Concrete Flashcards
What is the maximum that sheathing should be cut back on tendons?
1 inch
What is the purpose for post-tensioning concrete?
To increase compressive strength in the bottom of a joist or girder to offset tensile stress
On concrete thickness up to about seven inches, which of the below are true about air content to slump?
Air content increases as slump increases
of the below would increase the abrasive strength of concrete?
1 – Low water-cement ratio 2 – Steel - troweled surface 3 – Harder aggregate 4 – All of these
What is the purpose of using vibrators when doing concrete work?
1 – Get rid of air voids 2 – Consolidates the concrete 3 – Moves fine aggregate to bottom 4 – Both 1 and 2
Usually air-entrained lightweight concrete has a slump of 2 to 4 inches, what would be the maximum slump you would seldom exceed?
5 Inches
– Tools used for finishing air-entrained concrete should be constructed from which of the below?
Aluminum or magnesium
Normal weight and lightweight concrete exposed to freezing and thawing or deicing chemicals shall be ________.
Air entrained
Samples for strength test for each class of concrete placed each day shall be taken not less than once a day or less than once for each ___________ .
150 cubic yards
What would you use to keep concrete forms from spreading apart?
Snap ties and brackets
Hotter than normal temperatures when pouring concrete has what consequences (or “What is true” about pouring concrete in hot weather?)
1 – Increased slump loss 2 – Increased tendency for cracking 3 – Long term strength loss4 – All of the above
temperatures have what effect (or it may have asked what is true) on fresh concrete.
Concrete that has been frozen will not be as impermeable as concrete that wasn’t frozen
Retempered concrete or concrete that has been remixed after initial set shall not be used unless approved by?
Registered design professional
what is the minimum cover on #5 rebar where permanently exposed to earth?
3 inches
Once water is added to a concrete mix at the mixing plant, what is the maximum time for delivery and discharge from truck?
1 ½ hours
The minimum strength of a concrete wall not exposed to the weather after 28 days is:
2500 psi
What is true about moisture in concrete?
1 – It is important to prevent moisture from leaving the concrete 2 – It is important to not prevent moisture from leaving the concrete3 – Excessive moisture loss will result in concrete cracking 4 – Both 1 and 3
Steps in footings should be used when slope exceeds:
10%
pumping concrete into tightly reinforced forms which of the below should be used?
Superplasticizers
What would you use to laterally support concrete forms?
Whalers and braces (tied and braced)
On concrete beams, what is the recommended distance between tendons/cables?
1 ½ inches
Concrete that is exposed to salt water, what is the correct mixture added to protect rebar?
Corrosion resistant mixture
How much concrete coverage is required when you have a # 10 rebar in a beam?
¾ inch
Low slump means that the concrete is;
Stiff
What is the minimum thickness required for a concrete slab placed against and permanently exposed to the earth?
3 ½ inches
Which of the following is not acceptable when grouting stressing pockets?
Having chlorides as an additive
When back stressing a barrier cable, the end anchor should be installed to a force of ___%
80%
The maximum number of or .60 of an inch tendons / cables in a group is _____.
4
For an encapsulated post-tensioned system, the watertight cap should be installed less than____.
24 hours after stressing
Which of the following is not a method to control plastic shrinkage?
Use of a richer mix design
High early strength concrete is made using Type _____ cement.
III
of the following are methods of curing concrete except _____.
1 – Keeping the surface moist 2 – Heated forms 3 – Covering with plastic sheets 4 - Using plastic forms
Which of the following concrete joints allows for both vertical and horizontal movement?
Isolation
For the strength of concrete to continue to increase with time, the relative humidity within the concrete should remain above____ percent.
80%
Which of the following factors does not influence workability of concrete?
Calcium chloride
Durability of concrete can be related directly to:
Compressive strength
What is the most effective method for controlling shrinkage cracking in concrete?
Providing joints (contraction joints)
The resistance of hardened concrete to freezing and thawing is significantly improved by adding:
Air – entrainment
What is the effect of vibratory action on a concrete mixture?
