Jointing Flashcards
What is shrinkage?
The volume reduction in concrete as the water is consumed by the chemical reaction with portland cement and evaporation.
When does shrinkage occur?
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.
Why does concrete crack?
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
What are the 3 types of joints that reduce the stresses in concrete?
- Contraction (control) joints
- Isolation (expansion) joints
- Construction joints
What is a contraction (control) joint?
Joints that subdivide large concrete sections into smaller units. (transverse = perpendicular to slab) (longitudinal = parallel to slab).
What is an isolation (expansion) joint?
Joints that isolation concrete sections of unusual shapes or to separate slabs from fixed objects.
what is a construction joint?
Joints that provide transition between one day’s work and the next.
How do contraction joints work?
Joints create weakened sections that forces the concrete to crack in predetermined locations in.
How deep must contraction joints be in SOG?
At least 1/4 of the slab thickness.
What is the general spacing for contraction joints in SOG?
- 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.
How is longitudinal cracking controlled in SOG?
- 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
How is movement reduced between adjacent concrete panels for pavements with heavy truck traffic?
Load transfer dowels are place in between concrete panels to prevent differential movements.
Whats the max spacing between contraction joints for highway pavements?
Highway pavements with welded wire meshing placed 2 inches from slab surface have joints up to 40 feet apart.
What is the general spacing between contraction joints in floors of buildings to control early shrinkage stress?
Between 10’ to 30’, depending on floor thickness, agg. size and concrete slump.
What is the max spacing for contraction joints in exterior walls with frequent openings?
No more than 20’ apart.
What is the max spacing for contraction joints in exterior walls without openings?
No more than 25’ apart.
How far should wall contraction joints be from a corner?
Within 10 to 15 ‘ if possible.
For small openings more than 25’ apart at the first story level, where should the contraction joint be located?
Control joints should be in line with each jamb below the openings.
For small openings more than 25’ apart above the first story level, where should the contraction joint be located?
There should be a single joint at the centerline of each opening.
How do Isolation joints work?
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
What material thickness is most commonly used to create isolation joints?
Could be as thin as 1/4” but 1/2” materials are most commonly used.
How do construction joints work?
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.
Where should transverse construction joints be located?
These are necessary at the end of a paving segment.
Where should longitudinal construction joints be located?
These occur between lanes that are paved at different times, or join through-lanes to curb and gutter.
When should control joints be cut into concrete?
- 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
