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Re: Joints in Slabs on Ground

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What makes you think people that look for info on this site would just blindly takes others' opinion and put in their specs?

It is interesting that you state" ...If a "professional Engineer" is waiting for information he or she receives over the Internet, from someone they have never met and who possibly is just quoting information from a book .." So is it OK if the info is from someone you have met and know but may not have the experience? Is the information you get from books from people "you know and have met" ?

I am a licensed structural engineer - however, I do not claim to know everything there is to know in engineering. I am willing to bet that neither do you. I value experience and am willing to learn from others - yes, from people I do not know and have never met and may never meet ( plenty of them on this forum). When I send in a question in this forum, I am looking for peoples' experience - what works, what doesn't, pitfalls,etc. If I all I wanted was just some info to put in my specs, I think I know how and where to find the info without coming to this forum. Moreover, I think most of us are smart enough to know when somebody is dishing out crap.

Gautam, SE

From: GSKWY(--nospam--at)
Reply-To: <seaint(--nospam--at)>
To: seaint(--nospam--at)
Subject: Re: Joints in Slabs on Ground
Date: Tue, 18 Nov 2003 17:10:25 EST

Actually,  I think you proved something I feel pretty strongly about.  If a
"professional Engineer" is waiting for information he or she receives over the
Internet, from someone they have never met and who possibly is just quoting
information from a book, to know what to put on the drawings, the owner should
be looking for a new engineer.  Real quick.

In many applications,  joints are left open.  It's an economic decision,
based on the required performance of the slab. Joints in a slab on grade in a parking structure for example, are likely to be left open. Joints in commercial and industrial applications are typically filled rather than sealed; joints in residential applications such as slab on ground foundations are typically sealed. With respect to joints in slabs on ground, filling and sealing are not
the same thing.

Comments on the attached are invited.

Filling and Sealing Joints

Although joints in exterior applications are often left open if they will not
be exposed to hard-wheeled traffic, joints in commercial and industrial
facilities are typically either filled or sealed. This is particularly true if the
slab will be exposed to moisture or there are hygienic or dust-control
requirements; open joints tend to attract dirt and debris. In slabs that are exposed
to large temperature changes, dirt and debris trapped in the joints may
prevent the joints from closing as the concrete expands.

Joints that will only be exposed to foot traffic or low-pressure pneumatic
tires can be sealed with an elastomeric material.  These materials do not
provide any support to the joint edge but they can accommodate fairly large slab movements without failure. Sealants are typically required to conform to ASTM
C920, Specification for Elastomeric Joint Sealants.  A typical sealant
installation would be a two-component polyurethane with a Shore A Hardness of 35, installed 1/2- in. deep over a compressible backer rod. Field-cured sealants should always be installed over backer rod or some kind of bond breaker so that there is no restraint from the bottom of the saw-cut. Preformed sealants are also sometimes used. These sealants can be installed quickly, do not require curing, and if properly chosen, they can maintain a tight seal in joints that are subject to opening and closing. They should not be used for joints that will be subjected to hard-wheeled traffic, however, because they do not provide any
support for the joint edges.

Joints that will be exposed to hard plastic casters, solid rubber tires, or
steel-wheeled traffic should be filled with a material that provides lateral support to the edges of the sawcut. Specifications typically require semi-rigid epoxy or polyurea fillers with 100% solids and a Shore A Hardness of at least 80. Some specifications refer to Shore D requirements; although both Shore A and Shore D tests measure hardness using a indenter, Shore D measurements are done with a pointed tip and do not really reflect the ability of the material
to carry load.  The correlation between Shore A and Shore D measurements
varies, depending on the material; for materials typically used as joint fillers, a
Shore D measurement of 50 corresponds to a Shore A measurement of 80 to 90.

The filler material should be installed full depth in sawcut joints, without a backer rod, with minimum and maximum depths as recommended by the material
manufacturer. Two-component materials work well because their curing is
relatively independent of job site conditions. To ensure that the joint will be flush with the slab surface, the joint is usually slightly overfilled, allowed to
cure, then shaved or ground flat.

Although concrete slabs continue to shrink for years, most shrinkage takes
place within the first year. Because fillers have limited extensibility, joint
filling should be delayed as long as possible to minimize the effects of
shrinkage-related joint opening. Typical recommendations are to wait at least 60 to
90 days after the concrete is placed.  Ideally, if the building is equipped
with an HVAC system, it should be run for two weeks before joint filling. If the joint continues to move after it is filled, there can be separation at the joint edge (adhesive failure) or within the filler itself (cohesive failure).
When filler separation occurs, the voids should be refilled with the filler
that was originally used or a compatible low-viscosity repair material. If the
filler separates from the concrete on both sides and becomes loose to the
touch, it should be completely removed and replaced. Epoxy fillers can sometimes be
sawcut to a depth of 1/2-in. and replaced.

If construction traffic requires that joints be filled early, the
specifications should require that the contractor return at a pre-established date to repair any separations using materials approved by the manufacturer of the joint filler. The earlier the joints are filled, the more likely it is that there will be separation requiring repair; this does not indicate a failure of the
filler but must be addressed to ensure proper performance of the joint.

Polyureas are sometimes promoted as having more extensibility (ability to
elongate) than semi-rigid epoxies and would appear to be better suited for joints
where movement is expected.  In general, however, if they are more
extensible, they will provide less support to the joint, even if they have the same Shore hardness. Polyureas set much faster than epoxies, however, and can usually be opened to light traffic within an hour. They are thus useful for repairs in
areas where access to the slab is critical. Polyureas are a fairly new
material; they are similar to polyurethanes but are based on slightly different
resins and thus have slightly different properties.

Because sealants are designed to accommodate movement, a separated sealant
typically indicates improper joint preparation or incomplete mixing of the
components. Separated sealants should be removed and replaced.

The construction drawings should clearly indicate which joints are to be
filled and which joints are to be sealed.  If the specifications are done
according to the Construction Specifications Institute (CSI) Masterformat, joint fillers should be specified in Section 03250, under concrete work. Sealants are
specified in Section 07900.

Before being filled or sealed, joints must cleaned to allow a good bond
between the filler or sealant and the concrete. Dirt, debris, saw cuttings, curing compounds, and sealers should be removed; vacuuming is better than blowing out
the joints with compressed air. If curing compound has been used, it is
typically necessary to clean off both edges using a dry-cut saw.

Construction joints detailed to act as contraction joints should be saw cut 1 in. (25 mm) deep before they are filled. Saw-cutting makes it easier to fill the joints, since typically the as-cast joint edges are somewhat raveled and
the opening is fairly narrow.  The saw-cut also provides a defined base to
support the filler.  A layer of uniformly graded (non-compactible) sand is
sometimes placed at the bottom of the saw cut to seal the crack.

Unstable joints will not retain any type of filler. Joints without dowels or
continuous reinforcement may become unstable if there is significant
horizontal movement due to shrinkage or temperature changes, or significant vertical movement due to inadequate load transfer. The repetitive cycling of the joint
under traffic will eventually fatigue the filler to failure.

Gail Kelley

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