Location & Spacing of Joints

The location of contraction joints is an important part of overall slab performance and should be considered early in the design stage. The use of Connolly key joint gives designers greater control over the placement of contraction joints. As the joints are set up prior to concrete placement they can be inspected at the pre-pour stage and they do not have to rely on any post pour operations to be effective.

A common question asked by many designers and builders is "How far apart should contraction joints be?" This is a question to which there is no simple answer. The best solution is to consider all of the following factors before deciding on the joint locations prior to pouring.

Slab design / layout: The slab sections or panels should have a length to width ratio of a maximum of 1.5:1 and preferably 1:1. Panel shapes such as "L" and "T" should be avoided where possible.

Thickness: Generally the thicker the slab the further apart the joints may be.

Reinforcement: Joints in un-reinforced slabs should be spaced at closer intervals than in reinforced slabs. Slabs with light gauge reinforcement also require closer joint spacing then slabs containing higher percentages of reinforcement.

Concrete mixture: Cement type and quantity, aggregate size, quantity and quality, water/cement ratio and the use of any admixtures will all affect the amount of shrinkage that will occur. The more shrinkage, the closer the contraction joints should be.

Base friction & restraints: The sub-base should be as smooth and even as possible to allow the concrete to move freely as its volume decreases. A rough sub-base means closer joints.

Environmental Factors: High air temperature, drying wind and low humidity will all accelerate the drying process and increase shrinkage. Some of these factors such as temperature and humidity are seasonal and therefore joints spacings may not be the same for all jobs poured throughout the year.

Curing methods: For various reasons, different curing methods may apply to different jobs. The slower the concrete is cured, the further apart the joints may be placed.

Despite all of the above variables the chart below is a guide to joint spacing for nominally reinforced slabs with square slab panel layouts. Please note that this chart is a guide only and the particulars of each individual job should be considered in determining contraction joint locations.