Concrete-Properties Flashcards
Primary steps to concrete production (5)
- Batching - weighing out ingredients
- Mixing - central mix, shrink mix, truck mix
- Transporting/Placement - mixer truck, dump truck, bucket,
buggy, conveyor, pump, chute, tremie - Finishing - compaction, strikeoff, floating, troweling, texturing
- Curing - sprinkling, wet covering, ponding, waterproof
membrane, steam
a. Prevent loss of moisture
b. Supply additional moisture
Optimum mixing time depends on many factors (7)
- Type of mixer
- Condition of mixer
- Speed of rotation
- Size of the charge
- Nature of ingredients
- Mix design/proportioning
- Ambient temperature
types of mixers (3):
- Drum Mixers – a sloping horizontal drum with fixed blades attached to the inner face. Uses gravity to induce mixing. Not very effective for low workability mixes.
- Pan Mixers – consists of a flat pan within which a series of rotating blades mixes the concrete. Good for low workability
mixes and FRC. - Continuous Mixers – a long trough containing a spiral blade. Conveyors continuously feed ingredients and the concrete is mixed as it passes along the trough. Allows continuous production of concrete.
ready mixed concrete
- Central Mixed – Completely mixed at plant. Truck mixer only used to agitate concrete during conveyance.
- Transit Mixed – Partially or completely mixed while being transported to the site.
- Shrink Mixed – Partially mixed at the plant to reduce overall volume, and mixing is completed in the truck mixer.
- Truck Mixed – Completely mixed within the truck mixer. Water can be added later, if aggregate is dry
segregation of concrete can lead to
the formation of rock pockets and honey combing
free falls can cause segregation
effect of vibration
reduces air content and makes aggregates uniform
(consolidation)
ideal fresh concrete
-easily mixed and transported
-uniform throughout a given batch
-capable of flowing such that it completely fills the forms
-compactable without the need for excessive energy
-resistant to segregation during placing and consolidation
-capable of being finished properly
define workability
the amount of mechanical work required to produce
full compaction of the concrete without segregation.
factors affecting workability
segregation and bleeding
define segregation
Segregation refers to the separation of the components of fresh concrete, resulting in a nonuniform mix.
factors of segregation
-large aggregate size
-high specific gravity of coarse aggregate relative to fine
-decreased amount of fines
-variations in particle shape away from smooth and rounded
-mixes that are either too wet or dry
define bleeding
refers to the upward movement of water after concrete is consolidate but before it has set
Generally caused by aggregates settling within the fresh concrete and being unable to hold all the mixing water
impact of bleeding
-distinct, localized channels that later act to increase the permeability of the hardened concrete
-The upper layer of the concrete may become rich in cement paste which has a very high W/C ratio.
-Both of these factors lead to weakness, porosity and reduced durability.
-Water pockets may form beneath inclusions in the concrete, leaving weak zones and reducing bond
plastic shrinkage
If bleed water evaporates more quickly than bleeding rates, plasticshrinkage cracking can form
Problematic in hot, dry and/or windy weather.
laitance
A scum of fine particles or dissolved salts may be carried to the surface, resulting in a weak and nondurable layer (laitance).
Laitance at the top of a lift can prevent proper bonding.
Laitance at the final surface can result in dusting
slump test
The slump test provides a rough measure of consistency - not an absolute measure of workability!!!
when cant you use the slump test
High slump concrete
Low slump concrete
Fiber reinforced concrete
Certain admixtures
In air-entrained concrete, there are two types of air ….
Entrained Air & Entrapped Air
3 tests for air content
the gravimetric, volumetric and pressure methods
gravimetric method
Compares the unit weight of concrete containing air with the calculated unit weight of air-free concrete
volumetric method
Based upon a comparison of the volume of concrete containing air with the volume of the same concrete after the air has been removed
Requires a significant amount of physical effort to remove the air from the concrete
Pressure method
The most common method for measuring the air content of fresh concrete in the field.
Based upon a measurement of the change in volume of the concrete when subjected to a given pressure.
