Test Review from Notes Flashcards
(32 cards)
Water of questionable origin can be tested and used to make concrete if ___________.
It produces a 28 day strength equal to at least 90% of those obtained from mortar made with known acceptable water.
What are chloride ions and how are they formed?
Chloride ions are electrically charged atoms of chlorine which form when salt dissolves in water. They cause problems in reinforced steel by speeding up the deterioration of reinforcing steel embedded in concrete.
What forms on the steel in a high (above 11) pH level?
A protective oxide forms on the steel preventing normal oxidation (rusting) of the steel.
What do chloride ions do to the protective coating?
The chloride ion disrupt the protective steel and rusting can proceed even in a high pH.
Controlling the chloride content in concrete is difficult because ___________.
Chloride ions can get into the concrete from many sources.
How can chloride ions get into the concrete?
From the aggregates, the admixtures, the cementing materials, and the environment (deicing salt, sea water, sal-laden air).
What does excess water cause? (6 points)
1) lower strength
2) less durability
3) higher permeability (less watertight)
4) increased shrinkage
5) less volume stability (more volume change from wetting and drying)
6) poorer bond to the reinforcing steel and successive layers of concrete
How much of the concrete is aggregate?
60%-70% of the volume, which is 70%-80% of the mass.
What does the aggregate influence? (4 points)
It influences:
1) properties of freshly mixed concrete
2) properties of hardened concrete
3) mix properties
4) economy (cost)
What does fine aggregate consist of?
Natural or crushed stone in which most particles are below 5mm.
What does coarse aggregate consist of?
River gravel and crushed stone and/or air cooled blast furnace slag. The largest particles are between 10mm and 40mm. Most concrete has a max. size of 20mm.
What is the gravel in southern ontario like?
It’s generally clean, durable, and rounded from the glacial process.
What are the different types of rocks and how are they formed? (3 types)
igneous-from molten rock
sedimentary-from deposits or sediments
metamorphic-igneous or sedimentary rocks changed by heat or pressure
Aggregates classified by size (chart).
boulders ->200mm cobbles -60mm-200mm gravel -coarse -20mm-60mm gravel -medium -6mm-20mm gravel -fine -2mm-6mm sand -coarse -0.6mm-2mm sand -medium -0.2mm-0.6mm sand -fine -0.06mm-0.2mm silt -0.002mm-0.06mm clay -0.002mm
What properties do aggregates need? (5 points)
1) strong-wear resistant
2) hard-wear resistant
3) clean-free from coatings in clay
4) durable-chemically and mechanically stable
5) free of absorbed chemicals (sulphates and chlorides)
Mohs Hardness Scale
1) talc
2) gypsum
3) calcite
4) fluorite
5) apatite
6) feldspar
7) quartz
8) topaz
9) corundum
10) diamond
How to choose a max aggregate size?
1/5 narrowest dimension between the sides between the sides of the forms (wall)
1/3 of the slab depth
The moisture content affects the requirement for adding additional water to the mix. What are the mixed proportions based on in regard to the aggregates?
The aggregates need to be dry on the surface but fully saturated.
What should batching be based on?
Batching should be based on mass and not volume.
What kinds of aggregates should be avoided for freeze and thaw resistance?
Moderately porous aggregate like chert should be avoided to avoid pop outs.
What is aggregate beneficiation?
It is the process used to improve the quality of available aggregates. This may include crushing, screening, and washing to obtain the proper gradation and cleanliness,
In marine areas what do they use for aggregate?
They use seashells which are made acceptable by crushing. This improves their shape. Washing them in fresh water reduces salt levels and removes organic materials.
Why is proper handling and storage of aggregates necessary?
To prevent segregation of the aggregates. Stockpiles should be built in layers of uniform thickness, not just a big pile.
Round aggregate segregate more than crushed or angular aggregates.
Large aggregate separates more than fine aggregates.
Dry aggregate separates more than moist/wet aggregates.
The aggregate must have a dry sample more than _______________.
nominal maximum aggregate size (mm)/minimum mass of sample (kg) 10/1 14/3 20/5 28/10 40/15 56/20 80/60
