Aggregates Flashcards
What is the typical promotion of aggregates in concrete vs asphalt? What does this mean for the price and workability?
Portland concrete: 65-80%
Asphalt: 90%
More aggregates means cheaper, but harder to place and less flowable
What are the functions of aggregates?
Economy - cheap filler
Dimensional stability - shrinkage and expansion control
Durability - wear resistance and don’t resist chemical attack (sulphates and asr)
What are the three classifications of aggregates? Which do we use the most and why
Lightweight, normal weight and heavyweight. Heavyweight is too expensive to transport although it could be useful because of its dead weight and it’s protection against radiation.
Are most aggregates natural or artificial stone?
Natural either naturally occurring or modified (crush and wash). They used to use blast furnace slag for aggregates but realized it’s more useful as cement replacement.
What are the types of rocks and their sources?
Igneous - volcanic
Sedimentary - deposition and built up by pressure (weaker because layers)
Metamorphic : heated and pressurized sedimentary rocks (eg slate and marble)
What are desirable characteristics of aggregates?
Hard strong and durable, free of impurities, chemical stability. Wash out twigs and leaves because they break down and leave voids. You don’t want dust left on your aggregates (only exception being **calciferous limestone **)
What are the important properties of aggregates?
Shape and texture, size gradation, moisture content, specific gravity, bull unit weight
How do marbles relate to aggregate shape and texture?
For the mixing process, marble would be ideal because less friction and rubbing but for hardened concrete not ideal because they can’t form bonds and aren’t very strong under compression (we want higher surface area)
Why do we want size gradation in our concrete?
Reducing amount of void space. Aggregates can pack in together better when there are different sizes. Misconception: smaller aggregates are better
What is gap grading and no-fines grading?
Gap: one of the sizes is missing (eg they took it for asphalt)
No-fines: only larger aggregates are found. Then you can pump in sand and water and cement. You can drive over this crater with a big truck and you get direct aggregate to aggregate compression which is actually really strong
Why do we want biggest aggregate size?
Cheaper (don’t need need to pay for crushing and for more rocks)
Stronger (path of highest resistance)
Limited only by the project
How can we separate aggregates by size and what could be a potential problem?
Through sieve analysis. Could be a problem because oval can be flipped on side
What are the two types of maximum aggregate size?
Maximum: smallest sieve opening through which the entire aggregate sample will pass
Nominal maximum: 5-10% retention of largest sieve size. Typically 1/5 of narrowest dimension between forms so they can fit but also slide through without causing a blockage
Grading curves?
What is fuller’s maximum density curves?
The greater the amount of solid particles that can be packed in a given volume of concrete, the higher will be its strength
What handling and storage situations cause segregation?
Storing in cone shaped piles (large aggregates go to bottom because of inertia)
Avoid windy conditions because fine aggregates get more swept away than than coarse aggregates
Runnings aggregates down a slope will separate the aggregates because large aggregates have more momentum
How to calculate fineness modulus and and what is cumulative amount retained?
Cumulative amount retained is a percentage of percentage retained on a sieve plus everything above it. The fineness modulus is the sum of all cumulative amounts remained decided by 100
ONLY REPRESENTATIVE FOR FINER AGGREGATES
Why is FM not used for coarse aggregates? Why is it important for fine aggregates?
Less relevant
Has high values
Low sensitivity (we don’t care as much)
Required for fine aggregates because necessary for mix proportioning and it’s effect on workability
What are the four moisture states?
Oven dry (OD)
Air dry (AD)
Saturated surface dry (SSD)
Wet
What is the oven dry moisture state?
No water at all. Absolute value of mass of aggregate
What is saturated surface dry?
Completely saturated aggregates, can’t absorb any more water but also no leaking (the surface of aggregate is dry). Very critical state we want all concrete to be in this state
What is AD?
Range between OD and SSD. No water on surface but pores aren’t full. Effective absorption in order to get to SSD
What is wet?
Too much water in aggregates and on it and that raises the w/c ratio. Surface moisture is process of going down to ssd
Why is SSD used as a reference state instead of OD?
It is the equilibrium moisture content (what you want), moisture contents in the field are usually closer to SSD and more specific and more also apparent
What is absorption and how is it calculated?
Maximum amount of water that aggregate can absorb to get to ssd. 2% moisture means 2% water mass proportional to mass of aggregate
What is effective absorption and why do we calculate it?
It’s the amount of water needed to bring the aggregate from AD state to SSD. We calculate this so we can compensate for this by adding more water since we know the aggregates will soak up some of the water that we add.
What is surface moisture and why do we calculate it?
Amount of excess water. We need to keep it in mind when adding our water to our concrete mix because we don’t want to mess up our wc ratio. This is a preferable condition (we want our aggregates to be wet) because the excess water is readily available as opposed to absorption which takes longer for the aggregates to soak up the extra water.
Can sand absorb water? Describe bulking of sand
Sand cannot absorb water because it’s very crystalline and pure silica. Sand keeps water between its grains (its volume increases when you add water but it stays in menisci not absorbed by sand). Reason we measure sand by mass not water. Don’t oversaturate or it will fall apart
What is the difference between apparent specific gravity (ASG), and bull specific gravity (BSG)?
BSG includes the volume and mass of the solid AND of the pores so it’s more accurate because volume changes with pores but mass doesn’t. Both are multiplied by 1/density of water
Mass/volume!! BSG volume > asg volune
What is unit weight?
Sometimes called bulk density. It’s the weight of a given volume of graded aggregate. Includes solids, voids in solids, and voids between solids. Bad because it depends on the packing of the mixture. Also depends on proportion of fine and coarse aggregates
What is the biggest problem for aggregate durability?
ASR silica in aggregates reacts with alkalis in HCP
What is soundness for aggregates?
Not a quantifiable measurement. Means it’s free from things that will cause it to deteriorate (eg freeze thaw damage). Three forms: cracking and pop outs and shrinkage (when they lose moisture)
What detrimental reactions is the aggregate exposed to?
Iron pyrite that causes rust spots (rust is water soluble). Excessive gypsum causing sulphate attack. Zinc or lead setting delay. Mainly ASR!!
Lightweight aggregates? Huhh
Slides 73-75
What is the challenge is using recycled concrete?
Used concrete has already been exposed to (contaminated) chlorides found in salts so we get accelerated corrosion with recycled concrete. Also they have been coated in cement paste so they are less strong. Steel is okay to be recycled! Use crushing to separate concrete from steel
What is a solution for concrete coated in cement paste?
Heat it up and rub it off to get clean aggregate. Expensive but works!
