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RE: Joints in Slabs on Ground

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I thought it was a good briefing on joints on slabs without being a 
full text on S.O.G.  It just points out the need for the designer to
full research before designing a slab.
Gary Hodgson

On 15 Nov 2003 at 13:11, Syed Faiz wrote:

> Since comments are solicited, here is what I have to submit:
> This is a good piece of lecture of a "generic" nature only. It would
> have been better if it had also been focussed on the "recommended
> practice" on the "joint spacing" (for SOG), which I think was the
> exact query from the gentleman on this LIST.
> Best regards, 
> Syed Faiz Ahmad; MEngg, M.ASCE 
> Senior Structural Engineer 
> Saudi Oger Ltd 
> Riyadh, Saudi Arabia 
> -----Original Message-----
> From: GSKWY(--nospam--at) [mailto:GSKWY(--nospam--at)]
> Sent: Thursday, November 13, 2003 11:31 PM
> To: seaint(--nospam--at)
> Subject: Joints in Slabs on Ground
> 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
> **************
> Isolation Joints
> 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.
> ****************
> Gail Kelley

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