Last updated July 28, 2026
Concrete moves. It shrinks as it cures, it expands and contracts with temperature, and different parts of a structure settle by different amounts. A slab joint is where you decide that movement will happen, instead of letting the concrete decide by cracking somewhere you did not want.
This covers the joint types you will be asked to draw, what each one is for, and the abbreviation problem that catches people out.
C.J. means two different joints
Start here, because it is the one that causes real trouble on site.
C.J. is used for both control joints and construction joints, and they are not the same thing at all. A control joint is a deliberate weak line that tells a shrinkage crack where to go. A construction joint is where one pour stops and the next begins, with reinforcement carried through so the two become one slab.
One is meant to open. The other is meant never to. Put the wrong one on a drawing and you either get an uncontrolled crack or a joint that was supposed to move and cannot.
The fix is not clever: spell the joint out in the legend on every drawing that uses the abbreviation, and if a project uses both, use distinct abbreviations and say so. Do not assume the reader shares your office’s habit.
The joint types
| Joint | Abbrev. | What it does |
|---|---|---|
| Control joint | C.J. | A deliberate line of weakness so shrinkage cracking happens where you put it. Slabs on ground. |
| Construction joint | C.J. | Where one pour stops and the next starts. Reinforcement continues through, permanently stitching the two pours together. |
| Sawn joint | S.J. | A control joint cut with a saw while the slab is still green, typically to about one third of the slab depth. |
| Tooled joint | T.J. | The same job as a sawn joint, but formed by hand in the wet concrete straight after the pour. |
| Expansion joint | E.J. | A full-depth gap with a compressible filler, letting adjacent areas move toward each other. |
| Isolation joint | I.J. | Separates the slab from something it must not be tied to — a column, a footing, a wall — so the two settle independently. |
| Key joint | K.J. | A construction joint with a formed key, so the two pours transfer shear across the joint. |
| Temporary movement joint | T.M.J. | Post-tensioned slabs only. Left open while the tendons compress the slab, then grouted and locked. |
| Permanent movement joint | P.M.J. | Splits a large structure into distinct areas that are allowed to move relative to each other for good. |
Why sawn joints are cut early
The timing is the whole point and it is worth understanding, because it explains why the note on your drawing says what it says.
Concrete shrinks as it dries. If you cut the joint before the shrinkage stress builds, the slab cracks at the bottom of your saw cut, which is exactly what you wanted. Cut too late and the slab has already cracked somewhere else, and your joint is decoration. Cut too early and the saw tears the aggregate out of a green slab and leaves a ragged edge.
Cutting to about a third of the slab depth is what makes the crack go where you put it — deep enough to create the weakness, shallow enough to leave the slab working.
The 56 days on a post-tensioned slab
Post-tensioned slabs behave differently and the temporary movement joint exists because of it.
The tendons put the slab into compression, and the slab shortens as they are stressed. On a large floor plate that shortening has to go somewhere. A temporary movement joint is left open to absorb it. Once the early shrinkage and creep have mostly happened — the figure usually quoted is around 56 days — the joint is grouted and becomes part of the slab.
Close those joints too early and the areas lock together while they are still trying to shorten, which is precisely the cracking the joints were there to prevent.
Drafting them
- Show them on the plan, detail them in section. The plan says where the joint runs; the section says what it is made of. Neither works alone.
- Give every joint a tag that matches the legend. If the plan says C.J. and the legend does not define it, the drawing has not communicated anything.
- Watch re-entrant corners. An internal corner is where a slab wants to crack. Joints usually run into them, and trimmer bars usually run across them.
- Model it or draw it. In Revit a joint can be modelled as a real break in the floor, or drawn as a line with a tag. Modelling gives you a coordinated result; drawing gives you a fast one. Which is right depends on whether anyone downstream needs the geometry.
- Do not invent spacings. Joint layout is a design decision. Take it from the engineer.
References
- Waffle pod slabs — the slab type most of these joints turn up on in housing
- Acronyms and abbreviations in engineering — where C.J. and the rest are listed
- Standards Australia — AS 3600, Concrete structures
- Standards Australia — AS 2870, Residential slabs and footings