Exposed concrete floors are becoming more and more important in today's building designs. The “bare” or decorative concrete floor can contribute to LEED points, as well as offer the facility owner a low maintenance surface that will last for the life of the structure.

Finished concrete floor aesthetics, therefore, become increasingly more important in the overall layout of the exposed surface.

Exposed cracks, curling, and unsightly diamond patterns around support columns detract from the intended beauty of the design. Increased attention is now being given to constructing joints to actually increase the eye appeal of the floor while eliminating the negative aspects.

There is no rule that says that joints must run parallel to the walls of the building! Laying out joints in a decorative manner or with closer spacing than the conventional ACI guidelines, enhances the look and feel of the exposed concrete surface while, at the same time, reducing the tendency for meandering cracks and curling from excessively wide spacing. When these joints are filled with a suitable joint filling material, the contrast and look of the floor is instantly improved.

Contractors and design professionals are encouraged to consider these possibilities when laying out pour sequences and joint cutting. One of the easiest improvements in proper construction techniques can be to eliminate the “diamonds” around support columns. Traditionally, support columns have been boxed out and the floor poured up to the diamond shaped forms, then filled in at a later date, isolating the two pours with expansion material.

This creates two problems: First, the curl of the surrounding primary slabs causes the two surfaces to soon be of different heights and 2) the isolation material is less than attractive.

To eliminate these problems, the pinwheel technique is now frequently employed.

The pinwheel joint pattern is a result of a specific form layout where the concrete contractor installs the forming materials on alternating sides of the columns and wraps the support with a suitable bond breaker, such as closed cell foam expansion material. Saw cuts are then installed at the two alternate sides of the column at an angle between 45 and 90 degrees. This creates a pinwheel effect around the joint and eliminates the previously mentioned negative impact of the diamond method. As slab curling occurs, it goes unnoticed because it is uniform on all sides of the support. This method can be employed regardless of the shape of the column.

The other all-important rule is to honor all re-entrant corners using these points as a starting point for the joint layout and working the balance of the pattern from that line. A re-entrant corner is any angle where the slab may have a corner to turn such as around a wall, trench drain, loading dock or other embedments. These corners create a stress rise in the slab and are virtually guaranteed to crack if not addressed.

Joint filling should be scheduled at the latest possible moment in the construction schedule to allow maximum curing and slab shrinkage to occur. Always re-saw joint wells prior to filling and remove all dust and contaminants. This will improve joint filler bond to the concrete and ensure that future separation of the joint filler material will be minimized.

For more information about low-maintenance joint fillers, please refer to www.lmcc.com or contact Bill Butler, L&M Regional Sales Manager, at bbutler@lmcc.com or by phone 920-450-2932.


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© 2008 L&M Construction Chemicals, Inc. | ConcreteNews September 2008.

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