Placing Contraction/Control Joints in Concrete
Flatwork: Why, How, and When
joints are placed in concrete slabs to control random cracking.
A fresh concrete mixture is a plastic (fluid) mass that can be molded
into virtually any shape, but as the material hardens there is a
reduction in volume (shrinkage). When shrinkage is restrained by
contact with supporting soils, granular fill, adjoining structures,
or reinforcement within the concrete, tensile stresses develop within
the concrete section. While concrete is very strong in compression
the tensile strength is only 8% to 12% of the compressive strength.
In effect, tensile stresses act against the weakest property of
the concrete material. The result is cracking of the concrete.
There are two basic strategies to control cracking for good overall
structural behavior. One method is to provide steel reinforcement
in the slab which holds random cracks tightly. When cracks are held
tightly or remain small, the aggregate particles on the faces of
a crack interlock thus providing load transfer across the crack.
It is important to recognize that using steel reinforcement in a
concrete slab actually increases the potential for the occurrence
of random hairline cracks in the exposed surface of the concrete.
The most widely used method to control random cracking in concrete
slabs is to place contraction/control joints in the concrete surface
at predetermined locations to create weakened planes where the concrete
can crack in a straight line. This produces an aesthetically pleasing
appearance since the crack takes place below the finished concrete
surface. The concrete has still cracked which is normal behavior,
but the absence of random cracks at the concrete surface gives the
appearance of an un-cracked section.
Concrete slabs-on-ground have consistently performed very well
when the following considerations are addressed. The soils or granular
fill supporting the slab in service must be either undisturbed soil
or well compacted. In addition, contraction joints should be placed
to produce panels that are as square as possible and never exceeding
a length to width ratio of 1 ½ to 1 (Figure 1). Joints are
commonly spaced at distances equal to 24 to 30 times the slab thickness.
Joint spacing that is greater than 15 ft. require the use of load
transfer devices (dowels or diamond plates).
|Figure 1a: Joint Spacing in Meters
Figure 1b: Joint Spacing in Feet
Contraction joints may be tooled into the concrete surface at the
time of placement. Joints may be tooled into the surface (first
pass) prior to the onset of bleeding or immediately with the first
pass of the floating operation. The longer the first pass for jointing
is delayed the more difficult it will be to shape clean straight
line joints. Tooled joints should be re-established with each successive
pass of finishing operations.
Joints may also be sawed into the hardened concrete surface. It
is important to understand that the longer sawing is delayed the
higher the potential for cracks to establish themselves before sawing
is complete. This means that any cracks that occur before the concrete
is sawed will render the sawed joint ineffective. Timing is very
important. Joints should be sawed as soon as the concrete will withstand
the energy of sawing without raveling or dislodging aggregate particles.
For most concrete mixtures, this means sawing should be completed
within the first 6 to 18 hours and never delay more than 24 hours.
Early-entry saws are available which may allow cutting to begin
within a few hours after placement.
Contraction/control joints must be established to a depth of ¼
the slab thickness (Figure 2). Proper joint spacing and depth are
essential to effective control of random cracking.
|Figure 2: Minimum Depth of Contraction Joints