Flashcards
Durability and Mixed Design
Cards in this deck (24)
- 1Durability- ability to last for the service life without unacceptable performance related to materials
- unacceptable performance could be: failure or risk of failure, loss of function, serviceability, unacceptable aesthetics
- durability helps environment: conserves natural resources, reduces GHG emissions from new materials, reduces waste associated with replacement
- concrete is designed for different levels of durability as required by: exposure environment, expected service life
- corrosion of rebar is #1 cause of concrete deterioration - 2Chloride Induced Corrosion
- 3Carbonation Induced Corrosion- concrete naturally has high pH from alkalis and Ca(OH)2
- CO2 from atmosphere diffuses in concrete and reacts with Ca(OH)2 which lowers pH and disrupts passive layer on steel
- measured with acid base indicator dye - 4Freeze Thaw Deterioration- non air entrained concrete susceptible
- concretes with high w/c
- see photo, top is non air entrained and high w/c, bottom is opposite
- 5Freeze Thaw Damage - Entrained Air- freeze thaw is prevented by allowing water to expand into entrained air voids or eliminating water saturation
- 6Acid Attack- ex) in sewage
- chemical plants
- microbial induced corrosion (MIC) - 7Sulphate Attack- sulphur from groundwater, industrial chemicals or trapped interanally can react with aluminate phases
- severe softening, expansion and cracking - 8Marine Exposure- reinforcing steel corrosin, abrasion and chem attack, freeze thaw damage, temp gradients, humididty gradients, AAR, chem decomposition of hydrated cement
- solutions: low w/c through use of a superplasticizer, SCM, waterproofing, proper placement/curing, air entrainment, cement/agg selection, shrinkage reduction, absorption reduction, coatings, cathodic protection, corrosino resistant rebar - 9Concrete Mix Design- factors for proportioning concrete mixtures: strength, durability, workability, sustainability, economy, appearance
- 10Design Criteria- strength/structural: compressive strength, tensile, rate of strength gain, density etc
- Durability: air content, w/cm, type of cement, SCM content, shrinkage, durability performance limits (ex) test limits on AAR, sulfate resistance, chloride diffusion etc), heat of hydration/temp rise limits
- Workability: slump, slump loss, flow, cohesiveness, agg ratio, coarse agg max size and slump for geometric requiremtns, rebar cover, set time, mix texture
- Sustainability: amount of SCM, reycled materials, reduce total cement, reduce trucking
- Economy: min cement content, use cheap SCM, use available materials, reduce variability
-Aesthetics: colourm surfface texture - 11Types of Materials- cement
-SCM
-water
-coarse agg, fine agg
-admixtures
-fibers
- obtain required info about materials: sieve analysis of agg, density and absorption for agg, density of cement, SCM, performance reports, recommended dosage rates of chem admixtures - 12Slump- choose a slump: depends on type of concrete element
- depends on method of placemnt and compaction
- note: many older eng specs have "rule of thumb" requirements for slump but increasingly these are out of date and the choice of slump is made by builder - 13Choose Nominal Max Size of Coarse Agg- spacing between rebar and formwork (ususlaly 20mm with 10-14mm for congested rebar areas)
- avail in local market
- experience in local market
- type of placing equipment - 14Nominal Max Size of Agg- CSA says shouldnt exceed:
- 1/5 than narrowest dimension between sides of forms
- 3/4 clear spacing between rebar
- 1/3 depth of slabs
- cover depth for concrete not exposed to earth or weather
- 2/3 cover for concrete exposed to earth or weather
- 1/2 cover for concrete exposed to chlorides - 15Concrete Cover
- 16Determine Air Content- exterior concrete in cold climate: all of canada considered freeze thaw zone
- degree of saturation: vertical wall elements will dry out quickly whereas horiz elements will become nearly saturated
- de-icing salts present? ex) bridges, sidewalks, marine works
- agg size: mixes w larger agg generally have less paste vol therefore less total air % required to protect paste fraction - 17Air Content function of agg size?- not really
- max agg influences amount of paste needed to get a given workability for given w/cm b/c larger agg mixes have less paste volume
- paste fraction is what needs to be protected with a specific vol of air bubbles
- thus a mix with less paste needs less total air
- kind of outdated method: better to measure air void spacing factor - 18air content from CSA
- 19Determine Amount of SCM- amount for "green building" may be prescribed
- used for economic reasons
- min amount may be set for durability requirements
- min amount may be set for reducing heat of hydration
- some may be used for pumping and workabiltiy
- amounts for concerns of cracking and scaling
- may need silica fume for: cohesiveness, rapid strength gain or high strength, high durabiltiy, chem protection - 20Special Cements- Type HS for sulfate exposure
- type LH for low heat of hydration
- type HE for very rapid strength gain - 21W/CM- based on required strength
- based on required durability: usually specified in prescriptive code, may be established by test data
- shrinkage considerations: low w/cm alone will not give low shrinkage without also reducing cement and water contents - 22Determine Environmental Exposure- Exposure classes:
- C1: structurally reinforced concrete exposed to chlorides with or without freeze thaw
- C2: non structurally reinforced or unreinforced concrete exposed to chlorides and freeze/thaw
- F1: concrete in a saturated condition exposed to freeze thaw but not chlorides
- F2: concrete in unsaturated condition exposed to freeze/thaw but not chlorides
- note: more advanced methods of determining effect of environmental exposure on service life are now common for critical infrastructure such as major bridges - 23Environmental Exposure contd
- 24Estimate Cement, water and agg contents/mix proportions- design guides
- by local practice and rule of thumb
- empirical strength vs w/c estimation programs can help
- mix proportions are always based on 1 m3
- see written notes for example of mix proportions
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- Flashcards on Durability and Mixed Design, created by Maddi Everton on 12/12/2017.
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- Maddi Everton · Dec 2017
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- Dec 12, 2017
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