10 Concrete Construction

Question 1

Heat of hydration

Answer 1

During concrete hardening heat is given off by chemical process of hydration.

Question 2

Concrete requires what to properly harden

Answer 2

Hydration with water

Question 3

Concrete can be cured using several techniques

Answer 3

Pounding, sprinkling with water, covering with plastic film, sealing to reduce evaporation

Question 4

Admixture

Answer 4

Ingredients or chemicals added to concrete to change its characteristics

Question 5

Concrete must be cured at certain temperatures:

Answer 5

Between 50-70 degrees, over 100 will not be strong and near freezing with take a while.

Question 6

Concrete can shrink while curing and can harden indefinitely slowly. Optimum time for design strength hardening is how many days

Answer 6

28

Question 7

Types of concrete

Answer 7

Ordinary stone
High early strength
Expansive
Water permeable
Structural light weight
Insulating light weight
Gypsum

Question 8

Concrete is compressive lay strong but weak in tensile strength. Compression strength can vary from

Answer 8

2,500-6,000 psi

Question 9

When tensile strength is needed concrete must be

Answer 9

Reinforced with rebar

Question 10

Mechanical vibrators are used when pouring concrete to ensure

Answer 10

It adheres fully with rebar and there are no air pockets.

Question 11

Instead of an I beam, concrete is often shaped

Answer 11

As a T with rebar at the bottom

Question 12

Pretensioned reinforcing.

Answer 12

Pretensioned concrete. Steel strands are stretched between anchors causing tensile force, concrete poured around it. Tension in steel is released, compressing concrete horizontally- Creates an upward bow in concrete that is flattened when compression forces are applied.

Question 13

Post tensioning

Answer 13

Steel on concrete is tensioned after concrete hardens. Steel covered with grease or tubing to allow them to slide around concrete. Once hard the steel is tightened, compressing the concrete horizontally.

Question 14

Cutting of tensioned concrete is not recommended because

Answer 14

Steel is under tension and may snap back like rubber band. Also cause structural failure

Question 15

Cast in place concrete

Answer 15

Use of forms and wet concrete to form concrete shapes, done at sight

Question 16

The single most important factor determining concrete strength

Answer 16

Water to cement ratio

Question 17

If too much water is used in the concrete mix what happens

Answer 17

It won’t be strong enough

Excess moisture will cause spalling during freeze or fire conditions

Question 18

Because large construction can’t be cast at on time, concrete needs to be formed in stages. To eliminate structural issues what is done

Answer 18

Rebar overlaps joints to be poured in stages.

Question 19

Common cast in place systems

Answer 19

Flat slab
Slab and beam
Waffle construction

Question 20

Flat slab system

Answer 20

Concrete slab supported by columns. Slab varied form 6-12” columns reinforced with drop panels or mushroom capitals

Question 21

Flat plate

Answer 21

No reinforcements for concrete columns supporting slabs

Question 22

Concrete Slab and beam framing

Answer 22

Slab supported by concrete beam. Lightweight, looks like wood framing on therms of spacing.

Question 23

Waffle construction concrete.

Answer 23

Waffle pattern on bottom of slab. Steel reinforced at bottom of slabs.

Question 24

Precast concrete.

Answer 24

Poured formed, and hardened at off site manufacturing facility, under controlled conditions to be shipped out for use.

Question 25

advantage to precast concrete

Answer 25

Quality control,
Can be sheltered prior to use,
Efficient uniform and consistent

Question 26

Precast concrete can be produced using polystyrene core causing

Answer 26

Improved insulation

Question 27

Disadvantage to precast concrete

Answer 27

Transportation of members limits size and shape and is expensive.

Question 28

Tilt up construction

Answer 28

Concrete wall sections cast laying down then tilted up into position.

Question 29

Precast slabs can be solid or hollow, singe t, or double t. They can span

Answer 29

Solid Appx 30 ft

Tee slabs up to 120 ft

Question 30

Roof integrity and strength is important for tilt up wall sections because

Answer 30

Failure of the roof can cause outward force causing wall collapse.

Question 31

In precast concrete, the sections must be joined. But they can simply rest on supports. Methods to tie include

Answer 31

Bolting, welding, post tensioning.

Question 32

When precast beams are to be supported by columns, a corbels is used, this is

Answer 32

A bracket cast into the column the holds the beam.

Question 33

Prestressed concrete is more vulnerable to failure than ordinary reinforced because

Answer 33

The high strength steel has lower yield at around 752 degrees and will fail sooner.

Question 34

Concrete structural systems can have fire resistance rating of 1-4 hrs. Resistance of concrete assembly is affected by these variables

Answer 34

Concrete density
Thickness
Quality
Load supported
Depth of concrete over rebar

Question 35

Spalling

Answer 35

Expansion of excess moisture in concrete when exposed to fire or freeze. Causes tensile force and spalling.

Question 36

New concrete as opposed to old is more subject to what when exposed to fire

Answer 36

Spalling because of the increased moisture that hasn’t been evaporated yet.

Question 37

Concrete is produced from

Answer 37

Portland cement, coarse and fine aggregates and water