Joints

Design Process for Selection of Joint Types

Free-end Joints or Isolation Joints

As this name implies there must be no restraint in linear movement. Choose the following from Table 1:

a) Where there are limited load transfers use FJ1

b) Where wheel load transfers use:

FJ6 Keyway Joint

FJ3 Dowel bar with sliding free end

c) As (b) but where adjoining slab is to be cast several weeks after the first slab or  where the slab has significantly different overall free joint dimensions use:

FJ5 Dowel bar with sliding free end in sleeve providing lateral horizontal movement

Table 1. Free Joints : Selection Table

There are two primary joint types:

  • Permitting shrinkage/contraction movement

  • Permitting expansion and shrinkage/contraction movement 

Free Joint Type FJ Typical Application
src=/images/cmsimage/joints1.jpg All of these joints will also allow some expansion
src=/images/cmsimage/joints2.jpg Adjoining walls columns etc where no load transfer is required across joint.
 
This joint can be used where limited load transfer is required where unreinforced slabs are used with restricted bay sizes.
Load transfer is required principally for wheel loads but also stack loading over joint. The joints illustrated are formed after construction. They can also be formed up joints. Using Dowels for slabs below 150mm thickness is not recommended. 
Load transfer required with full expansion movment option. Using dowels for slabs below 150mm thickness is not recommended.

Where the free joint is required to accommodate both wheel loading and separation from an adjacent bay shrinking parallel to the original free joint direction, a special dowel system is required. Using dowels for slabs below 150mm is not recommended.
Tongue and groove joint not recommended for heavy wheel load transfers but useful for slabs under 150mm where dowels are not recommended. Limited draw on the groove is needed. Recommended maximum joint spacing is 5m.

Tied Joints

There are a variety of tied joints to primarily suit the methods of construction. However, as the name implies, there is always some reinforcement passing through the joint:

a) Where there are no wheel load transfers use TJ1 in Table 3. The grooves to a depth of D/4 may be formed during construction or cut generally within 24 hours of casting. Recommended for slabs not thicker than 150mm. The reinforcement is what was determined in the calculation from Table 2. The load transfer on these joints relies on aggregate interlock. A variation of this joint that can by used where thicker slabs than 125mm are used or where fixed forms and a strip method of construction applies is shown as TJ2 in Table 3. The mesh reinforcement is stopped and a tie bar is used.

b) Where there are significant wheel load manoeuvres, it is recommended that a tied keyway joint TJ2 is used.

Steel  Steel mm² Area Slab thickness (mm
100 125 150 175 200
665 145 26 21 17 15 13
664 186 33 27 22 19 17
663 205 37 30 25 21 18
662 260   37 31 27 23
661 290     35 30 26
661/0 330       34 30
H12 diam at 250mm 452       41 36
H12 diam at 225mm 503         40
For mesh Fly = 485 mPa; for H12 = 430 mPa

Table 2. Maximum spacing of free joints (m)

c) An alternative joint for heavy wheel loads is to use 16-20mm dowels. There are some risks associated with this detail because the reinforcement area at the joint is higher than that provided in the slab, so the shrinkage movement can form at the weakest point i.e. near the end of the dowel (see Table 3 TJ2). Also when the dowelling technique is used in two directions at right angles to the bay, the bay becomes locked in position with an inevitable 45° crack developing with shrinkage movement.

Table 3. Tied Joints : Selection Table

The principal concern relates to the suitability of construction (i.e. formed or induced) and the level of load transfer - particularly with wheel loads required.

Tied Joints Type TJ Typical Application
Induced crack by sawing the concrete. Reinforcement passes through the joint.
Alternative detail using bars. Illustrated as formed detail but can be created as an induced crack/sawn method. Steel bar same area as reinforcing mesh. Roughen interface with surface retarder on formwork.
Tongue and groove free joint modified as a tied joint. Steel bar size/spacing to give some area as reinforcing mesh.