TUT 05 Reinforced concrete durability Flashcards
how does transport in concrete work
- most deterioration processes require aggressive agents to enter the concrete
why is water the source of many types of durability concerns
- it directly attacks or facilitates the ingress of harmful ions into the concrete
what are the three different water transport mechanisms
- absorption
- diffusion
- permeation
what is absorption
- movement of water driven by surface tension in unsaturated capillary pores
- flow from unsaturated to saturated regions
what is diffusion
- random movement of individual liquid (ions) or gas particules driven by a concentration gradient
- flow from high to low concentration
what is permeation
- movement of gases or liquids driven by a pressure gradient
- high pressure to low pressure
what is the difference between porosity and permeability
- porosity is the proportion of voids in a material
- permeability is the ability of a fluid such as water to flow in a material
- if there are pores in a material, even up to 99% porosity, if these pores are not interconnected or connecting to the surface then there will be no permeability because water can’t flow between them
what are the different physical attacks (FATESW)
- Freeze-thaw
- abrasion/Wear
- temperature change
- erosion and cavitation
- salt scaling
- wet-dry
what type of damage does freeze-thaw cause
damage through attrition
- damage induced by internal tensile stresses which are a direct result of repetitive cycles of freezing and thawing
how does freeze-thaw cause damage
- expansion during water to ice phase change
- hydraulic pressure from differential pore freezing
- solar heating
- litvan’s model
explain more in detail the hydraulic pressure process in the freeze thaw
- freezing begins in large cavities and progresses to successively smaller ones due to effect of pore pressure
- effect or pore pressure: produces hydrostatic pressure as the expansion forces unfrozen water ahead of the freezing front
what is solar heating’s effect on freeze-thaw
- radiation causes surface thaw, water freezes during the night
- produces thermal gradient in concrete which induces stresses
what is litvan’s model and its effect in freeze-thaw cycle
- vapour pressure gradient between high pressure water and low pressure ice surface (how fish survive during winter)`
- ice lenses forcing pop out
what are the macroscopic effects of freeze-thaw
- progressive mass loss from surface
- pop outs
- crackings/ spalling
how to protect against freeze-thaw
- air entrainment: provides space for water to expand and freeze into
what is salt scaling
- when concrete pore water contains chlorides or dissolved salts
- the salts crystallize right below the surface, chipping off concrete at the surface
how does wet-dry cause damage
- damage due to constrained expansion:
- volume changes when water is absorbed/ released
- damage is cyclic
how to prevent wet- dry damage
- low W/C ratio , lowers permeability
- put a moisture barrier
why does thermal extremes (fires) affect concrete
- concrete has a low thermal conductance for construction material and is therfore less susceptible to fire damage
why does concrete have a low thermal conductance?
- heat is consumed in the evaporation of water (evaporation in concrete can cause shrinkage and cracking)
- heat is consumed in the decomposition of hydration products (partially reverses hydration reaction)
- some aggregates decompose and consume heat
- decomposed material has even lower thermal conductivity
what does a low thermal conductance mean
- allows core (reinforcement) to remain at lower temperatures and will be relatively unaffected
how does hot/cold damage concrete
- differential temperatures through-out bulk concrete can create stresses that may crack or damage concrete
- larger problems on mass concrete during early hydration
how to prevent hot/ cold damage in concrete during early hydration
- use low heat of hydration cement to minimize damage
examples of physical abrasion
- car tires, snowplows, traffic, construction loads
what is erosion in concrete
solid particles suspended in water repeatedly rubbing against concrete
what is cavitation and how does it affect concrete
- due to fast water flow around irregular geometry
- tiny bubbles form when water lifts from surface
- popping bubbles create shockwaves that damage concrete
what are the different chemical attacks to concrete (SALCA)
- Sulphate attack
- alkali-silica reaction
- leaching/efflorescence
- acid attack
- carbonation
how does alkali-silica reaction (ASR) damage concrete
- pore water becomes alkaline due to hydration
- active aggregates react with alkaline pore water solution to produce amorphous silicate gel
- amorphous silicate gel imbibes pore water and expands a lot
- gel expands into surrounding hydrated cement paste (HCP) causing volumetric expansion damage
what deos alkali silica reaction require to react
specific kinds of active silicate aggregates
what are the alkali-silica reaction macroscopic effects
- map cracking
- mass loss at surface
- volumetric expansion
how to prevent alkali-silica reaction
- use non-reactive aggregates!
