Operating a profitable precast concrete plant hinges on the ability to move product through the facility in a timely manner. A key part of the precast manufacturing process that has a direct relationship to profit is being able to “turn the beds” as often as possible. The phrase “turn the beds” refers to removing completed product so that the next batch of product can be started.The speed at which the producer can engage in this repetitive cycle is what allows them to be competitive in the market. In order to do this, the precaster must attain 75% of the ultimate 28 day designed compressive strength of the concrete within 12 to 15 hours from final placement of the precast product.This 75 % is the level a precast piece needs to attain in order to be stripped and handled.

Air Curing

The most basic and least expensive technique is air curing. This involves simply allowing the concrete to dry naturally without any additional treatments/actions. Most producers will not use this type of curing due to the development of surface cracks and increased strength will not occur earlier in the cycle.

Adding Additional Cement

Adding additional cement to the mix to obtain a higher strength earlier in the curing process. Some plants also change the type of cement used from Type I/II to a Type III to achieve the advanced strength needed to “strip” or remove the product in the 12 to 15 hour time frame. The drawback of this method is an increase in price, as cement is one of the more expensive ingredients in the concrete mix.

Using Chemical Additives

The next curing option is the use of chemical additives, primarily, the use of accelerators. Accelerators increase the rate of hydration, which speeds up the set time, thus kickstarting the curing cycle earlier. The major drawback to accelerators is the increased cost. Accelerators require a seasoned technician with experience and knowledge of the process.

Curing Compounds

Curing compounds are also options when applied after finishing and used in conjunction with curing blankets to trap moisture and elevate the internal temperature of the product...allowing for a greater rate of hydration speeds up the curing cycle and increases the strength earlier. A typical drycast product uses this type of system. 

Water Curing

The last two types of systems are active systems that require equipment and procedures. Water curing is a system by which burlap and soaker hoses are used for a period of time “to soak” the concrete, thus keeping it moist and trapping internal moisture at the optimal level for hydration. This system is also used in hot climates to keep the internal curing temperature from reaching levels that are not conducive to good curing practices. This system is used on DOT projects/products. 

Water and Steam Curing

Finally, a commonly employed technique is a type of water/steam curing system that allows a precast piece to be stripped within the 12-15 hour time. In this case, low pressure steam is introduced into a controlled environment, such as an enclosed structure (or temporary structure like a tarp), at elevated temperatures for a predetermined time and then allowed to be cooled at a controlled rate. This provides a number of  advantages to the curing cycle, such as maintaining relative humidity between 75%-90%. The greatest advantage to this system is the degree of control a producer has with the concrete curing cycle. Many producers customize the steam curing system for changes in the seasons or plant production cycles.

In conclusion, properly cured concrete will, in most cases, out perform design parameters and allow a properly designed structure to increase its life cycle and ultimately be a sustainable and resilient structure far into the future.

References: NPCA Tech Notes, Curing Wet-cast precast concrete