Concrete

Question 1

Answer 1

robert maillart

-swiss engineer used conrete in his bridges and buildings

Question 2

most common form of forming material

Answer 2

plywood

• usually 3.4” thick
• coated with oil and water resistant glue or plastic to prevent water penetration into wood; help reusability of formwork

Question 3

form ties

Answer 3

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

Question 4

Slip Forming

Answer 4

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

Question 5

flying forms:

Answer 5

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)

Question 6

for concrete columns, piers, and walls, the maximum variation in plumb will be

Answer 6

plus ot minus 1/4 in in any 10 ft length.

-the same tolerances applies for horizontal elements

Question 7

the maximum variation out of plumb for the total height of the structure is

Answer 7

1 in for interior columns

1/2 in for corner columns for buildings up to 100 ft tall

Question 8

Ideally, concrete slabs should be allowed to cure and dry for a minimum of

Answer 8

six weeks before resilient flooring is installed

Question 9

THe best way to stop the migration of moisture though a conrete slab is with a vapor barrier placed…

Answer 9

directly below the conrete slab and on top of any sand cushion layer or subbase.

Question 10

vapor barrier

Answer 10

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

Question 11

vapor retarder

Answer 11

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

Question 12

two types of reinforcing steel for cast in place concrete

Answer 12

1 deformed bars

2 welded wire fabric

Question 13

rebar comes in 2 common grades

Answer 13

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)

Question 14

How to read rebar

Answer 14

• 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

Question 15

Formwork

Answer 15

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.

Question 16

Reinforcing

Answer 16

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

Question 17

how to read welded wire fabric designations

Answer 17

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

Question 18

concrete components

Answer 18

• cement
• fine a nd course aggregate
• water

portalnd cement: binder; made of lime, silica, iron oxide, alumina

Question 19

5 types of cement

Answer 19

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) !

Question 20

Air Entrained Cements

Answer 20

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

Question 21

concrete strength

Answer 21

• 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

Question 22

Aggregates

Answer 22

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

Question 23

Air entraining admixtures:

Answer 23

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

Question 24

The size of the aggregates in concrete is determiend by…

Answer 24

…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

Question 25

Methods used to specify the proportions of concrete mix:

Answer 25

- 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

Question 26

strength of final mix is specified by...

Answer 26

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

Question 27

Admixtures...

Answer 27

... are ingredients other than cement, aggregates, and water that are added to the mix to alter it in various ways

Question 28

Accelerating admixtures:

Answer 28

cause concrete to cure more rapidly

Question 29

Hydration:

Answer 29

chemical reaction between water and cement which creates heat

Question 30

High range water reducing admixtures (Superplasticizers):

Answer 30

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

Question 31

• Water reducing admixtures:

Answer 31

allow a reduction in the amount of mixing water while retaining the same workability, results in a higher strength concrete

Question 32

Retarding admixture:

Answer 32

slow curing to allow more time for working with wet concrete help reduce the heat of hydration

Question 33

Waterproofing admixtures:

Answer 33

decrease the permeability of the conrete

Question 34

Fly ash:

Answer 34

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

Question 35

Silica fume (microsilica):

Answer 35

a powder 100x finer than portland cement, a byproduct of electronic semiconductor chip manufacturing that when added produces extremely high strength and low permeability

Question 36

Blast furnace slag:

Answer 36

byproduct of iron manufacture that improves workability, increases strength, reduce permeability, reduce temperature rise during curing, and improve sulfate resistance

Question 37

Pozzolans:

Answer 37

varicose natural/artificial material that react with calcium hydroxide in wet concrete to form cementing compounds

Question 38

Workability agents

Answer 38

: improve the plasticity of wet concrete to make it easier to place in forms and finishes

Question 39

Corrosion inhibitors:

Answer 39

used to reduce rusting of rebar in structures that are exposed to road deicing salts or other corrosion causing chemicals

Question 40

Fibrous admixtures:

Answer 40

short fibers of glass or steal added to a concrete mix to act as microreinforcing

Question 41

Freeze protection admixtures:

Answer 41

allow concrete to cure at temperatures as lot as 20 degrees F

Question 42

Extend set control admixtures:

Answer 42

used to delay the curing reaction in concrete for up to 7 days.

Question 43

Coloring agents:

Answer 43

dyes and pigments used to alter the color of concrete

Question 44

True or false. Water used to mix with concrete must be potable\>

Answer 44

True. Mixing water for concrete must be free of harmful substances, clay, salts, etc. If it’s good enough to drink, it’s good enough to be in concrete.

