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Joints in Slabs on Ground[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Joints in Slabs on Ground
- From: GSKWY(--nospam--at)aol.com
- Date: Thu, 13 Nov 2003 15:30:34 EST
In cleaning out my e-mail box, I came across a question on slabs on ground joints. Here's an opinion or observation or both. Comments are welcome
All concrete, even concrete with a very low w/c ratio, will shrink over time as it loses excess moisture from the original mixture design. This shrinkage (drying shrinkage) is usually not a problem by itself; however, when this shrinkage is restrained, tensile stresses develop. These tensile stresses can result in cracking, particularly if they occur before the concrete has achieved its design strength. A properly implemented curing program will reduce the shrinkage, but the main objective of curing is to delay shrinkage long enough that the concrete has developed some tensile strength.
Restraint is primarily caused by friction with the subgrade, but any construction feature that increases restraint will tend to increase cracking. Isolation joints are used to eliminate restraint from adjoining building elements. This includes walls that do not require lateral restraint from the slab as well as columns, machinery bases, footings, and other points of restraint such as drains, manholes, sumps, and stairways. Isolation joints permit horizontal and vertical movement between the abutting faces of a floor slab and fixed parts of the building, allowing each part to move independently without damage to the other.
Isolation joints are typically formed using preformed joint filler such as insulation board, asphalt-impregnated fiber materials, or wax coated cardboard. The joint material is installed for the full depth of the slab, before the slab concrete is cast. It should not protrude above the slabs. Where the joint is likely to be exposed to moisture, or there are aesthetic, hygienic, or dust-control requirements, the top of the joint material is typically removed, and the joint is sealed with an elastomeric sealant.
Columns on separate footings can be isolated from the floor slab with either circular or square isolation joints. Square isolation joint are typically rotated 45 deg so that the corners align with contraction and construction joints. This can result in a fairly large slab blockout, however. With steel columns, it is sometimes more efficient to use a "pin-wheel" joint. A pin-wheel joint simply requires that the column be wrapped with compressible isolation joint material. The concrete in the area between the flanges is typically placed at the same time as the slab is placed. Contraction joints are then cut on alternate sides of the column. The joints in both direction are offset by the column dimensions, thus resulting in a layout that resembles a pin-wheel.
The terms "isolation joint" and "expansion joint" are often used interchangeably. They are not the same, however. Although slab on ground isolation joints are typically designed to be thick enough to allow for some slab movement, they are not considered to be expansion joints. Expansion joints are used in elevated slabs to allow the expansion and contraction caused by temperature changes to occur without affecting the serviceability or structural integrity of the building. Because of subgrade friction, slabs on ground expand and contract much less than elevated slabs. Conventional (non-shrinkage compensating) concrete slabs will never expand to more than their original, as-cast dimensions. In slab on ground construction, expansion is accommodated at contraction joints.
There is some confusion in the literature, however, and specifications for pavement and sidewalk work often require "expansion joints." In many cases, these are actually isolation joints that isolate the pavement from the building slab. Specifications may also require that isolation joints be filled with "expansion joint filler." Typically this is simply referring to a premolded joint filler.
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