Durability Flashcards
What is the objective of durability?
To design and build concrete structures that meet their
Expected Service Lives with Minimum Repair
What is deterioration?
Physical manifestation of failure of a material (e.g., cracking, spalling, delamination, pitting)
Decomposition of material (disintegration, weathering)
WHAT ARE THE CAUSE OF CONCRETE DETERIORATION?
PHYSICAL DETERIORATION
CHEMICAL DETERIORATION
REINFORCEMENT CORROSION
What causes PHYSICAL DETERIORATION?
Frost
Cracking
Fire
Abrasion
What causes CHEMICAL DETERIORATION?
Sulphate
AAR
Sea water
Leaching
What are the causes REINFORCEMENT CORROSION?
Chlorides
Carbonation
How much of steel produced annually in the world is used to repair corrosion prob?
40% (25% to 30% for corrosion within reinforced conrete)
What is corrosion?
it is a cycle caused by iron oxyde (rust) that expands the concrete –>cause stress–> causes failure —> causes spalling of concrete (écaillage) –>causes more rust (cycle goes back to expansion)
What is passivation?
thin oxide film on reinforcement
What causes de-passivation?
Entering of chlorides ION
Carbonation of concrete
How much can volume increase with rust?
Up to 6 times
How to differentiate carbonation from chloride infiltration?
Phenolphthalein test
Phenolphthalein test –>PINK
NOT CARBONATED/Chloride infiltration
Phenolphthalein test–> not pink
Carbonated
What is the diffusion coeff?
rate of chloride ingress
how to reduce diffusion coeff?
by reducing voids–>reducing permeability–>reducing water–>reducing diffusion coeff?
What to use to reduce corrosion?
Epoxy coated bars (if defected in coat, it can accelerate the corrosion affect (concentrated in one pts))
Stainless Steel rebar ($$$)
FRP rebar (brittle)
How to repair corrosion affected concrete?
Remove concrete–>corrosion inhibitors (adm) –> cover concrete –> coating–> impregnation
some cases, replace reinforcement/add new
How much water expands as it freezes?
Expands up to 9% (tensile stress)
What are the most common signs of freeze/thaw deterioration?
-Development of cracks sub-parallel to the surface
-Cracks throughout the concrete (delamination)
-Gaps around aggregates in the surface regions of concrete.
How much freeze&froze cycle can concrete resists?
300 cycles before damages
What is paste failure? (spalling)
Paste failure: Fracture surface consists of broken paste and occasionally undisturbed agg. faces
What is agg. failure? (spalling)
spalled section contains a broken agg. at the bottom of fracture surface pit
in sidewalks, where does D-cracking and scaling occurs bc of freeze-thaw?
expansion joints
How to protect concrete from F-t?
keep concrete dry (not always possible)
Reduce amt of freezable water
Provide a relief for pressure (AEA)
Combination of above
Where does the Alkali in AAR comes from?
from cement
Where does the silica in ASR comes from?
Agg (silica mineral)
ACR or ASR worst?
ACR is worst (alkali carbonate reaction)
Avoid it by using agg in Kingston (limestone)
What are the field symptoms of ASR?
Longitudinal cracking
Cracks starts from the 1st layer of agg
Map cracking
White powder (silica gel extracted from concrete)
How to prevent AAR (ASR & ACR)
use non reactive agg (100% for ACR)
selective aff
Limit alkali in cement & concrete
Lithium adm (earlier apply –> less damage)
use SCM (SF, FA, Slag)
Tight the concrete (confinement–>increase strength of struc; NOT permanent)
How is the sulfate dangerous?
Sulfate is aggressive with water/moisture bc attack hydration product for cement
Hydration products; what are the others?
Ettringite —> Monosulfate (stable phase of ettiringite)
How is ettringite and monosulfate created?
C3A + gypsum+water =ettringite (unstable)
C3A+ettringite+water = monosulfate (stable)
Volume ett> volume mono (about 700times)
How can monosulfate go back to ettringite?
CH+sodium sulfate= GYPSUM +sodium hydroxide
GYPSUM +monosulfate +Water= ettringite
WHY is it a problem the conversion of ettringite to monosulfate?
Very large volume expansion and contraction
How to prevent sulfate attacks?
prevent with type 20 and type 50 cement (low C3A)
adding SCM (CH to CSH; pozzolanic activity)
How to prevent plastic and drying shrinkage?
Wet curing
Wind break
Shade concrete surface
What causes Drying shrinkage?
By volume change (decrease) due to loss of moisture in combination with restraint of soil or structure (struc applies stress by stoping mvt)
Thermal effects(expansion)
How to control shrinkage in sidewalks?
Seperation because of agg. that will restrain the slab
(limit cracking)
