By Claude Goguen, P.E., LEED AP
It is not uncommon for precast concrete manufacturers to face situations where production needs exceed capacity. Double pouring can be a viable solution to the problem.
Double pouring is the process of casting in the same form two times in the same day, effectively multiplying the output of the form. The key to success when double pouring is planning. It is important to recognize production capacity issues long before they occur. Strategies and processes can be set in advance to be better prepared when the need arises. To effectively execute double pouring, consider the following:
- Mix design
- Production processes
- Quality control
Embarking on a double-pouring schedule does not always involve a change in mix design. However, precasters should nevertheless examine the mix design to ensure it is adequate for a significant production modification. When double pouring, precasters usually need to strip products sooner than normal and, therefore, need to accelerate concrete’s strength gain. This may require modifying the concrete mix.
Water-to-cementitious-materials ratio (w/cm)
Strength gain occurs when the water and cementitious materials chemically react and create a paste. That paste hardens at a certain rate due to many factors. One main factor is the amount of water used in the mix.
When a cement grain reacts with water, the hydration products build up on the outside of the grain and progress inward. The volume of the hydrated product is larger than the original cement grains, so as multiple grains hydrate, the space between them shrinks. Soon, adjacent hydrating grains of cement connect and start forming a network of hydration products. This starts the setting process (Figure 1). When there is more water in the fresh concrete mix, the cement grains are farther apart, and it takes longer for connections to occur and the network of hydration products to form because of the added space. More water means slower strength gain and, ultimately, lower strength.
Strategies to lower water content include monitoring aggregate moisture content and using set-accelerating and water-reducing admixtures.
The type of cementitious materials used in precast concrete production affects set time and strength gain. Portland cements come in many different types and blends to address specific demands. Type III cement, also called high early strength cement, provides expedited early-age strength because of its fineness. This can be helpful in double pouring but is not a necessity. Many producers double pour successfully with Type I or Type I/II cement.
Supplementary cementitious materials (SCMs) such as fly ash and slag generally retard concrete set. The degree of set retardation depends on numerous factors, including fineness, composition and amount used. Other SCMs, such as silica fume and metakaolin, can cause faster set. Assessing SCMs ahead of time is recommended when considering double pouring.
There are a few options to consider when looking at admixtures as a strategy for facilitating double pouring. The first involves water content. Water reducers work well when looking not only at accelerating concrete strength gain but also enhancing concrete quality.
Set-accelerating admixtures are effective at increasing strength gain at early ages. Consider non-chloride-containing accelerators when working with reinforced concrete.
Other options are available to precast manufacturers, including hardening or strength-enhancing admixtures. Those focus more on strength development while preserving workability at early ages and are well suited for a double-pouring scenario.1
Mix design modification through testing
The key to mix design modification is testing. To accurately assess the impact of incorporating one change or addition to the mix, samples or cylinders should be made and tested. Strength tests and temperature gain can be measured with each modification and lead to better-informed decisions.
“We will test different cylinders ahead of time to see how they perform,” said Mark Sinicrope, director of technology at Master Builders Solutions. “For example, we will test cylinders with no accelerator, with just an accelerator and then with a mix of accelerator and strength enhancer. The producer can see the results and analyze what option works best for them.”
Time, moisture and temperature play a crucial role in the strength gain of precast products. When double pouring, the goal is to not only reach sufficient strengths earlier but also maintain optimal curing conditions for the structure to continue gaining strength even after it is stripped from the forms.
When looking to strip a product within a few hours rather than the next morning, there are a few approaches to consider.
Raising the temperature
Hydration reactions, like most chemical reactions, are temperature dependent. Higher temperatures lead to faster reactions. That is why an effective double-pouring strategy can include maintaining higher temperatures during placement and curing of precast structures. Most of these temperature-control methods come both prior to and after stripping the product.
The temperature of concrete constituents can have a significant impact on concrete temperature. Consider using warm water and heating aggregates during colder weather. Any surfaces that come in contact with concrete also can affect temperature, so heating formwork and reinforcement is also helpful.
A simple option to preserve or achieve optimal concrete temperatures includes adding insulating blankets over the structures once they are cast, consolidated and finished.
This encloses the heat released by the hydration process – raising the concrete temperature – and helps prevent moisture loss. Using insulating blankets enclosed by moisture-proof coverings is optimal. If this is not sufficient, you can direct heat into an enclosure to help accelerate the curing process. Some producers use an enclosure made of canvas or reinforced polyethylene film to confine the formwork and source of heat.
