A customer recently divided the placement of a large slab-on-grade warehouse floor into 2 pour days. The plans required the contractor to apply a metallic-aggregate dry-shake hardener. For each pour, a standard 3500-psi concrete mix, without air, was pre-ordered. But on the second day, the crew was running late, so the foreman called and added 2% calcium chloride to the order in an attempt to speed up the job.
The first pour area looks great, but the second pour area is covered with a red residue that looks like rust. In addition to the blotchy red spots, the concrete poured on the second day is significantly darker. The contractor has tried to pressure wash off the red spots, but couldn't remove them.
The owner has refused payment until the general contractor resolves these surface problems. Adding fuel to fire, the owner's engineer reported that he worried about the slab's structural integrity. The engineer told the owners that the rust spots are from the calcium chloride corroding the #6 wire mesh embedded in the slab. He's advising a complete tear-out and replacement.
As if this wasn't enough, the concrete contractor just told me he found out that the job specifications forbade the use of calcium chloride and claims the general contractor never informed him of this fact.
What is the cause of the surface discoloration, and what should we advise our customer?
Your customer, the concrete contractor, is in a poor bargaining position. By failing to follow the job specifications, not fully following standard procedures for metallic dry-shake applications, and ordering calcium chloride, the concrete contractor may be held responsible for any costs associated with these problems.
We'll address the color tone difference first. According to ACI 302.1R-96, "Guide for Concrete Floor and Slab Construction," while the addition of calcium chloride accelerates the overall hydration process, it also retards the hydration of the ferrite compounds in portland cement. Thus, concrete batched with calcium chloride will often appear darker than calcium-chloride-free concrete. You should advise your dispatcher to alert customers of this potential color difference when they place orders for calcium-chloride and calcium-chloride-free concrete for the same job. The color-tone difference can be very apparent in side-by-side pours.
The calcium chloride is probably the main cause for the red streaks, too. These surface defects most likely are not caused by corrosion of the reinforcement wire, which should be at least 2 inches beneath the top surface, but by the oxidation of the iron particles in the dry-shake metallic hardener.
Whether the general contractor called attention to the no-calcium-chloride spec is a moot point. Metallic hardener manufacturers recommend using them only in concrete that doesn't use admixtures. This pertains to calcium chloride in particular. ACI 302 states that "metallic hardeners should not be placed over concrete with chloride ion contents greater than 0.1% by weight of cementitious material."
Since this oxidation process will continue for the life of the floor, there are few alternatives to removal and replacement. Since the problem is probably not structural, the contractor might offer to reduce the cost of the portion containing chloride. This would cost him less than removing and replacing all of the concrete.