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
