Jointing

Question 1

What is shrinkage?

Answer 1

The volume reduction in concrete as the water is consumed by the chemical reaction with portland cement and evaporation.

Question 2

When does shrinkage occur?

Answer 2

1/3 of the shrinkage takes place during the first month then the remaining shrinkage takes place throughout the year, up until the end of the year.

Question 3

Why does concrete crack?

Answer 3

Stresses from volume change (expansion/contraction) > tensile strength of concrete. volume changes are due to concrete shrinkage, thermal contraction, and moisture/thermal gradients w/in concrete

Question 4

What are the 3 types of joints that reduce the stresses in concrete?

Answer 4

• Contraction (control) joints
• Isolation (expansion) joints
• Construction joints

Question 5

What is a contraction (control) joint?

Answer 5

Joints that subdivide large concrete sections into smaller units. (transverse = perpendicular to slab) (longitudinal = parallel to slab).

Question 6

What is an isolation (expansion) joint?

Answer 6

Joints that isolation concrete sections of unusual shapes or to separate slabs from fixed objects.

Question 7

what is a construction joint?

Answer 7

Joints that provide transition between one day’s work and the next.

Question 8

How do contraction joints work?

Answer 8

Joints create weakened sections that forces the concrete to crack in predetermined locations in.

Question 9

How deep must contraction joints be in SOG?

Answer 9

At least 1/4 of the slab thickness.

Question 10

What is the general spacing for contraction joints in SOG?

Answer 10

• The spacing in feet should not exceed 2 to 3 times the slab thickness in inches.
  ie. a 5” slab will be jointed @ 10 to 15 feet intervals in both directions.
• Plain pavements with short joint spacings designed to control cracking are usually limited to spacings of 15 to 20 feet.

Question 11

How is longitudinal cracking controlled in SOG?

Answer 11

• Pavement lane widths must be limited to 12 to 13 feet.
• Longitudinal contraction joints are usually tied together with deformed tie bars to keep lanes from separating at joints

Question 12

How is movement reduced between adjacent concrete panels for pavements with heavy truck traffic?

Answer 12

Load transfer dowels are place in between concrete panels to prevent differential movements.

Question 13

Whats the max spacing between contraction joints for highway pavements?

Answer 13

Highway pavements with welded wire meshing placed 2 inches from slab surface have joints up to 40 feet apart.

Question 14

What is the general spacing between contraction joints in floors of buildings to control early shrinkage stress?

Answer 14

Between 10’ to 30’, depending on floor thickness, agg. size and concrete slump.

Question 15

What is the max spacing for contraction joints in exterior walls with frequent openings?

Answer 15

No more than 20’ apart.

Question 16

What is the max spacing for contraction joints in exterior walls without openings?

Answer 16

No more than 25’ apart.

Question 17

How far should wall contraction joints be from a corner?

Answer 17

Within 10 to 15 ‘ if possible.

Question 18

For small openings more than 25’ apart at the first story level, where should the contraction joint be located?

Answer 18

Control joints should be in line with each jamb below the openings.

Question 19

For small openings more than 25’ apart above the first story level, where should the contraction joint be located?

Answer 19

There should be a single joint at the centerline of each opening.

Question 20

How do Isolation joints work?

Answer 20

They permit both vert. and horiz. differential movements at a joining parts of a structure. Separate the slab from the more rigid parts of the structure.

Examples:

• around the perimeter of a floor on ground
• around columns
• Around machine foundation

Question 21

What material thickness is most commonly used to create isolation joints?

Answer 21

Could be as thin as 1/4” but 1/2” materials are most commonly used.

Question 22

How do construction joints work?

Answer 22

Construction joints join concrete that is paved at different times. True construction joints bond existing concrete with new concrete, preventing movement b/w the two often using deformed tie-bars.

Question 23

Where should transverse construction joints be located?

Answer 23

These are necessary at the end of a paving segment.

Question 24

Where should longitudinal construction joints be located?

Answer 24

These occur between lanes that are paved at different times, or join through-lanes to curb and gutter.

Question 25

When should control joints be cut into concrete?

Answer 25

• Not to early: loose aggregate will move around (not clean cut).
• Not too late: the concrete will begin to crack without the control joints in patter.
• Determined by maturity test