Aggregates
Where do we use mineral aggregates?
- as BASE in pavement structures; pavements consist of layers of material laid on top of each other. It is common to have an aggregate base - or a layer of compacted aggregate - underneath the asphalt concrete or portland cement concrete surface layer. Compaction is the process of reducing voids and increasing the density of such a material usually by the use of pressure and vibration applied with heavy steel drum rollers.
- as the surface layer of gravel roads
- as BACKFILL behind retaining walls; if the amount of FINES in aggregate is low (less than 15%), the aggregate will allow the water to drain through and prevent the accumulation of water behind the retaining wall which may results in failure of the structure. FINES are particles smaller than 0.075 mm which would pass through a No. 200 US Sieve (a sieve that has 200 openings per linear inch).
- as BACKFILL for drainage applications such as trench drains
- in FOUNDATIONS, as imported fill
- in asphalt CONCRETE and portland cement CONCRETE, to fill the volume and contribute to concrete strength and durability. Note that 70-80% of the volume of concrete or asphalt concrete is in fact occupied by aggregates.
- Mineral aggregates are therefore the most used construction material. Aggregates are a non-renewable natural resource. Luckily, aggregates are recyclable:
- RAP is reclaimed asphalt pavement
- RCA is recycled concrete aggregate
- TDA is tire derived aggregate
- CRM is crumb rubber modifier
- LIFE CYCLE and CARBON FOOTPRINT
| Item | declared unit | carbon footprint (kg CO2e) | unit weight (pcf) | per pound of material (for comparison purposes) |
| mineral aggregate | ||||
| portland cement concrete | ||||
| asphalt concrete | ||||
| structural steel | ||||
| structural timber | ||||
| beef burger |
Where do aggregates come from?
- Sand and gravel deposits
- Quarries
- mine
- crush
- sieve/sort
- wash
- store in STOCKPILES
Here is an aerial view of the CEMEX aggregate facility in Rialto, CA. Feel free to explore. Can you find a concrete plant adjacent to the facility? Can you spot an asphalt plant adjacent to the facility?
In the photograph below, can you estimate the weight of aggregate in US tons for the stockpile inside the red circle in the photo? A 20-feet long segment is drawn on the bottom-right corner of the photograph to help you estimate lengths. Assume the stockpile has the shape of a straight cone with a side slope of 45 degrees. Once you figure out the volume you will need to use an estimate for the BULK UNIT WEIGHT of the aggregate. Use 90 pounds per cubic feet (pcf).
What are typical tests for aggregates?
- Shape
- Texture
- Size: Sieve Analysis
- Coarse
- Fine
- Fines
- Dense-graded
- Gap-graded
- Open-graded
- Uniform (one-size)
- D-values
- Plasticity Index
- Cleanliness: Sand Equivalent
- Soundness
- Abrasion Resistance
Advanced tests for aggregates
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