Difference between a construction joint and an expansion joint? 3

[GeotechRic]

What is the difference between a construction joint and an expansion joint?

I've done some search on the web but have found limited info on their definitions.
Thanks in advance.

 

[UcfSE]

A construction joint is used at pour stops. Without a construction joint you end up with a cold joint that may or may not be finished suitably. The construction joint is usually a "key" strip, sometimes formed with metal, and will be located at predetermined pour stops or where pour stops occur in the field. The idea is to help provide continuity between pours to help maintain structural integrity in shear and reduce cracking.

An expansion joint is exactly that: a joint provided to allow for expansion or contraction without overstressing the concrete and causing cracking problems. The joint maybe about 0.5 inches wide and is sometimes filled with expansion joint material or maybe a 1x wood member and caulk.

 

[boffintech]

From ACI-318

R22.3 — Joints
Joints in plain concrete construction are an important design consideration. In reinforced concrete, reinforcement is provided to resist the stresses due to restraint of creep, shrinkage, and temperature effects. In plain concrete, joints are the only means of controlling and thereby relieving the buildup of such tensile stresses. A plain concrete member, therefore, should be small enough, or divided into smaller elements by joints, to control the buildup of internal stresses. The joint may be a contraction joint or an isolation joint. A minimum 25 percent reduction of member thickness is considered sufficient for contraction joints to be effective. The jointing should be such that no axial tension or flexural tension can be developed across a joint after cracking, if applicable, a condition referred to as flexural discontinuity. Where random cracking due to creep, shrinkage, and temperature effects will not affect the structural integrity, and is otherwise acceptable, such as transverse cracks in a continuous wall footing, transverse contraction or isolation joints are not necessary.

Contraction joint — Formed, sawed, or tooled groove in a concrete structure to create a weakened plane and regulate the location of cracking resulting from the dimensional change of different parts of the structure.

Isolation joint — A separation between adjoining parts of a concrete structure, usually a vertical plane, at a designed location such as to interfere least with performance of the structure, yet such as to allow relative movement in three directions and avoid formation of cracks elsewhere in the concrete and through which all or part of the bonded reinforcement is interrupted.

From The Concrete manual

Construction Joints. In most structures, it is not possible to place all of the required concrete at one time. The volume of concrete, structural limitations and time limitations make it necessary to discontinue placing concrete at some point, to be resumed later after the concrete has hardened. The plane of separation between the placements is called a construction joint. A construction joint may be horizontal, as between lifts in a wall or column, or it may be vertical, as one formed by a header board in a slab or beam. Locations of construction joints are shown on the plans. If because of a stoppage of the work for several hours or longer it becomes necessary to install a joint, the joint should be made in a plane normal to the main reinforcing bars and in a region of minimum shear. Minimum shear in simply supported slabs or girders is at or near the center of the span. The design engineer should be consulted with respect to location of construction joints, as a wrongly located joint can seriously alter the loadcarrying properties of the structure.

The ACI 318 Standard, in Section 6.4, provides as follows: Construction joints must be made and located as not to impair the strength of the structure. Where a joint is to be made, the surface of the concrete must be cleaned and laitance removed. Immediately before new concrete is placed, all construction joints must be wetted and standing water removed. Construction joints in floors must be located within the middle third of spans of slabs, beams and girders. Joints in girders must be offset a minimum distance of two times the width of intersecting beams.

Isolation Joint. When new slab concrete is placed against existing concrete, such as walls, columns, footings, steps, and abutting driveways and sidewalks, there must be a separation to allow for vertical as well as horizontal movement of the new concrete relative to the old. The same holds true when concrete is placed against any other area of restraint caused by structures of any kind. Horizontal movement is caused mainly by drying shrinkage, or occasionally by thermal contraction and expansion. Vertical movement is usually the result of settlement. The new concrete must be separated or isolated from the old, and the joint is called an isolation joint (sometimes called an expansion joint). The joint is made by inserting premolded joint material between the old and the new construction. (See Figure 16-14.) The joint material must be as wide as the slab is thick, and it must not extend above the slab elevation. It can be attached to the old construction with asphalt or a similar adhesive before the slab concrete is placed.