Question 45

Autoclaved aerated conrete (AAC)

Answer 45

precast conrete product manufactured by adding aluminum powder to conrete hardening it in molds, and then curing the molds in pressurazed steam chambers (autoclaves) - 1/5 the density of regular conrete - has good sound control qualities and thermal mass - not as strong thefore should only be limited to non load bearing application and low rise buildings - must be protected from weather

Question 46

self consolidating concrete (SCC)

Answer 46

can be placed purely by means of its own weight without the use of vibration - made posible with the use of superplasticizer admixture called polycarboxyplate polymer - placement is increased, less labor required, productivity increased - uniform smooth surface

Question 47

carbon fiber conrete

Answer 47

uses epoxy coated carbon fiber mesh in place of standard steel meash - carbon fiber is less corrosive requiring less conrete cover - resulting panels require smaller foundation and support structures - carbon fiber are bundled together to form pieces resembling yarn, called tows; tows laid perpendicular to each other, forming a grid; grid could have nearly seven times the tensile strength of standard steel mesh

Question 48

poured gypsum deck

Answer 48

used for roofs and similar to conrete in that liquid is poured on a reinforcing deck -Poured gypsum and concrete deck fill insulation are economical & applied directly to the corrugated steel decking and rough concrete decks, easily tapered during installation

Question 49

Slump test

Answer 49

measures the consistency of the conrete - conrete is placed in a 12 in high cone and compacted with a rod - the distance the conrte slumps from the original 12 in is measured in inches - the amount of slump depends on how the conrete will be used (typically 2-6 in) - too much slump indicates too much water in the mix

Question 50

cylinder test

Answer 50

measures compresive strength - concrete is placed samples are put in cylinder molds that are 6 in diamter and 12 in high, and are moist cured and tested in the lab - cylinders are tested at specified number of days, normally 7 - 28 days; seven days are suually about 60-70% of the 28 day strength

Question 51

core cylinder test

Answer 51

- usde when a portion of the structure is placed and cured but needs to be tested - cylinder is drilled out of the conrete and tested in a lab to determie compressive strength

Question 52

kelly ball test

Answer 52

- also known as the ball penetration test - a hemispheric mass of steel with a calibrated stem is dropped onto a slab of freshly laid concrete - the amount of penetartion of the ball into the concrete is measured and compared to one-half the values of the slump test

Question 53

impact hammer test

Answer 53

- nondestructive way to test concrete strength after is has hardened - spring loaded plunger is smalled against a conrete surface, and the amount of rebound is measured. - the rebound gives an apporoximate measure of conrete strength

Question 54

k slump test

Answer 54

- uses a 3/4 in tube that contains a floating scale - the tube is palced on the wet concrete and the scale is pushed into the mixture and released. - the distance the scale floats out is read directly and is a measure of the consitency of the conrete, comparable to the slump

Question 55

calcium chloride test

Answer 55

sometimes called the moisture dome test - used to measure moisture emission from conrete - a standard mass of calcium choloride is placed below a plastic cover and sealing it to the concrete floor - after the 60 to 72 hours the calcium chloride is weighed and compared to its pre test weight

Question 56

hygrometer

Answer 56

- sometimes called the relative huidity test - determines the moisture emission by measuring the relative humidity of the atmosphere confined adjacent to the concrete floor - a pocket of air is trapped below a vapor impermeable box and a probe in the box measures RH - moisture sensitive flooring shouldn't be installed unless RH is 75% or less

Question 57

polyethelene sheet test

Answer 57

qualitative test conducted by sealing an 18 in by 18 in sheet of plastic to the floor to trap excessive moisture - after a min. of 16 hours a visual inspection of the floor and sheet is performed - the presence of visible water indicates the concrete is insuficiently dry

Question 58

mat test

Answer 58

similar to polyethelene sheet test a 24 in x 24 in smaple of vapor retardant floor finish is applied to floor with adhesive and edges are sealed with tape - after 72 hours, is floor is firmly bonded or difficult to remove, conrete is considered sufficiently dry

Question 59

electrical impedance test

Answer 59

-uses proprietary meaters to measure moisture by measuring conrete conductance and capacitance

Question 60

alkalinity in concrete can cause problems in two ways

Answer 60

high alkalinity on the surface of a slab can damage a tile installation by causing the adhesive to reemulsify also respnsible for the phenomenon known as alkali silica reaction (ASR); high alkaline cement begins to dissolve sand and rock within the concrete this reaction produces a gel like materal that creates pressure in the pores of the conrete surface -ASR can be reduced by specyfiying aggregates not suceptible to ASR, using low lime cement, proper curing not finishing conrete with a hard trowel surface

