Controlling concrete setting times
Concrete chemistry is complicated, and the makeup of the mix can impact its behavior – from its workability to the set times. Often, contractors need concrete that has an extended set time when concrete is being transported over long distances or when it will be placed during high temperatures.
Temperature plays a role in almost every aspect of producing concrete. As concrete temperatures increase, concrete sets faster. Ready-mix producers often automatically add concrete admixtures called set retarders (“retarders” for short) to the mix to help offset this temperature-related set acceleration. However, the trick is knowing which type of concrete admixture to use and what dosage.
Understanding the limits of set retarders
A common misconception is that retarders can control temperature increases. Concrete gains temperature for one of two reasons: Heat of hydration or ambient temperature increases. Conventional set retarders typically slow the hydration process. This action provides a means of manipulating when temperature gains occur due to the heat of hydration. In contrast, if the ambient temperature is hot, the concrete will be hot. The common cooling options are either chilled water, ice, or liquid nitrogen. Although retarding admixtures do not cool concrete temperatures, at higher dosages, they can be used to offset thermal acceleration. This allows warmer concrete to be adequately placed.
In addition, set retarders cannot hold a concrete slump level indefinitely; at some point, the retarding effect will wear off. Retarders are useful for preventing normal slump loss that occurs during the hydration process, but they cannot help with slump loss due to moisture loss in the concrete mix. A solution for this type of situation is to use set retarders in conjunction with slump extenders. This provides additional benefits, such as:
- Enabling predictable extended set times for continuous placement on mass concrete and tremie projects
- Supporting long hauls to remote sites while preserving slump
- Allowing for extended truck discharge times
- Reducing the need for portable batch plants at the job site
Moving beyond traditional set retarders
Conventional set retarders form a barrier around hydrated products that temporarily stall further hydration from occurring. They typically have a narrow dosage range due to the unpredictability in controlling concrete set times. Conventional retarders work relatively well under low dosages, but when used at high dosage rates, the set extension can be unpredictable, and over-retardation can occur. Often, conventional retarders will hold hydration, then wear off somewhat quickly, giving a short window for finishing operations. This is where advanced hydration stabilizers are useful.
Solutions such as the RECOVER® concrete admixture provide advanced hydration stabilization. The organic chelating agents in the RECOVER® admixture coat the hydrating cement grains. The water and ions needed for further hydration are blocked to suppress cement surface activity. With the hydration reaction suspended, the set time is postponed, and the mix retains its slump, plasticity, air content, as well as a stable temperature for the duration of the set extension. By suppressing all major hydration events, the admixture provides more predictable control over the setting process.
Hydration stabilizers are also less cement-specific than traditional set retarders, which makes the setting effect more consistent across a broader range of cements.
The RECOVER® concrete admixture works well in a variety of applications, including:
- Drill shaft concrete – Typically, concrete for drill shafts must be able to maintain a minimum slump of 4” for the duration of the placement (typically from 2 to 12 hours). For one bridge project in South Carolina, the land-based drill shafts had a dosage rate ranging from 4 to 10 ounces per 100 pounds of cement, with a slump life is 4 to 8 hours. For the drill shafts located in the river, the ready-mix producer used 13 oz/cwt of RECOVER® admixture to achieve a slump life in excess of 12 hours.
- Slip form concrete – For a project in Alabama, a combination of 3 oz/cwt of WRDA® 64 water-reducing admixture and 3 oz/cwt of RECOVER® admixture was used for slip formed curb and gutter mixes and for slip form paving. The addition of the RECOVER® admixture allowed the ready-mix supplier to maintain the low slump required and increase the ease of placement. The mix design was easy to discharge from the truck mixer, requiring less handwork than traditional concrete mixes.
- Roller-compacted concrete – In Mississippi, RECOVER® admixture was used to produce zero-slump concrete that was discharged from the truck mixer with minimal difficulty.
- Slab on grade – In the Florida panhandle, low doses (1.5 oz/cwt) of RECOVER® admixture was used along with WRDA®64 admixture in large slab placements, providing a four to five-hour set time in high-temperature conditions.
- Pervious concrete – Florida Concrete Products Association guidelines for pervious concrete require the use of a hydration control admixture. In most applications for pervious concrete, RECOVER® admixture is used at 6 oz/cwt of cement. This allows for easy discharge of the mix, reduced buildup in the mixer, and increased hydration efficiency of the mix during the seven-day curing period.
- Self-consolidating concrete – RECOVER® admixture extends slump flow life and reduced “stickiness” of self-consolidating concrete.
Determining concrete admixture dosage
Hydration stabilizers provide a linear dosage response for a set of materials. As a rule of thumb, with concrete temperatures around 70-75° F, advanced hydration stabilizers such as the RECOVER® admixture typically provide roughly a 30-minute delay for a 1 oz/cwt dosage. However, if the concrete is naturally setting faster or slower, then the stabilizer dosage will have to be adjusted. Also, the dosage further depends on the mix design, materials, and other factors.
Using supplementary cementitious materials
In the past, it has proven problematic to extend set times when there are supplementary cementitious materials such as slag and fly ash in the mix. In many of these instances, the addition of RECOVER® admixture extended set times in a more consistent manner.
Work with your GCP representative to come up with the ideal solution, based on your specific application, desired concrete performance, and the environment in which the concrete will be poured and placed.
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