Minimum concrete cover for balconies with PT cables

Q. What cover depth should be used for balconies with post-tensioned (PT) and mild reinforcement? I’m designing a condominium on the Florida coast, and it has balconies that are extensions of a PT slab. The PT tendons serve as primary reinforcement for the balconies, so should my specification indicate concrete cover per Table 20.5.1.3.1 or Table 20.5.1.3.2 of ACI 318-19? Furthermore, if I specify the cover per either, do the tolerances listed in Table 26.6.2.1(a) apply?

A. Because your balconies are prestressed, you can require cover per Table 20.5.1.3.2, which specifies a minimum cover of 1 in. (25 mm) for members exposed to weather or in contact with ground. However, it would be prudent to consider additional cover beyond the minimum requirements in Table 20.5.1.3.2 to provide protection of reinforcing bars.

It’s particularly important to have adequate cover on the reinforcement because the balconies will be in a warm, humid, coastal environment. In addition to exposure to chlorides and moisture from seawater spray, the balcony extensions on the floor slabs will create thermal bridges that could increase the incidence of condensation and raise the moisture content in the concrete. Balconies are difficult to repair, so it makes sense to avoid corrosion of the reinforcing bars.

Your construction documents must also specify tolerances, and the cover tolerances should be at least as stringent as those indicated in Table 26.6.2.1(a). However, although Commentary Provision R26.6.2 states that more restrictive tolerances provided in ACI ITG-7 may be desirable for prestressed concrete, note that ITG-7 applies only to precast concrete produced in a factory. The cited tolerances would be difficult to achieve in cast-in-place concrete.

References:  ACI 318-19; ITG-7-09

Topics in Concrete:  318 Building Code; Corrosion; Prestressed; Tolerance