- minimize alkalis available in solution
- lower concrete permeability
- use SCMs like fly ash to react with the silicates
how does sulphate attack cause damage
- reaction between sulphate ions and certain components of the hydrated cement paste (HCP)
- damage may include expansion and cracking of the concrete and softening and disintegration of the paste
what is the sulphate attack reaction (3 steps)
- Sulphates enter concrete via pore water solution and diffusion/absorption/permeation
- Sulphates react with CH to produce gypsum
- Gypsum then reacts with monosulphoaluminate in HCP to produce ettringite
what is leaching in concrete
dissolution of components of cement paste (CH) in pore water solution
what is efflorescence in concrete
when water reaches surface, it evaporates and leaves residue on surface
- requires water without many ions already dissolved in it
what are the macroscopic effects of leaching and efflorescence
- weakening of the cement paste (removes CH)
- staining on surface
how to prevent leaching and efflorescence
- minimize water flow through structure
what is hard water
- ground water, lakes and rivers
- contains dissolved minerals
- not detrimental to concrete
what is soft water
- rain, melting snow and ice
- does not contain calcium ions or other minerals
- readily dissolves calcium containing products
- the rate fo leaching depends on the amounts of dissolved salts in the water and the temperature of the water
how does acid attack affect concrete
- direct dissolution of cement matrix in acid (usually on an issue with exposure to acid for extended periods of time)
what are the macroscopic effects of acid attack on concrete
- weakening or destruction of paste
how deos carbonation damage concrete
- infiltration of CO2 into concrete (as gas or ion)
- progresses from surface layer inwards
- (lowers pH or pore solution) : CO2 molecules react with CH to form calcium carbonate (lowers pH or pore solution)
macroscopic effects of carbonation on concrete
- increases corrosion rate of reinforcing steel (deprivation)
- carbonates can strengthen matrix
what is an electrochemical attack on concrete
corrosion of reinforcing steel
how does corrosion of reinforcing steel work
- electrochemical process that requires formation of a cathode and an anode and an electrical current between them
- chemical reaction between water, oxygen and metallic iron that produces rust
- rust volume larger than original metal’s volume
how does concrete protect against electrochemical attack? physically
- concrete restricts the entering of the basic components required to start corrosion (Water and oxygen)
how does concrete lose its ability to protect agains electrochemical physical attack
- insufficient cover
- concrete with poor transport properties
- damage to concrete (cracking , spalling, scaling)
how does concrete protect against electrochemical attack? chemically
- concrete pore solution has a high PH which leads to the formation of a protective iron oxide film around the steel bar called the passivation film at pH>10
how does concrete lose its ability to protect againstelectrochemical chemical attack
- passivation film is lost because of lowering of the pore ph solution (carbonation or acid attack)
- penetration of chlorides into the concrete
what are the macroscopic effects of electrochemical attack (reinforcement corroding)
- volumetric expansion of embedded steel (induces tension cracks, spalling, delamination which all increases permeability)
- reduction of cross-sectional area at anode (structural level issues due to under-reindorcement)
how to prevent agains electrochemical attack
- maintain high ph environment around reinforcement (more OH- ions in pore solution minimize kinematics)
- lower permeability of concrete (decreases chloride ingress)
exam questions: expain the consequences of reinforcement corroding
- expansion of rebar cracks concrete and thus increases permeability
- steel’s cross-sectional area is reduced which reduces steel’s load carrying capacity
exam question: what is the difference between leaching and efflorescence
- leaching is the dissolution of calcium hydroxide into soft water
- efflorescence is the physical deposition of crystals on the surface of concrete, leaving white strains
exam question: explain the process of damage in sulphate attack
- sulphates enter concrete via pore water solution and diffusion,absorption/permeation
- sulpahtes react with CH to produce gypsum
- gypsum reacts with monosulphoaluminate in HCP to produce ettringite
- reactions are expansive and restrained expansion causes damage through cracking
explain the process of damage in alkali-silica reaction
- alkalis in pore solution react with silica from aggregates to form amorphous silicate gel
- amorphous silicate gel imbibes (absorbs) pore water and greatly expands
- once pores are filled by expansion, expansion becomes restrained and cracks occur
Concrete chemically protects embedded reinforcing steel from corrosion through the formation of a passivation film. Describe two ways in which this protection can be lost.
- penetration of chlorides - will attack and break down the passivation film once a sufficient concentration accumulates at the steel surface
- carbonation (co2 reaction with calcium hydroxide) - leads to a drop in pH of the pore solution. Passivation film breaks down as pH approaches 11
list and describe two ways in which concrete protects embedded reinforcing steel from corrosion
chemical: passivation film
physical: prevents reactive agents from entering
