Concrete-Durability Flashcards
durable
able to exist for a long time without significant deterioration
Durability of PC Concrete
“The ability to resist weathering action,
chemical attack, abrasion, or any other process of deterioration.”
How long does concrete last
Under Ideal Conditions: Virtually forever.
Under Normal Conditions: Depends on exposure conditions
(i.e. deterioration mechanisms)
General Categories of Deterioration Mechanisms:
Chemical Attack
Physical Attack
how does deterioration initiate
Generally, surface attack of concrete is an extremely slow deterioration process.
In most cases, aggressive agents must enter the concrete to cause significant damage
three primary transport mechanism to allow penetration of aggressive agents
Absorption
Permeation
Diffusion
porosity effect on attack
as capillary porosity increases, fraction connected increase, making it more susceptible to deterioration
absorption
Transport of liquids into unsaturated porous solids
due to surface tension acting in capillaries
Permeation
Movement of gases or liquids through a saturated
porous medium due to a pressure gradient
Diffusion
Transfer of mass by random motion of free molecules
or ions in the pore solution due to a concentration gradient
Absorption and Diffusion are affected in a similar manner
a denser paste acts to restrict movement
Leaching
- the hydrolysis of cement paste components
(particularly calcium hydroxide) by water flowing through the
concrete
define hard water and impact on concrete
Hard Water (Groundwater, Lakes, Rivers) contains chlorides, sulfates, bicarbonates of calcium and magnesium. Not detrimental to concrete
define soft water and impact on concrete
Soft Water (Rain, Melting Snow & Ice) contains no calcium ions or other minerals. Readily dissolves calcium containing products
rate of leaching depends on
the amount of dissolved salts in the water and the temperature of the water
prevention of leaching
- Minimize transport properties (low W/C, SCMs)
- Minimize calcium hydroxide content of hcp (SCMs)
efflorescence
migration of salt to surface
alkali silica reaction
chemical reaction between the
soluble alkalis contained in the hcp and certain reactive forms of silica found in the aggregates
factors affecting reaction
- nature of the reactive silica
- amount of reactive silica
- particle size of reactive material
- amount of alkalis available
- amount of moisture available
pessimum amount
max amount of expansion for reactive silica in aggregate
effect of particle size on ASR
small particles have higher surface area, so more extensive reaction
prevention of alkali silica reaction
- Identify and avoid reactive aggregates.
- Limit the amount of alkalis available in the hcp:
Na2O + 0.65 K2O < 0.60 - Add an SCM to the concrete mix
how do you test for ASR?
UV fluorescence technique
Alkali Carbonate Reaction (ACR)
- Expansive reactions involving carbonate rocks (dolomitic limestone)
Carbonate rocks susceptible to expansive reactions possess the following features (4):
- Very fine grained dolomite (small crystals)
- Considerable amounts of fine-grained calcite
- Abundant interstitial clay
- Dolomite and calcite crystals evenly dispersed in clay matrix
Sulphate attack
- A chemical reaction between a sulphate ions
and certain components of hcp
damage during sulphate attack
Damage may include expansion and cracking of the concrete, as well as softening and disintegration of the paste
primary forms of sulphate attack
– External sulphate attack
– Physical sulphate attack
– Thaumasite
– Internal sulphate attack (DEF)
– Waste/Sewage
3 step reaction of sulphate attack
- Sulphates must first enter the concrete, usually from an
outside source. - Sulphates react with CH to produce gypsum:
- The gypsum reacts with the monosulphoaluminate in the hcp to form ettringite
(2 and 3 are expansive)
effect of seawater on sulphate attack
though high levels of sulphates are present in
seawater, sulphate attack is mitigated to some extent.
- Magnesium hydroxide chemically protects against sulphate attack.
- Gypsum and ettringite are more soluble in solutions containing chloride ions
Internal Sulphate Attack
– Delayed Ettringite Formation (DEF)
Curing at elevated temperatures destroys ettringite and the sulphate is absorbed by the C-S-H.
After cooling, the sulphate again becomes available to form ettringite, resulting in expansion and cracking
Acid attack
a chemical reaction between an external source of
acidic liquid and hcp and, in some cases, aggregates.
Acid Attack Sequence
Attack is normally limited to surface of concrete only. Progresses inward.
Dissolution of compounds soluble in the given acid takes place virtually instantaneously.
In most cases, this reaction forms insoluble calcium salts which build up and protect the concrete from further attack
Freezing and Thawing
Damage is induced by internal tensile stresses which are a direct result of repetitive cycles of freezing and thawing.
Freeze/thaw damage is through attrition - one cycle does very little damage, it takes many cycles before the damage adds up to significant levels
4 contributing factors
- Expansion of water
- Hydraulic pressure
- Solar heating
- Litvan’s model
Expansion of water
Just before freezing, volume increases 9%
Hydraulic pressure
All of the water in concrete does not freeze at the same time, but follows a gradual process as freezing begins in the larger cavities and progresses to successively smaller ones due to the effect of pore pressure.
