Summer concreting at 110 degrees F in an arid climate and on a massive scale resembles any other hot weather concreting. But these rigorous conditions require better planning, better organization, closer attention to detail, constant checking and supervision, and use of the most modern technology. This degree of care characterized operations at the Palo Verde Nuclear Generating Station, constructed in the desert 45 miles west of Phoenix, Arizona, in a climate where even wintertime temperatures called for some summertime precautions.

Planning for summer concreting at Palo Verde began in the mix design phase of the project, when the prime contractor chose to replace 15 percent of the cement in the structural mixes with a natural pozzolan. The natural pozzolan was chosen because: it would increase workability at reduced water content; it would contribute to increased long-term ultimate strength (6000 psi in 91 days); it would prevent any possibility of harmful alkali-silica reactivity (which was an added benefit, as the primary safeguard against such reactivity was the use of nonreactive aggregates imported from elsewhere in the state); and finally, as a mineral admixture, it would afford better mix control than fly ash.

For massive summer placements in the desert another attractive attribute of a mix incorporating a pozzolan is that it is slower to set and has a lower early heat of hydration. On the Palo Verde project, some base slabs are 8 feet thick and containment structure walls up to 4 feet thick. The mix design also includes a water-reducing retarder as an admixture. This further reduces the early heat of hydration.