1 – It permits the use of a stiffer mixture 2 – It causes the mix to be more liquid 3 – Removes trapped air 4 – All of the above
What is the effective period of time for increased workability that can be expected when placing concrete using super plasticizer?
30 to 60 minutes
What is most closely related to abrasive strength of concrete?
Compressive strength
How do higher temperatures at time of placement affect concrete?
Concrete has a lower slump
Besides the water to cement ratio, what else determines the final compressive strength of concrete?
Age
The most common plasticizer used in concrete is:
Lime
For the most part you do not need to add anything to the concrete mix other than the standard ingredients, but on occasion additives do help to make the job easier. Which of the below is not considered an additive?
1 – Air entraining powder 2 – Plasticizers 3 – Retarders 4 – Water
Concrete in which wires are tensioned on the jobsite after the concrete has been hardened by running the wires through ducts is called
Post-Tensioned concrete
What is the minimum 28 day compressive strength for lightweight aggregate concrete?
2500 psi
If you order and are pouring one cubic yard of concrete 4 inches thick, which of the following forms would it fill?
9 feet ½ inch x 9 feet ½ inch Design and Control of Concrete Mixtures Calculations Divide 27 cubic feet (one yard) by inches (.33) and then square root to get the square dimensions. You may have to take answers given (if not square) and multiply and then multiply by inches and divide by 27
The concrete arrives on a construction project stiffer that needed and the workers add water to the mix. How will this affect the mix?
1 – Does not affect the mix 2 – Makes the mix weaker 3 – Increases the water to cement ratio 4 – Both 2 and 3
A footing needs to have a load supported on it of 252 kips, with a soil bearing capacity of 6,000 pounds. How many square feet will this footing have to be?
42 square feet
When placing concrete, what is the maximum distance it is allowed to fall before a tremie chute is required?
4 feet
What is the purpose for vibrating concrete?
1 – To remove trapped air 2 – To consolidate the concrete 3 – To separate the ingredients 4 – Both 1 and 2
When truck concrete mixers are used, the time between batching and complete discharge after water has been added to the cement and aggregates is ____ hours or before the drum has revolved ___times.
1 ½ hours and 300 revolutions
When mixing concrete, the reason for adding water to the cement is called:
Hydration
What temperature range would be considered a hot weather condition for concrete?
77 degrees to 90 degrees
What is the definition of creep in concrete?
The continuous deformation of the concrete due to continued sustained loads
If you have a pier or pier footing that is 7’0” x 7’0” by 1’6” deep using 4,000 psi concrete. The soil that this footing is sitting on has a bearing capacity of 2,000 pounds per square foot. What is the maximum load that soil can bear with this footing in kips?
98 kips
What is the definition of pre-cast concrete?
that is cast and poured off site and transported to its final position
What is the best temperature for curing concrete?
55 to 73 degrees (actually 50 to 70)
Which of the below would be the correct splice lap for splicing reinforcement rods (rebar) together in masonry?
40 times the bar diameter, minimum one foot of splice
What is the main reason for taking a slump test?
Check the workability and consistency of the concrete
When pouring concrete in hot weather, this is a condition that can occur:
1 – Shrinkage cracks 2 – Loss of strength 3 – Fast hydration (stiff very quickly) 4 – All of the above conditions
To resist the deterioration of concrete due to deicers, freezing and thawing, which of the following admixtures need to be added?
Air-entrainment
What is the purpose for using form ties when pouring concrete in forms?
To resist the lateral pressure of the concrete 4 – Neither of the above
When installing post tension cables, you are allowed to have 1 inch of the cable exposed in the concrete at the end tying into the form without plastic sheathing. What procedures are required if more than 1 inch is exposed?
Grease cable and tape
How can you stop reverse curvature of a post tension cable?
– Add additional cable supports
does post tension cables have to have before concrete can be poured on them?
Covered in grease and plastic covering in place
What is the definition of a post tension cable?
1 – Reinforcing steel is housed in a plastic coating or sheath that prevents the concrete from bonding with the cable so it can be stressed after the concrete has hardened
What type of concrete is used to allow stripping forms the quickest?
