When can pre-stressed concrete beams be transported to the job site?

The correct choice indicates that pre-stressed concrete beams can be transported to the job site once the concrete has reached the minimum required design strength and has cured for at least 7 days. This timeframe is generally accepted based on the significant gains in strength that concrete obtains shortly after being cast.

In the context of pre-stressed concrete, ensuring that the concrete achieves sufficient strength allows for the safe handling and transportation of the beams without risking structural integrity. The initial strength gain in concrete typically occurs rapidly due to hydration, and by the end of a seven-day curing period, most concrete mixes will have achieved enough strength to accommodate stresses from lifting and transportation.

Longer curing periods, such as 14, 21, or 28 days, are often unnecessary for the transportation phase, particularly for pre-stressed beams, as this can lead to inefficiencies and delays in the construction schedule. Therefore, the 7-day minimum is established based on industry standards and practices that balance safety, efficiency, and the physical curing properties of concrete.