CONCRETE SLABS - FREE JOINTS OR SHRINKAGE CONTROL JOINTS???   Concrete expansion joint

For non-specific design concrete slab-on-ground, when do you use a free joint or a shrinkage control joint and what are they anyway?  It is easy to get confused with these types of joints, as some published explanations can be vague or in a few cases incorrect. In this article we will attempt to explain it simply, referencing NZS 4229: 2013, Concrete Masonry Buildings Not Requiring Specific Engineering Design, and NZS 3604: 2011, Timber-Framed Buildings* (as modified by NZBZ B1/AS1).

DEFINITIONS

Free Joint

The old definition from NZS 3604 was;

  • A construction joint where no reinforcement passes through the joint linking both sides of the concrete slab and the vertical faces of the joint are not in bonded contact with each other.

There is no new definition, however NZBC B1/AS1, 3.1.13 would require a modification to this by adding unbonded reinforcement dowels across the slab joint (see the construction detail below).

Shrinkage Control Joint

Definition for NZS 3604 and NZS 4229;

  • A line along which the horizontal strength of the slab is deliberately reduced so that any shrinkage in the slab will result in a crack forming along that line.

 

FREE JOINT SPACINGS

The purpose of a Free Joint is to allow a bigger slab than the design criteria of the Standard allows. For instance in NZS 3604, 7.5.1 the maximum size of a slab is 24m, or 24m either way between free joints, or between free joints and the slab edge. Clause 7.5.8.5 goes on to say slabs may be of unlimited size provided the requirements of 1.1.2 (l) and 7.5.1 are met.  NZS 4229 indicates in C7.8.3 this is also how it should be interpreted although the dimension is a maximum of 18m. So subject to other limits (NZS 3604, 1.1.2.(l) and NZS 4229, 1.1.3.(e)) your slab can be any size as long as it is divided up with Free Joints at maximum spacing of 24m for NZS 3604, 7.5.8.3*, and 18m for NZS 4229, 7.8.3.

When planning free joint locations you must consider finishes and how they will bridge the joint and allow for movement. If this is an unacceptable issue consider a specific designed slab.


FREE JOINT CONSTRUCTION

Both NZS 3604, 7.5.8.8* and NZS 4229, 7.8.5.3, have the same construction description;

  • At free joints, slab reinforcement shall be terminated and there shall be no bonding between vertical concrete faces (prevented by using building paper or a bituminous coating). R12 dowel bars 600 mm long shall be placed at 300 mm centres along the free joint and lapped 300 mm with slab reinforcement on both sides of the joint. All dowel bars on one side of the joint shall have a bond breaker applied, for example by wrapping dowel bars for 300 mm with petrolatum tape. Joint dowel bars shall be installed in a single plane, in true alignment and parallel.


SHRINKAGE CONTROL JOINT SPACINGS

Concrete cures and dries over a long period of time, resulting in shrinkage which causes stresses within the slab and may result in cracking. Shrinkage control joints attempt to control where the cracks occur by providing weak paths for the crack to follow. Irregular slab shapes and load points can concentrate stress at particular points like internal corners and columns etc, so these are strategic places to introduce control joints. When planning shrinkage control joint locations it is also a good idea to think about floor finishes and how they can handle subsequent cracking. Alternatively try to avoid the problem by locating them at the changes in floor finishes, or running them under walls and only exposing them at door openings where floor finish joints may be more acceptable. Reducing spacings in some cases may reduce the size of cracking and this may allow the floor finishes to handle the movement. If location of joints becomes an unacceptable issue consider a specific designed slab.

Both NZS 3604, 7.5.8.6.4 and NZS 4229, 7.8.5.2, have the same description for joint location;

  • Shrinkage control joints in reinforced concrete ground slabs shall comply with the following criteria: (a) Shrinkage control joints shall be positioned to coincide with major changes of plan. See NZS 4229 Fig 7.4, or NZS 3604 Fig 7.19.; (b) Supplementary steel shall be placed as shown in NZS 4229 Fig 7.5, or NZS 3604 Fig 7.18, but not across shrinkage control joints; (c) Supplementary shrinkage control joints shall be used such that intermediate bay sizes do not exceed 6 m; (d) Panels shall be formed as close as practicable to length to width ratios of between 2:1 and 1:1 and shall have a maximum length in any direction of 6m.

Although 6m is the maximum, in their concrete sections Masterspec have industry recommendations for smaller spacings in some cases.


  Floor situation  Maximum spacing of
 saw cuts both ways
  Industrial floor       5m
  Architectural, exposed floor, thin finishes, rigid finishes  4m
  Carpet on underlay flooring  6m
  Supermarket floor  5m


SHRINKAGE CONTROL JOINT CONSTRUCTION

Both NZS 3604, 7.5.8.6.1 and NZS 4229, 7.8.5.1, have the same construction description;

  • Shrinkage control joints shall either be formed by saw cutting the slab after it has hardened, or by casting-in a crack inducer into the slab. Crack inducer placement shall not damage the DPM. The inducer or saw cuts shall extend to a quarter of the depth of the slab. Saw cutting shall take place no later than 24 hours after initial set for average ambient temperatures above 20°C, and 48 hours for average ambient temperatures below 20°C. Shrinkage control joints may be cut at an angle as long as the included angle is not less than 60°. Shrinkage control joints should be positioned where possible below walls.

Consideration must be given to the depth of the cut and the depth of the reinforcing, normally for a non-specific design 100mm thick slab, cover to the top of the reinforcing is 30mm minimum, a cut to quarter depth is 25mm (and usually 5mm wide) which leaves 5mm over the reinforcing, which is OK. If dimensions or proportions change slightly it may lead to cut reinforcing which will be an issue, so ensure you adjust dimensions and proportions to suit the situation. Although some specific designed slabs do have alternate reinforcement cut, it requires Engineer advise.

Saw cuts may need to be filled and the fill needs to be compatible with the floor finishes, so clarify this in your documents.


   
      

Note - Only one shrinkage control joint is required at each corner

7.4 Positioning of shrinkage control joints (see 7.8.5.2(a))

 



Fig 7.5 - Supplementary Steel (see 7.8.5.2)

 

 Figures taken from NZS 4229

NZS 4229 Fig 7.4, is the same as NZS 3604 Fig 7.19

NZS 4229 Fig 7.5, is the same as NZS 3604 Fig 7.18


CONCLUSION

Very simply, free joints are a means of increasing the size of a slab beyond the design parameters of the Standards, and shrinkage control joints are a means of controlling (to a degree) cracking caused by natural shrinkage as the concrete cures and dries.

Masterspec concrete sections cover both free joints and shrinkage control joints, refer to:

3110  CONCRETE WORK in Masterspec Minor

3101  CONCRETE WORK - BASIC in Masterspec, Basic, Services, Landscape, Interiors, Structural & Civil

3102  CONCRETE WORK - STANDARD in Masterspec Standard

Note; the Masterspec Structural and Civil only concrete sections that are related to 3105 CONCRETE - COMMON REQUIREMENTS are for specific design projects and treat joints slightly differently.

 

* as modified by NZBC B1/AS1.

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