Concrete Flashcards
robert maillart
-swiss engineer used conrete in his bridges and buildings
most common form of forming material
plywood
- usually 3.4” thick
- coated with oil and water resistant glue or plastic to prevent water penetration into wood; help reusability of formwork
form ties
metal wires or rods used to hold opposite sides of the form together and also to prevent their collapse
-when the forms are removed the wires remain in the conrete; ecess wire is twisted or cut off
Slip Forming
is useful for tall walled structures such as elevator shafts, stairwells,
and storage silos, a ring of formwork is pulled by jacks supported on rebar while
concrete is added
flying forms:
large fabricated sections of framework that are removed, once the concrete has cured, to be resued in forming an identical section above
-used in constuctung buildings with repetitive elements (e.g. hotels and apartments)
for concrete columns, piers, and walls, the maximum variation in plumb will be
plus ot minus 1/4 in in any 10 ft length.
-the same tolerances applies for horizontal elements
the maximum variation out of plumb for the total height of the structure is
1 in for interior columns
1/2 in for corner columns for buildings up to 100 ft tall
Ideally, concrete slabs should be allowed to cure and dry for a minimum of
six weeks before resilient flooring is installed
THe best way to stop the migration of moisture though a conrete slab is with a vapor barrier placed…
directly below the conrete slab and on top of any sand cushion layer or subbase.
vapor barrier
a thin sheet material, generally plastic, designed to prevent water vapor from passing though it.
permeance (measure of a materials resistance to water vapor transmission) not exceeding 0.04 perm
10 mils thick
perm is the passage of one grain of water vapor per hour through one square foot of material at a pressure differential of one inch of mercury between the two sides of the material
vapor retarder
is not vapor proof; allows vapor to pass
it slows the rate of water vapor transmission
- is a membrane of metal foil, plastic, or treated paper placed on the warm side of thermal insulation to prevent water vapor from entering the insulation
and condensing into liquid
• Many batt insulation materials are furnished with a vapor retarder layer of treated paper or aluminum already attached
-essential in colder climates to prevent moisture from
accumulating within the insulation
two types of reinforcing steel for cast in place concrete
1 deformed bars
2 welded wire fabric
rebar comes in 2 common grades
grade 40 and grade 60
- grades 50 and 75 are also sometimes available
- these numbers refer to kips per square inch
- grade 60 is the type most used in construction
- rebars are classified as axle, rail, and billet (billet is most common)
How to read rebar
- top symbol: producing mill
- second marking: size (given
in eights of an inch)
-Third marking: Manufacturing
material ( “S” for carbon-steel or “W” for low- alloy steel)
-bottom marking: Grade marking
Formwork
shapes and supports concrete until it cures enough to support itself
• Formwork for a beam or slab is also a temporary working surface during construction
• Must be strong enough to support weight concrete without deflection
• Usually repeatedly used and must be protected
• Coated with a form release compound that prevents adhesion of concrete to the form
• Formwork is basically an entire temporary building that has to erected and
demolished to construct the actual building. It’s a major cost of the project.
Reinforcing
is required because concrete has no useful tensile strength
• Steel rebar is placed where there are tensile forces in a structural member, and concrete resist the compressive forces
• Rebar is hot rolled like structural shapes, round in cross section with ribs for better bonding to concrete
• Bars are numbered based on the number of eights of an inch the bar’s diameter is • Ex: number 6 rebar is 6/8” (or 3/4”) in diameter, number 8 rebar is 8/8” or 1”
• Structural engineer knows how to
calculate the amount of rebar required
for a given cross section and can be achieve with a few big bars or many small bars
• Rebar in structures exposed to salts like seawater are prone to rust
how to read welded wire fabric designations
example 6 x 6 -W1.4 x 1.4 means that the grid is 6 in by 6 in and the size of the wire is 1.4 hundredths of a square
-the letter preceding the gage is either W for smooth wire or D for deformed wire
concrete components
- cement
- fine a nd course aggregate
- water
portalnd cement: binder; made of lime, silica, iron oxide, alumina
5 types of cement
type I: standard or normal cement (used for most construction)
type IA: Normal, air entraining
type II: modified cement; used in plaecs (Moderate resistance to sulfate attack (used when in contact with water with a high concentration of sulfates)
type IIA:Moderate resistance, air entraining
type III:High early strength (hardens more quickly, used when reduced curing time is required)
type IIIA:High early strength, air entraining
type IV:Low heat of hydration (used in massive structures like dams, where heat emitted might raise the temperature of the concrete to damaging levels)
type V:High resistance to sulfate attack (used when in contact with water with a high concentration of sulfates) !
