While it might seem reasonable to believe that the constructed tolerance of a new slab on grade is permanent, it is not. As a new concrete floor dries during the first year, surface tension caused by this drying of the concrete creates shrinkage, and more specifically a lifting or curling of the floor at all floor joints. Data from floor slabs indicates that tolerances losses from curling can result in up to a 50% reduction in tolerances that were originally measured at the time of construction.
One of the most common myths about curling is that it is related to poor curing. This is not the case. It has been repeatedly proven that even extremely well cured floors curl.
Curling is a major concern both covered and exposed floors. Floor slab designs are normally based upon applied loading conditions and do not account for problems associated with curling. We strongly encourage you to specify the addition of rebar or steel fibre reinforcing to minimize or even eliminate curling by restraining this drying shrinkage of the concrete (steel is the best thing that ever happened to concrete!). The control of curling is critical to avoid material handling operations in exposed floors and to avoid visible curvatures in the floor under applied floor finishes.
Curling also creates a diving board effect at floor joint edges, which if not restrained through the use of reinforcing, may lead to significant joint instability (requiring expensive and time consuming repairs).
Examples of curling
| Slab Description: | Observed Curling (Maximum): |
| 150mm (6″) thick, unreinforced, wet cured | 12mm (1/2″) |
| 100mm (4″) thick, unreinforced, wet cured | 35mm (1.5″) |
| 125mm (5″) thick, steel fibre reinforced (25kgs/m3), wet cured | 6mm (1/4″) |
Both the ASTM and CSA requirements for tolerance measurements require that they take place within 72 hours of floor slab construction. These fail to inform the unwary specifier and owner of potential changes which will occur in the future due to curling. Owners and specifiers must be aware that tolerances used in specifications must take these losses into consideration in order to produce a final tolerance that meets the needs of the building user.
Please refer to our technical bulletin on curling for further information.