Type III
What is the purpose for vibrating concrete?
Separate the course aggregate
What admixture needs to be added to concrete in hot weather conditions?
Retardant
When installing post tension cables, there needs to be at least ____ inches of cable protruding out of the form
12
What is the minimum reinforcing bar lap or splice for a # 5 bar with 40D?
25 inches: A # 5 bar is 5/8 of an inch or .625 of an inch x 40 diameters splice is 25 inches.A # 4 bar is 1/2 of an inch or .50 of an inch x 40 bar diameters is 20 inches
What is the minimum percent of the 28 day compressive strength of concrete before post tension cables can be stressed?
75%
Definition of Type II concrete
– Moderate / Low sulfate resistance
Definition of Type III Concrete
High early strength – early set
Definition of Type IV concrete.
Low heat of hydration
which of the below is true about the volume of concrete once it sets up?
Stays about the same (changes slightly)
What is the concrete coverage for folded plate members on # 6 thru # 18 reinforced bar not exposed to earth?
¾ inch
How are post tension cable / tendons installed?
Wrapped in plastic with PT applied
What are the proportions of concrete made up of?
Sand, cement, water and course aggregate
What effect does low slump have on concrete?
More stiff, hard to work
What is the definition of air-entrained concrete?
1 – Concrete that will have resistance to freeze – thaw conditions when exposed to water and de-icing conditions
What is the definition of superplasticizers?
High range water reducers that are added to concrete with a low-to-normal slump and water ratio to make high slump concrete.
What is the definition of slump?
It is a test for the rigidity (consistency) of uncured concrete.
What is the minimum thickness at the edge for plain concrete footing?
8 inches
What is a characteristic of pouring concrete at low (cold) temperatures?
Concrete gains very little strength at low temperatures
What is a common admixture to add to concrete to help hydration and strength when pouring in cold weather?
Accelerating admixture
What is the average size of course aggregates (rock/gravel) in concrete?
3/4 inch
How many days does it take for concrete to reach its minimum design compressive strength?
28 days
When using air-entrainment in concrete as an admixture it will help with the freezing and thawing conditions over the life of the concrete, air-entrainment also will:
Improve the workability
What is the next process after completion of the screeding of a concrete slab?
Bull float finish
What type of concrete finish does a driveway require?
Broom finish
When is reinforcement not needed in concrete?
Driveways
How do concrete forms maintain their shape?
Braced and wall tied
Where are construction joints located in concrete beams and slabs?
Middle third
What is the best method to brace and align concrete forms in masonry work?
Turnbuckles and pins
Turnbuckles and pins
TRUE
Insulating concrete is a light weight concrete with the aggregate of:
Perlite
What is the definition of low permeability of concrete?
– Low permeability will produce acid and corrosion resistance of concrete
What is the definition of high permeability of concrete?
– High permeability of concrete will cause reinforcing to corrode.
What is the most preferred method to consolidate concrete?
Mechanical vibrators
What is placed directly under a concrete slab on grade?
1 – Vapor barrier 2 – Crushed rock 3 – Compacted sub grade 4 – All of the above
What type of admixture is high early concrete?
Accelerating admixture
What is the diameter of a # 5 reinforcing bar?
5/8 inch
When pouring concrete with reinforcement that will be exposed to seawater, what type of admixture should be used?
Corrosion inhibitors
For lightweight concrete with an overdry density of 50 pcf (pounds per cubic feet) which of below is correct?
Maximum slump of 5 inches
– Type II concrete is used for what kind of construction?
Moderate sulfate resistance
Which of the following may be used to hold foundation wall forms together?
Snap ties and metal brackets
When performing the final operation where two extensions of the stressing jack are used, what is the percent of the final stroke of the jack?
60%
which of the below statements is true about moisture when curing concrete?
It is important to prevent the loss of moisture from concrete during curing
All of the following procedures apply during the preparation for concrete placement except:
Concrete should be retempered or remixed as needed
What should be applied to hardened concrete to protect from deicing chemicals?
Air - entrainment
What additive is used for concrete at high temperature or too hot?