Avoid pointing heaters directly at the formwork or exposed concrete, as this can create a hotspot and cause subsequent quality issues because of extreme differences in concrete temperature. With gasoline-powered heaters, be wary of carbon monoxide accumulation, not just for a dangerous buildup in the air but also for carbonation of exposed concrete surfaces. Make sure to provide adequate ventilation.
The most efficient means of accelerating strength gain of precast concrete through temperature and moisture control is steam curing. Live steam curing requires enclosures where steam is circulated throughout. Avoid raising concrete temperatures too high.
“You don’t want to exceed an internal temperature of 155 F to 160 F,” GCP Applied Technologies Technical Services Manager Brett Harris said. “You can monitor the temperatures with sensors or wires.”
There are several options in the marketplace for temperature sensors, including thermocouples, wired sensors and fully embedded wireless sensors.
Do not expose a structure to rapid temperature rise when curing or an extreme temperature decline when yarding. Applying heat and then exposing a structure to cold temperatures could lead to thermal cracking. Consider using curing enclosures outside in colder conditions.
“In South Carolina, we ran into a situation where an 8-inch base was poured outside in the sun and cooled at night,” Harris said. “This resulted in cracking due to thermal stresses. Adding Styrofoam to the top of the base solved this issue.” In this case, the Styrofoam served as an insulator.
Stripping and moving the structure
The minimum required stripping strength of a precast structure depends on the structure’s weight, dimensions, how it is stripped, and how it will be lifted and handled. In trying to accelerate the production process, allowing a lower stripping so the product can be stripped sooner could be acceptable if the structure can still be stripped and handled safely at a lower strength. That requires lowering the stresses on the freshly hardened product. There are a few ways to achieve this.
The optimal situation is leaving the product in place and removing all formwork. Dislodging formwork from a stationary structure still applies stress to the product but far less than having to pick up the structure from the formwork. This may require a look at other types of formwork that open, swing and/or collapse, thus enabling the poured structure to stay in place on the production floor or pallet.
Plant conditions may require moving a structure a short distance, even if formwork is stripped away. Casting the product off the floor on dunnage or a pallet system can permit easy movement without exposing the structure to high stresses. Some producers move structures over from their formwork onto dunnage that includes foam or other types of soft supports for added protection. If the product can be left adjacent to the open, soon-to-be-used-again form, that helps maintain a controlled environment for further strength gain.
If the structure must be moved outside and the concrete’s strength development is slower than during normal operations, additional measures may be needed to reduce dynamic loads during transport, such as storing the structures near the plant.
During double-pour periods, the quality control team still has the same function of making sure all products meet or exceed applicable standards. However, they will be harder at work conducting twice the number of pre- and post-pour inspections as well as fresh- and hardened-concrete tests.
Double pouring may require additional qualified QC personnel, which in turn may require additional training and ACI certification.
Modifying the manufacturing landscape and adding more hours to the production day to accommodate a double-pouring schedule comes with added safety considerations.
Safety personnel should be included in planning and implementation so they can identify added risks earlier to remove them or properly train employees.
Additional trained safety personnel may be necessary to monitor longer and busier production days.
Double pour examples
To meet one project’s demands, Andy Hayward of Panhandle Concrete Products in Scottsbluff, Neb., had to pour multiple median barriers each day to meet the project timeline.
“We used both Type III cement and accelerators in our mix,” Hayward said. “We created an assembly line production layout and were able to avoid picking up the median barrier by its lifters. We had two 8-hour shifts, and each form was poured twice per shift, meaning we would pour four times in 16 hours.
“This forced us to look at ways to be more efficient and implement some lean production practices.”
Drew Wieser of Wieser Concrete Products double poured box culverts at their Roxana, Ill., plant.
“We poured two box culverts in the same mold over 12 hours.” Wieser said. “We used SCC with Type I/II cement, a water reducer and accelerator. We would reduce our spread a bit.
“We aimed to reach 2,000 psi in four to five hours. We would strip the culvert without having to lift it so we can just leave it on the pallet.”
Going from a conventional, single-pour-per-day operation to a double-pour operation requires plenty of careful analysis and planning. Waiting until the need for double pouring arrives can make it difficult to plan effectively and push the producer into a learn-as-you-go scenario. This can be costly – even counterproductive.
Take the time now to start planning for this potential manufacturing shift. Even those who have double poured before can re-examine their processes and improve efficiency, discover useful strategies and increase the chances of success when that next big job arrives. PI
Claude Goguen, P.E., LEED AP, is NPCA’s director of technical education and outreach.