Concrete Flashcards
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:
2:1
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 : 1
What design procedure allows for the anticipated downward deflection of formwork when the concrete fills the form?
Camber
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
