Which of the following circumstances warrants contacting the contractor immediately?

Contacting the contractor immediately regarding the concrete temperature exceeding 90 degrees Fahrenheit is crucial due to the potential adverse effects this can have on the concrete mix and its ability to cure properly. High temperatures can accelerate curing, leading to difficulties in workability, increased risk of cracking, and compromised strength and durability of the finished product. Immediate communication allows for timely interventions, such as adjusting the mix design or altering the pouring schedule to mitigate these effects.

In contrast, while the other scenarios do raise important considerations (such as slump and rebar lap splice issues), they do not present as immediate of a risk as elevated temperatures. Slump at the maximum allowed does not necessitate urgent action unless it is leading to significant workability problems. The exceedance of minimum rebar lap splices is serious, but typically involves follow-up discussions rather than urgent action. Air content within acceptable limits does not warrant immediate contractor notification either, indicating that it is more manageable and less critical than the implications of high concrete temperatures.