This produces a hydraulic pressure as the expansion forces unfrozen water ahead of the freezing front.
Magnitude of hydraulic pressure is a function of:
- Concrete’s resistance to flow
- Distance to void boundary
- Rate of freezing
Solar heating
Two-directional freezing at surface due to daily thawing from incident solar radiation
Litvan’s Model
A vapor pressure gradient is created between surface ice and super-cooled pore water. Induces movement of water toward surface.
A dense, impermeable surface layer will restrict this movement and potentially cause mechanical failure
scaling
removal from surface
heat/fire relationship with concrete
Low rate of heat penetration due to:
- Low thermal conductivity.
- Heat is consumed by evaporation of water.
- Heat is consumed in decomposition of hydration products.
- Some aggregates also decompose and consume heat.
- Decomposed material has even lower thermal conductivity
PRODUCTS REINFORCING STEEL BEAMS
corrosion of reinforcement
An electrochemical attack mechanism affecting the reinforcing steel which results in a volume increase, thus inducing tensile stresses in the concrete.
Structural concrete requires steel reinforcement to carry the applied tensile stresses.
Concrete is normally capable of providing excellent protection to the steel and prevent it from corroding
how does concrete prevent corrosion in steel bars
Physically: the concrete restricts ingress of the basic components required to initiate corrosion (water, oxygen, chlorides)
Chemically: the pore solution in concrete typically has a very high pH, which leads to the formation of a protective iron oxide film around the steel bar
passivation film
protective iron oxide film around bar caused by high pH in concrete
Primary physical reasons for loss of protection (4)
- Insufficient cover over reinforcement.
- Concrete with poor transport properties.
- Failure to protect concrete from chloride sources.
- Damage to concrete (cracking, spalling, scaling)
Primary chemical reasons for loss of protection (2)
- Penetration of chlorides into concrete. Passivation layer is destroyed when chloride ion content reaches 0.2 – 0.4% in region adjacent to steel.
- Carbonation (due to CO2 exposure) of concrete leads to a reduction in pH. Depassivation occurs as pH approaches 11
why is corrosion an electrochemical process
– it requires the formation of a cathode and an anode, with an electrical current flowing between them
what is the anode in corrosion of reinforcement
iron metallic atoms are oxidized to fe 2+ ions, which dissolve into the surrounding solution, producing electrons
what is the cathode in corrosion of reinforcement?
electrons are consumed and OH- ions formed. water and oxygen required for this to occur
deleterious effects of corrosion of steel
- Reduction of the crosssectional area of the steel at
the anode - Spalling or cracking of the concrete due to the expansion stresses created by rust formation
prevention in design (8)
- Sufficient Cover
- Improved Transport Properties
- Corrosion Inhibitors
- Corrosion Resistant Reinforcement
- Galvanized Steel
- Stainless Steel
- Epoxy Coated Steel
- Fibre Reinforced Plastics
Protection/Repair of Existing Structure (9)
- Remove, Clean, Replace
- Corrosive Resistant Reinforcement
- Corrosion Inhibitors
- Cathodic Protection
- Sacrificial Anode
- Cancellation Current
- Protective Overlay
- Waterproof Membrane
- Watertight Concrete Overlay
surface wear
- Progressive mass loss from a concrete surface
due to repetitive attrition cycles
Surface wear is divided into three primary mechanisms:
- Abrasion
- Erosion
- Cavitation
Abrasion
Refers to dry attrition as another solid objects moves
along or rubs against the concrete surface.
Primarily relates to vehicular traffic or mechanical devices but can also occur in walls of silos or bins
Erosion
Wear caused by the abrasive action of solid particles
suspended in fluids.
Caused by the physical action of debris impacting, rubbing, rolling, and grinding against the concrete surface.
Common on canal linings, spillways and pipes for water or sewage transport
cavitation
– Loss of mass caused by the formation of vapor
bubbles and their subsequent collapse due to sudden changes of direction in rapidly flowing water
how does cavitation work and what does it require
Requires:
- Rapid water flow (exceeding 12 m/s)
- Surface irregularities
At irregularities, water flow separates from concrete surface creating zone of lowered vapour pressure causing bubbles to form.
As bubbles move downstream to regions of normal pressure they collapse violently, creating a shock wave.
Shock wave can induce high tensile stresses in concrete if this occurs near the concrete surface
deleterious effects of sea water
-Leaching - Constant exposure to seawater and/or flow
-AAR - Alkalis in seawater (if reactive aggregate is present)
-Sulphate Attack - chemical reaction + crystallization (W/D)
-Acid Attack - High CO2 contents possible (pH < 7.5)
-Freeze/Thaw - Accentuated in tidal zone
-Corrosion - High Cl- content
-Surface Wear - flow, waves, sediment, floating objects
destruction of concrete in sea water- shape
makes an almost hour glass shape
