First of all, there is no getting around the fact that the ideal temperature range for placing concrete is 40° to 80° F.  However, high quality concrete work can be done at temperatures above that range. Often specifications will state that cooling is required if ambient or concrete  temperatures exceed a certain limit. My experience has been that most of the reduction in quality occurs between 80° and 90° and there is very little difference between 90° concrete and 100° concrete. Therefore a limit of 90° does not make much sense, and a limit of 80° would be very expensive to implement.

My suggestion is to emphasize prompt delivery and placement. The Concrete Construction  web site says it best:  "A concern regarding delivery and placement times in hot weather is the rate of slump loss in the concrete. One study showed that after 90 minutes of mixing, concrete at 90° F had a 2 3/4-inch slump loss (ACI Journal, August 1977). Specifications on delivery time usually aim to limit the amount of slump loss, so concrete can be placed and finished without adding excess water to the mix. ASTM C 94, "Standard Specification for Ready Mixed Concrete," states that the time from concrete batching to placement should be 90 minutes or the time it takes for 300 drum revolutions, whichever comes first. However, it also states that this requirement may be waived by the purchaser if, after 90 minutes, the concrete still can be placed without adding water to the batch. The 90-minute window is not precise. The key is getting workable concrete without exceeding maximum water requirements. Even if the slump has fallen out of specification, ACI 305, "Hot Weather Concreting," allows for additional water to increase slump as long as the maximum allowable water content or water-cement ratio is not exceeded. High-range water-reducing admixtures also can be used to return the concrete to a workable condition if excessive slump loss occurs. Source: Concrete Construction, August 1994"

Most of the time, quality concrete can be poured without cooling, even on days when temperatures reach 100° or more. Planning ahead, wetting the subgrade adequately, appropriate use of water reducing/retarding admixtures and flyash, evaporation retardants, and proper curing can produce a good job without the use of expensive cooling techniques. The cheapest, most effective way to cure concrete is through the use of a membrane curing compound. Concrete should always be cured regardless of temperature. This is one of most frequently omitted steps in finishing concrete, but one of the most important.

Concrete poured in hot weather will have higher early strength gain, but lower ultimate strengths. Proper mix design can compensate for these conditions, and in combination with protective measures to prevent rapid evaporation, quality concrete can be poured in hot temperatures.