Retardant
which concrete joint allows both horizontal and vertical movement of concrete?
Isolation joint
What is a common characteristic of low slump concrete?
Stiff
Concrete that is low in heat and hydration is:
Type IV
What is the last step before pouring concrete on post tension cables?
Install support bars
What is true about how to finish light weight concrete?
Need to start finishing earlier than normal concrete
What is the effect of algae in mixing water used for concrete?
Reduced strength
What is the effect of high heat in concrete?
Reduced slump
The abrasion resistance of concrete is dependent upon __________________.
Adequate curing
The minimum overdry density for lightweight insulating concrete is _______pcf.
15
Concrete cover is the minimum distance from the _______________.
Outside of the reinforcing bar to face of concrete
What is the finish calculation for figuring the cubic yards of concrete if the beginning calculation is Length x?
Width x Depth (in feet) divided by 27
What type of concrete has a low heat of hydration?
Type IV
What type of concrete has a moderate sulfate resistance?
Type II
What classification of reinforcing rod supports is used in situations where there will be maximum exposure but no grinding or sandblasting is required?
Class 1A
The maximum spacing for rebar in a 6 inch thick wall should not exceed _______inches on center.
18 inches
When placing reinforcing steel in a steel mat, where may the wire ties be placed if the spacing is not specified?
At every fourth or fifth intersection
Short bars tied at right angles to main reinforcing bars in a floor slab are called “___________ bars”.
Temperature
Which of the following items is NOT required to be identified by the markings on reinforcing steel?
A set of numbers indicating the date of fabrication
Which of the below would be the best order to install post tension cables?
Forms set, install sleeves and embeds, cables, cable supports and install plumbing and electrical sleeves
Stirrups are used in a concrete beam to resist:
Shear
many pieces of rebar does a beam bolster chair support?
2
Unreinforced structural plain concrete is used in all of the following except:
Columns
Why should you use a chute for placing concrete instead of dropping it in place?
Helps concrete to not separate (without segregation)
Which admixture is added in concrete to accelerate the rate of hydration and increase strength development at an early age?
Accelerating admixture
The minimum standard reinforcing bar size is a ______bar.
2 – # 3 3 – # 4 The above answer # 3 bar is correct for this question but there is an exam question where a # 4 bar is the correct answer for the same question. I think that on that exam question that a # 3 bar is not listed as a possible answer
What is used in concrete walls to hold metal reinforcing in place?
Tie wire
What is the minimum spacing between bundles of tendons?
1 ½ inches
Which of the below would be an admixture in concrete?
Air - Entrainment
What is the range of air entrainment of light weight moderate exposure structural concrete?
5 to 8 %
What is the best slump used for light weight concrete?
What is the definition of Type V concrete?
What is the definition of Type V concrete?
High sulfate resistance
What is the slump used for lightweight concrete that can be placed under conditions that would normally require a slump of 3 to 5 inches for normal-weight concrete?
2 to 3 inches
Which of the below is correct for the standard reinforcing bar sizes?
3 through # 11, # 14 and # 18
How many years of experience are needed to be a lead man for post tension cables?
5 years
Which of the below would more likely require using a vibrator to consolidate concrete?
Mixture with aggregate
What is the minimum support bar needed for cables in a post tension beam?
4
What type of support would be used to hold a rebar mat 3 feet off the ground?
Standee
Where are the temperature bars placed in a slab?
Right angles
What is the purpose for putting composite sheathing over a post tension tendon?
Cable can move more freely in concrete
What is the best type of joint to control cracking in a driveway?
Contraction
What is the most widely used supplementary cementitious material in concrete?
Fly ash
How many different sizes are there in reinforcing bars?
11
What is the best type of concrete mix to use in a drain structure that requires moderate sulfate concrete?
II
Which group of concrete uses expanded aggregates such as perlite?
I
What is the best method for curing concrete?
Moist condition for 7 days at 50 degrees
What is the proper method for vibrating concrete?
Insert vibrator quickly and remove when large air bubbles stop surfacing
What is the most effective way to prevent water evaporation when curing concrete?
