Flatwork Faux Pas
By Christopher E. Carter, P.E.Concrete does not have to be what it is sometimes cracked up to be. Post-construction problems with flatwork are common warranty items. In general, residential concrete in Colorado has three issues: isolation, support and finishing.
Isolation involves separating concrete from whatever it touches due to our native expansive soil. Ironically, the term, “expansive soil” is sort of an oxymoron, since soil can also shrink. Nonetheless, whenever soil under concrete expands or shrinks, the overlaying flatwork is displaced. If it moves up, anything touching it will move up too or be crushed. If the soil moves down, anything sitting on it will lose support or hang in midair.
Most flatworkers today know this and are pretty good at isolating whatever they pour. The problem generally lies with those who come later – such as the bricklayers, stone masons, siding subcontractors, and trim guys. Some don’t seem to know about these movement dynamics and view new concrete as a good starting or terminating point. Finishes which sit on soil-supported concrete are usually not a good thing in Colorado.
Supporting structural members on flatwork is acceptable as long as the flatwork is supported on a foundation. If a structural column, for example, is supported on flatwork that is supported by expansive soil, the column will likely perform like a piston or a Minuteman missile – depending on clay mineralogy, of course.
Again, the flatworkers don’t really have control over what gets built on their slabs, but the architects, engineers and superintendents do. Oftentimes building plans don’t clearly indicate how minor structural elements, like porch roof columns, are to be supported, so they end up being put on a porch or patio slab. Architects and engineers need to clearly specify their intent on building plans, and superintendents need to enforce these plans.
With Colorado’s ever-changing weather and semi-arid climate, it is often very difficult to finish and cure concrete properly. One common finish defect is scaling – a cosmetic condition that occurs when surface mortar flakes off, which exposes the aggregate underneath. It is most often found on driveways and is caused by freeze-thaw cycles which break the surface cement bond. The best preventative defense is to ensure the concrete mix has air-entrainment, usually about 6 percent, and that finishing and curing compensate for environmental conditions.
Many warranty dollars are spent to correct problems. But these known concrete issues shouldn’t be among those problems.