Question 62

titration test

Answer 62

used to determine level of alkalinity in concrete -involves grinding portions of the concrete, mixing with demineralized water and performing lab analysis to determine PH level

Question 63

tremie

Answer 63

a long, cylndircal steel chute used to palce concrete underwater

Question 64

placing conrete

Answer 64

If concrete slurry is vibrated excessively, dropped from a height, or moved too far horizontally in formwork its likely to segregate • Aggregate works it way to the bottom of the form and water/cement to the top • Result is non uniform and unsatisfactory, and typically has to be redone • Segregation is prevented by depositing fresh from the mixer as close as possible to the final position • Should not be dropped more than 3’ - 4’ • If it must be moved to an inaccessible location, it should be pumped through hoses or conveyed in buckets

Question 65

form liner

Answer 65

conrete is shaped with liners of plastic, wood, or metal - parallel rib liners are common type - joints and form tie holes are treated as desired-either left exposed or patched

Question 66

scrubbed finish

Answer 66

the surface of the conrete is wetted and scrubbed with a wire or fiber bush to remove surface mortar and expose the soarse aggregate

Question 67

acid wash finish

Answer 67

the surface of the conrete is wetted with muriatic acid to expose and bring out the full color of the aggregate

Question 68

water jet finish

Answer 68

a high pressure water jet mixed with air is used to remove some of the mortar and expose the aggregate

Question 69

brush hammering

Answer 69

a brush hammer finish gives a rugged, heavy texture by removing a portion of the surface made with form liners

Question 70

grinding

Answer 70

finishing technique that smoothes out the surface of the concrete, similar to terrazzo in appearance

Question 71

applied finishes

Answer 71

appication of other materials, sucha s stucco

Question 72

sand blasted finish

Answer 72

produced by removing surface material of conrete -sposes the fine and course aggregate to varying degrees

Question 73

smooth rubbed finish

Answer 73

concrete surface is wetted and rubbed with carborundum brick to produce a smooth uniform color

Question 74

grout cleaned

Answer 74

grout is applied over concrete and smoothed out -conceals defects

Question 75

float finish

Answer 75

- sandpaper like texture - appropriate of exterior surfaces

Question 76

light steel troweled finish

Answer 76

achieved by using a steel trowel several hours after floating

Question 77

hard steel troweled finish

Answer 77

continues the consolidation of the conrete and greatly densifies the top 1/8 in, making a very smooth surface

Question 78

broom finish

Answer 78

- achieved by running a broom over the surface of the conrete - process dislodges fine aggregate and produces slip resistant surface on outdoor slabs

Question 79

super flat floor finish

Answer 79

- hard steel trowl finish - used in industrial warehouses where forklift vehicles are used - vehicles must travel on very smooth and level floors.

Question 80

four primary tyoes of concrete joints:

Answer 80

1) control joints: joints that allow for shrinkage of large areas, and controls and inducescracking to occur along the joint: cut to a depth of 1/4 the slab thickness; intentionally created cracks that can open to accommodate shrinkage in surfaces made of concrete masonry 2) construction joints: occur wherever thera are two successive pours ( a new pour against a cured section of concrete); should be located at point of minimum shear; suceptible to water leakage, thus wa waterstop should be cast into the first pour 3) expansion joints:joints that allow free movement of adjacent parts due to expansion or contraction of concrete. Typically waterproof, watertight, filled with an elastic filler; Required at buildings over 200’ long, at joints of building wings, and additions 4) Isolation Joints: joints located at slab on grade, columns, or walls to allow for independent movement

Question 82

weld plates

Answer 82

steel plates cast into conrete to attach other materials and components

Question 83

the designation for america standard sizes of rebar is based on...

Answer 83

...the number of eights of an inch in the nominal diameter of a bar, up to 1 inch

Question 84

soft metric sizes are base on ...

Answer 84

the apporximate number of millimeters in the nominal diameter example: no. 5 bar in the american designation is 5/8 in which in soft metric is 15.9 mm or a no. 16 bar

Question 85

if too much water is placed in a concrete mix, which of the following problems might develop laitance efflorescence hydration segregation

Answer 85

laitance laitance is the surface deposite of low strength material containing cement and fine aggregates (sand) brought to the surface of concrete. it is caused by having to much water in the concrete mux, which results in water bleeding to the top

Question 86

efflorescence

Answer 86

• Sometimes appears on the surface of a wall of brick, stone, or concrete masonry • Consist of one or more water soluble salts that were originally present in the masonry unit or in the mortar • Brought to the surface and deposited there by water that seeped into the masonry, dissolved the salts, then migrated to the surface and evaporated • Can be avoided by choosing masonry units that are lab tested and don’t contain water soluble salts • Will eventually diminish and disappear with time as the salt is gradually leached out of the wall