Air Entrained Cements
contain ingredients that cause microscopic air bubbles to form in the concrete during mixing which give improved workability during placement, and greatly increases the resistance of the cured concrete to damage caused by repeated cycles of freezing and thawing
• Commonly used for pavings and exposed architectural concrete in cold climates
• Can reach the same structural strength a regular concrete
concrete strength
• Concrete cures/hardens due to hydration, where considerable heat is given off as the
! !
cement combines with water to form strong crystals that bind the aggregates together
• Concrete Strength
• Commonly designed on the basis of the s strength that is reaches after 28 days • Typical strength ranges from 3,000 psi - 10,000 psi
• Early strength concrete can reach normal 28 day strength in 7-14 days
• 7 day strength should be about 75% of the designed 28 day strength
• Nominal weight is 150 lbs per cubic foot
• •
• •
Concrete shrinks lightly during the process
Majority of the volume consist of coarse and fine aggregates, proportioned and graded so that fine particles complete fill the spaces between the coarse ones
Each particle is completed coated with cement/water paste that joins it to the surrounding particles
Aggregates
take up about 3/4 of the volume of concrete
• Structural strength is heavily dependent on the quality of its aggregates
• Must be stone, clean, resistant to freeze-thaw deterioration, chemically stable, and
properly graded for size distribution
• A range of sizes must be included and properly proportioned to achieve close packing
of the particles
• The largest particle in a concrete mix must be small enough to easily pass between
the most closely spaced reinforcing bars and to fit easily into the formwork
• Maximum aggregate size should not be more than 3/4 of the clear spacing between
bars or 1/3 the depth of the slab
• 3/4” or 1 1/2” maximum is common for most slab and structural work
• Can be up to 6” for dams and other massive structures
• Structural lightweight aggregates are made from minerals like shale and reduce the
density of concrete by about 20%
• Vermiculite or perlite used in non structural lightweight concrete (used for insulating
roof toppings) are less dense than expanded shale
Air entraining admixtures:
increase the workability of wet concrete, reduce freeze/
thaw damage, and (when a lot is used) create very lightweight non structural
concretes with thermal insulating properties
The size of the aggregates in concrete is determiend by…
…the size of the forms and the spacing between the reinforcing
-in most cases, it should not be larger than three fourths of the smallest distance between reinforcing bars or larger than 1 /5 of the smallest dimension of forms, or more than 1/3 the depth of slabs
Methods used to specify the proportions of concrete mix:
- ratio of cement to sand to gravel, such as 1:2:4; amount of water also has to be specified
- specify weight of materials, including water, per 94 lbm bag of cement
- for large batch quantities, define the wieght of materials needed to make up one cubic yard of conrete
strength of final mix is specified by…
the compressive strength of concrte after it has cured and hardened for 28 days (known as the design strength of concrete)
-typical specified design strengths are indicated as 2000 psi, 3000 psi, and 4000 psi; 12000 is available but it’s more expensive
Admixtures…
… are ingredients other than cement, aggregates, and water that are added to the mix to alter it in various ways
Accelerating admixtures:
cause concrete to cure more rapidly
Hydration:
chemical reaction between water and cement which creates heat
High range water reducing admixtures (Superplasticizers):
organic compounds
that transform a stiff concrete mix into one that flows freely into forms, used to help place concrete in challenging circumstances, or to reduce the water content in a mix in order to increase its strength
• Water reducing admixtures:
allow a reduction in the amount of mixing water while
retaining the same workability, results in a higher strength concrete
Retarding admixture:
slow curing to allow more time for working with wet concrete
help reduce the heat of hydration
Waterproofing admixtures:
decrease the permeability of the conrete
Fly ash:
a fine power that’s a waste product from coal-fired power plants, which
increases concrete strength, decreases permeability, increase sulfate resistance, reduces temperature rise during curing, reduces mixing water, and improves workability
Silica fume (microsilica):
a powder 100x finer than portland cement, a byproduct of electronic semiconductor chip manufacturing that when added produces extremely high strength and low permeability
Blast furnace slag:
byproduct of iron manufacture that improves workability, increases strength, reduce permeability, reduce temperature rise during curing, and improve sulfate resistance
Pozzolans:
varicose natural/artificial material that react with calcium hydroxide in wet concrete to form cementing compounds
Workability agents
: improve the plasticity of wet concrete to make it easier to place in forms and finishes
Corrosion inhibitors:
used to reduce rusting of rebar in structures that are exposed to road deicing salts or other corrosion causing chemicals
Fibrous admixtures:
short fibers of glass or steal added to a concrete mix to act as microreinforcing
Freeze protection admixtures:
allow concrete to cure at temperatures as lot as 20 degrees F
Extend set control admixtures:
used to delay the curing reaction in concrete for up to 7 days.
Coloring agents:
dyes and pigments used to alter the color of concrete
what are these joints
b - construction joint, which wold be used to physically connect a new pour of concrete to a previously poured section; uses keyed joint; common to used rebar to tie sections together
a-control joint; control crackin on raised ruface
c- isolation joint; used for successive pours of concrete but does not connect the two pairs
d-expansion joint; used to allow two section of a building to move independently