This change in volume is assumed to be entirely caused by compression of the air.
steps to time of set
Setting refers to the onset of rigidity in fresh concrete.
Hardening is the development of useful and measurable strength.
Both are gradual changes controlled by the continuing hydration of the cement
why is time of set important
- To help regulate mixing and transit times.
- To gauge the effectiveness of various set-controlling
admixtures. - To help plan the scheduling of casting and finishing
operations. - As a QC test to monitor the accelerating/retarding effects of additives.
how to measure time of set?
measure penetration resistance at different times
define curing
Affects both mechanical properties and durability characteristics
and reduces volume changes due to shrinkage
two components of curing
- Prevents loss of moisture through evaporation
- Supplies additional water for continued hydration
Excessive evaporation from concrete surface must be avoided since they can cause plastic shrinkage cracking
water cement ratio ideal
0.42
how is water lost in concrete curing
Water is lost due to evaporation or by absorption by the aggregates, formwork and/or subgrade
Once internal relative humidity drops below ~80%, the hydration reaction stops … as does strength development
curing of SCMs
Slags or pozzolans require even longer curing periods to ensure that there is enough water present for the pozzolanic reaction to take place.
factors affecting curing (3)
– Duration of Curing
Once moist curing stops, the rate of strength gain slows down (as water is lost from the concrete) and eventually stops
- Interrupted curing can be restarted with a corresponding increase
in strength. Ultimate strength will not reach the same level as continuously cured concrete
-Temperature affects the rate of strength development and thus the length of curing necessary.
At early ages, cement hydrates more rapidly, resulting in increased early strength.
However, high temperatures produce a nonuniform distribution of hydration products.
These leave weak zones in the hcp that govern strength.
concrete should always be tested in
fully saturated condition
water curing
Continuous supply of water to the concrete surface
Can be accomplished by ponding, spraying, fogging, sprinkling, or saturated coverings.
Must maintain water in concrete as a continuous liquid system
sealed curing
Sealing of the concrete surface to prevent evaporation
Can be done using waterproof paper, plastic sheeting or membrane forming curing compounds
Sprayed curing compounds consist of solid materials dissolved in a solvent that form a membrane as the solvent evaporates.
Colored pigments are added to reflect heat and clearly indicate coated areas
what should not be used with sealed curing
Curing compounds should never be used between lifts or bonded overlays, or when the surface is to be painted.
Curing compounds should not be used during the fall on pavements to be exposed to deicing salts
Self-desiccation is a common problem with sealed curing procedures
define plastic shrinkage cracking
A phenomenon induced when the rate of evaporation of free water from the concrete surface becomes excessive.
As the concrete begins to stiffen and set, shrinkage is restrained and tensile stresses develop.
Under extreme conditions (hot, dry, windy weather), can lead to severe cracking of the concrete surface if left unprotected.
factors of plastic shrinkage cracking factors
- Concrete and air temperature
- Relative humidity
- Wind velocity
hot weather effects on fresh concrete (7)
Accelerated slump loss
Increased rate of setting
Difficulty in controlling air content
Increased loss of moisture during curing
Increased risk of plastic shrinkage cracking
Higher concrete temperatures
Greater thermal variation within concrete
precautions when hot weather concreting (10)
Cool concrete materials
Cool/protect handling equipment
Cool/moisten forms
Increase relative humidity of air
Avoid hottest periods of the day
Reduce time between mixing and curing
Protect from wind
Implement proper curing methods
Use set retarding admixture
Use low heat of hydration cement
cold weather on concrete
Accelerated slump loss
Decreased rate of setting
Increased risk of plastic shrinkage cracking*
Extreme damage if concrete allowed to freeze
precautions for cold weather concreting
Heat concrete materials
Insulate forms
Avoid coldest periods of the day
Reduce time between mixing and curing
Protect from wind
Use shelters/heaters to keep concrete warm
Use air entraining admixture
Use set accelerating admixture
Use high early strength cement
concrete is a ________ material
brittle
ultimate strength is a function of:
paste and aggregates
Increasing water content=
higher W/C ratio
lower strength
