Joints in Concrete

WHAT ARE JOINTS Joints are PLANNED BREAKS in concrete which allow it to move
and prevent random cracking.


WHEN TO MAKE JOINTS Joints can be made at two different times:
BEFORE any concrete is poured. As for Construction joints or Isolation joints.
AFTER concrete has been placed and compacted, as for Control joints.
Joints are used to control CRACKING in concrete. Random
cracking can weaken the concrete and spoil its appearance.


TYPES OF JOINTS
Control Joints Wet formed joints are inserted with the use of a grooving tool to create a
plane of weakness which conceals where the shrinkage crack will occur. To be effective the
joint must be tooled to a minimum depth of 1/4 to 1/3 the depth of the concrete,
eg for 100 mm thick concrete – joint depth should be a minimum of 25 mm to 35 mm.
Control joints may be made while concrete is
hardening by slicing it with a thin piece of metal.
The edges of the joints should be finished with a
grooving or edging tool.
See CHAPTER 9 Finishing Concrete


Alternatively a crack inducer may be cast, or pressed into, the concrete.
Control joints may also be sawn, but timing is very important.
Too early and the sawcut can ravel and too late the
concrete will have already cracked randomly.
A joint may be filled with a flexible filler to minimise
water entry and to prevent stones etc entering which
may later cause spalling of the concrete.


The position and number of control joints must be carefully planned. Control joints in an
unreinforced concrete slab should divide it into roughly square areas. (ie A one-metre-wide
path needs a control joint about every one metre).
Control joints in unreinforced concrete should be located at a spacing of a width to depth
ratio of about 20 (to 25) to 1 ie a 100 mm deep slab should have joints every 2 to 2.5 m.
In steel reinforced slabs the joint spacing is controlled by the area of steel. The more steel
there is, the further apart the joints can be.
Isolation Joints An isolation joint totally separates a concrete element from another
concrete element, or a fixed object such as a wall or column, so that each can move and
not affect the other. The joint filling should be full depth and soft. It can be made of cork,
foam rubber, or some other flexible material.




Construction Joints A construction joint is a place where work finishes temporarily.
Formwork is used to support the edge of the concrete already in place so that it doesn’t just
collapse. Concrete is finished square and the reinforcement normally runs through the joint.
When placing begins again:
Remove the formwork and brush any loose material from the old surface.
Roughen the old surface, to expose the coarse aggregate, to help the new
concrete bond properly.
Pour the new concrete against the old surface.
PLANS The position of ALL JOINTS should be shown on the plans
for any concrete slab.