Question 87

what are these joints

Answer 87

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

Question 88

expansion joints in concrete walks should be located at a maximum spacing of

Answer 88

20 ft control joints placed where spearate sections of concrete are poured and in walks are placed 5 ft apart -expansion joints with a joint filler are placed a maximum of 20 ft apart

Question 89

What cement would be used in slip form construction type 1 type 2 type 3 type 4

Answer 89

type 3 type 3 is high ealry strength --the type needed for rapid slipd form construction type 1 is normal cement type 2 is low heat and sulfate resistant type 4 is slow setting and low heat for massive structures

Question 90

what is used to minimize conrner chipping of a concrete

Answer 90

chamfer strips a chamfer strip is a small, triangular piece of material placed in the corners of forms to prevent sharp 90 corners, which are difficult to cast and have a tendency to break off during use or when the forms are removed

Question 91

what is the primary purpose of the voids in a cored slab? a. to allow electrical services to be concealed in the slab b. to make a more efficient load carrying member c. to make erection easier d. to minimize weight

Answer 91

b. to make a more efficient load carrying member as with any beam, the deeper the member, the more efficient the beam. using a cored slap rather than a solid slab allows the depth to be increased without increasing the weight in the center of the beam where it is not needed option A and D are also correct but B is the best choice

Question 92

at what temperature doe workers need to to take to protect concrete when cold weather is predicted?

Answer 92

around 40 deg F -construction operations generally discontinue or switch from regular activity to cold weather mode at 40 deg F. A concrete pour can proceed at temperature below 40 deg F. However the water and sand must be heated to ensure that none of the constituents have frozen, and the concrete must be heated for at least 7 days after placement, during the early curing stage

Question 93

temperature sensitive materials, such as sealants should not be applied when the temperature dips below..

Answer 93

...40 deg F

Question 94

When specifuing small batches of conrete what does 1:2:5 mean?

Answer 94

it is the ratio of cement:sand:aggregate C:S:A

Question 95

Which of the following statements if FLASE? a. type iii cememnt is often used for precast concrete members b. precast concrete members can be a maximum of 14 ft wide c. concrete used in precast members is typically 300 psi d. conditions ar emosre controlled during the production of precast concrete members than they would be for similar structural systems

Answer 95

c. concrete used in precast members is typically 300 psi concrete used in precast members is generally higher strength mix than the 3000 psi concrete typically used for site cast applications

Question 96

aggregate shall not be larger than...

Answer 96

... 1/3 of the slab thickness or 3/4 of the minimum space between reinforcing bars, whichever is smaller

Question 97

epoxy coated reinforcing bars are used when ...

Answer 97

...the concrete will be exposed to cholrides (salts) such as deicing salts and those in sea water. -epoxy coating helps keep that salts from corroding the steel by chemical reaction

Question 98

chairs

Answer 98

small wire supports that help to keep rebar a specified distance from the outside of the concrete and help ensure adequate concrete cover

Question 99

bolster

Answer 99

a type of chair ued in broad slabs or beams

Question 100

float finish

Question 101

during which period in the curing process does concrete gain the most compressive strength

Answer 101

0 to 3 days - concrete gains the most compressive strength during the first few days of curing - most critical time in curing process - the compressive strength of concrete is always referred to as its strength 28 days after it is palced

Question 102

What is the most common method of splicing reinforcing bars

Question 103

-Capillary break

Answer 103

crused stone placed over compacted subsoil and serves as a drainage layer

Question 104

-expanded shale aggregate

Answer 104

- density of about 20% less than normal concrete; made by heating shale, causing water in the shale to turn into steam; shale become plastic and expands

Question 105

-pozzolans

Answer 105

-materials that react with the calcium hydroxide in wet concrete to form cementing compounds; (e.g. fly ash, silica fume, natural polozzans

Question 106

-formwork costs for self consolidating concrete may be higher than those for conventional concrete because

Answer 106

-the greater fluid pressures exerted by the concrete require stronger and stiffer formwork

Question 107

-In post tensioned concrete construction, what does it mean to drape reinforcing

Answer 107

- reinforcing is draped along the length of a beam; after tensioning, the drape creates for structural efficiency ## Footnote

Question 108

true or false. the strength og concrete increases as the water cement ratio decreases

Answer 108

true

Question 109

calcium chloride is used as an admixture for concrete in order to...

Answer 109

accelerate the strengthening of concrete