Concrete ARC 133 NVCC

What is concrete made up of?

Cement, Aggregate, Water

What is Cement?

Any material that will bind two or more non adhesive materials together

Example of natural cement?

Limestone

what is the composition of Portland Cement

60-65% Lime, 10-25% Silica, 2-4% Iron oxide, 5-10% alumina

What is type I cement?

Normal quality, used for general purposes

what is air entraining cement?

cement that has microscopic air bubbles that makes it resistant to freezing or thawing

What is type II cement 

Cement with moderate resistance to silica, used when concrete will be in contact with water that has a high concentration of sulfates

What number of types does air entraining go up to

I-III types of cement that can have this feature of admixture

what is type III cement

High early cement, hardens quicker (accelerator cement)

what is type IV cement

Low heat P.C., used in massive structures like dams

what is type V cement

High resistance to sulfates, used when concrete will be in contact with water that has a high concentration of sulfates

What is White Portland Cement 

 used in Architectural applications, controls quantities of oxides of iron and manganese to make it white

What is special Portland cement

Mixture of P.C. and pozzolanic materials like volcano ash, used for stucco and plastering (light weight aggregate)

What is Hydration

When cement hardens in a chemical reaction with water

How heavy is a bag of cement

94lb

How many gallons of water does a bag of cement need

2 and a half to 3 gallons

In the field how many gallons is actually added to a bag of cement 

4 to 8 gallons

What is Bleeding concrete

When water is added for special finish, creating a fine textured finish

What percentage of aggregate takes up the volume of concrete 

66% to 78%

What’s the usual diameter of fine sand aggregate

¼ or less in diameter

what’s the usual diameter of gravel aggregate 

3/8  to 1 ½ 

The shape of the aggregate affects what?

The plasticity, more rounded aggregate moves easier

What are the two categories of Standard weight Aggregate?

Natural Aggregate (sand and gravel, more rounded) and By Product aggregate (crushed rocks from quarries, sharp and angular)

What is Light weight Aggregate

Volcanic materials processed by heat, during the heating process the gas that forms inside the materials create a porous form

What is Light weight Aggregates use?

Insulating roof toppings

What is an Admixture?

Any material added to concrete to alter the properties in various ways

What’s Air Entraining Admixture?

Increases the workability of wet concrete, used for roads subject to freezing temperatures.

Water reducing Admixtures

Allows a reduction in mixing water, Stronger concrete

Retarder admixtures

Delays setting times in hot weather or delay because concrete requires slower pour but reduces the strength of concrete

Accelerator admixtures

Speed up setting, doesn’t reduce strength

Damp proofing agents 

Minimize absorption of water

Fly Ash

Stronger more durable and chemical resistant concrete mix, reduces amount of cement in mix

Silica Fume

Improves its properties, helps in reinforced steel from corrosion

Color Pigments

Adds color, if color mixture goes more than 10% in mix weight it reduces strength of concrete

Freeze Protection

Allows concrete to cure as low as 20 F, may reduce strength

Strength depends on what?

Water/Cement Ratio, Quality of Aggregate, and Proportion of fine and coarse aggregate

1:3:4 ratio refers to what?

1 part cement, 3 parts sand, 4 parts gravel

What is Cement factor

The percent of cement in a cubic yard, the # of cement bags that will need to form one cubic yard of concrete

Water should be added at what mixing time?

Before one fourth of mixing time has passed

What should the speed of revolution of the mixer be?

100 to 200 ft per min

How much mixing time should a one cubic yard mixer have?

It shouldn’t be less than one minute, each additional cubic yard of the mixer is +15 seconds

Concrete must be discharged in less than what after water is added?

1 and a half hour after water is added

What should happen at site prep before concrete is poured?

Forms and soil must be moisturized so they don’t take the concretes moisture/water

What is the slump test

Done to measure consistency, wet concrete is measured on how much it slumps after being molded into a cone

How much should normal concrete and reinforced concrete slump down?

2” to 5” inches for Normal concrete and 3” to 6” for reinforced

What is the Strength test?

Concrete compressed at different days of its curing process to see strength over time when it cures

Direct Placement 

Concrete is placed in layers of 6” to 18”, must be poured very closed no further than 3” to 4”

Drop Chutes

When concrete must go into deep forms this must be used to reduce the amount of free fall 

Pneumatic Placement

Air pressure is used to place concrete, dry mixture of cement and sand is blown through hoses and water is ejected at the nozzle

What is used for revolving elements in the concrete

Vibrators, the pipes are approx 18” in length and 1” to 4” in diameter connected to an engine

When new concrete is poured next to old concrete what does it need

A watertight construction joint between the two

What must be done before concrete is poured over new concrete

The hardened concrete is roughened and cleaned by jets of water 

What’s the expansion and contraction inch per degree of concrete?

1/55,000,000 of an inch per degree but hairpin cracks still show up

What are relief joints for

To absorb movement

What's a built in joint

Rubber or plastic strips cast into concrete

What’s a Filled Joint

Concrete joints that are filled with formed in place sealant

What’s a false joint

Creating weaker lines in concrete to control where the cracks form, found on sidewalks, the false joint depth can not be more that 1/5 of total depth

T/F concrete slabs don’t have to be finished manually

False, concrete slabs need to be finished manually

What’s the three stages of Concrete slab on grade finishing

Leveling with screeds, Floating, and final finish

What’s leveling with screeds

when concrete is poured it must be leveled with screeds, a long plank that’s placed horizontally to see is the level is even in concrete and there are no points that have too much. 

What’s hooking operation

Wire mesh being pulled inside concrete after leveling, after that it is jitterbugged

What’s Floating?

Workers use a tool called a hand float that pushes down aggregate and brings finer cement paste to the top, fills in small voids that was missed in screeding.

Whats Final finish

Happens about 4-6 hours after placement, makes it look good, a power trowel is used for the final finish while edges and anchor bolts are a hand trowels job

What’s exposed aggregate

Can be made using special sized gravel ( pea gravel) and sand, retarders are sprayed on the surface after the final finish

What’s the best temp to cure concrete

between 60-80 degrees

When is the most critical period of concrete curing

the first week

What’s water curing

covering a flat slab with a thin layer of water, prevents evaporation of moisture from concrete

what’s burlap curing

wet burlaps are covered over the curing concrete

what’s paper curing

waterproof paper is covered over the concrete to prevent evaporation, a layer of wet sand is placed on top to control the temp

What’s plastic curing

plastic sheets are used to seal the top of concrete

What is curing compounds

Can be sprayed or brushed on freshly placed concrete, drys very fast

how much psi can concrete withstand

10,000 psi

What’s the purpose of reinforced concrete

concrete takes car of the compression while steel takes care of the tension

what is shearing action

takes place near the support areas which is trying to cut and separate the beam from the column to resist this shearing action steel is used (STIRRUPS)

what should reinforced steel be tied with

10 gauge soft wire

What variants does rebar come in

deformed or smooth

The number of deformed bar starting at #3 goes up at what measurement rate till #8?

it goes up 1/8”,  #3 starts at 3/8” till #8 that ends at 1” since 8/8 equals 1

what is wire mesh used for

to reinforce concrete slabs

what does 6×6 10/10 wire mesh mean

the distance between wires are 6” each way and the thickness is 10 gauge

What is precast concrete 

Casting beams, columns, slabs, etc. in industrial plants then transporting it to the construction job in order to save money when making a tall building

what is prestressed concrete

steel that is put in tension before the concrete is cured, after it is set the tension is release, the concrete will withstand a force of 250,000 PSI

what is pretension

tension is applied before pouring concrete

what is post tension

the tension is applied after the concrete hardened