current passed by the specimen in 6 hours to chloride diffusion
What parameters does the rapid chloride permeability test depend on?
-Chloride ions present
-Other dissolved ions in the pore solution
What is a disadvantage of the rapid voltage permeability test?
Cannot be used to compare materials with different chemistry, but is useful for quality control of one material
Describe the hybrid method to test chloride permeability.
Use electrical acceleration of chloride mitigation, but split the sample and measure actual chloride depth at the end of the test
What are the advantages of the hybrid chloride permeability test?
-24 hour testing time, without dealing with the resistivity of the material
-Can compare different types of cement
-More reproducible
What are the two reaction pathways for sulphate attack in concrete?
-Internal: Sulphates present within concrete, causing phase evolution after a long period
-External: Sulphates from the environment, entering the material and causing phase changes
Describe a Thaumasite Sulphate attack.
-Needs cool, wet conditions, with both carbonates and sulphates present
-C-S-H is converted to thaumasite
-Concrete loses its strength and becomes soft and ‘mushy’
-Relatively rare worldwide, but important in UK conditions
How do you test for Sulphate resistance in concrete?
-Immerse the concrete in a sulphate rich solution (usually 5 wt% Na2SO4)
-Measure specimen length regularly
-Testing for conversion of AFm to AFt phases
-Crystallisation of sulphate salts can cause additional (physical) damage
What are the properties of Sulphate resistant concretes?
-Low Alite content, or high slag content
-Favours the formation of C-S-H rather than AFm phases during hydration, so does not expand
-High slag content also reduces permeability in the long-term
