Sunday, November 20, 2011

Concrete Floor Finishing

Fellow engineers,
        I have an architect client specializing in high-end housing.  His client, the owner, is having a residence built that is intended to have an "industrial" look, namely exposed structure consisting of exposed metal deck supported on wide flange beams and topped with a 5" concrete slab.  The problem is finishing the floor.
        The 5" (from bottom of deck) is intended to contain in-floor heating (3/4" o.d. water pipes) plus any reinforcing required.  The owner wants to grind off and polish the floor surface.  About 1/16" (or 1mm if you prefer metric) is to be cut off for this purpose.  The desire is to have a perfectly finished floor; any cracks or other blemishes are exceedingly undesirable.
        The building is nearly closed in; the steel beams and 1.5" High-Bond" steel floor deck spanning 5 feet c/c are in place; the concrete will not be placed until after Christmas due to low temperature possibilities.  We have already had overnight temperatures down to -18 degrees C or 0 degrees F; the architect wants a fully heated building before any concrete floors are placed.
        Given that I can guarantee that the concrete WILL crack and that I can not even strongly suggest that cracking WILL NOT occur how can I best keep the owner happy?
Ideas that have already been discussed include:
1.) Add sufficient top steel over the beams (or over all) and calculate crack width.  Here I am open to suggestions as to what crack width I should accept.
2.) Cast the floor in two pours creating a topping.  Increasing the overall thickness to achieve this end is possible.
3.) We can add a shrinkage reducing agent (to one or both layers if we use the topping rout).
4.) The idea of providing shoring at midspan to ensure a shored composite concrete construction effect has been discussed
5.) We could use fibre reinforcing, although I am inclined to reject this option, at least for the top layer because the grinding could expose the fibres with bad results.
6.) With an owner willing to spring (-+)$25.00 per square foot for grinding and polishing the floor I don't think cost is a serious object.
        Thank you in advance for any suggestions you might make.
H. Daryl Richardson