Joints in Concrete | Isolation joints | Expansion Joints

Joints in Concrete

Types of joints in concrete constructions are:

1. Construction Joints
2. Expansion Joints

Construction Joints in Concrete: 

Construction joints are placed in a concrete slab to define the extent of the individual placements, generally in conformity with a pre-determined joint layout.

Construction joints must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to transfer flexural stresses produced in the slab by external loads.

Construction joints must allow horizontal displacement right-angled to the joint surface that is normally caused by thermal and shrinkage movement. At the same time they must not allow vertical or rotational displacements. summarizes which displacement must be allowed or not allowed by a construction joint.

Expansion Joints: 

These are structural separation between building elements that allow independent movement without damage to the assembly. Expansion joints are designed to safely absorb the thermal expansion and contraction of various construction materials, to absorb vibration. They are commonly provided in bridges, railway tracks, piping systems, and other structures.

Contraction Joints or Control Joints:

Control Joints (often confused with expansion joints) are cuts or grooves made in concrete or asphalt at regular intervals. These joints are made at locations where there are chances of cracks or where the concentration of stresses are expected, so that when a concrete does crack, the location will be known to you. In such a way a concrete will not crack randomly but in a straight line (i.e. control joint). In other words Contraction or Control Joints are Pre-Planned Cracks. The cracks may be due to temperature variations or drying shrinkage or other reasons.

Joints depth should be 25% of the depth of the slab. For instance a 4" thick slab should have 1" deep cut. Joints Interval (taken in feet) should not be more than 2 - 3 times the slab thickness (in inches). Let  say a 6" slab should have joints 2 x 6=12 to 3 x 6 = 18 feet apart. For fresh concrete grooving tools are used while saw is used for hardened concrete.

Isolation joints:

Isolation joints have one very simple purpose—they completely isolate the slab from something else. That something else can be a wall or a column or a drain pipe. Here are a few things to consider with isolation joints:

1. Expansion joints are virtually never needed with interior slabs, because the concrete doesn't expand that much—it never gets that hot.
2. Blowups, from expansion of concrete due to hot weather and sun, are more commonly caused by contraction joints that are not sealed and that then fill up with non-compressible materials (rocks, dirt). They can also be due to very long unjointed sections.
3. Expansion joints in concrete pavement are also seldom needed, since the contraction joints open enough (from drying shrinkage) to account for temperature expansion. The exception might be where a pavement or parking lot are next to a bridge or building—then we simply use a slightly wider isolation joint (maybe ¾ inch instead of ½ inch).
4. Walls and columns, which are on their own footings that are deeper than the slab subgrade, are not going to move the same way a slab does as it shrinks or expands from drying or temperature changes or as the subgrade compresses a little.
5. If slabs are connected to walls or columns or pipes, as they contract or settle there will be restraint, which usually cracks the slab—although it could also damage pipes (standpipes or floor drains).
6. At columns, contraction joints should approach from all four directions ending at the isolation joint, which should have a circular or a diamond shaped configuration around the column. For an I-beam type steel column, a pinwheel configuration can work. Always place the slab concrete first and do not install the isolation joint material and fill around the column until the column is carrying its full dead load.
7. Isolation joint material should go all the way through the slab, starting at the subbase, but should not extend above the top.
8. Isolation joints are formed by placing preformed joint material next to the column or wall or standpipe prior to pouring the slab. Isolation joint material is typically asphalt-impregnated fiberboard, although plastic, cork, rubber, and neoprene are also available.
9. For a cleaner looking isolation joint, the top part of the preformed filler can be cut off and the space filled with elastomeric sealant. Some proprietary joints come with removable caps to form this sealant reservoir.