Concrete Flashcards
Hydration of concrete is exothermic. What factor affects the total heat produced?
The volume of concrete produced
Larger volumes generate more heat
What happens as the time taken for free water to evaporate increases?
Strength increases
What is the standard design drying time for concrete?
28 days
Fundamentally, how do you increase concrete strength?
Decrease the number and size of voids
What is a standard proportion of aggregates and voids in concrete?
Aggregates: 60-70%
Voids: 1-2%
What happens when a concrete mix has too much water? What about when it has too little?
Too much: Large voids form
Too little: Concrete is not workable
How much of global CO2 production is due to cement production?
7%
How is Limestone typically mined?
With explosives
Which parts of the cement process are the most energy-intensive?
Clinker production - requires kiln to reach 1480C
Clay & Limestone grinding
Why must Gypsum be added to cement? How much is added?
To delay the hydration process
2-3% by weight
What is sometimes used as a cheaper alternative to clinker?
Similar substances which are byproducts of other industrial processes
Which chemical structure contributes most greatly to cement strength?
Calcium Silicate Hydrate (C-S-H)
Why does cement have a high pH value?
To resist corrosion, such as acid rain
What are the 2 types of cement setting failures?
Flash Setting: Too little gypsum is added, hence hydration produces excessive heat which dries & sets the cement too quickly
False Setting: Cement stiffens without generating any heat
What are the 3 aggregate weight classes, roughly?
Heavy: 2900-8900kg/m3
Normal: 2100-2550kg/m3
Light: 1450-1900kg/m3
How are heavyweight aggregates composed? What are they often used for?
They can contain iron and lead ores
Often used for blast-proof walls/structures
How does aggregate shape affect concrete properties?
Sharper aggregates produce stronger structures, but they are harder to cast than rounded aggregates
What does saturated surface dry mean?
The aggregate is filled with exactly as much water as it can absorb, no more
Why must aggregate be saturated surface dry for mix design?
Dry aggregates cause free water to evaporate too quickly, leaving voids
Wet aggregates increase the total water content of the mix and thereby change the cement/water ratio
What are the 3 main aggregate size classes?
Coarse: Average diameter > 4mm
Fine: Average diameter < 4mm
All-In: Both of the above mixed together
What are the 2 setting stages of concrete?
Initial set: Gel layers which form on aggregate surfaces expand to overlap with eachother and begin stiffening (occurs about 2 hours into drying)
Finial set: One hour after initial set, gels fully expand to comprise the whole mix, causing the whole structure to stiffen.
What are the 2 most important properties of fresh concrete?
Workability and Stability
What happens to workability and strength as water/cement ratio (i.e. amount of water) increases?
Workability increases at a decaying rate
Strength decreases at a decaying rate
What are the 3 workability tests?
Slump Test
Degree of Compatibility Test
Vebe Test
Describe Slump Testing
A mould is used to create a cone of concrete, which is left to sit and slump over time. Slump is measured as the decrease in height of the cone. Collapse or shearing are counted as failures requiring a retest
Describe Degree of Compatibility Testing
A tube is filled with concrete and mechanically compacted until it cannot compact any more.
The degree is the original height divided by the compacted height
What is the key difference between Slump and Vebe testing?
Vebe testing uses a vibrator table
After failure, how is concrete behaviour usually plotted?
Stress against Crack opening
As opposed to Stress/Strain
