The process of mixing concrete dates back to 5000 BC. Mixed correctly, concrete bonds tighter than any other natural material known to mankind, even though it contains just four simple elements: cement, sand, gravel and water. The key, however, to both strength and longevity is in the process and conditions in which it is mixed. Temperature is critical. Concrete remains a vital resource in construction.

When a large water authority in Georgia decided to construct a new dam just north of the City of Atlanta, a demanding project schedule was proposed with an aggressive completion date. Contractors were tasked with construction of a 950-foot wide and 180-foot high dam, making it one of the largest privately constructed dams in the state of Georgia. The reservoir behind the dam would hold five billion gallons of water intended to provide an additional source of drinking water for the rapidly growing metropolitan Atlanta area.

Expertise & Scale

Given the contractors’ needs for a wide range of jobsite equipment and services, United Rentals was awarded the project as the primary rental supplier. The depth and breadth of United’s technical expertise and fleet played a critical role in allowing the project to move forward on timeline.

United provided general rental equipment such as trucks and forklifts, aerial work platforms, trench safety and shoring solutions, and most critically, Power & HVAC equipment and engineering services.

The Dam project required approximately 235,000 cubic yards of roller-compacted concrete and over 12,000 cubic yards of castin-place concrete. All concrete for the project had to be mixed on site and transported to the worksite via truck and an enormous 1500-foot conveyor system.

Dams, bridges and parking decks are built according to specifications to prevent and eliminate cracking, ensuring structural integrity. During the concrete curing process, temperatures can rise due to a chemical reaction known as hydration.

The concern is that a concrete batch will arrive on the job too warm – above 70 degrees Fahrenheit.

Starting with a cooler temperature means the curing temperatures will be lower. Inconsistencies in temperature – heating and cooling during the curing process – can cause cracks and jeopardize the structural integrity of the concrete structure. Inspectors and engineers will often monitor delivery temperature and reject a batch simply based on the temperature.

For the Georgia Dam project, water used in the batch process was from a municipal water supply which meant water temperatures were too high to conform to the specifications of the concrete mixture.

Because of this constraint, United Rentals Power & HVAC group engineered and designed a chilled water system to accommodate the demanding 24-hour construction schedule. The Power & HVAC technical team utilized a 60-ton chiller along with a 2300-gallon tank, outfitted with large pumps and special electrical controls. The plant operators also devised a sprayer system utilizing the chilled water to wash down and cool the stone aggregate prior to use in the batches.