Fogging
What is the relationship between concrete strength and hard aggregate?
Hard aggregate is more wear resistant
What admixture in needed when placing concrete in areas of closely spaced and congested reinforcing steel to improve concrete flow without weakening the concrete mix?
Superplasticizer
As a concrete beam is loaded and has the tendency to bend down, where in the beam would the reinforcing be placed to resist this tension?
Bottom
Where are temperature bars placed in a joist?
On top of all other bars with support bars underneath them parallel to joist bars
How much pcf of perlite is used in Group I lightweight concrete?
15 and 50 pcf
Which of the below would prevent surface scaling of concrete?
Air entrainment
What is the minimum depth of construction joints?
25% of slab (1/4 of slab)
The maximum number of or .50 of an inch tendons / cables in a group is _____.
5
A rebar splice is 25 inches using a 40D splice, what is the size of the rebar?
5Placing Reinforcing Bars Page 10-12Formula = 25 divided 40 = .625 x 8 = 5
What of the below is not a characteristics of a slab on grade?
Grass and sod underneath
What can be added to Group I light weight concrete that has 15 pcf of perlite?
Additional perlite
Which of the following is not considered an aggregate in concrete and should be avoided?
Chert
What is used to hold reinforcement together in a concrete wall?
Wrap and snap
What type of tie wire is used to hold reinforcement together in a concrete wall?
Black soft-annealed
What is the required temperature range for test cylinders to be stored in the field?
60 to 80 degrees F
What is the minimum to maximum water to cement ratio for a concrete mix?
.40 to .50
What are concrete cylinders tests used for?
To estimate concrete strength at a given time
When you need concrete to set quickly, what type of concrete should be used to produce high early strength?
Type III
What chemical reaction takes place when Portland cement is hardening?
Hydration
What is the difference between fine and coarse aggregate?
The coarse stone will not pass through a 1/4” sieve
What should be added to concrete in cold weather?
Calcium chloride
Which of the following is not a reason to use an admixture?
To increase permeability
What is the most noticeable characteristic of air-entrained concrete?
Increased workability
What is the effect of adding too much water to a concrete mix?
Higher slump and lower strength
Can fly ash be introduced into a concrete mix specifying a minimum cement content?
Yes, if called for by the specifier
The maximum delivery time for ready mix concrete is:
90 minutes or 300 revolutions of the mixer drum
Mat, raft, or floating foundations are specified when:
The allowable bearing capacity of the soil is very low to great depths
What type of forms are used to place concrete by extrusion?
Slip-forms
What is the minimal quality plywood that can be used when building concrete forms?
BB
What is the best type of nail to use for nailing bracing and forms when the nails must be removed when the pour is completed?
Double headed or duplex nails
Which of the following is the most economical wood to use for formwork?
Kiln-dried pine
The usual safety factor for new form ties is:
2:01
The purpose for form liners is to provide:
Special textures
How much does concrete weigh per cubic foot?
150 lbs
What is the best way to reduce lateral pressure on concrete forms when placing concrete?
Slower placement
When an engineer’s specifications for removal of concrete forms is not available, the ACI has recommendations for the length of time concrete should remain in the forms when the aire temperature is above:
50 degrees
What is the main reason for steel reinforcement in a concrete member?
To resist tensile forces
Placing drawings show details for fabrication and:
Placing of reinforcing steel
What are the characteristics of a #5 bar?
625” diameter, .31% area and 1.043 lbx. per LF
What do the markings on rebar represent?
Producers mill, bar size, steel type and grade
Lowering the top bars or raising the bottom bars by 1/2” more than specified in a 6” concrete slab could reduce it’s load carrying capacity by:
Approximately 20%
What is the maximum variation of stirrup location in a floor slab?
+ or - 1”
Which of the following is not a type of rebar splice?
Bundled
What is the minimum length of lap in a lapped splice for reinforcing bars?
12”
The most effective waterstops are made of
PVC
To what depth should a contraction joint be cut into a concrete slab?
One-fourth the slab thickness
How soon after pouring a slab should it be sawcut?
