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Re: Joints in Slabs on Ground

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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