Lecture #2 - Concrete Ingredients - Part 2 Flashcards
Name the three different types of failure for a poor mix.
Aggregate, paste, bond
What is the ideal failure mode?
Combined failure (aggregates, paste, and bond all fail a bit)
Where do normal weight natural aggregates come from?
Natural gravel and sand, weathering and erosion of rocks, sometimes requires a bit of processing
What are sound sources of natural aggregates? (rock type)
metamorphic and igenous
Define metamorphic.
Recrystallization and reorganization of minerals due to pressure, volume, and termperature changes.
Define igneous.
Solidifcation of magma and lava
What are unsound sources of natural aggregate? (rock type)
shale and siltstone (sedimentary)
Define sedimentary.
consolidation and compaction of sediments
What is a sound transport mechanism for natural aggregate?
materials at higher elevations than glacier and are untouched
What is a questionable transport mechanism for natural aggregates? Explain.
water transport, smoothed by prolonged agitation in water, harder than less abraded sand and gravel but smooth surface can reduce bond with cement paste
How is manufactured aggregate produced?
crushing sound parent rocks or crushing air-cooled blast-furnace slag
What are some features of manufactured aggregate?
customizable in terms of grading and higher quality as particle elongatoin and layered flakes can be reduced and less likely to be contaminated by clay minerals and organic material
Which has a rougher texture, manufactured or natural aggregate?
Manufactured
Which is more uniformly graded: manufactured or natural aggregate?
manufactured
How is recyled concrete aggregate made?
Demolishing and removing existing concrete, crushing it in crushers, removing embedded items and reinforcement and dirt, wood, etc., grading and washing
What is recycled-concrete-aggregate?
Recycled concrete from buildings, pavements, and other structures used as aggregate to be more sustainable
What are the features of recycled concrete aggregate?
higher absorption because the hardened cement paste is more porous, lower specific gravity
What do you need to do if absorption is high in your aggregate?
add more water for workability, adjust water for concrete mix
Which is less angular, cubical or long: manufactured or natural aggregate?
Natural
What is recycled concrete aggregate mostly used for?
pavement reconstruction
What might marine-dredged aggregate contain that is not good?
seashells (weak), and salt (ruins)
What are the sources of marine-dredged aggregate?
estuaries, seabed
What weight of aggregate needs to be prewetted or kept moist?
lightweight
Why do we need grading limits?
affects proportions, workability, pumpability, economy, porosity, durability, shrinkage
Why would you use marine-dredged aggregate even though it sucks?
if other resources are not available in your region, as you need to use whatever’s close by for sustainability and cost
What is grading?
particle size distribution of aggregate
What kind of curve does single-sized aggregate grading have?
single sharp curve
What kind of grading curve does poorly graded aggregate have?
two sharp cruves
What kind of grading curve does well graded aggregate have?
two smooth curves
What does the ideal grading curve distribution look like?
like a parabola of different sizes
Is the volume of paste greater than the voids between aggregates? Why?
yes, to provide workability, cohesiveness of the paste matters
What is the fineness modulus?
sum of percent retained on each sieve (except pan)/100
Why can’t we only have very fine sand as aggregate?
too expensive
Why can’t we only have very coarse gravel as aggregate?
mix is harsh and unworkable
What is the maximum aggregate size?
Smallest sieve that all aggregates pass through (100% passing)
What is the nominal maximum aggregate size?
smallest sieve that majority of aggregate passes through (85 to 95% passing), first sieve that retains aggregate
What does maximum aggregate size depend on?
Shape and size of concrete member, amount and distribution of reinforcing steel, need to have enough workability so that the concrete can be placed properly without honeycomb or voids
Explain why low maximum aggregate size is not economical.
Surface covered by paste increases which causes required water and cement to increase so more cement paste is required to provide enough bond which is expensive.
What is air-entrainment?
Microscopic air bubbles in concrete.
What are the features of air entrainment?
Increased resistance to freeze and thaw durability, increased resistance to scaling caused by de-icing chemical, improved workability, reduced segregation and bleeding
How does air entrainment help vs no air entrainment in concrete?
Water in capillary pores freezes which expands and creates stress in concrete which then tends to crack. Entrained air allows water to expand which relieves stress and prevents cracking
What is water content in concrete?
Water in cement paste and water absorbed by aggregates
How much moisture is in oven dry aggregate?
none
How much moisture is in air dry aggregate?
less than potential absorption
How much moisture is in saturated surface dry (ssd) aggregate?
equal to potential absorption
How much moisture is in damp or wet aggregate?
greater than absorption, has free water
What needs to adjusted to account for the moisture condition of aggregates?
water content
Describe how aggregates are prepared for water absorption capacity test.
Put aggregate in water for 24h then dry it with towel then it becomes SSD then put it in the oven
Describe how aggregates are prepared for moisture content test.
Sample aggregate from stockpile (can have any moisture condition) is then oven dried
Should moisture content and absorption capacity be equal? What do you do if they’re not equal?
Yes. If moisture content is greater than absorption capacity remove water. If moisture content is less than absorption capacity add water.
What is bulking?
Increase in total volume of moist fine aggregate over the same mass in a dry condition
What is abrasion?
The physical wearing and grading of a surface through friction and impact on the surface