Within 4 to 12 hours
What type of joint is used when concrete placement is interupted?
Construction joint
What is the sole purpose for installing welded wire mesh in a concrete slab?
To prevent cracks
What is the proper location of welded wire mesh in a concrete slab?
2” below the slab surface, in the upper third of the slab
Most specifications allow only one addition of water as long as you do not exceed what?
The water to cement ratio
What is the best shape of aggregate to use when pumping concrete?
Rounded
When using a 5” diameter pump line, what is the ideal mix to pump?
4” slump with air entrainment containing some fly ash
A concrete containing a lightweight aggragate should be presoaked to 70 to 80% total absorption. If not, it will:
Make pumping difficult
When using a vibrator on concrete, it should not be kept in one location more than:
10 to 15 seconds
What type of concrete finish creates a skid-resistant surface?
Broom finish
Air-entrained concrete is most vulnerable to what tupe of problem?
Blistering
What is the slump test used to measure in concrete?
Consistency
Concrete that is specified to be exposed to freezing and thawing conditions should have what added to the mix?
Air entrainment
What percent of the 28 day strength of concrete should be obtained in the first 7 days?
65 to 70 percent
What is the most common type of Portland cement used in the construction field?
Type I
The quality of the cement mix depends primarily on the:
Water-cement ratio
What type of sand is not a suitable substitute for aggregate?
Fine sand
What additive is used for concrete at high temperature?
Retardant
What is the range of moderate slumps for most concrete?
3 to 4 inches
Air entrainment should always be specified for concrete exposed to what?
Freezing and thawing
What is the minimum temperature concrete should be poured on thin slabs?
60 degrees
What is the maximum temperature that concrete should be when delivered to the jobsite?
90 degrees
What report is required before driving test piles?
Geotechnical data
What type of footing is used for columns on the exterior of the building when projection outside of the building is restricted?
Combined footing
What is the most common type of spread footing mainly used to support a column?
Isolated
Who is responsible for checking concrete loads on formwork in large structures and multistory buildings?
Structural engineer
What type of concrete form will create a void inside of the floor or beams?
Internal
Which type of form should be used when forming waffle slabs in a multi-story building?
Pan Forms
What fabricated metal or plastic units hold forms secure against the lateral pressure of freshly placed concrete?
Form ties
The working load of new ties used in formwork should have a safety factor of:
2:01
Breaking concrete cylinders is the most common method for measuring concrete strength, what would be an alternative method for strength measuring?
Penetration
What is the correct procedure for installing reshores under a newly stripped slab?
Snug, but not tight
Concrete forms for slab-on-grade can be boards or metal frame and are set at:
Finish elevation
What is the percent of the tensile strength of unreinforced concrete in relationship to the compressive strength of unreinforced concrete?
10 percent
What is a primary use of welded wire reinforcement in flatwork?
Extending control joint spacing
What are three types of concrete joint?
Construction, Isolation, and Contraction
What type of joint permits the separate segments of the structural frame to expand and contract with temperature and moisture changes?
Contraction Joint
Which type of concrete joint allows for horizontal and vertical movement?
Expansion
What is the primary use of a mechanical vibrator on freshly poured concrete?
To consolidate the concrete
What is used to cut off high points and fill in minor low points on a freshly poured concrete slab?
Checkrod (bullfloat)
What type of bullfloat is best used to finish air-entrained concrete?
Aluminum or magnesium
What problems are caused by pouring concrete in hot weather?
Lower strength, rapid drying time, shorter setting time
Why is calcium chloride (added at a rate of one to two percent of the weight of the cement in the mix) used?
An Accelerator
What is the most effective way to prevent water evaporation when curing concrete?
Fog spraying
What will alleviate excessive bleeding in concrete?
Adding fly ash, adding fine sand, increasing cement
What temperature should concrete cylinders be maintained at during the first 48 hours prior to testing concrete?
60-80 degrees
Which types of concrete slabs are suitable for air-entrained concrete?
Driveway, Sidewalk, and Exterior slabs
What is the most important factor in preventing surface scaling in concrete flatwork?
Air Entrainment
