Concrete Basics: The Three Types of Joints
While it may seem that concrete installation is a fairly simple process of pour, smooth, and dry, in reality, it is a very complex procedure that requires a large amount of knowledge and skill from everyone involved, from the designers to the engineers to the installers.
In order to ensure that the concrete is properly put into place and that it lives up to its durable characterization, many factors must be considered. One of the most important is ensuring that proper concrete joints are installed to allow for proper installation of the concrete and for movement over time.
There are three types of joints in concrete: control joints, isolation joints and construction joints. It is important to know which joint is used in which situation. Generally, the engineer and drafters will work together to decide on the type, number and placement of the concrete joints required in the job specifications.
These joints are also called contraction joints, which adequately describes their purpose. Control joints are strategically placed throughout concrete members or slabs to provide room for movement due to weather and time, such as temperature changes, shrinkage and deformation.
The joint is not a complete break in the concrete. Instead, it is a joint that goes one-third of the way through the concrete. This weakens the surface of the concrete while maintaining the structural integrity. The idea is that the joint will allow any cracks that may form to occur along the control joint and not in other areas of the surface.
Control joints are usually placed partially through the underlying foundation, whether it is a slab, wall or other, in order to ensure that the concrete retains its structural integrity and water tightness. Because of the naturally uneven nature of concrete cracks and the steel that is used to reinforce the joint, a control joint ensures that no movement occurs along the joint as time progresses.
Also known as expansion joints, isolation joints are similar to control joints except that the joint goes straight through the concrete. Isolation joints allow the concrete to move along the joint without compromising the strength of the structure.
Generally, these joints are positioned at stress points within a concrete slab or at junctures, such as where walls meet the concrete. When construction involves the use of columns, isolation joints are necessary at the base of the column where it meets the concrete slab below. As time passes and the pressure of the structure as well as weathering impacts the column, it will naturally rotate and settle. The isolation joint endures that the column can move while remaining in place and providing the required support to the overall structure.
During the concrete pour, any break in the work can cause the already poured concrete to harden enough to prevent new concrete from bonding adequately. This requires the use of construction joints, also known as pour joints.
Reinforcement is placed in the old concrete prior to hardening, which extend into the space where the new concrete will be poured. This ensures that the entire structure acts as one piece rather than moving independently over time. Construction joints may also be used to provide additional support between floor levels or at corners where they will be hidden in the final product. It is important to understand how to place construction joints, especially at stress points, to ensure that structural integrity is not compromised.