Precast Concrete Construction

Question 1

What is precast concrete?

Answer 1

concrete elements cast and cured in a manufacturing plant, then transported to the construction site

Question 2

What advantages does precast concrete have over site-cast concrete? (4)

Answer 2

• high-quality forms can be reused hundreds of thousands of times, reducing formwork cost
• concrete mixing and pouring operations can be highly-mechanized
• work is performed in a climate-controlled environment at ground level
• use of Type III Portland cement and steam curing expedites the manufacturing process

Question 3

How is precast concrete more efficient than site-cast concrete?

Answer 3

when compared to site-cast concrete, precast concrete assembly on site is faster and less affected by adverse weather conditions

Question 4

How is the finish quality improved in precast concrete?

Answer 4

controlled casting conditions and high-quality forms allow for greater control of surface finishes

Question 5

What is prestressing?

Answer 5

reinforcing with tightly stretched, high-strength, pretensioned steel strands produces greater structural efficiency

Question 6

Which concrete members can be precast? (5)

Answer 6

• slabs
• beams/girders
• columns
• wall panels
• specialty items

Question 7

What are precast concrete slabs used for, and what sizes do they include? (2)

Answer 7

• used for floors, roofs, bridge decks

- deeper elements span further than those that are shallower

Question 8

When are solid flat slabs used?

Answer 8

used for short spans and locations where minimum slab depths are required

Question 9

When are hollow core slabs used, and what makes them unique? (2)

Answer 9

• suitable for intermediate spans

- longitudinal voids replace much of the nonworking concrete

Question 10

When are single tee slabs used, and what makes them unique? (2)

Answer 10

• used for longer spans

- most precast slabs have a rough top surface to accommodate a concrete topping later

Question 11

When are double tee slabs used, and what makes them unique? (2)

Answer 11

• best suited for the longest spans

- double tee design allows for faster assembly at less cost

Question 12

Describe the form and function of beams and girders. (2)

Answer 12

• provide support for slabs

- the projecting rebars will bond with concrete cast on site

Question 13

Describe the form and function of precast columns. (3)

Answer 13

• usually square or rectangle
• provide support for beams and slabs
• typically reinforced without prestressing

Question 14

Describe the form and function of precast wall panels. (2)

Answer 14

• precast wall panels may be solid, hollow, or sandwiched with an insulating core
• wall panels can be ribbed to increase their vertical span capacity while minimizing weight

Question 15

What other concrete elements can be precast? (2)

Answer 15

• special concrete elements such as stairs can also be precast
• uniquely shaped structural elements

Question 16

What sort of reinforcing do precast elements contain?

Answer 16

many precast elements contain both high-strength prestressing strands and conventional reinforcing

Question 17

How are precast concrete elements assembled?

Answer 17

once on site, precast concrete elements are lifted into place and assembled

Question 18

What determines the type of slab elements used in construction?

Answer 18

in all cases, choice of slab elements depends largely on span requirements

Question 19

What framework can be used in conjunction with precast elements?

Answer 19

precast elements may also be mixed with site-cast concrete, steel, and masonry systems

Question 20

What are the benefits of a site-cast concrete topping over precast slabs? (5)

Answer 20

• greater floor strength and stiffness
• greater fire resistance
• greater acoustic isolation
• easier integration of electrical services into floor system
• smoother, flatter floor surface

Question 21

How are precast concrete members joined together? (4)

Answer 21

• bolted, welded, or grouted connections are commonly used
• connections can also be post-tensioned to produce continuous beam action at points of support
• the simplest connections rely on gravity
• with spanning members, bearing pads are usually inserted at points of contact to prevent grinding and allow for limited movement

Question 22

How are precast column-to-column connections established? (2)

Answer 22

• metal-bearing plates and embedded anchor bolts are cast into the ends of the columns
• connections are dry-packed with grout to provide full-bearing and protect the metal connectors

Question 23

How are precast beam-to-column connections established? (2)

Answer 23

• beams are set on bearing pads at the column corbels

- steel angles are welded to metal plates cast into the beams and columns

Question 24

How are precast slab-to-beam connections established? (3)

Answer 24

• hollow core slabs are set on bearing pads on precast beams
• steel reinforcing bars inserted into the slab keyways span the joint
• good example of a gravity connection